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1.
Short term spatial and temporal variations in seismicity prior to the three sequences of earthquakes of mb 5.8 of the Burma—Szechwan region are studied. Six years (1971–1976) of ISC seismicity data, as reported in the Regional Catalogue of Earthquakes, are considered. During the period, six earthquakes of body wave magnitude mb 5.8 occurred in four sequences. Of these, three sequences are preceded by swarm activity in the epicentral regions. Evison (1977b) suggested that the swarm before the sequences of large shocks is a possible long-term precursor. He derived the conclusion by analyzing earthquakes in New Zealand and California. The analysis of the seismicity data for the region under investigation supports Evison's view and suggests that a relation between swarms and sequences of large events exists. The precursory time period (i.e. the time from beginning of the swarm to the main shock) for the Szechwan earthquakes of mb = 5.9 (Feb. 6, 1973) and mb = 5.8 (May 10, 1974) and the Burma earthquake of mb = 6.2 (Aug. 12, 1976) are 305, 317 and 440 days, respectively.  相似文献   

2.
Dextral transtensional deformation is occurring along the Sierra Nevada–Great Basin boundary zone (SNGBBZ) at the eastern edge of the Sierra Nevada microplate. In the Lake Tahoe region of the SNGBBZ, transtension is partitioned spatially and temporally into domains of north–south striking normal faults and transitional domains with conjugate strike-slip faults. The normal fault domains, which have had large Holocene earthquakes but account only for background seismicity in the historic period, primarily accommodate east–west extension, while the transitional domains, which have had moderate Holocene and historic earthquakes and are currently seismically active, primarily record north–south shortening. Through partitioned slip, the upper crust in this region undergoes overall constrictional strain.Major fault zones within the Lake Tahoe basin include two normal fault zones: the northwest-trending Tahoe–Sierra frontal fault zone (TSFFZ) and the north-trending West Tahoe–Dollar Point fault zone. Most faults in these zones show eastside down displacements. Both of these fault zones show evidence of Holocene earthquakes but are relatively quiet seismically through the historic record. The northeast-trending North Tahoe–Incline Village fault zone is a major normal to sinistral-oblique fault zone. This fault zone shows evidence for large Holocene earthquakes and based on the historic record is seismically active at the microearthquake level. The zone forms the boundary between the Lake Tahoe normal fault domain to the south and the Truckee transition zone to the north.Several lines of evidence, including both geology and historic seismicity, indicate that the seismically active Truckee and Gardnerville transition zones, north and southeast of Lake Tahoe basin, respectively, are undergoing north–south shortening. In addition, the central Carson Range, a major north-trending range block between two large normal fault zones, shows internal fault patterns that suggest the range is undergoing north–south shortening in addition to east–west extension.A model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes, one mode characterized by an east–west minimum principal stress and a north–south maximum principal stress as at present. In this mode, seismicity and small-scale faulting reflecting north–south shortening concentrate in mechanically weak transition zones with primarily strike-slip faulting in relatively small-magnitude events, and domains with major normal faults are relatively quiet. A second mode occurs after sufficient north–south shortening reduces the north–south Shmax in magnitude until it is less than Sv, at which point Sv becomes the maximum principal stress. This second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains, which dominate the overall moment release in the region, producing significant east–west extension.  相似文献   

3.
The temporal and spatial distribution of the aftershock sequences of the Ruwenzori (February 5, 1994, Mb (5.8)), Masisi (April 29, 1995, Mb (5.1)) and Kalehe (October 24, 2002, Mb (5.9)) earthquakes have been studied. It has been found that most of the aftershocks of the Ruwenzori earthquake are located on the eastern flank of the main escarpment and those of the Masisi earthquake are confined to the northwest of Lake Kivu margin where earthquake occurrence of swarm-type was normally observed. The Kalehe earthquake occurred in the central part of Lake Kivu and it was the largest earthquake observed in the Lake Kivu basin since 1900. The rate of decrease in aftershock activity with the time has shown that the p-value for Ruwenzori and Masisi earthquake equals 0.6, somehow smaller than that found in other geotectonic zones where p is close to 1. The p-value of the Kalehe earthquake is a normal value equal to 1.From an area delimited by spatial distribution of aftershocks, the linear dimension of the fault was estimated. The fault area determined in this study correlates well with those of previous studies which occurred in the Western Rift Valley of Africa including the Tanganyika and Upemba Rift.  相似文献   

