The depth of the deepest historical earthquakes

We useP andS times listed in the International Seismological Summary to relocate 23 historical earthquakes (1927–1963) reported as occurring at or below 670 km. In all cases, our relocated hypocenters are shallower than the starting depths; furthermore, all events converge to 691 km or less, with a precision estimated at ±10 km. This study upholds the results of Stark and Frohlich, who had usedpP–P times of post-WWSSN earthquakes to constrain reliable hypocentral depths to no greater than 684 km. In particular, we reject Rothé's claim that a 1963 event in the vicinity of New Guinea occurred at a depth of more than 780 km.     

The statistical study of precursory anomalies observed in Chinese Seismological Network

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Ground-motion predictions in Shillong region, northeast India

We deliver ground-motion prediction equations for Shillong region, northeast India, based on a database generated by finite-fault stochastic simulations. An examination of the regional seismic source characteristics is carried out beforehand. Micro/minor earthquakes (M _W?<?5.0) nucleating at hypocentral depth <21?km in the region recorded at broadband seismic stations are observed to have Brune stress-drop ranging between 2.8 and 99.9?bars. Likewise, macroseismic intensity data for the 1897 Shillong Earthquake that nucleated at a hypocentral depth of ~35?km places the associated stress-drop at 100?200?bars. The apparent variation of the stress-drop parameter with depth is considered with two source zones namely lower-crust and upper-crust. Equations for the lower-crust predict higher ground-motion levels and exhibit affinity to those developed for stable continental region of Eastern North America. The ground-motion levels predicted by the equations for the upper-crust are relatively lower but are still higher compared with those predicted for tectonically active regions, viz., the Himalayas and Western North America.     

晋冀鲁豫交界地区震源位置及震源区速度结构的联合反演

利用邯郸数字台网记录到的2001—2008年间460次M_L≥1.0地震的1861条P波到时数据, 采用震源位置和速度结构联合反演方法确定晋冀鲁豫交界地区(35.0°~38.0°N, 113.0°~116.0°E)地震的震源位置分布和该区域的速度结构。 结果表明: ① 经过重新定位后, P波走时的均方根残差(RMS)由反演前的1.35 s降到反演后的0.45 s。 定位偏差在EW方向上平均为0.031 km, 在NS方向上平均为0.029 km, 在垂直方向上平均为0.060 km。 ② 邢台震区的中小地震明显呈NEE向分布, 深度主要集中分布在7~14 km范围内; 磁县震区中小震分布相对复杂, 具有NEE和NWW两个展布方向, 震源深度主要集中在8~18 km范围内, 总体上晋冀鲁豫交界地区中小地震深度呈现北部浅南部深的趋势。 ③ 反演得到了晋冀鲁豫交界地区的速度结构, 在邢台地震极震区下方7~14 km处存在低速层, 与1966年邢台7.2级地震的震源深度一致;在磁县地震极震区下方13~18 km处也存在低速层与1831年磁县7.5级地震震源深度一致, 且磁县震区下方的速度结构比邢台震区更为复杂。     

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 20.6km, determined by the double-difference earthquake location method, 18.1km by the PTD method, and 19.2km 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.     

2015年新疆皮山M_S 6.5地震序列震源深度测定

采用震源深度测定的确定性方法(PTD)和HypoSAT方法,使用新疆地震台网记录的原始地震波形数据,计算2015年7月3日皮山M_S 6.5主震和余震序列震源深度。结果显示,对于主震,采用PTD方法得到震源深度为21 km,采用HypoSAT方法得到震源深度为20 km;对于地震序列,采用PTD方法得到的震源深度主要分布在15—35 km,平均深度为23 km,而采用HypoSAT方法得到的震源深度主要分布在5—30 km,平均深度为19 km;采用PTD方法计算得到的震源深度较深,与中国地震局地球物理研究所采用矩张量反演得到的矩心深度(24 km)相差不大。     

Determining hypocentral parameters for local earthquakes under ill conditions using genetic algorithm

