The first ML scale for North of Vietnam

Abstract The first local magnitude scale ( M L ) for Northern Vietnam has been derived using a portable broadband seismic network in Northern Vietnam as part of the cooperation between the Vietnam Institute of Geophysics and the Institute of Earth Sciences at Academia Sinica, Taiwan. The composite horizontal peak amplitude data used in this study is comprised a total of 202 amplitude records from 14 broadband stations, which measure 36 shallow earthquakes with focal depths less than 36 km, occurred in and around north-western Vietnam during 01/2006–10/2007. The new distance-correction function obtained in this study is - log A 0 = 1.74 log ( r ) + 0.00048 r - 0.522 , where A 0 and r are the empirically determined distance correction and hypocentral distance, respectively. This distance-correction relation for Northern Vietnam is quite similar to one for southern California, implying relative efficient attenuation with distances. The correlation between the new M L and duration magnitudes ( M d ), which is still being used for making the official earthquake bulletin in Vietnam, is expressed as M L = 0.955 M d + 0.17 .