Karl K. Irikura

The Importance of the Dynamic Source Effects on Strong Ground Motion during the 1999 Chi-Chi, Taiwan, Earthquake: Brief Interpretation of the Damage Distribution on Buildings

The 1999 Chi-Chi, Taiwan, earthquake, that originated on a low-angle reverse fault, showed complexity and uncommon characteristics. The records show that the hanging-wall side is characterized by larger particle motions than the foot- wall, and the ground motion is stronger in the northern part than in the southern part of the causative fault. Although the strongest ground motion occurred near the north- ern part of the trace, structural damage was heavier in the southern part. In order to get a better understanding of the complex damage distribution caused by this earth- quake, the dynamic rupture process was numerically simulated. Because of the dif- ferences between the observed features of the rupture process in the northern and southern parts of the fault, each part was modeled independently by using a 2D discrete element model (DEM). The principal results of the simulation show that the velocity ground motions in the northern part, in the frequency range of 0.5-2 Hz (natural frequency range of standard structures), are small near the surface break, thus, light structural damage might be predicted near the surface rupture. Moreover, in the northern part the fault rupture propagation reaches the surface with a very slow velocity (about 1.2 km/sec); however, in the southern part the rupture propa- gation reaches the surface with higher velocity (about 3.0 km/sec). These differences between the models could explain why the ground motion near the surface rupture in the northern part caused less damage in structures than the ground motion in the southern part.

Estimation of Dynamic Rupture Parameters from the Radiated Seismic Energy and Apparent Stress

We propose a new procedure for estimating the critical slip weakening distance, Dc, by evaluating the spatio-temporal distribution of the apparent stress calculated from the spatio-temporal distribution of the slip velocity function on the fault plane obtained from a kinematic inversion of the earthquake source. The idea is based on the fact that the apparent stress can be related to the difference between the earthquake average stress and the frictional dynamical stress on the fault plane during the rupture process. From the cumulative slip-apparent stress relationship we estimate the critical slip, Dc, of the 1992 Landers earthquake. We find that the critical slip weakening plays an important role in controlling the rupture velocity. In the case of the Landers earthquake the very low rupture velocity in the central segment could be explained by the large Dc-value obtained for the main asperity on that segment.

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

(1) We investigated the rupture process of the 1999 Duzce 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 theforwardmodellingofhigherfrequencyground motions (0.3-10.0 Hz) by the Empirical Greens 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. INDEX TERMS: 7212 Seismology: Earthquake ground motions and engineering; 7215 Seismology: Earthquake parameters; 7223 Seismology: Seismic hazard assessment and prediction. Citation: Birgoren, G., H. Sekiguchi, and K. Irikura (2004), Rupture model of the 1999 Duzce, Turkey, earthquake deduced from high and low frequency strong motion data, Geophys. Res. Lett., 31, L05610, doi:10.1029/ 2003GL019194.

Fault dynamic rupture simulation of the hypocenter area of the thrust fault of the 1999 Chi-Chi (Taiwan) Earthquake

A 2D Discrete Element Model was employed to simulate the rupture propagation and near-source ground motion in the epicentral area of the 1999 Chi-Chi (Taiwan) earthquake. The observations show that the hanging wall side is characterized by larger particle motions than the footwall side. The simulation results reproduce the main features of the recorded ground motion and show that the particle velocity on the hanging wall side and on the footwall side are symmetric in the deeper fault region, however, as the crack approaches the free surface, the hanging wall side is characterized by larger particle motions than the footwall side. These results suggest that the difference in the particle motion on the hanging wall and on the footwall is due mainly to the asymmetric geometry of hanging wall and footwall. Thus, the model used leads to good approximations in the vicinity of the epicenter.