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1.
Whether intraplate earthquakes have different average source properties, compared to interplate events, has been long debated. It has been proposed that intraplate events tend to rupture smaller areas with higher stress drops, compared to the average interplate earthquake. Here we estimate the rupture lengths of several Brazilian earthquakes by accurately locating their immediate aftershocks. The sparsity of stations in low-seismicity regions, such as Brazil, hinders accurate epicentral determination. We use cross-correlation of P, S and Lg waves to accurately locate the aftershocks relative to a reference event. In several cases, it was possible to infer the rupture length by the distribution of the early aftershocks; with the later aftershocks tending to span a larger area. We studied six different aftershock sequences using regional stations up to several hundred km distance. The mainshock occurs close to the foreshocks, which act as triggers to the main rupture. The immediate aftershocks tend to occur in a circle around a central (presumably stress-free) zone, which we interpret as the rupture of the mainshock. Published data from other events, based mainly on local networks, were added to provide an empirical relationship between rupture length and magnitude. These data suggest that stress-drops in Brazil vary mostly between 0.1 and 10 MPa, a similar range to many other studies worldwide. However, the mean stress drop (about 1 MPa) is smaller than the mean values of both interplate and intraplate events globally (mostly between 2 and 10 MPa). A possible dependence of stress drops with hypocentral depth may explain this difference: Brazilian intraplate earthquakes tend to be shallower than most other mid plate regions giving rise to smaller stress drops, on average. This result has important implications for seismic hazard estimation when GMPE equations from other intraplate regions are used in Brazil.  相似文献   

2.
A large earthquake, by British standards, occurred near Bishop's Castle in the Welsh Borders on 2 April 1990 at 13:46 GMT. This magnitude 5.1 ML event was felt over a wide area of Britain, from Ayrshire in the north to Cornwall in the south, Kent in the east and Dublin in the west. The epicentre was near the village of Clun, 7 km SSW of Bishop's Castle. Damage was minor and limited to the epicentral area, north to Wrexham and in particular Shrewsbury, which suffered most. Results from a macroseismic survey by BGS revealed that the maximum intensity in the epicentral area was 6 MSK. The mainshock had a focal depth of 14.3±4.7 km; however, better located aftershocks further constrained the mid-crustal seismicity to 15±0.2 km in the best cases. The marked lack of aftershocks contrasts with some previous similar magnitude events for intraplate earthquakes in Britain and throughout the world and may represent a large stress drop due to almost total relief of strain energy by the mainshock. The aftershock epicentral distribution shows a preference for an approximately N-S orientation which is consistent with one of the focal planes of the mainshock focal mechanism and suggests that this is the fault plane. Movement on this plane was predominantly strike-slip with a component of thrust and was consistent with a maximum compressive stress axis orientated NW-SE. The NE striking Welsh Borderland Fault System dominates the epicentral area; however, there is no surface fault which can clearly be related to the seismicity.  相似文献   

3.
In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ⩾ 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is MLH7.6 for a strike-slip event in the NE part of the Baikal rift system and MLH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W–E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.  相似文献   

4.
The most of shallow earthquakes are followed, just after the main shock, by increased residual seismicity known as “aftershocks” or “aftershock sequences”. Because of their disparity in time and space, aftershock sequences are more or less obvious and their productivity is spread out in time. Several studies have been regularly proposed to explain or to understand the mechanisms of the occurrence and the behaviour of these small earthquakes. In a theoretical context, many factors can induce the aftershock triggering: residual friction, subcritical crack growth, pore fluid flow etc. Just after the occurrence of the most destructive main shock of the 21 May 2003 Boumerdes (Algeria) earthquake, a wide sequence of aftershocks was recorded at different geographical locations and with various magnitudes. Based on the fact that the region of Boumerdes (40 km east of the capital Algiers) did not develop major earthquakes in the past, a geostatistical investigation of the data for this aftershock sequence is a valuable input for better seismogeological identification of this area. In the present analysis, after an overview of the geological factors in the likely occurrence of the earthquake, fundamental statistical parameters were chosen: the b value from the Gutenberg–Richter law, the p factor of the extracted respectively from the b value and the fractal variogram defined as a graphic tool to describe the continuity or the roughness of data. Jointly to the geostatistical parameters provided by the variogram like the fractal dimension. The main objective of the calculation and interpretation of these parameters is oriented towards a better understanding of the seismicity of the region of Boumerdes (Algeria) now classified as seismogenic zone.  相似文献   

