SOURCE CHARACTERIZATION OF INLAND CRUSTAL EARTHQUAKES FOR NEAR-SOURCE GROUND MOTIONS

Transcription

1 SOURCE CHARACTERIZATION OF INLAND CRUSTAL EARTHQUAKES FOR NEAR-SOURCE GROUND MOTIONS Hiroe Miyake *, Tomotaka Iwata **, and Kojiro Irikura *** ABSTRACT To obtain the characterized ource model for prediction of trong ground motion, we etimated trong motion generation area and it rupture geometry of inland crutal earthquake from near-ource ground motion record. The ource parameter related to the trong ground motion were etimated by waveform imulation uing the empirical Green' function method. The ynthetic waveform from the ource model with large and uniform lip velocity a trong motion generation area fit well to the obervation (0.2~10Hz). We found a elf-imilar caling relationhip between the trong motion generation area and eimic moment, and clarified that thi area wa coincident with the aperity area characterized from heterogeneou patial lip ditribution etimated by waveform inverion uing low frequency (<1Hz). Introduction A lot of waveform inverion uing low frequency (<1Hz) of the trong ground motion record have been performed to examine ource procee preciely. A a conequence of thee analye, it became clear that mot large inland crutal earthquake have heterogeneou patial lip ditribution. In order to catch the feature of heterogeneou ource model, ource characterization and caling for heterogeneou lip ditribution have been jut tarted (e.g Somerville et al., 1999; Mai and Beroza, 2000). Characterized ource model i expreed quantitatively including not only macrocopic fault parameter but alo roughne of lip ditribution uch a aperity or effective ource dimenion. Somerville et al. (1999) characterized ource model compoed of aperitie and background lip urrounding the aperitie, both of which are expreed a finite extended area with homogeneou lip ditribution. The ource characterization o far done i available only for lower frequency motion le than 1Hz becaue of the reolution of the waveform inverion. For the purpoe of the ource characterization applicable to broad frequency band, we etimate ource parameter related to trong ground motion uing the empirical Green' function method. Then we clarify * Graduate Student, Diater Prevention Reearch Intitute, Kyoto Univerity ** Reearch Aociate, Diater Prevention Reearch Intitute, Kyoto Univerity *** Profeor, Diater Prevention Reearch Intitute, Kyoto Univerity

2 the relationhip between the trong motion generation area and heterogeneou patial lip ditribution. Data We examined even inland crutal earthquake (M JMA 4.9~6.5) occurring in Japan from 1996 to 1999 (Fig. 1). Strong ground motion record were provided by K-NET (Kinohita, 1998), and nearet (within 20km) four tation urrounding the ource area were ued for trong ground motion imulation. A the empirical Green' function for imulating the mainhock, we elected the record of afterhock whoe focal mechanim and hypocentral ditance were imilar to their repective mainhock. The fault plane of the mainhock were referred to focal mechanim olution uing moment tenor inverion (e.g. Dziewonki et al., 1996, 1997; Fukuyama et al., 1998) and afterhock ditribution. Procedure Strong ground motion imulation uing empirical Green' function method Here, the trong motion generation area i defined to be a finite extended area with large and uniform lip velocity in the total rupture area. We aumed that rupture propagate radially from the hypocenter with the peed of 90% of S-wave velocity. In order to etimate the ize and poition of the trong motion generation area and the lip duration, we imulated trong ground motion uing the empirical Green' function method by Irikura (1986). Thi method i baed on the caling law of ource parameter (Kanamori and Anderon, 1975) and ource pectra, i.e. the omega-quared model (Aki, 1967). Strong ground motion imulation for the mainhock wa carried out uing acceleration, velocity, and diplacement record from 0.2Hz (uable lowet frequency of afterhock waveform) to 10Hz. The parameter for the bet ource model were determined to minimize the ummation of reidual of the diplacement waveform fitting and thoe of the acceleration envelope fitting, by the Genetic Algorithm method or forward modeling. We contructed ource model for the earthquake hown in Fig. 1. Mot ource model were expreed a a ingle trong motion generation area. The ynthetic waveform from the ource model fit well to the obervation. Fig. 2 how comparion of the oberved waveform with the ynthetic for the ource model of the Kagohima-ken Hokueibu earthquake of March 26, 1997 (M JMA 6.5, M w 6.0). Relation for parameter of trong motion generation area We evaluate the relation between the trong motion generation area and the eimic moment, and compare it with the caling relationhip for the characterized ource model by Somerville et al. (1999). The eimic moment for the mainhock wa determined by the moment tenor inverion uing broadband eimic waveform (e.g. Dziewonki et al., 1996, 1997; Fukuyama et al., 1998). We determined the eimic moment for the afterhock relatively by the ource pectral ratio of mainhock to afterhock in the lower frequency range. We found a elf-imilar caling relationhip between the trong motion generation area and the eimic moment in thi tudy where the range of the eimic moment wa le

