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Macroseismic Interpretation of the 1812 Earthquakes in Venezuela Using

Intensity Uncertainties and A Priori Fault-Strike Information

by Jos E. Choy, Christl Palme, Carlos Guada, Mara Morandi, and Stephanie Klarica

Abstract We applied the Bakun and Wentworth (1997) method to the 26 March1812 Venezuelan earthquake. Previous studies have shown that it had at least two largesubevents, one associated with the Bocon fault and an intensity center betweenBarquisimeto and San Felipe, the other associated with the San Sebastin fault nearCaracas. We used the method with two modifications. First, we introduced intensityintervals to accountfor theuncertainties associated with historical information.Second,we incorporated a correction that considers the faults strike, but not its actual position.These modifications were tested with three earthquakes: the 1989 Loma Prieta; a 1980Mw 5:2 event in Tchira, Venezuela; and the 1967 earthquake in Caracas, Mw 6:6.Thethird one was important for the interpretation of the 1812 Caracas subevent. For theBarquisimetoSan Felipe subevent we obtained MWI 7:4 0:35, and an intensity cen-

ter 10.20 N, 69.95 W that lies between Barquisimeto and San Felipe. Our preferredrupturelengthwasbetween90and100km.TheCaracassubeventhad MWI 7:1 0:33,with an intensity center 10.60 N, 67.10 W, close to the Venezuelan north coast. Theexpected rupture length was on the order of 70 km.

Online Material: Three excerpts of contemporary accounts of the 1812 earth-quake, and intensity centers, magnitudes, and minimum and maximum values.

Introduction

The1812earthquakehasprobablybeenthemostdestruc-

tive seismic event during Venezuelan history. It occurred onHoly Thursday, 26 March at 4:07 p.m. local time when themajority of the people were attending the religious services.This hour is well-defined because the cathedrals clock inCaracas stood still. The historical accounts tell about heavydamages in three regions of the country: CaracasLa Guaira,BarquisimetoSan Felipe, and Mrida (Fig. 1).

It is widely accepted that this earthquake broke theBocon and San Sebastin faults. These faults form part ofthe tectonically complex boundary between the Caribbeanand the South American plates (Perez et al., 1997a; Perezet al., 1997b; Audemard et al., 2000; Audemard and Aude-mard, 2002). Several authors (Fiedler, 1961, 1972; CentenoGra, 1969; Altez, 2005, 2006) considered the event as a mul-tifocal one. Centeno Gra proposed three foci, one in theCaribbean Sea between the Los Roques Islands and the coastof La Guaira, near Caracas; a second one in front of SanFelipe, Barquisimeto, and El Tocuyo; and a third one southof the Maracaibo Lake. Fiedler supported the hypothesis ofthree foci: one north of La Guaira, another one on the Boconfault between San Felipe and Barquisimeto, and the third onesouthwest of Mrida. Altez (2005, 2006), who publishednumerous new documentary references about the damages

caused by the earthquake, holds that there was one double

focus seism with epicenters near La Guaira and near SanFelipe and another local earthquakenear Mridathat occurredapproximately one hour later.

Fiedler (1961) assigns surface wave magnitude Ms 6:3to the Caracas event; Ms 6:2 to the one of San Felipe; and 7.0to the one of Mrida. In a reevaluation of the earthquake,Fiedler (1972) assigns body wave magnitude mC 7.1 to eachof the three events that corresponds to a Ms value of 7.3 (forthis equivalence see Richter, 1958). Grases (1990) estimatesthe Ms magnitudes of the ruptures in Mrida, BarquisimetoSan Felipe, and Caracas to 7.0, 7.2, and 6.3, respectively.Grases and Rodrguez (2001) give the following Ms magni-tude values for the events in Mrida, San Felipe, and Caracas:6.56.7, 6.97.2, and 7.1, respectively. Audemard (2002)states that the magnitude of the San Felipe subevent shouldhave been larger than Ms 7:0. Laffaille and Ferrer (2003)consider that the Mrida rupture had Ms 5.15.4 and thata local fault that crosses Mrida and forms part of the Boconsystem (Laffaille and Ferrer, 2005) was probably associatedwith this seismic event.

These estimations were inferred by visual inspection ofisoseismals, a procedure that introduces a strong amount ofsubjectivity that makes it almost impossible to test the results

241

Bulletin of the Seismological Society of America, Vol. 100, No. 1, pp. 241255, February 2010, doi: 10.1785/0120080345

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statistically (Gasperini et al., 1999). Bakun and Wentworth(1997) developed a statistical method to get more objectiveinferences from the intensities of historical earthquakes. Thismethod uses the individual intensity values for estimatingintensity centers and magnitudes. It requires a calibrationprocess involving intensities, magnitudes, and epicentral dis-tances, which is achieved by means of instrumentally reg-isteredmodern earthquakes. It presents some advantages withrespect to the traditional isoseismal method such as: (1) objec-tive analysis of intensities, which is of special importance inthe cases of nonuniform distributions of the intensity datapoints or when the number of intensity points is scarce;(2)assignment of weights forintensityobservations;(3) estab-lishment of confidence intervals for the intensity centers andmagnitudes based on statistical methods.

Palme et al. (2005a) carried out the calibration processfor central and western Venezuela. The results were used in(Palme et al., 2005b) to analyze the intensities related to the1812 Mrida event, obtaining an intensity center betweenMrida and Tabay with a moment magnitude MWI 6:0 0:5.This result agrees well with the one obtained by Laffaille andFerrer (2003) with nearly identical epicenters. Converting themagnitude of Laffaille and Ferrer to a moment magnitudevalue, one obtains Mw 5.65.9 (for the conversion from Msto Mw we used the equation given in Shedlock, 1999); theintervals for the magnitudes overlap, though the confidenceinterval obtained by Palme et al. (2005b) accepts highermagnitude values.

Recently, a significant amount of information about thedamages caused by the Caracas and BarquisimetoSan Felipesubevents was made accessible (Grases et al., 1999; Altez,2006, Sistema de Teleinformacin de la Sismologa Histricade Venezuela, STSHV; see the Data and Resources section);this allowed for an increase in the number of places with

intensity observations and establishment of uncertainty inter-vals for previously reported intensities.

In this article the Bakun and Wentworth (1997) methodis used with two modifications: (1) The first one considersthe cases where the intensity distributions do not show a cir-cular symmetry, but the isoseismals are elongated in the faultdirection and a lack of intensity data occurs at one side of

the fault. (2) The second one relates to the uncertaintiesof the intensity values, owing to the fact that different authorsassign different values to the same place or that damagescannot be clearly defined as a consequence of insufficienthistorical information.

