Engg Seismolog Ak

ELEMENTS OF ENGINEERING SEISMOLOGY

Department of Earthquake Engineering, IIT Roorkee

INTRODUCTION

Earthquakes have been an integral component of the geologic evolution of planet earth. Since the dawn of civilization, mankind has been continuously suffered due to their disastrous effects. Earthquakes usually strike without warning and have several direct effects (e.g., ground shaking and permanent crustal movements) and induced effects (e.g., landslides, avalanches, ground subsidence, liquefaction, ground fissuring, tsunamis, seiches, and fire). Broadly speaking seismology is defined as the science of earthquakes and related Phenomena. Earthquake engineering deals with the study of earthquake effects on people and their built environment and device ways and means to reduce these effects. Therefore, in the study of earthquake engineering the knowledge of seismology is required to understand various aspects of earthquake phenomena including the ground motion characteristics and ground shaking hazards associated with the occurrence of earthquakes.The planet earth on which we live has been evolved over millions of years and is undergoing a continuous change. The earth is both rotating around its axis and revolving around the sun. It is a significant member of our solar system because it is capable of sustaining various forms of lives. The earth is not only rotating and revolving but also slowly deforming due to internal and external forces. The earth is roughly spherical with an equatorial diameter of 12,740 km and a polar diameter of 12,700 km. The earth weighs about 5.4 x 1021 tons which indicate an average specific gravity of about 5.5. One of the important contributions of seismology has been in the determination of the internal structure and composition of the earth from the study of seismic waves produced by large earthquakes.

DEFINITIONS OF SOME COMMONLY USED TERMS

Earthquake: An earthquake is perceived as a sudden transient motion of the ground. This motion originates from a limited region inside the earth and spreads from there in all directions. This motion occurs on account of sudden release of elastic strain energy stored along active geological features.

Elements of Engineering Seismology / 1

Macroseismic Effects: Macroseismic effects of earthquake are those that can be observed on the large scale in the field without the aid of instruments.Microseismic Effects: Microseismic effects are small scale effects that are observable with the help of instruments.Teleseism: An earthquake recorded by a seismograph at a great distance is termed teleseism. By international convention this distance is more than 1000 kilometers from the epicenter. Earthquakes originating nearer the recording station are termed as local earthquakes.Intensity: It refers to the degree of shaking at a specified locality. It is rating assigned to a locality using a descriptive scale, with grades indicated by Roman numerals from I to XII.Magnitude: It is the measure of size of an earthquake and is intended to be a rating of an earthquake independent of the place of observation. Normally It is calculated from measurements of amplitudes of seismic waves observed on the seismograms.Isoseismals: These are curves connecting localities where equal intensity was observed in a given earthquake. They are now more commonly mapped as boundaries between regions of successive intensity ratings.Meizoseismal Area: The area within the isoseismal of highest intensity is the meizoseismal area.Hypocenter or Focus: It is the point from where the elastic disturbance constituting an earthquake originates. This point is believed to represent the position of the initial rupture of the rocks. The depth of this point from the earth’s surface is called focal depth (d) of an earthquake.Epicenter: The point on the surface of the earth vertically above hypocenter is called epicenter.Epicentral Distance (): It is the distance from the place of observation or recording station to the epicenter. Due to large distance involved this distance is expressed in terms of angle between the two radii joining the epicenter and place of observation to the center of the earth. However, for local earthquakes it is stated in terms of kilometers.Focal Distance or Hypocentral Distance: The distance (d2+2)1/2 from hypocenter to the recording station is called the focal distance or hypocentral distance.Origin Time: The onset time of an earthquake at the hypocenter.Transit or Travel Time: The time taken by a seismic wave to travel from hypocenter to recording station. This time is the difference between the origin time and the arrival time of a given seismic wave.


Seismograph: An instrument that records as many details as possible of the ground motion in the vicinity of particular point following an earthquake is called a seismograph.Seismogram: It is a time-wise record of the ground motion written by a seismograph.Accelerograph: An earthquake recording instrument that measures the ground motion in terms of acceleration in the epicentral region of strong shaking. It writes the time wise amplitude of ground acceleration at a particular locality.Accelerogram: The ground motion record produced by an accelerograph is called accelerogram.Microearthquakes: Very small earthquakes having magnitude less than three on Richter scale are called microearthquakes. High gain high frequency seismographs are employed to monitor these for seismological and engineering applications.Lithosphere: The outer rigid shell of the earth, situated above the asthenosphere and containing, the crust, continents and plates.Asthenosphere: The world wide layer below the lithosphere which is marked by low seismic velocities and high seismic attenuation. The asthenosphere is a soft layer, probably molten. It may be site of convection.Plate: One of the dozen or more segments of the lithosphere that are internally rigid and move independently over the interior, meeting in convergence zones and separating at divergence zones.

Plate Tectonics: Theory and study of plate formation, movement and destruction; the attempt to explain seismicity, volcanism, mountain-building and paleomagnetic evidence in terms of plate motion.

