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In a previous study published in this journal, the authors developed a comprehensive methodology for modelling the shear wave velocity profile in crustal rock, for purposes of seismic hazard assessment. The derived shear wave velocity profile was used to estimate the amplification and attenuation mechanisms in the transmission of seismic waves. The ability to conduct seismic hazard assessments in regions of low and moderate seismicity is greatly enhanced by this new modelling approach, given that developing a local attenuation model based on curve-fitting strong motion data is generally not feasible under such conditions. This paper reports a follow-up study conducted to evaluate the significance of near-surface attenuation in bedrock (as distinct from attenuation in unconsolidated soft soil sediments). The κ parameter is used to characterize the extent of this attenuation mechanism. Empirical correlations of κ with two forms of near-surface shear wave velocity parameter in crustal rock have been developed, employing information obtained from global sources in conjunction with that from local studies. The resulting development of two simple equations to predict median values of κ as functions of readily available shear wave velocity parameters represents the key outcome of this study. Applications of the proposed empirical approaches to determine κ have been provided, taking Hong Kong and Melbourne as case studies to illustrate different aspects of the proposed methodology. Consistency between the results obtained by the two recommended approaches has thereby been demonstrated. 相似文献
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Site effects characterize the filtering mechanisms within the soil sedimentary layers overlying bedrock. In regions of high seismicity such as California where strong motion records are relatively abundant, site coefficients can be developed by regression of recorded ground shaking parameters. In regions of low‐to‐moderate seismicity or of high seismicity but with a paucity of recorded strong motion data, such empirical models cannot be obtained in the same way. This study describes the theoretical development of a simple, rational manual procedure to calculate site coefficients, based on a single period approximation (SPA), and to construct displacement response spectra (RSD) for soil sites. The proposed simplified model, which takes into account the non‐linear behaviour of soil that is dependent on the level of shaking, impedance contrast at the soil–bedrock interface and the plasticity of soil material, has been verified by comparison with results obtained from non‐linear shear wave analyses and data recorded during the 1994 Northridge earthquake. The proposed model is believed to be a convenient tool for calculating non‐linear site responses and constructing site‐specific response spectra, which has the potential of being incorporated into code provisions. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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Awad Hassoup 《Journal of Seismology》2002,6(4):459-467
The data described here are obtained from the continuous record of earthquakeactivity and lake water-level variation in the Lake Aswan area in Egypt between 1982 and 1997. The seismicity is monitored by a local telemetered seismograph network. The hypocentral parameters of earthquakes have been determined using Hypo71. The earthquake foci are distributed in two seismic zones, shallow and deep in the crust. Shallow events have focal depths of less than 10 km. Deep events extend from 10 to 30 km. The temporal sequence of seismicity iscorrelated with both the water-level variation and the average daily change of the water level in the lake Aswan. The temporal variation of the seismicityindicates that there are only six sequences of increased seismic activity during 1982–1997. The correlation between the seismic activity and the daily variation of the lake water level is poorly observed except with the June 1987 events swarm, which was accompanied by the presence of an anomaly in rate of water level decreasing. It is concluded that the increase in seismic activity in the Aswan reservoir is demonstrating an example of rapid reservoir-triggered seismicity. The deeper seismic sequence in this area, which was associated with the November 14, 1981 mainshock (MD = 5.7), and the earlier seismicity (1981–82), has been correlated with a deeper high velocity anomaly (Awad and Mizoue, 1995-b). 相似文献
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人工诱发地震现象已经有很久的历史.水库蓄水、采矿、地热开发、从地下提取液体或气体,或将液体注入地球内部都可能诱发地震.大量地震监测数据与科学分析结果显示:美国俄克拉何马州的地震剧增主要与页岩油气开采的废水回注量相关;加拿大阿尔伯塔省的地震剧增主要与页岩油气开采水力压裂的工作量相关;而荷兰罗宁根天然气田的传统天然气开采也同样诱发了较强的地震活动.在中国四川盆地的页岩油气开发区域,地震活动近几年也大幅度增强,但目前监测与科研工作较少,对某些地震成因尚有争议.目前研究诱发地震问题已成为学术界与工业界的一门专业学科.推断诱发地震,除了分析时空分布与工业活动的相关性之外,本文综述了该领域基于地震学、地质动力学、构造地质学的多种分析方法.如何在油气开采过程中减少诱发地震的灾害影响成为当前相关各界极为关注的科研问题,本文介绍了多个国家或地区建立的控制诱发地震的管理系统、基于地震大数据的诱发地震概率预测方法,以及基于地球物理与地质信息的综合诱发地震风险评估方法,并对我国控制诱发地震问题提出建设性意见. 相似文献