Method

In a previous publication (Palme et al., 2005a) we pre-sented the calibration relation (1) between moment magni-tude, MWI, and MSK intensities, IMSK, for seismic eventsthat occurred in western and central Venezuela applying themethod proposed by Bakun and Wentworth (1997)

IMSK 2:2237 1:6684 MWI 0:04121 med:

(1)

MWI is the magnitude determined using intensities andcalibrated with moment magnitudes; (med ) is the medianof the distances between the intensity center and the indivi-dual intensity points for a value of IMSK. This equation wasobtained using 7 instrumentally recorded earthquakes with atotal of 241 intensities. The magnitudes of these eventsranged from Mw 5.2 to 6.8. It was tested with 7 other instru-mentally recorded events with a total of 213 intensity pointsand with Ms ranging from 5.2 to 7.0. This linear relation was

shown to give good results for epicentral distances up toapproximately 150 km. As has been shown in Palme et al.(2005a), for longer distances the intensitydistance relationis no longer linear in the regions of western Venezuela. So forour data processing we took into account only intensitypoints up to approximately 150 km from the trial epicenter.On the other hand, we did not apply site-effect corrections,as the number of intensity data was too small for a statisticevaluation of site effects and the systematic studies of theinfluence of soil effects on seismic intensities were justbeginning in Venezuela. Strong site effects occurred inCaracas in 1967, but these sites, as we explain later, werenot inhabited in 1812.

In this article we used EMS-98 intensities as explainednext. We determined the intensity centers and the magnitudesof the earthquakes following Bakun and Wentworth (1997)and Hinzen and Oemisch (2001). First, we calculated over agrid of trial source locations using equation (1):

MWIjk IEMSk 2:2237 0:04121 jk=1:6684;

(2)

where IEMSk was the intensity at the kth observation pointand jk was the distance between the jth grid point and

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7

8

9

10

11

12

100 km

CaracasBarquisimeto

San Felipe

Mrida

El Tocuyo

Tabay

Maracaibo

Barinas

Valera

La Guaira

Boc

onFa

ultValencia

Coro

San Cristbal

Los Roques

San Sebastin Fault

MaracaiboLake

Caribbean Sea

Colombia

Venezuela

Brazil

Figure 1. The studied region indicates the main cities andthe principle fault systems in central and western Venezuela, theBocon fault and the San Sebastin fault.

242 J. E. Choy, C. Palme, C. Guada, M. Morandi, and S. Klarica

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the kth observation point. The grid spacing used was 0.05.Then we calculated an average for every grid point:

MWIj meanMWIjk; (3)

and

rmsMWIj rmsMWIj MWIjk rms0MWIj MWIjk;

(4)where rms0MWIj MWIjk was the minimum value ofrmsMWIj MWIjk. The intensity center corresponded tothe grid point where rmsMWIj 0. The resulting magni-tude value, MWI, corresponded to the MWIj of the intensitycenter. The confidence levels for the root mean square (rms)and for the magnitude values were taken from Bakun andWentworth (1999).

Fault-Strike Correction for Large Seismic Events

The equations of the Bakun and Wentworth (1997)

method depend only on the distances between the intensitypoints and the intensity center and not on the angle that thisline forms with the strike of the rupture. However, earth-quakes of Mw 7 produce an average subsurface rupture ofapproximately 50 km (Wells and Coppersmith, 1994). Asa consequence, the intensity distributions should be betterapproximated by elliptical rather than circular isoseismals.This does not represent a problem for the Bakun and Went-worth procedure as long as the intensity points are more orless uniformly distributed around the fault, but a significant

bias may be introduced in the location of the intensity centerwhen there are very few or no intensity points at one side ofthe fault. For example, this is the case with nearly all the largehistorical earthquakes near Caracas, because the principalseismogenic structure, the San Sebastin fault, runs parallelto the coastline; there are very few intensity data from theislands north of the San Sebastin fault. For the reason

explained previously, a systematic shift in the location of theevent, moving the intensity center away from the fault, wasintroduced. To show this effect we designed the followingexample: We assumed an MWI 7 earthquake with a focuslocated at 10.2 N, 70.0 W, and we calculated a grid ofintensity values up to a distance of 150 km from the focususing equation (1). Then we assumed the rupture on an east-west striking fault symmetrical with respect to the epicenter.To obtain elliptical isoseismals, we reduced the distances ofthe grid points from the fault by 20% keeping the originalintensity values. Finally, we eliminated all intensity pointsnorth of 10. The result of this intensity distribution is shown

in Figure 2. Applying the Bakun and Wentworth (1997) pro-cedure to this distribution without modifications resulted inan intensity center at 10.4 N, 70.0 W, which was 0.2 northof the supposed one indicating the influence of the extendedsource.

To stabilize the inversion we introduced an additionalweight function for the intensity observations that dependedon the fault strike, although not on its actual position. Foreach trial epicenter we assumed a trial fault line; in thisway we obtained a set of tentative parallel faults that covered

9.0

9.5

10.0

10.5

11.0

latitudeN

68.569.069.570.070.571.071.5

longitude W

I=7

I=6I=5

I=4I=8

I=7

I=6

I=5

I=4

I=3

E1

E2

E3

LI

Figure 2. This diagram shows the effect of a distribution of intensity values asymmetric with respect to a supposed horizontallystriking fault (straight line LI). The supposed epicenter used to calculate the intensities is point E1 with Mw 7. Applying the conventionalmethod ofBakun and Wentworth (1997), the intensity center E3 is obtained; E2 is the result of the application of the algorithm proposed inthis article.

Macroseismic Interpretation of the 1812 Earthquakes in Venezuela Using Intensity Uncertainties 243

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the epicentral area. In principle the fault lengths were infi-nite, but the influence of the points far from the trial epicenterwere damped by two effects, the equation (1) and the limita-tion on the distance of the intensity points from the trialepicenter to 150 km as explained in the Method section.The error function was calculated as in Bakun and Went-worth (1997); however, the additional weight function

depended on the distance of each intensity point from thefault trace. We used an exponential form for the weight func-tion given by:

pjk wjk expc djk; (5)

where wjk was the weight function given by Bakun andWentworth (1997), c was a parameter we determined empiri-cally, and djk was the distance of the intensity point from thetrial fault with the selected strike. So the error function wasnow as in Bakun and Wentworth (1997):

rmsMWIjMWIjk fkMWIj MWIjk pjk2=Pjg

1=2;

(6)with

Pj kpjk2: (7)

In order to find an appropriate value for the parameterc,we used the same grid of intensity points mentioned beforewith the east-west striking fault (see Fig. 2). We appliedthe Bakun and Wentworth (1997) procedure, but now withthe weight function. The parameterc was varied between 0.0and 0.07, and for values between 0:03 km1 and 0:035 km1

the correct location of the epicenter was found. Larger andsmaller c-values were over and under the correct location,respectively.

The procedure was tested with data of the 1989 LomaPrieta earthquake. Bakun (1999) applied the Bakun andWentworth (1997) method to this earthquake before. Theinstrumental epicenter was near the Californian coast, so themajority of the 560 intensity points were east of the SanAndreas fault. We processed the intensity data, which wetook from the web site of Bakun (see the Data and Resourcessection). We did the calculations using Bakuns own equa-tions for California published in Bakun and Wentworth(1997). First, we ran the program without fault-strike correc-tion obtaining the following results: 37 N, 122.1 W;

MWI6

:92

, which was in agreement with the one of Bakun(1999). Then we ran the program including the correction,assuming a fault strike of 130 clockwise from the north,obtaining for the intensity center a latitude of 37.05 N anda longitude of 121.95 W, which was close to the instrumen-tally determined epicenter: 37.04 N, 121.88 W (Fig. 3), anda magnitude ofMWI 6:8, consistent with the instrumentallyobtained result of Mw 6:9. The localization of the intensitycenter was stable with respect to small changes in the as-sumed fault strike. A variation up to 20 in the fault strikechanged neither the intensity center nor the magnitude.

