Peih-Lin Leu

Precursor times of abnormal b-values prior to mainshocks

Seismic observations exhibit the presence of abnormal b-values prior to numerous earthquakes. The time interval from the appearance of abnormal b-values to the occurrence of mainshock is called the precursor time. There are two kinds of precursor times in use: the first one denoted by T is the time interval from the moment when the b-value starts to increase from the normal one to the abnormal one to the occurrence time of the forthcoming mainshock, and the second one denoted by Tp is the time interval from the moment when the abnormal b-value reaches the peak one to the occurrence time of the forthcoming mainshock. Let T* be the waiting time from the moment when the abnormal b-value returned to the normal one to the occurrence time of the forthcoming mainshock. The precursor time, T (usually in days), has been found to be related to the magnitude, M, of the mainshock expected in a linear form as log(T) = q + rM where q and r are the coefficient and slope, respectively. In this study, the values of T, Tp, and T* of 45 earthquakes with 3 ≤ M ≤ 9 occurred in various tectonic regions are compiled from or measured from the temporal variations in b-values given in numerous source materials. The relationships of T and Tp, respectively, versus M are inferred from compiled data. The difference between the values of T and Tp decreases with increasing M. In addition, the plots of T*/T versus M, T* versus T, and T* versus T-T* will be made and related equations between two quantities will be inferred from given data.

Studies on Aftershocks in Taiwan: A Review

We reviewed studies on aftershocks in Taiwan for the following topics: the spatial-temporal distributions and focal-plane solutions of aftershocks fromof thirty larger earthquakes with magnitudes > 5; the correlations between the mainshock and the largest aftershock based on dependence of the differences in magnitudes (∆M), occurrence times (∆T), epicenters (∆H), and focal depths (∆D) upon the mainshock magnitude, Mm; magnitude-dependence of p-value of Omori’s law of aftershocks; the correlation between the b-value of the Gutenberg-Richter’s frequency-magnitude law and the p-value; application of the epidemic-type aftershock sequences (ETAS) model to describe the aftershock sequence; the mechanisms of triggering aftershocks; and dynamic modeling of aftershocks. The main results are: (1) The spatial distribution of aftershocks for some earthquakes is consistent with the recognized fault; (2) Unlike Båth’s law, ∆M slightly increases with Mm; (3) ∆T does not correlate with Mm; (4) ∆D does not correlate with Mm; (5) ∆T somewhat increases with ∆D; (6) The p-value slightly increases with Mm; (7) There is a negative correlation between the band p-values. (8) There was seismic quiescence over a broader region of Taiwan before the 1999 Chi-Chi earthquake; (9) Both the static and dynamic stress changes trigger aftershocks; and (10) Dynamic modeling shows that a decrease in elastic modulus is a significant factor in triggering aftershocks.