GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,Dynami Landslide Pro esses Revealed by Broadband Seismi Re ordsMasumi Yamada,1 Hiroyuki Kumagai,2,3 Yuki Matsushi,1 Takanori Matsuzawa2We use broadband seismi re ordings to tra e the dy-nami pro ess of the deep-seated Akatani landslide that o - urred on the Kii Peninsula, Japan, whi h is one of the bestre orded large slope failures. Combining analyses of the seis-mi re ords with pre ise topographi surveys done beforeand after the event, we an resolve a detailed time historyof the mass movement. During 50 s of the large landslide,we observe a smooth initiation, a eleration with hanges inbasal fri tion, and reversal of the momentum when the mass ollides with the opposite valley wall. Of parti ular impor-tan e is the determination of the dynami fri tion duringthe landslide. The oeÆ ient of fri tion is estimated to be0.56 at the beginning of the event and drops to 0.38 for mostof the sliding. The hange in the fri tional level on the slid-ing surfa e may be due to liquefa tion or breaking of roughpat hes, and ontributes to the extended propagation of thelarge landslide.1. Introdu tionAssessing and managing the risks posed by deep-seated atastrophi landslides requires a quantitative understand-ing of the dynami s of sliding ro k masses. Previously,landslide motion has been inferred qualitatively from to-pographi hanges aused by the event, and o asionallyfrom eyewitness reports. However, these onventional ap-proa hes are unable to evaluate sour e pro esses and dy-nami parameters. In this study, we used ground shakingdata re orded away from the landslide for re onstru ting thedynami landslide pro esses. The deep-seated atastrophi landslide sequen e indu ed by heavy rainfall in 2011 in theKii Peninsula, Japan, was the �rst instan e in whi h 1) seis-mi signals radiated by landslides were re orded by denselydistributed near-sour e seismometers [Yamada et al., 2012℄,and 2) the pre ise volume of the landslide material was ableto be measured by omparing pre- and post-landslide to-pographi data obtained using airborne laser s anning. Weperformed a sour e inversion with the long-period seismi re ords [Kanamori and Given, 1982; Brodsky et al., 2003;Lin et al., 2010; Moretti et al., 2012℄ of one of the largestevents, and from this obtained a for e history of the land-slide. Here we reveal the dynami pro esses of the landslide:smooth initiation of sliding, a eleration a ompanied by a1Disaster Prevention Resear h Institute, KyotoUniversity, Gokasho, Uji, Kyoto 611-0011, Japan2National Resear h Institute for Earth S ien e andDisaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki305-0006, Japan3Now at Graduate S hool of Environmental Studies,Nagoya University, Chikusa, Nagoya, Ai hi 464-8601, JapanCopyright 2013 by the Ameri an Geophysi al Union.0094-8276/13/$5.00

