Abstract
The attenuation function, log A(subscript o)(△), used in the calculation of local magnitude is derived for the Taiwan area. The simulated Wood-Anderson seismograms are constructed by using digital three-components short-period seismogram of the Central Weather Bureau Seismic Network, (CWBSN). The decay of peak amplitude with distance is the attenuation characteristic of seismic energy. Also, it essentially represents the distance correction term, log A(subscript o) (△), after a proper normalization. Considering the focal depth of earthquakes in the Taiwan area, the log A(subscript o)(△)functions are: (The equation is abbreviated) for shallow earthquakes (focal depth, h≤35 km) and log A(subscript o)(△)=-0.00326R-0.83 logR-1.01 for deep earthquakes (h＞35 km) where △ is epicentral distance, R((The equation is abbreviated)) is the hypocentral distance. Results also show that the local magnitude of a deep earthquake is underestimated by using the Richter's log A(subscript o)(△) values (1935, 1958) with comparison to the M(subscript L) value obtained from the revised log A(subscript o)(△)values of this study. By applying the revised attenuation function, a compatible local magnitude can be calculated from the strong motion data. The conversion of duration magnitude, M(subscript D) which is currently used in the Taiwan area, to M(subscript L)is in the form: M(subscript L)=1.12 M(subscript D)+0.03±0.21
Highlights
The ML formulation was originally designed for the region of southern California where the function of logA0(L.\) was empirically detennined
The network consists of 68 stations. including 43 digital stations belonging to the Central Weather Bureau (CWB) and 25 one-component short-period stations of the Taiwan Telemeter{!d Seismograph Network (TISN). which is operated by the Institute of Earth Sciences (JES)
The squares are the attenuation function obtained by Yeh et aL (1982) by synthetic Wood-Anderson seismogram (SWAS) of strong motion data of Taiwan area
Summary
To study the attenuation of seismic wave, a new logA0(.6.) curve can be developed for Taiwan area and calibrated so that logAo(lOO) = -3. T he local station of new system is installed with a three-component short-period digital seismographs while a real time operation is performed in the center. Including 43 digital stations belonging to the CWB and 25 one-component short-period stations of the Taiwan Telemeter{!d Seismograph Network (TISN). The current local magnitude is calculated from the simulated Wood-Anderson seismogram from the short-period digital seismogram while the Richter's logA0(Ll) curve (1935, 1958) is used. The Richter's logA0(Ll) function (1935, 1958) is of depth-independence since almost all earthquakes in southern California are shallow. It is not so for earthquakes in Taiwan.
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