Research About Focal Parameters Using CAP And Depth Phases

Earthquake focal depth, as a focal mechanism parameter, is important in many aspects. For example, fast focal depth determination can provide preliminary information for earthquake damage, which is crucial for earthquake rescue. Further, accurate focal depth is useful for better understanding of earthquake nucleation environment in earth and for exploring the tectonic setting and stress background in the specific areas. However, accurate focal depth determination has long been a problem. At present, CAP(cut and paste) method has been recognized as an effective way of inverting focal parameters including focal mechanism and magnitude by waveform fitting. However, by now the accuracy of focal depth inversion using CAP has not been discussed and examined systematically. In this article, we test the effect of duration, crustal velocity structure, surface topography to the inversion of focal depth and other focal parameters. And we found that the deviation of duration and crustal velocity structure (±10%) has nearly no effect on the inversion result, but surface topography can cause deviation of focal depth inversion.In this article, we do researches about three earthquake events:Rongchang earthquake in China at September 10,2010 of magnitude Mw4.1, Feidong earthquake in China at April 6,2011 of magnitude Mw3.3 and Arkansas earthquake in America at February 18,2010. By method of CAP, we locate the focal depth of the three earthquakes mentioned above. At the same time, by using sPL, pP and sP depth phases to locate the focal depth as a comparison, we concluded that the focal depth inversion result by CAP method is accurate when the 1D velocity model is accurate (within ±10% deviation).If the magnitude of an earthquake is too small(<Mw3.7), then focal parameters inverted by CAP may have higher deviation. At this time, sPL is very useful for constraining focal depth even with one station. In a serious of study, we found that sPL phase cannot develop well when the velocity is low at top of the ground, as for the area covered by low velocity sedimentary materials. For Arkansas earthquake at February 18,2010, sPL phase is not obvious at all for the reason of low velocity in the first layer of the ground.The teleseismic depth phase-pP and sP, observed at distance from 30°to 80°, is not sensitive with velocity structure and is useful for constraining focal depth. Even for the earthquake of magnitude at Mw4.0, some stations at 30° to 80° with high signal-noise-ratio can also record the earthquake signal clearly, thus focal depth can be located after recognition of pP and sP. For Rongchang earthquake at September 10, 2010 of magnitude Mw4.1, we constrain the focal depth in this way.