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 100 km 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 that 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,  
formula
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 −2.80 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 0.2 magnitude units larger than their Mw. The station correction factors also determined are large, from −0.40 to 0.52 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 overestimates of the magnitude by up to half a magnitude unit.
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