Incorporation of Intensity Intervals in the Bakunand Wentworth Method

The first step was the preparation of a table with thecoordinates of the geographic locations of the intensity obser-vations and the maximum and minimum intensity values foreach location. If a location had only one intensity value as-signed, this wastaken forboth maximum andminimum value.Next, we supposed N intensity observations for a seismicevent, M < N, with two different intensity values assignedby two or more authors; for N-M localities the differentauthors agreed on one value. The Mlocalities were randomly

permuted and the maximum values of intensity were assignedto the first P of them, where P was a random number thatfulfilled the condition 0 P M. To the rest of the places(M-P) the minimum intensity values were assigned. Thentheprocedure from Bakun and Wentworth (1997) was appliedusing all the N intensity data. This process was repeated alarge number of times, each time a permutation of the Mplaces was applied and a new P value used. The final rmsvalues were the rms of all the individual inversions.

This method was tested with an instrumentally recordedearthquake that occurred on 26 November 1980 in the

-122

37

30 km

San Jose

Santa Cruz

Monterey

Palo Alto

6.8

7

7.2

Figure 3. This map depicts the geographical region of the 18October 1989 Loma Prieta earthquake. The black square representsthe intensity center determined by Bakun (1999); the black starrepresents the intensity center determined using the strike correctionprocedure of this article; and the black triangle represents the instru-mentally determined epicenter. The continuous lines represent con-stant levels for the rms (MWI) corresponding to confidence levels of95%, 80%, and 50%. Dashed lines represent constant magnitudevalues.


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Venezuelan Andes near the frontier with Colombia. Thecoordinates of the epicenter given from different laboratorieswere: International Seismological Center (ISC) 7.87 N,72.40 W; National Earthquake Information Center (NEIC)8.05 N, 72.44 W; and Fundacin Venezolana de Investiga-ciones Sismolgicas (FUNVISIS) 8.05 N, 72.47 W (see theData and Resources section). The magnitude published by

the Global Centroid Moment Tensor Project (see the Dataand Resources section) was Mw 5:2. Intensity values of 42different places were published by the Centro Regional deSismologa de Amrica del Sur database (CERESIS, seethe Data and Resources section); 20 of them showed two dif-ferent values, the differences were one unity (Table S1, inthe electronic edition ofBSSA). The coordinates the intensitycenter obtained were 7.85 N, 72.45 W, the magnitudeMWI 5:2; Figure 4 shows this result and those from ISC,NEIS, and FUNVISIS. This showed the Bakun and Wentworth(1997) method gave robust results even if we admitteduncertainty intervals in the intensity values.

Intensity Data Processing for the 1812 Earthquake

We made a careful analysis of the intensity data pub-lished for this earthquake. These data were found in theSistema de Teleinformacin de la Sismologa Histrica deVenezuela, (STSHV, see the Data and Resources section).In cases of differences between the values assigned by differ-ent authors, we turned to the historical descriptions publishedrecently by Altez (1998, 2005, 2006) and reinterpreted thedescriptions. Moreover, using the Information System wewere able to assess intensities for additional places. Most

of the traditional intensity values in Venezuela are expressedin the Modified Mercalli Intensity (MMI) scale, but there is atendency now to use the European Macroseismic Scale 1998(EMS-98, see the Data and Resources section). Both scaleshave 12 levels. The European scale is newer; it was devel-oped with the intention to keep the internal consistency withrespect to the previous 12 scales, so that previous intensity

assessments would not be in contradiction with this scale(Grnthal, 1998). For the new evaluations we used theEMS-98 scale; we complemented it in some cases with thedescriptions of the MMI scale and the environmental effectsfrom the INQUA Environmental Seismic Intensity Scale (ESI2007; see the Data and Resources section), the last one alsofollowed the basic structure of the 12 macroseismic scales.

During this process we confronted four particular prob-lems common in the seismological analysis of historical data.First, there was a tendency to overestimate the damagescaused by earthquakes in the historical descriptions; thiswas analyzed by Rodrguez and Audemard (2003) and Laf-faille et al. (2007) for the case of Venezuela. In those cases

where we assessed our own intensity values, especially whenthese differed from those of other authors, we preventedexaggeration by reading the documents very carefully andconsidering their historical and social contexts. This contextin 1812 was a particular one, because Venezuela found itselfin a state of war and the interests of the writers had to betaken carefully into account. The publication ofAltez (2006)helped to understand this complex situation. In cases ofdoubts, we used the lower value or, as our method allowedit, we used an intensity interval.

Second, the EMS-98 scale required the definition ofthe vulnerability class of the buildings; the MMI scalealso classified the buildings as well-built or poorly built.The common buildings at that time in western and centralVenezuela were constructions of adobe, bahareque (a con-struction technique that used earth reinforced with inter-twined sticks of cane), or tapias (compacted earthconstruction material). The adobe and bahareque houseshad straw roofs, while the constructions of tapias mostlyhad tile roofs. The vulnerability of those constructions de-pended strongly on the quality of the building materialsand on the construction quality itself. Adobe constructionswere classified as A in the EMS-98 scale. The bahareque con-structions, the traditional way used by the natives, were con-sidered to be very ductile and for this reason rarely collapsed

(see the Data and Resources section). Nevertheless, in theCariaco earthquake of 1997 in eastern Venezuela, the major-ity of the collapsed houses and those that were extremelydamaged were bahareque constructions (FUNVISIS, 1997).This report assigned intensity 8 and locally 9 to the center ofCariaco. However, bahareque constructions rarely causedcasualties because of the light construction materials. More-over, in the Cariaco 1997 event, most casualties were causedby the collapse of modern concrete buildings. Mocquet(2005) reported from the same event in Cariaco an intactbahareque house nearby to two completely collapsed

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8

30 km

5.5

5.5

66

6.5

1

3

4

2

Colombia Venezuela

Figure 4. This figure represents the seismic event of 26 Novem-ber 1980. The continuous and dashed lines follow the pattern ofFigure 3. (1) The intensity center is determined by the methoddescribed in the article; (2) the epicenter is determined by ISC,(3) or determined by NEIS, or (4) by FUNVISIS. (see the Dataand Resources section). The circle indicates the 90% confidenceinterval indicated by ISC.


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unreinforced brick houses. Laffaille (2008) classified bahar-eque construction as less vulnerable than those of adobe. Soit was impossible to find a general rule for the vulnerabilityof bahareque constructions as long as we did not know moredetails about the individual construction quality. Moreover,as these were the houses of the native people, the historicaldescriptions did not pay much attention to them. So we

preferred to consider the bahareque houses as belonging tovulnerability class A, avoiding an overestimation of thedamage. On the other hand, the constructions of tapias hadvery heavy roofs and very little resistance to shearing forces(Castillo, 1983), thus, their tile roofs were prone to collapseduring earthquakes. Both the adobe and the tapias construc-tions were very vulnerable; we considered them both as be-longing to vulnerability class A.

