Results of seismological data processing for the territory of Armenia

The distribution of earthquake hypocenters in the territory of Armenia is obtained based on data from seismological bulletins for 1970–2012. It is shown that in addition to crustal earthquakes, there are deep earthquakes with a hypocenter depth down to 300 km. The seismicity is concentrated primarily in the northwest of the republic in the region adjacent to the Javakheti Upland. The positions of the main shock and the aftershocks of the December 1988 Spitak earthquake are determined separately. It is shown that the aftershocks of this earthquake cover a big area, extending to a depth of 100 km, although most of them are located at a depth of ~27 km.     

On the Problem of Deep Earthquakes in Crimea–Black Sea Region

It is a common opinion that only crustal earthquakes can occur in the Crimea–Black Sea region. Since the existence of deep earthquakes in the Crimea–Black Sea region is extremely important for the construction of a geodynamic model for this region, an attempt is made to verify the validity of this widespread view. To do this, the coordinates of all earthquakes recorded by the stations of the Crimean seismological network are reinterpreted with an algorithm developed by one of the authors. The data published in the seismological catalogs and bulletins of the Crimea–Black Sea region for 1970–2012 are used for the analysis. To refine the coordinates of hypocenters of earthquakes in the Crimea–Black Sea region, in addition to the data from stations of the Crimean seismological network, information from seismic stations located around the Black Sea coast are used. In total, the data from 61 seismic stations were used to determine the hypocenter coordinates. The used earthquake catalogs for 1970–2012 contain information on ~2140 events with magnitudes from–1.5 to 5.5. The bulletins provide information on the arrival times of P- and S-waves at seismic stations for 1988 events recorded by three or more stations. The principal innovation of this study is the use of the original author’s hypocenter determination algorithm, which minimizes the functional of distances between the points (X, Y, H) and (x, y, h) corresponding to the theoretical and observed seismic wave travel times from the earthquake source to the recording stations. The determination of the coordinates of earthquake hypocenters is much more stable in this case than the usual minimization of the residual functional for the arrival time of an earthquake wave at a station (the difference between the theoretical and observed values). Since determination of the hypocenter coordinates can be influenced by the chosen velocity column beneath each station, special attention is focused on collecting information on velocity profiles. To evaluate the influence of the upper mantle on the results of calculating the velocity model, two different low-velocity and high-velocity models are used; the results are compared with each other. Both velocity models are set to a depth of 640 km, which is fundamentally important in determining hypocenters for deep earthquakes. Studies of the Crimea–Black Sea region have revealed more than 70 earthquakes with a source depth of more than 60 km. The adequacy of the obtained depth values is confirmed by the results of comparing the initial experimental data from the bulletins with the theoretical travel-time curves for earthquake sources with depths of 50 and 200 km. The sources of deep earthquakes found in the Crimea–Black Sea region significantly change our understanding of the structure and geotectonics of this region.     

京津唐地区地壳三维P波速度结构与地震活动性分析

本文利用华北遥测地震台网和首都圈数字地震台网112个台站记录到的1993~2004年发生在首都圈地区3983次地震的P波绝对到时资料和相对到时资料,采用双差地震层析成像方法联合反演了京津唐地区地壳三维P波速度结构和震源参数.京津唐地区的三维P波速度结构图像在浅层上很好地反映了地表地质、地形的特征.在平原和凹陷的盆地处呈现P波低速速度异常,而在隆起的山区或基岩出露区显示为P波高速速度异常.在研究区域内震级M≥6.0历史地震和经过重新定位后的震级M_L≥3.0的地震的震源位置在10 km深度和15 km深度处的P波相对速度扰动图上的投影都显示出相似的特点,即:绝大部分的地震的震源位置在P波相对速度扰动图上的投影分布在低、高速异常的交界地带,且偏高速体一侧,只有极少数的地震分布在P波速度异常体内部.     

Seismicity of the North of the Russian Plate: Relocation of Recent Earthquakese

The hypocenters of the earthquakes recorded in the north of the Russian Plate from 1982 to 2013 are relocated. The relocation of the hypocenters is based on the common velocity section, common methodology, and the entire set of the initial data and bulletins available from the Russian and foreign seismic stations. The efficiency of the algorithm for calculating the hypocentral parameters and the velocity section is demonstrated by the example of two nonmilitary nuclear explosions in July 18, 1985 and September 6, 1988 in the northern part of the European Russia. For the first time, two earthquakes of July 19, 1982 and October 7, 2012, which have not been previously reported in the catalogs for the north of the Russian plate, are included in the seismic catalog.     

