Comparison of two earthquake early warning location methods

According to earthquake catalog records of Fujian Seismic Network, the Tnow method and the fourstation continuous location method put forward by Jin Xing are inspected by using P-wave arrival information of the first four stations in each earthquake. It shows that the fourstation continuous location method can locate more seismic events than the Tnow method. By analyzing the results, it is concluded that the reason for this is that the Tnow method makes use of information from stations without being triggered, while some stations failed to be reflected in earthquake catalog because of discontinuous records or unclear records of seismic phases. For seismic events whose location results can be given, there is no obvious difference in location results of the two methods and positioning deviation of most seismic events is also not significant. For earthquakes outside the network, the positioning deviation may amplify as the epicentral distance enlarges, which may relate to the situation that the seismic stations are centered on one side of epicenter and the opening angle between seismic stations used for location and epicenter is small.

The MW5.5 earthquake on August 6,2023,in Pingyuan,Shandong,China:A rupture on a buried fault

On August 6,2023,a magnitude MW5.5 earthquake struck Pingyuan County,Dezhou City,Shandong Province,China.This event was significant as no large earthquakes had been recorded in the region for over a century,and no active fault had been previously identified.This study collects 1309 P-wave arrival times and 866 S-wave arrival times from 74 seismic stations less than 200 km to the epicenter to constrain the spatial distribution of the mainshock and its 125 early aftershocks by the double difference earthquake relocation method,and selects 864 P-waveforms from 288 stations located within 800 km of the epicenter to constrain the focal mechanism solution of the mainshock through centroid moment tensor inversion.The relocation and the inversion indicate,the Pingyuan MW5.5 earthquake was caused by a rupture on a buried fault,likely an extensive segment of the Gaotang fault.This buried fault exhibited a dip of approximately 75°to the northwest,with a strike of 222°,similar to the Gaotang fault.The rupture initiated at the depth of 18.6 km and propagated upward and northeastward.However,the ground surface was not broken.The total duration of the rupture was~6.0 s,releasing the scalar moment of 2.5895×1017 N·m,equivalent to MW5.54.The moment rate reached the maximum only 1.4 seconds after the rupture initiation,and the 90%scalar moment was released in the first 4.6 s.In the first 1.4 seconds of the rupture process,the rupture velocity was estimated to be 2.6 km/s,slower than the local S-wave velocity.As the rupture neared its end,the rupture velocity decreased significantly.This study provides valuable insights into the seismic characteristics of the Pingyuan MW5.5 earthquake,shedding light on the previously unidentified buried fault responsible for the seismic activity in the region.Understanding the behavior of such faults is crucial for assessing seismic hazards and enhancing earthquake preparedness in the future.

Comparison of Two Earthquake Location Methods for Seismic Early Warning

In this paper,according to the Fujian Seismic Network earthquake catalog records,the T now method and the Four Stations Continuous Location method( hereinafter called FSCL)put forward by Jin Xing are inspected by using P-wave arrival information of the first four stations of each seismic event. Results show that for earthquakes within the network,both methods can obtain similar location results and location deviations are small for the majority of the events. For earthquakes outside the network,the location deviation may be amplified as the epicentral distance increases,owing to the seismic station distribution which spread toward the side of the epicenter and the small opening angle between seismic stations used for locating and epicenter. For the FSCL method,the impacts of the wave velocity on the location results may be significant for earthquakes outside the network.Thus,selecting a velocity model which is similar to the actual structure of the wave velocity will contribute to improving location results of earthquakes. The FSCL method can locate more seismic events than the T now method. It concludes that the T now method makes use of mistake information from some non-triggering stations in earthquake catalog,and some P-wave arrivals are not included in the earthquake catalog due to discontinuous records or unclear records of the seismic phase,which induces incorrect location.