We demonstrate that GA-MHYPO determines accurate hypocentral parameters for local earthquakes under ill conditions, such as limited number of stations (phase data), large azimuthal gap, and noisy data. The genetic algorithm (GA) in GA-MHYPO searches for the optimal 1-D velocity structure which provides the minimum traveltime differences between observed (true) and calculated P and S arrivals within prescribed ranges. GA-MHYPO is able to determine hypocentral parameters more accurately in many circumstances than conventional methods which rely on an a priori (and possibly incorrect) 1-D velocity model. In our synthetic tests, the accuracy of hypocentral parameters obtained by GA-MHYPO given ill conditions is improved by more than a factor of 20 for error-free data, and by a factor of five for data with errors, compared to that obtained by conventional methods such as HYPOINVERSE. In the case of error-free data, GA-MHYPO yields less than 0.1 km errors in focal depths and hypocenters without strong dependence on azimuthal coverage up to 45°. Errors are less than 1 km for data with errors of a 0.1-s standard deviation. To test the performance using real data, a well-recorded earthquake in the New Madrid seismic zone and earthquakes recorded under ill conditions in the High Himalaya are relocated by GA-MHYPO. The hypocentral parameters determined by GA-MHYPO under both good and ill conditions show similar computational results, which suggest that GA-MHYPO is robust and yields more reliable hypocentral parameters than standard methods under ill conditions for natural earthquakes.     

江苏及邻区地震重新定位和构造特征分析

本文将双差定位和遗传算法定位两种方法相结合,对江苏及邻区1980～2005年的地震进行重新定位研究.重新定位后大大改善了原地震定位的精度,地震在空间上更加集中分布在某些区域,地震震源深度分布结果更加合理.江苏及邻区地震主要发生在上地壳和中地壳;研究区不同构造单元的震源深度统计分析表明,下扬子断块江苏段、冀鲁断块和豫皖断块的地震震源深度特点相似,在10～11 km、15～17 km深处存在二个明显的地震优势分布,推测分别在上地壳底面和中地壳;在25 km深处也存在一小的地震优势分布面,但地震频次较低.大别山地区地震的震源深度与下扬子断块、冀鲁断块和豫皖断块内的地震震源深度存在明显的差异,主要差异为大别山地区10 km以上的浅源地震十分发育,在6～7 km的深处有一地震优势分布,该深度附近地震波速度可能较高,而在10 km以下差异不大.     

The earthquake sequence of May 1951 at Jucuapa, El Salvador

Three earthquakes that happened over two days in May 1951 caused extensive damage to villages in a small area of eastern El Salvador (Central America). Contemporary hypocentral solutions indicated focal depths, confirmed by re-calculations using available seismic data, of the order of 90 km, suggesting events associated with the subducted Cocos plate. Macroseismic observations strongly indicate that the earthquakes were of very shallow focus and this is supported by wave-form modeling and the appearance of seismograms recorded in Guatemala. A re-evaluation of the location and source characteristics for these events is presented, together with a fault plane solution and additional macroseismic evidence. The implications for seismic hazard and risk assessment in Central America, where shallow earthquakes of moderate magnitude, frequently occurring in clusters, pose the greatest threat to settlements which, like the area affected by these events, are concentrated along the axis of Quaternary volcanoes.     

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.     

The location and mislocation of subcrustal earthquakes

In regions, such as subduction zones, with systematic velocity variations from the travel-time model, the estimated hypocentres may be significantly mislocated and the estimated hypocentres may depend strongly on the stations and phases used. The estimated depths of earthquakes beneath the North Island, New Zealand, as determined by the International Seismological Centre and the New Zealand Seismological Observatory are used to illustrate this effect. Compared with ISC depths determined using local P phases only, the NZSO depths using local P and S phases are shallower by about 15 km while ISC depths using local and releseismic P phases are shallower again by about 30 km (and up to 70 km). Thus ISC depths which do not use teleseismic observations are not consistent with those which do and so care must be exercised when using both types of solution.     

