| China is an earthquake-prone country with high occurrence of earthquakes.According to statistics,30%of the mainland earthquakes in the word occurred on mainland China and its adjacent regions.Many cities in our country are threatened by earthquake disasters.Therefore,analysis of the pre-seismic deformation and anomaly are of great significance to the Earthquake prevention and disaster reduction in China.In essence,the development and occurrences of earthquakes are the process of the stress accumulation and release.Based on this point,studying the changing of crustal stress and strain can be one of the most important methods to analyze seismic hazard.However,because of the limitation of observed technology,it is hardly to obtain crustal stress and strain in earlier period.During the last two decades,with the development of space geodesy technology,especially the GPS observed technology;people get a new method to obtain stress and strain information.Much more researches have studied the preseismic,coseismic and postseismic deformation with GPS observations.Because of the scarcity of continuous stations,few researches showed anamoly from preseismic deformation,which is very important to understand seismogenic process and earthquake forecasting.Therefore,base on previous works,new data and properate area,this study focuse on the characteristic of regional strain rate and anomaly of preseismic deformation.This can not only help people understand seismogenic process and occurrence mechanism of earthquakes but also provide theoretical basis for earthquake monitoring,prediction and early warning.Based on the consideration above,for the mid-long term seismic hazard studying,strain rate field around the north-south seismic zone is studied with GPS observations and locations of earthquakes?Mw?6.0?.With continuous observations before and after the Lushan earthquake,viscosities of southern segment of Longmenshan Thrust Belt?LTB?are studied and postseismic deformation of the Wenchuan earthquake are excluded from observations before the Lushan earthquake.Finally,anomalous deformation before the Lushan earthquake is analyzed.Main contents of this paper are list as follow:1.Strain Characteristics of North-South Seismic ZoneAround the North-south Seismic Zone in China,strain rates fields of the year 1999-2007,2009-2013 and 2013-2016 are calculated and analyzed with GPS velocity field and the least square collocation method.With epicenters of earthquakes?Mw?6.0?that occurred from the year 2008 to 2018,the relationship between strain rate and epicenters is summarized.Finally,with geological and seismological results,the seismic hazard in the studied area is analyze.The results are as follows:?1?Around the Bayan Har Block,strain rates changed strongly from 2009 to 2016,which shows the weakening of maximum shear strain rates around Xianshuihe fault zone and the strengthening of the East Kunlun fault zone;From 2009 to 2013,the maximum value of the maximum shear strain moved southward to the sounthern segment of LTB.The maximum shear strain rate around the western segment of the Xianshuihe fault zone that is about 2.0×10-8/a,decreased significantly.nn the contrary,the value of the maximum shear strain rate around the eastern segment of Kunlun fault zone increased from 1.5×10-8/a to 2.0×10-8/a.These shows the Wenchuan earthquake strongly influenced this two area.After the Wenchuan earthquake,the strain rate around the southern segment of the LTB kept high value.That indicate faults around this area were still locked.After the Lushan earthquake,the value of strain rate around this area was still higher.This may indicate the Lushan earthquake did not unlock the sounthern segment of the LTB completely.?2?In the studied area,strike-slip earthquakes?Mw?6.0?all occurred on the area with higher value of the maximum shear strain rate or occurred on the boundry of higher and lower value.The direction of principal strain rate deviate from the direction of long-term stress before the Wenchuan earthquake.This may indicate anomaly of deformation before large earthquakes.Strike-slip earthquakes?Mw?6.0?all occurred on the area with higher value of the first or second shear strain rate.Seismic hazard zones can be recognized as eastern part of Qilian Mountain,Maqin-Maqu segment of east Kunlun fault,Anning-Xiaojiang fault zone and Mid-Southern segment of Honghe fault zone.The elapse rates of 4 areas above have reached or exceeded 1.0.In addition,the characteristics of strain rates around these 4 areas show high seismic harzard.?3?The strain accumulation rates in Qilian block gradually increase.Area that surface strain rates are larger than 2.0×10-8/a extends gradually.In addition,around the Zemuhe fault,the surface strain rates also increase during the studied period.These two phenomenons may indicate that the Tibetan Plateau has accelerated its extension.The strain