Strong Motion Generation Areas Research Articles

Modeling of strong motion generation areas of the 2011 Tohoku, Japan earthquake using modified semi-empirical technique

Modification in the semi-empirical technique for the simulation of strong ground motion has been introduced to incorporate the strong motion generation areas (SMGA) in the modeled rupture plane. Strong motion generation areas identified within the rupture plane of the Tohoku earthquake of March 11, 2011 (M w = 9.0), have been modeled using this modified technique. Two different source models having four and five SMGAs, respectively, are considered for modeling purpose. Strong motion records using modified semi-empirical technique have been simulated at two near-field stations located at epicentral distance of 137 and 140 km, respectively, using two different source models. Comparison of the observed and simulated acceleration waveforms is made in terms of root mean square error (RMSE) at both stations. Minimum root mean square error of the waveform comparison has been obtained at both the stations for source model having five SMGAs. Simulations from same rupture model have been made at other four stations lying at epicentral distance between 154 and 249 km. Comparison of observed and simulated records has been made in terms of RMSE in acceleration records, velocity records and response spectra at each six station. Simulations have been made at six other stations to obtain distribution of peak ground acceleration and peak ground velocity with hypocentral distance. Peak ground acceleration and velocity from simulated and observed records are compared at twelve stations surrounding the source of Tohoku earthquake. Comparison of waveforms and parameters extracted from observed and simulated strong motion records confirms the efficacy of the developed modified technique to model earthquake characterized by SMGAs.

Source model for strong ground motion generation in the frequency range 0.1–10 Hz during the 2011 Tohoku earthquake

The source model of the 2011 Tohoku earthquake, which is composed of four strong motion generation areas (SMGAs), is estimated based on the broadband strong ground motion simulations in the frequency range 0.1–10 Hz using the empirical Green’s function method. Two strong motion generation areas are identified in the Miyagi-oki region west of the hypocenter. Another two strong motion generation areas are located in the Fukushima-oki region southwest of the hypocenter. The strong ground motions in the frequency range 0.1–10 Hz along the Pacific coast are mainly controlled by these SMGAs. All the strong motion generation areas exist in the deeper portion of the source fault plane. The stress drops of the four SMGAs range from 6.6 to 27.8 MPa, which are similar to estimations for past M 7-class events occurring in this region. Compared with the slip models and aftershock distributions of past interplate earthquakes in the Miyagi-oki and Fukushima-oki regions since the 1930s, the SMGAs of the 2011 Tohoku earthquake spatially correspond to the asperities of M 7-class events in 1930s. In terms of broadband strong ground motions, the 2011 Tohoku earthquake is not only a tsunamigenic event with a huge coseismic slip near the trench but is also a complex event simultaneously rupturing pre-existing asperities.

Strong Motion Records of the 2011 Tohoku Earthquake and its Attenuation Characteristics

Many strong motion records were obtained during the 2011 Off the Pacific Coast of Tohoku earthquake owing to the implementation of dense strong motion observation in Japan. The earthquake provides an opportunity to examine the characteristics of strong ground motion from a gigantic earthquake. Attenuations of peak acceleration and velocity are examined by comparing them to curves from existing attenuation relationships. When the shortest distance from the fault plane of the slip distribution model is used, curves for Mw8.0 to 8.3 give the smallest deviation from data, suggesting the saturation of ground motion intensity at large magnitudes. When the shortest distance from the strong motion generation areas is used, however, the scattering of data becomes smaller and the curve for Mw9.0 fits acceleration data. Results thus change with the definition of distance. This suggests that a consideration of the rupture heterogeneity is important in strong motion prediction for gigantic earthquakes.