HOW AN EARTHQUAKE IS PRODUCED

An earthquake occurs when the elastic strain energy stored in the rocks is suddenly released. The question arises as to how this energy is produced and how it is stored. If we take a sample of rock and apply sudden force the rock will break into pieces. However, if the force is applied very slowly the rock mass will not break, but would experience a change or start deforming. This deformation process will lead to changes in the shape and size of the rock mass. Therefore, under slow varying stresses the rocks behave in an elastic manner and theory of elasticity is applied to study their behavior. When the rocks deform, the strain energy is stored in them like it is stored in a compressed spring. It has also been observed in the laboratory experiments that brittle rocks when subjected to triaxial compression, microcracks are formed parallel to the axis of


maximum compression when applied stress reaches one third to two third the fracture stress at a given confining pressure. When we apply this concept on a much larger scale i.e., crust of the earth or lithosphere, then several factors may participate in the overall deformation process. Some of the important factors are: the time over which the stresses are applied, the rate at which the stresses are applied, compressive and tensile strength of rocks. Other possible factors are the temperature, confining pressure and presence of liquids at crustal depths. When the applied stresses exceed the compressive or tensile strength of rocks, the rocks fail by rupturing. This is also called brittle deformation. This sudden failure will produce a fracture surface and stored elastic strain energy in the rocks is suddenly released producing an earthquake.

SEISMIC WAVES

The strain energy released from an earthquake source propagates in the form of elastic waves also called seismic waves. These waves are recorded by the seismographs placed on the surface of the earth. The seismic or elastic waves propagate through the whole of the earth interior, or along its surface layers. A typical seismogram of an earthquake is shown in Figure 1. The seismic waves are of two main types namely, the body waves and the surface waves. The seismic waves that propagate through the interior or body of the earth are called body waves. These are of two types, namely, i) the longitudinal waves or P-waves, and ii) the transverse or shear waves or S-waves. The seismic waves that propagate along the surface of the earth are called surface waves. These are also of two types, viz., i) the Love waves (LQ) and the Rayleigh (LR) waves.

The total sequence of seismic waves is termed as the wave train. The beginning of each new burst of energy is called a phase. The amplitude is one half the trace height. The time between the first wave arrival and the drop off to background noise is called earthquake duration or coda length.

Important parameters of each wave type are as follows:Longitudinal waves or P-waves

1. Possess longitudinal motion analogous to sound waves.2. Propagate through all material types (solid, liquids and gases).3. Are the first type, in the total wave train, to arrive at a

seismological station, hence are called primary waves. The velocity (Vp ) is determined by

Vp = {(K + 4 / 3)/ }1/2

Where K = bulk modulus (resistance to volume change) = rigidity modulus (resistance to change in shape) = density


4. Create a push (compression) and pull (dilation or relaxation) effect on materials through which they pass in a vector parallel to the wave path.

5. Can be perceived, if the earthquake is large enough, as a sudden shaking emanating from below the surface.

Transverse waves or S-waves1. Are similar to electromagnetic waves in that there are vertical (SV)

and horizontal (SH) components each at right angles to the direction of wave propagation.

2. Propagate only through solid substances because gases can not be sheared and hence Vs is determined by

Vs = ( / ) 1/2

3. Shear and twist crustal material as they move through it.4. Often more damaging to the works of construction than the P-waves.

Surface Waves1. Originate near the epicenter and move outward in all directions.

Surface waves can circle the earth several times and generate 1mm ground amplitudes in large earthquakes.

2. These waves are generally having 10-20 sec wave periods, 20-80 km wavelengths and 3 km/ sec velocities.

3. Even though love waves contain substantial amount of energy their long periods smooth there imparted ground motion, thereby greatly reducing their damage potential.

4. Wave amplitudes are largest at the surface, decreasing with depth, and the largest amplitudes are associated with shallow focus earthquakes.

5. Are reflected or refracted from original wave paths when geological contacts are encountered.

Love Waves1. Love waves result from the interaction of SH-waves with the

earth’s surface layer. Therefore, they are similar to SH waves and the ground motion is in the horizontal plane, and particles vibrate at right angle to the direction of wave propagation. These waves cannot pass through water bodies.

2. Usually faster than Rayleigh waves. The velocity of Love waves, (CL) is determined, by the relation,

Vs1 < CL < Vs2where Vs1 and Vs2 are the velocity of S waves at the surface and subsurface layers.


Rayleigh WavesRayleigh waves are produced by interaction of P-waves and SV-

waves with the earth’s surface They are having both vertical and horizontal particle motion. Due to this they introduce retrograde elliptical motion similar to ocean waves The velocity of Rayleigh waves (CR ) is determined by the relation: CR < 0.92 Vs, where Vs is the velocity of surface layer.PLATE TECTONICS

Most of the global earthquake activity can be explained within the framework of plate tectonics model of the earth. According to the theory of plate tectonics the lithosphere is broken into a number of moderately rigid plates whose outlines are shown in the Figure 2. These plates slide over a partially molten plastic asthenosphere. The relative directions of plate motions are also shown in this figure. There are six continental-sized plates namely, Africa, America, Antarctica, India-Australia, Eurasia, and Pacific and about 14 sub-continental sized plates. Due to relative motion of plates at plate boundaries the plates deform. The relative deformation between plates occurs in narrow zones near their boundaries. The deformation can occur slowly and continuously (aseismic or creep type) or suddenly (seismic slip or stick-slip type) in the form of earthquakes. According to the relative motions of adjacent plates we can define three kinds of plate boundaries:

(i ) Diverging plate boundaries or spreading ridges, typically ocean ridges.

