The source characteristics for subduction zone earthquakes and The nature of D” discontinuity beneath Central America
Date Issued
2014
Date
2014
Author(s)
Ko, Yen-Ting
Abstract
Part I.
An improved inversion technique is needed to effectively separate the frequency dependence of the source from the intrinsic attenuation of the medium. We developed a cluster-event method (CEM) in which clusters of nearby events, instead of individual events, pair with stations to form the basis for measurements of Q value and corner frequency (fc). We assume that the raypaths from one cluster to a station share an identical Q while each event in the same cluster is allowed for only one fc in the inversion process. This approach largely reduces the degrees of freedom to achieve a robust inversion. We use an optimization algorithm of simulated annealing to solve the non-linear inverse problem. The CEM was applied to events at 70 – 150 km depths in the Japan subduction zone recorded by F-net. We show that the method proposed here leads to better constraints on both source parameters and attenuation. The resultant Q’s in the mantle wedge increase from lower than 300 beneath the arc and back-arc to greater than 600 in the fore-arc region. The fc’s satisfy a self-similar scaling relationship with seismic moment of Mo ~ fc^-3 with a best-fit stress drop of 21.9±6.9 MPa in Madariaga’s form. This contrasts to the stress drop of 1.4±1.1 MPa for a global data set composed of prior measurements for crustal events. The results of this study agree with results from previous studies, except with an upward deviation due to higher corner frequencies and stress drops.
Part II.
Widespread and high rate of intermediate-large earthquakes in the Japan subduction zone provide an opportunity to systematically investigate depth-varying characteristics of earthquake sources. We utilize the cluster-event method to ensure a robust inversion for source and path parameters for 813 events spreading over a depth range of 0 – 150 km and seismic moments 4 orders of magnitude different. Static and dynamic source parameters are measured and compared for three depth regions: I, 0-25 km; II, 25-50 km; and III, > 50 km, where distinct rupture mechanisms may dominate. Our resulting corner frequency, stress drop, and radiated energy provide strong evidence of depth-dependent characterizations of earthquake sources. The estimations of higher corner frequency and stress drop at greater source depth with sufficient of short-period energy radiation may be attributed to earthquakes occurring on rougher and more immature faults.
Part III.
We constrain topography, impedance contrast, and shear velocity gradient of the D” layer beneath the Caribbean region using Scd-S and ScS-S times, Scd/S and ScS/S amplitude ratios, and the full waveforms between S and ScS recorded by the USArray. Our results reveal an east-west trending, V-shaped undulation of D” topography strongly correlated with shear velocity anomalies within the D” layer: The D” discontinuity elevates from the lowest point beneath the north South America by 80-180 km over a lateral distance of 600 km, while the shear velocity anomalies increase in the same trend. This can be explained by the fluctuation of temperature and post-perovskite phase boundary in the environment where a horizontally lying slab, probably from the ancient Farallon plate, is trapping heat and substantially warmed at its center.
Subjects
震源參數
隱沒帶地震
震源自我相關性
群組地震法
剪力波三重波像
波形模擬
D”不連續面
隱沒板塊與熱柱交互關係
Type
thesis
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