Many of the intensity data of the 1812 earthquake re-ferred to monumental buildings, churches, and other publicbuildings. The construction of those buildings was probablyof better quality; generally, they used some reinforcing ele-ments, but the structural and nonstructural complexity of

such buildings is such that they may be more likely to bedamaged than ordinary buildings, even though they maybe better built (Grnthal, 1998, seccion 3.5). Referring tothis problem, Altez (2006) cited Ibarra (1862, p. 325, trans-lation provided): The bad quality of the constructions of themonumental buildings in Caracas was in a big percentageresponsible for the damages produced in Caracas. We as-sumed that in the villages and small towns of the countrythe monumental buildings were even more vulnerable anddecided to classify the monumental buildings as vulnerableaccording to class A, though this could have, eventually, ledto some subestimation of intensity values.

Yamazaki et al. (2005) in their estimation of seismic in-tensities in Caracas during the 1812 earthquake used a modelwith 75% of the buildings of vulnerability class A and 25%of the buildings of vulnerability class B. This could havebeen a good approximation for Caracas, but generally, in theinterior of the country a higher percentage of very vulnerablebuildings was expected, as outlined previously.

The third problem in the assessment of intensity was re-lated to the fact that at many places we only had descriptionsreferring to the damages of a monumental building. As theEMS-98 scale applied statistical criteria we also consultedthe MMI scale in these cases, but always permitted an inten-sity interval to indicate the uncertainty in interpretation

(Grnthal, 1998).The fourth problem was related to site effects. The influ-ence of soft soils and strong impedance contrasts on macro-seismic effects was an important point for the evaluation ofhistoric earthquakes. But in Venezuela there existed only afew places where systematic studies had been carried out.Caracas is known for the strong site effects observed in the1967 earthquake. These were due to deep sedimentary basinfills in some places of Caracas (Los Palos Grandes, SanBernardino), whose existence was confirmed by recent stud-ies (Kantak et al., 2005), Nevertheless, none of the heavily

affected places formed part of the town in 1812. Yamazakiet al. (2005) published a map indicating the intensity distribu-tion in the historical town of Caracas during the 1812 earth-quake; those values varied between 7 and 9 MMI. Theauthors did not specify whether this was a statistical effector a consequence of site effects. In any case the interval ofintensity variation was slightly more than one, which did

not imply a strong site effect. There are current studies inBarquisimeto to obtaininformation aboutpossible site effects.According to Rocabado et al. (2005), the sediment deposits inthe center of Barquisimeto had relatively high shear wavevelocities (400600 m=sec)anddepthsbetween30and50m,without strong impedance contrasts (the shear wave velocityin the underlying layers reached 1000 m=sec), so strong siteeffects were notexpected in thearea occupied by thehistoricaltown. These were the only data available in western and cen-tral Venezuela. Therefore, we could not take into account siteeffects for our article. According to Bakun and Wentworth(1997), who compared results with and without site-effectcorrections, the areas bounded by confidence-level contours

were significantly larger without site corrections for eventswith few MMI observations. Bakun and Wentworth affirmedthat while the results without site corrections were good, theywere significantly better with site corrections.

In Tables S2b and S3b ( in the electronic edition ofBSSA) we published the intensity data assessed by otherauthors and in this article, together with the most importantquotations and their references for the BarquisimetoSanFelipe subevent and for the Caracas subevent, respectively.The intensity values we used are shown in Tables S2a andS3a ( in the electronic edition of BSSA); in Figures 5aand 5b those values are mapped. We used clocklike symbolsto represent the intensity values. They may indicate onesingle value or an intensity interval as demonstrated inFigure 5c.

Observations about Intensities of SomeSelected Places

In this section we present a discussion of some intensityvalues about which we found major disagreements amongthe different authors.

BarquisimetoSan Felipe Subevent. Barquisimeto: Theintensity values assigned for Barquisimeto by different

authors varied between 8 (Altez, 2006) and 9 (CERESIS). Butthe opinions about the grade of destruction in Barquisimetodiffered widely. Altez justified his relatively low intensityvalue with the arguments that reconstruction of Barquisimetowas carried out at the same place, that the city was not movedafter the earthquake, and that reconstruction started nearlyimmediately after the earthquake, much more rapidly thanin San Felipe and Caracas. Also economic and administrativeactivities in Barquisimeto, according to Altez, were not inter-rupted by the earthquake. Contrary to that opinion, Querales(1995), a chronicler of Barquisimeto, gave evidences in his


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publication that reconstruction of Barquisimeto occurred notabove the ruins, but at a new place called La Paya, not so faraway from the original one; and at least until 1831 (Jefepoltico del Cantn, 1831, cited by Querales; Table S2b, in the electronic edition ofBSSA), the old town of Barqui-simeto was still in ruins with only very few houses standing.According to Querales andother sources (Table S2b, intheelectronic edition ofBSSA), the destruction of Barquisimetowas nearly total. To accountfor the divergences between thesetwo opinions, we decided to assign intensity interval 810.

San Felipe: It is evident from the historical descriptionsthat San Felipe was heavily destroyed by the earthquake(Table S2b, in the electronic edition of BSSA). Thereis a discussion (see Altez, 2005) whether this was a primaryeffect of the earthquake or whether it was a secondary effect

of the Yurub River, in which the river was repressed by alandslide and a few days later the natural dam was brokenand San Felipe was devastated by the flood. As Altez con-firmed, however, the situation was not clear from historicaldocuments. In any case, it was clear that the devastation ofSan Felipe was vast and, as Altez noted, it took years for thepopulation to come back to live in San Felipe. Altez assignedonly intensity 8 to this place because of the doubts men-tioned. We believed that total destruction of at least manybuildings and probably of the majority of them occurredin San Felipe. Even if repression and flooding by the YurubRiver was the reason, this would require at least intensity 9because many landslides occurred (Table S2b, in the elec-tronic edition ofBSSA). Big landslides and vast inundationscaused by the rivers La Virgen and Guarebo were also

-70 -69 -689

10

11

12

30 km

Yaritagua

Albarico

San Felipe

Cocorote

Chivacoa

Guama

Barquisimeto

Cabudare

UrachicheJoval

Puerto Cabello

Cojedes

Montalbn

Valencia

Nirgua

El Tocuyo

Canoabo

Carora

Arenales

San Carlos

Coro

Ospino

El Pao

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10

11

12

30 km

La Orchila

MacutoMaiqueta

La Guaira

Choron ChuaoCaraballeda

TocomeCaracas

Los TequesBaruta

La Vega

San SebastinCagua

La Victoria CaucaguaSan Diego

Guarenas

El HatilloOcumare Costa

Carmen de Cura

Guatire

ChaguaramalSan JoaqunOcumare Tuy

Capaya

Magdaleno

Mamporal

Cpira

MaracayTurmero

San Mateo

Taguay

= 5 = 9

4 5 8 10

(a) (b)

(c)

Figure 5. Intensity values used in this article are shown for (a) the BarquisimetoSan Felipe subevent and (b) the Caracas subevent of the1812 earthquake. Clock symbols represent intensity values. (c) The clocks may indicate one single value or an intensity interval.


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reported from Cocorote, a village a few kilometers to thesouthwest of San Felipe.