(ii ) Transform plate boundaries or fracture zones.(iii) Converging plate boundaries or subduction zones.

Along diverging plate boundaries plates separate and in the process of plate separation, partially molten mantle material upwells along linear ocean ridges. New lithosphere is created along the trailing edges of the diverging plates. Such boundaries are characterized by basaltic volcanism, shallow focus earthquakes and high rates of heat flow. About 2.4 km2/year of oceanic material has been generated for the last 180 million years. The current estimated rate is about 3.1 km2/year. At diverging plate boundaries the spreading rate ranges from 2 to 18 cm per year. The ridge earthquakes are characterized by normal faulting and strike-slip faulting occurs along connecting transform faults. Fast spreading centers are seismically less active compared to the slow spreading centers which are seismically more active.

Along transform plate boundaries plates slide past one another, with neither creation nor destruction of lithosphere. Occasionally marked by scarps, transform faults are characterized by shallow focus earthquakes


with horizontal slips. Faults of this type include Motagua fault of Central America, San Andreas Fault of North America, and Fairweather fault along coastal Alaska. The Motagua fault separates the North American and Caribbean plates whereas the San Andreas and Fairweather faults separate the American and Pacific plate. Plate motions are retarded at shallow depths along transform faults. When in a locked position, strain accumulates near the faults and when the frictional strength of the rocks is exceeded the plate edges snap past each other creating often major to great earthquakes. Earthquakes along transform faults occur at very shallow depths usually less then 20 km.

Along converging plate boundaries the leading edge of one plate overrides another. The overridden plate is being subducted or thrust into the mantle where lithosphere is resorbed. The thrusting mechanism that operates along these collision boundaries tend to produce deep-sea trenches, shallow and deep focus earthquakes, adjacent mountain ranges of folded rocks, and both basaltic and andesitic volcanism.

Each plate is bounded by some combination of these three kinds of plate boundaries. For example the Nazca plate in the Pacific is bounded on three sides by diverging plate boundaries along which new lithosphere forms and on one side by Peru Chile trench, where lithosphere is consumed. Continental plates are thicker and more buoyant and are not readily subducted. Where two plates with continents at their leading edges coverage the crust thickens to form great mountain ranges like the Himalaya.

OCCURRENCE OF EARTHQUAKES

Earthquake Date: The knowledge regarding the present pattern of distribution of earthquakes over the earth has been known since early this century. The recording and interpretation of data obtained from the establishment of world wide system of seismological stations made this possible. Seismological stations installed all over the world are equipped with seismographs which record seismic waves generated due to the occurrence of earthquakes. The simplest possible means of recording is to mount a paper on a drum, which rotates at a constant speed, and a stylus produces a time versus amplitude record of seismic waves generated from the earthquakes. Various types of recording media (e.g., photographic, smoke paper, heat sensitive and pressure sensitive) are employed for recording. To facilitate reading of seismograms time marks in terms of hours and minutes are normally produced employing very accurate chronometer. Now-a-days due to advancement in technology the earthquake data is recorded employing digital seismographs or P. C. base systems and in such cases the recorded data has to be retrieved


employing suitable softwares. However, the record is not accessible to direct examination as in the case of analog recording.

From the analysis of recorded earthquake data the earthquake parameters and source parameter are computed and the earthquake catalogues are prepared. These catalogues provide information on origin time, epicenter location (in terms of latitudes and longitudes) focal depths, magnitudes and other related parameters of the recorded earthquakes. The most substantial catalogues of the world seismicity based on instrumental records is that of international seismological center located at Edinburgh (U.K.). This catalog lists earthquake hypocenters and origin times, recorded travel time and other data since 1918. Other important international lists are Bulletin Mensuel de Bureau Central Seismologique produced at Strasbourg, France and catalogues of United States Coast and Geodetic survey (USCGS) Washington D. C.

The United States Geological Survey’s (USGS) National Earthquake Information Service (NEIS) with headquarters in Golden Colorado, has the major responsibility for collecting, analyzing and publishing world wide earthquake data. 4000 to 6000 moderate magnitude earthquakes are analyzed every year at NEIS adopting the following procedure:

1. Earthquakes are recorded at a number of seismological stations (more than 650 in number) which include 116-stations of Worldwide Standard Seismograph Network (WWSSN).

2. An observer at each station interprets the earthquake record (seismogram) for phase identifications, arrival times, wave periods, and wave amplitudes.

3. The data is mailed to NEIS and is fed into computer twice daily and the earthquake parameters, namely, epicenters, hypocenters and magnitudes are computed.

4. NEIS provides information on the location and magnitudes of all worldwide earthquakes of magnitude 6.5 and larger in the United Stated within 1 to 2 hr after the occurrence of earthquake.

5. Disaster assistance agencies, field scientists and news media are notified worldwide. Some of the agencies contacted include the International Tsunami Information Service (ITIS), the Red Cross, and the Federal Disaster Assistance Administration.

Several government publications are devoted to the reporting on the worldwide occurrence of earthquakes; namely,

i) The preliminary determination of Epicenter (PDE) published weekly with a monthly summary.

ii) The earthquake data report is a biweekly issue which gives data used in computing the listing of PDE.


