Observations of tectonic microdisplacements in Europe in relation to the Iran 1997 and Turkey 1999 earthquakes

Long-term repeated measurements of microdisplacements in tectonic fault structures of various parts of Central Europe and the Balkan Mountains showed that displacement trends changed significantly at several points in the period from 1997 through 2000; afterward, long-term trends were restored. This phenomenon took place in the periods of strong (M > 7) earthquakes in Iran (1997) and in the North Anatolian fault zone in Turkey (1999). Two strong Izmit earthquakes of 1999 in Turkey at distances of 600 km from the Balkan Peninsula and 1400 km from observation points in Central Europe were the main seismic events of the period studied. Apparently, the crustal deformation due to the sources of the aforementioned earthquakes reaches the central part of the European craton. Anomalous displacements in some areas occurred due to deformation propagating for great distances in the heterogeneous block medium of the West European part of the Eurasian plate. Changes in stresses can be caused by impulsive deformations of various intensities acting on some structural units (fault segments) at various distances.     

Holocene slip rate and paleoearthquake records of the Salt Lake segment of the Northern Zhongtiaoshan Fault,Shanxi Province

The Northern Zhongtiaoshan Fault is a major deep fault at the southern margin of the Yuncheng Basin. There have been few studies on the fault, and the historical earthquakes are few and weak. However, the intensity of activity on the fault should never be underestimated. Through interpretations of aerial images, topography measurements and excavation of trenches, this paper studied the fault distribution, the surface deformation and the activity of the normal fault south of Salt Lake near the city of Yuncheng. By tracing faults in the three trenches, it was found that there had been at least three large paleoseismic events, at 1–3.5, 3.6–4.4 and 7.4–8.8 ka BP. Employing 14 C dating, we determined the same gravel layers in the uplifted side and downthrown side. Making differential Global Positioning System measurements of the vertical difference and topographic profile, we obtained the mean slip rate of the Northern Zhongtiaoshan Fault since 24.7 ka BP(0.75±0.05 mm/a). Using the results of relevant studies, we calculated the possible vertical fault displacement of one earthquake(2.35 m) and obtained the recurrence interval of characteristic earthquakes as 2940–3360 a after dividing the displacement by the mean slip rate.     

Rayleigh wave phase velocity tomography and strong earthquake activity on the southeastern front of the Tibetan Plateau

To investigate the relationship between velocity structure and earthquake activity on the southeastern front of the Tibetan Plateau, we make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from the regional Yunnan Seismic Network. These data are used to compute Rayleigh wave Green's Functions by cross-correlating between two stations, extracting phase velocity dispersion curves, and finally inverting to image Rayleigh wave phase velocity with periods between 5 and 34 s by ambient noise tomography. The results show significant lateral variations in crustal and uppermost mantle structures in the studied region. Phase velocity anomalies at short periods(5–12 s) are closely related to regional tectonic features such as sediment thickness and the depth of the crystalline basement. The Sichuan-Yunnan rhombic block, enclosed by the Honghe, Xiaojiang and Jianchuan faults, emerges as a large range of low-velocity anomalies at periods of 16–26 s, that inverts to high-velocity anomalies at periods of 30–34 s. The phase velocity variation in the vicinity of the Sichuan-Yunnan rhombic block suggests that the low-velocity anomaly area in the middle-lower crust may correspond to lower crustal channelized flow of the Tibetan Plateau. The spatial distribution of strong earthquakes since 1970 reveals that the Yunnan region is inhomogeneous and shows prominent characteristics of block motion. However, earthquakes mostly occur in the upper crust, with the exception of the middle-Yunnan block where earthquakes occur at the interface zone between high and low velocity as well as in the low-velocity zones, with magnitudes being generally less than 7. There are few earthquakes of magnitude 5 at the depths of 15–30 km, where gather earthquakes of magnitude 7 or higher ones which mainly occur in the interface zone between high and low velocities with others extending to the high-velocity abnormal zone.     

