The 2008 West Bohemia earthquake swarm in the light of the WEBNET network

A swarm of earthquakes of magnitudes up to M _L = 3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Novy Kostel focal zone, where also all recent earthquake swarms (1985/1986, 1997, and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET (West Bohemia seismic network), which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicenters. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s². A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/1986 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms that occurred in the West Bohemia/ Vogtland region since the M _L = 5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (4 weeks) and rapidity and, consequently, the fastest seismic moment release compared to previous swarms. Up to 25,000 events in the magnitude range of 0.5 < M _L < 3.8 were detected using an automatic picker. A total of nine swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude–frequency distribution of the complete 2008 swarm activity shows a b value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Novy Kostel focal zone that was activated by the 2000 swarm (M _L ≤ 3.2) in a depth interval from 6 to 11 km and also by the 1985/1986 swarm (M _L ≤ 4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Furthermore, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously, it would represent an earthquake of M _L ~5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past.     

Refined Locations of The Swarm Earthquakes In The Nový Kostel Focal Zone and Spatial Distribution of The January 1997 Swarm In Western Bohemia,Czech Republic

In order to improve the accuracy of the spatial distribution of earthquake foci in the principal Novy Kostel focal zone, refined focal locations of about 1500 micro-earthquakes of the 1991 – 1997 period were determined using the relative Master-Event location method. To estimate the reduction in the scatter of located hypocentres, the results were compared with those obtained by routinely used FASTHYPO method and cluster analysis (the nearest-neighbour method) was applied to the located foci to evaluate the spatial distribution of the foci. Based on the results of refined location and of the cluster analysis, a concept of seismic energy release in space and time in the main focal zone was developed. Especially the January 1997 earthquake swarm was studied in detail: 946 events were located with the Master-Event location method, and the dimensions and geometry of focal clusters were determined. Type analysis was applied to waveforms to divide approximately 800 located events into eight multiplet groups to each of which a characteristic source mechanism was assigned. The spatial distribution of the foci as well as of the eight types of source mechanisms was revealed in this way and also the planes fitting the clusters of foci with two predominant source mechanisms were determined fairly well.     

Relation of surface movements in West Bohemia to earthquake swarms

GPS observations in the Western Bohemia/Vogtland earthquake swarm region revealed indications of horizontal displacements of low amplitude, and no clear long-term trend in 1993–2007. On the other hand, in 1998–2001 there was relatively significant active movement along NNE-SSW oriented line that we called the “Cheb-Kraslice GPS Boundary” (ChKB), identical with an important limitation of earthquake activity. The most impressive were dextral (right-lateral) movements in the 1998–1999 period followed by reverse sinistral (left-lateral) movements in 1999–2000 that correlate with prevailing motion defined by fault plane solutions of the Autumn 2000 earthquake swarm. Before the February 2004 micro-swarm, two points located on opposite sides of the Mariánské Lázně fault showed extension in the order of about 7 mm in the same NNE-SSW direction of ChKB. The new NOKO permanent GPS station in Novy Kostel showed the peak-to-peak vertical changes up to 10 mm before and during the February 2007 micro-swarm. Annual precise levelling campaigns in the local network around Novy Kostel revealed regular vertical displacements during the 1994, 1997 and 2000 earthquake swarms. The points around the Novy Kostel seismological station showed uplift during the active periods, including the micro-swarm February 2004. However, no such indication was observed on levelling points in the period of the February 2007 swarm. Long-term vertical displacements depend on the same direction NNE-SSW (ChKB) as the GPS displacements. Both geodetic techniques have revealed oscillating displacements, GPS horizontal, and levelling vertical, rather than any long-term trends in the study period 1993–2007. The displacements exhibited significant spatial and temporal relation to tectonic activity (earthquake swarms) including their coincidence with the seismologically determined sense of motion along the fault plane during earthquakes.     