The accurate location of the injection- induced microearthquakes in German Continental Deep Drilling Program

From August 21, 2000 to October 20, 2000,a fluid injection-induced seismicity experiment has been carried out in the KTB (German Continental Deep Drilling Program). The KTB seismic network recorded more than 2 700 events. Among them 237 events were of high signal-to-noise ratio, and were processed and accurately located. When the events were located, non KTB events were weeded out by Wadatis method. The standard deviation, mean and median were obtained by Jackknife's technique, and finally the events were accurately located by Gei-gers method so that the mean error is about 0.1 km. No earthquakes with focal depth greater than 9.3 km, which is nearly at the bottom of the hole, were detected. One of the explanation is that at such depths the stress levels may not close to the rocks frictional strength so that failure could not be induced by the relatively small perturbation in pore pressure. Or at these depths there may be no permeable, well-oriented faults. This depth may be in close proximity to the bottom of the hole to the brittle-ductile transition, even in this relatively stable interior of the in-teraplate. This phenomenon is explained by the experimental results and geothermal data from the superdeep bore-hole.

Study on the earthquake fault of the Lulong ML=6.2 earthquake by precise relocation of aftershock

We have selected 171 near-field records from 391 aftershock records of the Lulong, Hebei Province, earthquake in October 1982 and relocated the hypocenter of 45 aftershocks using the program Hypoinverse. The distribution of aftershocks reveals a set of earthquake faults: a WNW stretching fault truncates two NNE stretching faults. The two branches of faults show the conjugate structure which is often seen in brittle fracture. The NNE stretching faults are connected together. The Luanhe river valley near Lulong developed to a rudiment rift basin surrounded by a series of faults. The fault of Lulong earthquake is a strike-slip fault with tension component. This fault type matches with the activity of Zhangjiakou-Bohai seismic belt (Zhang-Bo belt) and also shows the action of Zhang-Bo belt as a boundary of two secondary active blocks that truncates the NNE fault.

Double Difference Location of the Mainshock and Aftershocks of the Hutubi MS6.2 Earthquake That Occurred on December 8,2016

The mainshock and aftershocks of the Hutubi M_S6.2 earthquake on December 8,2016 were relocated by applying the double difference method, and we relocated 477 earthquakes in the Hutubi region. The earthquake relocation results show that the aftershocks are distributed in the east-west direction towards the north side of the southern margin of the Junggar Basin fault,and are mainly distributed in the western region of the mainshock. The distance between the mainshock after relocation and the southern margin of the Junggar Basin fault is obviously shortened. Combined with the focal mechanism and the spatial distribution of the mainshock and aftershocks,it is inferred that the southern margin of the Junggar Basin fault is the main seismogenic structure of the Hutubi earthquake.

An eikonal equation-based earthquake location method by inversion of multiple phase arrivals

The precise determination of earthquake location is the fundamental basis in seismological community,and is crucial for analyzing seismic activity and performing seismic tomography.First arrivals are generally used to practically determine earthquake locations.However,first-arrival traveltimes are not sensitive to focal depths.Moreover,they cannot accurately constrain focal depths.To improve the accuracy,researchers have analyzed the depth phases of earthquake locations.The traveltimes of depth phases are sensitive to focal depths,and the joint inversion of depth phases and direct phases can be implemented to potentially obtain accurate earthquake locations.Generally,researchers can determine earthquake locations in layered models.Because layered models can only represent the first-order feature of subsurface structures,the advantages of joint inversion are not fully explored if layered models are used.To resolve the issue of current joint inversions,we use the traveltimes of three seismic phases to determine earthquake locations in heterogeneous models.The three seismic phases used in this study are the first P-,sPg-and PmP-waves.We calculate the traveltimes of the three seismic phases by solving an eikonal equation with an upwind difference scheme and use the traveltimes to determine earthquake locations.To verify the accuracy of the earthquake location method by the inversion of three seismic phases,we take the 2021 M_(S)6.4 Yangbi,Yunnan earthquake as an example and locate this earthquake using synthetic and real seismic data.Numerical tests demonstrate that the eikonal equation-based earthquake location method,which involves the inversion of multiple phase arrivals,can effectively improve earthquake location accuracy.