Cocorote: A considerable number of people (no numberis given) died beneath the ruins of their houses and many ofthem were heavily wounded in this place. To describe thedamages, three witnesses (Table S2a, in the electronicedition of BSSA) used qualifications such as: annihilated

the church and the village

,general wreckage of churches,houses . . . , and total extinction (of the village). These

statements can be interpreted that the damages of grade 5belong to the category most of EMS98 and intensitywas 10. On the other hand, Altez (2005) assigned intensity9 to Cocorote. The landslides and inundations mentioned inthe previous paragraph occurred near Cocorote. So, as in SanFelipe, an intensity interval between 9 and 10 was assignedto this place.

Albarico, Guama, and Chivacoa: The historical re-ports about the effects of the earthquake in these three places,though they were quite poor, had similar characteristics:destroyed houses with some people killed by the ruins of the

falling houses and churches converted to dust. Aftershockswere felt from the first day of the event. Only in Albariconoise was reported (frequent explosions). The number ofpeople that were killed by the earthquake in these villagesseemed to be quite low (9 in Albarico, 15 in Guama, and9 in Chivacoa; see Table S2b in the electronic editionof BSSA). Altez (2006) mentioned that at that time Guamahad 3293 parishioners, and Chivacoa together with Guarahad 2856). There were several reasons for this. First, aswas explained by Juan Domingo Linares (Table S2b, in the electronic edition of BSSA), a significant part of themen were in the barracks of San Felipe since Holy Tuesday,where many of them died. Second, as Altez (2006) explained(p. 396, translation provided): . . . the natives and the slavesresided in the poorest houses (adobe walls and straw roofs)and the pardos (the cheap labor force for the white Criollos),because of their economic condition, had to live in the sub-urbs of the towns and villages in houses of bahareque andstraw roofs, therefore a big part of the population shouldhave escaped from dying buried beneath heavy tile roofs.Houses made of bahareque with light roofs had a high prob-ability to permit survival (Laffaille C., internal report forCIGIR, Mrida, unpublished, 2008). So we concluded thatthese three places still formed part of the most heavily dam-aged region and we assigned intensity interval 910.

Puerto Cabello: The historical descriptions for thistown clearly indicated less damage to the buildings than forthe localities previously described (Table S2a, in the elec-tronic edition ofBSSA). Only three people died oppressed bythe ruins. Puerto Cabello was an important colonial harborwith heavy colonial constructions; a death rate of only 3 peo-ple out of 6000 (Grases, 1979) meant that only a few housescollapsed; this was in accordance with the information givenby one of the historical sources, an eyewitness, that a dozenhouses collapsed. On the other hand, the same sourcereported that all the houses were affected and their roofs were

detached. Even if the expression all was an exaggeration,we considered that most of the houses suffered damagesbetween grade 3 and 4, which meant an intensity intervalbetween 7 and 8.

Caracas Subevent

Caracas: The CERESIS intensity catalog showed inten-sity values between 8 and 10 MMI for Caracas. Yamazakiet al. (2005) performed a detailed study of the damage dis-tribution for the 1812 earthquake in Caracas and they simu-lated this damage pattern by an earthquake near the coast ofCaracas. The intensity interval obtained by these authors wasbetween 7.9 and 9.4 MMI. When we rounded these values, asGrnthal (1998) suggested, and accepted the equivalencebetween the labels of EMS-98 and the MMI scale, we ob-tained an intensity interval 89 for Caracas. As previouslyexplained, there was no reason to think that strong site effectscould be expected in the historical center of Caracas as thesites that showed strong effects in 1967 (four 10-story build-

ings collapsed in Los Palos Grandes), because of their rela-tion to the deep sedimentary layers were far to the east of thehistorical Caracas (Seed et al., 1970).

La Guaira: CERESIS published 8 MMI and Altez(2005) 9 MMI for this place. From the analysis of the histor-ical documents we concluded that the destruction in LaGuaira was even stronger than the destruction in Caracas.There were at least two documents written by eye-witnessesclearly expressing this opinion. Four of the sources we cited(Table S3b, in the electronic edition of BSSA) used theexpression total to describe the destruction of La Guaira;one said that it was laid in ruins. These comments impliedthat intensity could have reached grade 10. There was also areport about enormous precipitations of material detachedfrom the summits and profound cracks at the base of therocks. The ESI 2007 (see the Data and Resources section)intensity definitions mentioned landslides from degree 8 on,but large landslides begin with degree 9. There was no pos-sibility that saturated soils could have facilitated the land-slides because the earthquake was preceded by five monthsof drought (Delpech, 1813). Therefore, intensity was at leastat degree 9 and could even have been as high as 10. Thisinterval agreed with the classification of the two aspectsrelated to the construction mentioned previously. In conclu-sion, we assigned an intensity interval from 9 to 10 for La

Guaira.Maiqueta: Altez (2006) assigned intensity 9 toMaiqueta, a few kilometers to the west of La Guaira. Theinhabitants of Maiquetia abandonedthevillage after theearth-quake because their houses were destroyed. Also, the churchwas totally destroyed. So we concluded that damage was verysimilar to that in La Guaira. On the other hand, the descriptionthat William Duane (Table S3b, in the electronic edition ofBSSA) gave about Cabo Blanco, a placeveryclose to Maique-ta, seemed to indicate a very big landslide above sealevel anda submarine one, though the phenomenon below sea level


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could also have been a liquefaction process. In both cases theintensity level would have been at least 9, but it could alsohave been 10.

All other intensity values for the Caracas subevent canbe taken from Tables S3a and S3b ( in the electronic edi-tion of BSSA).

ResultsThe BarquisimetoSan Felipe Subevent

We used intensities from 23 places, 20 of them withtwo different values; at three places the differences were 2(Table S2a, in the electronic edition ofBSSA). Data wereinverted without and with the fault-strike correction. Withoutthe strike correction the coordinates of the intensity centerwere 10.38 N, 69.0 W, and the magnitude was MWI 7:50:35. The intensity center was approximately 20 km north ofthe northern trace of the principal fault in the region, theBocon fault (Fig. S6, in the electronic edition ofBSSA).However, we considered that this location suffered from biasbecause nearly all the intensity points (Fig. 5a) were south-east of the Bocon fault, and there were only four points tothe northwest. Assuming that the strike of the rupture was thesame as that of the Bocon fault (approximately 55 clock-wise from the north), the inversion was repeated with strikecorrection. It is important to point out that we were notimposing the condition that the source would lie on theBocon fault as we only constrained the strike of the causa-tive fault, not its actual position. For the coefficientc of equa-tion (5), we used the value 0:035 km1 as in the exampleexplained previously. The result is shown in Figure 6; thecoordinates of the intensity center were now 10.20 N,

68.95 W, and the magnitude was MWI7

:4

0

:35

.In Figure 7 we show the intensity (individual andmedian) values versus distances from the calculated intensitycenter, separately for the minimal (Fig. 7a) and the maximal(Fig. 7b) intensities. The theoretical relation from Palme et al.(2005a) is also shown. Even in case of the minimumintensity values the inverted magnitude of the earthquakeis MWI 7:05.

The Caracas Subevent

The Caracas subevent showed a pronounced effect ofasymmetry of the distribution of the intensity points.