浙江珊溪水库2014年震群活动发震机理研究

本文使用双差定位法对2014年9月12日至12月30日浙江珊溪水库发生的4184次地震进行重定位,并采用CAP方法对11次M_L≥3.0地震事件的震源机制解进行反演,讨论了震群序列的活动特征及其与断裂之间的关系,分析了水库水位与地震之间的关系.重新定位的结果显示,在空间分布上,2014年震群序列发生在2006年震群序列NW向延伸的方向上,两者形成一条线性条带,该条带平行分布于双溪—焦溪垟断裂南侧.重定位得到的震源主要在0.7—6 km深度范围内分层分布,垂直于地震条带走向的震源剖面刻画出的结构面以高角度倾向SW.震源机制解结果显示多数地震为走滑型,均存在一个与地震条带走向一致的NW向节面,呈右旋走滑错动性质.考虑到断裂的定位误差,线性分布的震群活动极有可能沿双溪—焦溪垟断裂的破裂面活动,精定位的震源位置和震源机制刻画出了该断裂的几何结构和活动性质.但由于多数地震的震源深度在6 km以上,因此震群活动不能归为双溪—焦溪垟断裂活动的结果,即双溪—焦溪垟断裂不是这两次震群的发震构造,而且仍然属于水库诱发地震,而水库地震存在激发该断裂发生构造地震的可能.水库水位上升或者下降与震群活动关系不大,震群活动有随时间进一步增强的趋势, 可能是库水沿库底断层破裂面长期渗透和扩散增加了孔隙压所致.      

Space-time distribution of earthquake swarms in the principal focal zone of the NW Bohemia/Vogtland seismoactive region: period 1985–2001

The NW Bohemia/Vogtland region situated at the western part of the Bohemian Massif is characteristic in a frequent reoccurrence of earthquake and micro-earthquake swarms. We present a comprehensive, integrated pattern of the space and time distribution of seismic energy release in the principal NK (Nový Kostel) focal zone for the period 1991–2001 and for the intensive 1985/1986 swarm. More than 3000 earthquakes, recorded by the WEBNET, the KRASLICE net and by temporary stations VAC, TIS and OLV operating during the 1985/1986 swarm, were located or re-located using the master event technique. Swarm-like sequences were identified and discriminated from solitary events by detecting local minima of the inter-event time using a standard short-time/long-time average (STA/LTA) detection algorithm. Most of the seismic energy in the NK zone was released during the two intensive 1985/1986 and 2000 swarms and in the course of the weaker January 1997 swarm. Further 27 swarm-like sequences (micro-swarms) and many solitary micro-earthquakes (background activity) were identified in the NK zone for the period 1991–2001 by the inter-event time analysis. Relative location revealed a pronounced planar character of the NK focal zone. Most of the events, including those of the intensive 1985/1986 and 2000 swarms, were located at the main focal plane (MFP) striking 169° N and dipping 80° westward at depths between 6 and 11 km. A singularity was the January 1997 swarm together with a micro-swarm that were both located across the MFP. The position and geometry of the MFP match quite well the Nový Kostel-Počátky-Zwota tectonic line. The space distribution patterns of larger events and of micro-swarms at the MFP differ: larger events predominantly grouped in planar clusters while the micro-swarms lined up along two parallel seismogenic lines. The temporal behaviour was examined from two aspects: (a) migration and (b) recurrence of the seismic activity. It was found that (a) the seismic activity in the time span 1991–2001 migrated in an area of about 12×4 km and (b) several segments of the MFP were liable to reactivation. The activity before, during and after the 2000 swarm took place in different parts of the MFP.     

