2019年12月26日湖北应城M4.9地震震源参数及余震活动性研究

2019年12月26日湖北应城发生M4.9有感地震,其震感波及武汉大部分地区。为了分析该地震的发震构造及余震活动性,本文利用波形拟合方法测定了不同速度模型下该地震的震源机制解和矩心深度,并用Bootstrapping抽样反演技术评价反演结果;此外,利用模板匹配技术匹配主震和目录余震波形,获取了更为完整的余震目录。结果显示,应城地震以走滑为主,矩心深度7.5km左右,矩震级M_W4.67;应城地震有1个前震和17个余震,余震序列缺少M2~4事件,表明应城地震为孤立型地震,M2以下地震的b值为0.8。     

Surface fault traces,fault plane solution and spatial distribution of the aftershocks of the September 13, 1986 earthquake of Kalamata (Southern Greece)

A shallow earthquake ofM _S=6.2 occurred in the southern part of the Peloponnesus, 12 km north of the port of the city of Kalamata, which caused considerable damage. The fault plane solution of the main shock, geological data and field observations, as well as the distribution of foci of aftershocks, indicate that the seismic fault is a listric normal one trending NNE-SSW and dipping to WNW. The surface ruptures caused by the earthquake coincide with the trace of a neotectonic fault, which is located 2–3 km east of the city of Kalamata and which is related to the formation of Messiniakos gulf during the Pliocene-Quaternary tectonics. Field observations indicate that the earthquake is due to the reactivation of the same fault.A three-days aftershock study in the area, with portable seismographs, recorded many aftershocks of which 39 withM _S1.7 were very well located. The distribution of aftershocks forms two clusters, one near the epicenter of the main shock in the northern part of the seismogenic volume, and the other near the epicenter of the largest aftershock (M _S=5.4) in the southern part of this volume. The central part of the area lacks aftershocks, which probably indicates that this is the part of the fault which slipped smoothly during the earthquake.     

2010年玉树地震序列ML≥4.0事件震源机制解特征分析

采用CAP方法反演2010年玉树7.1级地震序列前震、主震及余震19个M_L≥4.0事件的震源机制解,19个结果以走滑类型为主,前震、主震的震源机制解十分接近,反映出前震、主震之间密切的联系;震源深度集中在7~12 km,震源最浅（4.5 km）与最深（34 km）的两个余震事件具有明显的逆冲性质,表现出明显的边界特征;19个事件的震中分布在甘孜-玉树断裂北支玉树-隆宝断裂上,目前已经证明该断裂即为玉树地震的发震构造。自SE-NW沿玉树-隆宝断裂走向拉一剖面,观察震源深度沿剖面的变化情况,可看出玉树-隆宝断裂西北段震源深度要大于东南段,该段主要是余震活动的中后期,因此在地震活动的中后期,余震向地壳深部扩展,断裂累积的应变能得到更进一步的释放;P轴方位角优势分布集中在220°~230°,T轴方位优势分布集中在310°~320°,两个优势分布互相垂直性与单个事件的沙滩球应力轴一样,说明玉树地震的震源机制解类型较为简单;玉树周边地区应力场分布比较均匀,并不像汶川周边地区那么复杂,本次玉树地震为巴颜喀拉地块与羌塘块体边界处甘孜-玉树断裂应变能量的正常释放。     

The time of occurrence and the magnitude of the largest aftershock over India

Summary The time of occurrence and the magnitude of the largest aftershock in relation to the main shock have been studied for India and its neighbourhood based on the USCGS data during the years 1963–1971. It is found that the largest aftershock occurs within 2 hours after the main shock in about 50% of the cases and frequency of occurrencen(t) of the largest aftershocks decreases hyper-bolically with the intervalt after the main event and could be represented by a law of the formn(t)=At ^–h whereA andh are constants. The probability of occurrence of the largest aftershock within 2 hours of the main shock is found to be higher over island are regions of the world. The difference (M ₀–M ₁) of the magnitude of the largest aftershockM ₁ to that of the main shockM ₀ as a measure of aftershock activity does not show any marked regional variation over India and its neighbourhood, as was reported by Mogifor Japan. Examination of the values ofM ₁/M ₀ and the constantb in Gutenberg-Richter's frequency magnitude relationship reveals a range of variation in both; high values ofM ₁/M ₀ have been found to be associated with high values ofb in many tectonic earthquakes and thus not, restricted to reservoir associated seismic activity.     