Following Fiedler (1961) and Audemard (2002), the eventoccurred near the Venezuelan Caribbean coast within theeast-west striking San Sebastin fault system. The only inten-sityvaluereportedfromthenorthofthecoastlinewasfromtheLa Orchila island. The intensity data with their minimal andmaximal values are listed in Table S3a ( in the electronicedition ofBSSA) and mapped in Figure 5b. We used a total of33 different sites, 15 of them had two intensity values with adifference of 1, and 18 had a single value. We corrected for afault strike of 90 and used for c the value 0:035 km1, thesame that gave us good results for the BarquisimetoSan

Felipe subevent. We obtained an intensity center with thefollowing coordinates: 10.60 N, 67.10 W, and MWI 7:10:33 (Fig. 8). For comparison, the result without fault-strikecorrection can be seen in Figure S8 ( in the electronic edi-tion ofBSSA).

As before, in Figure 9 we show the intensity (individualand median) values versus distances from the calculated

intensity center separately for the minimal (Fig. 9a) and themaximal (Fig. 9b) intensities.

The Mrida Subevent

Palme etal. (2005b) published results for the reevaluationof the Mrida subevent applying the method of Bakun andWentworth (1997), in that case without considering intensityintervals or applying fault-strike corrections (Fig. 10a).Confidence contours were not well constrained in the direc-tion perpendicular to the Bocon fault system as all theintensity points were aligned with the fault, which extendedalong thecentral valleyof theMrida Andes with theprincipal

populated places. In Figure 10b a map with the intensitypoints (Table S4, in the electronic edition of BSSA) isshown. In this case applying a fault-strike correction wouldnot have improved the result as the magnitude of the eventwas only moderate and therefore a significant rupturelength was not expected. Also, as the intensity points werealigned with the fault trace there was no asymmetry in theirdistribution.

-70 -69 -68

10

11

30 km

7.5

8

8

8.59

9.5

Barquisimeto

San Felipe

Figure 6. The intensity center is calculated for the Barquisime-toSan Felipe subevent of the 1812 earthquake constraining thefault strike. The star represents the intensity center determined inthis article; the triangle is the epicenter determined by Fiedler(1961). The continuous and dashed lines follow the pattern ofFigure 3.


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Discussion

BarquisimetoSan Felipe and Caracas SubeventsThedetermined intensity center of the BarquisimetoSan

Felipesubeventwasclosetotheepicenterproposedby Fiedler(1961) (10.2 N, 69.1 W) and was located in between SanFelipe and Barquisimeto close to the northern trace of theBocon fault. The magnitude MWI 7:4 exceeded all pre-viously published values published for this earthquake,though it overlapped with the magnitude interval given byGrases and Rodrguez (2001). This magnitude implied a sub-surface fault rupture length of 104 km with a 95% confidenceinterval from 43 to 172 km following Wells and Coppersmith(1994). The heaviest damages were reported (Fig. 5a) in the

region between Barquisimeto in the southwest and the villageof Albarico, which was 75 km northeast of Barquisimeto.However, Albarico was still within the zone of highest inten-sities (910), therefore the rupture probably extended to thenortheast of this locality. It was unlikely that the rupturereached Puerto Cabello, as the damages reported there weresignificantly less and the aftershocks were felt only slightly(Table S2b, in the electronic edition ofBSSA). In Canoabo,37 km northeast from Albarico, intensity 7 was assessed,therefore the fault rupture probably finished somewherebetween Albarico and Canoabo. The nearest village to thesouthwest from Barquisimetowas El Tocuyo with a felt inten-sity of only 67 and therefore also was outside of the rupturezone. So an estimation of a rupture length of 90100 km wasreasonable.

For the Caracas subevent we noted that the comparisonwith published results (Fiedler, 1961; Grases and Rodrguez,2001) showed that our intensity center was quite near to theepicenter determined by Fiedler, 10.6 N, 66.90 W (Fig. 8),and there was a good coincidence with the most recently pub-lished magnitude by Grases and Rodrguez, Ms 7:1. Also thescenario earthquake used by Yamazaki et al. (2005) to simu-late the destructions in Caracas caused by the 1812 earth-

quake had Mw 7:1, but there was a tendency in recentpublications to localize the epicenter more to the west fromthe one obtained in this article, just in the middle between LaGuaira and Puerto Cabello (Audemard, 2002). The subsur-face rupture length for a strike-slip earthquake of Mw 7:1would have been between 29 and 115 km (95% confidencelevel, average 68 km; Wells and Coppersmith, 1994).

Because of their magnitudes and the related rupturelengths, we attributed, as well as other authors (for example,Fiedler, 1972; Audemard, 2002), these two subevents to theBocon fault system (BarquisimetoSan Felipe) and theSan Sebastin fault system (coast of Caracas). Both faults

3

4

5

6

7

8

9

10

11

Iminimum

0 50 100 150 200

distance from the intensity center (Km)

3

4

5

6

7

8

9

10

11

Imaximum

0 50 100 150 200


(a) (b)

Figure 7. Graphs depict the intensity center of the BarquisimetoSan Felipe subevent: intensity (black diamond is individual intensity;white square is median intensity) values versus distance from the calculated intensity center are shown separately for (a) the minimal and(b) the maximal intensities. Heavy lines represent the best fit to the median intensity values. Dashed lines represent the theoretical lines fromPalme et al. (2005a).

-68 -67 -66

10

11

30 km

7

7.5

7.5

8

8

8

8.5

Caracas

Maracay

Valencia Lake

Figure 8. The intensity center is calculated for the Caracassubevent of the 1812 earthquake. The star represents the intensitycenter calculated in this article constraining the fault strike. Thetriangle represents the epicenter proposed by Fiedler (1961). Thecontinuous and dashed lines follow the pattern of Figure 3.


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(Audemard et al., 2000) are part of the most importantfault system in Venezuela, a right lateral strike-slip system

that goes from the southwest of Venezuela (frontier withColombia) at first striking in a northeast direction throughMrida, Barquisimeto, San Felipe, and Puerto Cabello(Bocon fault) and then in a west-east direction parallel tothe northern coastline passing north of Caracas (San Sebas-tin fault), near Puerto la Cruz, Cuman, Cariaco, and ElPilar (El Pilar fault). From our results we affirmed that theBarquisimetoSan Felipe event (MWI 7:42 0:35) was larg-er than the Caracas event (MWI 7:08 0:33), though theconfidence limits of both magnitudes overlapped.

To break both faults, Bocon and San Sebastin, fromBarquisimeto up to La Guaira, a rupture length of approxi-

mately 380 km was required. With our average magnitudevalues and the upper extremes of the 95% confidence levels

from the data ofWells and Coppersmith (1994), this rupturelength was still not reached. However, we estimated an errorof0:35 for the magnitudes. This meant that a continuousrupture was acceptable if we assumed magnitude values nearthe upper end of the confidence interval. As we mentionedbefore, our magnitude values were among the highest of allthe ones published. So we could not exclude the possibilityof a continuous rupture from Barquisimeto to the coast nearCaracas, but it seemed to be less likely than a discontinuousone. In the former case, probably, the fault-slip was unevenalong the fault, with maxima near San Felipe and nearCaracas.

