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1.
Papadimitriou P. Voulgaris N. Kassaras I. Kaviris G. Delibasis N. Makropoulos K. 《Natural Hazards》2002,27(1-2):15-33
On 7 September 1999 at 11:56 GMT a destructive earthquake (Mw = 6.0) occurred close to Athens (Greece). The rupture process is examined using data from the Cornet local permanent network, as well as teleseismic recordings. Data recorded by a temporary seismological network were analyzed to study the aftershock sequence. The mainshock was relocated at 38.105°N, 23.565°E, about 20 km northwest of Athens. Four foreshocks were also relocated close to the mainshock. The modeling of teleseismic P and SH waves provides a well-constrained focal mechanism of the mainshock (strike = 105°, dip = 55° and rake = -80°) at a depth of 8 km and a seismic moment M0 = 1.01025 dyn·cm. The obtained fault plane solution represents normal faulting indicating an almost north-south extension. More than 3500 aftershocks were located, 1813 of which present RMS < 0.1 s and ERH, ERZ < 1.0 km. Two main clusters were distinguished, while the depth distribution is concentrated between 2 and 11 km. Over 1000 fault plane solutions of aftershocks were constrained, the majority of which also correspond to N–S extension. No surface breaks were observed but the fault plane solution of the mainshock is in agreement with the tectonics of the area and with the focal mechanisms obtained by aftershocks. The hypocenter of the mainshock is located on the deep western edge of the fault plane. The relocated epicenter coincides with the fringe that represents the highest deformation observed on the differential interferometric image. The calculated source duration is 5 sec, while the estimated dimensions of the fault are 15 km length and 10 km width. The source process is characterized by unilateral eastward rupture propagation, towards the city of Athens. An evident stop phase observed in the recordings of the Cornet local stations is interpreted as a barrier caused by the Aegaleo Mountain. 相似文献
2.
The 13 March 1992 Erzincan earthquake, M=6.8, occurred in the eastern half of the Erzincan basin. The largest aftershock took place near Pülümür on 15 March 1992. No clear surface breaks were observed, although teleseismic studies suggested that it was a strike-slip earthquake striking parallel to the North Anatolian fault, with a focus of approximately 10±2 km depth, 30 km rupture length, 95 cm of slip, and a 1.16×1026 dyn.cm seismic moment. The aftershock distribution concentrated at an area of the intersection between the North Anatolian fault and the Ovacik fault. These results indicate that the previously suggested seismic gap along the North Anatolian fault, east of Erzincan, still remains unruptured. 相似文献
3.
The East Anatolian Fault Zone is a continental transform fault accommodating westward motion of the Anatolian fault. This study aims to investigate the source properties of two moderately large and damaging earthquakes which occurred along the transform fault in the last two decades using the teleseismic broadband P and SH body waveforms. The first earthquake, the 27 June 1998 Adana earthquake, occurred beneath the Adana basin, located close to the eastern extreme of Turkey’s Mediterranean coast. The faulting associated with the 1998 Adana earthquake is unilateral to the NE and confined to depths below 15 km with a length of 30 km along the strike (53°) and a dipping of 81° SE. The fixed-rake models fit the data less well than the variable-rake model. The main slip area centered at depth of about 27 km and to the NE of the hypocenter, covering a circular area of 10 km in diameter with a peak slip of about 60 cm. The slip model yields a seismic moment of 3.5?×?1018 N-m (Mw???6.4). The second earthquake, the 1 May 2003 Bingöl earthquake, occurred along a dextral conjugate fault of the East Anatolian Fault Zone. The preferred slip model with a seismic moment of 4.1?×?1018 N-m (Mw???6.4) suggests that the rupture was unilateral toward SE and was controlled by a failure of large asperity roughly circular in shape and centered at a depth of 5 km with peak displacement of about 55 cm. Our results suggest that the 1998 Adana earthquake did not occur on the mapped Göksun Yakap?nar Fault Zone but rather on a SE dipping unmapped fault that may be a split fault of it and buried under the thick (about 6 km) deposits of the Adana basin. For the 2003 Bingöl earthquake, the final slip model requires a rupture plane having 15° different strike than the most possible mapped fault. 相似文献
4.