A study in the velocity structure of December 5, 1997, M w = 7.8 Kronotskii rupture zone, Kamchatka

The object of the present study was to obtain and investigate the 3D velocity structure of the rupture zone of a large earthquake, to be specific, the great ( _Mw = 7.8) Kronotskii earthquake that occurred in Kamchatka on December 5, 1997. The event was preceded by a foreshock swarm (December 3–5, 1997) and followed by a long aftershock sequence. We investigated the V _P velocity distribution for different time periods: December 3–7, 1997 (when the chief events occurred, viz., the main shock and the larger aftershocks) and for subsequent periods of decaying aftershock activity until December 1998. The velocity distribution in the rupture zone proved to be inhomogeneous. Three regions have been identified: the northeastern (the main shock and foreshocks), the central, and the southwestern, which differ both in the character of seismicity and in velocity. The V _P distribution was found to be time-dependent. The velocity was below the standard values in the foreshock-aftershock area in December 1997, subsequently the velocity increased. These results may indicate the absence of a continuous rupture zone, with the main shock and the two largest aftershocks that occurred in the southwest probably being independent events rupturing a transverse fault during the stress rearrangement following the main shock.     

2018年9月4日新疆伽师MS5.5地震序列及发震构造讨论

2018年9月4日新疆伽师发生M_S5.5地震,震中处于塔里木地块西北缘,位于1997~1998年伽师强震群震区内。此次伽师地震前发生了M_S4.7前震,截至9月30日最大余震震级为M_S4.6（M_L5.0）,初步判定为前-主-余型地震序列。序列精定位结果显示,余震沿近NE向展布,主震震源深度与1997~1998年伽师强震主震基本一致,发震断层陡立。本文从区域的构造环境、地震震源机制解和余震分布特征等方面分析认为,地震发生在伽师隐伏断裂东南端部,为1997~1998年伽师强震群震区的一次新的构造活动。序列参数、视应力等计算结果显示,伽师M_S5.5地震的预测最大余震震级与最大余震震级M_S4.6接近,表明序列最大余震已经发生。     

Strong motion records and synthetic isoseismals of the Pyrgos,Peloponnisos, Southern Greece,Earthquake Sequence of March 26, 1993

In this work, several seismological observations are presented in order to explain characteristic features of the earthquake sequence which occurred in March 1993 in southwestern Greece, very close to the city of Pyrgos.Fault plane solutions of the largest fore- and aftershocks and the main shock, as well as the directions at which the maximum ground accelerations were recorded suggest that this earthquake sequence has been developed by rupturing three distinct focal planes with different focal mechanisms. The first focal plane, located in the off-shore area, strikes NW-SE, dips SE and includes most of the foreshock activity. The foreshock activity migrated to the northeastern part of the city of Pyrgos and took place on planes with a predominant direction NE-SW. The main shock ofM _x =5.5 occurred in a focal plane located between the two above-mentioned areas. Strong motion records of significant shocks of the sequence show peak acceleration values on components consistent with the relevant fault plane solutions.Furthermore, the observed macroseismic field has been compared with synthetic isoseismals computed by using a certain velocity model and the focal mechanism parameters of the main shock.     

The Tolud Burst of Seismicity and the Earthquake of November 30, 2012 (<Emphasis Type="Italic">M</Emphasis><Subscript>C</Subscript> = 5.4, <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> = 4.8) that Accompanied the Start of the 2012‒2013 Tolbachik Eruption

This paper reports a study of the Tolud earthquake sequence; the sequence was a burst of shallow seismicity between November 28 and December 7, 2012; it accompanied the initial phase in the Tolbachik Fissure Eruption of 2012?2013. The largest earthquake (the Tolud earthquake of November 30, 2012, to be referred to as the Tolud Earthquake in what follows, with K_S = 11.3, M_L = 4.9, M_C = 5.4, and M_W = 4.8) is one of the five larger seismic events that have been recorded at depths shallower than 10 km beneath the entire Klyuchevskoi Volcanic Cluster in 1961?2015. It was found that the Tolud earthquake sequence was the foreshock–aftershock process of the Tolud Earthquake. This is one of the larger seismicity episodes ever to have occurred in the volcanic areas of Kamchatka. Data of the Kamchatka seismic stations were used to compute some parameters for the Tolud Earthquake and its largest (M_L = 4.3) aftershock; the parameters include the source parameters and mechanisms, and the moment magnitudes, since no information on these is available at the world seismological data centers. The focal mechanisms for the Tolud Earthquake and for its aftershock are consistent with seismic ruptures at a tension fault in the rift zone. Instrumental data were used to estimate the intensity of shaking due to the Tolud Earthquake. We discuss the sequence of events that was a signature of the time-dependent seismic and volcanic activity that took place in the Tolbachik zone in late November 2012 and terminated in the Tolud burst of seismicity. Based on the current ideas of the tectonics and magma sources for the Tolbachik volcanic zone, we discuss possible causes of these earthquakes.     

