On the seismicity of central Kamchatka before the 1997, M = 7.9 Kronotskii earthquake

We report results from a detailed study of seismicity in central Kamchatka for the period from 1960 to 1997 using a modified traditional approach. The basic elements of this approach include (a) segmentation of the seismic region concerned (the Kronotskii and Shipunskii geoblocks, the continental slope and offshore blocks), (b) studying the variation in the rate of M = 4.5–7.0 earthquakes and in the amount of seismic energy release over time, (c) studying the seismicity variations, (d) separate estimates of earthquake recurrence for depths of 0–50 and 50–100 km. As a result, besides corroborating the fact that a quiescence occurred before the December 5, 1997, M = 7.9 Kronotskii earthquake, we also found a relationship between the start of the quiescence and the position of the seismic zone with respect to the rupture initiation. The earliest date of the quiescence (decreasing seismicity rate and seismic energy release) was due to the M = 4.5–7.0 earthquakes at depths of 0–100 km in the Kronotskii geoblock (8–9 years prior to the earthquake). The intermediate start of the quiescence was due to distant seismic zones of the Shipunskii geoblock and the circular zone using the RTL method, combining the Shipunskii and Kronotskii geoblocks (6 years). Based on the low magnitude seismicity (M≥2.6) at depths of 0–70 km in the southwestern part of the epicentral zone (50–100 km from the mainshock epicenter), the quiescence was inferred to have occurred a little over 3 years (40 months) before the mainshock time and a little over 2 years (25 months) in the immediate vicinity of the epicenter (0–50 km). These results enable a more reliable identification of other types of geophysical precursors during seismic quiescences before disastrous earthquakes.     

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.     

On the influence of earthquake energy range on the estimated seismicity parameters before the magnitude 7.9 Kronotskii earthquake of 1997

Results are reported from a detailed study of central Kamchatka seismicity for the period 1962–1997 based on a modification of the traditional approach. The approach involves (a) a detailed structure of the seismic region that recognizes the Kronotskii and Shipunskii geoblocks and two further blocks, the continental slope, and the offshore portion, (b) a study of variations in the rate of M = 3.0–7.2 earthquakes and the amount of seismic energy released at depths of 0–50 and 51–100 km, (c) a study of seismicity variability, and (d) separate estimates of the recurrence of crust-mantle earthquakes (depths 0–50 km) and mantle events (51–100 km). As a result, apart from corroborating the fact of a quiescence preceding the December 5, 1997 Kronotskii earthquake (M 7.9), we also found that a relationship exists between its beginning and the position of the earthquake-generating region relative to the mainshock epicenter. The quiescence dominates the seismic process during the pre-mainshock period and is characterized by a decreased rate of earthquakes (the first feature) and a decreased amount of seismic energy release (the second feature). Based on the first feature, we found that the quiescence started in 1987 throughout the entire depth range (0–100 km) in both parts of the Kronotskii geoblock close to the rupture zone of the eponymous earthquake. As to the Shipunskii geoblock, which is farther from the rupture zone, the quiescence began in the mantle of the inner area first (1988) and somewhat later at depths of 0–50 km within the continental slope (1989). By the second feature, the quiescence began at shallower depths in the inner area of the Kronotskii geoblock at the same time and later on (a year later) in the mantle (1988). Under the continental slope of the trench in the Shipunskii geoblock the shallower quiescence also began in 1987, while it was 3 years late in the inner zone (1990) and involved the earthquake-generating earth volume at depths of 0–100 km. These data are identical with or sufficiently close to the estimate for the beginning of this quiescence using a circular area of radius 150 km that combines the Kronotskii and Shipunskii geoblocks by the RTL method (1990).     