A 10-station portable seismograph network was deployed in northern Greece to study aftershocks of the magnitude (mb) 6.4 earthquake of June 20, 1978. The main shock occurred (in a graben) about 25 km northeast of the city of Thessaloniki and caused an east-west zone of surface rupturing 14 km long that splayed to 7 km wide at the west end. The hypocenters for 116 aftershocks in the magnitude range from 2.5 to 4.5 were determined. The epicenters for these events cover an area 30 km (east-west) by 18 km (north-south), and focal depths ranges from 4 to 12 km. Most of the aftershocks in the east half of the aftershock zone are north of the surface rupture and north of the graben. Those in the west half are located within the boundaries of the graben. Composite focalmechanism solutions for selected aftershocks indicate reactivation of geologically mapped normal faults in the area. Also, strike-slip and dip-slip faults that splay off the western end of the zone of surface ruptures may have been activated.The epicenters for four large (M 4.8) foreshocks and the main shock were relocated using the method of joint epicenter determination. Collectively, those five epicenters form an arcuate pattern convex southward, that is north of and 5 km distant from the surface rupturing. The 5-km separation, along with a focal depth of 8 km (average aftershock depth) or 16 km (NEIS main-shock depth), implies that the fault plane dips northward 58° or 73°, respectively. A preferred nodal-plane dip of 36° was determined by B.C. Papazachos and his colleagues in 1979 from a focal-mechanism solution for the main shock. If this dip is valid for the causal fault and that fault projects to the zone of surface rupturing, a decrease of dip with depth is required. 相似文献
5.
We present the estimated source parameters from SH-wave spectral modeling of selected 463 aftershocks (2002–06) of the 26 January 2001 Bhuj earthquake, the well-recorded largest continental intraplate earthquake. The estimated seismic moment (Mo), corner frequency (fc), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 1.7 to 5.6 range from 3.55×1011 to 2.84×1017 N-m, 1.3 to 11.83 Hz, 107 to 1515 m and 0.13 to 26.7 MPa, respectively, while the errors in fc and Δσ are found to be 1.1 Hz and 1.1 MPa, respectively. We also notice that the near surface attenuation factor (k) values vary from 0.02 to 0.03. Our estimates reveal that the stress drop values show more scatter (Mo0.5 to 1 is proportional to Δσ) toward the larger Mo values (≥1014.5 N-m), while they show a more systematic nature (Mo3 is proportional to Δσ) for smaller Mo values (<1014.5 N-m), which can be explained as a consequence of a nearly constant rupture radius for smaller aftershocks in the region. The large stress drops (= 10 MPa) associated with events on the north Wagad fault (at 15–30 km depth) and Gedi fault (at 3–15 km depth) can be attributed to the large stress developed at hypocentral depths as a result of high fluid pressure and the presence of mafic intrusive bodies beneath these two fault zones. 相似文献
6.
2022年1月8日青海门源MS 6.9地震发生在青藏高原东北缘的祁连山断块内部,仪器震中位于海原活动断裂系西段的冷龙岭断裂带上,是该断裂系自1920年海原8.5级大地震后再次发生M>6.5的强震。考察结果的初步总结表明,此次门源地震产生了呈左阶斜列分布、总长度近23 km的南北两条破裂,在两者之间存在长约3.2 km、宽近2 km的地表破裂空区。南支破裂(F1)出现在托来山断裂的东段,走向91°,长约2.4 km,以兼具向南逆冲的左旋走滑变形为主,最大走滑位移近0.4 m。北支主破裂(F2)出现在冷龙岭断裂的西段,总长度近20 km,以左旋走滑变形为主,呈整体微凸向北东的弧形展布,包含了走向分别为102°、109°和118°的西、中、东三段,最大走滑位移出现在中段,为3.0±0.2 m。此外,在北支主破裂中—东段的北侧新发现一条累计长度约7.6 km、以右旋正断为主的北支次级破裂(F3),累计最大走滑量约0.8 m,最大正断位移约1.5 m。综合分析认为,整个同震破裂以左旋走滑变形为主,具有双侧破裂特点,宏观震中位于北支主破裂的中段,其地表走滑位移很大可能与震源破裂深度浅有关,其中的右旋正断次级破裂可能是南侧主动盘向东运移过程中拖曳北侧块体发生差异运动所引起的特殊变形现象。印度与欧亚板块近南北向强烈碰撞挤压导致南祁连断块沿海原左旋走滑断裂系向东挤出,从而引发该断裂系中的托来山断裂与冷龙岭断裂同时发生破裂,成为导致此次强震的主要动力机制。在此大陆动力学背景下,以海原左旋走滑断裂系为主边界的祁连山断块及其周边的未来强震危险性需得到进一步重视。 相似文献
7.