Relocation of the MS5.7 Earthquake Sequence in Songyuan, Jilin Province, 2018 and the Analysis of Its Seismogenic Structure

The 2018,Songyuan,Jilin M_S5. 7 earthquake occurred at the intersection of the FuyuZhaodong fault and the Second Songhua River fault. The moment magnitude of this earthquake is M_W5. 3,the centroid depth by the waveform fitting is 12 km,and it is a strike-slip type event. In this paper,with the seismic phase data provided by the China Earthquake Network, the double-difference location method is used to relocate the earthquake sequence,finally the relocation results of 60 earthquakes are obtained. The results show that the aftershock zone is about 4. 3km long and 3. 1km wide,which is distributed in the NE direction. The depth distribution of the seismic sequence is 9km-10 km. 1-2 days after the main shock,the aftershocks were scattered throughout the aftershock zone,and the largest aftershock occurred in the northeastern part of the aftershock zone. After 3-8 days,the aftershocks mainly occurred in the southwestern part of the aftershock zone. The profile distribution of the earthquake sequence shows that the fault plane dips to the southeast with the dip angle of about 75°. Combined with the regional tectonic setting,focal mechanism solution and intensity distribution,we conclude that the concealed fault of the Fuyu-Zhaodong fault is the seismogenic fault of the Songyuan M_S5. 7 earthquake. This paper also relocates the earthquake sequence of the previous magnitude 5. 0 earthquake in 2017. Combined with the results of the focal mechanism solution,we believe that the two earthquakes have the same seismogenic structure,and the earthquake sequence generally develops to the southwest. The historical seismic activity since 2009 shows that after the magnitude 5. 0 earthquake in 2017,the frequency and intensity of earthquakes in the earthquake zone are obviously enhanced,and attention should be paid to the development of seismic activity in the southwest direction of the earthquake zone.     

2017年九寨沟MS7.0地震序列活动特征分析

中强地震序列的主震发生后,短时间内受台站距震中较远、尾波干扰和波形重叠等因素的影响,往往会遗漏大量的地震,而地震目录的完整性会直接影响到震后趋势判定和余震序列特征分析的科学性和可靠性。本文利用基于GPU加速的模板匹配方法对2017年8月1～12日的连续波形进行扫描计算,检测九寨沟M_S7.0地震前后遗漏的地震事件,选取台网目录中信噪比较高的1033个地震事件作为模板,在主震前7天至震后5天期间识别出4854个检测地震事件,为台网可定位目录的3.3倍,除去对台网单台地震事件的修正外,还检测到1797个遗漏地震事件,将完备震级从1.6级降低到1.4级。基于补充了遗漏地震的完整地震目录,对2017年8月8日九寨沟M_S7.0地震序列活动特征进行分析。结果表明,前震序列在主震前短时间内出现了地震活动的密集增强,b值也显示为低值状态,可能是深部断层发生破裂之前的加速蠕动的结果。随着时间的推移,余震序列的完备震级逐渐下降并趋于稳定,b值存在缓慢升高的趋势,未来较长时期内余震序列仍将处于持续衰减的状态。     

汶川8.0级大震的余震最大强度用科里奥利力效应方法的判定

用科里奥利力效应预测强余震是一种震源物理的方法。回顾2008年汶川8.0级大震时用该方法判定余震最大强度的过程,半定性与综合判定为可能发生的最大余震强度为6.5级左右,实际发生了6.4级地震,与主震震级相差大于1级（M_主—M_余=1.6）。验证结果进一步说明该方法的科学性,给科里奥利力效应判定余震增加了一个可靠的判例。     

A study on the focal parameters of Shidian M =5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian M _S=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 10¹⁰–10¹⁶ N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×10⁵–20×10⁵Pa. Through the statistical analyses of variation of corner frequency f _c and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses. Foundation item: Scientific and Technological Key Project of Yunnan Province (2001NG46)     

