| Earthquake swarms are a series of earthquakes clustered in time and space,without obvious main shock.Earthquake swarms usually occur in the area with high activity of crustal fluids,such as geothermal fields,volcanoes and subduction plates.Which is one of the manifestations of plate motion and regional stress field adjustment and is also important basic data for regional seismic activity and seismic hazard assessment.The research of earthquake swarm mechanism is of great scientific significance for understanding the evolution of the earth's interior medium,the process of earthquake preparation and occurrence,and the interaction between fluid and earthquake.In recent years,two significant swarms have erupted in Shandong Peninsula,the Rushan earthquake swarm started in October 2013 and the Chang Island earthquake swarm started in February 2017.Chang Island swarm has almost ended and Rushan swarm is still active.Seismic activity in Shandong Peninsula is relatively low.There have been averagely only 2.3 earthquakes with M_L(>3.0)per year since 1970.However,in the Chang Island and Rushan swarms more than70 earthquakes with M_L(>3.0)occurred during the period of 2013-2017.There are few large earthquakes occurred in Shandong Peninsula the history,but small-scale swarms often occur.These swarms usually last for a short period of time,for several days,not more than one month.However,the Chang Island earthquake swarm lasts for nearly one year,especially the Rushan earthquake swarm has lasted for more than five years,and its frequency and intensity are relatively large,which is very rare in eastern china.The Chang Island and Rushan swarms occur in the sea and land respectively.Deep study of these swarms is not only conducive to clarity the seismicity of Shandong Peninsula,but also conducive to understand the mechanism of swarms.The following studies have been carried out in this paper:1.We use the matched filter technique to detect the Chang Island earthquake swarms from February 9 to August 20,2017,and 15286 earthquakes were finally detected.The detected events are more than six times the number in catalogue of the Shandong seismic network,and the complete magnitude decreased from 1.0 to 0.5.Based on the detected catalogue,we use ETAS model to quantitatively detect the seismic activity in different periods of swarm activity,and discuss the effects of fluid action and self-excitation on swarm development.The proportion of earthquake triggered by fluid ranges from 16.9%to 63.5%,and the initial stage is 31.9%,that shows a trend of first rising and then declining.This trend may be related to the fluid permeation process.The result shows that fluid plays an important role in the initial stage of swarm,while the proportion of earthquake triggered by fluid decreases significantly in the later stage of swarm,so seismic activity is dominated by self-excitation.The b value of swarm is between 0.85 and 1.3,which is higher than the background b value.Since the b value is usually inversely proportional to the stress,the stress level of the Chang Island swarm is low.It is possible that the fault rupture strength decreases due to the intervention of fluid,and the clustering earthquakes can occur under low stress.2.We use the double difference location method based on waveform cross correlation to study the spatial distribution and epicenter migration characteristics of Chang Island swarm.The swarms are NWW-oriented and show obvious spatial migration characteristics.The swarm initiated in the NE-oriented central region,where at the end near the Penglai-Weihai fault.Then the swarm migrates from northeast to southwest,and then earthquakes occur successively on four parallel faults along the NW direction.The wave front of swarm migration conforms to the fluid diffusion migration model,and the fitted diffusion coefficient D=1 m~2/s,within the typical diffusion coefficient range(0.1-10 m~2/s).3.We use generalized superposition inversion method to estimate the stress drop of the Chang Island swarm.The result show that the stress level of the swarm is lower than the background,and the stress drop varies widely,positively correlate with the seismic moment,deviates from the self-similarity theory,and the spatial distribution is not uniform.These characteristics are similar to the stress drop characteristics of earthquake swarms in fluid active areas.4.Using the same methods and ideas,we study the Rushan earthquake swarm between May2014 and June 2015.More than 7500 earthquake events are detected,which are nearly four times the catalogue of Shandong seismic network,with the complete magnitude decreasing from 0.9 to0.1.The proportion of earthquake triggered by fluid is between 24.3%and 45.9%,and also shows a trend of first rising and then declining;the b value is between 0.75 and 0.95,which is also higher than the background b value;the swarms are NWW-oriented and show a certain spatial migration.However,the migration is not obvious.The swarm show the fluid diffusion characteristics only in the early stage of the study period,and the fitted diffusion coefficients are between 0.01 and 0.03m2/s;the stress level is also lower than the regional background value.The apparent stress varies largely,large difference in spatial distribution and positive correlation with seismic moment.5.After comprehensive analysis,we suppose that earthquakes in the initial stage of the Chang Island swarm are triggered by fluid,while earthquakes in the later stage of the swarm are the result of the diffusion of fluid along permeable faults and the stress adjustment caused by preceding earthquakes.However,the stress adjustment plays a more important role during the study period of Rushan earthquake swarm,but fluid diffusion runs through the process of swarm,which reduces the strength of fault rupture and also plays a certain role. |
PDF Full Text Request