3

4

5

6

7

8

9

10

11

Iminimum

0 50 100 150 200


3

4

5

6

7

8

9

10

11

Imaximum

0 50 100 150 200


(a) (b)

Figure 9. Graphs represent the intensity and distance of the Caracas subevent: intensity (black diamond is individual intensity; whitesquare is median intensity) values versus distance from the calculated intensity center are shown separately for (a) the minimal and (b) themaximal intensities. Heavy lines represent the best fit to median intensity values. Dashed lines represent the theoretical lines from Palme et al.(2005a)

-72 -71 -70

8

9

30 km

6.5

6.5

7

7

7

7.5

8

Maracaibo

Lake

Mrida

Boco

nFa

ult

-72 -71 -70

8

9

30 km

Maracaibo

Lake

MridaTabay

Ejido

La Mesa

Lagunillas

Chachopo

Mesa Bolvar

Timotes

La Grita

Trujillo

Barinas

(a) (b)

Figure 10. (a) Intensity center for the Mrida 1812 subevent is taken from Palme et al. (2005b). The triangle represents the epicenterpublished by Fiedler (1961) and the star is the intensity center determined by Palme et al. The continuous and dashed lines follow the patternof Figure 3. (b) Intensity values used. The clock symbols follow the pattern of Figure 5.


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Mrida Subevent

The Mrida event was only of local importance. How-ever, due to the close proximity of the epicenter the damagesin this town were quite heavy. The sparse information did notallow a determination as to whether it was caused by therupture of the Bocon fault or another local fault relatedto the Bocon system.

Though the results for the intensity centers and the mag-nitudes of the 1812 subevents were reasonable, there mightstill be a systematic error in the magnitude values as thecalibration equation (1) was obtained with earthquakemagnitudes from Mw 5:2 to 6.8, because we had no largermagnitude values for the calibration (Palme et al., 2005a).The implicit extrapolation could lead to a light increase inour magnitude values for events with magnitude larger thanMw 6:8.

Relation between the BarquisimetoSan Felipeand the Caracas Subevents

None of the documents related to BarquisimetoSanFelipe told us anything about the temporal sequence of theshaking, but we know from several documents (Altez, 1998)that the earthquake was felt in this region approximately atfour oclock in the afternoon (only one out of eight docu-ments cited by Altez mentioned three oclock as the hourof occurrence of the earthquake, all others said it was at fouroclock or a little later). This hour was very similar to the onereported from Caracas, though there is also an exception inCaracas; Delpech (1813), whose publication in Le Journalde Paris will be analyzed here, said that the earthquakeoccurred at five oclock in Caracas. Anyway, it was tradi-tional knowledge in Venezuela that the earthquake stoppedthe cathedrals clock at 4:07 p.m. and we accepted that bothsubevents occurred approximately at the same time. Butwhich one was first? There is an interesting article fromLouis Delpech, a French official stationed in Caracas whowitnessed the earthquake. The article gave many detailsabout the time sequence of the movement; it was publishedin Le Journal de Paris in December 1812 (translationprovided):

On 26 March 1812 at five oclock in the afternoon thefirst concussion was perceived. The air was calm, the warmthimmoderate: nothing preceded or announced the terribleevent. The first tremor was sufficiently strong to put the bells

in movement. It lasted just some 6 seconds. During the fol-lowing interval of 10 to 12 seconds, the earth was agitated bya wave movement similar to one of the ocean in a moment ofquiescence. People thought then that the crisis had passed,but very soon a subterranean noise was heard together withelectrical detonations much more violent than the explosionof thunder. The earth was agitated with a velocity that I couldnot describe, it seemed to boil as water exposed to a violentfire. During three or four seconds a deaf noise could beheard; some minutes afterward the earth suffered new agita-tions in the opposite direction, from north to south and from

east to west. This short time was sufficient to destroy the cityof Caracas from up to down.

Delpech subdivided the first movement into two phases,a shock of 6 seconds was followed by a wave movementsimilar to one of the ocean in a moment of quiescence thatlasted 1012 seconds. A very heavy shock that put the city ina violent agitation followed. Delpech told that a subterranean

noise was heard, together with electrical detonations muchmore violent than the explosion of a thunder. We believedthat this violent movement accompanied by an explosion-like noise was the principal shock produced by the rupturenear Caracas. The heavy agitation of the earth like boilingwater, as described by Delpech, seemed to be related to ver-tical high-frequency waves. So what would be the movementof the first 1618 seconds? According to our interpretation,the first 1618 seconds of movement felt in Caracas werecaused by the combination ofSwaves and surface waves thathad their origin in the focus of BarquisimetoSan Felipe.

We included the French original of the article and twomore descriptions of the temporal sequence felt in Caracas

(see Documents, in the electronic edition ofBSSA). Theyagreed with Delpech in that there were at least two groups ofarrivals and that the second one was stronger.

Relation between the 1812 Caracas Subeventand the 1967 Caracas Earthquake

It is interesting to observe that the intensity center for the1812 Caracas subevent was very close to the epicenter of theearthquake that destroyed Caracas on 29 July 1967. In orderto better understand the relationship between these twoevents, we also present here our results for this earthquake.

For the intensity values of this earthquake we used twodifferent sources, the publication of Surez and Nbelek(1990) and the values published in the CERESIS database(see the Data and Resources section). We combined both listsusing the minimum and maximum values for every place(Fig. 11 and Table S5, in the electronic edition ofBSSA).So we obtained 28 intensity points, 9 of them with two dif-ferent values; in some cases the difference was 2. For theazimuth correction we supposed an azimuth of 90, whichwould have been approximately the strike of the San Sebas-tin fault system in front of the coastline near Caracas, and avalue of0:035 km1 for the constantc. We obtained an in-tensity center in 10.60 N, 67.20 W, with a magnitude of

MWI6

:4

0

:3

. Without the strike correction we got an in-tensity center farther offshore (Palme, Morandi and Choy,2005a) in 10.95 N, 67.20 W, and consequently a biggermagnitude of MWI 6:7. The result of our localization canbe seen in Figure 12a. In the same figure the result of themost recent relocation done by Engdahl et al. (1998) isshown; the coordinates of their epicenter are 10.558 N,67.31 W, about 13 km to the east of our intensity center.

Surez and Nbelek (1990) made an inversion of longperiod World-Wide Standard Seismograph Network seismo-grams for this earthquake and found that it was a multifocal


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event: three subevents with a strike-slip motion triggeringsequentially from west to east and one dip-slip event slightlydeeper and more to the north than the others. For the locationof the first subevent they used the ISC epicenter (10.68 N,67.4 W). The distances between the subevents were in theorder of 50 km from one another, while the individual rup-ture lengths were not supposed to be longer than 12 km.Therefore, Surez and Nbelek concluded that a ruptureof four different fault segments had occurred that accountedfor the main part of radiated seismic energy and no energywas irradiated in the segments between faults. Due to the

short time delay between the individual ruptures, they sug-gested that there was not the stress concentration at the end ofthe individual ruptures that triggered the next event, but theyproposed that it was the passage of the surface waves thatcaused the chain reaction. In the opinion of these authors thiskind of rupture was indicative of a complex and severelyfaulted plate margin where the faults were highly stressed.

In a similar way the 1812 Caracas subevent was probablyalso a complex rupture of the San Sebastin system. How-ever, the event that triggered the chain reaction was located inthe Bocon fault rather than in the San Sebastin system.

