Studies On Rupture Dynamic Phase Diagram And Source Model Of Slow Earthquakes

Both low-frequency earthquakes and very low-frequency earthquakes are considered slow seismic events different from regular earthquakes.They mainly occur in subduction zones and are deemed local accelerated deep slip events induced by underlying slow slip.As the smallest seismic events of the slow earthquake family,low-frequency earthquakes are a series of small repeating earthquakes that are assumed to compose tectonic tremors.Compared with regular earthquakes of the same magnitude,low-frequency earthquakes have longer source durations and smaller stress drops and slip rates.On the other hand,very low-frequency earthquakes have lower dominant frequencies and longer source durations than low-frequency earthquakes.Very low-frequency earthquakes,which exhibit longer source durations and smaller stress drops than regular earthquakes of similar magnitude,possess abundant low-frequency energy but are depleted in high-frequency energy.It is because low-frequency earthquakes and very low-frequency earthquakes have similar characteristics of lower radiation energy efficiency and longer source duration than regular earthquakes,they are classified as slow earthquake events.Although they are slow earthquakes,both of them have similar focal mechanisms to regular earthquakes,and both radiate identifiable seismic waves,both caused by shear slip.It indicates that there is a certain relationship between the hypocenters of slow earthquakes and ordinary earthquakes.However,the mechanism that drives their unusual type of stress accumulation and release processes is still unknown.In this study,the full-space dynamic rupture phase diagram was recalculated.The phase diagram shows that there are four different types of rupture:super-shear rupture,sub-Rayleigh rupture,self-arresting rupture,and slow self-arresting rupture.Slow self-stop rupture shows special source characteristics.Different from other rupture types,slow self-arresting rupture arrests spontaneously in the nucleation patch with a much longer source duration and its stress drop and slip are smaller than those of regular earthquakes of the same magnitude.The characteristics of its source parameters are very similar to those of slow earthquakes.In order to compare the source characteristics of slow self-arresting ruptures with those observed slow earthquakes,we estimated the seismic moment and stress drop range of deep low-frequency earthquakes near the San Andreas fault from 2002 to 2016,as well as the range of shallow low-frequency earthquakes in Shikoku,Japan.The results show that the average moment magnitude of the deep and shallow low-frequency earthquakes is 1,and the range of the stress drop is 103～105 Pa.This result is close to the source parameters of low-frequency earthquakes obtained by previous works.It shows that low-frequency earthquakes in different deep regions have similar source characteristics.We also discussed different methods for estimating low-frequency earthquakes source parameters and the influence of parameter selection on the estimation results.In order to obtain accurate source parameters,we used the empirical Green’s function method to estimate the source parameters of low-frequency earthquakes in this region again,and the results are similar.It shows that the average stress drop of low-frequency earthquakes is 2-3 orders of magnitude smaller than that of regular earthquakes of the same magnitude.Based on the estimated rupture area of low-frequency earthquakes,we designed a series of numerical simulation rupture experiments under the same condition and calculated the simulated rupture source parameters.We compare the source parameters obtained from the simulations with the source parameters of observed low-frequency earthquakes and very low-frequency earthquakes.By comparison,we find that slow self-arresting rupture can explain the source parameter characteristics of low-frequency and very low-frequency earthquakes.Therefore,we conclude that both low-frequency earthquakes and very low-frequency earthquakes are earthquakes that self-arrest within the nucleation patch without any introduced interference.Our result shows that low-frequency earthquakes,very low-frequency earthquakes and regular earthquakes can be explained under the same source dynamics framework.Low-frequency earthquakes,very low-frequency earthquakes and regular earthquakes are different rupture types under different slip-weakening conditions,and the friction characteristics on the fault determine whether the slip mode is slow or fast.We also explain the scaling property of low-frequency earthquakes and very low-frequency earthquakes and explore the interpretation of our model for multiple low-frequency earthquakes characteristics.Our findings suggest a framework for fault deformation in which nucleation asperities can release stress through slow self-arrest processes and the underlying mechanism connects with the origin of regular seismic events.