5.
基于集集强震群序列地震特征的地震追踪预测   总被引：1，自引：0，他引：1  
郑魁香  赵汝仁 《地学前缘》2002,9(2):493-498
分析集集强震群前余震序列的 7年 (1993/ 0 9/ 2 1— 2 0 0 0 / 0 9/ 2 0 )中震级规模在M =3 0以上的地震目录 ,可以找到前震类型、孕震空区特征、孕震条带特征、前震丛集性活动与信号震特征、主震前平静以及余震序列的二次余震等至少 6项清楚的地震序列特征。利用已发展出的年度强震趋势分析步骤的经验 ,佐以依据地震序列特征进一步加以追踪的观念 ,以集集地震序列分析为例 ,试图将地震趋势分析由年的时间尺度 ,追踪到更短的月的时间范围 ;并尝试建立台湾地区西部地震带浅源强震的追踪分析步骤 ,并为以测震学为基础的地震预测提供逼近短临时间尺度的分析方法。  相似文献   

6.
Seismicity of Gujarat   总被引：2，自引：2，他引：0  
Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M w7.8), 1956 (M w6.0) and 2001 (M w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.  相似文献   

7.
We analyzed the seismicity of central and western France, using historical data, a compilation of all recorded earthquakes from 1962 to 2002 (4574 events, relocated), and all published focal mechanisms (119 focal solutions). The aim is to understand what are the causes of earthquakes and stress accumulation in a slowly deforming intraplate region. The distribution of earthquakes and focal mechanisms is first correlated with recognized faults, geological structures and tomographic images. Then, in order to better understand the distribution of hypocenters and seek deeper crustal sources for stress accumulation, Euler solutions are computed from the available Bouguer anomaly data. The analysis of the obtained pattern for heat flow values, provides a better understanding of the concentration of seismicity in some particular zones.Two different behaviors of this slowly deforming intraplate region are evidenced. One is linked to the presence of a hot spot under the Massif Central, the other to reactivation of the Hercynian structural heritage. Our results highlight that several possible sources for earthquake clustering can be invoked in central–western France.  相似文献   

8.
Locally recorded data for eighteen aftershocks of a magnitude(mb) 4.6 earthquake occurring near Ukhimath in the Garhwal Himalaya were analysed. A master event technique was adopted to locate seventeen individual aftershock hypocentres relative to the hypocentre of the eighteenth aftershock chosen as the master event. The aftershock epicentres define an approximately 30 km2 rupture zone commensurate with the magnitude of the earthquake. The distribution of epicentres within this zone and the limited amount of first motion data support the view that a group of parallel, sub-vertical, sinistral strike-slip faults oriented N46°, transverse to the regional NW-SE trend of the Garhwal Himalaya, was involved in this seismic episode. Since the estimated focal depth range for aftershocks of this sequence is 3–14 km, we infer that this transverse fault zone extends through the upper crustal layer to a depth of 14 km at least.  相似文献   

9.
The aftershock process induced by the Ms = 7.0 Uureg Nuur earthquake, one of the largest events in the Altai, has been studied comprehensively. As an additional experiment, a temporary local network of seismic stations was deployed in 2006 in the epicentral area of the earthquake to gain more insights into the current tectonic activity. The aftershocks of the Uureg Nuur event were restricted to small faults in the interior of fault blocks rather than those being localized along border faults. Seismic activity across the directions of large faults has apparently been generated by a fault (in the Tsagaan Shuvuut Range) reactivated during the Uureg Nuur earthquake. The aftershock process, at its final phase, involved an adjacent crust block.  相似文献   

10.
Correlations in space and time play a fundamental role in earthquake processes. One direct manifestation of the effects of correlations is the occurrence of aftershocks due to the stress transfer in the vicinity of a main shock. Less obvious and more speculative changes in correlations may occur in the background seismicity before large earthquakes. Using statistical physics it is possible to introduce a measure of spatial correlations through a correlation length. This quantity characterizes how local fluctuations can influence the occurrence of earthquakes over distances comparable with the correlation length. In this work, the physical basis of spatial correlations of earthquakes is discussed in the context of critical phenomena and the percolation problem. The method of two-point correlation function is applied to the seismicity of California. Well defined variations in time of the correlation length are found for aftershock sequences and background seismicity. The scaling properties of our obtained distributions are analyzed with respect to changes in several scaling parameters such as lower magnitude cutoff of earthquakes, the maximum time interval between earthquakes, and the spatial size of the area considered. This scaling behavior can be described in a unified manner by utilizing the multifractal fit. Utilizing the percolation approach the time evolution of clusters of earthquakes is studied with the correlation length defined in terms of the radius of gyration of clusters. This method is applied to the seismicity of California.  相似文献   