Detection of earthquake clusters on the basis of waveform similarity: An application in the monferrato region (Piedmont, Italy)

In August 2000 and July 2001 two seismic sequences, characterized by mainshocks with Ml (local magnitude) respectively 5.1 and 4.8, occurred in the Monferrato region (Italy). The regional seismic network of North-western Italy (RSNI) recorded more than 250 foreshocks and aftershocks. The routine locations, obtained from the Hypoellipse code, show a seismic activity concentrated in a circular area, of a radius of about 15 km, located near Acqui Terme, and randomly distributed in depth. Location errors, due to an uneven azimuthal station distribution of the regional seismic network, prevent recognition of the geometry of the active zone. Waveform analysis revealed the presence of several multiplets. In order to discriminate and successively relocate them, an automatic cross-correlation procedure was applied. Normalized cross-correlation matrixes, for the RSNI stations which recorded almost 90% of considered events, on the basis of a signal to noise ratio analysis, were computed using only S wave time windows. The use of a relocation procedure, based on the double-difference method which incorporates ordinary absolute travel-time measurements and/or cross-correlation differential travel-times, allowed us to overcome the constraints of the uneven distribution of stations giving a quite different frame of seismicity. The improved locations showed that the seismic activity is mainly arranged along a NE-SW oriented volume, at a depth range of 8–20 km, involving the basement crystalline units. This orientation is confirmed by the analysis of the focal mechanisms: most focal solutions show a strike slip component with one of the nodal planes consistent with the main orientation of the seismic events.     

Vibration Effect of Near Earthquakes at Different Depths in a Shallow Medieval Mine

The shallow medieval Jeroným Mine is located at a distance of about 25 km southeast of the Nový Kostel focal zone where the most intensive seismic activity in West Bohemia (Czech Republic) has been documented. Permanent seismological monitoring has been carried out since 2004 in this mine. During the 2011 and 2014 seismic swarms, more than 1000 triggered records comprising almost 1500 earthquakes were recorded at the permanent station in the mine. Three short-term seismological experiments were accomplished during these swarms. Several temporary seismic stations were simultaneously placed in different parts of underground spaces which enabled comparison of vibration effect caused by near earthquakes in different parts of the mine. Although the depth of the lowest parts of mine is only about 60 m, a vibration effect generated by earthquakes from the Nový Kostel focal zone is not the same for the whole underground complex.     

由现今小震资料研究琼北地区区域应力场和发震构造

利用2000年以来海南地震台网记录的琼北地区的波形资料,采用双差法对103个地震进行重新定位,采用振幅比方法测定震源机制.在此基础上,反演了琼北地区的区域应力场,由震源位置拟合出两个震源断层面,并且计算在区域应力场作用下的滑动方式.对照历史大震的等震线,WNW走向的震源断层位于极震区中部稍偏东的地方,与等震线长轴方向相同,表明该震源断层是1605年琼山大震的发震构造;高倾角震源断层的北东盘向东南斜下方滑动,对于该盘NNW象限产生强烈拉张,以致出现罕见的陆陷成海现象.另外一条NS向震源断层恰好位于第四系火山岩和第四系盆地交界处,是区域升降运动最为强烈的地方,正断层类型的震源断层东盘下降,与沉积盆地一致.研究表明由现今小震反演的两条震源断层分别与历史大震及构造运动有关,而与浅表断裂并不重合,存在深浅构造不协调的现象.     

鲜水河断裂带南北构造差异性的地球物理分析

对鲜水河断裂带重磁异常进行向上延拓,通过计算观测面高度异常与延拓后异常之间的相关系数得出最佳向上延拓高度,该延拓高度所对应的延拓结果即为研究区构造背景产生的异常值。向上延拓结果显示布格重力异常值沿鲜水河断裂带自北西向南东逐渐增大,反映出下地壳底边界沿该方向呈升高趋势,可能由青藏高原地壳软弱物质 “东向逃逸” 所致。化极后ΔT磁异常延拓结果表明鲜水河断裂带南东段的康定—石棉以东为强磁性刚性基底。以鲜水河断裂带为分界的不同地块之间基底岩石及地层物性的不同是断裂带南北磁性差异的主导原因。对鲜水河断裂带两侧各约50 km范围内地震的震源深度进行统计,经投影至剖面及线性拟合求出鲜水河断裂带的三维几何形状。结果表明鲜水河断裂带总体倾向南西,倾角近乎直立,范围约为57°—88°。      