Low-frequency and broad-band source models for the 2009 L’Aquila, Italy, earthquake

SUMMARY The 2009 L’Aquila, Italy, earthquake (Mw 6.3) produced an unprecedented number of seismological records for a normal faulting event, and hence represents an important case study of damaging moderate magnitude earthquakes occurring in densely inhabited areas. We have investigated the source process of the 2009 L’Aquila earthquake in both the low- and broad-band frequency ranges by analysing the teleseismic and strong motion data. A low-frequency (<0.5 Hz) source model has been determined by waveform inversion of teleseismic and near-field strong motion data, yielding slip distributions with a major asperity located about 8 km southeast of the hypocentre. The broad-band (0.2–10 Hz) source characteristics were estimated using the empirical Green's function method providing the strong motion generation area of about 42 km2 located southeast of the hypocentre below the village of Onna. This coincides with the location of the asperity from the inversion analysis, indicating that the low-frequency and broad-band ground motions produced during the earthquake were generated at similar locations on the fault plane. The position of the strong motion generation area also correlates well with the distribution of observed macroseismic (MCS) intensities reported by INGV. The maximum macroseismic intensities of IX–X were reported southeast of the epicentre, with an intensity of X measured in Onna. The source properties of the 2009 L’Aquila mainshock reveal the estimated size of the strong motion generation area to be in agreement with predictions made using empirical source scaling relation for inland crustal earthquakes, implying a stress drop of approximately 10 MPa. This further indicates that the 2009 L’Aquila earthquake involved a stress drop similar to that of previously recorded earthquakes in the Central and Southern Apennines.

Strong ground motion simulation of the 2003 Bam, Iran, earthquake using the empirical Green’s function method

The 2003 Bam, Iran, earthquake caused catastrophic damage to the city of Bam and neighboring villages. Given its magnitude (Mw) of 6.5, the damage was remarkably large. Large-amplitude ground motions were recorded at the Bam accelerograph station in the center of Bam city by the Building and Housing Research Center (BHRC) of Iran. We simulated the Bam earthquake acceleration records at three BHRC strong-motion stations—Bam, Abaraq, and Mohammad-Abad—by the empirical Green’s function method. Three aftershocks were used as empirical Green’s functions. The frequency range of the empirical Green’s function simulations was 0.5–10 Hz. The size of the strong motion generation area of the mainshock was estimated to be 11 km in length by 7 km in width. To estimate the parameters of the strong motion generation area, we used 1D and 2D velocity structures across the fault and a combined source model. The empirical Green’s function method using a combination of aftershocks produced a source model that reproduced ground motions with the best fit to the observed waveforms. This may be attributed to the existence of two distinct rupture mechanisms in the strong motion generation area. We found that the rupture starting point for which the simulated waveforms best fit the observed ones was near the center of the strong motion generation area, which reproduced near-source ground motions in a broadband frequency range. The estimated strong motion generation area could explain the observed damaging ground motion at the Bam station. This suggests that estimating the source characteristics of the Bam earthquake is very important in understanding the causes of the earthquake damage.

近地強震記録を用いた海溝型繰り返し地震の震源過程の推定と比較―茨城県沖で１９８２年と２００８年に発生したＭ７の地震を対象として―

At the Ibaraki-ken-oki region where the Paci.c plate subducts beneath the North American plate, repeating M7 class subduction-zone earthquakes have occurred with an interval of approximately 21 years. We estimated the source rupture processes of two M7 Ibaraki-ken-oki earthquakes from broadband strong motion records and compared them to investigate the source characteristics of repeating subduction-zone earthquakes. First, the source model of MJ 7.0 earthquake occurring on May 8, 2008 (2008MS) was estimated by the forward modeling through strong motion simulations using the empirical Green’s function method. Strong ground motions were simulated in the broad.band frequency range (0.3-10 Hz). A single rectangular Strong Motion Generation Area (SMGA) was assumed. The synthetic waveforms at stations within 150 km from the hypocenter reproduced the observed ones well. Then, we estimated the source model of another MJ7.0 earthquake occurring on July 23, 1982 (1982MS). We compared the observed waveforms of the 1982MS and the 2008MS at the same stations and identi.ed the initial rupture phase for the 1982MS. We assumed a single rectangular SMGA away from the hypocenter of the 1982MS. The SMGA was located about 33 km westward from the hypocenter and ruptured about 11s after the origin time; this location was just 7 km northward to the 2008MS hypocenter and so inside its SMGA. We analyzed the spectral ratio of the strong motion records of the 1982MS to the 2008MS and estimated the size, stress drop, and slip amount of the SMGA of the 1982MS. The estimated size was the same as that of the 2008MS, but the stress drop and slip amount were approximately 1.5 times larger. Synthetic waveforms reproduced the observed ones well when we assumed the rupture propagation characteristics were the same as that of the 2008MS. This simulation did not have the resolution to several kilometers’change of the location of the SMGA. Our results indicate that the two SMGAs may overlap. They were the same size, but had different stress drops and slip amounts. The hypocentral locations and initial rupture processes were also different. These results show the variety of the repeating earthquakes occurring in this region.