substantial de rease in fri tional for e, and de eleration dueto ollision. The approa h demonstrated here o�ers an inno-vative method for understanding the sliding pro esses asso- iated with atastrophi landslides, enabling us to simulatethe motion of su h events.2. Data and MethodsOn 3{4 September 2011, extensive slope failures o urreda ross a wide region of the Kii Peninsula as Typhoon Talasprodu ed heavy rainfalls a ross western Japan. The Akatanilandslide, one of the largest events, o urred at 16:21:30 on4 September 2011 (JST) in Nara prefe ture, entral Japan(135.725ÆN, 34.126ÆE) [Yamada et al., 2012℄. There was ex-tensive mass movement on a slope approximately 1 km long,in lined at an angle of 30Æ (Figures 1a and 1b). We wereable to al ulate the volume of the landslide from pre isemeasurement of topographi data using airborne laser s an-ning (LiDAR) whi h was done both before and after theevent. The volume was 8.2�106 m3 [Yamada et al., 2012℄and the total mass (m) of displa ed material is estimatedto be 2.1�1010 kg, assuming an average soil density of 2600kg/m3 [Iwaya and Kano, 2005℄. The verti al displa ementof the enter of the mass is al ulated as 265 m, and thepotential energy released from this event is estimated to be5:5 � 1013J .We performed a waveform inversion using six broadbandseismi stations to obtain the sour e time fun tion using thestation distribution shown in Figure S1. The sour e-stationdistan es of these stations range from 35 to 200 km. Wepro essed the broadband re ords a ording to the followingpro edure. First, we removed the mean from the time seriesand orre ted for the instrumental response in all waveforms.The re ords were then integrated on e in the time domain,and a non- ausal fourth order Butterworth �lter (0.01 and0.1 Hz) was applied. We determined these orner frequen iesto maximize the frequen y band and minimize the e�e t ofstru tural heterogeneities. We then de imated the data bya fa tor of 20, redu ing the sampling frequen y to 1 Hz. Weused these �ltered displa ement re ords for the inversion.Following the method of Nakano et al. [2008℄, we per-formed a waveform inversion in the frequen y domain todetermine the sour e pro ess of the landslide. We al u-lated Green's fun tions at sour e nodes on the grid shownin Figure S1, using the dis rete wavenumber method [Bou- hon, 1979℄ and the Japan Meteorologi al Agen y (JMA)one-dimensional stru ture model [Ueno et al., 2002℄. As-suming a single-for e me hanism for the landslide sour e[Hasegawa and Kanamori, 1987℄, we estimated the least-squares solution in the frequen y domain. We performedan inverse Fourier transform on the solution to determinesour e time fun tions for three single-for e omponents atea h sour e node [Nakano et al., 2008℄. We performed a gridsear h in spa e and the normalized residual ontour is shownin Figure S1. The best-�t sour e lo ation was determined ata position very lose to the a tual Akatani landslide, and theestimated sour e time fun tions shown in Figure S2a werestable in the region of the best-�t lo ation.1

X - 2 YAMADA ET AL.: DYNAMIC LANDSLIDE PROCESSES REVEALED BY SEISMIC RECORDS3. Estimated Sour e Time Fun tionsThe resultant sour e time fun tions for the three singlefor e omponents along with the waveform �ts between ob-served and syntheti seismograms are shown in Figure S2.The parti le motion of the horizontal sour e time fun tionsin Figure 1 indi ates that the for es a t in one dominantdire tion (130Æ from North), onsistent with the dire tionof mass sliding. Therefore, we rotated the two horizontal omponents to the 130Æ and 220Æ dire tions (Figure 2a).Note that the positive dire tion of the N130ÆE omponentis upslope.The sour e time fun tions estimated from the long-period waveform inversion reveal the dynami history ofthe landslide. The notably small amplitude of the N220ÆE(N140ÆW) omponent indi ates that mass movement in thisdire tion is negligible. The waveforms of the N130ÆE (ra-dial) and verti al omponents are highly oherent. We inter-pret these waveforms as being representative of three stagesin the landslide pro ess (see Figures 2a and 2b).During the �rst stage (90{112 s), the mass starts mov-ing and a elerates down the slope. The for e a ting on theground is in the opposite dire tion to the inertial for e a tingon the mass. The parti le motion derived from the verti aland radial omponents in Figure 1d shows that the dire tion

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Figure 1. Topography of the Akatani landslide. (a) El-evation hanges at the Akatani landslide estimated fromairborne LiDAR topographi surveys. Blue and Red on-tours show the extent of the soil mass before and afterthe landslide, respe tively. (b) Verti al se tion along theA{B line (see (a) for lo ation) before (blue) and after(red) the slide event. Small ir les show the entre ofmass before and after the landslide. Arrows orrespondto the dire tion of for es in (d). The verti al axis showsthe elevation above sea level. ( ) Parti le motion of thesour e time fun tion between 90 and 140 s in a horizon-tal plane. (d) Parti le motion of the sour e time fun tionbetween 90 and 140 s in a verti al plane along the A{Bline (see (a)).