Slug Flow--A Possible Explanation for Hydrogeochemical Earthquake Precursors at Bad Brambach, Germany

--Extensive hydrogeological, geochemical, radiometric and hydro-isotope investigations in the Vogtland region, Germany, since 1989 suggest a fluid connection between a special epicentral area (focal depth: 3-15 km; M_L < 5) and a mineral spring at Bad Brambach. Twenty-six hydrogeochemical anomalies are related to earthquakes/swarmquakes of that epicentral area near Novy Kostel (CZ) during the last 9 years. The anomalies were originated by a slug-flow process in the fluid-filled fracture system near the surface. The gas component of the observed fluid (99 vol. % CO₂) is of upper mantle/crustal origin. The fluid transport pathway to the surface is the seismically active Mariánské Lázné fault zone. The interpretation suggests an influence of the fluid system due to earthquake preparation processes.     

Local seismic observations at stations Nový Kostel and Skalná, and their iterpretation: Period 1986–93

Summary Systematic observations of natural seismic activity in the West-Bohemian earthquake-swarm region began at two autonomous seismological stations of the Geophysical Institute, the digital station Novy Kostel (NKC) and the analogue station Skalná (SKC), in May 1986 and December 1989, respectively. This paper presents the station data of NKC and SKC, the method of processing the records, and the database structure. It also includes the interpretation of observations made at these stations in the period 1986–1993. It was found that the seismic activity in the West Bohemian region, in the Vogtland (Saxony) and NE Bavaria was continuous. Between two strong earthquake swarms, the energy in this area was released in the form of a large number of micro-earthquakes of a markedly swarm-like nature, mostly concentrated in six focal regions.     

Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)

We summarise the results of seismological studies related to triggering mechanisms, driving forces and source processes of the West Bohemia/Vogtland earthquake swarms with the aim to disclose the role of crustal fluids in the preparation, triggering and governing of the swarms. We present basic characteristics distinguishing earthquake swarms from tectonic mainshock-aftershock sequences and introduce existing earthquakes swarm models. From the statistical characteristics and time-space distribution of the foci we infer that self-organization is a peculiarity of West Bohemia/Vogtland swarms. We discuss possible causes of the foci migration in these swarms from the viewpoint of co-seismic and/or post-seismic stress changes and diffusion of the pressurized fluids, and we summarize hitherto published models of triggering the 2000-swarm. Attention is paid to the source mechanisms, particularly to their non-shear components. We consider possible causes of different source mechanisms of the 1997-and 2000-swarms and infer that pure shear processes controlled solely by the regional tectonic stress prevail in them, and that additional tensile forces may appear only at unfavourably oriented faults. On data from the fluid injection experiment at the HDR site Soultz (Alsace), we also show that earthquakes triggered by fluids can represent purely shear processes. Thus we conclude that increased pore pressure of crustal fluids in the region plays a key role in bringing the faults from the subcritical to critical state. The swarm activities are mainly driven by stress changes due to co-seismic and post-seismic slips, which considerably depend on the frictional conditions at the fault; crustal fluids keep the fault in a critical state. An open question still remains the cause of the repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones in a wider area of West Bohemia/Vogtland. The analysis of the space-time relations of seismicity in the area between 1991 and 2007 revealed that during a significant part of this time span the seismicity was switching among distant focal zones. This indicates a common triggering force which might be the effect of an increase of crustal-fluid pore-pressure affecting a wider epicentral region.     