Accurate relocation of earthquakes in central-western China using the double-difference earthquake location algorithm

The double-difference earthquake relocation algorithm (DD algorithm) has been applied to the accurate relocation of 10057 earthquakes in the central-western China (21°-36°N, 98°-112E°) during the period of 1992-1999. In total, 79706 readings for P waves and 72169 readings for S waves were used in the relocation, and the source parameters of 6496 events were obtained. The relocation results revealed a more complete picture of the hypocentral distribution in the central-western China. In several seismic belts the relocated epicenters present a more defined lineation feature, reflecting the close correlation between the seismicity and the active tectonic structures. The relocated focal depths confirmed that most earthquakes (91 percent of the 6496 relocated events) in the central-western China were located at shallower depths not deeper than 20 km. The distribution of focal depths indicates that the seismogenic layer in the central-western China is located in the upper-mid crust with its thickness no deeper than 20 km.

Fault plane parameters of Tancheng M81/2 earthquake on the basis of present-day seismological data

The great Tancheng earthquake of M81/2 occurred in 1668 was the largest seismic event ever recorded in history in eastern China. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We relocated those earthquakes with the double-difference method and found focal mechanism solutions using gird test method. The inversion results are as follows： the strike is 21.6°, the dip angle is 89.5°, the slip angle is 170°, the fault length is about 160 km, the lower-boundary depth is about 32 km and the buried depth of upper boundary is about 4 km. This shows that the seismic fault is a NNE-trending upright right-lateral strike-slip fault and has cut through the crust. Moreover, the surface seismic fault, intensity distribution of the earthquake, earthquake-depth distribution and seismic-wave velocity profile in the focal area all verified our study result.

Determination of Focal Depths of the MS5.8 Alxa Left Banner,Inner Mongolia Earthquake Sequence

Using the double-difference earthquake location algorithm,the deterministic method (PTD method) and the CAP seismic moment tensor inversion method,the paper selects the primary waveform data of 78 earthquakes recorded by the "China Earthquake Science Array Probe Project in the Northern Part of North South Seismic Belt ",the "China Earthquake Scientific Exploration Array Data Center"of Institute of Geophysics,China Earthquake Administration,and the Inner Mongolia Digital Seismic Network to calculate the focal depths of the mainshock and the seismic sequence of the M_S5.8 Alxa Left Banner earthquake in Inner Mongolia. The results show that the focal depth of the main shock is 20. 6 km,determined by the double-difference earthquake location method,18. 1 km by the PTD method,and 19. 2 km by the CAP method. The focal depth of the earthquake sequence calculated by the double-difference location method is larger. The deterministic method (PTD method) and double-difference location method are the methods that fit the tectonic characteristics of the seismic source area,and the CAP method is suitable for larger earthquakes.

Spatiotemporal evolution of the 2021 M_(S)6.4 Yangbi earthquake sequence

Based on the seismic phase reports of the Yangbi area from January 1 to June 25,2021,and the waveform data of M≥4 earthquakes,we obtained the relocation results and focal mechanism solutions of the M_(S)6.4 Yangbi earthquake sequence using the HypoDD and CAP methods.Based on our results,our main conclusions are as follows:(1)the M_(S)6.4 Yangbi earthquake sequence is a typical foreshock-mainshock-aftershock sequence.The fore-shocks of the first two stages have the obvious fronts of migration and their migration rate increased gradually.There was no apparent front of migration during the third stage,and the occurrence of the mainshock was related to stress triggering from a M5.3 foreshock.We tentatively speculate that the rupture pattern of the Yangbi earthquake sequence conforms to the cascading-rupture model;and(2)the main fault of the M_(S)6.4 Yangbi earthquake sequence is a NW-trending right-lateral strike-slip fault.As time progressed,a minor conjugate aftershock belt formed at the northwest end of this fault,and a dendritic branching structure emerged in the southern fault segment,showing a complex seismogenic fault structure.We suggested that the fault of the Yangbi earthquake sequence may be a young sub-fault of the Weixi-Weishan fault.

