| As an integral part of seismology, the focal mechanism plays an important role innot only interpreting the earthquake preparation, but also describing the nature ofseismic source and the rupture process to some extent. The earthquake monitoringagencies routinely provide the magnitudes, the origin time, the focal depth and thelocations of earthquakes immediately after they occur. However, in many instancesthe reliable focal mechanism, which potentially increases the efficiency of rescueoperations, seismic timely analysis and aftershock prediction, shows its realisticmeaning, since it speeds up the velocity of the inversion and provides more effectivesource information. The automatic or semi-automatic inversion system, which canprovide more reliable results, is an effective way to use seismic network records andextend information service. In recent years, several groups in the United States, Japan,European and Taiwan of China have been implemented automatic focal mechanismprocedures based on the diffenent seismic waveform data recorded by variousobservactory network.In this project, we adopt the Cut-And-Paste (CAP) method (Zhao et al，1994；Zhu et al，1996) of waveform inversions to develop a system for the near real-timeand semi-automatic determination of focal mechanisms and source depth in CapitalCircle area. The procedure can automatically detect near real-time earthquakeprovided by the waveform data service system in the Institute of Earthquake Science，China Earthquake Administration. Each event with magnitude M≥3.5would triggeran automatic request for near real-time data in waveform data service system.Relevant preliminary solutions about source inversion may be produced within20minutes after an earthquake occurrence.The system consists of four parts: the establishment of Green’s function databasein the Capital area, the observed waveform data processing, the inversion of focalmechanism and the technology of information publishing. The Green’s function usedin this procedure had been computed with the Frequency-Wavenumber (F-K) package(Zhu et al,2002), in which the layered velocity structures was based on the1-D velocity model of Beijing metropolitan area. To reduce the computing time of theautomatic procedure, Green’s functions are pre-computed and stored in a library.Specifically, Green’s functions in study area are computed at5km intervals (from5upto400km) along distance and at1km intervals (from1down to50km) along depth.And the observed waveform data processing includes data acquisition and dataprocessing. A few minutes after an event occurrence, an automatic earthquake alertprovided by the waveform data service system is generally available. Subsequently,available seismic event waves are received at our server to be downloaded and storedautomatically within500seconds. Therefore, complete broad-band waveformsrecorded within the epicenter distance of400km can be inverted by multiple iterationsto achieve automatic inversion after the seismic waveform data pre-processing.Finally the relevant inversion solutions are automatically processed and published onthe World Wide Web as soon as possible.The region did not occur the earthquakes (M≥3.5) during the operation of thewaveform data service system since late2012. To simulate the application of theprocedure previously, we study Wen’an earthquake and Luan’xian earthquake whichoccurred within Capital Circle by using this constructed system. The results ofWen’an earthquake which occurred on July4th,2006showed fault solution of118/42/21(strike/dip/rake) for the event, and the focal mechanism of Luan’xianearthquake was inverted as follows: Strike=143°, dip=79°, slip=-18°. Those tworesults, comparable to existing related studies, indicate that the system has certainfeasibility. In order to test the whole system whether can run practically andcoherently or not, the Lu’shan main earthquake and several aftershocks withmagnitude above4.0had been done by using this near real-time automatic system.Based on existing related works about this region, we computed the temporaryparameters and got the mainshock’s focal mechanism as follows: Strike=14°, dip=37°, slip=71°, which shows that the rupture is dominated by left-lateral reverse faultand such solution is close to USGS, gCMT and Institute of Geophysics, ChinaEarthquake Administration and so on. Futhermore, the focal mechanism results ofLu’shan aftershocks are similar to mainshock though the fitting waveforms are not tooideal, which needs further works in the future. Nowadays, the near real-time automatic system has been implemented to invertthe focal mechanism rapidly and automatically. However, the attempts at automaticdetermination of focal mechanisms have not been very successful in terms ofreliability, stability, accuracy and efficiency. |
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