GLOBAL DISTRIBUTION OF EARTHQUAKES

Majority of earth’s earthquakes occur at plate boundaries due to plate deformations. These deformations result on account of slow plate movements at plate boundaries. Based on the distribution of epicenters of earthquakes it is revealed that almost all earthquakes occur in one of three major seismic belts or zones. These belts are: the Circum-Pacific Belt, The Alpine-Himalayan Belt or Alpide Belt, and the Mid-Oceanic Ridge Belt.

Gutenberg and Richter (1954) on the basis of distribution of 5100 earthquakes (mb> 6.5) divided the earth into seven belts of decreasing seismicity. These belts are: Circum-Pacific, Alpide-Asia, ocean ridges, continental rift zones, active areas marginal to continents, areas of older orogenies, and stable continental masses.

The available data of earthquakes indicate that destructive earthquakes have occurred in several regions. The frequency of occurrence varies considerably from region to region. Because of limited data sample, it is not possible to estimate the inter-occurrence time. Moderate to large earthquakes have occurred in China, Japan, India, Iran, Turkey, South and West of USSR, Palestine, Italy, Spain, Yugoslavia, Morocco, Parts of USA (California, Missouri), Central America, Alaska and New Zealand.

Selected earthquake locations from 1977 to 1994 and plate boundaries are shown in Figure 3a. It is interesting to note that from the locations of earthquakes the plate boundaries can be demarcated or delineated (Figure 3b). From the study of distribution of epicenters of earthquakes following inferences can be drawn:

Almost all earthquakes occur in one of the three major seismic belts, namely, the Circum Pacific belt, the Alpine-Himalayan (or Alpide ) belt and the Mid-Oceanic ridge belt.

The Circum-Pacific belt is confined to the margins of the Pacific Ocean. This is the most important seismicity belt in terms of energy release and frequency of occurrence of earthquakes. Most of the great earthquakes (M >8.0) have occurred in the Circum-Pacific belt and because most of the earth’s active volcanoes are associated with this belt, this belt is called Ring of Fire.

The east-west trending Alpide-Himalayan belt runs from Asia to Mediterranean. This belt coincides with the probable boundary separating the African, Arabian, and Indian-Australian plates from the Eurasian plate.

Most of the ocean basins are aseismic but the part of mid-oceanic ridges are accompanied by small to moderate magnitude earthquakes. In fact the trend of ridges can be mapped from the trend of epicenters. These mid oceanic ridge belts occur in the Indian, Pacific and Atlantic


oceans and present few earthquake hazards except at their continental intersections (e.g., Siberia, Iceland, New Zealand, Chile, west coast of North America).

The old pre-Cambrian shields of Africa, India, Siberia, Australia, Canada and Brazil are essentially aseismic as are Antarctica and most areas of the ocean basins.

Deep focus earthquakes occur only in few geographical regions, namely, South America, Tonga, New Hebridge, Japan sea. Tonga and Kermadec trenches in the south western Pacific accounts for most of the deep focus earthquakes within the Circum-Pacific belt. For focal depths exceeding 500 km the most active area in the world is between the Tonga trench and Fiji Islands.

75% of energy is released due to earthquakes occurring in the crust (focal depths less than 60 km). Frequency of occurrence of earthquakes falls off rapidly above 200 km depth.

From the study of global distribution of earthquakes following important conclusions can be drawn:

(1) Earthquakes are global but their present geographical distribution is structured and follows defined patterns. There are extensive aseismic regions and belts of high seismicity.

(2) Earthquakes occur in continents as well as in oceans, and cluster strongly in both space and time.

(3) The earthquakes occur up to 700 km depth. (4) Major and great earthquakes (where field observations are possible)

are usually accompanied by ground deformation, fault rupture, subsidence and surface uplift.

(5) Tsunamis are large sea waves and are caused by large submarine earthquakes.

EARTHQUAKE INTENSITY AND INTENSITY SCALES

Intensity represents direct or macroseismic effects of an earthquake on humans and their products and features on the earth surface at some locality. Intensity is determined by direct observations of the effects of earthquakes in the field. It is a measure of the severity of an earthquake. Intensity is a highly variable quantity and depends upon following factors:

Magnitude of the earthquake,Distance from the Epicenter,Focal depth of the earthquake,Geologic and soil conditions of the locality, Type of construction (including age and workmanship),Expertise of the observer measuring the intensity The earlier efforts to determine the size of an earthquake were

based upon personal observations of severity of the earthquake effects.


Investigators found that human reaction and characteristic pattern of damage were very similar for most earthquakes. These effects were categorized to make comparative studies of earthquakes occurring at different times in the same or different locations. Intensity scales were developed by grouping earthquake effects characteristic of each scale value. Intensity scales: A number of intensity scales have been introduced in countries having high seismic risk. In 1880s De Rossi of Italy and Forel of Switzerland introduced the first intensity scale, which became very popular. Both men had devised their own intensity scales earlier and 1883 scale resulted from their joint efforts. The Rossi-Forel (RF) scale comprised of 10 effect descriptions. Each designated by Roman numeral with I indicating the least amount of earthquake effect. For assigning the intensity value, the procedure was to compare observation assessments to those of the standard scale description. The number representing the closest match was the numerical intensity assigned to the locality. With advancement of technology it was realized that RF scale had several shortcomings.