云南盐津地区两种地震序列的地震活动性分析

2020年9月—2021年1月,云南盐津地区出现了密集中小地震活动。为获取更完备的地震目录和更准确的震源参数,利用2020年1月—2021年2月云南及其邻区地震台网记录到地震事件的观测报告和连续波形,采用匹配定位方法检测到了1340次地震事件。利用联合波形互相关的双差地震定位方法进行重定位,获得了检测目录中351次地震的精确位置,对精定位后的地震活动时空分布特征进行分析研究,结果表明:盐津地区发生的密集中小地震活动为先后发生的2种不同类型的地震序列,沿两条不同走向和倾角的断裂分布,其中2020年9—10月的M_(L)3.9震群型地震序列发生在NNW走向、倾角约为75°的盐津—筠连断裂;2021年1月的M_(L)5.2孤立型地震序列发生在近SN走向、断层线与盐津—筠连断裂呈约30°夹角且倾角约为50°的隐伏断裂。

起始破裂点位置对1932年昌马地震动的影响分析

起始破裂点位置是强地面运动模拟和预测的关键输入参数。为了进一步探究起始破裂点位置对强震模拟结果的影响,本文将非均匀凹凸体模型引入随机有限断层法,模拟了1932年昌马地震的强地面运动。结果显示:震中PGA随着起始破裂点位置的变化呈现明显的差异,产生高PGA值的起始破裂点大部分分布在断层左侧。起始破裂点对区域内PGA≥800 cm/s^(2)的空间分布影响有限,并且震中区PGA是整个区域内的最大值;对100~200 cm/s^(2)的PGA空间分布有较大影响;对其余范围PGA的空间分布影响较小。PGA衰减关系受起始破裂点位置的影响较大,起始破裂点位于断层中部和中下部偏右时,计算结果与第五代中国地震动参数区划图中的预测衰减关系较为符合。起始破裂点位置对反应谱影响的结果表明:震中的反应谱受到的影响有限。对不同周期反应谱空间分布的研究发现:反应谱值分布均呈现随周期增大而衰减的趋势。相较于其他部位的起始破裂点,起始破裂点位于断层底部时的反应谱值分布随周期的变化规律有明显不同。本文讨论了起始破裂点位置对昌马地震研究区域内PGA和反应谱的影响,研究结果可为地震动模拟时起始破裂点位置在断层上的选择提供参考。

Aftershock sequence relocation of the 2021 M_(S)7.4 Maduo Earthquake, Qinghai, China

The 2021 Qinghai Maduo M_(S)7.4 earthquake was one of the strongest earthquakes that occurred in the Bayan Har block of the Tibetan Plateau during the past 30 years,which spatially filled in the gap of strong earthquake in the eastern section of the northern block boundary.In this study,the aftershock sequence within 8 days after the mainshock was relocated by double difference algorithm.The results show that the total length of the aftershock zone is approximately 170 km;the mainshock epicenter is located in the center of the aftershock zone,indicating a bilateral rupture.The aftershocks are mainly distributed along NWW direction with an overall strike of 285°.The focal depth profiles indicate that the seismogenic fault is nearly vertical and dips to southwest or northeast in different sections,indicating a complex geometry.There is an aftershock gap located to the southeast of the mainshock epicenter with a scale of approximately 20 km.At the eastern end of the aftershock zone,horsetaillike branch faults show the terminal effect of a large strike-slip fault.There is a NW-trending aftershock zone on the north side of the western section,which may be a branch fault triggered by the mainshock.The location of the aftershock sequence is close to the eastern section of the Kunlun Mountain Pass-Jiangcuo(KMPJ)fault.The sequence overlaps well with surface trace of the KMPJ fault.We speculate that the KMPJ fault is the main seismogenic fault of the M_(S)7.4 Maduo earthquake.

2021-09-29宁夏中宁M_(L)3.6地震震源参数测定

基于宁夏区域地震台网资料,采用初至P震相定位法测定2021-09-29中宁M_(L)3.6地震震源深度,同时利用gCAP反演方法测定该地震震源机制解及矩张量解,然后根据震源机制与应力场模拟方法计算现今应力场体系在中宁M_(L)3.6地震震源机制两个节面的相对剪应力和正应力。结果表明,初至P震相定位法和gCAP方法测定的中宁M_(L)3.6地震震源深度均为11 km,震源机制节面Ⅰ走向242°、倾角63°、滑动角8°,节面Ⅱ走向148°、倾角83°、滑动角153°,矩张量解结果表明该地震的双力偶成分占全矩张量解的97.07%,表明该地震为天然地震。结合震中附近2009年以来M_(L)1.0以上地震重定位后的空间分布、区域地质构造和相对剪应力等分析推测,中宁M_(L)3.6地震发震断层可能与天景山断裂东南段附近NNW向断裂薄弱带有关,该地震是在青藏高原东北缘NE向挤压作用下,沿着NNW向断裂剪切滑动引起的一次右旋走滑型地震事件。

