地壳速度结构对极浅源地震深度反演的影响——以荣昌地震为例

Inverting effect of crustal velocity structure on focal depth of very shallow earthquakes——Case study of the Ｒongchang earthquake

摘要:

研究表明，大陆地震很少发生在距地表5km 以浅，然而，近年来四川、重庆等地区发生了一系列3km 以浅的极浅源地震，据推测可能与新生断层形成或者工业开采活动有关。如果仅凭直达体波震相到时信息，要较好测定极浅源地震深度，需以震中距在2 倍震源深度范围内有台站为前提，而目前的台站密度显然不足。在这种情况下，地震的波形信息可为深度测定提供更有效地约束。目前已经广泛采用的CAP(Cut and Paste)方法利用了地震波形信息反演震源机制解、震级和深度，此方法对速度模型虽然没有严格要求，但是速度模型误差太大时不利于地震深度定位。本文以2010 年9 月10 日荣昌地区发生的Mb4. 7 地震为例，结合远震深度震相约束震源深度，测试了一系列地壳速度模型对CAP 方法反演极浅源地震深度的影响。研究发现浅层速度模型误差达10% 时，震源深度反演误差不大，但当误差超过50% 时，震源深度误差则很大。因此在研究极浅源地震时，需要对浅层速度结构进行较为准确的测定。

Abstract:

As most mature earthquake faults are filled with gouge at shallow part and it’s difficult to accumulate pressure，continental earthquakes hardly occurred shallower than 5km in depth． However，several very shallow earthquakes at a depth of about 3km occurred in Sichuan and Chongqing in recent years，which might be related to the formation of new faults or industrial activities． Generally speaking，to locate focal depth accurately using arrival time of direct body waves，stations within the epicentral distance of two times the focal depth are needed． Thus，by now the density of station is not enough to locate the focal depth of very shallow earthquakes ( shallower than 5km) ． Under this circumstance，utilizing the earthquake waveform is very useful to constrain the focal depth． CAP (Cut and Paste) is a method widely used to invert earthquake focal mechanism，magnitude and depth using earthquake waveform． Although this method is not strict with accuracy of crustal velocity structure model，large deviation of the velocity structure model can lead to bad focal depth inversion result． We took the Ｒongchang earthquake as an example，which happened on September 10，2010 with Mb4． 7，to test the accuracy of focal depth inversion using a series of velocity structure models，with the teleseismic depth phases as a constraint． We found that the deviation of focal depth inversion is small if the deviation of velocity structure model is within 10% ． Nevertheless，the deviation is large if the shallow part of velocity structure model has a deviation of about 50% ． Thus，it is necessary to ensure the accuracy of shallow part of velocity structure model to locate focal depth of a very shallow earthquake．