of the for e is parallel to the updip dire tion of the slope. Inthe se ond stage (112{132 s), the toe of the mass rea hes theend of the slope and the mass starts de elerating. The slid-ing mass pushes toward the opposite side of the valley, andthe dire tion of the for e a ting on the ground is reversedfrom that in the �rst stage. Due to this hange in dire -tion, the phase of the horizontal and verti al omponents isshifted (see Figures 1b and 1d). When the leading edge ofthe mass rea hes the end of the slope, a downward-dire tedverti al for e a ts on the ground. The mass ontinues tomove toward the other side of the valley, produ ing a on-ta t for e in the horizontal dire tion. The amplitudes of thewaveforms during the third stage (132{140 s) is relativelysmall, and the for e in the verti al ross-se tion is dire tedapproximately parallel to the sliding slope. This may indi- ate that the mass slightly ran up on the sliding slope andthe movement terminated with some plasti deformation.4. Dynami History of the LandslideAs the estimated sour e time fun tions are equivalent tothe inertial for e of the sliding mass [Kanamori and Given,1982℄, the velo ity and displa ement of the mass an be al- ulated from the sour e time fun tions. The velo ity (v) isthe integral of the single for e (f) divided by the mass (i.e.v = R fdt=m), and the displa ement (d) is the integral ofvelo ity.For these al ulations, we assumed that m is onstantover time. Field observations prior to the Akatani landslideindi ate that there may have been earlier deformations onthe sliding surfa e preparatory to the large failure. A pre- ursory gravitational deformation feature was observed ontop of the slide area from LiDAR topographi data [Chigiraet al., 2012℄. This small s arplet suggests that strain due todeformation was a umulating prior to the slide event, in ef-fe t preparing a large portion of the sliding surfa e for failure

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YAMADA ET AL.: DYNAMIC LANDSLIDE PROCESSES REVEALED BY SEISMIC RECORDS X - 3in 2011. These observations are onsistent with our assump-tion that the entire mass moved relatively uniformly fromthe beginning. The onstant mass assumption is also jus-ti�ed by a typi al motion of deep-seated atastrophi land-slides, i.e., sour e-to-deposit translational blo k movementas observed in a videotaped landslide in 2004 [Suwa et al.,2010℄ and in a landslide of the 2011 event!Ohto-Shimizulandslide) sighted by residents nearby the Akatani landslide.Additionally, during the �rst 10 se onds shown in Figure 3e,radiation of high-frequen y energy is limited, indi ating thatdestru tive failure did not o ur at the onset of sliding.We integrated the estimated three- omponent sour e timefun tions individually, and omputed their ve tor sum toobtain the velo ity and displa ement (Figure 3b and 3 ).Stage 1 2 3

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DcFigure 3. Time histories of dynami parameters. (a)Sour e time fun tion in the N130ÆE omponent. (b) Es-timated absolute velo ity of the mass. ( ) Estimated ab-solute displa ement of the mass. (d) Time history of thedynami oeÆ ient of fri tion obtained from equation (1).The data after the se ond stage annot be used for thispurpose. (e) Envelope of high-frequen y velo ity wave-forms. (f) Relationship between the dynami oeÆ ientof fri tion and velo ity in the �rst stage. (g) Relationshipbetween the dynami oeÆ ient of fri tion and displa e-ment in the �rst stage.

