On the problem of magnitude calibration of palaeoearthquakes

Abstract

Some factors affecting the accuracy of magnitude calibration of palaeoearthquakes are discussed. The general problem is illustrated by two case studies. First, the 2003 strong earthquake in the Altai region (Russia–Mongolia border), which has generated co-seismic surface faulting is considered. An analysis is conducted to determine how accurately its magnitude could be defined after 2000 years based on surface faulting parameters, retrieved when future palaeoearthquake studies are launched. This example demonstrates that accurate evaluation of magnitude is possible if specific regional seismotectonic features are taken into account and surface faulting parameters are measured over a dense grid. Next, the Muya earthquake that occurred in the Baikal seismic region in 1957 is studied with respect to the problem of palaeoearthquake magnitude calibration in this region. The intensity of the Muya earthquake is assessed based on macroseismic and geological data. Macroseismic effect distribution confirms source depth at 20–22 km, agreeing with a seismic rupture length of 25 km: only a part of the source is exposed on the surface. Comparison with length of paleoseismodislocations shows that it is a regional feature. The source mechanism with three sub-sources is in agreement with the segmentation of surface ruptures. Sub-sources are of strike-slip type with small normal components. Essential normal slip at surface is probably not representative for the source and is due to accommodation of strike-slip movement along with a system of sub-parallel en echelon ruptures under tension.