Simulation of a strong ground motion exceeding 4G during the 2008 Iwate-Miyagi Nairiku earthquake, Japan
Abstract
The Iwate-Miyagi Nairiku earthquake, a powerful reverse fault event that occurred on the southern Iwate prefecture Japan (2008/6/14), produced the largest peak ground acceleration recorded to date (4G), at the West Ichinoseki, KiK-net/NIED strong motion station (IWTH25), which is located immediately above the hypocenter. This KiK-net station which is equipped with surface and borehole accelerometers (GL -260m), also recorded very high peak accelerations up to 1G at the borehole level, despite being located in a rock site (Vs ~ 1800m/s). Preliminary analyses of these waveforms show a very strong content of high frequencies (HF). To understand the generation process of this extreme shaking we simulated the three components of ground motion at the IWTH25 borehole and surface levels, by applying a strong motion simulation methodology based on a dynamic model of fault rupture (Pulido and Dalguer 2008, PD08). Following PD08 we investigated the contribution of rupture velocity changes (Δ Vr) and stress drop (Δσ) across the fault plane to the HF ground motion radiation of the earthquake at the borehole level, and then calculated the non-linear response of the shallow layers to obtain the ground motion at the surface. To calculate the stress drop distribution of the earthquake we apply the methodology of Ripperger and Mai (2004), by using the slip model obtained from an inversion of near-source strong motion recordings (Suzuki et al. 2008), and a 1D velocity model for the Kanto region. The dislocation model of this earthquake is characterized by two patches of large slip, the first one located at the hypocenter and the second at ~7 km south of the hypocenter. Our results for the calculation of stress drop follow a similar pattern, namely a very large stress drop of ~80MPa concentrated at the hypocenter, as well as a large value of ~50MPa for the southern patch. To calculate the HF ground motion we follow PD08 who found that the HF radiation from earthquakes is confined to regions in the fault plane where the product ΔσΔ Vr is relatively large. Based on this result we calculate a stochastic distribution of Δ Vr values in the ±3.0 km/s range for areas in the fault plane where Δσ > 15 MPa, and in the ±0.5 km/s range for the remaining areas. We calculate the ground motion at the IWTH25 station following the methodology of PD08, by using the mapped HF radiation distribution and an average rupture velocity of 2 km/s. Our simulated waveforms incorporate the contribution of the P and S waves to the HF ground motion at the horizontals and vertical component. Our simulation is able to reproduce the overall characteristics of the observed waveform at IWTH25, including the observed ω -2 spectral radiation. Our results show a strong contribution of the P wave radiation pattern to the relative amplitudes of ground motion for the three components at IWTH25. In the present calculation our assumption of Δ Vr is entirely ad hoc. Future research includes the calculation of a dynamic model of the earthquake to put physical constraints on the fault rupture process and the simulation of near-source ground motion. ReferencesRipperger, J., and P.M., Mai (2004), Geophys. Res. Lett., 31, L18610. Pulido, N., and L.A. Dalguer (2008), Estimation of the high-frequency radiation of the 2000 Tottori (Japan) earthquake based on a dynamic model of fault rupture: Application to the strong ground motion simulation, Bull. Seism. Soc. Am., 2008 (in review). Suzuki, W., S. Aoi, and H. Sekiguchi (2008), 2008 AGU fall meeting.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S33C..02P
- Keywords:
-
- 7200 SEISMOLOGY;
- 7203 Body waves;
- 7209 Earthquake dynamics (1242);
- 7212 Earthquake ground motions and engineering seismology;
- 7215 Earthquake source observations (1240)