云南盐津地区中小地震活动性研究

1研究背景丰富的中小地震记录和准确的震源参数可以为研究地震活动性、发震断层及破裂特征、震情趋势判定、探索地震成核及触发机制等提供重要的地震学依据。Liu等(2019)利用模板匹配滤波和双差定位研究2016年青海门源M_(S)6.4地震的早期余震时空分布特征发现,冷龙岭北部断层破裂带中出现了16组重复地震群,可能是由主震震后滑移触发。李姣等(2020)对2017年漾濞M_(S)4.8和M_(S)5.1地震序列进行微震检测及重定位,确定了发震断层,并认为“震群(多震)型”相对于“主余型”地震序列,具有余震数量相对更少、余震活动衰减更快的特征。

江苏天宁M_(S)4.2地震震源深度分析

应用不同地震定位方法,对2021年12月22日江苏天宁M_(S)4.2地震震源深度进行重新测定。使用NonLinLoc方法对地震进行绝对定位,得出震源深度为11.9 km。以绝对定位结果为初值,利用该区域范围内历史地震事件,使用双差定位法对地震进行重定位,得出震源深度为10.1 km。结合该地区构造环境等进行分析,认为震源深度为10—12 km相对合理。

P-wave velocity structure beneath reservoirs and surrounding areas in the lower Jinsha River

The lower reaches of the Jinsha River are rich in hydropower resources because of the high mountains,deep valleys,and swift currents in this area.This region also features complex tectonic structures and frequent earthquakes.After the impoundment of the reservoirs,seismic activity increased significantly.Therefore,it is necessary to study the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds,thus providing seismological support for subsequent earthquake prevention and disaster reduction work in reservoir areas.In this study,we selected the data of 7.670 seismic events recorded by the seismic networks in Sichuan.Yunnan,and Chongqing and the temporary seismic arrays deployed nearby.We then applied the double-difference tomography method to this data,to obtain the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds.The results showed that the Jinsha River basin has a complex lateral P-wave velocity structure.Seismic events are mainly distributed in the transition zones between high-and low-velocity anomalies,and seismic events are particularly intense in the Xiluodu and Baihetan reservoir areas.Vertical cross-sections through the Xiangjiaba and Xiluodu reservoir areas revealed an apparent high-velocity anomaly at approximately 6 km depth:this high-velocity anomaly plays a role in stress accumulation,with few earthquakes distributed inside the high-velocity body.After the impoundment of the Baihetan reservoir,the number of earthquakes in the reservoir area increased significantly.The seismic events in the reservoir area north of 27°N were related to the enhanced activity of nearby faults after impoundment:the earthquakes in the reservoir area south of 27°N were probably induced by additional loads(or regional stress changes),and the multiple microseismic events may have been caused by rock rupture near the main faults under high pore pressure.

2014年新疆于田MS7.3级地震序列重定位

2014年新疆于田发生MS7.3地震,这一地区6年来连续发生2次强烈地震,震中相距不到110km.由于初始定位误差较大,于田地震的发震断层仍不清楚.本研究的主要目标是利用地震精定位方法对于田地震序列及其背景地震活动进行重新定位,确定于田地震的发震断层.本研究使用双差定位方法对于田地震序列进行重新定位.这一方法假设两个地震的震源距小于事件到台站的距离,两个事件到同一台站的走时差主要归因于其空间位置的偏移,因此可消除由于速度模型不准确引起的定位误差.重定位后得到了435个地震的位置参数.结果表明,2014年于田MS7.3级地震发生在阿尔金断裂带的西端,余震分布的优势方向为北东向,展布长度约33km,震源深度主要集中在4~12km,多数余震位于主震的西南侧.NS,EW和UD方向的定位误差分别为0.5km,1.1km和1.7km.于田地震余震序列总体衰减较慢.根据余震分布特征和震源机制解,认为此次地震的断层面为北东向的节面,阿尔金断裂的西南延伸分支断层是这次地震的主要发震构造.于田地震的发生与巴颜喀拉块体的东南向运动有关.