1989—1999大同地震序列的隐伏断层研究:库仑应力分析和余震JHD重定位

1989年到1999年,大同—阳高地区发生了一系列MS≥5的中强地震.本文基于前人对1989年三次MS≥5地震的震源机制反演的结果,通过建立不同断层模型,利用库仑应力方法,计算前震对于主震,以及前震和主震对于余震的库仑应力触发关系,提出了一种可能的破裂模型,即1989年前震沿北西西方向发生左旋破裂,之后主震和余震沿北北东方向发生右旋破裂.根据这种破裂模式计算得出,前震发生后,主震震源处的库仑应力增加了约2×105 Pa,余震震源处的库仑应力出现下降;主震发生后,余震处的库仑应力出现回升,最后余震处的库仑应力几乎没有变化.基于大同地震台网的近场观测数据,用JHD(Joint Hypocentral determination)定位方法,对1999年11月1日MS=5.6地震后一个月的余震进行重定位,得到一条走向118°,倾角85°的左旋走滑断层,余震的深度分布在5km至20km范围内,显示该断层是隐伏断层.另外提出对主震震中位置约10km的修正.本文对1989年三次MS≥5地震序列和1999年MS=5.6地震余震空间分布的研究揭示该地区存在两条活跃的共轭隐伏走滑断层(1989年主震的北北东方向和1999年地震的北西西方向),并且推断已知的大王村断裂和团堡断裂是地下这两条共轭的隐伏走滑断层构造/地震活动在地表的响应.     

A study on the focal parameters of Shidian <Emphasis Type="Italic">M</Emphasis>=5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian M _S=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 10¹⁰–10¹⁶ N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×10⁵–20×10⁵Pa. Through the statistical analyses of variation of corner frequency f _c and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses.     

Properties of the 1979 Monte Negro (southwest Yugoslavia) seismic sequence

Focal properties of the Monte Negro earthquake (15 April 1979,M=7.1) and its seismic sequence (foreshocks and aftershocks), which occurred near the southwestern coast of Yugoslavia, are investigated. Fault plane solutions of the main shock and its largest aftershock (24 May 1979,M=6.3) and the spatial distribution of the shocks of this sequence show that the seismic fault strikes about southeast-northwest (parallel to the coast) and dips northeast (towards the continent). It is a strike-slip left-lateral fault with a considerable thrust component. Its length is 95 km and its width 12 km. An aseismic belt, which separated the aftershock foci into two groups (the northwestern and southeastern), is interpreted as a section of the fault that slipped smoothly during the main shock. The aftershock foci were barriers where stress had been induced. One of these barriers broke later and produced the largest aftershock of 24 May.     

2020年新疆伽师MS6.4地震震源区 微震检测与目录完备性分析

中强地震发生后,地震检测因受到尾波的干扰可能会遗漏部分微震事件,影响地震目录的完备性。文章利用波形模板匹配方法对2020年新疆伽师M_S6.4地震序列开展微震检测,相比原始的中国地震台网中心统一地震目录,新检测出1 756个微震事件,地震数量增加了1.3倍。基于检测后的余震目录计算最小完备震级为M_L1.2,地震活动性b值为0.76,较原始目录的M_L1.6和0.77均有所降低。通过伽师震源区地震序列活动特征分析,结果表明前震序列在主震前短时间内(前36小时)出现地震活动的密集增强,相应的b值显示为低值;主震发生后地震序列完备震级较高,随着时间的推移,完备震级缓慢降低并趋于稳定,并且呈周期性的波动。本研究提高了伽师震源区地震目录的完备性,为精细化描述该地区地震序列时空演化特征提供了关键数据基础。     

Statistical testing of the relation between burst of aftershocks and strong earthquakes

According to geological tectonics and seismic activites this paper devided North China (30°–45°N, 105°–130°E) into four areas. We analyzed the North China earthquake catalogue from 1970 to 1986 (from 1965 to 1986 for Huabei, the North China, plain region) and identified forty-two bursts of aftershock. Seven of them occurred in aftershock regions of strong earthquakes and seventeen of them in the seismic swarm regions. The relation between strong earthquakes with the remaining eighteen bursts of aftershocks has been studied and tested statistically in this paper. The result of statistical testing show that the random probabilityp of coincidence of bursts of aftershock with subsequent strong earthquakes is less than six percent. By Xu’sR scoring method the efficacy of predicting strong earthquake from bursts of aftershock is estimated greater than 39 percent. Following the method proposed in the paper we analyzed the earthquake catalogue of China from 1987 to June, 1988. The results show that there was only one burst of aftershock occurred on Jan. 6, 1988 withM=3.6 in Xiuyan of Northeast China. It implicates that a potential earthquake withM _S⩽5 might occur in one year afterwards in the region of Northeast China. Actually on Feb. 25, 1988 an earthquake withM _S=5.3 occurred in Zhangwu of Northeast China. Another example is Datong-Yanggao shock on October 18, 1989 which is a burst of aftershock. Three hours after an expected shock withM =6.1 took place in the same area. Two examples above have been tested in practical prediction and this shows that bursts of aftershocks are significant in predicting strong earthquakes. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 273–280, 1991. Part of earthquake catalogue is from Jinbiao Chen, Peiyan Chen and Quanlin Li.     