Xinjian Shan Guohong Zhang Chisheng Wang Chunyan Qu Xiaogang Song Guifang Zhang Liming Guo 《Journal of Asian Earth Sciences》2011,40(4):935-942
On 21 March 2008, an Ms7.3 earthquake occurred at Yutian County, Xinjiang Uygur Autonomous Region, which is in the same year as 2008 Mw 7.9 Wenchuan earthquake. These two earthquakes both took place in the Bayar Har block, while Yutian earthquake is located in the west edge and Wenchuan earthquake is in the east. The research on source characteristics of Yutian earthquake can serve to better understand Wenchuan earthquake mechanism. We attempt to reveal the features of the causative fault of Yutian shock and its co-seismic deformation field by a sensitivity-based iterative fitting (SBIF) method. Our work is based on analysis and interpretation to high-resolution satellite (Quickbird) images as well as D-InSAR data from the satellite Envisat ASAR, in conjunction with the analysis of seismicity, focal mechanism solutions and active tectonics in this region. The result shows that the 22 km long, nearly NS trending surface rupture zone by this event lies on a range-front alluvial platform in the Qira County. It is characterized by distinct linear traces and a simple structure with 1–3 m-wide individual seams and maximum 6.5 m width of a collapse fracture. Along the rupture zone are seen many secondary fractures and fault-bounded blocks by collapse, exhibiting remarkable extension. The co-seismic deformation affected a big range 100 km × 40 km. D-InSAR analysis indicates that the interferometric deformation field is dominated by extensional faulting with a small strike-slip component. Along the causative fault, the western wall fell down and the eastern wall, that is the active unit, rose up, both with westerly vergence. The maximum subsidence displacement is ～2.6 m in the LOS, and the maximum uplift is 1.2 m. The maximum relative vertical dislocation reaches 4.1 m, which is 10 km distant from the starting rupture point to south. The 42 km-long seismogenic fault in the subsurface extends in NS direction as an arc, and it dipping angle changes from 70° near the surface to 52° at depth ～10 km. The slip on the fault plane is concentrated in the depth range 0–8 km, forming a belt of length 30 km along strike on the fault plane. There are three areas of concentrating slip, in which the largest slip is 10.5 m located at the area 10 km distant from the initial point of the rupture. 相似文献
8.
The source process and parameters for a moderate earthquake of magnitude Ml 4.1 that occurred on the Kalabsha fault at the Aswan area are analyzed. The derived focal mechanisms of this event and other two aftershocks using polarities of P, SV, and SH waves show strike-slip fault with minor vertical movement of normal type. The solutions give two nodal planes trending ENE–WSW and NNW–SSE in close agreement with the surface traces of the faults crossing the area. The movement is right lateral along the first plane while left lateral along the second one. The rupture process characterization of this event has been investigated by using the empirical Green’s function deconvolution method. By inversion only for the P wave part of the records of these three events (main and other two aftershocks), the source time function for the master events and the azimuthally variations in the (RSTF) pulse amplitude are retrieved for estimating the rupture directivities. The estimated rupture direction is combined with the P-wave focal mechanisms for the three events to identify the fault plane solution for these earthquakes. Based on the width, amplitudes, and numbers of the isolated source time functions, a complex bi-lateral rupture of the studied earthquake is delineated. The source parameters of the master event is calculated and the derived corner frequencies f o for P-wave spectra show a value of 6.6 Hz; the seismic moment (M o ) is 4.2?×?1022 Nm; the average displacement (U) is 0.5 m; fault radius (r) 40 m; the average value of the stress drops (Δσ) is 0.6 Mpa, and the moment magnitude (M w ) is 4.4. 相似文献
9.
