Research On Source Papameters And Rupture Mechanism Of Small Earthquakes

Rupture phase diagrams indicate that there are two rupture types for small earthquakes,magnitude of which are usually less than about 6 Mw.One is self-arresting rupture with smooth seismic spectra and simple source time function(STF).Another is runaway rupture with complicated STF,and holes exist on seismic spectra at specific frequency.In this paper,we calculate seismic spectra and source time function of small earthquakes,and estimate corner frequency and stress drop to understand rupture styles and rupture mechanism of small earthquakes.Empirical Green function(EGF)method is used to extract seismic spectra and STF from P waveform of multi-component of multiple stations.The Boatwright source model and Brune source model are used to estimate corner frequency and stress drop.There are a lot of factors to select a good EGFs including waveform correlation coefficient,EGFs earthquake magnitude,signal-to-noise ratio and fitting stability of seismic spectra.Waveform correlation coefficient,representing similarity of the source location and focal mechanism between target earthquakes and EGF earthquakes,is a significant factor.We apply multitaper spectral analysis method to obtain stable seismic spectra and Nelder-Meade method to fifing seismic spectra.We investigate seismic spectra and source parameters of the medium-deep earthquakes(125-165 km)with 3-5.7 Ml in Bucaramanga nest and the shallow earthquakes(less than 20 km)with 3-6.7 Mw in Fukushima-Ibaraki.In Bucaramanga nest,we totally obtain seismic spectra and STF of 714 earthquakes,516 of which are self-arresting rupture and 198 of which are runaway rupture.In Fukushima-Ibaraki,we totally obtain seismic spectra and STF of 575 earthquakes,283 of which are self-arresting rupture and 292 of which are runaway rupture.We find that there are more one holes exist on seismic spectra of a few runaway rupture earthquakes.Hole-frequency of seismic spectra calculated from different stations and same EGF earthquake are different.In Fukushima-Ibaraki,seismic spectra's amplitude of some earthquakes reduces at low frequency,which deviate normal source model.In this paper,only corner frequency and stress drop of self-arresting rupture earthquakes are estimated.In these two regions,corner frequency of same magnitude earthquakes vary a lot and the highest corner frequency decrease with magnitude.Stress drop increase with seismic moment and the range variation of earthquake's stress drop with same seismic moment is up to 2 order of magnitude.In Bucaramanga nest,stress drop of most of earthquakes are between tens and hundreds Mpa and increase slightly with depth,and only a few earthquakes' stress drop are over 1000 Mpa,spatial and temporal distribution of which has no obvious characteristics.In Fukushima-Ibaraki,stress drop of all earthquakes are between a few and hundreds Mpa and increase slightly with depth,spatial and temporal distribution of which has no obvious characteristics.Stress drop estimated by Boatwright source model are generally less than that by Brune source model.The best n value of Boatwright source model is 2.The best n value of Brune source model is over 2 and it is not obvious.