Quasi-automatic Waveform Inversion Of Focal Mechanisms Of Moderate And Small Earthquakes On High-performance Cluster Computing System And Its Applications

The earthquake disaster is directly related to the source parameters of the earthquake,especially its focal mechanism and depth,and its moment magnitude. The focal mechanism solution can reflect the geometry and kinematics features of the earthquake rupture,which is very significant to analyze the activity of aftershocks and to understand the stress status of hypocentral area and seismogenic structure.The seismic networks routinely determine the locations and magnitudes of earthquakes immediately after they occur. However, many networks are still unable to provide the earthquake mechanisms which are very important to the analysis of the seismogenic environment and potential earthquake hazard in the near future.At present, the focal mechanisms of earthquakes are mostly determined by the first-motions and S/P amplitude ratios of seismic waves from good station coverage. When an earthquake happens at the edge or outside of a seismic network, its focal mechanism determination is often unacceptable.Nevertheless, the seismic waveforms(P, S and surface waves)can provide much better constraints on the earthquake source parameters.In recent years, methods and algorithms have been widely developed to use waveforms to invert the focal mechanisms of moderate and small earthquakes,and some have been incorporated into the routine operating networks in USA,Japan,some European countries and Chinese Taipei for earthquakes with magnitudes3.0-5.0.However, most of seismic networks in China are unable to automatically determine the focal mechanisms of moderate and small earthquakes.In this paper, the author will develop an automatic system to quickly and reliably determine the earthquake mechanism based on the Cut-And-Paste (CAP) waveform inversion method (Zhu and Helmberger1996),and test this system with the focal mechanism inversions of small earthquakes and deep earthquakes to check its applicability.The quasi-automated moment tensor inversion system based on CAP method had been built in a high-performance cluster computing system(HPCCS) with the capability of given reference epicentral distances or without distance calibration. Furthermore,the automated script program named run.cap.auto.mpi had been set in the HPCCS to work with different inversion parameters (as frequency ranges of waveform filtering) for source parameters determinations with various magnitude ranges of earthquakes. Finally, a graphical user interface of the CAP inversion has been deployed on the HPCCS and SUSE Unix system. It allows a user with very limited amount of training to near-real-time determine the focal mechanism of an earthquake.Frequency-Wavenumber(F-K) package suitable for Windows and Unix/Linux environments had been compiled to compute Green’s function libraries for specific velocity model of study region. For the Green’s function computation using1-D velocity model, all computing jobs can be created automatically for each depth of the study region and be submitted to different computing nodes to obtain all of Green’ function libraries of all depths on HPCCS as soon as possible.The built HPCC CAP inversion system was used for the2009earthquake swarm occurred in the seismic network of Xiaolangdi Reservoir (XRSN in short) of Yellow river. The ML magnitude of this swarm ranges from1.0to2.6. The inverted focal mechanisms of this swarm show that all of24events, there were15events (62.5%) with strike-slip mechanisms, and seldom normal and thrust type earthquakes with only12.5%and25%respectively. The dominated shear mechanisms of this swarm occurred in the hanging wall of the middle segment of Shijinghe fault are in accordance with regional tectonics. Our results indicate that CAP method is suitable to inverse the focal mechanisms of small earthquakes with high signal-to-noise ratio waveforms from dense network.Then the CAP method was first time tried to invert source parameters of2deep earthquakes occurred in Northeast China in2010and2011respectively. The results with well fitted synthetic and observed waveforms indicate that this method is also suitable to invert the mechanism of deep earthquake. Our result of the2010event is a good agreement with the results from USGS and gCMT. Our low-angle thrust result for the2011deep earthquake reveals the activity of dynamic response of the subducting Pacific plate, and is associated with the tectonic stress adjustments of great coseismic displacements and postseismic slip induced by the2011Mw9.0Tohoku-oki earthquake in Japan.Furthermore, the Linear Stress Inversion with Bootstrapping resampling (LSIB) method had been used to invert for tectonic stress field from23historical focal mechanisms within study region. Our results show that the direction of maximum principal stress is NWW (the trend of272.6°) with a near horizontal axis (the plunge of26.4°)while the orientation of minimum principal stress SEE with an axis plunge of62.4°.The near EW direction of stress field is highly caused by the subducting Pacific plate beneath the European-Asian plate from Japan trench to Northeast China.