Source Studies

Martin VALLEE : martin.vallee@cea.fr Julien VERGOZ : julien.vergoz@cea.fr (Laboratoire de Détection Géophysique, CEA, Paris, France) Based on the method defined by Vallée and Bouchon (2003), we describe the rupture as a collection of slip patches. In this case, one patch is dominant and insure a good fit to teleseismic P and SH waves (see figure 1). The inferred mechanism has a (strike,dip,rake) equal to (259°,28°,129°). The strike is close to the azimuth of the trench near Hokkaido (235-240°). The rake is consistent with NW-SE convergence between Japan and Pacific Ocean. The rupture initiation (denoted by a star in figure 1) occurred approximately at 32 km depth, 60km off the coast of Hokkaido and 130km from the coastal town of Kushiro. Rupture propagated mainly in the Western direction. The rupture extension is about 80km along strike and 130km along dip with an amount of slip of 2-3m. The global duration is approximately 42s and the inferred rupture velocity is about 3.3km/s. The relatively low and smooth slip on a broad zone could be partly responsible of the relatively low damage induced by such an earthquake (Mw =8.1).

Figure 1. 3-D geographical visualisation of the fault slip

Figure 2. Geographical distribution of the used stations (IRIS and LDG)

Figure 3 Model and fit to teleseismic body waves. The left part of the figure presents the comparison between data coming from IRIS and LDG stations (black) and synthetics (red). Teleseismic SH waves are presented in the inner ring and teleseismic P waves in the outer ring of the figure. Azimuths and names of each station are indicated and their positions on the ring are approximately representative of these azimuths. Both data and synthetics have been lowpass filtered at 13s but data do no exhibit large higher-frequency features. The considered time windows are indicated by vertical bars on each seismogram.

References: Vallée, M. and M. Bouchon, Imaging coseismic rupture in far field by slip patches, accepted in Geophys. J. Int., 2003.