玉树地震震源区速度结构与余震分布的关系

利用玉树震区21个应急流动地震台站和青海省地震台网固定地震台站的观测数据,采用双差层析成像方法,对2010年4月14日至6月15期间发生的地震进行了重定位,并反演得到了玉树地震震源区的三维速度结构.重定位结果揭示余震主要沿NW向成窄带状分布在断层的两侧,表明脆性破裂应力释放主要集中于一个狭窄的区域内.在西北端,余震偏离玉树—甘孜断裂分布,在SW向也有分布,推测可能与南西向次级断裂有关.双差层析成像得到的速度结构在浅部与地表地质构造相一致,中上地壳的速度结构显示巴颜喀拉地块为高速异常,羌塘地块为低速异常.玉树地震余震分布与特定的速度结构存在相关性：主震发生在高低速过渡带偏高速体的一侧,余震主要分布在高速体外围,高速体内部几乎没有余震分布.一般说来,中上地壳的高速体通常具有较高的强度,可以积累较强的孕震能量.主震发生后,高速体内积累的弹性能量向周边释放,可能是导致高速体周边余震发生的主要原因.     

1997年玛尼79级地震的构造环境和地表破裂带特征

1997年11月8日西藏玛尼79级地震发生在羌塘盆地北缘.本文利用LANDSAT影像，研究地震的地质构造背景，研究表明玛尼79级地震发生在NEE向玛尔盖茶卡—若拉错断裂带上，这是一条全新世明显活动的地壳深断裂.利用CBERS_1影像，研究地震地表破裂带的几何特征，100000分之一CBERS_1影像上由地震裂缝、地震陡坎和断塞塘组合显示的线性影像清楚地反映出地震地表主破裂带的形迹，可有效地进行破裂带的分段和长度量测.结果表明玛尼79级地震形成的地震地表主破裂带西起羌塘盆地北缘，绥加山南麓的白雪湖湖积平原上，向东延伸到双端湖西岸，长110km，走向N70 80°E.可分为白雪湖—玛尔盖茶卡、玛尔盖茶卡—朝阳湖、朝阳湖—双端湖３段.多时相MSS、TM影像分析表明，1997年玛尼79级地震是先存地震地表破裂带再次破裂的结果.     

A study of the velocity structure for the Benioff zone of Kamchatka: The Avacha Bay-Kronotskii Bay Segment

We present computations of 3D P-wave velocity field for the segment of the Benioff zone of Kamchatka between 51.5° N to Avacha Bay to Cape Shipunskii to Kronotskii Bay. P-wave travel times for regional earthquakes were used to compute and construct 3D models of the velocity field for this segment of the Benioff zone. We examined the velocity structure to interpret it in conjunction with neotectonic morphostructures. The computations showed a complex structure of the field and the presence of inhomogeneities, both along and across the Benioff zone. Of special interest are the results for the Avacha and Kronotskii basins, where high velocity masses are unusually shallow beneath the low velocity layers in the top of the earth section. This creates high-gradient zones that can possibly generate large earthquakes.     

Spatial relation between source regions of strong earthquakes (M ≥ 7) on Southern Kamchatka and the distribution of velocities and elastic moduli in the Benioff zone