11.
We have analysed three recent earthquake sequences in the northern part of the Taupo Volcanic Zone. A 1998 sequence at Haroharo with a largest event of ML 4.8, and a 2004 sequence near Lake Rotoehu (largest event ML 5.4), had normal b-values, and displayed an aftershock decay pattern, with most of the activity within the first few days. In contrast, a 2005 sequence a few tens of kilometres away at Matata (largest event ML 4.1), had very different characteristics, with a slow development and decay, no tendency for enhanced seismicity after the larger events, and a very high b-value.The focal mechanisms of the Rotoehu and Matata events are normal, and have stress patterns consistent with the geodetically observed extension of the Taupo Volcanic Zone in a northwest–southeast direction. The extensive recent volcanism in the Okataina Volcanic Centre does not seem to have affected the stress pattern in this area.The Rotoehu sequence showed a strong resemblance, particularly in the time distribution of events, to the well-known swarm activity in the Vogtland region on the German/Czech border, in which larger events were followed by a burst of seismicity, as in a normal aftershock sequence. Some of the arguments that have been advanced to explain the Vogtland swarm as seismicity induced by fluid injection apply to Rotoehu, but there is no direct evidence of fluid involvement. The Matata sequence appears to have a continuing trigger mechanism, either a slow injection of fluid, or a slow slip event, in an environment in which opening pore spaces prevent high overpressures developing. The Matata sequence occurred close to the area of the 1987 ML 6.3 Edgecumbe Earthquake, so exhibiting two extremes of seismic temporal pattern, namely mainshock–aftershock and a swarm with many events of similar magnitude, within a small area.  相似文献   

12.
Himalayan seismicity is related to continuing northward convergence of Indian plate against Eurasian plate. Earthquakes in this region are mainly caused due to release of elastic strain energy. The Himalayan region can be attributed to highly complex geodynamic process and therefore is best suited for multifractal seismicity analysis. Fractal analysis of earthquakes (mb ?? 3.5) occurred during 1973?C2008 led to the detection of a clustering pattern in the narrow time span. This clustering was identified in three windows of 50 events each having low spatial correlation fractal dimension (D C ) value 0.836, 0.946 and 0.285 which were mainly during the span of 1998 to 2005. This clustering may be considered as an indication of a highly stressed region. The Guttenberg Richter b-value was determined for the same subsets considered for the D C estimation. Based on the fractal clustering pattern of events, we conclude that the clustered events are indicative of a highly stressed region of weak zone from where the rupture propagation eventually may nucleate as a strong earthquake. Multifractal analysis gave some understanding of the heterogeneity of fractal structure of the seismicity and existence of complex interconnected structure of the Himalayan thrust systems. The present analysis indicates an impending strong earthquake, which might help in better hazard mitigation for the Kumaun Himalaya and its surrounding region.  相似文献   

13.
Yaghmaei-Sabegh  Saman  Wang  Hongwei 《Natural Hazards》2022,111(3):2579-2599

The characteristics of aftershocks can be quite different from those of mainshocks. However, most of the works in the past have generally focused on the aftershocks of strong earthquakes, while some moderate earthquakes can cause aftershocks with magnitudes close to the mainshocks. This paper focuses on the aftershocks characteristics of the 2012 Varzaghan–Ahar Doublet event (Mw?=?6.4, 6.2), northwest of Iran. Numerous aftershocks with magnitudes 3.7–5.5 recorded by the local seismic stations provide useful data to study the spectral characteristics at various periods. The predictive capability of the 2008 Next Generation Attenuation along with two regional models has been examined by several aftershock records obtained during these events. This paper is the first attempt made to compare the predictive capability of modern models based on significant aftershock data obtained for these two moderate events. Results confirmed that the ground motion models which have been derived based on larger-magnitude events should not directly be extrapolated to predict aftershock ground motions with magnitude smaller than 5.5, especially when we use relationships that developed without considering aftershock data. The residual analysis showed that the model of Abrahamson and Silva (Earthq Spectra, 24(1): 67–97, 2008) performed the best overall predictions in this study. However, this model performs poorly in the short period of 0.2 s at distances greater than 50 km which underestimates the spectral values for this case.

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14.
The August 1, 1975 earthquake near Oroville, California, occurred along the Sierra foothills in a region characterized by occasional moderate earthquakes. Several earthquakes in the general region, including those in 1869, 1875, and 1909, appear to have had significant aftershock sequences. The general character of the aftershock sequence of the Oroville earthquake thus does not appear to be anomalous when measured against the known seismic history of this area.

Four smoked-paper micro-earthquake recorders were deployed immediately following the occurrence of the main earthquake to attempt to define the structural associations of the principal earthquake by location and analysis of aftershocks. Focal locations for 243 micro-earthquakes in the magnitude range of 1–3 were selected from the 30-day period (August 2–September 1), during which monitoring was continued. The aftershocks clearly define a planar surface striking north–south and dipping west at 62° from the surface to a depth of about 12 km. Aftershocks during the first two days of monitoring defined a surface of active faulting of approximately 100 km2. Extension of this surface both to the north and south began on August 5 at focal depths of 5–10 km, resulting in a total ruptured area of approximately 125 km2. The number of aftershocks per day decreased at the rate oft−1.1, but the decay curve was punctuated by several secondary aftershock sequences. No. direct relationship between the aftershock sequence and the presence of Oroville Reservoir was observed.  相似文献   