Sin e long-period noise a umulates when we integrate inthe time domain, the in reasing trends in the velo ity anddispla ement after the third stage (Figures 3b and 3 ) maybe due to noise e�e ts. We infer that the motion of thelandslide mainly stopped at about 140 se where the a el-eration values are near zero. The maximum velo ity was28 m/s (101 km/h), attained at approximately 116 se onds.The amplitude of the displa ement at the end of the se ondstage was 570 m, whi h is onsistent with the distan e the enter of the mass travelled during the slide (530 m). Theenvelope of the high-frequen y waveform (1{4 Hz) [Yamadaet al., 2012℄ is also shown in Figure 3e for omparison. Thisis the average of the 15 normalized verti al velo ity wave-forms for whi h station distan e was less than 40 km fromthe landslide lo ation.The motion of the sliding mass in the dire tion of land-slide propagation an be expressed as:f = mg(sin � � � os �): (1)where f is the inertial for e of the sliding mass along theslope, � is the angle of the slope, and � is the oeÆ ient offri tion. As m and � are assumed to be onstant, the onlyunknown variable is �. Therefore, the oeÆ ient of fri tionduring sliding an be obtained dire tly from equation (1).The estimated dynami oeÆ ient of fri tion is shown inFigure 3d. Note that equation (1) only holds in the �rststage, as the additional stopping for e a ts on the mass inthe se ond stage. Therefore, the data from the se ond stageonwards in Figure 3d annot be used in this ontext. Dur-ing the �rst 10 se onds, � drops from 0.56 to 0.38, thenremains at approximately 0.4 during sliding. In order to as-sess the dependen y of the oeÆ ient of fri tion on slip, therelationships between � and displa ement, and � and velo -ity are shown in Figures 3f and 3g. The �gure shows learslip-weakening and velo ity-weakening, and the steady-statedistan e (D ) is estimated to be 25 m.5. Dis ussionThe de rease in sliding fri tion has also been observedduring earthquakes [e.g. Ide and Takeo, 1997; Heaton, 1990℄and laboratory ro k experiments [e.g. Hirose and Shi-mamoto, 2005; Han et al., 2007℄. For large earthquakes,D is estimated to be 0.2{1.0 m [Ide and Takeo, 1997℄, anorder of magnitude or more lower than our estimate. Thisdi�eren e may be aused by the greater normal stress duringearthquakes. For the Akatani landslide, normal stress dueto gravitational for e is 0.4{0.5 MPa on average, while atdepths of about 5 to 20 km where most seismi events o ur,the normal stress an be several orders of magnitude larger.Larger normal stress generally equates to a smaller D [Wib-berley and Shimamoto, 2005℄. The steady-state oeÆ ient offri tion and steady-state distan e depend on several fa tors,in luding normal stress, slip rate, and material propertiesof the onta t surfa es [Togo et al., 2011℄. These onditionsare very di�erent from those in laboratory experiments, so itis diÆ ult to use small s ale testing to infer dynami prop-erties of real landslides. Therefore, using this approa h to orre tly determine parameters of the sliding mass an helpin understanding the dynami fri tion of a tual landslides.The dynami oeÆ ient of fri tion is diÆ ult to mea-sure in the �eld. Most measurements of the fri tion oef-� ient are obtained from the ratio of drop height to runoutlength [S heidegger, 1973℄. S heidegger [1973℄ proposed anempiri al relationship between volume of the landslide andthe apparent oeÆ ient of fri tion based on the �eld mea-surements. A ording to that equation, the apparent oef-� ient of fri tion of the Akatani landslide is estimated to