Surface dynamic deformation estimates from local seismicity: the Itoiz reservoir,Spain

We analyze the ground motion time histories due to the local seismicity near the Itoiz reservoir to estimate the near-source, surface 3D displacement gradients and dynamic deformations. The seismic data were obtained by a semipermanent broadband and accelerometric network located on surface and at underground sites. The dynamic deformation field was calculated by two different methodologies: first, by the seismo-geodetic method using the data from a three-station microarray located close to the dam, and second, by single station estimates of the displacement gradients. The dynamic deformations obtained from both methods were compared and analyzed in the context of the local free-field effects. The shallow 1D velocity structure was estimated from the seismic data by modeling the body wave travel times. Time histories obtained from both methods result quite similar in the time window of body wave arrivals. The strain misfits between methods vary from 1.4 to 35.0 % and rotational misfits vary from 2.5 to 36.0 %. Amplitudes of displacement gradients vary in the range of 10^?8 to 10^?7 strains. From these results, a new scaling analysis by numerical modeling is proposed in order to estimate the peak dynamic deformations for different magnitudes, up to the expected maximum M _w in the region (M5.5). Peak dynamic deformations due to local M _w5.5 earthquakes would reach amplitudes of 10^?5 strain and 10^?3 radians at the Itoiz dam. The single station method shows to be an adequate option for the analysis of local seismicity, where few three-component stations are available. The results obtained here could help to extend the applicability of these methodologies to other sites of engineering interest.     

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10^?8 yr^?1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.     

Seismotectonic studies of the pleistoseist area of the <Emphasis Type="Italic">M</Emphasis> <Subscript> <Emphasis Type="Italic">s</Emphasis> </Subscript> = 6.9 Ilin-Tass earthquake in northeast Yakutia

The complex seismotectonic studies of the pleistoseist area of the Ilin-Tas earthquake (M_s = 6.9), one of the strongest seismic events ever recorded by the regional seismic network in northeastern Russia, are carried out. The structural tectonic position, morphotectonic features of present-day topography, active faults, and types of Cenozoic deformations of the epicentral zone are analyzed. The data of the instrumental observations are summarized, and the manifestations of the strong seismic events in the Yana–Indigirka segment of the Cherskii seismotectonic zone are considered. The explanation is suggested for the dynamical tectonic setting responsible for the Andrei-Tas seismic maximum. This setting is created by the influence of the Kolyma–Omolon indenter, which intrudes into the Cherskii seismotectonic zone from the region of the North American lithospheric plate and forms the main seismogenic structures of the Yana–Indigirka segment in the frontal zone (the Ilin-Tas anticlinorium). The highest seismic potential is noted in the Andrei- Tas block—the focus of the main tectonic impacts from the Kolyma–Omolon superterrane. The general trend of this block coincides with the orientation of the major axis of isoseismal ellipses (azimuth 50°–85°), which were determined from the observations of macroseismic effects on the ground after the Uyandina (M_s = 5.6), Andrei-Tas (M_s = 6.1), and Ilin-Tas (M_s = 6.9) earthquakes.     

Gravity and Displacement Variations in the Areas of Strong Earthquakes in the East of Russia

The modern gravimetry methods are capable of measuring gravity with an accuracy of up to 10^–10 of the normal value, which is commensurate with the accuracy of the up-to-date methods of displacement measurements by satellite geodesy. Significant changes, e.g., in the coseismic displacements of the Earth’s surface are recorded in the zones of large earthquakes. These changes should manifest themselves in the variations of gravity. Absolute measurements have been conducted by various modifications of absolute ballistic gravimeters GABL since the mid-1970s at the Klyuchi point (Novosibirsk) in the south of the West Siberian plate. Monitoring observations have been taking place in the seismically active regions since the 1990s. In this paper we consider the results of the long-term measurements of the variations in gravity and recent crustal displacements for different types of earthquakes (the zones of shear, extension, and compression). In the seismically active areas in the east of Russia, the longest annual series of absolute measurements starting from 1992 was recorded in the southeastern segment of Baikal region. In this area, the Kultuk earthquake with magnitude 6.5 occurred on August 27, 2008, at a distance of 25 km from the observation point of the Talaya seismic station. The measurements in Gornyi (Mountainous) Altai have been conducted since 2000. A strikeslip earthquake with magnitude 7.5 took place in the southern segment of the region on September 27, 2003. The effects of the catastrophic M = 9.0 Tohoku, Japan, earthquake of March 11, 2011 were identified in Primor’e in the far zone of the event. The empirical data are consistent with the results of modeling based on the seismological data. The coseismic variations in gravity are caused by the combined effect of the changes in the elevation of the observation point and crustal deformation.     

