Coulomb stresses imparted by the 25 March 2007 Mw=6.6 Noto-Hanto, Japan, earthquake explain its ‘butterfly’ distribution of aftershocks and suggest a heightened seismic hazard

Shinji Toda

doi:10.1186/bf03352866

Abstract

AbstractThe well-recorded aftershocks and well-determined source model of the Noto Hanto earthquake provide an excellent opportunity to examine earthquake triggering associated with a blind thrust event. The aftershock zone rapidly expanded into a ‘butterfly pattern’ predicted by static Coulomb stress transfer associated with thrust faulting. We found that abundant aftershocks occurred where the static Coulomb stress increased by more than 0.5 bars, while few shocks occurred in the stress shadow calculated to extend northwest and southeast of the Noto Hanto rupture. To explore the three-dimensional distribution of the observed aftershocks and the calculated stress imparted by the mainshock, we further resolved Coulomb stress changes on the nodal planes of all aftershocks for which focal mechanisms are available. About 75% of the possible faults associated with the moderate-sized aftershocks were calculated to have been brought closer to failure by the mainshock, with the correlation best for low apparent fault friction. Our interpretation is that most of the aftershocks struck on the steeply dipping source fault and on a conjugate northwest-dipping reverse fault contiguous with the source fault. Since we found that the Coulomb hypothesis works well for the Noto Hanto sequence, we subsequently computed stress changes on the nearby active faults. Although the calculated stress changes were found to be negligible on the major faults south of the Noto Peninsula, several short active faults near the epicentral area were calculated to have been brought several bars closer to failure. Thus, the probability of strong shaking in and around the epicentral area may still be high due to the transfer of stress to the adjacent faults by a short blind thrust fault.

Highlights

Calculations of static Coulomb stress changes caused by an earthquake have been used to explain the spatial distribution of the subsequent aftershocks (Stein, 1999; Steacy et al, 2005; Freed, 2005 and the references therein)
On the Noto Peninsula there are many short discrete reverse faults associated with vertical movements, resulting in approximately 10- to 20-km-long small tilted crustal blocks that developed during the late Tertiary period to the Quaternary (Ota, 1975)
In order to examine the relationship between aftershocks and stress changes, we assume that most small to moderate earthquakes dominating in the earthquake catalog are caused by thrust faults under NW-SE compressional regional stress

Summary

Introduction

Calculations of static Coulomb stress changes caused by an earthquake have been used to explain the spatial distribution of the subsequent aftershocks (Stein, 1999; Steacy et al, 2005; Freed, 2005 and the references therein). If the hypothesis is sound, Coulomb stress change provides a tool to forecast the subsequent large earthquake triggering nearby. On the Noto Peninsula there are many short discrete reverse faults associated with vertical movements, resulting in approximately 10- to 20-km-long small tilted crustal blocks that developed during the late Tertiary period to the Quaternary (Ota, 1975). One of these crustal blocks slipped in the 2007 shock. We first examine the spatial correlation between aftershocks and Coulomb stress change due to the Noto Hanto earthquake. We estimate how many stresses are loaded onto major active faults nearby

Static Coulomb Stress Change Throughout the Crust

Coulomb Stress Change on Nodal Planes of Moderately Sized Earthquakes

Findings

Discussion and Conclusions