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Revised M_L Determination for Crustal Earthquakes in Taiwan

¹ Department of Geosciences
National Taiwan University
Taipei, Taiwan
drymwu{at}ntu.edu.tw
 (Y.-M.W.)

² University of California Berkeley
Berkeley, California
 (R.M.A.)

³ Central Weather Bureau
Taiwan
 (C.-F.W.)

The local magnitude scale M_L 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 M_L when compared to moment magnitude, M_w. We therefore define a revised M_L scale for crustal earthquakes in the Taiwan region that is more consistent with M_w. Using observed peak ground shaking and M_w, 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,

(001)

Using this new log A₀ curve and station corrections, the new M_L is more consistent with M_w, 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 M_L estimates in Taiwan (which use Richter’s definition) average 0.2 magnitude units larger than their M_w. 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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