Temperature observations of Crater Lake,Mt Ruapehu,New Zealand,using Temperature Telemetry Bouys

The temperature of the Crater Lake of the active volcano Ruapelm has been recorded by Temperature Telemetry Buoys, to determine if lake temperature is correlated with volcanic activity. These buoys had to be specially designed to cope with the unfavourable environment of Crater Lake. A buoy contains a thermistor to measure the lake temperature, and a radio transmitter to transmit a short signal every few minutes, the interval between signals being a function of temperature. The temperature records obtained from these buoys show that the temperature near the lake surface can vary considerably within a few hours. Some of these variations appear to be caused by disturbances in convective heat transfer occurring in the lake. The occurrence of these short term temperature variations means that there is no simple relation between Crater Lake temperatures and the volcanic activity of Ruapehu. Some rapid increases in temperature followed volcanic earthquakes, but one of the biggest increases in temperature occurred just before a group of earthquakes upder the lake.     

Large-scale aseismic creep in the areas of the strong earthquakes revealed from the GRACE data on the time variations of the Earth’s gravity field

The refinement of the accuracy and resolution of the monthly global gravity field models from the GRACE satellite mission, together with the accumulation of more than a decade-long series of these models, enabled us to reveal the processes that occur in the regions of large (M_w≥8) earthquakes that have not been studied previously. The previous research into the time variations of the gravity field in the regions of the giant earthquakes, such as the seismic catastrophes in Sumatra (2004) and Chile (2010), and the Tohoku mega earthquake in Japan (2011), covered the coseismic gravity jump followed by the long postseismic changes reaching almost the same amplitude. The coseismic gravity jumps resulting from the lower-magnitude events are almost unnoticeable. However, we have established a long steady growth of gravity anomalies after a number of such earthquakes. For instance, in the regions of the subduction earthquakes, the growth of the positive gravity anomaly above the oceanic trench was revealed after two events with magnitudes M_w=8.5 in the Sumatra region (the Nias earthquake of March 2005 and the Bengkulu event of September 2007 near the southern termination of Sumatra Island), after the earthquake with M_w=8.5 on Hokkaido in September 2007, a doublet Simushir earthquake with the magnitudes M_w = 8.3 and 8.1 in the Kuriles in November 2006 and January 2007, and after the earthquake off the Samoa Island in September 2009 (M_w=8.1). The steady changes in the gravity field have also been recorded after the earthquake in the Sichuan region (May 2008, M_w = 8.0) and after the doublet event with magnitudes 8.6 and 8.2, which occurred in the Wharton Basin of the Indian Ocean on April 11, 2012. The detailed analysis of the growth of the positive anomaly in gravity after the Simushir earthquake of November 2006 is presented. The growth started a few months after the event synchronously with the seismic activation on the downdip extension of the coseismically ruptured fault plane zone. The data demonstrating the increasing depth of the aftershocks since March 2007 and the approximately simultaneous change in the direction and average velocity of the horizontal surface displacements at the sites of the regional GPS network indicate that this earthquake induced postseismic displacements in a huge area extending to depths below 100 km. The total displacement since the beginning of the growth of the gravity anomaly up to July 2012 is estimated at 3.0 m in the upper part of the plate’s contact and 1.5 m in the lower part up to a depth of 100 km. With allowance for the size of the region captured by the deformations, the released total energy is equivalent to the earthquake with the magnitude M_w = 8.5. In our opinion, the growth of the gravity anomaly in these regions indicates a large-scale aseismic creep over the areas much more extensive than the source zone of the earthquake. These processes have not been previously revealed by the ground-based techniques. Hence, the time series of the GRACE gravity models are an important source of the new data about the locations and evolution of the locked segments of the subduction zones and their seismic potential.     