重庆荣昌诱发地震区精细速度结构及2010年ML5.1地震序列精确定位

Based on data collected from a temporal seismic network, and in addition to the data from some nearby permanent stations, we investigate the velocity structure and seismicity in the Rongchang gas field, where significant injection-induced seismicity has been identified. First, we use receiver functions from distant earthquakes to invert detailed 1-D velocity structures beneath typical stations. Then, we use the double-difference hypocenter location method to re-locate earthquakes of the 2010 ML5.1 earthquake sequence that occurred in the region. The re-located hypocenters show that the 2010 ML5.1 earthquake sequence was distributed in a small area surrounding major injection wells and clustered mostly along pre-existing faults. Major earthquakes show a focal depth less than 5km with a dominant depth of ～2km, a depth of major reservoirs and injection wells. We thus conclude that the 2010 ML 5.1 earthquake sequence might have been induced by the deep well injection of unwanted water at a depth ～3km in the Rongchang gas field.     

Historical seismogram reproductions for the source parameters determination of the 1902, Atushi (Kashgar) earthquake

The majority of original seismograms recorded at the very beginning of instrumental seismology (the early 1900s) did not survive till present. However, a number of books, bulletins, and catalogs were published including the seismogram reproductions of some, particularly interesting earthquakes. In case these reproductions contain the time and amplitude scales, they can be successfully analyzed the same way as the original records. Information about the Atushi (Kashgar) earthquake, which occurred on August 22, 1902, is very limited. We could not find any original seismograms for this earthquake, but 12 seismograms from 6 seismic stations were printed as example records in different books. These data in combination with macroseismic observations and different bulletins information published for this earthquake were used to determine the source parameters of the earthquake. The earthquake epicenter was relocated at 39.87° N and 76.42° E with the hypocenter depth of about 18 km. We could further determine magnitudes m _B = 7.7 ± 0.3, M _S = 7.8 ± 0.4, M _W = 7.7 ± 0.3 and the focal mechanism of the earthquake with strike/dip/rake ? 260°± 20/30°± 10/90°± 10. This study confirms that the earthquake likely had a smaller magnitude than previously reported (M8.3). The focal mechanism indicates dominant thrust faulting, which is in a good agreement with presumably responsible Tuotegongbaizi-Aerpaleike northward dipping thrust fault kinematic, described in previous studies.     

Media velocity model and macroseismic effect: The Spitak earthquake of December 7, 1988

Relationship between the intensity of seismic shaking on the surface and the velocity structure of the medium at large depth is studied. The Spitak earthquake of December 7, 1988 is chosen as an object of study. A method to correlate the intensity of shaking in the localities to the geophysical parameters specified in the nodes of regular spatial grid is proposed. Formalized definition of anomalous intensity is suggested; it takes into account the distribution of distances from the localities with given intensity degrees to the hypocenter or to the nearest segment of the surface fault. It is found that the seismic wave velocities at a depth of 1 km are higher (up to 0.2–0.6 km/s) under the localities with anomalously high intensity. No any certain regularity is found in deeper layers.     

The earthquake of February 3, 2015, near Sumy,Ukraine: Source parameters and focal mechanism

The parameters of the earthquake that took place February 3, 2015, near the city of Sumy, Ukraine, were calculated from an analysis of records obtained by both Russian and Ukrainian seismic stations (Poltava, Skvira, Nikolaev, Dneropetrovsk, and Desna). The calculated hypocenter depth of 54 km was verified by several approaches: isolation of deep PP, SP phases from the records of remote stations and solution of the kinematic problem for the Poltava station. The focal mechanism as shear with a complex fault component was determined by the first arrivals of P-waves. The data on the azimuthal travel-time curve confirm the focal mechanism. We have calculated the earthquake parameters; they are as follows: length gap L₁ = 8.08 km, L₂ = 6.68 km, a destruction rate of C = 2 km/s. We have obtained the dynamic parameters of the event. The calculated fault length (L = 5.46 km) within the accuracy limits of the method coincides with the early result obtained by the azimuthal travel-time curve. On the basis of these results, we suggest that elastic energy release and formation of the dislocations in the earthquake source occurred on a smooth, prefractured fault (σ_r > 0). Association of the hypocenter with the tectonic node of the northern marginal fault of the Dnieper–Donets graben and northern branch of Kryvyi Rih–Kremenchuk suture confirm this. Here, we observe a considerable Moho depth, structural alteration, and high gradients of the temperature and magnetic and electric rock properties in the lower Earth’s crust and upper mantle. These circumstances are favorable for the earthquake occurring here.     

