Fine Velocity Structure and Relocation of the 2010 gL 5. 1 Earthquake Sequence in the Rongchang Gas

Earthquake Research in ChinaVolume 26, Number4, 2012Fine Velocity Structure andRelocation of the 2010 M[ 5. 1Earthquake Sequence in theRongchang Gas Field'Wang Xiaolong'), Ma Shengli?), Lei Xingln2.3), Guo Xin”, Wang Qiang’，Yu Guozheng',Gou Xianbin'，Kuwahara Yasuto') , Imanishi Kazutoshi) ，and Jiang Xiadong1) Earthquake Administration of Chongqing Municipality , Chongqing 401 147, China2) Stale Key Laboratory of Earthquake Dynamics, Institute of Geology, CEA, Beijing 100029, China3) Geological Survey of Japan, Tsukuba 305 ~ 8567, Japan4) Hohai University, Changzhou, Changzhou 213022, ChinaBased on data collected from a temporal seismic network , and in addition to the data fromsome nearby permanent stations, we investigate the velocity structure and seismicity in theRongchang gas field, where significant injection-induced seismicity has been identified.First, we use receiver functions from distant earthquakes to invert detailed 1-D velocitystructures beneath typical stations. Then, we use the double-difference hypocenter locationmethod to re-locate earthquakes of the 2010 M[5. 1 earthquake sequence that occurred inthe region. The re-located hypocenters show that the 2010 M,5.1 earthquake sequence wasdistributed in a small area surrounding major injection wells and clustered mostly alongpre-existing faults. M ajor earthquakes show a focal depth less than 5km with a dominantdepth of ~2km, a depth of major reservoirs and injection wells. We thus conclude thatthe 2010 M, 5.1 earthquake sequence might have been induced by the deep well injectionof unwanted water at a depth ~ 3km in the Rongchang gas field.Key words: Inducedearthquake;Receiverfunction; Double difference location ;RongchangINTRODUCTIONThe Rongchang gas field is located at the southern edge of Sichuan basin in the border area' Received on March 15, 2011; revised on April 21, 2012. This reegoroh“Program of China Earthquake Administration ( XH12038Y), the中国煤化工lquake Dynamic(LED2008B04)，and Science and Technology Program of ChongdMHCNMH Gant No. CSTC，2011AC0149)，Research on the New Patterm and Key Technique。Lraliyuant uicigoncy Decision inSouthwest China ( 201108013).468Earthquake Research in Chinabetween Sichuan Province and Chongqing Municipality. Tectonically, it is located in thesouthwestem range of the Huaying Mountains on the eastern Sichuan fault-fold zone, being a partof the boundary belt between the two tectonic units, i. e. the central Sichuan dome structure andthe eastern Sichuan fault-fold zone (Fig. 1). The strata in the central Sichuan dome structure isnear-horizontal，with weak deformation， characterized by nose-like or short and domelikeanticlines. The eastern Sichuan fault-fold zone has experienced intensive deformation ，characterized by the elongated narrow anticlines and wide gentle synclines spreading parallel androughly equally-spaced, being typical Jura-type and comb-like folds. The anticlines form themountains and the synclines form the valleys; surface faults and anticline structures are aattendant phenomenon, and most faults show offset crossing of the axes or steep flanks of theanticlines ( Bureau of Geology and Mineral Resources of Sichuan Province, 1991 ). The majorfaults of this region are the Huayingshan basement faults, which strike N40° ~ 45°E on thewhole, dipping SE with dip angles 30° ~ 70°. This fault zone starts at the north of Dazhou in thenorth, running southwestward through Dazhu, Linshui, Hechuan, Tongliang and Rongchang tothe south of Yibin, with a total length of about 460km , being the largest fault zone in the Sichuanbasin. The fault zone is characterized by deep incision, multi-episode activity, and also withpossible late Quatemary activity. Attendant secondary faults developed in the overburden of theHuayingshan basement fault are located mainly at the axes or steep flanks of the anticlines , shownas a series of parallel faults with lengths from a few to tens of kilometers , resulting in a number ofen- echelon struetures. These faults have played a key role in controlling the earthquake activity ofthis region (Ding Renjie, et al.