Geophysical evidence of the Eastern Marginal Fault of the Cheb Basin (Czech Republic)

The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Oh?e/Eger Graben and the NNW-SSE trending Mariánské Lázně Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of M_L 3+ earthquake swarms. The Eastern Marginal Fault of the Cheb Basin (northern segment of the Mariánské Lázně Fault) separates the basin sediments and underlying granites in the SW from the Kru?né Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth. The survey was conducted at the Kopanina site near the Nový Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction. We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60° towards SW. At greater depths, the slope turns to subvertical with dip angle of about 80°. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys.     

采用2021年青海玛多地震序列震源机制节面聚类确定发震断裂几何形态

地震断裂带形状是活动构造和地球动力学的重要资料。2021年发生的玛多地震序列提供了丰富的震源机制资料,为统计震源机制节面并估计玛多地震发震断层面形状提供了很好机会。文章对地震的震源机制资料进行基于密度的聚类来确定断层的几何形态。首先对收集到的玛多地震序列的震源机制解进行中心解求解，从而获得更为精准的数据,然后对其进行DBSCAN方法的聚类分析，得到断层的走向为113.5°，倾角为88.2°,通过震源机制反演应力场,并将应力场投影到断层上，得到断裂带的相对剪应力和相对正应力分别为0.84和-0.79,剪应力强度较大,滑动角为-0.72°。结果表明玛多地震是发震断裂受NE-SW的挤压和NW-SE的拉张,形成了较大的剪切力,从而促使近东西的江错断裂发生左旋走滑错动所致。     

High precision locations of multiplets on south-eastern flank of Mt. Etna (Italy): reconstruction of fault plane geometry

On 9 January 2001, a seismic swarm, located on the south-eastern flank of Mt. Etna and with nearly identical waveforms, caused some damage to Zafferana Etnea village, 3 km from the epicentral area.An analysis of the seismicity occurring in the last 8 years in this area has revealed other earthquakes with the same characteristics; some pre-empted and followed (up to a few months) the 2001 January swarm, others were recorded more than five years beforehand.Using similarity of waveforms, these earthquakes were classified into three families.The use of a multiplet-technique has allowed to obtain the spatial distribution of the events with higher precision (mean error of 10-20 m) with respect to traditional localization techniques.Mt. Etna earthquakes relocation clearly describes the geometry of the seismogenic tectonic structure; the hypocenters lie on a NE-SW oriented plane that is coincident with one of the focal planes obtained by first-arrival polarities. This alignment is also coherent with one of the main regional tectonic trends cutting the Mt. Etna area, and can be interpreted as a right-lateral strike seismic source on the south-eastern flank of Mt. Etna, distant from eruptive centres, which repeats from time to time and is able to produce strong energy releases.     

Study of Seismic Clusters at Bahía de Banderas Region,Mexico

The coast in the state of Jalisco and south of Nayarit is located within a region of high seismic potential, increasing population, and tourism development. This motivated Civil Defense authorities of Jalisco and the Universidad de Guadalajara to launch in the year 2000 the assessment of the seismic risk of the region. This work focuses in the seismicity study of the area of Bahía de Banderas and northern coast of Jalisco, which is actually a seismic gap. We perform an analysis of available seismograms to characterize active crustal structures, their relationship to surface morphology, and possible extent of these structures into the bay shallow parts. The data consist of waveforms recorded during 2003 when the seismograph network spanned the region. Our method is based on the identification of seismic clusters or families using cross-correlation of waveforms, earthquake relocation and modeling of fault planes. From an initial data set of 404 located earthquakes, 96 earthquakes with M_L < 3.6 are related to 17 potentially active continental structures. We present fault plane model for 11 structures. A subgroup of 7 structures is aligned parallel to the Middle America Trench, as a possible consequence of oblique subduction. The foci of the earthquakes were grouped into clusters corresponding to fault dimensions of hundred of meters, may be considered as asperities or barriers in tectonic structures with lengths between 10 and 30 km. These structures could generate shallow earthquakes with magnitudes between 5.0 and 6.0 and represent an additional seismic threat to the region.     