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Summary In this paper an attempt has been made to analyse the World shallow focus earth-quakes by the method ofGutenberg andRichter. Frequency-magnitude analysis of various earthquakes indicate that in the relation logN=a–b M, the ratio (b/a)–1 satisfies fairly well the seismicity of a region and it is felt that this ratio may be used as a measure of seismicity for a given region. 相似文献
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J. Almendros J. M. Ibez G. Alguacil J. Morales E. Del Pezzo M. La Rocca R. Ortiz V. Araa M. J. Blanco 《Journal of Volcanology and Geothermal Research》2000,103(1-4)
Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S–P time and seismogram shape has permitted to establish three groups of events: local seismicity (S–P time between 3 and 5 s), very local earthquakes (S–P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag Cross-Correlation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide–Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task. 相似文献
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Five ground-tilt stations were established on the flanks of the Soufriere of St. Vincent; two in 1977, two in 1980, and one in 1983. Four of these stations have survived; two consist of lines oriented radially to the volcano, and the other two consist of cross-shaped arrays. Collectively, this network showed that the volcano inflated gradually before the eruption of 1979 and then deflated rapidly during the eruption and for about a year after it had ended. The volcano then reinflated during much of 1981 and inflated only slightly, if at all, during the 7-year period 1982–1989. The measured amount of ground tilt from 1977 to 1989 was relatively small; the maximum recorded change of radial tilt at a station 6.5 km from the summit of the volcano totaled about 20 rad. Local seismicity correlates well with measured ground deformation: numerous earthquakes accompanied the 1979–1981 deflation/inflation cycle; relatively few earthquakes occurred during the 1982–1989 period of little or no ground deformation. In the hope that the experience we have gained might be of use to others who are considering a program of ground-tilt monitoring on volcanoes elsewhere in the humid tropics, we describe many practical aspects of our program that have evolved over the years. For example: (1) stainless steel bench marks cemented into solid bedrock appear to be stable for at least 12 years, (2) bench marks can be located in concrete-filled drums where bedrock is absent, (3) bench marks should be concealed to protect them from vandalism, (4) care must be exercised where sight lines graze the ground (<0.5 m), and (5) automatic levels are preferable because of their efficiency of operation, especially with inexperienced personnel. 相似文献
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Faulting, shallow seismicity (0–30 km), and seismic hazard of the Costa Rican Central Valley were analyzed. Faults in the study area are oriented northwest or northeast. There is an active fault system in the south flank of the Central Volcanic Ridge and another in the north flank of the Talamanca Ridge. Faults of these systems have generated 15 destructive earthquakes in the area during the last 228 years all of them shallow and their locations show one cluster near the Poas Volcano and another southward the Central Valley. These earthquakes have damaged cities of the Central Valley, two of them destroyed Cartago city and almost 1000 people were killed. Regarding recent seismicity, there are three main seismic sources at the Central Volcanic Ridge: Irazu, Bajo de la Hondura and Poas and other three in the Talamanca Ridge: Puriscal, Los Santos and Pejibaye.A seismic hazard map for the Metropolitan Area of San José has been elaborated, based on local tectonic and seismic information. The area for the hazard computation covers an area of 20×15 km2 and includes the zone where the most population and socioeconomic activities are concentrated. The computation analysis are based on areas zones and faults, each one characterized by recurrence parameters, geometry, minimum and maximum magnitude and source depth. A recent local spectral attenuation model, which includes relations for shallow crustal sources and subduction zone earthquakes, has been applied in this study. The seismic hazard results are presented in terms of contour plots of estimated peak ground acceleration (PGA) for bedrock conditions for return period of 50, 100 and 500 years. In the Central Park of San Jose City the following PGA values were found: 0.29g for 50 years, 0.36g for 100 years, and 0.53g for 500 years. 相似文献