，2004).At the end of the 1980s, the Rongchang gas field started to inject waste water producedduring gas exploitation into underground through several abandoned wells 2 ~ 3km deep.Meanwhile, seismicity increased significantly. However, there were no historical earthquakes ofM≥5.0 recorded in this region, and the seismicity level was very low before the water injection.Since then, the water. injection induced seismicity in this region has been the concern of manyresearchers. Cheng Shi et al. ( 1992) made a field investigation and found that the seismicity inthe Rongchang area is related, to a certain extent, to the oil wells nearby. Ding Renjie et al.( 2004) made a preliminary analysis on the water injection-induced seismicity in Rongchang. In2006, the seismicity in this area was enhanced again, and studies found that it also had a certainrelationship to the oil wells nearby ( Huang Shiyuan et al. , 2006). These studies have enrichedour knowledge about the water-injection induced seismicity in the Rongchang area. Lei et al.( 2008) made a detailed study on the characteristics of seismicity of the Rongchang area before2006 and its relationship with water injection using statistical analysis methods, such as theepisodic type aftershock sequence ( ETAS) model. They revealed the temporal evolution processof seismicity of the region in the past 30 years, established the methods to distinguish water-injection induced earthquakes and common tectonic earthquakes based on the ETAS model andother statistical methods， and discussed the triggering mechanism of water injection and therelationship between the induced earthquakes and the regional geological structures.In the Rongchang gas field area, there have been more than 30 , 000 earthquakes observedduring the period from 1988 through 2006 including two earthquakes with M≥5.0 and 14 withM≥4.0 (Lei et al. , 2008). At present, the seismicity in this region is slightly lower than that inthe peak period in the 1990s, but still frequent. The induced seismicity in the Rongchang gasfield provides a valuable opportunity to gain some im itn. the manhanisms of inducedearthquakes. However, since there was only one ea中国煤化工inn this area, itwas impossible to get detailed spatial distribution of eeMHCNMHGtothestudiesofthe seismogenic faults, the interior structure of the Earth, earthquake forecast, and the causativemechanism of induced earthquakes. Therefore, in cooperation with the Japan Geological Survey中国煤化工MYHCNMH G470Earthquake Research in Chinaseismic phases are applied. This algorithm is superior to the traditional methods in anti-interference and robustness ( Zhao Jinhua et al.，2007) , especially as it has been widely appliedin the earthquake observations and studies, in which small-scale networks are utilized.On the other hand, an accurate velocity model is a prerequisite for earthquake relocation.The double-difference location algorithm adopts a horizontally layered velocity model and thelocation result is influenced mainly by the velocities in the layer where the source is located. Theseismic velocity model has great impact on the accuracy of earthquake location. The more detailedthe knowledge about the crust structure is, the higher the location accuracy of earthquake will be.Though a velocity model does not have any effect on the azimuth distribution betweenearthquakes, it affects the size of the distribution pattern of event groups. Therefore, we need touse the crustal velocity model as close to reality as possible ( Yang Zhongshu et al. , 2007). Asthe network built for this study adopts the continuous recording mode，many teleseismic waveformswere recorded，which provide necessary conditions for the inversion of velocity structures beneaththe network using the receiver function method.In this paper, we firstly use natural earthquake data recorded by the mobile network and theChongqing regional seismic network to invert the one-dimensional velocity structure in the crustbeneath the network with the receiver function method. We then re-locate the 2010-09-10 M[5. 