With the Engdahl et al. (1998) relocation of the 1967epicenter, the first two subevents were on the San Sebastinfault trace if the error in the epicenter location was taken intoaccount. Our intensity center fell in between these two sub-events (Fig. 12b). This made sense as these two subeventsreleased approximately 72% of the total moment producedby the earthquake and they were the ones closer to the dam-age areas; the other two subevents were not only smaller, but

also farther away from the coast and therefore they had lessinfluence on the intensity distribution. This explained thelower value obtained by us for the magnitude.

According to these results, both the 1812 and the 1967events broke the San Sebastin fault. Moreover, the ruptureof the first one included the segment between the two firstsubevents of the 1967 earthquake, which in turn rupturedthose fault segments that were not broken or where the stresswas not totally released in 1812. On the other hand, this laterevent probably did not rupture the San Sebastin fault east ofthe second 1967 subevent as this zone was broken in 1900 by

-68 -67 -66

10

11

30 km

CaracasCarayaca

Los TequesPetare

Antmano

Santa Teresa

Santa Luca

Ocumare del Tuy

Charallave

Naiguat

GuarenasPuerto Cabello

Valencia

Caucagua

Turmero

El Sombrero

HigueroteMaracay

Guatire

Ca

CaraballedaMaiquetaCatia La Mar

Tucacas

Boca de Aroa Osma

S. Juan Cayos

Figure 11. Intensity values used in this article represent theCar-acas 1967 earthquake. Clock symbols follow the pattern of Figure 5.

-68 -67 -66

10

11

30 km

6.56.5

7

7

7.5

7.5

7.5

San Sebastin Fault

Valencia Lake

Caracas

La Victoria Fault

Maracay

-68 -67 -66

10

11

30 km

San Sebastin Fault

Valencia Lake

Caracas

La Victoria Fault

Maracay

(a) (b)

Figure 12. (a) The star represents the intensity center determined in this article for the Caracas 1967 event with correction for the faultstrike. The triangle represents the epicenter relocated by Engdhal et al. (1998). The continuous and dashed lines follow the pattern of Figure 3.(b) The square is the intensity center of the 1812 Caracas subevent. The triangles represent the four subevents of the 1967 Caracas earthquakeaccording to Surez and Nbelek (1990) relocated with respect to the Engdhal localization. The westernmost triangle and the star have thesame meaning as in (a).


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a Ms 7:6 event (Lugo, 1984; Pacheco and Sykes, 1992; Perezet al., 1997b; Audemard, 2002).

Conclusions

The incorporation of intensity intervals in the Bakun andWentworth method gives reliable results. This is important

for the reevaluation of historic earthquakes because the un-certainties of the intensity levels are a well-known difficultyin assessing intensity values using historical information ofdamages.

When we have knowledge of the potential fault strike, itis possible to get a considerable improvement for the local-ization and magnitude of large historical earthquakes in thosecases where the intensity points are located mostly at oneside of the fault, for example, when the epicenter of the earth-quake occurs near a coastline. This approach was validatedwith the intensity data from the 1989 Loma Prieta earthquakein California and the 1967 Caracas earthquake; it improvedthe results for the 1812 earthquake in Venezuela.

The earthquake of 26 March 1812 in Venezuela had threesubevents: the biggest one between Barquisimeto and SanFelipe with MWI 7:4 0:035, a second one in front ofCaracas with MWI 7:1 0:33, and a local event near Mridawith MWI 6:0 0:5. We cannot exclude the possibility of acontinuous rupture from Barquisimeto to the coast nearCaracas, but it seems to be less likely than a discontinuousone. The BarquisimetoSan Felipe subevent broke theBocon fault, whereas the San Sebastin fault was brokenby the 1812 Caracas subevent as well as by the 1967 Caracasearthquake. The rupture lengths estimated from the magni-tude values agree quite well with the area of the heaviestdamages. There are good reasons to assume that theBarquisimetoSan Felipe subevent happened first and theCaracas subevent occurred only seconds later.

Data and Resources

The information system Sistema de Teleinformacin dela Sismologa Histrica de Venezuela (STSHV) was searchedusing http://sismicidad.ciens.ula.ve (last accessed 10November 2008).

The Global Centroid Moment Tensor Project databasewas searched using http://www.globalcmt.org/(last accessed10 November 2008).

The intensity database from the Centro Regional de Sis-mologa para Amrica del Sur (CERESIS) was searched usinghttp://www.ceresis.org (last accessed 10 November 2008).

TheEuropean Macroseismic Scale (EMS-98) was search-ed using http://www.gfzpotsdam.de/pb5/pb53/projekt/ems(last accessed 10 November 2008).

Information about bahareque constructions wasobtained from http://es.wikipedia.org/wiki/Bahareque (inSpanish, last accessed 26 May 2009).

The INQUA Environmental Seismic Intensity Scale (ESI2007) was searched using http://www.apat.gov.it/site/_Files/

Inqua/ESI07_definition_intensity_degrees.pdf(last accessed10 November 2008).

All figures were made using the Generic Mapping Tools(Wessel and Smith, 1998).

Fundacin Venezolana de Investigacin Sismolgicas(FUNVISIS) http://www.funvisis.gob.ve/.

The fault map of California was taken from http://

earthquake.usgs.gov/regional/qfaults/ (last accessed 10 No-vember 2008).International Seismological Center online bulletin was

found athttp://www.isc.ac.uk, Thatcham, United Kingdom.National Earthquake Information Center (NEIC) data

were found athttp://neic.usgs.gov (last accessed 10 Novem-ber 2008).

Intensity data for the Loma Prieta earthquake were takenfrom Bakuns web site at http://quake.wr.usgs.gov/~bakun/(last accessed 24 May 2008).

Acknowledgments

We thank the CDCHT of the Universidad de Los Andes, Mrida,Venezuela, for the financial support through the projects N. C-1178-03-02-A, and C-1202-03-02-C. Most of the historical data were taken fromthe Sistema de Teleinformacin de la Sismologa Histrica de Venezuela(STSHV), a project that was supported by the Fondo Nacional de Ciencia,Techonologa e Innovacin (FONACIT) of Venezuela. The fault maps fromVenezuela are a courtesy of FUNVISIS. We acknowledge K. G. Hinzen andan anonymous reviewer for valuable comments that helped us to improve themanuscript.

References

Altez, R. (1998). Cronometrizacin extempornea: Los sismos del 26 deMarzo de 1812 en Caracas y en Mrida, Rev. Geog. Venez. 39,209326 (in Spanish).

Altez, R. (2005). Todo lo que se movi en 1812 en la placa del Caribe:Sismos, volcanes y transmisin de energa, Rev. Geog. Venez Nmeroespecial, 143170 (in Spanish).

Altez, R. (2006). El Desastre de 1812 en Venezuela: Sismos, Vulnerabil-idades y Una Patria No Tan Boba, Universidad Catlica Andrs Belloy Fundacin Polar, Caracas, Venezuela, 522 pp. (in Spanish).

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Laboratorio de GeofsicaFacultad de CienciasUniversidad de Los AndesLa Hechicera MridaEstado Mrida Venezuela

Jos. E. Choy, [email protected] Palme, [email protected] Guada, [email protected] Morandi, [email protected] Klarica, [email protected]

Manuscript received 30 November 2008