GPS观测得到的大地震前兆地壳形变震例

综合介绍2008年汶川大地震以来,GPS观测得到的国内外10多次6—9级,不同构造、不同类型的大地震前兆地壳形变震例:2008年汶川8级大地震、2011年东日本9级巨震、2013年芦山7级,直至2020年6月墨西哥7.4级地震和7月美国阿拉斯加州以南海域7.8级地震等。利用GPS连续观测站区域参考框架水平位移时间序列和水平位移场,特别是水平位移向量时间序列的研究证明,同震水平位移是研究地震前兆形变存在的关键;利用垂直位移和水平位移向量时间序列、同震垂直位移及同震水平位移向量的分解,揭示地震弹性回跳真实方式;提出了符合GPS观测和岩石破裂试验结果的地震压-剪弹性回跳模型;根据已有震例,提出预报不同震级地震的可能性和监测临震前兆形变的GNSS站布设设想。      

Rupture model of the 2013 M_W 6.6 Lushan (China) earthquake constrained by a new GPS data set and its effects on potential seismic hazard

Vertical records are critically important when determining the rupture model of an earthquake, especially a thrust earthquake. Due to the relatively low fitness level of near-field vertical displacements, the precision of previous rupture models is relatively low, and the seismic hazard evaluated thereafter should be further updated. In this study, we applied three-component displacement records from GPS stations in and around the source region of the 2013 MW6.6 Lushan earthquake to re-investigate the rupture model.To improve the resolution of the rupture model, records from both continuous and campaign GPS stations were gathered, and secular deformations of the GPS movements were removed from the records of the campaign stations to ensure their reliability. The rupture model was derived by the steepest descent method(SDM), which is based on a layered velocity structure. The peak slip value was about 0.75 m, with a seismic moment release of 9.89 × 10~(18) N·m, which was equivalent to an M_W6.6 event. The inferred fault geometry coincided well with the aftershock distribution of the Lushan earthquake. Unlike previous rupture models, a secondary slip asperity existed at a shallow depth and even touched the ground surface. Based on the distribution of the co-seismic ruptures of the Lushan and Wenchuan earthquakes, post-seismic relaxation of the Wenchuan earthquake, and tectonic loading process, we proposed that the seismic hazard is quite high and still needs special attention in the seismic gap between the two earthquakes.     

Classification of seismic events using linear discriminant function (LDF) in the Sakarya region,Turkey

The Sakarya prefecture is an interesting area with various seismicity types. This activity comes from earthquakes occurring at the North Anatolian Fault Zone and from a few quarry blast areas in the region. These quarry blast recordings produce errors in the determination of active faults and mapping of the microearthquake activity. Therefore, to recognize the tectonic activity in the region, we need to be able to discriminate between earthquakes and quarry blasts in the catalogues. In this study, a statistical analysis method (linear discriminant function) has been applied to classify seismic events occurring in the Sakarya region. We used 110 seismic events that were recorded by Sakarya University Seismic Station between 2012 and 2014. Time and frequency variant parameters, maximum S wave and maximum P wave amplitude ratio (S/P), the spectral ratio (Sr), maximum frequency (f_max), and total signal duration of the waveform were used for discrimination analyses. The maximum frequency (f_max) versus time duration of the seismic signal gives a higher classification percentage (94%) than the other discriminants. At the end of this study, 41 out of 110 events (44%) are determined as quarry blasts, and 62 (56%) are considered as earthquakes.     

Repeated Geodetic Measurements in the West Bohemia Seismoactive Region: Period 1993 - 1996

The western part of the Bohemian Massif is characterized by repeated occurrences of intraplate earthquake swarms. To study surface deformations of this anomalous region, a network covering about 2000 square kilometres for repeated geodetic measurements was established in 1993 - 1994. The positions of the individual points of the network were carefully picked with respect to local tectonic structure and earthquake foci distribution. GPS and precise levelling measurements were performed 1-2 times a year. The GPS data were processed by Bernese GPS software. No tendency to any displacement - either horizontal or vertical - of geological blocks was derived from the geodetic data for the period 1993 - 96. Only displacements of less than 5 mm/year in average could occur in the whole region in that period; larger displacements would have been revealed by our measurements.     

Internal co-seismic deformation and curvature effect based on an analytical approach

In this study, we present a new method to compute internal co-seismic deformations of a homogeneous sphere, based on our previous approach (Dong et al. 2016). In practical numerical computations, we consider a strike-slip point source as an example, and compute the vertical co-seismic displacement on different internal spherical surfaces (including the Earth surface). Numerical results show that the internal co-seismic deformations are generally larger than that on the Earth surface; especially, the maximum co-seismic displacement appears around the seismic source. The co-seismic displacements are opposite in sign for the areas over and beneath the position of the seismic source. The results also indicate that the curvature effect of the internal deformation is pretty large, and larger than that on the Earth surface. The results indicate that the dislocation theory for a sphere is necessary in computing internal co-seismic deformations.     