据中国地震台网测定，2021年5月21日21时48分在云南省大理州漾濞县发生MS6.4地震，及时查明此次地震的发震构造及震源破裂特征，可为认识该区孕震条件和判别未来强震危险性提供关键依据。采用双差定位方法对漾濞地震序列进行重新定位，得到3863次地震事件的精确震源位置。结果显示:漾濞地震序列整体呈北西—南东向分布，长约25 km；整体走向135°；MS6.4主震震中位置为25.688°N，99.877°E；震源深度约9.6 km。综合地震序列深度剖面和震源机制解结果可知，发震断层应为北西走向、整体向西南方向陡倾的右旋走滑断层，倾角具有自北西向南东逐渐变缓的趋势。进一步分析地震序列的时空演化过程发现，该地震具有典型的"前震-主震-余震型"地震序列活动特点，其破裂过程主要包括3个阶段。破裂成核阶段:首先在发震断层10~12 km深度处相对脆弱部位产生小尺度破裂，之后失稳加速破裂，发生MS5.6地震；主震破裂阶段:在构造应力场持续加载和周围小尺度破裂的共同影响下，促使浅部较高强度断层闭锁区破裂，形成MS6.4主震；尾端拉张破裂阶段:主震破裂向东南扩展过程中，在东南端形成与之呈马尾状斜交的、具有正断性质的次级破裂，并产生MS5.2余震。而且此次地震还在源区北东侧触发了北北东向的左旋走滑破裂。综合分析认为，漾濞地震是兰坪-思茅地块内部北西向草坪断裂在近南北向区域应力挤压作用下发生右旋走滑运动的结果，具有明显的新生断裂特征。近年来兰坪-思茅地块内部一系列中强地震的发生表明，青藏高原物质向东南持续挤出的过程中，遇到该地块的阻挡，正在导致地块内部早期断层贯通形成新的活动断裂。因此，川滇地块西南边界带上或相邻地块内部老断层的复活和新生断裂的产生是区域中强地震危险性分析评价中值得关注的重要课题，同时建议需重视未来该区中强地震进一步向东南和向北的迁移或扩展的可能性。 相似文献
10.
This paper presents the computation of time series of the 22 July 2007 M 4.9 Kharsali earthquake. It occurred close to the Main Central Thrust (MCT) where seismic gap exists. The main shock and
17 aftershocks were located by closely spaced eleven seismograph stations in a network that involved VSAT based real-time
seismic monitoring. The largest aftershock of M 3.5 and other aftershocks occurred within a small volume of 4 × 4 km horizontal extent and between depths of 10 and 14 km.
The values of seismic moment (M
∘) determined using P-wave spectra and Brune’s model based on f
2 spectral shape ranges from 1018 to 1023 dyne-cm. The initial aftershocks occurred at greater depth compared to the later aftershocks. The time series of ground motion
have been computed for recording sites using geometric ray theory and Green’s function approach. The method for computing
time series consists in integrating the far-field contributions of Green’s function for a number of distributed point source.
The generated waveforms have been compared with the observed ones. It has been inferred that the Kharsali earthquake occurred
due to a northerly dipping low angle thrust fault at a depth of 14 km taking strike N279°E, dip 14° and rake 117°. There are
two regions on the fault surface which have larger slip amplitudes (asperities) and the rupture which has been considered
as circular in nature initiated from the asperity at a greater depth shifting gradually upwards. The two asperities cover
only 10% of the total area of the causative fault plane. However, detailed seismic imaging of these two asperities can be
corroborated with structural heterogeneities associated with causative fault to understand how seismogenesis is influenced
by strong or weak structural barriers in the region. 相似文献
11.
Surface Rupture of the 1515 Yongsheng Earthquake in Northwest Yunnan,and Its Seismogeological Implications 总被引：2，自引：0，他引：2
The 1515 M7? Yongsheng earthquake is the strongest earthquake historically in northwest Yunnan. However, its time, magnitude and the seismogenic fault have long been a topic of dispute. In order to accurately define those problems, a 1:50000 active tectonic mapping was carried out along the northern segment of the Chenghai–Binchuan fault zone. The result shows that there is an at least 25 km–long surface rupture and a series of seismic landslides distributed along the Jinguan fault and the Chenghai fault. Radiocarbon dating of the ~(14) C samples indicates that the surface rupture should be a part of the deformation zone caused by the Yongsheng earthquake in the year 1515. The distribution characteristics of this surface rupture indicate that the macroscopic epicenter of the 1515 Yongsheng earthquake may be located near Hongshiya, and the seismogenic fault of this earthquake is the Jinguan–Chenghai fault, the northern part of the Chenghai–Binchuan fault zone. Striations on the surface rupture show that the latest motion of the fault is normal faulting. The maximum co–seismic vertical displacement can be 3.8 m, according to the empirical formula for the fault displacement and moment magnitude relationship, the moment magnitude of the Yongsheng earthquake was Mw 7.3–7.4. Furthermore, combining published age data with the ~(14) C data in this paper reveals that at least four large earthquakes of similar size to the 1515 Yongsheng earthquake, have taken place across the northern segment of the Chenghai–Binchuan fault zone since 17190±50 yr. BP. The in–situ recurrence interval of Mw 7.3–7.4 characteristic earthquakes in Yongsheng along this fault zone is possibly on the order of 6 ka. 相似文献
12.