3 than 1.38*10 17 Nm (M w 6.0). For everal earthquake, we confirmed that the trong motion generation area i located at almot the ame poition a the aperity area characterized from heterogeneou patial lip ditribution uing the waveform inverion from 0.1 to 0.5Hz (Miyakohi et al., 2000) (Fig 3). They extracted aperity area baed on the criterion by Somerville et al. (1999). Our caling of the trong motion generation area wa cloe to that of combined area of aperitie by Somerville et al. (1999) (Fig. 4). Thi ugget that the trong motion generation area i coincident with the aperity area characterized from heterogeneou patial lip ditribution etimated by the waveform inverion uing low frequency (<1Hz) motion. The effective area for generating trong ground motion in near-ource area i conidered to be fairly maller than the total rupture area by the waveform inverion. Fig. 5 how the caling between the lip duration and the eimic moment. The lip duration wa alo found to have a elf-imilar caling in thi tudy (i.e., le than 1.38*10 17 Nm (M w 6.0) of the eimic moment), and cloe to the empirical relationhip between lip duration and eimic moment derived by Somerville et al. (1999). They pointed out that lip duration wa imilar to the rupture duration of the larget aperity. We conider that trong ground motion generation i mainly controlled by the lip duration at lower frequencie. Phyical interpretation on trong motion generation area Baed on the characterized ource model determined by Miyakohi et al. (2000), we imulated ground motion from 0.2 to 10Hz for the 1997 Kagohima-ken Hokueibu earthquake uing the empirical Green' function method. The imulated waveform for the characterized ource model wa almot the ame that for the trong motion generation area. The contribution of the aperity area to imulated waveform wa much larger than the background lip area in the frequency range from 0.2 to 10Hz (Fig. 6). Da and Kotrov (1986) compared ource amplitude pectra for a ingle aperity model and dilocation (crack) model (Fig. 7). They howed that the level of ource amplitude pectra at high frequencie wa almot equal for both model, although that at low frequencie wa different. We conider that the main ground motion in our imulation correpond to the higher frequency motion in Da and Kotrov' aperity model. It i the reaon why the ground motion only for the trong motion generation area agree well with the obervation. That i, the trong motion generation area in our imulation i uppoed to have correpondence to the aperity itelf in Da and Kotrov (1986). In thi paper, we only dicu moderate ize of inland crutal earthquake. We need to check whether the trong motion generation area correpond to the aperity area or not for much larger earthquake. Concluion The trong motion generation area etimated in thi tudy i coincident with the aperity area characterized from heterogeneou patial lip ditribution etimated by the waveform inverion uing low frequency (<1Hz) motion. We found the trong motion generation area and the lip duration have a elf-imilar caling to the eimic moment where the range of analye wa le than 1.38*10 17 Nm (M w 6.0). Our analye ugget that ource

4 characterization for broadband frequency ground motion need trong motion generation area which reproduce higher frequencie a well a total rupture area which reproduce lower frequencie. Acknowledgement We deeply appreciate K-NET for providing the trong ground motion record. We would like to thank Keiko Kuge and FREESIA project for giving focal mechanim information, and Faculty of Science, Kyuhu Univerity and JMA for providing hypocentral information. We are alo grateful to Ken Miyakohi for giving u reult of the waveform inverion, Jorge Aguirre for hi helpful upport uing Genetic Algorithm. Some figure were drawn by GMT Ver.3.0 (Weel and Smith, 1995). H.M. wa upported by JSPS Reearch Fellowhip for Young Scientit. Thi work wa partially upported by Grant-in-Aid for Scientific Reearch from the Minitry of Education, Science, Sport and Culture of Japan (No , ). Reference Aki, K. (1967). "Scaling law of eimic pectrum", J. Geophy. Re., 72, Da, S. and B. V. Kotrov (1986). "Fracture of a ingle aperity on a finite fault: A model for weak earthquake?", Earthquake Source Mechanic, Am. Geophy. Union., Dziewonki, A. M., G. Ektrom, and M. P. Salganik (1996). "Centroid-moment tenor olution for January-March 1995", Phy. Earth Planet. Interior, 93, Dziewonki, A. M., G. Ektrom, N. N. Maternovkaya, and M. P. Salganik (1997). "Centroid-moment tenor olution for July-September", 1996, Phy. Earth Planet. Interior, 102, Fukuyama, E., M. Ihida, D. S. Dreger, and H. Kawai (1998). "Automated eimic moment tenor determination by uing on-line broadband eimic waveform", Ziin2, 51, (in Japanee with Englih abtract). Irikura, K. (1986). "Prediction of trong acceleration motion uing empirical Green' function", Proc. 7th Japan Earthq. Eng. Symp., Kanamori, H. and D. L. Anderon (1975). "Theoretical bai of ome empirical relation in eimology", Bull. Seim. Soc. Am., 65, Kinohita, S. (1998). "Kyohin Net (K-NET)", Seim. Re. Lett., 69, Mai, P. M. and G. C. Beroza (2000). "Source caling propertie from finite-fault rupture model", ubmitted to Bull. Seim. Soc. Am. Miyakohi, K., T. Kagawa, H. Sekiguchi, T. Iwata, and K. Irikura (2000). "Source characterization of inland earthquake in Japan uing ource inverion reult", Proc. 12th World Conf. Earthq. Eng., Somerville, P., K. Irikura, R. Grave, S. Sawada, D. Wald, N. Abrahamon, Y. Iwaaki, T. Kagawa, N. Smith, and A. Kowada (1999). "Characterizing crutal earthquake lip model for the prediction of trong ground motion", Seim. Re. Lett., 70, Weel, P. and W. H. F. Smith (1995). "New verion of the Generic Mapping Tool releaed", EOS Tran. Am. Geophy. Union., 76, 329.