X - 4 YAMADA ET AL.: DYNAMIC LANDSLIDE PROCESSES REVEALED BY SEISMIC RECORDSbe 0.35. A similar oeÆ ient was obtained by Ashida et al.[1985℄. These values are onsistent with our observation ofthe steady-state oeÆ ient of fri tion.The de rease in the oeÆ ient of fri tion as a fun tion ofslip an be interpreted in several ways. One possible expla-nation is that liquefa tion on the sliding surfa e is indu edby ex ess pore pressure, a pro ess whi h is widely observedin laboratory experiments [Sassa, 1996℄. When the Akatanilandslide o urred, the hillslope had re eived in ex ess of1000 mm of rainfall over 3 days. As the mass starts sliding,the loosened bedro k is fra tured and ompa ted, ausingpore pressure to rise, and triggering liquefa tion on the slid-ing surfa e. This phenomenon an drasti ally de rease thefri tional for e on the sliding surfa e. Another possibilityis that roughness on the sliding surfa e a�e t the fri tionlevel. Previous studies have dis ussed how the roughness ofsliding surfa es an ontrol the movement of slow landslides[Mizuno, 1989℄. Deformation of hillslopes [Chigira et al.,2012℄ may be indi ative of topographi features on the slidesurfa e whi h an ontribute to the initial slope support. Asthe slide initiates and progresses, breaking of these featuresmay ause a drop in the fri tion and signi� antly a�e t themotion of rapid landslides.We note that the pattern of high-frequen y energy ra-diation is quite di�erent from the low-frequen y radiation(Figures 3a and 3e), so the sour e me hanisms are likelyquite di�erent. The high-frequen y energy may be relatedto pro esses on the sliding surfa e [Yamada et al., 2012℄ orinternal to the landslide mass. The high-frequen y has itspeak amplitude during the third stage when the for e esti-mated from the low-frequen y waveforms is relatively small.As shown in Figure 1, the sliding mass as ended the oppo-site valley wall during the se ond stage. We spe ulate thatthis mass fell ba k and returned in the opposite dire tion,as suggested by Evans et al. [1994℄. The internal ollisionwith the returning mass and other ompli ated pro esses onthe slipping surfa e may ontribute to the high-frequen iesduring the stopping pro ess.6. Con lusionsIn this paper, we analyzed the seismograms re ording thesignal produ ed by the Akatani landslide due to TyphoonTalas, passing Japan Island in September 2011. The high-quality dataset enables us to perform a sour e inversion andobtain a detailed time history of the landslide pro ess. Weshowed the hanging values of the dynami fri tion duringthe landslide, starting at a relatively high value, and dropsto a lower value as the landslide progresses. The hange inthe fri tional level on the sliding surfa e may be due to liq-uefa tion or breaking of rough pat hes, and ontributes tothe extended propagation of the large landslide.The approa h demonstrated here o�ers an innovativemethod for understanding the sliding me hanism of land-slides and determining parameters, su h as slide a elera-tion and oeÆ ient of fri tion. In future resear h, we willa umulate seismi re ords from slope failures of varyingsize, and obtain a generalized fri tional onstitutive law forlandslides. Su h physi al models will help to simulate themotions of mass failure, and ontribute to the mitigation of atastrophi landslides hazards.A knowledgments. We a knowledge the National Resear hInstitute for Earth S ien e and Disaster Prevention for the use ofF-net data. High-resolution DEM data, whi h have been used to al ulate landslide volumes, were provided by the Nara Prefe -tural Government and the Kinki Regional Development Bureauof the Ministry of Land, Infrastru ture and Transport. We thank

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YAMADA ET AL.: DYNAMIC LANDSLIDE PROCESSES REVEALED BY SEISMIC RECORDS X - 5Togo, T., T. Shimamoto, S. Ma, and T. Hirose (2011), High-velo ity fri tional behavior of Longmenshan fault gouge fromHongkou out rop and its impli ations for dynami weakeningof fault during the 2008 Wen huan earthquake, EarthquakeS ien e, 24, 267{281.Ueno, H., S. Hatakeyama, T. Aketagawa, J. Funasaki, and N.Hamada (2002), Improvement of hypo enter determinationpro edures in the Japan Meteorologi al Agen y, QuarterlyJournal of Seismology, 65, 123{134.Wibberley, C. and T. Shimamoto (2005), Earthquake slip weaken-ing and asperities explained by thermal pressurization, Nature,436, 689{692.Yamada, M., Y. Matsushi, M. Chigira, and J. Mori (2012),Seismi re ordings of landslides aused by Typhoon Ta-las (2011), Japan, Geophys. Res. Lett., 39, L13301,doi:10.1029/2012GL052174.M. Yamada, Kyoto University, Gokasho, Uji, Kyoto 611-0011,Japan (masumi�eqh.dpri.kyoto-u.a .jp)