The presence of a phenomenological relationship between high velocity regions in the Benioff zone and sources of relatively strong earthquakes (M ≥ 6) was established for the first time from the comparison of such earthquakes with the velocity structure of central Kamchatka in the early 1970s. It was found that, in the region with P wave velocities of 8.1–8.5 km/s, the number of M ≥ 6 earthquakes over 1926–1965 was 2.5 times greater than their number in the region with velocities of 7.5–8.0 km/s. Later (in 1979), within the southern Kurile area, Sakhalin seismologists established that regions with V _P = 7.3–7.7 km/s are associated with source zones of M = 7.0–7.6 earthquakes and regions with V _P = 8.1–8.4 km/s are associated with M = 7.9–8.4 earthquakes. In light of these facts, we compared the positions of M = 7.0–7.4 earthquake sources in the Benioff zone of southern Kamchatka over the period 1907–1993 with the distribution of regions of high P velocities (8.0–8.5 to 8.5–9.0 km/s) derived from the interpretation of arrival time residuals at the Shipunskii station from numerous weak earthquakes in this zone (more than 2200 events of M = 2.3–4.9) over the period 1983–1995. This comparison is possible only in the case of long-term stability of the velocity field within the Benioff zone. This stability is confirmed by the relationship between velocity parameters and tectonics in the southern part of the Kurile arc, where island blocks are confined to high velocity regions in the Benioff zone and the straits between islands are confined to low velocity regions. The sources of southern Kamchatka earthquakes with M = 7.0–7.4, which are not the strongest events, are located predominantly within high velocity regions and at their boundaries with low velocity regions; i.e., the tendency previously established for the strongest earthquakes of the southern Kuriles and central Kamchatka is confirmed. However, to demonstrate more definitely their association with regions of high P wave velocities, a larger statistics of such earthquakes is required. On the basis of a direct correlation between P wave velocities and densities, the distributions of density, bulk modulus K, and shear modulus μ in the upper mantle of the Benioff zone of southern Kamchatka are obtained for the first time. Estimated densities vary from 3.6–3.9 g/cm³ in regions of high V _P values to 3.0–3.2 g/cm³ for regions of low V _P values. The bulk modulus K in the same velocity regions varies from (1.4–1.8) × 10¹² to (0.8–1.1) × 10¹² dyn/cm², respectively, and the shear modulus μ varies from (0.8–1.0) × 10¹² to (0.5–0.7) × 10¹² dyn/cm², respectively. Examination of the spatial correlation of the source areas of southern Kamchatka M = 7.0–7.4 earthquakes with the distribution of elastic moduli in the Benioff zone failed to reveal any relationship between their magnitudes and the moduli because of the insufficient statistics of the earthquakes used.     

1970年通海7.7级大地震强余震触发

1970年1月5日云南通海发生了M_S7.7地震, 震后发生了多次M_S＞5.0的强余震。 文中计算了1970年通海7.7级大地震后, 主震分别在5次强余震破裂面上诱发的库仑破裂应力变化(ΔCFS)。 结果表明, 有4次强余震发生在库仑破裂应力增加(ΔCFS＞0)的地区, 增加的范围为10^-2～10^-1 MPa; 有1次强余震按2种震源机制解结果给出的破裂面计算, 得到2种结果, 分别发生在库仑破裂应力变化为正和在库仑破裂应力变化为负的地区。 研究结果表明, 主震位错产生的库仑破裂应力变化可能是1970年通海7.7级大地震强余震活动的重要原因。     

龙门山和青藏高原的隆起与2008年汶川(M=7.9)地震

龙门山山脉,即2008年5月12日汶川（M=7．9）毁灭性地震的发生地,界定了喜马拉雅造山带的东缘,并显现出比青藏高原任何地方都大的地形起伏。然而,在这次地震之前,大地测量与地质调查工作都没有测到横跨该山系前缘有明显的缩短变形（Shenetal,2005;Meade,2007;Chenetal,2000）,从而引起了关于该山岳带地形的形成与发展过程的激烈争论。现已提出两个端元模型：（1）脆性地壳增厚,即具有大量滑动的逆冲断层切穿岩石层并引起隆起（Ta—pponnieretal,2001）;（2）地壳流动,即青藏高原下地壳的低粘度物质向外挤出,抬升了喜马拉雅山东北部的地壳（Roydenetal,1997;Burehfiel,2004;Bird,1991）。在此,我们使用平衡地质剖面来说明该山系前缘的地壳缩短、构造起伏与地形地貌是密切相关的。这表明,地壳缩短是位于青藏高原东缘的龙门山的隆起及其地形形成的主要驱动力。致使沿该山系前缘大型逆冲断层发生破裂,并造成数万人死亡和大范围破坏的2008年汶川地震（M=7．9）就是这种地壳缩短过程的作用表现。     

Tempo-spatial rupture process of the 1997 Mani, Xizang (Tibet), China earthquake of MS=7.9

IntroductionAnearthquakeofMs=7.9occurredinMaul,Xizang(Tibet),Chinaat10:02f55.4(UTC),No')ember8.1997.TheepicenterdeterminedbyChinaNationalSeismographNetwork(CNSN)is87.33"E.3>.26'N,thefocaldepthis40km,andthemagnitudeisMs=7.4.Accordingtothedeterllllnati...     