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Seth Stein Sierd Cloetingh Douglas A. Wiens Rinus Wortel 《Earth and Planetary Science Letters》1987,82(1-2)
The possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest. This effect is suggested by three key observations: a decrease in seismicity with lithospheric age, the fact that focal mechanisms show extension perpendicular to the spreading direction, and a depth stratification of mechanism types. A difficulty with this idea is that although thermoelastic stresses should be comparable in different regions, the intraplate seismicity seems to occur in local concentrations. In particular, the ridge-parallel extensional seismicity occurs preferentially in the Central Indian Ocean region.We explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects. In particular, the dramatic concentration of extensional seismicity in the Central Indian Ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the Himalayan collision and the various subduction zones. The ridge parallel extensional stresses show a decrease with age similar to that of the seismicity. As earthquakes in this area provide a major portion of the data for both ridge-parallel extension and depth stratification, these effects may be due more to the regional stress. We thus propose that thermoelastic stresses provide a low level “background” in all plates, but that the dominant effect is that of individual plate geometry and local boundary effects. 相似文献
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Restricted parts of the Valley of Mexico have been plagued by swarms of small earthquakes during recent years. The cause of these events is puzzling, but they may relate to massive artificial changes in the hydrology of the lake-bed which underlies most of Mexico City. If the valley responds elastically, plane strain finite element calculations can be used to relate geologic and seismic structure to observed seismicity and overall stability of the valley. These calculations show that unstable zones and areas of local seismicity coincide if the triggering mechanism of the seismicity is related to variations of pore pressure and water load.In order to make such a connection it is necessary to ascribe the generating mechanism of the seismicity to the density and elastic modulus anomalies associated with the valley structure, and to suggest that the phenomenon that triggers this seismicity is related to changes in water content of the near surface formations. 相似文献
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Seismic and eruptive activities that occurred at Etna volcano during the decade 1978–1987 have been analyzed statistically. The seismic activity consists of about 7500 events. This catalog has been found complete above the magnitude threshold 2.8. On the basis of the complete catalog (1458 earthquakes), the clustering features of seismicity have been investigated. The hypothesis of a Simple Poisson process is rejected. Applying a Generalized Poisson process of the Shlien and Toksoz (1970) type, the “E” parameter of cluster size appears to be strongly dependent on the chosen time interval. The application of Gasperini and Mulargia (1989) algorithm for identifying the single earthquake sequences indicates that the whole period is composed of only three sequences. Etnean seismicity appears therefore characterized by a “diffuse” low-magnitude (less than about 3.0) earthquake occurrence. From the volcanological point of view, two time series of eruptions (flank and flank + summit) have been analyzed in order to identify different regimes in both magma output and inter-event time. No change-points are apparent in the magma output series, while both inter-event time series of flank and flank + summit eruptions are characterized by one change-point each. No evident relation between the series of eruptions and the identified earthquake sequences is apparent. 相似文献
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Hetty Triastuty Masato Iguchi Takeshi Tameguri 《Journal of Volcanology and Geothermal Research》2009,187(1-2):1-12
