Coulomb Stress Based Earthquake Risk Estimates in North China after the Mw9 Tohoku Earthquake, 2011
Shi, Yaolin; Zhang, Bei; Zhang, Siqi; Zhang, Huai
University of Chinese Academy of Sciences, CHINA

North China is a seismic active region. Historically, more than 20 earthquakes with magnitude greater than 7, including 5 of them
greater than M8, occurred in this region. The M7.8 Tangshan earthquake in 1976 killed 242000 people, and Beijing was affected.
Because the political and economic importance of this region, earthquake risk estimates is a sharp problem, especially after the
Mw9.0 Tokoku earthquake, Japan, 2011.

Traditionally seismic risk based on statistics of earthquake catalogs by seeking the relation of Japanese seismicity and North China
seismicity. The advance of space geodetic observation and high performance computing capacity enable us to make a new approach by
calculating the displacement and stress field variation produced by Tohoku earthquake, and discuss the possible changes of North China
seismicity based on earthquake mechanics.

Slip of earthquake fault can produce displacement and stress variation at surrounding regions. These changes may facilitate or prevent succeeding
earthquakes depending on the variation of stresses as well as geometry and mechanical nature of faults. Coulomb stresses are usually
calculated to make the evaluation. Analytical solutions for layered half space are available for Coulomb stress calculation. However, for giant
earthquakes of Mw greater than 9, the curvature of the earth must be considered in far field stress computation. In this study, we make finite
element numerical simulation to calculate Coulomb stresses in North China due to the Tohoku earthquake.

A whole earth finite element model can avoid uncertainties in assigning boundary conditions for regional model, but a huge number of elements
are needed to ganranttee the accuracy of computation. We construct a layered finite element earth model of 4 million elements, with dense grids
near the fault zones produced adaptively. Element size is 2 km near the fault and about 20-40km in North China. Fault slip model from seismic
data, GPS data and InSAR data have been proposed by many researchers, although they differ in details, they all work well in the
calculation of far field deformation. Calculated displacements are in agreement with GPS observed co-seismic deformation (Figure 1).
Calculated co-seismic stress variation is characterized by a nearly E-W direction tensile stress of magnitude ~ kPa, about 0.5kPa in western
North China increased to 4kPa in eastern North China (Figure 2). The principal stress directs to the epicenter (Figure 3).

North China is under NEE compression from earthquake mechanism and in-situ stress measurements with annual increment of ~0.25kPa deduced
from 10-9 annual strain rate suggested by GPS measurement. Therefore, the co-seismic changes due to Tohoku
earthquake will reduce seismic risk in North China in a period of at least a decade for most seismic generating faults in the region. Coulomb
stresses on different active faults in North China were calculated too. Almost all of them have negative values of a few hundred Pa, that means
seismic risk is reduced.

In conclusion, the Mw9.0 Tohoku earthquake may reduced seismic risk in North China for a decade duration.

Figure 1 Comparison of calculated (blue arrows) and GPS observed far field co-seismic displacement (white arrows)

Figure 2 Co-seismic change of E-W direction stress component.

Figure 3 Principal stresses of co-seismic stress changes

Figure 4 Calculated Coulomb stresses on major active faults in North China. Negative means reduction of earthquake risk