Variations in shear wave splitting during aftershocks of the Luquan earthquake in Yunnan Province

ＶａｒｉａｔｉｏｎｓｉｎｓｈｅａｒｗａｖｅｓｐｌｉｔｔｉｎｇｄｕｒｉｎｇａｆｔｅｒｓｈｏｃｋｓｏｆｔｈｅＬｕｑｕａｎｅａｒｔｈｑｕａｋｅｉｎＹｕｎｎａｎＰｒｏｖｉｎｃｅＢＡＩ－ＪＩＬＩ（李白基）（ＳｅｉｓｍｏｌｏｇｉｃａｌＢｕｒｅａｕｏｆＹｕｎｎａｎ...     

玛尼Ms7.9级地震三维动态模拟及余震解释

以1997年玛尼地震为例,利用三维非线性动态有限元方法,再现了玛尼地震的破裂过程.研究结果表明：（1）玛尼地震产生的动态应力波随时间变化呈明显波动性且随距离衰减,沿断裂带衰减相对慢一些.其波动性是震源区不同相位应力波及一些反射波相互叠加造成的.震源区动应力强烈的波动性使破裂表现“愈合脉冲”特征.（2）玛尼地震与其后几年发生的中强余震关系密切. 2001年3月5日在大震西南方向发生M_s6.4级地震、阿尔金断裂西段1999年与2000年的两次中强地震可能均主要是玛尼地震的产物.（3）玛尼地震对昆仑大震孕震区的动应力影响,卸载前产生的动应力变化大约为0.05 MPa,其远大于静应力的地震触发阈值,卸载后动应力剧烈波动,上下波动达0.2 MPa左右,其大约相当于0.02 MPa的静应力,这是静应力触发地震阈值的2倍,揭示玛尼地震很大程度上触发了昆仑大震的提前发生.     

Tempo-spatial rupture process of the 1997 Mani, Xizang (Tibet), China earthquake of M S=7.9

An earthquake of M _S=7.4 occurred in Mani, Xizang (Tibet), China on November 8, 1997. The moment tensor of this earthquake was inverted using the long period body waveform data from China Digital Seismograph Network (CDSN). The apparent source time functions (ASTFs) were retrieved from P and S waves, respectively, using the deconvolution technique in frequency domain, and the tempo-spatial rupture process on the fault plane was imaged by inverting the azimuth dependent ASTFs from different stations. The result of the moment tensor inversion indicates that the P and T axes of earthquake-generating stress field were nearly horizontal, with the P axis in the NNE direction (29°), the T axis in the SEE direction (122°) and that the NEE-SWW striking nodal plane and NNW-SSE striking nodal plane are mainly left-lateral and right-lateral strike-slip, respectively; that this earthquake had a scalar seismic moment of 3.4×10²⁰ N·m, and a moment magnitude of M _W=7.6. Taking the aftershock distribution into account, we proposed that the earthquake rupture occurred in the fault plane with the strike of 250°, the dip of 88° and the rake of 19°. On the basis of the result of the moment tensor inversion, the theoretical seismograms were synthesized, and then the ASTFs were retrieved by deconvoving the synthetic seismograms from the observed seismograms. The ASTFs retrieved from the P and S waves of different stations identically suggested that this earthquake was of a simple time history, whose ASTF can be approximated with a sine function with the half period of about 10 s. Inverting the azimuth dependent ASTFs from P and S waveforms led to the image showing the tempo-spatial distribution of the rupture on the fault plane. From the "remembering" snap-shots, the rupture initiated at the western end of the fault, and then propagated eastward and downward, indicating an overall unilateral rupture. However, the slip distribution is non-uniform, being made up of three sub-areas, one in the western end, about 10 km deep ("western area"); another about 55 km away from the western end and about 35 km deep ("eastern area"); the third about 30 km away from the western end and around 40 km deep ("central area"). The total rupture area was around 70 km long and 60 km wide. From the "forgetting" snap-shots, the rupturing appeared quite complex, with the slip occurring in different position at different time, and the earthquake being of the characteristics of "healing pulse". Another point we have to stress is that the locations in which the rupture initiated and terminated were not where the main rupture took place. Eventually, the static slip distribution was calculated, and the largest slip values of the three sub-areas were 956 cm, 743 cm and 1 060 cm, for the western, eastern and central areas, respectively. From the slip distribution, the rupture mainly distributed in the fault about 70 km eastern to the epicenter; from the aftershock distribution, however, the aftershocks were very sparse in the west to the epicenter while densely clustered in the east to the epicenter. It indicated that the Mani M _S=7.9 earthquake was resulted from the nearly eastward extension of the NEE-SWW to nearly E-W striking fault in the northwestern Tibetan plateau. Contribution No. 99FE2016, Institute of Geophysics, China Seismological Bureau. This work is supported by SSTCC Climb Project 95-S-05 and NSFDYS 49725410.     