Characterization of the Heterogeneous Source Model of Intraslab Earthquakes Toward Strong Ground Motion Prediction

We characterize the heterogeneous source slip model of intraslab earthquakes to compare source scaling properties with those of inland crustal and subduction-zone plate-boundary earthquakes. We extracted rupture area (S), total area of asperity (S a), average slip (D) and average slip on asperity (D a) of eleven intraslab earthquakes following the procedure proposed by Somerville et al. (Seism Res Lett 70:59–80, 1999) and proposed the empirical scaling relationship formula of S, S a, and D for intraslab earthquakes. Under the same seismic moment, an intraslab earthquake has a smaller rupture area and total area of asperity, and smaller average slip than an inland crustal earthquake. The area ratio of asperity area and total rupture area of intraslab earthquakes are similar to those of inland crustal earthquakes. The strong motion generation area (SMGA) scaling of intraslab earthquakes appears self-similar, and those results support the idea the characterized source model of intraslab earthquakes can be modeled in a manner similar to that of inland crustal earthquakes.

Strong ground motions from an Mj 6.1 inland crustal earthquake in Hokkaido, Japan: the 2004 Rumoi earthquake

AbstractAn inland crustal earthquake (Mj6.1) occurred on December 14, 2004 in the northern part of Hokkaido, Japan. A large ground acceleration of 1127 cm/s2and a large pseudo-velocity response of over 100 cm/s were recorded at the nearest strong-motion station, HKD020, about 10 km from the hypocenter. This large ground motion is considered to be attributable to the source effect and the site effect. The site effect is investigated using the traditional spectral ratio technique and the theoretical evaluation based on the subsurface structure model. The results imply that the site effect has an insignificant effect on the large ground motion at HKD020. The source effect is investigated by constructing the source model that explains the broad-band strong-motion records at four stations around the epicenter using the empirical Green’s function method. The estimated source model satisfies the empirical relationship between the strong motion generation areas and the seismic moment for inland crustal earthquakes. The high-frequency level of the acceleration source spectrum is also consistent with the empirical relationship. These results suggest that this earthquake is a normal crustal event and that the large ground motion at HKD020 is mainly attributable to the source effect, short distance from the strong motion generation area and the forward directivity effect. Finally, the temporal change of the site response at HKD020 is examined using long duration records including the main shock and several aftershocks. The site response based on theS-wave horizontal-to-vertical spectral ratio method shows the nonlinearity for the main shock and an aftershock occurring about 20 s after the main shock. However, the site response shows linearity for other later aftershocks. This site response change is attributed to the difference in ground motion amplitude.

Source parameters of the 2007 Noto Hanto earthquake sequence derived from strong motion records at temporary and permanent stations

AbstractA large crustal earthquake, the 2007 Noto Hanto earthquake, occurred west off the Noto peninsula in Ishikawa prefecture, Japan, on March 25, 2007. We started temporary strong motion observation at five sites within the aftershock area from about 13 h after the main shock occurrence. We first applied the spectral inversion method toS-wave strong ground motion records at the temporary and permanent stations. We obtained source specta of the main shock and aftershocks,Qs(quality factor forS-wave) values (Qs= 34.5f0.95) and site responses at 22 sites. We then estimated a source model of the main shock using the empirical Green’s function method. The source model consists of three strong motion generation areas and well explains the observed records. Finally, we examined the consistency of the main-shock source models estimated from the above two analyses. The high-frequency level of the acceleration source spectrum based on the main-shock source model is consistent with the source spectrum estimated from the spectral inversion. The combined area of strong motion generation areas is approximately half of the value expected by the empirical relationship, and the high-frequency level of acceleration source spectrum is approximately 2.5 times larger than the empirical relationship for shallow inland and inter-plate earthquakes.