Shallow volcano-tectonic (VT) earthquakes recorded at the Kuchinoerabujima island volcano in southwest Japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity. From analysis of shallow VT earthquakes in 2006, two specific episodes of elevated seismicity are observed in April and November 2006. The VT earthquakes have hypocenters at depths of 0–0.4 km beneath the summit crater, and normal fault focal mechanisms with WNW–ESE extension consistent with the tensional stress field indicated by the alignment of craters and fissures. Although the hypocenters and focal mechanisms are found to be largely invariant during these episodes, the corner frequencies of the VT earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates. The corner frequencies increased to 20–25 Hz approximately one month prior to the onset of elevated seismicity, and then decreased to 10–15 Hz in the period of peak seismicity. The rupture length also decreased at the onset of seismicity, thereafter increasing as the seismicity continued. The peak seismicity in terms of the daily number of VT events was accompanied by inflation around the crater, suggestive of a pressure increase in the volcanic system. It is inferred that the increase in shallow VT seismicity and rupture length is related to the development of a fractured zone. The pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids, which is supported by an observed increase in fumarolic temperature and activity. The preceding monochromatic events are thus considered to be generated by the effect of fluid-filled cracks. The shortening of rupture length is then inferred to be related to the closing of non-fluid-filled cracks in the fracture zone under the increasing pressure field, leading to a transition from monochromatic events to low-frequency and shallow VT seismicity. 相似文献
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Over the last twenty years, Chagos Bank has a seismicity rate disproportionate to its supposed intraplate location. Earthquake relocation also shows a high seismicity rate in pre-WWSSN time (1912–1963), with seven events located off of the Central Indian Ridge, including large events in 1912 (M = 6.8) and 1944 (M = 7.2). This study uses the moment variance technique, a systematic search for the mechanism which best fits P, PP, SH, Love and Rayleigh amplitudes, to determine the focal mechanisms of two pre-WWSSN earthquakes. A test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available. Moment variance analysis shows a thrust faulting mechanism for the 1944 event, northeast of Chagos Bank near the Chagos-Laccadive ridge, and a strike-slip focal mechanism for a smaller 1957 event west of Chagos Bank. The 1944 event, one of the largest oceanic “intraplate” earthquakes known (moment 1.4 × 1027 dyne-cm), indicates that the Chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought, but rather regional tectonic deformation extending northeast of Chagos Bank and including thrust, normal and strike-slip events. This seismicity and previously studied seismicity near the Ninetyeast Ridge and Central Indian Basin suggest a broad zone of deformation stretching across the equatorial Indian Ocean. This zone contains all known magnitude seven oceanic “intraplate” earthquakes not associated with subduction zones or continental margins, suggesting that elsewhere such extensive deformation occurs only along plate boundaries. This study proposes that a slow, diffuse plate boundary extends east from the Central Indian Ridge to the Ninetyeast Ridge and north to the Sumatra Trench. A recent plate motion study confirms this boundary and suggests that it separates the Australian plate from a single Indo-Arabian plate. 相似文献
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Although Singapore is located in a low‐seismicity region, huge but infrequent Sumatran subduction earthquakes might pose structural problems to medium‐ and high‐rise buildings in the city. Based on a series of ground motion simulations of potential earthquakes that may affect Singapore, the 1833 Sumatran subduction earthquake (Mw=9.0) has been identified to be the worst‐case scenario earthquake. Bedrock motions in Singapore due to the hypothesized earthquake are simulated using an extended reflectivity method, taking into account uncertainties in source rupture process. Random rupture models, considering the uncertainties in rupture directivity, slip distribution, presence of asperities, rupture velocity and dislocation rise time, are made based on a range of seismologically possible models. The simulated bedrock motions have a very long duration of about 250 s with a predominant period between 1.8 and 2.5 s, which coincides with the natural periods of medium‐ and high‐rise buildings widely found in Singapore. The 90‐percentile horizontal peak ground acceleration is estimated to be 33 gal and the 90‐percentile horizontal spectral acceleration with 5% damping ratio is 100 gal within the predominant period range. The 90‐percentile bedrock motion would generate base shear force higher than that required by the current design code, where seismic design has yet to be considered. This has not taken into account effects of local soil response that might further amplify the bedrock motion. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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Linear Amplification of Horizontal Strong Ground Motion in Zagreb (Croatia) for a Realistic Range of Scaled Point Sources 总被引：2，自引：0，他引：2