1Rongchang，Chongqing earthquake sequence， and discuss the relationship between thisearthquake sequence and the tectonic setting, as well as the possible causative mechanism.1 SEISMIC VELOCITY MODEL INVERSIONIn 1979, Langston proved that under the equivalent seismic source assumption the impulseresponse, in other words, the receiver function， of the crust beneath a given station can beobtained from the long period teleseismic P waves. In 1984, Owens et al. expanded this methodto the newest broad-band seismic data and developed the linear inversion method of the receiverfunction. Nowadays, it is also the most widely-used method in studying the velocity structure ofthe crust beneath a station. The method of receiver function inversion of crustal thickness hasbecome more and more fully developed, and has also been developed and widely applied inChina. A number of results have been achieved (Liu Qiyuan et al. ，1996; Wu Qingju et al.，1998; He Chuansong et al. , 2003; An Zhanghui et al. , 2006; Li Yonghua et al. , 2008; Zhu etal.，2000). It uses three- component teleseismic P waveforms to de-convolve the radial andtangential components to obtain the time series, which mainly represent the response of velocitystructure of the crust and upper mantle beneath a station and is basically independent of thesource and ray paths, and invert the velocity structure of the crust and upper mantle beneath thestation. In this study, 63 teleseismic events of M≥6.0 and epicentral distance of 30° ~ 95° areselected, which were recorded by Rongchang station ( ROC) of the Chongqing seismic networkand Panlong station (PAL) of the mobile network in the period from July 2008 to October, 2010( Fig.2). Station ROC uses CMG-3ESPC ( 60s) seismometer and TDE -324CI data acquisitionsystem made by Guralp Systems， Ltd. Station PAL is equipped with a LS-7000 type high-performance 24-bit portable data acquisition system made by Hakusan Corporation of Japan andthe FSS-3 M seismometer produced by Beijing Geodevice Co. The epicentral distance data selectedcan avoid the interference of triplicate phases of the upper mantle and the core/ mantle boundary-induced P waves with low S/N ratio and weak energy. From the epicenter distribution, we findthat the azimuths of earthquakes are fairly well-distl中国煤化工to reduce thedeviations arising from lateral variation of crustal stru:YHCNMHG'aanalysis andprocessing.Volume 26, Number 4471.1 339kmFig.2Epicenter distribution of distant earthquakes used for receiver function inversionThe two eircles represent the epicenter distance of 30° and 90° ,respectively;★represents the Rongchang mobile station(105.50E, 29.5°N);●represents earthquake event1. 1 Receiver Function EstimationIn this paper, pre-processing of the seismograms is done using the GSAC software, which isdeveloped by Saint Louis University for seismic data analysis and processing. Firstly, records of100s from 20s before the direct P-wave are extracted. This time window is sufficient to cover themultiple reflected waves , even from the deepest interface of the lithosphere. Then, after removingthe tilt and DC components and instrument response, the record is digitally filtered using a 4-poleButterworth band-pass filter of0. 1 ~ 3Hz. The NS and EW coordinates are rotated to obtain radialand tangential components. Finally ，the maximum entropy deconvolution is performed in the timedomain on the radial and tangential components with the vertical component to get the receiverfunctions beneath the station. In addition, to suppress the high-frequency noise and stabilize thedeconvolution calculation it is generally necessary to try different Gaussian low -pass filterparameters of 0.5， 1.0 and 2.5 ( corresponding to different cut-off frequencies) in thecalculation. In order not to lose the effective components in the teleseismic seismograms, ourstudy selects the Gaussian filter parameter of 2.5 ( the corresponding cut-off frequency is about1.2 Hz).I.2 Receiver Function Inversion for Velocity StructureIn order to increase the signal-to-noise ratio, we stacked multi teleseismic receiver functionsrecorded at a same station to obtain the average receiver function. The velocity structure beneaththe station is inverted by ftting the synthetic to the average receiver function. The unknownparameters include S-wave velocity