S. G. Gaonkar B. V. Srirama S. R. Samaddar D. V. Punekar Sagina Ram Reena De J. R. Kayal 《Journal of Earth System Science》2003,112(3):401-412
The Geological Survey of India (GSI) established a twelve-station temporary microearthquake (MEQ) network to monitor the aftershocks
in the epicenter area of the Bhuj earthquake (M
w7.5) of 26th January 2001. The main shock occurred in the Kutch rift basin with the epicenter to the north of Bhachao village,
at an estimated depth of 25 km (IMD). About 3000 aftershocks (M
d ≥ 1.0), were recorded by the GSI network over a monitoring period of about two and half months from 29th January 2001 to
15th April 2001. About 800 aftershocks (M
d ≥ 2.0) are located in this study. The epicenters are clustered in an area 60 km × 30 km, between 23.3‡N and 23.6‡N and 70‡E
and 70.6‡E. The main shock epicenter is also located within this zone.
Two major aftershock trends are observed; one in the NE direction and other in the NW direction. Out of these two trends,
the NE trend was more pronounced with depth. The major NE-SW trend is parallel to the Anjar-Rapar lineament. The other trend
along NW-SE is parallel to the Bhachao lineament. The aftershocks at a shallower depth (<10km) are aligned only along the
NW-SE direction. The depth slice at 10 km to 20 km shows both the NE-SW trend and the NW-SE trend. At greater depth (20 km–38
km) the NE-SW trend becomes more predominant. This observation suggests that the major rupture of the main shock took place
at a depth level more than 20 km; it propagated along the NE-SW direction, and a conjugate rupture followed the NW-SE direction.
A N-S depth section of the aftershocks shows that some aftershocks are clustered at shallower depth ≤ 10 km, but intense activity
is observed at 15–38 km depth. There is almost an aseismic layer at 10–15 km depth. The activity is sparse below 38 km. The
estimated depth of the main shock at 25 km is consistent with the cluster of maximum number of the aftershocks at 20–38 km.
A NW-SE depth section of the aftershocks, perpendicular to the major NE-SW trend, indicates a SE dipping plane and a NE-SW
depth section across the NW-SE trend shows a SW dipping plane.
The epicentral map of the stronger aftershocksM ≥ 4.0 shows a prominent NE trend. Stronger aftershocks have followed the major rupture trend of the main shock. The depth
section of these stronger aftershocks reveals that it occurred in the depth range of 20 to 38 km, and corroborates with a
south dipping seismogenic plane. 相似文献
13.
The Jiashian earthquake (ML 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a Mw 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake. 相似文献
14.
Tsuneji Rikitake 《Tectonophysics》1974,23(3):299-312
Statistics of ultimate strain of the earth's crust are obtained on the basis of levelling and triangulation data over earthquake areas. The mean value of ultimate strain e0 is obtained as 5.3 · 10?5 with a standard deviation σ amounting to 3.3 · 10?5 on the assumption that the deviation from the mean value is described by a Gaussian distribution.Assuming that crustal strain increases linearly with time t from an approximately zero value immediately after a large earthquake, which occurred at t = 0, the probability of having a crustal rupture or an earthquake occurrence during a time-interval from 0 to t can be calculated from e0 and a along with the data for strain accumulation over the area concerned as brought out by repetitions of geodetic survey.Applying the above theory to an area southwest of Tokyo, where an earthquake of magnitude 7.9 took place in 1923, the probabilities for repetition of an earthquake there are estimated as 0.2, 0.5 and 0.8 respectively for periods 1925–1980, 1925–2030, and 1925–2080.Similar studies are made for the areas off eastern Hokkaido and the Tokai district in Central Japan. No geodetic data over focal regions are available in these cases because observations are made only on land more than 100 km distant from epicentral area off the coast. In the circumstances theoretical land deformations caused by a plate subduction, which is believed to be taking place at the trench axis, are compared to the deforma tions actually detected by repeated surveys. Although the reliability of probability calculated on the basis of such processes may be substantially lower than that based on data taken in an area immediately covering a focal region, it is striking that the probabilities of reoccurrence of a large earthquake for a time-interval from the last shock to the present are so high that they exceed 0.8 ~ 0.9 for reasonable values of parameters involved. 相似文献
15.
