Methods For Precise Determination Of Rupture Directivity Of Small Earthquakes And Source Parameters Of Moderate Earthquakes And Their Applications

Many factors can cause the earth to quake,such as a tectonic earthquake,a volcanic eruption,an induced earthquake,a mining collapse,a nuclear explosion,etc.Study on the rupture directivity and focal mechanism of seismic source is the key point of seismology and is also an important basis for assessing the earthquake disaster and post-earthquake rescue,having high scientific research value and social significance.The rupture directivity of seismic source can be used as an important basis for disaster assessment,geological analysis,nuclear test monitoring and earthquake early warning.Firstly,as an important characteristic of seismic source property,rupture directivity could significantly enhance the ground motion in the direction of rupture propagation,resulting in strong damage.Study on rupture directivity of the seismic event can provide key information for disaster assessment,disaster prevention and mitigation.Secondly,studying the rupture directivity of natural earthquakes can help to characterize the distribution of geological faults,providing support for the study of geological structure.Thirdly,different from the tectonic earthquakes,nuclear tests are explosive and non-directional.Analyzing the directivity of the target event in the nuclear test area can help to identify the nuclear test in the nuclear test area and provide key information for monitoring the nuclear test.Fourthly,a large earthquake and its neighboring small earthquakes may have similar waveforms and rupture directions.The determination of the rupture directions of small earthquaks can help us better understand the physical mechanism of the large earthquake and provide key information for the earthquake early warning.However,it is difficult to study the rupture directivity of small earthquakes(ML<3.3)because of their small rupture dimensions and the complicated path effect from the source to stations.Accurate determination of source parameters can help to identify type of the moderate seismic event.With the constraint of waveform information recorded by the seismic station,the source parameters of the target event can be accurately determined.Published studies have shown that sources of many special seismic events and the natural earthquakes rupturing on multiple faults exhibit significantly higher nondouble-couple(non-DC)components than that of the natural earthquake rupturing along a single fault.The accurate source parameters of the target event,especially the non-DC components,can suggest the physical mechanism of the target event,providing seismological evidence for distinguishing dangerous artificial events(e.g.,nuclear tests.explosions)or natural disasters(e.g.,natural earthquakes,volcanic eruptions).Thus,it has high application value of national defense and people’s livelihood guarantee.However,due to the multiplicity of source solutions in source inversion based on fitting of waveforms only,it is still a very challenging task to determine the non-DC components of a seismic source accurately.In this paper,we approach the two problems theoretically and observationally.In the theoretical front,we have developed the following two methods:(1)Relative Directivity Inversion(RDI)method which is applicable to small seismic events and(2)a waveform-polarity moment-tensor(WPMT)method which is developed to study the source components of moderate seismic events based on the polarities of first-arriving P waves and long period waveforms.Theoretically,the RDI method can resolve the rupture direction and length of the seismic event with any magnitude,and thus characterize rupture process of the target event.The greatest advantage of the RDI method is its applicability to small seismic events(ML<3.3).The WPMT method is able to identify events that are more complex than a point source and resolve the source solution for a point source with true non-DC components.Besides,the simulated annealing algorithm used in the WPMT inversion takes at least 2 order of magnitude of lesser time to obtain the correct solution in comparison to the grid search method.affording a tectonic potential for its real-time application.In the observational front,we have made the following four research achievements.(1)Validate the applicability of the RDI method.We apply the RDI method to determine the rupture directivity of a ML 3.2 earthquake in Japan occurred on April 11,2010,and resolve its rupture direction and rupture length,thus validating the applicability of the RDI method.(2)Validate the applicability of the WPMT method.We apply the WPMT method to study the focal mechanisms of a Mw 5.6 northern Osaka earthquake occurring on northern Osaka Plain,Japan on June 17,a Mw 5.4 California earthquake occurring in Central California on July 5,2019 and the Mw 5.5 Iceland event occurring in Iceland on September 21,2014.The WPMT method successfully identified the rupture complexity of the Osaka earthquake and resolved the double couple source of the California earthquake.The conclusions are consistent with the published studies.The WPMT method reveals a 35%term of isotropic component and a 65%term of a tilted compensated-linear-vector-dipole component in source of the Iceland event.The WPMT confirmed the exixtence of the isotropic component in source of the Iceland event,which has been neglected by the previous studies.The three data applications provide further support for the validity and applicability of the WPMT method.(3)Employing the WPMT method,we study the focal mechanism of the largest event(Mw 5.6)occurring on June 1,2012 in west Antarctica.The inferred source of the Antarctic event consists of 38%double couple,24%isotropic expansion and 38%compensatedlinear-vector-dipole with a vertical tension axis,located at a depth of 12 km.In addition,a dominant frequency of 0.34 Hz is observed in the seismic energy of the event.Based on the anomalous source solution and spectra,we propose that the Antarctic event is related to magma activities in the region.(4)Employing the RDI method and the WPMT method,we perform detailed source analysis of the 2019 Changning Ms 6.0 earthquake sequence based on both seismic data and interferometric synthetic aperture radar(INSAR)observation.The inverted results indicate that the Changning Ms 6.0 sequence has very complicated rupture directivities and rupture ways,reflecting the complex fault system in Changning salt mining area.Besides,events in this sequence have complicated focal mechanisms and shallow source depths consistent with the drilling depth of salt mining,which suggests that its occurrence is closely related to the local salt mining activities.