Mercalli updated and expanded RF scale descriptors in 1902 but the ten levels of intensity were kept. Cancani later expanded the Mercalli scale to 12 levels. This scale was called Mercalli-Cancani scale.

Seiberg published an expanded version of the Mercalli-Cancani scale in 1923. In 1931, Wood and Neumann made significant modifications in this scale to by and large suit California conditions. This intensity scale is called Modified Mercalli (MM) Scale of 1931 (Appendix 1).

Richter made further modifications to the MM scale and this version is called MM scale, 1956 version. This scale is currently used by the U. S. Geological Survey. The 1956 version incorporates effects on four types of masonry buildings.

The MSK scale (devised by Soviet geophysicists Medvedev, Sponheuer, and Karnik) is more comprehensive and describes the earthquake intensity more precisely. This scale is based on classifying the structures into three types, damage into five grades, and quantity or number into three types. The scale was proposed for international use in 1964. This scale is similar to GEOFIAN and is used in Soviet Union and on experimental basis in Japan. In India we use both the MM scale and MSK scale for measuring the intensity of earthquakes.Collection and Evaluation of Intensity Data: In order to measure the intensity at different localities the special expeditions and field parties of experts are sent to the epicentral area or the affected area immediately after an earthquake. The felt area may be very large and it may not be possible to visit all the localities. In view of this the questionnaires are distributed to the public for recording their response on the effects of


earthquakes. The responses collected on felt effects of earthquakes through questionnaires and from personal observations are evaluated by team of experts, and intensities are assigned to various localities.Isoseismal Maps: A map is constructed to show the variation of intensity with distance from epicenter for a particular earthquake. The procedure is to plot the intensity values at their respective localities on a base map. The localities having equal value of intensity are connected. These lines are called isoseismals and the map is termed the isoseismal map. Following observations are made from the isoseismal maps:

1. Isoseismals and size of intensity zones depend on epicentral distance.

2. Generally isoseismals are not symmetrical and this is caused due to variations of fault geometry, local geology and soil conditions.

3. Intensity is usually higher at the epicenter and in loosely consolidated soil then in hardrock

4. Closely spaced isoseismals normally indicate a shallow focus earthquake.

Empirical Relationships: Several empirical intensity-epicenter distance relationships have been developed and are of the following form: I0 – I = n log(r/h) + absorption termwhere I0 = epicenter intensity; I = intensity at a distance X from the epicenter; h = focal depth; r = epicenter distance; n = constant to be determined. The reported values of n are 3,5 and 6. The absorption term is not considered for distances less than 100 km. Following relationship has been developed between ground acceleration (a in cm/sec2) and intensity for California region:

Log a = I/3 – ½Gutenberg and Richter derived following relation between the local

magnitude (M) and epicentral intensity (I0) for southern California earthquakes (where focal depths were usually about 16 km).

M = 1 + 2I0/3Using the world wide intensity and acceleration data following

regression relationships have been developed to compute the horizontal component of peak acceleration and horizontal component of peak velocity:

log a = 0.30 Imm + 0.014 (IV Imm X)log v = 0.25 Imm – 0.63 (IV Imm X)

where a = estimated horizontal component of peak acceleration in cm/sec2

v = estimated horizontal component of peak velocity in cm/sec


Imm = Earthquake intensity on MM scale

EARTHQUAKE MAGNITUDE

To measure the size of an earthquake, American seismologist C. F. Richter introduced the magnitude scale in 1930’s, while issuing the listing of earthquakes occurring in Southern California. Richter started with two assumptions. First, if two different sized earthquakes having the same hypocenter and recorded at the same station, the larger earthquake will have larger amplitude on the seismogram. However, if the hypocenters differ, the smaller earthquake may occur closer to a given station and hence, will have larger amplitude on the seismogram. Second, seismographs of the same type are placed at varying distances to record these two earthquakes. If a graph is plotted with the maximum ground motion on the Y-axis vs. the epicentral distances on the X-axis for each station two curves can be constructed by connected X-Y station coordinates. The higher curve represents the larger earthquake. With these assumptions Richter plotted logarithm of maximum observed amplitudes virus epicentral distances for various located earthquakes in Southern California falling within an epicentral distance of 600 km. The logarithm plots for different earthquakes showed parallel curves. A dashed curve was drawn parallel to these curves at an arbitrary level to define the threshold value (Figure 4). These curves showed that the difference between the logarithms of amplitudes for any two given shocks is independent of distance, and amplitudes are in constant ratio. With this concept a quantity, called local magnitude or magnitude on Richter scale (ML), was defined as: ML = Log A – Log A0

Where, A is the recorded trace amplitude on a standard type of seismograph for a given earthquake at a given distance and A0 is the amplitude of a zero magnitude earthquake selected as standard. The magnitude is thus a number characteristic of a given earthquake.

Definition of Local Magnitude (ML): The local magnitude on Richter Scale is defined as the logarithm of the maximum trace amplitude (expressed in microns) recorded on a Wood-Anderson torsion seismograph with specified constants: free period = 0.8 sec, static magnification = 2800, damping = 0.8, when the seismograph is placed at an epicentral distanced of 100 km. A0 is the amplitude of zero magnitude earthquake and is taken as one thousand of a mm at an epicentral distance of 100 km. The magnitudes of earthquakes at other distances can be calculated from knowledge of the variation of maximum amplitude with distance.