B. K. Rastogi Santosh Kumar Sandeep K. Aggrawal Kapil Mohan Nagabhushan Rao N. Purnachandra Rao Girish Ch. Kothyari 《Natural Hazards》2013,65(2):1197-1216
The Talala (Sasangir) area in the Saurashtra region of Gujarat, western India, is experiencing tremors since 2001. The swarm type of earthquake activity in 2001, 2004, and every year from 2007 onward has occurred after the monsoon and lasted 2?C3?months each time. In 2007 some 200 shocks (largest Mw 5.0) and in 2011 about 400 shocks down to M1 are well recorded with 1?C2?km location error. The focal depths are about 2?C10?km and shocks are accompanied by blast-like subterranean sounds. The epicenter (21.09?N 70.45E, focal depth: 5?km from location program, 3?km from MTS) of the October 20, 2011 mainshock occurred about 12-km WNW of Talala town or 8-km SSW of the 2007?M w 5.0 earthquake epicenter. The epicentral trends deciphered from local earthquake data indicate two ENE trends (Narmada trend) for about 50?km length and a conjugate 15-km-long NNW trend (Aravali trend). The focal mechanisms by moment-tensor analysis of full wave forms of two 2007 events of Mw 4.8 and 5.0 and the 2011 event of Mw 5.1 indicate rupture along either of the two trends. The ENE trends follow a gravity low between the gravity highs of Girnar mounts. Seismic reflections also indicate a fault in the area named Girnar Fault. Most of Saurashtra region including the Talala area is covered by Deccan Trap Basalt forming plateaus and conical ridges. There is no major fault within Saurashtra Peninsula though it is believed to have major faults along the boundaries that are non-seismic. The intensity of the October 20, 2011 Talala earthquake is estimated to be 6.5 in MM scale while isoseismals of 6, 5, and 4 and felt distance give Mw 5.1 based on Johnston??s 1994 empirical regressions. The source parameters of the 2011 Talala earthquake are estimated using data from 14 broadband seismograph stations. Estimated seismic moment, moment magnitude, stress drop, corner frequency, and source radius are found to be 1016.6 N-m, 5.1, 1.6?MPa, 1.3?Hz, and 2,300?m, respectively. The b and p values are obtained to be low, being 0.67 and 0.71, respectively. PGA of 35?cm/sec2 is noted and the decay rate of acceleration has been estimated from strong motion data recorded at 5 stations with epicentral distances ranging from 32 to 200?km. 相似文献
16.
Earthquakes in Kenya are common along the Kenya Rift Valley because of the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy, which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the M w = 7.2 southern Sudan earthquake of 20 May 1990 and aftershocks of M w = 6.5 and 7.1 on 24 May 1990, the 1937 M s = 6.1 earthquake north of Lake Turkana close to the Kenya-Ethiopian border, and the 1913 M s = 6.0 Turkana earthquake, among others. Source parameters of the 20 May 1990 southern Sudan earthquake show that this earthquake consists of only one event on a fault having strike, dip, and rake of 315°, 84°, and ?3°. The fault plane is characterized by a left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment M o = 7.65 × 1019 Nm, and moment magnitude, M w = 7.19 (?7.2). The fault rupture started 15 s earlier and lasted for 17 s along a fault plane having dimensions of ?60 km × 40 km. The average fault dislocation is 1.1 m, and the stress drop, ?σ, is 1.63 MPa. The distribution of historical earthquakes (M w ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW–SE alignment of epicenters is characterized by earthquakes of local magnitude M l ≤ 4.0, except the 1928 Subukia earthquake (M s = 6.9) in central Kenya. This NW–SE alignment of epicenters is consistent with the trend of the Aswa-Nyangia Fault Zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that the NW–SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general. 相似文献
17.