of each layer, the laver thickness and the velocity ratio. In theinversion, the velocity structure of the crust and up中国煤化工lation is equallydivided into thin layers of 2km-thick each; P-wave veYHC NMH Gaity P satisfy theempirical relationship with S-wave velocity Vs, Vp =kvs, ρ =U.3LVp +U. 11 In determining theP-wave velocity, and ht is velocity ratio (Li Yonghua et al. , 2009). The H ~ K stacking method,Earthquake Research in ChinaROCBHR8期Gaussian0.5BHNGaussian1.0一+BZGaussian2.5MWwr. 900180027003600年4500206080100Dec 25 (359), 2010 13:05:04 809TimeBHT小ψ小明(d)3600408(Dec 25 (359) 2010 13:05.04.809 TimelsTime/sFig.3Teleseismic waveforms and receiver functions(a) Shows the primary waveforms of the 2010-12-25 Ms 7.3Vanuatu earthquake recorded by station ROC; (b) is the waveforms after removal of tilt,DC component and instrument response and rotation of coordinates; (c) and (d) are theradial and tangential receiver functions obtained by using different low-pass Gaussianfilter parameters of0.5, 1. 0 and 2.5, respectivelywhich uses the receiver functions， is used in selecting the layered crust model ( Zhu et al. ，2000) to invert the crustal thickness and the velocity ratio beneath the station ( W ang Xiaolong etal.，2010). The global travel-time model ( AK135) is used as the initial upper mantle model toreduce the non-uniqueness of inversion. Fig. 4 shows the velocity structure of the crust beneathstations PAL and ROC.Fig. 5 is the fiting effect diagram for the waveform inversion of receiver functions atRongchang station. As can be seen from Fig. 5, the fitting of observed receiver functions tnumerically calculated ones is fairly good. All receive functions show significant Ps convertedphases from the Moho interface at around 4. 8s after the first arrival of P-wave. Fig. 4 indicatesthat the wave velocity in the crust of this area increases gradually with depth. The depth of theMoho is ~ 42km, corresponding to a relatively sharp velocity interface. There is a high-velocityzone at the depth of 6km ~ 8km beneath station PAL. Gm中indicates that中国煤化工。the depth and properties of the crystalline basement aretwo sides ofthe Huayingshan basement fault. The basement of centraMYHCNMHGstsmainlyofaset of basic, neutral and strong magnetic igneous rocks characterized by high density and strongmagnetism, suggesting that the basement is a deeply metamorphic and highly hardened rigidVolume 26, Number 4473V.(km/s)Vp(km/s)2.40 3.00 3.60 4.20 4.805.00 6.00 700 8.00100F20oF40-? 40[60F 二鼢60F =船Fig.4The 1-D velocity structures beneath stations ROC and PALblock, with a depth of 5km ~ 6km. The basement of the eastern Sichuan on the east consistsmainly of a set of thick metasedimentary clastic rocks ，intercalating flysch formations containingcarbonate rocks and pyroclastic rocks, as a low-density， weak to non-magnetic ductile basementstructure with depth of 7km ~ 9km in general, and the maximum depth up to 12km ( ZhaoCongjun et al.，1989). Station PAL is located on the west side of the basement fault and stationROC located at the east of the basement fault ( Fig. 1). Our results of a high-velocity zone atdepths of 6km ~ 8km beneath station PAL and a low-velocity transition zone at depth of 6km ~10km beneath station ROC are basically consistent with the results of previous studies. Thisprovides an accurate velocity model for earthquake hypocenter location. As the 2010 M5. 1earthquake sequence occurred near station ROC on the south of the Huayingshan basement fault,this study uses station ROC's model as the reference velocity model in double-difference location.2RE-LOCATION OF THE EARTHQUAKE SEQUENCE2.1 Selection of CatalogsEarthquake data used in this study is mainly obtained from the mobile seismic network, inaddition with the data from stations of western Chongqing area as a supplement. During the periodfrom the September 10, 2010 M.5. 