rates distribution around the northeastern margin of the Tibetan Plateau show obvious inhomogeneity.It indicate inhomogeneity of medium properties within the area,which is consistent with seismological researches.2.Viscoelastic structure of southern segment of LTBDistance between epicenters of the 2008 Wenchuan earthquake and 2013 Lushan earthquake is so small that observations recorded before the Lushan earthquake should include influences from the Wenchuan earthquake.Therefore,viscosities of the southern segment of the LTB should be obtained to further estimate the postseismic deformation of the Wenchuan earthquake.After these steps,anomalous deformation before the Lushan earthquake can be analyzed better.In this paper,the viscoelastic structure of the mid-lower crust is studied with GPS observations after the Lushan earthquake from 7 continous GPS stations.The results are as follows:?1?After many tests and data experiments,exponential fitting is the best method to extract the postseismic deformation of the Lushan earthquake from observations.Error range is[-0.49,0.49]?mm?.By fitting observed data,the secular motion and postseismic deformation of the Wenchuan earthquake can be excluded.During the fitting,characteristic time of the Wenchuan and Lushan earthquakes are 1.5 and 0.4years,respectively.?2?Displacements time series that observed from LS06 station showed obvious rebound phenomenon after the Lushan earthquake within the first 123 days.The epicenter distance of LS06 is about 17.6 km,which is nearer than other stations without obvious rebound phenomenon.The postseismic rebound phenomenon observed near the Lushan epicenter may represent an over-slip mechanism during the coseismic movement,which was also observed after the Wenchuan earthquake.?3?The crustal viscosities are inversed by the grid research method,Maxwell body and a layerd structure with a low velocity zone.Based on our results,the viscosities of the low velocity zone and the middle and lower crust in the southern Longmenshan region are 2.75×1017(6?and 2.56×1018(6?,respectively.The 95%and 99%confidence interval of viscosities are estimated by T test.Based on the T test,the result also shows that the viscosity of upper mantle has few influences on postseismic deformation of the Lushan earthquake.Compared with previous works,the viscosities of different segments of LTB have the same order.That means the different fault activity is not caused by viscoelastic structure.With previous works,the more complex structural style may be one of the reason that produced the weaker fault activity around the southern segment.3.preseismic anomaly of the Lushan earthquake10 GPS stations that located around the epicenter of Lushan earthquake are investigated to figure out anomalous deformation before the earthquake.Because the Lushan earthquake is caused by the collision of Bayan Har block and the South China block.nbservations are first transformed into South China block reference frame to highlight the interaction of these two blocks.After filterting the common mode errors and extract the low frequency information,the preseismic anomaly of the Lushan earthquake is analyzed.The results are as follows:?1?Epicenter distances of LS05 and LS06 are nearer than other stations.These two stations showed obvious anomaly that velocity decayed before the Lushan earthquake.During the studied period,direction of observations are opposite to the postseismic deformation of the Wenchuan earthquake.It indicates that the decayed phenomena is not caused by the Wenchuan earthquake but the preseismic anomalous deformation of the Lushan earthquake.nbservations recorded from LS02 and LS07 also showed anomalous characteristics but less obvious than LS05 and LS06.A negative correlation between anomalous characteristics and epicenter distance is concluded.?2?Anomalous deformation before the Lushan earthquake may reflects the limitation of regional deformation.During seismogenic process,strain accumulate around the fault.When the accumulation of strain reaches its limited value,the strain can not accumulate any more.Thus,stations within near-field area shows few cumulative displacements after 2011.?3?The 2008 Wenchuan earthquake and the 2013 Lushan earthquake did not rupture the LTB completely.A rupture blank zone exits between two earthquakes.Among GPS stations in this study,LS02 and LS05 were both set adjacent to the LTB.LS02 is located on the rupture blank zone and LS05 is located near the epicenter of the Lushan earthquake.Before the Lushan earthquake,the anomalous characteristic of LS05 is more obvious than LS02.It indicate the accumulation of strain near the Lushan earthquake has reached its limitation compared with the rupture blank zone.To some extent,this may explain the reason why the Lushan earthquake did not occur on the rupture blank zone. |
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