Foreshocks and aftershocks of the M W = 7.1, 1992, earthquake in the Atrato region, Colombia

We study the October 18, M _W = 7.1, 1992 Atrato earthquake, and its foreshocks and aftershocks, which occurred in the Atrato valley, northwestern Colombia. The main shock was preceded by several foreshocksof which the M _W = 6.6, October 17 earthquacke was the largest. Inparticular, we examine foreshocks and aftershocks performing joint-hypocenter relocations using high quality Pn and Sn wave readingsfrom permanent regional networks. We observed a few hours prior to the main shock a sudden increase of foreshocks. Maybe this could be used as a predictor since foreshocks have been known for other major events in the region. Our locations align for 90 km with a trend of 5^° ±4^° in agreement with the Harvard CMT solution showing the faultplane trending 9^° to be the plane of rupture. In relation to theepicenter of the main shock, maximum intensities were located to thesouth, consistent with a rupture that traveled from north to south witha larger energy release in the south as suggested by an empirical Green'sfunction study (Li and Toksöz, 1993; Ammon et al., 1994). The boundarybetween the Panama and North Andes blocks has been placed close to thePanama-Colombia border as either a sharp boundary or a diffuse zone. TheAtrato earthquake, however, shows that the plate boundary between thePanama and North Andes microblocks is a diffuse deformation zone. Thiszone has a width of at least 2^° stretching from 78^°W to 76^°W. Quantification of earthquake moment release (during the past30 years) in this zone shows a similar amount of moment release in thewestern and eastern parts of this zone.     

1605年琼山大地震余震分布质疑

1995年地震出版社出版的中国历史强震目录将1605年琼山大地震余震震中位置较以往版本作了大幅度变更.根据该震的历史资料,1966～1976年我国大陆发生的9次强震余震间距,东南沿海地震带6级以上强震的主震与余震位置关系,琼山大地震的发震构造等,我们认为变更琼山大地震余震位置依据不足.     