Reconnaissance surveys of near-event seismic activity in the volcanoes of the Cascade Range,Oregon

Surveys of near-event seismic activity were made at two principal locations in the Cascade Range in Oregon during the summers of 1969 and 1970. A tripartite array of ultrasensitive high frequency seismometers was deployed about 7.5 km north of the Mt. Hood summit with one of the 1-km legs oriented broadside to the dormant volcano. Seismometers were emplaced over olivine andesite flows associated with the Pinnacle, one of the parasitic cones formed on the flanks of the strato-volcano. During 16 days of operation on the north slope, 53 near events were recorded, most of which originated within the upper crust and were associated with the north-south trending zone of the Cascade Range. Event magnitudes for these near events range from ?1.7 to +1.8 and determination of b-values in the Gutenberg-Richter relationship was ?0.80, indicating a probable tectonic mechanism for the shocks. During the late summer of 1970, a four-station array was operated at Crater Lake Park about 13 km south-southwest of the caldera rim. In addition, an ultraportable outlier station was operated at two locations north of the caldera that resulted from the collapse of ancient Mt. Mazama some 6,600 years ago. Only a limited number of near events with S-P intervals of 4 sec or less were detected at Crater Lake; a larger number were recorded with S-P intervals longer than 4 sec. Event epicenters for the Crater Lake area are broadly distributed in azimuth, indicating the complex structure underlying the Cascade Range in southern Oregon. Crater Lake is located astride the broad upwarp of crystalline pre-Cenozoic rocks believed to extend northeast from the Klamath Mountains to the Ochoco Uplift of central Oregon. Major regional structural trends are also shown by the north-south trending belts of the Cascade volcanoes, probably related to deep fracture zones, and by the northeast-trending shear zones that exist in the Basin and Range province to the southeast of Crater Lake. Regional gravity and aeromagnetic surveys indicate that Crater Lake lies at the intersection of these zones that probably provided the conduits for the rise of magma that ultimately led to the collapse of Mt. Mazma and the formation of Crater Lake. Epicenters for near events recorded at this juncture do not reflect these linear trends and, indeed, a generally smaller incidence of near-event activity was recorded at Crater Lake than was recorded at Mt. Hood. Magnitudes for Crater Lake events with S-P intervals of 4 sec or more range from +0.25 to +2.19, and an examination of the relationship between cumulative frequency and magnitude for these events yields a b-value in the Gutenberg-Richter relationship of ?1.16, indicating the events at Crater Lake, like those detected at Mt. Hood, are associated with tectonic rather than volcanic sources. Events for which depth determinations were made show these sources to be within the crust, occurring in the upper 10 km of the earth’s crust. The relatively low incidence of small magnitude near events within the Oregon Cascade Range shows the aseismicity of the mountain chain which is consistent with the low incidence of earthquakes of a magnitude of 4.5 or greater detected for the volcanic range. The volcanoes of the Cascade Range in Oregon are dormant, and only small numbers of shocks are now being generated, probably from isostatic adjustments within the crust. The Cascade volcanic range, which once was a seismically active island are chain associated with subduction zones off the northwestern coast of America, has moved into a passive phase in which most seismic activity in western Oregon now occurs along the ridge and fracture zones offshore and within the Willamette Downwarp west of the dormant chain.     

Seismic Empirical Relations for the Tellian Atlas, North Africa, and their Usefulness for Seismic Risk Assessment

