Abstract

In this study, we establish the empirical relationships between the spatial dimensions of the aftershock zones and moment magnitudes (Mw) for the Taiwan region. The length (l) and width (w) of the aftershock zone of an earthquake is measured by the major and minor axes, respectively, of the ellipse of a two‐dimensional Gaussian distribution of one‐day aftershocks. Our data is composed of 649 mainshocks (depth≤70  km, Mw 4.0–7.6) between 1990 and 2011. The relationships between aftershock zone dimensions and Mw were obtained by least‐squares method with the corresponding uncertainties estimated by bootstrap. Our study confirms that aftershock zone dimensions are independent of faulting types and the seismic moment is proportional to l3. The ratio (w/l) increases slightly with Mw and is independent of faulting types. Together with previous study, our results suggest that earthquakes of both small (Mw=4.0) and large (Mw≥7.0) magnitudes have similar focal zone geometrical parameters. By using the MwS relation, where the aftershock zone area S is estimated from l and w, we also provide an independent examination of the variations in the median stress drop. We find that the median stress drops of strike‐slip earthquakes are higher than those of thrust events. Moreover, the median stress drops are independent of the moment magnitudes for normal and strike‐slip events but decrease for large thrust events. These results are consistent with the latest global observations. Regardless of faulting types, the median stress drop decreases for larger (6≤Mw≤7.6) and relatively deep (depth∼60–70  km) earthquakes.

Online Material: Effect of up‐dip rupture propagation on estimated maximum slip duration, details on teleseismic and geodetic stations, and figures of far‐field displacements, temporal change in slip, and snapshots of the semblance‐value distribution.

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