Ground motion and rupture process of the 2003 Tokachi-oki earthquake obtained from strong motion data of K-NET and KiK-net

Abstract

A great earthquake, named the 2003 Tokachi-oki earthquake, occurred in the southern Kuril subduction zone on 26th September 2003, 4:50 JST (41.7797 • N, 144.0795• E, 42 km depth; Japan Meteorological Agency). Its ground motion was recoreded at 655 stations of the nationwide strong motion networks, K-NET and KiK-net. A maximum peak ground acceleration of 988 cm/s 2 (gal) was observed at station HKD100 and amplitudes greater than 200 cm/s 2 were observed over a wide area of eastern Hokkaido. We used a multi-line linear waveform inversion method to estimate the rupture process from the strong motion data of supplied by 15 stations. We assumed a fault plane model of 140 km × 160 km with strike and dip angles of our fault model are N246• E and 18• , respectively, placed on the estimated upper boundary of the subducting Pacific Plate. The estimated total slip distribution consisted of three major slip areas; (a) around the hypocenter, (b) the northwest part of the fault with the maximum slip of 5.9 m, and (c) the northeast edge of the fault plane. The major asperity (b) was composed of two large slip areas with different slip rate functions: the duration of moment release in the sourtheast part is longer than 15 sec, but in contrast most of the seismic moment of the northwest part was released in a short period of less than 10 sec. Our estimation of the total seismic moment was 2.9 × 10 21 N·m which corresponded to Mw = 8.2.

Modeling the rupture process of the 2003 September 25 Tokachi‐Oki (Hokkaido) earthquake using 1‐Hz GPS data

“…[14] We also found significant slip in the northeast area of the fault plane, as inferred by Yamanaka and Kikuchi [2003] and Honda et al [2004] but not by Yagi [2004]. Koketsu et al [2004] show a smooth broad slip distribution centered near our highest slip but lacking any clear slip locus in the northeast region of the fault.…”

“…The larger slip patch is located where neither significant afterslip nor aftershocks [Ito et al, 2004] were observed; the region of smaller slip is located at the deeper edge of the afterslip. On the other hand, seismic waveform inversion studies [Honda et al, 2004;Yagi, 2004] find a maximum downdip where more than 20 cm of afterslip was observed. We suggest that almost all of the accumulated stress was released by highspeed rupture in ''asperities'' [Yamanaka and Kikuchi, 2004], and that afterslip and some aftershocks were concentrated in the surrounding regions due to stress transfer.…”

“…Yagi [2004] found that the rupture occurred at 3 asperities. Koketsu et al [2004] inferred a single large asperity; Honda et al [2004] found two major loci to the northwest of the epicenter, one minor locus near the epicenter, with one minor locus at the northeast edge of the fault. Our coseismic slip model makes a clear contrast with afterslip ( Figure 3a) estimated for the following 30 days .…”

“…These kinds of comparisons are difficult because each study uses a different station distribution, parameterization and procedures. Four slip models based on seismic data were recently published: Yamanaka and Kikuchi [2003] used 25 teleseismic records; Yagi [2004] combined 12 teleseismic stations with 12 surface strong motion sites on Hokkaido; Honda et al [2004] used 15 surface strong motion sites; and Koketsu et al [2004] used 11 strong motion borehole instruments on Hokkaido and static GPS displacements. All four seismic models and our seismo-geodetic study find a large area of slip northwest of the epicenter, although the location of the maximum slip region varies by as much as $20 km among the five models.…”

“…The rupture was close to the location of the 1952 TokachiOki earthquake [Yamanaka and Kikuchi, 2003]. Peak ground accelerations of $1g were observed at the coastal sites nearest the epicenter [Honda et al, 2004], with resulting coseismic deformation exceeding 1 meter [Miura et al, 2004]. Seismic moments for this event range from 0.8 to 3.5 Â 10 21 Nm, corresponding to Mw = 7.9 -8.3 [Honda et al, 2004].…”