Source Parameters Study Of Small To Moderate Earthquakes

Earthquake prediction, one of the most important fields in seismology, depends on the information on stress state, fault feature and rheology in the crust. Source parameters including depth, focal mechanisms, rapture directivity and rupture velocity provide abundent such information. Focal mechanism reflects regional stress state and focal depth shows rheology feature of medium, while rupture directivity can be used to determine the fault on which the earthquake occurs. Large numbers of small and moderate earthquakes (M2-M5) distribute widely, which make it possible to explore the crust physical state. Moreover, most aftershocks of strong earthquake are small and moderate events, which provide important information on focal mechanism of mainshock.For macroseism, the mature long period waveforms inversion methods are applied to determine source parameters. For small and moderate events, energy of P waves concentrate at short period, so Signal Noise Ratio is high enough only at this band. But the amplitudes of observed P waves are quite different from those of synthetics at this band, thus it can not be used to invert source parameters directly. With determined source parameters, short period (0.5-2Hz) P waves of the these events are compared with synthetic ones to handle amplitude variability with Amplitude Amplification Factor (AAF) correction, which is then used in modeling source mechanism of the small and moderate earthquakes. On the other hand, interfaces in the crust may affect body wave amplitude, and hence interfaces such as Moho discontinuity and basin basement were also investigated in this study.Receiver functions were calculated with teleseismic P waveforms recorded at broadband and short period stations of the Capital Digital Seismic Network. H-Kappa method was adopted to obtain the thickness and Poisson ratio of the crust. The results show that the crust thickeness gradually thickens from SE to NW with an average thickness of 34 km. The crustal Poisson ratios are sensitive to geological structure and high ratios are considered to be related to active faults. S wave velocity in the top 100m of the crust is lower than 300 m/s, and it reaches 800 m/s as depth increases to 500 m with an average gradient of 0.8 m/s which are obtained from differential travel time between direct S wave and reflected wave at the free surface with borehole station records. To demonstrate the effect of velocity structure on amplitude of high frequency waves, the relationship between velocity models and SmS amplitude was studied, and the effect of SmS amplitude over the critical distance on the strong ground motion was investigated. The foreshock and aftershocks of the 21 August 2008 Yingjiang earthquake (Mw5.9) were used in this study. For the data recorded at Wangding station which is 100 km far away from epicenter, large SmS phases were observed with amplitudes of 2 to 5 times greater than the direct S wave. Comparisions with the synthetic seismograms indicate that such large-amplitude SmS phases were produced by simple crustal structure of Wangding region which allows a large Moho reflection. This suggests that when damaging earthquakes occur in regions of simple crustal structures, Moho reflections will produce amplified strong motions at distance around 100km depending on the local structure.Focal depth is one of the most important source parameters, which can be well determined by depth phases of P-SV series such as sPL, sPg, sPmP, sPn, greatly improving the resolution of depth determination. But so far, the depth phases of SH series have been rarely used to determine focal depth. In fact, SmS and sSmS phases could be easily indentified due to there are no P-SV coupling with relatively simple waveforms on tangential componet. To test the feasibility of determing focal depth with SmS and sSmS phases, we calculated synthetic seimograms on a set of 1D crustal velocity models, finding out that SmS and sSmS are obvious phases only in high frequency(>0.2Hz), simple crustal velocity model, and deep focal depth(>10km). Therefore, we drawed the results that the depth phases of P-SV series are preferential, while those of SH series are supplementary.Since depth phases of SH series is feasible only in regions with simple velocity structures and the geological structures near Longmen Mountain faults are very complicated that the crustal structures of eastern and western sides near LongMengShan faults are very different, we determined focal depths of over 300 small to moderate afthershocks of the Wenchuan earthquake using depth phases and CAP inversion at long period which employed amplitude ratios between surface waves and S wave. The results show the epicenter mainly locates at 5-15km deep and the largest depth is around 21km, while the shallowest one is only 1km. A Ms 5.7 aftershock occurred at Qingchuan, northeast end of the LongMengShan fault on Jul 24, 2008, featuring thrust mechanism with a 3km source centroid depth. The shallow focal depth is confirmed using the sPL phase recorded by a station at an epicentral distance of 15km. The observed large amplitude of Rg at a distance of 15km implies a depth of 3km or less. Dozens of aftershocks' sPL waveforms are also analyzed and less than 3km depths were obtained from it. On the other hand, both geological observations and InSAR studies report there is no surface breaks in this region. It is strongly suggested that this aftershock sequence initiates a fresh rupture in intact rocks which were triggered by stress increment raisen from the deep co-seismic rupture of the Wenchuan mainshock.Shallow earthquakes with potential of great earthquake disaster are very dangerous. The depth phases are very close to Pg in arrival time because of shallow depth, which makes it difficult to identify them. However, generally the shallow earthquakes are characterized by developmental surface waves. Waveform modeling of short period surface waves and teleseismic body waves is performed to study 2010 Suining-Tongnan M5.0 earthquake. The source depth of 1-3km indicates that this event occur in the over six kilometer's sediment layer. In general, only basement rocks are strong enough to accumulate strain energy for destructive earthquake. While sediment layers are porous and full of fluids, thus they are too soft to accumulate enough strain energy. This event provides an unusual but typical case for seismogenic mechanism.Besides source depth, focal mechanism, rupture directivity, rupture length and rupture speed are also important parameters for describing source. Short period body waves calibrated with station amplitude amplification factor (AAF) inversion approach is applied to study the 2009 Inglewood M4.7 earthquake sequence in California. Using this approach, we have determined 14 focal mechanism solutions of small events with ML down to 2.0. Then records of small events with well determined source mechanism was taken as Empirical Green's Function (EGF), and the source parameters including rupture directivity, rupture length and rupture speed of two largest events (M>3.5) obtained with such EGF. The results show a source depth of 8km and fault solution of 145/77/139(strike/dip/rake) for mainshock. One of the fault planes has similar strike to that of Newport-Inglewood fault. The rupture propagation directions of the two largest events are N154 and N143 respectively, coinciding with strike of Newport-Inglewood fault. The high rupture velocity may be related to the fact that the Newport-Inglewood fault is a mature fault. To confirm the results from P wave inversion, we first used the rupture velocity and rupture scale to obtain duration time of SH wave and then calculate SH synthetics. Comparison between synthetics and observed SH waveforms demonstrated validity of results inferred from P waves.We try to provide a frame of systematically studying source parameters of small to moderate earthquakes. The amplitudes and arrival times of main phases could be corrected on the base of the well understood fine-scale velocity model, which is required by studying source parameters of small and moderate earthquakes with good resolution.