Richter scale was a major breakthrough in quantitative seismology. It became possible to rank earthquake size by a numerical scale and to


carry out detailed study of seismicity of a region. However, there are following three main shortcomings of the Richter scale.

1. The Wood-Anderson seismographs had to be used.2. The scale was accurate for only shallow earthquakes (focal

depth of about 16 km). 3. The scale could be used only for earthquakes occurring within

epicentral distance of 600 km of a seismograph station and for the region around California. In view of above a need was felt to extend the magnitude scale to other areas of the world, to deeper earthquakes and to other instruments. For this purpose several other magnitude scales have been developed. These scales are based on the measurements of amplitudes of body waves and surface waves and have a general formula:

M = Log A + f(d,h) + Cs +CrIn this formula M is the magnitude, A is the ground amplitude of

seismic waves measured after removing the instrument response, f(d,h) is a function which accounts for attenuation of amplitudes with epicentral distance and focal depth, and Cs and Cr are station and regional corrections. These corrections are applied to account for local and regional effects. The amplitude term may be normalized by the period of ground motion of the seismic wave. Although many magnitude scales have been developed, the most widely used are: the surface-wave magnitude scale and the body-wave magnitude scale.Surface Wave Magnitude (Ms ): Ms is the magnitude computed from surface wave amplitudes (Rayleigh waves). Gutenberg extended local magnitude scale considerably making it applicable to any epicentral distance and to any type of Seismograph. This development required a better knowledge of the variation of wave amplitude with distance, and in order to use different seismograph types, it was desirable to use ground amplitudes instead of the trace amplitudes. It was observed that for shallow focus earthquakes, surface waves have well-developed amplitudes in the period range of 20± 2 sec. These amplitudes were used to calculate surface wave magnitude. Values for the standard shock taken as zero magnitude were calculated with epicentral distance. These values are to be added to the observed value for estimating the surface wave magnitude. The U.S. Geological Survey and many other organizations use the following equation to determine surface wave magnitude.

Ms = log (A/T) + 1.66 log D +3.3Where A = maximum horizontal surface wave magnitude in microns; T = wave period in seconds (18 T 22); D = distance from the epicenter to a station in geocentric degrees (20 D 160); and no correction is made for focal depths up to 50 km.


Surface wave magnitude scale also suffered from certain limitations because well-developed surface waves are observed only for relatively shallow focus earthquakes and most easily observed at distance greater than 1000 km from earthquakes grater than Ms = 5. In view of this surface wave magnitude scale cannot be used to measure the size of deep-focus earthquakes and small regional and local earthquakes.Body Wave Magnitude (mb ): Magnitude computed from body waves is called mb. In 1945 Gutenberg extended magnitude determinations also to body waves employing amplitude of P, PP and S waves, and to the earthquakes occurring at any depth. Body wave magnitude is determined by

mb = log (A/T) +Q(D,h)

Where A = ground amplitude in microns extracted from the P wave group (not necessarily the maximum of group); T = wave period in seconds, restricted to 0.1 T 3.0; Q = correction factor, a function of distance (D) and focal depth (h). Set of charts are available from which correction factor for the earthquake can be read as a function of focal depth and epicentral distance, for P, S or PP. Later on ‘mb’ was termed as the ‘Unified Magnitude’ by Gutenberg. Normally mb is computed from maximum amplitude of the first few cycles of P waves observed on the vertical component seismograms. These waves have period around one sec. This scale is widely used to measure the size of deep focus earthquakes. However, as mb is computed from short period seismic waves it cannot fully describe the strength of large earthquakes.

In view of above consideration the magnitude computed using various magnitude scales do not agree with each other. This means that the same earthquake will have different magnitude values on the different scales. However, relationships between these scales have been deduced, which allow conversion of magnitude from one scale to another. These relationships are given below:

mb = 1.7 + 0.8 ML - 0.01 ML

mb = 2.5 +0.63 MS Ms = 1.59mb -3.97

Magnitudes agree at 6.75 to about 6.8; for higher magnitudes, Ms is larger than mb and for lower magnitudes mb is larger than Ms.Seismic Moment (M0): Because of the limitations of various magnitude scales a magnitude scale has been developed which can be applied to earthquakes of all sizes depths and locations. This magnitude scale is based on the concept of seismic moment which is a description of the extent of deformation at the earthquake source. The seismic moment is defined as M0 = AD, where is the rigidity modulus, A is the fault rupture area and D is the average dislocation (slip) between the opposite side of a


fault. Seismic moment can be determined from seismogram by utilizing long period seismic waves. Because it is measured from long period seismic waves it can be used to quantify very large earthquakes.Moment Magnitude (Mw): Kanamori (1977) and Hanks and Kanamori (1979) devised a moment magnitude scale (Mw) which is based on seismic moment. Mw is given by the following expression:

Mw = (Log M0 – 16.05)/1.5It has been found that the moment magnitude scale is consistent

with other magnitude scales over a wide range of magnitudes. Figure 7 shows a comparison of moment magnitude with some other magnitude scales. It is important to note that different magnitude scales, except moment magnitude, saturate or stop increasing with earthquake size or moment. This happens because each magnitude scale is determined from seismic waves of a particular period. For example body wave magnitude is computed from P-waves with period around one second and wavelength less than 10 km. This cannot give correct energy release from faults having rupture dimensions of tens of kilometers or grater. In view of this body wave magnitude saturates at about magnitude 6.5. Similarly Ms is computed from surface waves of about 20 seconds period and 80 km wavelength and saturates at about magnitude 8.5. Except for Ms less than about 5.5, all the magnitude scales become almost equal to moment magnitude below their respective saturation points. Saturation explains the observation that earthquakes of different sizes and energy releases often have the same magnitude.