1 Rongchang earthquake to October 30, 2010, a total of 724earthquakes were recorded. In order to increase accuracy, we select 247 earthquakes, of whichcomplete records are available at 4 or more stations for the relocation with the double-differencelocation method.2.2 Procedures of the Re-location中国煤化工Firstly, we use GELOR and LOCSAT methods to.CHC N M H Gations ofM <2.0and M≥2.0 earthquakes, respectively. As there is a dtterence In precision 0I readings of seismicphase, it is necessary to assign appropriate weights in the inversion according to the data quality.Generally speaking, the P-wave first arrival is easier to recognize, and its reading precision is474Earthquake Research in Chinahigher than that of the S-wave. Thus we assign a weight of 1 for the P-wave and a weight of 0.75for the S-wave. In the double- difference location calculation， there are many parameterscontrolling the forming of event pairs. Appropriate selection of parameters will directly affect theaccuracy of location.We firstly determine the travel time differences between pairs ofearthquakes, then select the earthquakes with the number of observed recordings (Pg, Sg) to beover8, a source spacing less than 5km ( because of the dense distribution of stations in thisstudy)，and a station distance of 150km to form a pair. Finally, through careful sorting andcontrol of parameters, earthquake clusters are formed. In total, 247 earthquakes are selected toparticipate in the double-difference location. The paper performs two rounds of iteration. The firstround has 15 iterations, with its WDCT ( the maximum distance between earthquake pairs) takingthe standard deviation of the distance distribution of the events (9km) as the cutoff value. Thesecond round has 10 iterations, and the WDCT takes the corresponding standard deviation (4 km )as the cutoff value. By repeated iterations, we finally obtain 178 high-quality hypocenters.3 RESULTS OF RE-LOCATIONFig. 6 shows map views of routinely determined hypocenters and re-located ones for acomparison. The earthquake distribution calculated with the absolute location method from theearthquake observation reports is relatively scattered ( Fig. 6 ( a))，while the earthquakedistribution after the relocation with the double-difference method shows strong clustering (Fig. 6(b)). The root mean square residual ( RMR) of arrival times decreases from 1. 215s before therelocation to 0.039s after the relocation. According to the statistics, the maximum error after thedouble-difference relocation is 1. 199km in NS direction， 1. 247km in EW direction， and1.439km in the vertical direction. The mean error in NS， EW and vertical directions is0.227km, 0.204km and 0.241km, respectively. As can be seen from Fig. 6(b)，seismicity ismainly concentrated near the north section of Yanziyan fault at the border area between Rongchangand Longchang, most of the focal depths are within the range up to 5km, and concentrated at adepth of around 2km.Fig. 7 shows hypocenter distribution on two cross sections which are perpendicular to eachother and shown in Fig.6(b) as A-B and C-D. Sect A-B is perpendicular to the NE strike ofmajor faults while C-D is parallel with the major faults. As shown in Fig. 7, the focal depth of themain shock is 1. 85km, aftershocks occurring in the first 10 days demonstrate upward migration.Aftershocks during the second 10 days show an outward ( but downward dominant) migrationpattern. After that aftershocks concentrated again at a depth less than 2km, overlapping with thedepth of aftershocks during the first 10 days.4 DISCUSSION AND CONCLUSIONBased on the teleseismic data recorded by Chongqing Rongchang seismic station and themobile seismic station, the present study inverts the 1-D velocity structure beneath the stationsusing receiver function method, and on this basis, re-locates the M[5. 1 Rongchang earthquakesequence using the double- difference location algorithm. As a result, the location accuracy isgreatly improved. The epicenter distribution is more concentrated and shows clusters, parallel tothe strike of fault.The M,5. 1 Rongchang earthquake sequence is distributed mainly along the north segment ofthe Yanziyan fault, particularly in its junctions with G中国煤化工buried faults.The Yanziyan fault is a thrust fault which strikes alongat an angle of75°. The Guangshunheng fault is characterized by right-MYHCNMH G3159. The twoburied faults revealed by oil prospecting and deep drilling data are both thrust faults, strikingNE45°, dipping in opposite directions to each other, about 1. 