Wentao Ma 《Natural Hazards》2012,62(1):141-148
Dongjing reservoir with storage capacity of 955 million m3 and 150 m dam height had been set up in Guizhou province, southeastern China on May in 2005. After filling with water in
August 20, 2009, the reservoir-induced earthquake in 20 km took place first in September 2009 at the 440 m water level. When
the water level changes, the number of earthquakes is increased rapidly. On January 17, 2010, the largest M 4.4 earthquake
with depth of 7 km has happen and month frequency achieved 21 events at the highest water level. M 4.4 earthquake caused rock
collapse with the disaster of killed six people and nine injure. After our investigation and study, the reason of higher epicentral
intensity of earthquake was the surface effect of near-field elastic wave transmission. The disaster of rock falls certainly
depended on the very very shallow earthquake, the height of valley and fault. Comparing as same magnitude of natural earthquake,
very shallow earthquake increased 1–2° of epicentral intensity I0, more than twice amplitude of S-wave at 200 m height of valley and the largest displacement on fault. The superposition of
three factors has increased the epicentral intensity of earthquake and directly caused rock collapse with the disaster of
killed six people and nine injure. 相似文献
18.
The Killari earthquake of September 29, 1993 (Mw=6.2) in peninsular India triggered several aftershocks that were recorded by a network of 21 stations. We computed the change in regional static stress caused by coseismic slip on the earthquake rupture and correlated it with the aftershocks with a view to constrain some of the rupture parameters of this earthquake. We evaluated the six available estimates of fault plane solutions for this earthquake and concluded that reverse slip on a 42° dipping, N112° trending fault, which extends up to the surface from a depth of 7 km, produces maximum correlation between the increased static stress and aftershock distribution. Our analysis suggests that the majority of coseismic slip occurred on the part of the rupture that lies in the depth range of 3–6.5 km. 相似文献
19.
Behnam Rahimi 《International Journal of Earth Sciences》2012,101(5):1441-1450
The Bam earthquake (2003 December 26, M W = 6.6) was one of the largest earthquakes that occurred in southeast of Iran during last century. It took place along an N–S trending right-lateral strike-slip fault, almost near the southern end of Nyband–Gowk fault. In this study, we mapped the frequency–magnitude distribution of aftershock events spatially across the Bam aftershock zone. The b-value varies between 0.6 and 1.1 across the Bam rupture zone. The overall depth distribution of b-value in Bam aftershock zone reveals two distinct increases in b-value: (1) at depths of 8–10 km and (2) shallower than 4 km beneath the Bam city. There is no correlation between high b- value anomalies found in this study and the region of largest slip, whereas the spatial correlation between high b-value anomalies and the zone of low V s and high σ (in earlier tomography study) is obvious. This correlation reveals that material properties and increasing heterogeneity are more important in controlling b-value distribution in Bam earthquake rupture zone. The high b-value anomaly near the surface of northern part of rupture zone may be related to unconsolidated and water-rich quaternary alluvial sediments and probable low-strength rocks beneath them. The high b-value anomaly at depth range 8–10 km can be correlated with fractured and fluid-filled mass, which may result from the movement of magma during Eocene volcanism in the Bam area. In this study, the induced changes in pore fluid pressure due to main shock are suggested as a mechanism for aftershock generation. 相似文献
20.
On 7 September 1999 the Athens Metropolitan area (Greece) was hit by a moderate size (Ms = 5.9) earthquake. The severely damaged area is localized in the northwestern suburbs of the city, at the foothills of Mt. Parnitha (38.1°N, 23.6°E), about 18 km from the historic centre of Athens. In this paper, we present our results on the surface expression of the seismogenic structure. Methods applied were: field observations, geological mapping, fault geometry and kinematics, evaluation of macroseismic data, interpretation of LANDSAT images, construction of a DEM and application of shading techniques. Aftershock distribution and fault plane solutions were also considered. Our results suggest that the earthquake source is located within the NW-SE trending valley bearing a few outcrops of Neogene-Quaternary sediments across the south foothills of Mt. Parnitha, never known in the past to have been activated by such strong earthquakes. The earthquake occurred along a 10 km long normal fault, striking N110°–133° and dipping 64°–85°SW, extending from the Fili Fort (4th century BC) in the NNW to the Fili town and then to Ano Liossia, to the SSE. Tensional stress field with 3 axis almost horizontal striking NNE-NE prevails in the area. The fault strike and the extensional direction (3) are compatible with the focal mechanism of the main shock. 相似文献