The magnitude scales were derived for computing the energy release of earthquakes. Approximate energy-magnitude relationships were determined and the most widely known relation is Log E = 1.5 Ms + 11.8, where E is the energy in ergs. It may be noted that as the magnitude increases by one unit, the energy increases by a factor of 31.6. An atomic bomb of 20,000 tons TNT would release energy equivalent to magnitude 5.5 earthquake. The magnitude 9.5 earthquake that occurred in Chile released about one million times as much energy.

EARTHQUAKE GROUND MOTION

The ground motion due to seismic waves can be divided into two general levels; small amplitude motion from distant or small earthquakes and large amplitude motion due to nearby or large earthquakes. Ground motion at a particular site is influenced by three main factors; characteristics of source of seismic waves, the path through which the seismic waves travel and local geological structure below the recording site. The estimation of large amplitude ground motion, also called the strong ground motion, is of interest to engineers and those concerned with mitigating the effects of earthquakes on society. Strong motion


accelerographs are used to record strong ground motion and provide records of acceleration time history. A typical record of accelerogram is shown in Figure 6. The peak of the maximum acceleration which is normally expressed in terms of acceleration due to gravity can be measured in three orthogonal directions from these records. However, these records are uncorrected and needs some processing for the purpose of removing ambient noise and instrument response. Figures.7a, b & c show the corrected acceleration, velocity and displacement time histories of one of the horizontal component of ground motion shown in Figure 6.

Strong ground motion is commonly described in terms of peak horizontal acceleration, sometime supplemented by the peak horizontal velocity or in terms of Fourier amplitude spectra. Another description which is widely used for engineering purposes is the response spectrum, which is defined as the peak response to the given motion of a set of single degree of freedom oscillators of different natural periods and damping. The oscillators are intended to represent simple models of structures and the response spectrum is thereby a compact description of the potential effect of ground motion on structures.

ESTIMATION OF SEISMIC HAZARD

Estimation of seismic hazard at a given site involves computing the level of strong ground motion (e.g., peak ground acceleration) which the site is likely to experience during the life time of structure.

This is accomplished from the careful evaluation of geologic and tectonic framework of the region around the site and from the study of available seismicity data. The seismic hazard analysis consists of two parts:

1. Characterizing the seismicity source zones (e.g., the sizes and spatial location of postulated earthquakes).

2. Computing the effects of these source zones at a particular site.Two fundamental type of analysis are probabilistic and deterministic.

The deterministic analysis requires the specification of three basic elements, an earthquake source, a controlling earthquake of specified size and a method of calculating the hazard in terms of say peak ground acceleration at a specified distance to the site. The following basic steps are involved in the deterministic analysis.

1. Identification of earthquake sources: Earthquake sources may range from clearly understood and well defined faults to less well understood or less well defined geologic structures to hypothetical seismotectonic provinces.

2. Selection of the controlling earthquake: The earthquake potential of each source zone is computed in terms of maximum earthquake.


Earthquake magnitude or epicentral intensity is usually used to define the earthquake size. For each source zone the distance which represents the distance between the source and site is computed. The controlling earthquake is the earthquake whose ground motion will dominate the effect of all the sources being considered.

3. Determination of earthquake effect: This is done by means of earthquake ground motion attenuation relationships which provide estimates of ground motion for an earthquake of given magnitude at different distances.

4. Definition of hazard at a site: This is usually a simple statement that the hazard at a site is a specific peak ground acceleration, velocity or any other measure that describes the earthquake effects.Assigning the maximum earthquake to a particular source zone is an

involved process and there are different kinds of maximum earthquakes. For example maximum credible earthquake is the maximum earthquake which is based upon the evaluation of tectonic processes that are expected to be associated with an earthquake source. Another kind of maximum earthquake is the maximum historic earthquake which is associated with a seismotectonic source for which historical or instrumental evidence exits. This earthquake normally defines the lower bound of the maximum credible earthquake.

For the purpose of determining the maximum earthquake for known active faults or seismotectonic source zones, regression equations have been developed based on the observed fault rupture characteristics, such as lengths, and displacements from the earthquakes of known or reliably inferred sizes. These regression equations relate the earthquake size to various fault characteristics, and allow estimating the maximum earthquake. Some of these regression equations are given in Table 1. In this Table, n specifies the number of observations used in estimating the regression equation, a & b are regression coefficients, and s, r, r2 are the standard deviations, correlation coefficients and coefficients of determinations.

Table 1. Summary of regression statistics, historical continental earthquakes, all slip types (adopted from Coppersmith, 1991).


REFERENCE

1. Berlin, G. L. (1980). Earthquakes and the urban environment, Vol. 1, CRC Press, Inc., Boca Raton, Florida.

2. Campbell, K.W. (1981). Near-source attenuation of peak horizontal acceleration. Bulletin of the seismological society of America, 71:2039-2070.