Seismic events that occurred during the past half century in the Tellian Atlas, North Africa, are used to establish fundamental seismic empirical relations, tying earthquake magnitude to source parameters (seismic moment, fault plane area, maximal displacement along the fault, and fault plane length). Those empirical relations applied to the overall seismicity from 1716 to present are used to transform the magnitude (or intensity) versus time distribution into (1) cumulative seismic moment versus time, and (2) cumulative displacements versus time. Both of those parameters as well as the computed seismic moment rate, the strain rate along the Tellian Atlas strike, and various other geological observations are consistent with the existence, in the Tellian Atlas, of three distinct active tectonic blocks. These blocks are seismically decoupled from each other, thus allowing consideration of the seismicity as occurring in three different distinct seismotectonic blocks. The cumulative displacement versus time from 1900 to present for each of these tectonic blocks presents a remarkable pattern of recurrence time intervals and precursors associated with major earthquakes. Indeed, most major earthquakes that occurred in these three blocks might have been predicted in time. The Tellian Atlas historical seismicity from the year 881 to the present more substantially confirms these observations, in particular for the western block of the Tellian Atlas. Theoretical determination of recurrence time intervals for the Tellian Atlas large earthquakes using Molnar and Kostrov formalisms is also consistent with these observations. Substantial observations support the fact that the western and central Tellian Atlas are currently at very high seismic risk, in particular the central part. Indeed, most of the accumulated seismic energy in the central Tellian Atlas crust has yet to be released, despite the occurrence of the recent destructive May 2003 Boumerdes earthquake (M _w = 6.8). The accumulated seismic energy is equivalent to a magnitude 7.6 earthquake. In situ stress and geodetic measurements, as well as other geophysical field data measurements, are now required to practically check the validity of those observations.     

The character and extent of seismic deformation in the focal zone of gazli earthquakes of 1976 and 1984,M>7.0

With objective of investigating the peculiarities of seismic process development and seismotectonic deformation character in the focal area of the Gazli earthquakes of 1976 (7.0<M<7.3) and 1984 (M=7.2), a local seismic network was installed. For the field observation period (May to June, 1991) more than 400 events with magnitudes –0.2<M<4.5 were recorded by at least 6 stations.Isometric presentation of earthquake hypocenters distribution allows us to define the depth and dipping planes orientation of seismoactive faults of the region.The focal mechanisms of 35 earthquakes for the period 1979–1988,M>2.8, connected to a gas extraction regime period, and 75 events 1<M<4.3 for the 1991 period (gas storage regime) are used to analyze the dynamics of seismotectonic deformation processes (SDP) in this region. It has been ascertained, that the earth's crust in the Gazli region is subject to complicated deformation processes, particularly below 4 km depth. The predominant kind of deformation is compression. Vertical velocities of deformation show uplift of most of the region during the period of field work. The maximum velocity of vertical deformations for the Gazli structure isV=0.41 mm/year. The comparison of the vertical velocities' displacements due to seismic flow with recent tectonic movements of the earth's crust has revealed their direct relation and high percentage of seismic flow contribution to the tectonic movement. The results obtained testify that the active seismic processes in the Gazli region are connected not as much as the residual stress release in the focal zone of the earthquakes 05. 1976 and 1984,M>7.0 but rather with the influence of the gas reservoir exploitational regime on the rocks with different rheologic properties.     

中蒙弧地区地震活动性及其地球动力学成因研究

应用中国地震局地球物理研究所和蒙古科学院天体和地球物理研究中心合作编制的蒙古国及邻区M≥3.5的地震目录,研究了中蒙弧地区的地震活动性特征.结果表明,与中国大陆的“南北地震带”相对应,研究区地震总体分布大致以107°E为界,呈现西强东弱的特点,7级以上的强震集中于贝加尔湖、萨彦、阿尔泰以及天山地区,107°E以东,除纬度40°线附近（燕山地震带）地震较集中且强度大之外,其它地区地震稀少,强度也低.通过断层的野外调查和本地区震源机制解,进一步研究了该地区地震活动性特征的构造应力场及地球动力学成因.研究区大部份地震都是走滑型断层活动的地震.逆断层活动的地震主要分布于中国的天山地区和中蒙边境一带的阿尔泰山地区,正断层活动的地震主要分布于俄罗斯的贝加尔湖裂谷带,走滑兼倾滑断层活动的地震主要分布在研究区域的西部地区.研究区域内的大部份地区主压应力轴（P）的倾角都小于30°,为水平或近水平的构造应力场,自西向东主压应力轴的走向从近南-北方向逐渐转为北东-南西方向.断层的野外调查、震源机制解和区域构造应力场的方向表明,中蒙弧地区主要来自西南面的印度洋板块向北偏东方向的碰撞挤压,通过青藏高原传递到本区,来自东面太平洋板块的影响已较微弱,这是研究区地震活动西强东弱、8级以上强震都发生在西部的主要原因.