| The Haicheng earthquake(Ms 7.3,122.8°E,40.7°N,12 km)which occurred at 19:36 local time on 4 February 1975,is the strongest recorded earthquake in Liaoning(39°-43°N,120°-126°E)China,with the epicenter located near the boundary between the counties of Yingkou and Haicheng.The earthquake caused extensive ground failure.The Liaoning area shows an uplift in both the western and eastern parts,and a depression in the central part.The uplift and depression continue in the Quaternary forming multistage tectonism with lots of faults.Considering the well-developed transport system and large population of this region,it is crucial to investigate the crustal structure and physical properties for hazard mitigation purposes.The paper includes two parts: seismogenesis of the Haicheng earthquake and seismic statistical characteristics of this area.The paper mainly focuses on the interpretation of 3-D velocity structures,seismic magnitude-frequency analysis and rescaled range analysis of seismic statistical parameters of Haicheng(HC)region.This paper first introduces the geological evolution history of Liaoning area,focusing on tectonic structure of the Jinzhou(JZ)fault and Haichenghe-Dayanghe(HD)fault.It also summarizes several previous study results conducted on the Haicheng region.The development history of geophysical inversion method is reviewed in detail,followed by seismic tomography technique,procedures and its application at home and abroad.We collected seismological travel time data from 1431 earthquakes.The seismic network that was used to monitor the earthquake events consisted of 53 broadband stations.Choosing proper initial velocity models and parameters,the double-difference(DD)tomography method was used to determinate the 3–D Vp and Vs structures as well as earthquake locations with high–resolution simultaneously.Tomography results rebuild the middle and upper crustal structures of Liaoning area and provided better evidence of mechanism of earthquakes here:(1)the earthquake relocations are observed to form three northwest–striking line zones associated with the JZ fault,and a northeast–striking zone associated with the HD fault vertically.The lower interface of upper crust is supposed to be at 13–15 km depth and middle–crust is at 25 km.(2)The JZ fault is believed to consist of three small faults from earthquake locations which are called Niuju–Youyangou(NY)fault,Anshan–Yingkou(AY)fault and Haicheng–Gaizhou(HG)fault,respectively.The HD fault extending along NWW broke through the JZ fault zone while intersected with the Kangjialing fault to the SE direction.(3)Prominent layered low–velocity zones(LVZs)with low Vp/Vs are found at 15–20 km depth beneath the HC region and at 20 km depth beneath XY region.Combined with electrical and temperature data,these are high conductivity ductility layers,which may change the stress concentration and weaken the seismogenic layer above them,contributing to the occurrence of large earthquakes.(4)The LVZs in the middle crust are interpreted as regions of fluids which may be indicative of rock dehydration processes in high temperature.The low velocity with low Vp/Vs in the upper crust may be from the cracks with porosities of several percent filled with spring water from groundwater circulation.We also deduce that the spring water circulation depth is at 8 km.Compared with precious results,the seismic distributions obtained are more closely related to the tectonic structure,and the velocity models are more accurate and clear.More reasonable explanations are given for the LVZs.We believe that large crustal earthquakes in this area are caused by both the intersection of faults and the fluids in the middle crust.Then,we use the spatial distribution of b-values to detect current stress state of HC region.We briefly introduced the origins of b and affirmed that the b values are not only a statistical parameters but also have direct physical meanings.After decreasing errors from the estimated b-values using gridding techniques,we finally obtain its temporal and spatial variations.The results show that:(1)if the magnitude-frequency relationship is relatively linear,the b values from the maximum likelihood estimation and the least square method are almost the same within measurement errors.(2)The errors increases when the loss rate of data reaches 25% or the magnitude error rate reaches 20%.(3)The distribution of b values corresponding with tectonic structure is directly affected by source location errors.The spatial distribution of b-values from routine catalogue and data after using DD method have been compared and analyzed to reveal that DD data can be used for b-value calculation and has higher accuracy when the earthquakes clustered.(4)We got the vertical and horizontal variation of b-values in HC region,revealing that the b-values were significantly spatial variations,ranging from 0.6 to 1.7 and decreased with depth.The Xiuyan and Gaizhou regions with low b-values may indicate the most likely seismogenic area of future large earthquakes.The Hunhe area and the southeast part of HD fault have high b-values indicating low seismic hazard.Near the intersection of JZ fault and HD fault,the b-value is closed to 1.0,suggesting that the stress keeps stable and has low probability of future earthquakes.(5)The b-value will decrease before large earthquakes that might be one of the technologies for earthquake prediction.Finally,we used R/S(Rescaled Range Analysis)method to analyze the variation of range of seismic parameters such as earthquake frequency and earthquake time interval.Our results indicate that the extreme difference and mean square deviation of time interval and earthquake frequency are the power law functions of the length of time.The exponent H are all much larger than 0.5,which means the earthquake series is not an ideal Poisson process,on the contrary,the earthquake as a phenomenon bears dual characteristics of randomicity and regularity.The greater H departs from 0.5,the more regularity the time series will show.As a result,we can deduce that the focal mechanism of these earthquakes might be the same with similar stress field,which also indicates that the earthquake prediction in this region is possible. |
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