3. Campbell, K.W. (1985). Strong motion attenuation relations: a ten year perspective. Earthquake Spectra, 1:759-804.

4. Coppersmith, K.J. (1991). Seismic source characterization for engineering seismic hazard analysis. Fourth international conference on seismic zonation, vol. I, 3-60, Stanford California.

5. Joyner, W.B. and D.M. Boore. (1981). Peak horizontal acceleration and velocity from strong-motion records including records from the 1979 Imperial Valley, California earthquake. Bulletin of the Seismological Society of America, 71:2011-2038.

6. Press, F. and R Siever. (1978). Earth, W.H. Freeman and Company. San Francisco

7. Richter, C.F. (1958). Elementary Seismology. San Francisco California. W.H. Freeman.

8. Reiter, L. (1990). Earthquake Hazard Analysis, Issues and Insights, Columbia University Press, New York.

9. Shearer, M. P. (1999). Introduction to seismology, Cambridge University Press.


Appendix 1

MODIFIED MERCALLI INTENSITY SCALE OF 1931 (Abridged)

Intensity Description

I. Not felt except by a very few under specially favorable circumstances. (I RF Scale.)II. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately

suspended objects may swing. (I to II RF Scale.)III. Felt quite noticeably indoors, especially on upper floors of buildings, but many people

do not recognize it as an earthquake. Standing motorcars may rock slightly. Vibration like passing of truck. Duration estimated. (III RF scale.)

IV. During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motorcars rocked noticeably. (IV to V RF Scale.)

V. Felt by nearly everyone, many awakened some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. (V to VI RF Scale.)

VI. Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight. (VI to VII RF Scale.)

VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motorcars. (VIII RF Scale.)

VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motorcars disturbed. (VIII + to IX-RF Scale.)

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. (IX+ RF Scale.)

X. Some well-built wooden structures destroyed; most masonry and frame structures with foundations; ground badly cracked. Rails bent. Landslides considerable from riverbanks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks. (X RF Scale.)

XI. Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII. Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into air.


MODIFIED MERCALLI INTENSITY SCALE OF 1931(1956 VERSION ABRIDGED AND REWRITTEN)

Intensity Description

I. Not felt. Marginal and long-period effects of large earthquakes.II. Felt by persons at rest, on upper floors, or favorable placed.III. Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration

estimated. May not be recognized as an earthquake.1. Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt

like a heavy ball striking the walls. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of IV, wooden walls and frames creak.

V. Felt outdoors; direction estimated. Sleepers wakened. Liquid disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate.

VI. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knickknacks, books, etc., off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes shaken (visibly, or heard to rustle).

VII. Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices (also unbraced parapets and architectural ornaments-CFR). Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.

VIII. Steering of motor cars affected. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.

IX. General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. (General damage to foundations- CFR). Frame structures, if not bolted, shifted off foundations. Frames cracked. Serious damage to reservoirs. Underground pipes broke. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, earthquake fountains, sand craters.

X. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes and embankments. Large landslides. Water thrown on banks of canals, rivers lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.

XI. Rails bent greatly. Underground pipelines completely out of service.


XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air.

Masonry Categories

Masonry A: Good workmanship, mortar and design; reinforced especially laterally, and bound together using steel, concrete etc., designed to resist lateral forces.

Masonry B: Good workmanship and mortar; reinforced but not designed in detail to resist lateral forces.

Masonry C: Ordinary workmanship and mortar; no extreme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces.

Masonry D: Weak materials such as adobe; poor mortar; low standards of workmanship; weak horizontally.

Fig. 1 Seismogram of an earthquake in EL Salvador as recorded near New York City. P, S, SS, LR and LQ indicate the first arrival of different kinds of seismic waves (adopted from Reiter, 1990).


Fig. 2 Map showing major and some minor tectonic plates. Arrows indicate the relative direction of motion of tectonic plates (after Bolt, 1978).


Fig. 3a. Selected global earthquake locations from 1977 to 1994 showing that earthquakes occur along well defined belts of seismicity (adopted from Shearer, 1999).

Fig.3b. Map showing plate boundaries that are delineated based on the earthquake locations given in Fig. 3a.


Fig. 4. Graph showing the procedure used by C. F. Richter to establish magnitude scale for local earthquakes (adopted from Berlin, 1980).


Fig. 5. A comparison of moment magnitude with other magnitude scales (Adopted from Reiter, 1990).


Fig. 6. Uncorrected accelerogram from April 24, 1984 Morgan Hill, California earthquake (adopted from, Reiter, 1990).

Fig. 7. Instrument corrected and noise filtered acceleration, velocity and displacement time history from the accelerogram shown in Fig. 6(a).


Fig. 8. Latest Seismic Zoning map of India for Indian Standard Earthquake Resistant design of Structures (IS:1893-(Part I) 2002: General Provisions and Buildings)


Fig. 8 Median estimates for peak horizontal acceleration from Campbell (1981a) and Joyner and Boore (1981). JB estimates have been reduced by 12% for comparison with Campbell estimates (after Campbell, 1981a).

Fig. 9 84th percentile estimates for peak horizontal acceleration from Campbell (1981a) and Joyner and Boore (1981). JB estimates have been reduced as in Fig. 8. for the purpose of comparison.


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