基于熵权法的我国西部地区历史地震灾害等级划分

选取1990—2019年我国西部地区258次破坏性地震事件的灾害损失资料进行空间化处理,以每次地震烈度区的面积、死亡人数、受伤人数、直接经济损失等 4 个灾害损失量为指标,运用熵权法确定各指标的权重系数,划分我国西部地区各市（州）的5个历史地震灾害等级。结果表明,历史地震灾害极重的区域是四川省阿坝藏族羌族自治州、青海省玉树藏族自治州、云南省昭通市、新疆维吾尔自治区和田地区、云南省丽江市、新疆维吾尔自治区喀什地区、新疆维吾尔自治区巴音郭楞蒙古自治州、四川省雅安市、云南省楚雄彝族自治州;历史地震灾害严重的区域是青海省海南藏族自治州、新疆维吾尔自治区克孜勒苏柯尔克孜自治州、青海省甘孜藏族自治州、甘肃省定西市、内蒙古自治区巴彦淖尔市。     

沉积盆地台网稀疏地区中等强度浅源地震起始深度测定——以2013年前郭MS 5.8和2016年Fairview MW5.1地震序列为例

沉积盆地中的中等强度浅源地震起始深度不仅是地震成灾研究的重要参数,还是深地下工程开采诱发地震成因研究的关键信息.然而在台网稀疏、沉积层结构复杂的盆地地区,仅利用直达波到时,常常难以获得较为准确的地震起始深度.基于参考事件的Pn/Pg相对定位方法加入Pn震相,可以有效提高震源深度的测定精度.本文针对松辽盆地中2013年前...     

Origin of cenozoic petrographic provinces of Japan and surrounding areas

Three petrographic provinces can be recognized in the Cenozoic volcanic fields of Japan and surrounding areas. A province of a tholeiite series lies on the Pacific side of the Japanese Islands and includes the Izu Islands, whereas that of an alkali rock series occupies the Japan Sea side of the Islands with a narrow offshoot extending across central Honsyū (Honshū) and a continuation westward to Korea and Manchuria. A province of a calc-alkali rock series is superposed on the two provinces and occupies the greater part of the Japanese Islands exclusive of the Izu Islands and the islands in the Japan Sea southwest of Honsyū and north of Kyūsyū (Kyūshū). The boundary lines between the tholeiite and alkali provinces are located very closely to those between the areas where earthquakes occur at depths shallower than about 200 km and those for deeper ones. It is suggested that the parental tholeiite magma is produced by partial melting of the periodotite layer at depths shallower than 200 km. In the Izu Islands, except Nii-zima(Nii-jima) and Kōzu-sima(Kōzu-shima) close to Honsyū, the magma erupts to the surface without assimilating granitic material because the granitic layer is absent, resulting in volcanoes made up exclusively of the tholeiite series. The parental alkali olivine basalt magma is produced by partial melting of the peridotite layer at depths greater than 200 km. In the Japan Sea region, Korea, and Manchuria, it erupts to the surface without assimilating the granitic material, although it passes through a thick granitic layer, resulting in volcanoes made up exclusively of the alkali series. However, in the Cenozoic orogenic belt of the Japanese Islands, both types of parental magma assimilate granitic material during passage to the surface and erupt to form volcanoes of the calc-alkali series.     

USGS地震目录中4～5级震源深度异常地震可靠性初步研究:以南北地震带若干地震为例

在南北地震带地区,USGS全球地震目录中存在一些震源深度大于30km的地震.这些地震的震源深度是否可靠,对于研究这一地区的孕震机制、岩石圈强度和构造演化等科学问题具有重要意义.本文以南北地震带2012年发生的5个4~5级地震为例,利用区域地震台网的波形数据,基于sPL深度震相、短周期瑞利面波以及CAP等独立方法测定了其震源深度.结果表明:sPL深度震相和CAP方法给出的震源深度比较一致,差别小于2~3km,能够得到比较可靠的震源深度;短周期瑞利面波及其与P波振幅比也确定了地震震源深度较浅的特征.本文研究结果显示:宁夏会宁4.7级、云南富民4.8级和四川会东4.7级地震的震源深度约为8~12km左右,仍为发生于上地壳的地震,USGS地震目录给出的30km甚至更深的震源深度存在明显偏差;对于四川隆昌4.6和4.9级地震,本文给出的震源深度为1~2km,属于极浅源地震,USGS地震目录给出的10km和35km的震源深度结果尚需进一步改进.     