The 2014–2015 earthquake series in the northern Upper Rhine Graben,Central Europe

Since March 2014, an unusually large amount of earthquakes occur southeast of the city of Darmstadt in the northern Upper Rhine Graben. During the period, until April 2015, we have recorded 356 earthquakes with magnitudes ranging from M_L?=??0.6 to 4.2. We identified two source clusters separated laterally by about 5 km. The hypocentres within these clusters are aligned vertically extending over a depth range from 1 to 8 km with a lateral extent of about 1 to 2 km. Focal mechanisms show left-lateral strike-slip movements; b values are changing with time between b?=?0.6 and b?=?0.9. This is the first time in almost 150 years that such high earthquake rates have been observed in the region. Historical accounts dating back to the nineteenth century report of over 2000 felt earthquakes over a time span from 1869 to 1871. From these, maximum intensities of VII have been estimated. Other seismic activities in the region were reported in the 1970s. The observations of the 2014–2015 earthquake series do not completely match a typical main shock–aftershock sequence or a typical earthquake swarm. Especially the activity at the beginning of the earthquake series may be considered as a mixture of a main shock–aftershock sequence and a short-lasting swarm event. Whether or not the time gap between the current seismic activity, which actually takes place at the same locations as parts of the seismic swarm in 1869–1871, and the seismic activity in the nineteenth century or the seismic activity in the 1970s can be interpreted as a seismic cycle remains unclear.     

The 9th of July 1998 Faial Island (Azores,North Atlantic) seismic sequence

The Faial earthquake (M _L 5.8) that occurred on the 9th of July, 1998, in the Azores region (north Atlantic), caused nine casualties and severe destruction affecting more than 5,000 people. The main shock was located at sea, 10 km NE of the Faial Island, and triggered a seismic sequence that lasted for several weeks and was characterized by an unusual high p-value of 1.40 for the modified Omori law. We present here the results of a joint inversion of hypocenters and 1D velocity model performed on the data collected by the permanent network complemented with a temporary network installed shortly after the occurrence of the main event. The 1D velocity model shows a heterogeneous upper crust, testified by the observed differences in site effects at the stations, while the middle crust from ∼2.5 to 8 km in depth is quite homogeneous. The Moho is located at a depth of about 12–13 km and the Vp/Vs ratio is found to be around 1.78. The events at depth are mainly concentrated in the middle-lower crust (8–12 km), while their spatial distribution shows a main cluster, visible after relocation, SSE trending. This direction of elongation is consistent with one of the fault planes (N151°E) of the centroid moment tensor (CMT) solution for the main shock. The same plane is the preferred main shock fault plane inferred after a Coulomb failure function analysis on the aftershock distribution. The main event relocation points to a focal depth shallower than 5 km. The aftershocks pattern shows that several fault systems were reactivated by the stress perturbation induced by the main shock. Besides the two main tectonic directions, trending WNW–ESE and NNW–SSE, observed in the tectonics of Faial, Pico, and S. Jorge, there is also evidence of a new tectonic direction trending WSW–ENE.     

Pattern Characteristics of Foreshock Sequences

—Earthquake clusterings in both space and time have various forms, in particular, two typical examples are the foreshock sequences and earthquake swarms. Based on the analysis of 8 foreshock sequences in mainland China during 1966–1996, this study concentrates on the pattern characteristics of foreshock sequences. The following pattern characteristics of foreshock sequences have been found (1) the epicenters of foreshock sequences were densely concentrated in space; (2) the focal mechanisms of foreshocks were similar to that of the main shock. Such consistency of focal mechanisms with main shocks did not exist in aftershock series as well as in several earthquake swarms; (3) we found no case in mainland China during the past thirty years that a main shock is preceded by an earthquake clustering with inconsistent focal mechanisms. Finally, we found 5% of the main shocks in mainland China are preceded by foreshock sequences.