7km deep, being mid PleistoceneVolume 26, Number 4475wwwluwuluwuulwwLuww20801500)19101 23020080526(147)031820081 124(329)17027908Wwh0.054.801820080106(006)1314 29020080529(150)2345空OC20081209(344)023980s0.0500.047208021404)1809 RoC20080531(152)1237 R26090104004)034350370.056_0.0520.075_20080214(045)2008 ROC20080608(160)2025200901 16(016)01497093hworor0.094RoC20080227(058)454 ROC20080623(175)0756ROC200902 12(043)01340.095myumrao0.073.0.079 .wnmoan20080303(063)173120080705(187)101220090306(065)18058042 .89.390050__ Jmyowwan20080315(075)0632 R0C20080724(206)0943 BOC20090320(079)0217880720.041oC20080323(083)0524 ROC20080910(254)1900 ROC20090407(097)122392.040.062_0.075Min20080409(100)204620081006(280)065620090421(1 1)13268182873mtrimrerio30。20080416(107)065920081029(303)0709 R020090701(182)17300.0630.057BOC20080416(107)1354 BOC20081029(303)1932 R020090706(187)22530.063_00- Jhunufrmen0.064Imfourorv20080502(123)0933 ROC2081102(307)214820090708(189)0311801190.0486270.051.20080510(131)0551 R30C2081132)0102 B.OC20090908(251)06410.0790.068Ar05101510 I5Time/sFig. 5W aveform fitting of receiver function inversionDot line refers to observed receiver function, solid line is for synthetic receiverfunction.At the left side of the receiver functions are the station name , Gaussian filter parameter ,waveform fiting rate, and ray parameter( s/km)active faults (Ding Renjie et al.，2004). Earthquakes are clustered in the junction of faults andthe focal depth of the main shock is only 1850m. Most of the earthquakes are concentrated at adepth around 2000m，consistent with the depth of major reservoirs and water injection wells(1. 45 ~3. 148km). The depths of aftershocks migrat中国煤化工downward. Suchmigration patterns may be related to the fluid dispersiHCNMHGck. Moreover,asis shown by investigations, earthquake clustering isIation of ijectionwells. The clustered seismicity near the buried fault of the south flank of Louguanshan decreased476Earthquake Research in China4TGUATGU _29936NOPALD④29930'ALU292408i.。.Rongcharg :M84QIS29*18"0卤。中;29*15018E 105924 105930 105-36105942' 105948' 105°18E 105024' 105930' 105936'105*42' 105*48'-12(a) Before relocation(b) After relocationFig. 6Distribution of earthquake epicenters before (a) and after (b) re-locationusing the double difference method (see Fig. 1 for other map iterms)A1.6-●-163.2-4.8-Magnitude-4.8吕6.4-.0≤M≥3.0-6.42.0≤M≥3.010 20 30 40dayM=3.0，km 2010-09-10 .10-31-8.08.0-Fig. 7Depth distribution of aftershocks in the profile C-D and A-BRed star stands for the main shock, O andO are for aftershock distributionsignificantly with the stopping of injection in well No.2 in 2009 ( Wang Xiaolong et al. , 2011)，and seismicity migrated clearly to the western Longchang area, which may be related to the re-injection of waste water, which was collcted through pipelines, into the abundant wells near theLongchang area. Therefore, it is preliminarily inferred that this earthquake sequence is related tothe water injection-induced fault activity， that is to say， the injection of water increased porepressure of the faults, especially the junctions of the faults which are prone to fluid penetration,and decreased the strength of the fault zone， creating rupturing and slip， hence， causingearthquakes. Studies show a good correlation between injection-induced seismicity and pressureand flow of water injection (Long Feng et al.，2010; Shapiro et al.，2009). However, due tovarious reasons， we have not yet acquired recent water injection data in the Rongchang andLongchang areas. We will continue our observation and present further discussion in later paperson the mechanism and source process of water injection中国煤化工8 region.This paper has been published in Chinese in the joMHCN MH Glogy, Volume34，Number 2, 2012.Volume 26, Number 4477REFERENCESAn Zhanghui, Wu Qingju, Zhou Mindou. 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A senior engineer at the EarthquakeAdministration of Chongqing Municipality, and a visiting scholar at the Institute of Geology 0China Earthquake Administration, his major research interests are on earthquake monitoring andcrustal structure. E-mail: cqwxl@ mail. ustc. edu. cn中国煤化工MYHCNMH G