Strong-Motion Generation Areas of a Great Subduction-Zone Earthquake: Waveform Inversion with Empirical Green's Functions for the 2003 Tokachi-oki Earthquake

A waveform inversion with empirical Green's functions was conducted to estimate strong-motion generation areas (SMGAs) of the 2003 Tokachi-oki, Japan, earthquake (Mw 8.3). Although the rupture process of this great subduction-zone earthquake has been revealed with waveform inversions based on low-frequency (lower than 0.2 Hz) ground motions, it is much more important from an engineering point of view to investigate how strong ground motions with higher frequencies were generated from the earthquake. Waveform data with higher frequencies have been excluded from the conventional inversions because of the difficulty in computing realistic theoretical Green's functions at higher frequencies. In this study, with the aid of empirical Green's functions, we extended the applicability of the waveform inversion to higher-frequency ground motions up to 1 Hz. We selected three after- shocks for use in the analysis, referring to the similarity of the group delay time be- tween the mainshock and the aftershock records. The resultant slip model has three SMGAs, each of which is close to the hypocenter of one of the aftershocks used. Gen- erally speaking, locations of the SMGAs identified in this study agree well with as- perities identified from the inverted results using low-frequency (lower than 0.2 Hz) ground motions plus geodetic data and tsunamis. It implies that strong ground motions up to 1 Hz were generated from almost the same asperities producing lower-frequency ground motions. In terms of the complexity of slip models, although our analysis is focused on high frequencies, our slip model is at least as simple as conventional low- frequency slip models. Such results would be useful in constructing source models of future great subduction-zone earthquakes for strong-motion prediction.

Source model of the 2005 Miyagi-Oki, Japan, earthquake estimated from broadband strong motions

AbstractWe estimate the source model of the 2005 Miyagi-Oki earthquake, which consists of two source patches called “strong motion generation areas (SMGAs)”, to explain the broadband strong motions. The location of the first SMGA is determined from the arrival time differences between the first motion and the first pulse in theP-wave portion. Based on the broadband waveform modeling obtained using the empirical Green’s function method, we estimate the parameters of the two SMGAs. The estimated location of the two SMGAs coincides with the two major large slip areas inferred from the inversion of strong-motion and teleseismic waveforms, which implies that the broadband strong motion of the 2005 Miyagi-Oki earthquake mainly radiates from the asperities. The deeper SMGA has a larger stress drop (34.1 MPa) than the shallower one (17.6 MPa). Comparison of the locations of SMGAs between the 2005 and 1978 Miyagi-Oki earthquakes suggests that they do not overlap with each other. The interplate earthquakes, including this event, were found to have smaller SMGAs than those predicted from the relationship of crustal earthquakes for a given seismic moment. This implies that the stress drop of the SMGAs for interplate earthquakes is larger than that for crustal earthquakes.

Source parameters of intermediate-depth Vrancea (Romania) earthquakes from empirical Green's functions modeling

Several source parameters (source dimensions, slip, particle velocity, static and dynamic stress drop) are determined for the moderate-size October 27th, 2004 ( M W = 5.8), and the large August 30th, 1986 ( M W = 7.1) and March 4th, 1977 ( M W = 7.4) Vrancea (Romania) intermediate-depth earthquakes. For this purpose, the empirical Green's functions method of Irikura [e.g. Irikura, K. (1983). Semi-Empirical Estimation of Strong Ground Motions during Large Earthquakes. Bull. Dis. Prev. Res. Inst., Kyoto Univ., 33, Part 2, No. 298, 63–104., Irikura, K. (1986). Prediction of strong acceleration motions using empirical Green's function, in Proceedings of the 7th Japan earthquake engineering symposium, 151–156., Irikura, K. (1999). Techniques for the simulation of strong ground motion and deterministic seismic hazard analysis, in Proceedings of the advanced study course seismotectonic and microzonation techniques in earthquake engineering: integrated training in earthquake risk reduction practices, Kefallinia, 453–554.] is used to generate synthetic time series from recordings of smaller events (with 4 ≤ M W ≤ 5) in order to estimate several parameters characterizing the so-called strong motion generation area, which is defined as an extended area with homogeneous slip and rise time and, for crustal earthquakes, corresponds to an asperity of about 100 bar stress release [Miyake, H., T. Iwata and K. Irikura (2003). Source characterization for broadband ground-motion simulation: Kinematic heterogeneous source model and strong motion generation area. Bull. Seism. Soc. Am., 93, 2531–2545.] The parameters are obtained by acceleration envelope and displacement waveform inversion for the 2004 and 1986 events and MSK intensity pattern inversion for the 1977 event using a genetic algorithm. The strong motion recordings of the analyzed Vrancea earthquakes as well as the MSK intensity pattern of the 1977 earthquake can be well reproduced using relatively small strong motion generation areas, which corresponds to small asperities with high stress drops (300–1200 bar) and high particle velocities (3–5 m/s). These results imply a very efficient high-frequency radiation, which has to be taken into account for strong ground motion prediction, and indicate that the intermediate-depth Vrancea earthquakes are inherently different from crustal events.