Marijan Herak Ivan Lokmer Franco Vaccari Giuliano F. Panza 《Pure and Applied Geophysics》2004,161(5-6):1021-1040
— The linear amplification of the larger horizontal component of strong ground motion along a selected profile in the city of Zagreb is estimated by examining the synthetic waveforms corresponding to a suite of 16 realistically chosen scaled point sources. The accelerograms, computed for the average bedrock model by modal summation, are propagated through local laterally heterogeneous anelastic models by the finite-difference algorithm. The ratio of peak ground acceleration (PGA) and of the response spectra (RS), obtained by using local and bedrock models, define the PGA and RS amplification AMP(PGA) and AMP(RS), respectively. Even variations of the order of commonly observed uncertainties of only dip and rake angles of the causative fault show that both AMP(PGA) and AMP(RS) vary at some sites by more than a factor of two. It follows that, especially for strongly laterally heterogeneous structures, local effects must be determined for each of the relevant sources considering all associated uncertainties as completely as possible. Such a conclusion certainly holds for the case of the microzonation of Zagreb, where the local geology is quite complex, and the seismicity is not confined to a single seismic source zone. 相似文献
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— A seismic data file of 3,740 earthquakes from January 1987 to December 1994 has been elaborated for Morocco and the border regions, with 10 main events registering magnitudes from 5 to 5.6. Such seismicity is particularly important for Morocco as the released seismic energy constitutes a considerable part of the total energy radiated during the 20th century. Relative seismicity maps confirm the persistence of the major features of the seismicity of Morocco. An important seismic activity is observed in the Alboran region continental crust, which absorbs the maximum deformation resulting from the convergence of the African and Iberian plates. However, in the longitude window 3.5°– 6° W at depths of 25 to 50 km, a seismic gap zone seems to take place. An explanation of this phenomenon may be provided by the slab breakoff model. Even if the seismicity of Morocco remains moderate, heavy damage is observed when the magnitude of earthquakes exceeds 4.5, especially in the case of traditional buildings. 相似文献
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R. Westaway 《Pure and Applied Geophysics》2002,159(1-3):7-62
—?During the late nineteenth century, coastal parts of northern California experienced seismicity which was more intense than at any other time in its historical record and was concentrated in Spring. The seasonal character of this anomalous seismicity was statistically significant from the 1880s, if not earlier, until the time of the great 1906 earthquake. It can be explained as a result of seasonal stress transients caused by the large seasonal sediment loads which accumulated in coastal bays that overlie active strike-slip faults, as a result of hydraulic mining of gold in inland areas. Such localised loading will increase the absolute vertical stress and reduce the absolute horizontal stress, thus facilitating shear failure by unclamping vertical fault planes. However, rather than simply influencing the timing of earthquakes which would otherwise have occurred anyway sometime later, this process appears to have affected the character of the region's seismicity, causing a sequence of magnitude 5–6 earthquakes which propagated outward from these coastal depocentres. These results have three important implications. First, the proposed physical mechanism is only feasible provided the deepest continental crust in this region has a viscosity no greater than ～1017?Pa?s, giving it a Maxwell time of the order of weeks. These observations of seasonal seismicity thus suggest that such low viscosities are reasonable, a conclusion which has been difficult to establish using other evidence. They indicate that future calculations of the stress field in this region, for instance for investigating the role of earthquake triggering, should incorporate this response by lower-crustal flow, not just the immediate elastic effect. Second, the conclusion that seasonal stress transients caused by seasonal changes in surface processes can affect seismicity, has important implications for hazard assessment. It raises the possibility that surface processes may influence the seasonal timing and character of seismicity in other regions also. Finally, it indicates that the decades of intense seismicity, widely interpreted as precursory activity to the 1906 earthquake, were atypical of the longer-term record. Analogous sequences of moderate-sized events cannot necessarily be expected to provide advance warning of future large earthquakes in this region. 相似文献