| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 97 === Rupture Directivity Analysis for Large Earthquakes
Postgraduate：Jo-Pan Chang
Adviser：Dr. Chien-Ying Wang
Abstract
Rupture directivity analysis for large earthquakes can provide some basic fault
parameters. Relationships between these fault parameters, the so-called scaling law,
lead to understand the physical properties of earthquakes. Many methods and seismic
data can investigate the rupture directivity of an earthquake. In this study, we use the
differences of surface-wave travel time between the mainshock and reference
earthquake to determine the fault parameters for large earthquakes. Above mentioned,
constraint on such work is that there must be reference earthquakes in the vicinity of
the mainshock. Here, we propose a new method to analyze the rupture directivity of an
earthquake without using reference earthquakes. That is, we calculate the surface-wave
travel time using the global surface-wave phase-velocity maps to be the travel times
from reference earthquakes. The concept is similar to the Green’s function.
In this study, 8 large earthquakes occurring from 1999 to 2008 with Mw
7.6-9.0(9.3) are analyzed using surface waves with epicentral distances between 30°
and 90°. We use the Rayleigh waves to perform the rupture directivity analysis for
trust-type earthquakes; on the contrary, using the Love waves to analyze the rupture
directivity for strike-slip-type earthquakes. Results show that the proposed method is
valid for the rupture directivity analysis of a large earthquake. On the whole, the
source duration and rupture length increase with seismic moment (or Mw). Among
these analyzed earthquakes, the 2004 Sumatra-Andaman earthquake (Mw 9.0-9.3) has
the longest source duration and rupture length. Besides, earthquakes with
strike-slip-type mechanism have relatively larger source duration and rupture length
than those with thrust-type mechanism. The optimal rupture azimuth can efficiently
provide the judgment on which is the fault plane from the beach ball for the strike-slip
earthquakes. However, it is ambiguous to judge the fault plane for the trust-type
earthquakes. The rupture velocity is easily underestimated from the whole source
duration (including the rupture time and rise time). By investigating the period of
spectral node, we can determine the rise time, and then derive the reasonable rupture
velocity. The advantage of our proposed method is that it is not in want of the
reference earthquakes and can quickly and efficiently determine the fault parameters of
large earthquakes from rupture directivity analysis. Also, the proposed method is
appropriate to re-examine some historical earthquakes, such as the 1960 Chile
earthquake and 1964 Alaska earthquake.
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