Spectral analysis and earthquake scaling of the Petatlan earthquake (M s =7.6) aftershocks,recorded at stations along the pacific coast and between the coast and Mexico City

Spectral parameters have been estimated for 214 Petatlan aftershocks recorded at stations between Petatlan and Mexico City and between Petatlan and Acapulco. The spectral parameters were used to obtain empirical relations for the estimation of seismic moment from coda length and fromM _L . Stress drops, using Brune's model, were calculated for these aftershocks. Six events with large stress drop are located within a previously suggested asperity, and seven more suggest a boundary zone at the intersection of the Petatlan and Zihuatanejo aftershock rupture volumes. Stress drops increase with increasing seismic moment up to 10²⁰ dyne-cm but appear to be constant at greater moment values. The peak horizontal velocity times distance of aftershocks recorded near the coast and between the coast and Mexico City (30 to 270 km away), scales linearly with seismic moment, and predicts well the peak horizontal values of large (M _s 7.0) coastal thrust events recorded on rock sites at Mexico City. Peak horizontal velocity is a straightforward measurement, thus this relation allows us to evaluate expected ground motion between the Pacific coast and Mexico City from the seismic moment of subduction related earthquakes along the coast.     

2003年云南大姚6.2级和6.1级地震余震S波分裂研究

利用2003年云南大姚6.2、 6.1级两次地震的余震资料对该区进行了S波分裂研究。 结果表明, 云南大姚地区的快剪切波偏振的优势方向约为N33°W, 与该区域的区域主压应力的方向一致。 通过对比6.2级余震和6.1级余震的偏振方向和延迟时间, 可以看到6.1级地震后明显的应力释放现象, 在几次强余震前, 均观测到延迟时间呈现增加的特征, 并且几次地震在即将发生之前的短时间内还观测到延迟时间有减小的现象, 这一现象符合震前应力长时间积累和短时间应力释放的结论, 时间延迟在强震前会出现下降具有重要的地震短临预测意义。 研究认为S波分裂参数可以反映区域应力场的动态变化信息, 为应力场研究与地震预测提供有用的信息。     

The GIS and analysis of earthquake damage distribution of the 1303 Hongtong M=8 earthquake

The geography information system of the 1303 Hongton M=8 earthquake has been established. Using the spatial analysis function of GIS, the spatial distribution characteristics of damage and isoseismal of the earthquake are studies. By comparing with the standard earthquake intensity attenuation relationship, the abnormal damage distribution of the earthquake is found, so the relationship of the abnormal distribution with tectonics, site condition and basin are analyzed. In this paper, the influence on the ground motion generated by earthquake source and the underground structures near source also are studied. The influence on seismic zonation, anti-earthquake design, earthquake prediction and earthquake emergency responding produced by the abnormal density distribution are discussed. Foundation item: National important fundamental research “The Basic Research of Important Project in Damage Environment” and The important project “The Seismic Hazard Assessment Research and Anti-earthquake Structure Research” from China Earthquake Administration during the 10th Five-year Plan. Contribution No. 04FE1008, Institute of Geophysics, China Earthquake Administration.     

The GIS and analysis of earthquake damage distribution of the 1303 Hongtong M=8 earthquake

Introduction The 1303 Hongtong earthquake is an important earthquake in the eastern China. There is a lot of information in historical documents about the earthquake and many traces destroyed by the earthquake in the southern Shanxi Province. Many scholars have studied the earthquake from dif-ferent aspects, but mainly limited the definitions of the isoseismal and three factors of the earth-quake. Owing to being limited by the analysis technique, many useful damage information of the earthqu…     

Process of source dynamics of the Jingtai earthquake (M=6.2)

In this paper, the process of source dynamics of the Jingtai earthquake (M=6.2) is studied on the basis of both earth resistivity changes before the event and the spatial directivity of single-observatory magnitude measurements of the seismic network of China. The following conclusions are obtained: the NW-SE directional tension stress (or the one about in the direction) inside the source region was enhanced during the short period from three to five months before the event, and done still further about in twenty odd days before the event, so that it finally leads the fault in about the EW strike to produce dislocation in NWW-SEE direction. Finally, this dislocation pattern caused the strong ground vibration to be received at the seismic observatories which were located in the NWW and SEE directions of the epicenter, while the weak vibration to be received at the ones in NE and south directions of the epicenter. At last, understandings on the imminent earthquake prediction through earth resistivity method are set forth.