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.     

Focal Depths of Earthquakes in the Crimea–Black Sea Region

The problem of determining focal depths of earthquakes in the Crimea–Black Sea region is considered. Based on the results of interannual studies, it is found that the focal depths of Crimean earthquakes are mainly crustal, with maximum values of up to 60 km. Some recent publications, however, have described deep-focus earthquakes with depths of up to 300 km which were “revealed” in the Crimean region. In this respect, there arose the need to study such a large difference in estimated focal depths. Convincing examples show that the sensational “revelation” of deep earthquakes in Crimea was caused by incorrect processing of the experimental data, in particular, due to (1) a sharp distortion in the recorded arrival times of body waves, (2) exclusion of data from stations nearest to a source, (3) unreasonable arbitrary selection of data from seismic stations, and (4) dropping of data from the worldwide seismological network, including those on deep seismic phases. Thus, the conclusions about the presence of deep mantle earthquakes in Crimea are erroneous. We have redetermined the parameters of hypocenters and verified that the focal depths of earthquakes in the Crimea–Black Sea region are no more than 60 km. Based on these data, we analyze the features of the spatial distribution of focal depths to show that earthquake sources are grouped along conduits that dip southeastward, from the continental part of Crimea toward the Black Sea Basin, in the case of grouping of sources in the Alushta–Yalta and Sevastopol areas. The seismic focal layer of the Kerch–Anapa area dips northeastward, from the Black Sea beneath the North Caucasus.     

新疆天山近7级地震前震群活动的时空分布演化特征

通过普查与系统研究发现, 新疆天山近7级强震之前, 震群活动携带了丰富震兆信息。 主要结果如下: ① 震前1年半至3年多, 包括未来强震震中在内的整个地震带或某一区段震群活动明显增强; ② 在未来强震震中附近, 震群的数量最多且空间分布最为集中, 这些大量密集的震群构成的震群环包围了未来主震震中, 即形成震群空区或空段。 震群空区或空段的几何形状大多为椭圆, 其最大尺度约200 km; ③ 强震之前, 震群数量的多少, 随未来强震震级的增加而增加; ④ 个别强震前几个月, 震群增加的速率有加快的趋势, 同时震群空区内出现填空, 可作为近7级强震进入短期异常阶段的参考标志。 最后, 对所得结果的物理机制以及在地震预报中的应用等问题进行了讨论。     

2005年2月15日新疆乌什6.2级地震的地震环境及强震加速度记录

2005年2月15日7时38分7．5秒，在新疆维吾尔自治区乌什县境内发生了6．2级地震，在4团、乌什设立的强震台记录到了地震加速度记录。本文介绍了乌什地区6级以上地震的历史地震环境，对比分析了衰减公式与实际事件的异同。     

唐海—商都地震台阵剖面下方的地壳上地幔S波速度结构研究

2006年底,我们沿“张渤地震带”布设了一条从唐海—北京—商都的宽频带地震台阵剖面.本文利用台阵记录的远震波形资料,通过接收函数和面波联合反演对剖面下方100 km深度范围内地壳上地幔S波速度结构进行了研究.结果表明剖面东段莫霍面深度约30～34 km,西段深度约38～42 km,平原与山区的过渡地带地壳厚度变化较快.地壳内部10～20 km深度范围内存在多个低速体.在唐山7.8级地震震区附近Moho面出现小幅度隆起,中地壳存在明显的S波低速体.张家口以西,剖面下方10～20 km范围内存在两个S波低速体,张北6.2级地震发生在这两个低速体之间狭小的高速区. 在观测剖面附近,历史上发生的4个大震都与壳内低速体的分布有关. 张家口以东,上地幔普遍存在低速层,顶部埋深在60~80 km之间,并表现出明显的东部浅西部深的特点.