Suboceanic earthquake location and seismic structure in the Kanto district,central Japan

We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P- and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P- and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A low-velocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region.     

中国东北的深源地震波形匹配检测及定位

中国东北珲春周边地区位于环太平洋地震带上,也是中国唯一存在的深源地震带.较大地震发生后常会有若干震级较小的余震发生,但在相同的地震震级情况下深源地震的余震一般比浅源地震的余震数量要少1~3个数量级,且在全球不同深震区的深震余震数量也存在显著差异.针对国际地震中心（ISC）2010年7月至2014年12月目录中给出的中国东北附近27次震源深度超过300 km的深源地震,我们首先利用区域固定地震台及NECsaids流动地震台阵的连续波形数据,选取已知地震事件作为模板,采用Match&Locate及Matched Filter方法进行波形互相关叠加分析来检测微小深震事件;然后对1966至2017年ISC目录中的东北地区深源地震进行双差重定位以提高震源位置的准确性,进一步分析深震活动与俯冲板片的关系.研究结果显示除ISC目录中给出的深震事件,我们未能检测出作为模板的深源地震的前震或余震活动,证明东北深源地震余震活动较少并不是由于台站分布有限而造成的漏检结果;重定位后震源延伸的角度与西太平洋板片在410~660 km地幔转换带内的俯冲角度较为一致,并且大部分深震震源位置位于俯冲板片中的亚稳态橄榄岩楔形区内部.结合双差定位结果、b值分析及前人研究成果,我们认为东北深源地震应不属于与俯冲非直接相关的"孤立地震",而是与西太平洋板块俯冲直接相关.     

Moment tensor inversion and source rupture process of the September 27, 2003 <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=7.9 earthquake occurred in the border area of China,Russia and Mongolia

We conducted moment tensor inversion and studied source rupture process for M _S=7.9 earthquake occurred in the border area of China, Russia and Mongolia on September 27 2003, by using digital teleseismic P-wave seismograms recorded by long-period seismograph stations of the global seismic network. Considering the aftershock distribution and the tectonic settings around the epicentral area, we propose that the M _S=7.9 earthquake occurred on a fault plane with the strike of 127°, the dip of 79° and the rake of 171°. The rupture process inversion result of M _S=7.9 earthquake shows that the total rupture duration is about 37 s, the scalar moment tensor is M ₀=0.97×10²⁰ N·m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust under the earth surface. The maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed that when the rupture propagated towards northwest and closed to the area around the M _S=7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. The consistence of spatial distribution of slip on the fault plane with the distribution of aftershocks also supports that the rupture is a heterogeneous process owing to the presence of barriers.     

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.     

The influence of one-dimensional velocity sections on determining the key parameters of the earthquake sources in Azerbaijan

The paper addresses the construction of one-dimensional (1D) velocity models in the seismogenic regions of Azerbaijan taken individually and the analysis of implications of these models for estimating the key parameters of earthquake sources in Azerbaijan. We considered and analyzed the seismological data from the local earthquakes, the arrival times of the P-, P-g, Pn-, S-, Sg-, and Sn-waves recorded by the network of telemetry stations during the period from 2005 to 2014 with ml ≥ 2.5. For constructing the models, we used the VELEST program which calculates 1D velocity models from travel times of seismic waves. As a result, the 1D models were built for ten regions of Azerbaijan; the key parameters of the hypocenters of the earthquakes were recalculated; and the corrections to the body-wave arrival times at the observation stations were obtained, which increased the accuracy of locating the hypocenter of earthquakes.     

2019年甘肃张掖M5.0地震余震序列重定位

本文使用祁连山主动源台网和甘肃省数字测震台网记录到的地震资料,应用双差定位方法和遗传算法对2019年甘肃张掖M5.0地震及其余震进行重定位,获得了30个地震事件的重定位结果,双差定位显示主震位置为38.502°N,100.254°E,震源深度14.7 km。重定位结果显示余震分布在昌马—俄博断裂,较为集中,震源深度主要分布在5～15 km范围内,余震序列沿SW—NE向空间分布。     

Rupture process of November 6,1988,Lan-cang-Gengma,Yunnan,China,earthquake of M_s=7.6 using empirical Green’s function de-convolution method

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龙门山断裂带精细速度结构的双差层析成像研究

利用川西流动地震台阵、汶川地震震后应急台网记录到的P波到时资料,对2008年5月至2008年10月期间发生的汶川地震余震序列应用双差层析成像方法进行了地震震源和三维P波速度结构的联合反演.结果显示,联合反演获得的地震重定位结果与基于一维地壳参考模型的双差定位方法结果相近;研究区15 km以上速度结构与地表断裂分布密切相关,20 km以下深度呈现北东向和北西向交错结构.汶川地震破裂带南段龙门山断裂带之间上地壳呈现高速异常,速度结构的非均匀变化是控制余震分布和主震破裂传播的主要因素;联合反演结果给出了小鱼洞-理县方向存在隐伏断裂的速度结构证据,同时发现,破裂带北东段可能沿新发断裂扩展;结果确认了汶川地震起始段的高角度逆冲断裂特征,也确认了前山断裂和中央断裂在约20 km深度合并到脆韧转换带的特征.     

Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50^°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.     

The M<Subscript>W</Subscript> 4.5 Vallorcine (French Alps) earthquake of 8 September 2005 and its complex aftershock sequence

On 8 September 2005 a moderate M_W 4.5 earthquake occurred in the north-western Alps midway between Chamonix (France) and Martigny (Switzerland). The focal mechanism corresponds to a right-lateral strike-slip on a N60°E fault plane. The foreshock–mainshock–aftershock sequence is investigated on the basis of data recorded by a temporary network of 28 stations deployed for 1 month just after the mainshock, and data from permanent, regional seismic networks. Absolute and relative locations of more than 400 events are obtained with a mean uncertainty of approximately 0.2 km. Small foreshocks, the mainshock, and early and late aftershocks are located relative to the main aftershock set. The seismic sequence exhibits a surprisingly complex structure, with at least five clusters on distinct fault planes. The main elongated cluster agrees with the location of the mainshock, its hypocenter being 4.3 km below sea level. We discuss the relationship between the right-lateral fault beneath the Loriaz peak (the source of the Vallorcine event), the nearby normal Remuaz fault, and the regional seismotectonic stress field.     

宽频带海底地震仪的地震重定位对马尼拉俯冲板片形态的约束

将宽频带OBS用于海底天然地震长期观测,在国内尚处于实验阶段.2015年在马尼拉俯冲带北段开展了为期6个月的宽频带海底天然地震观测试验.根据回收的1台海底地震仪(OBS04)与国际地震台网的718台陆地地震台站,共记录到7562个P波走时和5002个S波走时数据,利用Hyposat地震定位方法,对马尼拉俯冲带北部(119°E—123°E,19°N—22°N)在2015年8月至2016年2月期间的264个地震进行了重定位.地震重定位的结果及定位误差分析表明,在海域布设的OBS04台站让地震观测的空间分布更为合理,提高了地震定位精度;重定位后的震中分布更为集中,与地质构造吻合良好;浅部的地震活动较为活跃,分布密集,与浅部断层发育有关;重定位后的4条震源深度投影剖面,从不同角度较好地约束了俯冲板片上边界的板片形态,板片倾角在浅部0～30km区间约为10°～22°,随着深度的增加,俯冲板片逐渐变陡,在深度120～180km处倾角约为41°～58°.该项研究为马尼拉俯冲带北段的板片形态提供了重要约束,而且为今后长期天然地震观测提供了重要而宝贵的经验与借鉴.     

The Lemnos 8 January 2013 (M w = 5.7) earthquake: fault slip,aftershock properties and static stress transfer modeling in the north Aegean Sea

We investigate mainshock slip distribution and aftershock activity of the 8 January 2013 M _w?=?5.7 Lemnos earthquake, north Aegean Sea. We analyse the seismic waveforms to better understand the spatio-temporal characteristics of earthquake rupture within the seismogenic layer of the crust. Peak slip values range from 50 to 64 cm and mean slip values range from 10 to 12 cm. The slip patches of the event extend over an area of dimensions 16?×?16 km². We also relocate aftershock catalog locations to image seismic fault dimensions and test earthquake transfer models. The relocated events allowed us to identify the active faults in this area of the north Aegean Sea by locating two, NE–SW linear patterns of aftershocks. The aftershock distribution of the mainshock event clearly reveals a NE–SW striking fault about 40 km offshore Lemnos Island that extends from 2 km up to a depth of 14 km. After the mainshock most of the seismic activity migrated to the east and to the north of the hypocenter due to (a) rupture directivity towards the NE and (b) Coulomb stress transfer. A stress inversion analysis based on 14 focal mechanisms of aftershocks showed that the maximum horizontal stress is compressional at N84°E. The static stress transfer analysis for all post-1943 major events in the North Aegean shows no evidence for triggering of the 2013 event. We suggest that the 2013 event occurred due to tectonic loading of the North Aegean crust.