Waveform-based Earthquake Location And Rupture Process Imaging

Obtaining accurate seismic source information is the essential task for earthquake monitoring,and it is also the foundation of earthquake physics,source dynamics,fault zone structure and its evolution.At the same time,it is of great significance for earthquake prediction,earthquake disaster assessment and mitigation.Therefore,recently many researches have been carried out on quickly and accurately determining the earthquake location,rupture directivity and fault plane parameters;imaging the rupture process and analysing the control of the fault plane structure on the source location and rupture behavior,which are also the main research focuses of this thesis.Seismic location is a classic problem in seismology.In this paper,the traditional imaging condition of the waveform migration location method is changed to the hybrid multiplicative imaging condition for a higher-resolution seismic location method.Compared with the arrival-based location methods,the waveform-based location methods do not require picking phase arrivals,and are more suitable for locating seismic events with low signal-to-noise ratio.Among the waveform-based location methods,an approach is to stack different attributes of P and S waveforms around arrival times corresponding to potential event locations and origin times,and the maximum stacking values are assumed to indicate the correct event location and origin time.In this study,in order to obtain high-resolution location results,we improve the waveform-based location method by applying the hybrid multiplicative imaging condition to the seismic waveform characteristic functions.In our new stacking method,stations are divided into groups;characteristic functions of seismic waveforms recorded at stations in the same group are summed,and then multiplied among groups.We found that this imaging condition can largely eliminate the cumulative effect of noise in summation process,thereby improving the resolution of the location imaging.We use both synthetic and real datasets that are related to induced seismicity occurring in petroleum/gas production to test the reliability of the new method.The test results show that the new hybrid multiplicative imaging condition can obtain higher resolution location images than the other three conventional imaging conditions.In addition to locating small and medium induced earthquakes,we also improved this method and applied it to locate moderate and strong earthquakes on the Gofar transform fault by combining P-wave first arrivals and S waveforms.For the first time,the focal depth of the 2008 Gofar earthquake is determined,providing the basis for studying its rupture process.An earthquake is not essentially a point source,but a process involving sequential rupturing of several asperities on the fault planes.In this study,the waveform-based location method mentioned above has been improved,and the near-field waveform data are used to quickly and accurately identify the earthquake rupture plane and the rupture directivity.Through the time window sliding,the subevents can be identified from the complex waveforms and located by using the above location method.Furthermore,the spatial positions with radiated seismic energy in different time periods can be obtained,and thus the rupture plane and rupture direction can be determined.We have applied this method to the 2004 Parkfield M6 earthquake in California,United States of America,and its rupture plane was clearly imaged and its rupture directivity from southeast to northwest was determined,which is consistent with previous research results.Since this method can identify the rupture plane only using seismic waveform data and does not need to calculate the Green’s function,the computational cost is low and it is very suitable for near real-time source imaging,which is of great significance for real-time monitoring of earthquake activities and rescue after a large earthquake.The conventional finite fault inversion method takes the seismic point source solution and the rupture fault parameters as prior conditions,and invert the coseismic slip distributions in time and space on the fault plane through waveform fitting,which can be used to better describe the earthquake rupturing process.We have used strong motion data and interferometric synthetic aperture radar(InSAR)data to jointly invert the rupture processes of the ML5.7.Xingwen earthquake on December 16,2018 and the ML5.3 Gongxian earthquake on January 3,2019 in Sichuan,which are two large-scale and highly destructive earthquakes in the Changning shale gas block,which caused certain casualties and economic losses to the surrounding areas.Previous studies have shown that these two events were induced by local hydraulic fracturing activities.Therefore,it is very important to determine the rupture process of such relatively large induced earthquakes.The results show that the Xingwen earthquake,with a magnitude of Mw 5.5,is a unilateral rupture near the north.The rupture is segmented,and there are two main rupture patches along the strike.One is the first stage of rupture(1-3s)at 0-5km near the source,and the other is at 6-8km far from the source which is the second stage of rupture(3-5s).The slip is mainly concentrated in the shallow area above 5 km,and the peak slip value is 0.27 m.The Gongxian earthquake,with a magnitude of Mw 5.1,was also a unilateral rupture near the north.The duration of the source rupture process is 8 s,and the energy release is mainly concentrated in the first 5 s.The main slip area is distributed near the hypocenter,which ruptures during 1-4 s,the rupture length is about 5 km,and the peak slip value is about 0.036 m.We take the 2008 Gofar Mw 6 earthquake as an example to study the control of the fault plane structure on the seismic location and rupture behavior.The Gofar transform fault located on east Pacific Rise can quasi-periodically generate Mw 5.5-6 earthquakes on fault segments straddled by rupture barriers.However,the distribution of earthquake rupture and barrier zones along the fault is mainly inferred from the distribution of foreshocks and aftershocks.In addition,the accurate location of the 2008 Mw 6.0 Gofar earthquake is not available.This makes it challenging to understand what factors control the quasiperiodic generation of Mw 5.5-6 earthquakes and the detailed rupture segmentation pattern.In this study,by using the near-field strong-motion data and teleseismic data of 2008 Mw 6.0 Gofar earthquake,for the first time we have determined its accurate location by a new location method combining P-wave first arrivals and S waveforms and its coseismic slip distribution by the finite-fault model inversion.By using high-resolution Vp/Vs variations,we find that physical property variations along the fault plane are spatially correlated well with the coseismic slip distribution of 2008 Mw 6.0 earthquake.In addition,the 2008 earthquake is located around the edge of a main slip patch with low Vp/Vs anomalies.This indicates the structure has control of the generation of 2008 Mw 6.0 earthquake and its coseismic slip distribution.In general,this study firstly improved the waveform migration location method by using a hybrid multiplicative imaging condition,and successfully applied it to seismic location and seismic rupture plane and directivity imaging.Then,the finite fault inversion was used to determine the coseismic slip distributions of two large induced earthqukes due to shale gas development.Finally,combined with the high-precision velocity structure,the control of the fault plane structure on the hypocenter location and rupture behavior was resolved.