Focal depths of earthquakes in the Baikal region: A review

Conditions and specific features in the determination of the focal depths of the earthquakes in the Baikal region are considered. Particular attention is given to the procedure of relocation of the hypocenters in accordance with the programs based on the method of minimization of rms residuals in the seismic wave traveltimes, which is widely used at present. The advantages and shortcomings of this approach are demonstrated. The available determinations of the source depths are reviewed, and examples for the interpretation of the results are presented. The averaged depth distribution of the hypocenters shows that the strongest seismic activity is observed in the depth range 10–20 km. The bottom of the seismically active layer corresponding to the level, above which 90% of sources are located, lies at a depth of 25 km. The trend of the deepening of seismic sources is observed at the northeast flank of the rift zone, where seismic activity involves the lower part of the crust.     

The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan MS8.0 earthquake

The seismicity of Longmenshan fault zone and its vicinities before the 12 May 2008 Wenchuan M_S8.0 earthquake is studied. Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined. Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state. No obvious phenomena of seismic activity intensifying appeared. According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE. The two hypocenter fault planes are NE-striking and NW-striking. The plane of NE direction is among N50°?70°E, the dip angles of fault planes are 60°?70° and it is very steep. The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip. The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock. The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock. Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake. The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging. The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake.     

Seismic doublets and multiplets at Polish coal and copper mines

The following criteria for selection of doublets at Polish coal mines were accepted: the difference in magnitude (based on seismic moment) of two events not larger than 0.15, the distance between their hypocenters not greater than 150 m, and the time interval between their occurrence not longer than 10 days. Similarly, the criteria for seismic events at copper mines are: the difference in magnitude not exceeding 0.15, the distance not greater than 200 m, and the time interval not longer than 20 days. Seismic events from the Wujek and Ziemowit coal mines that occurred between 1993 and 1995, and seismic events from the Polkowice copper mine that occurred between 1994 and 1996 and from the Rudna copper mine that occurred between 1994 and 2004 were considered. Their source parameters and focal mechanisms were known in most cases from previous studies. Altogether 108 seismic pairs from coal mines and 118 pairs from copper mines were found, forming doublets, triplets and quadruplets, within the magnitude range from 0.7 to 3.5. The distance and time intervals between two events forming pairs are not dependent on magnitude of these events. The focal mechanism of seismic events forming pairs is similar in over 60 percent of pairs at coal mines and in about one third of pairs at copper mines. Spatial distributions of doublets in particular sections of coal and copper mines display dominant linear trends, characteristic for a given area, which are often in conformity with the direction of nodal planes determined by fault plane solution of one or both the events forming a doublet. In such cases, the rupture plane can be discriminated among the nodal planes.     

1970年以来鲜水河断裂带地震活动特征与2014年康定M_S6.3地震

本文基于1970年以来的地震目录及四川地区4.0级以上地震的震源机制解资料,分析鲜水河断裂带分段(炉霍段、道符段、康定段、石棉段)的地震活动特征及研究区现代构造应力场,结合深部速度结构,探讨鲜水河断裂带上地震活动频度与龙门山断裂带地震活动的关系及康定地段6.3级地震的孕震环境。结果发现:(1)鲜水河断裂带北段和南段地震活动性存在差异,炉霍段和道孚段的地震活动频度1981年前要高于2000年后,康定段和石棉段的地震活动频度2000年以后高于1981年前;(2)分析地壳P波速度结构发现康定震区西侧川滇块体表现出低速异常,东侧表现出高速异常;(3)对构造应力场的分析结果表明龙门山断裂带主要以NW-SE向挤压为主,鲜水河断裂带构造应力场以NWW-SEE向为主。综合鲜水河断裂带应力场特征、深部速度结构、断层间的相互作用等信息推断,康定M6.3地震的发生与该地区应力积累及深部孕震环境相关,同时由于龙门山断裂带地震活动性影响,导致鲜水河断裂带康定段的能量释放。