Revised ML determination for crustal earthquakes in Taiwan

Yih-Min Wu
Department of Geosciences, National Taiwan University, Taiwan

Richard M. Allen
University of California Berkeley

Chien-Fu Wu
Central Weather Bureau, Taiwan

Bull. Seismol. Soc. Am. 95 (6), 2517-2524, doi: 10.1785/0120050043, 2005.

Download a reprint: WuAllenWu2005BSSA.pdf (0.5 Mb)

Abstract The local magnitude scale ML is an empirically derived scale anchored to a zero magnitude reference event. Richter defined the amplitude of ground shaking at an epicentral distance of 100km for a zero magnitude earthquake in southern California. The crustal attenuation characteristics of Taiwan result in a relatively high ML when compared to moment magnitude, Mw. We therefore define a revised ML scale for crustal earthquakes in the Taiwan region which is more consistent with Mw. Using observed peak ground shaking and Mw we determine a new log A0 curve of ground shaking versus hypocentral distance, R, for a zero magnitude earthquake, and station correction factors. The log A0 curve determined in this study is,

log A0(R) = 0.156 - 0.000796R - 1.424 log R

Using this new log A0 curve and station corrections the new ML is more consistent with Mw with a 0.2 magnitude unit uncertainty. The new log A0 curve has a value of -2.77 at the distance of 100 km compared to the anchor point of log A0 = -3 at the same distance as defined by Richter for southern California. This means that the current ML estimates in Taiwan (which use Richter’s definition) average 0.23 magnitude units larger than their Mw. The stations correction factors also determined are large, distributed in the range –0.39 to 0.55 magnitude units. The use of station corrections in routine seismic network operation in Taiwan will improve magnitude estimates. This is particularly important for smaller events when recording stations may be predominantly on either hard rock or soft soil sites which could lead to under or over estimates of the magnitude by up to half a magnitude unit.