Modeling of the post-seismic slip of the 2003 Tokachi-oki earthquake M 8 off Hokkaido: Constraints from volumetric strain

Tetsuo Takanami,Alan T Linde,Hui Peng,Genshiro Kitagawa,Selwyn I Sacks

doi:10.5047/eps.2012.12.003

Abstract

A Sacks-Evertson borehole volumetric strainmeter (SE strainmeter) at a site located 105 km from the epicenter of the mainshock recorded a clear slow strain event following the 2003 M w 8.0 Tokachi-oki earthquake (September 25, 2003, 19:50:06 UTC). This consisted of an episode of contraction for 4 days followed by expansion for 23 days. GPS sites in southeastern Hokkaido also recorded displacement changes during the same time interval. We use quasi-static calculations to generate synthetic waveforms for the measured quantities. All the data are satisfied by a propagating line source 2-stage model of slow reverse slip, uniform amplitude of 50 cm, with rupture propagation velocities of constant 9 cm/s (first stage) and exponentially decreasing from 3 to 0.7 cm/s (second stage). This post-seismic slip event is taken to be coplanar with the main shock rupture on the upper plane of the double Wadati-Benioff seismic zone (DSZ), and largely overlaps the seismic rupture. Regular earthquakes release only about 30% of the plate motion in this section of the subduction zone; post-seismic slip appears to account for at least some of the deficit.

Highlights

The Hokkaido corner in the southernmost Kuril trench is the site of great earthquakes due to the subduction of the Pacific Plate beneath Hokkaido, Japan, at a rate of 8.3 cm/yr (Fig. 1)
When we apply the very reasonable constraint that the source of the post-seismic slip is coplanar with the mainshock rupture and the slip vector has the same rake angle (124◦) as for the mainshock, as given by Geospatial Information Authority of Japan (GSI) (2004), we immediately find that the strain record supplies strong constraints on the extent of the source in the dip direction and requires that the rupture propagates down-dip
Relationship between Post-Seismic Slip and Aftershock Activity In Fig. 5, we show aftershock activity during the first 10 days of the post-seismic slip event

Summary

Introduction

The Hokkaido corner in the southernmost Kuril trench is the site of great earthquakes due to the subduction of the Pacific Plate beneath Hokkaido, Japan, at a rate of 8.3 cm/yr (Fig. 1). 3. Propagating Fault Model for Post-Seismic Slip Previous studies of post-seismic strain changes in locations of GPS sites (Ozawa et al, 2004; Miyazaki and Larson, 2008; Fukuda et al, 2009) solved for source models coplanar with the mainshock rupture. We minimize the number of free parameters by taking a constant slip amplitude over the whole slip surface, so the observed strain change amplitude for Stage 2 determines the lower limit of slip This coincidentally results in the down-dip rupture width being the same for both stages. The northeast extent is less well controlled by the data in Fig. 4; rather, we limit the northeast extent to fit well with the extent of coseismic rupturing This limit, for our very simple source model, is rather similar to that in Ozawa et al (2004) and in Miyazaki et al (2004); those studies use data from many more GPS sites. Previously-published models are in conflict with the observed strain change data whereas our very simple model, derived primarily to fit the strain data, fits the GPS data very well for those sites in the vicinity of our model

Relationship between Post-Seismic Slip and Aftershock Activity

Findings

Conclusion