15.
Paper describes triggered seismicity to 200?km distance and for a decade due to the 2001 M w7.7 Bhuj earthquake. The Kachchh region is seismically one of the most active intraplate regions of the World due to the occurrence of two large earthquakes 1819 (M w7.8) and 2001 (M w7.7). Though, it has high hazard but was known to have low seismicity in view of the occurrence of fewer smaller shocks. However, the status seems to have changed after 2001. Besides the strong aftershock activity for over a decade, seismicity has spread to nearby faults in Kachchh peninsula and at several places southward for 200?km distance in Saurashtra peninsula. Beyond the rupture zone of the 2001 Bhuj earthquake, more than 40 mainshocks of M w?~?3?C5 have occurred at 20 different locations, which is unusual. The increased seismicity is inferred to be caused by stress perturbation due to the 2001 Bhuj earthquake by viscoelastic process. In Saurashtra, over and above the viscoelastic stress increase, the transient stress increase by water table rise in monsoons seems to be affecting the timing of mainshocks and associated sequences of earthquakes.  相似文献   

16.
We observe the spatial distributions of the magnitude of aftershocks following the six earthquakes of focal depth shallower than 20 km with magnitude more than 5.0 from 1983 to 1987 in Japan. The upper limit of the aftershock magnitude is examined as a function of the distance from mainshock hypocentre. The observed spatial distributions of the upper limit are bimodal, with a tendency of the upper limit to decrease as the distance from mainshock hypocentre increases. Moreover, we observe the correlations between the aftershock spatial distribution and earthquake fault length. We focus on the largest aftershocks in each of two aftershock sequences constituting the bimodal distribution. The distances of the two largest aftershocks from the mainshock hypocentre are equal to the fault lengths of shallow earthquakes in Japan and to the maximum earthquake fault lengths.  相似文献   

17.
A catalogue of aftershocks of the 2007 Nevelsk earthquake (M w = 6.2) was prepared on the basis of the data from the local network of digital seismic stations established on the southern part of Sakhalin Island. The parameters of the aftershock hypocenters were determined using the method of the seismic wave travel time inversion. The errors in the determination of the coordinates of the seismic events were analyzed. The particularities of the spatiotemporal distribution of the aftershocks in the source zone of the earthquake were established. It was shown that a strong aftershock was a subsource earthquake with its own source zone. This explains the disagreement between the energetic characteristics and the size of the aftershock zone of the Nevelsk earthquake.  相似文献   

18.
In this paper the features of seismic process in the southern depression of Lake Baikal are considered. By the data on focal mechanisms of the earthquakes of February 25, 1999 (M w = 6.0), and August 27, 2008 (M w = 6.3), as well as based on configuration of their aftershock fields, it is determined that foci of strong seismic events in southern Baikal are controlled by the greatest structural elements of sublatitudinal and submeridional strikes. It has been shown that a substantial role in the formation of focal zones is played by low-scale destruction of the Earth’s crust, revealed by geological-geophysical data and proved by clustering of seismic shocks. New data on the August 27, 2008, earthquake have proved the high level of seismic danger of this part of the Baikal Rift Zone and allowed us to determine generation conditions of strong earthquakes more precisely.  相似文献   

19.
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20.
A combination of double couple (DC) and non-double couple (non-DC) earthquakes hit Eastern Turkey, in the vicinity of Lake Van, in October–November 2011. Teleseismic waveform inversion was used to find the best fitting double couple and deviatoric moment tensors on four large and medium sized events of this sequence. The aftershocks of the Mw = 7.1, 2011/10/23:10:41 earthquake built a NE–SW aftershock zone where the Mw = 5.7, 2011/10/25 aftershock was located. The Mw = 6.0, 2011/10/23:20:45 event was located around the terminal section of the Mw = 7.1 aftershock zone which might be triggered by this event (aftershocks of this event propagated from W to E to build a W–E aftershock zone where the Mw = 5.7, 2011/11/09 event was located). For these events the calculated best fitting grid search parameters are not very different from GCMT results, but DC components, after deviatoric moment tensor inversion, represent much more difference with GCMT and grid search. The important feature of deviatoric moment tensor inversion is the existence of a notable compensated linear vector dipole (CLVD) component on the Mw = 5.7, 2011/10/25:14:55 and Mw = 5.7, 2011/11/09:19:23 aftershocks. According to the regional seismotectonics, these CLVD components could be related to crustal rheology and volcanic activities. Based on the results, the existence of a cylindrical aftershock distribution could be taken as an indication of induced seismic activity on complex-ring structures resulted from magma or water–magma injection. However, the existence of Karst like structures suggests that the CLVD components may be under the influence of high-pressure water or gas injection rather than magma.  相似文献   

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