5 50 N 120 E130 E 140 E150 E 'E 'E 97 / 06 / 27 M J 6.3 km KGS N depth 8.3km 99 / 03 / 16 M J / 09 / 03 M J 'N KGS004 M J 6.5 depth 12.0km depth 9.6km M J / 08 / 11 M J 5.7 KGS / 03 / 26 M J N depth 9.8km 20 N depth 8.2km 97 / 05 / 13 M J 6.3 depth 7.7km 96 / 08 / 11 M J 5.9 depth 8.6km km 'N KGS007 rock ite ediment ite mainhock afterhock ued a empirical Green function afterhock (< 1 day) Fig. 1. Left: Epicentral location and focal mechanim ued in thi tudy. Black dot indicate oberved tation of K-NET. Right: Map howing the dataet ued for the Kagohima-ken Hokueibu earthquake of March 26, 1997 (M JMA 6.5) and afterhock ditribution. Acc. (cm/**2) EW component Vel. (cm/) Dip. (cm) Acc. (cm/**2) NS component Vel. (cm/) Dip. (cm) ob. KGS yn. ob. KGS004 yn ob. KGS005 yn ob. KGS yn. Fig. 2. Comparion of oberved and ynthetic waveform (0.2~10Hz) for the ource model of Kagohima-ken Hokueibu earthquake of March 26, 1997 (M JMA 6.5) at nearet four tation. Number between oberved and ynthetic waveform indicate the maximum value of oberved waveform. Fig. 3. Superpoition of trong motion generation area on characterized ource model derived from heterogeneou patial lip ditribution (Miyakohi et al., 2000). From left to right, the Kagohima-ken Hokueibu earthquake of March 26, 1997 (M JMA 6.5), the Yamaguchi-ken Hokubu earthquake of June 27, 1997 (M JMA 6.3) and Iwate-ken Nariku Hokubu earthquake of September 3 of 1998 (M JMA 6.1). Thick line on the rupture area indicate the boundary of the aperity, and dot line how the boundary of the trong motion generation area. Number on each ubfault indicate lip value etimated in the waveform inverion.

6 10 4 trong motion generation area combined area of aperitie 1 2 trong motion generation area (km 2 ) lip duration (ec) lip duration in thi tudy lip duration by Somerville et al.(1999) M 0 (dyne*cm = 10-7 Nm) M 0 (dyne*cm = 10-7 Nm) Fig. 4. Scaling between trong motion generation area and eimic moment. Cloed circle how the trong motion generation area, and open circle how the combined area of apeirtie (Somerville et al., 1999). The line indicated by 1 and 2 correpond to the rupture area and the combined area of aperitie, a a function of eimic moment (Somerville et al., 1999), repectively. Fig. 5. Scaling between lip duration and eimic moment. Cloed circle how the lip duration etimated in thi tudy, and open circle and line correpond to the lip duration and the empirical relation a a function of eimic moment (Somerville et al., 1999), repectively. ob. yn.1 SMGA KGS002 EW 0.2~10Hz Acc.(cm/**2) Vel.(cm/) Dip.(cm) Schematic lip ditribution W u L yn.2 characterized ource model yn.3 background lip area Fig. 6. Comparion of oberved with ynthetic waveform (0.2~10Hz) of EW component at KGS002 for the Kagohima-ken Hokueibu earthquake of March 26, From top to bottom, the trace how oberved waveform, contribution of trong motion generation area, characterized ource model and background lip area to ynthetic waveform, repectively. (a) (b) (c) Log A tre free field r R 2 Log R/r ASPERITY DISLOCATION Fig. 7. (a) Geometry of the aperity model (left) and dilocation (crack) model (right) by Da and Kotrov (1986). (b) Far-field diplacement pectra for aperity and dilocation (crack) model howing relation between eimic moment and corner frequency for the two model at a given magnitude. (d) Far-field acceleration pectra for aperity and dilocation (crack) model. 0 Log ω