Empirical Green’s function simulation of broadband ground motions on Genkai Island during the 2005 West Off Fukuoka Prefecture earthquake

Abstract The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.

Investigation of the earthquake sequence off Miyagi prefecture with historical seismograms

Abstract We investigated the old seismograms of several earthquakes which occurred offshore of Miyagi prefecture in 1933 (June 18, 21:37 UT), 1936 (Nov. 2, 20:45 UT), 1937 (July 26, 19:56 UT), 1978 (June 12, 08:14 UT), and 2005 (Aug. 16, 02:46 UT). A characteristic earthquake model for the sequence of the 1936, 1978, and several other earlier events is the basis of the recent forecast by the Earthquake Research Committee (2003) that “The probability of the occurrence of another similar earthquake in the next 30 years is 99%”. To assess the validity of the characteristic earthquake model, we compared the waveforms, size and other characteristics of these earthquakes recorded at Pasadena, DeBilt, Abuyama, Aso, Weston, Strasbourg, and Christchurch. We conclude that (1) The 1978 event is 3 to 4.5 times larger (in seismic moment) than the 2005 event; (2) The 1936 and the 2005 events are about the same size and are fairly close in location; (3) The 1937 event is smaller than the 1936 event, and is significantly deeper, possibly as deep as 90 km. In contrast, the 1933 event is significantly shallower than the 1936 event. The differences between these events are too large to justify the use of a simple characteristic earthquake model for the probabilistic forecast. The seismic slip rate in this area and along the adjacent subduction zone to the south is about 1/4 of the plate convergence rate, which has an important implication for the long-term seismic hazard in this area.The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.

Source model of the 2005 west off Fukuoka prefecture earthquake estimated from the empirical Green’s function simulation of broadband strong motions

AbstractWe estimated strong motion generation area (SMGA) source model which is responsible for broadband (0.2, 0.3–10 Hz) strong motions of the 2005 west off Fukuoka prefecture earthquake and its largest aftershock using the empirical Green’s function (EGF) method. The estimated SMGAs are almost corresponding to the large slip area deduced by kinematic waveform inversion. The SMGA rupture for the mainshock propagated mainly upward and toward the northwest with rupture velocity of 3.15 km/s. The size of the SMGA follows the self-similar empirical formula of the asperity size derived by Somervilleet al.(1999). The stress drop of the mainshock SMGA is calculated to be 10.7 MPa, which is larger than those of the largest aftershock SMGA and the aftershocks used as EGF.

Estimation of the source model for the foreshock of the 2004 off the Kii peninsula earthquakes and strong ground motion simulation of the hypothetical Tonankai earthquake using the empirical Green’s function method

AbstractWe estimated the source model for the foreshock of the 2004 off the Kii peninsula earthquakes by empirical Green’s function modeling. The size and the rise time of the strong motion generation area (SMGA) were estimated to be 30 × 15 km, and 0.9 sec, respectively. The stress drop of the SMGA was calculated to be 8.3 MPa. This model could reproduce long-period ground motions following the direct S-wave observed in the Osaka basin well. Using the derived source parameters, we simulated strong motions of the hypothetical Tonankai earthquake. Distribution of the seismic intensity derived here is similar to that obtained by the previous report. We could predict long-period ground motions which last for a long duration at the basin sites.

Rupture model of the 1999 Düzce, Turkey, earthquake deduced from high and low frequency strong motion data

We investigated the rupture process of the 1999 Düzce Earthquake by applying the multi time‐window linear waveform inversion method to low‐frequency ground motions (≤0.5 Hz) in the near source area. Then, we performed the forward modelling of higher frequency ground motions (0.3–10.0 Hz) by the Empirical Green's Function (EGF) method to image the strong motion generation area (SMGA). We also examined the source model to check whether the supershear phenomenon occurred inside the asperities or not. Slip distribution model from the inversion suggests 2 asperities. The maximum slip is obtained as 5 m near the hypocenter. The high first time window front propagation velocity (FTWFPV) and consequently, high apparent rupture velocity found from the investigation of rupture progression indicated that eastern propagation partially showed supershear behaviour. The EGF simulation, which identifies an average rupture velocity (Vr) in SMGA, supports this finding.

2003年5月26日に宮城県沖で発生したスラブ内地震の震源モデルと強震動シミュレーション

A large intraslab earthquake (MJMA 7.1) with the focal depth of about 70km occurred in the Pacific slab below off Miyagi Prefecture, northeastern Japan on May 26, 2003. This earthquake source brought strong high-frequency ground motion to the near-source region. At first, a source model is estimated by the forward modeling through strong motion simulations using the empirical Green's function method. Strong ground motions are simulated in the broadband frequency range (0.3-10Hz). Four borehole station data of KiK-net are used. A simple source model having several rectangular “strong motion generation areas” (Miyake et al., 2003) is assumed. The fittings of acceleration, velocity, and displacement waveforms are fairly well. Next, strong ground motion simulation is conducted at all the stations of K-NET and KiK-net (surface) within about 100km distance from the epicenter to validate the obtained source model. The spatial distributions of peak horizontal velocity and acceleration are well reproduced, which gives an evidence that our obtained source model can describe principal characteristics of the actual source process of this earthquake. However, synthetic peak horizontal accelerations are much larger than observed ones at several stations, which may be caused by nonlinear effect of the superficial layers during the mainshock. The obtained source model contains three strong motion generation areas. The spatial distribution of the strong motion generation areas is almost corresponding to the large slip regions from the kinematic waveform inversion analysis. The total size of the strong motion generation areas against a given seismic moment follows the empirical relationship with source-depth for intraslab earthquakes by Asano et al. (2003). Each strong motion generation area of this source model has 105MPa of stress drop. The varieties and uncertainties in estimation of the stress drop value on strong motion generation areas are also discussed.

Source Characterization for Broadband Ground-Motion Simulation: Kinematic Heterogeneous Source Model and Strong Motion Generation Area

We estimate strong motion generation areas that reproduce near-source ground motions in a broadband frequency range (0.2-10 Hz) using the empirical Green's function technique. The strong motion generation areas are defined as extended areas with relatively large slip velocities within a total rupture area. Four M 6 class (the 1997 Kagoshima on March and May, 1997 Yamaguchi, and 1998 Iwate) and several moderate-size earthquakes in Japan were analyzed. We examine the relationship between the strong motion generation area and the area of asperities, which is characterized based on heterogeneous slip distributions estimated from low-frequency (<1 Hz) waveform inversions (Somerville et al. , 1999). We performed waveform fitting in acceleration, velocity, and displacement, then obtained the strong motion generation area occupying about a quarter of the total rupture area. The size and position of the strong motion generation area coincide with those of characterized asperities. We find self-similar scaling of seismic moment to both the size of the strong motion generation area and the rise time for a magnitude range of analyzed earthquakes. Based on our results, we propose a characterized source model for the broadband ground-motion simulation, which consists of strong motion generation areas with large slip velocities and a background slip area with a small slip velocity. Waveform modeling of the characterized source model suggests that the strong motion generation areas play important roles in simulating broadband ground motions and have the potential to be an about 10-MPa stress-release area on the fault. Manuscript received 22 August 2002.

Source characteristics of shallow intraslab earthquakes derived from strong-motion simulations

Source modeling of recent shallow intraslab earthquakes is studied using strong-motion data. The total area of strong-motion generation areas obtained is smaller than the prediction from the empirical relationship for inland crustal earthquakes by Somerville and co-workers. Moreover, the ratio between the combined area of strong-motion generation areas obtained in this study and the area predicted by the empirical relationship, tends to decrease with focal depth. The stress drops on strong motion generation areas (or asperities) of shallow intraslab earthquakes increase with focal depth.