4.
The maximum magnitude, the activity rate, and the Gutenberg-Richterb parameter as earthquake hazard parameters, have been evaluated for Sweden. The maximum likelihood method permits the combination of historical and instrumental data. The catalog used consists of 1100 earthquakes in the time interval 1375–1989. The extreme part of the catalog contains only the strongest historical earthquakes, whereas the complete part is divided into several subcatalogs, each assumed complete above a specified threshold magnitude. The uncertainty in magnitude determination was taken into account. For southern Sweden, the calculations giveb-values of 1.04 (0.05) for the whole area south of 60° N and 0.98 (0.06) for a subregion of enhanced seismicity in the Lake Vänern area. For the whole area north of 60° N, theb-value is 1.35 (0.06) and for the seismicity zone along the Gulf of Bothnia 1.26 (0.06). The number of annually expected earthquakes with magnitude equal to or larger than 2.4 [ML(UPP) or MM(UPP)] is 1.8 for the whole southern Sweden, 1.3 for the Lake Vänern region, 3.7 for northern Sweden, and 2.4 for the region along the Gulf of Bothnia. The maximum expected regional magnitude is calculated to 4.9 (0.5) for a time span of 615 years for southern Sweden and the Lake Vänern subregion, and 4.3 (0.5) for a time span of 331 years for northern Sweden and the Gulf of Bothnia subregion. However, several historical earthquakes with magnitude above 5 in nearby areas of Norway indicate that the seismic potential may be higher.  相似文献   

5.
We summarize seismogenic structures in four regions of active convergence, each at a different stage of the collision process, with particular emphases on unusual, deep-seated seismogenic zones that were recently discovered. Along the eastern Hellenic arc near Crete, an additional seismogenic zone seems to occur below the seismogenic portion of the interplate thrust zone—a configuration found in several other oblique subduction zones that terminate laterally against collision belts. The unusual earthquakes show lateral compression, probably reflecting convergence between the subducting lithosphere's flank and the collision zone nearby. Along oblique zones of recent collision, the equivalence between space and time reveals the transition from subduction to full collision. In particular, intense seismicity beneath western Taiwan indicates that along the incipient zone of arc–continent collision, major earthquakes occur along high-angle reverse faults that reach deep into the crust or even the uppermost mantle. The seismogenic structures are likely to be reactivated normal faults on the passive continental margin of southeastern China. Since high-angle faults are ineffective in accommodating horizontal motion, it is not surprising that in the developed portion of the central Taiwan orogen (<5 Ma), seismogenic faulting occurs mainly along moderate-dipping (20–30°) thrusts. This is probably the only well-documented case of concurrent earthquake faulting on two major thrust faults, with the second seismogenic zone reaching down to depths of 30 km. Furthermore, the dual thrusts are out-of-sequence, being active in the hinterland of the deformation front. Along the mature Himalayan collision zone, where collision initiated about 50 Ma ago, current data are insufficient to distinguish whether most earthquakes occurred along multiple, out-of-sequence thrusts or along a major ramp thrust. Intriguingly, a very active seismic zone, including a large (Mw=6.7) earthquake in 1988, occurs at depths near 50 km beneath the foreland. Such a configuration may indicate the onset of a crustal nappe, involving the entire cratonic crust. In all cases of collision discussed here, the basal decollement, a key feature in the critical taper model of mountain building, appears to be aseismic. It seems that right at the onset of collision, earthquakes reflect reactivation of high-angle faults. For mature collision belts, earthquake faulting on moderate-dipping thrust accommodates a significant portion of convergence—a process involving the bulk of crust and possibly the uppermost mantle.  相似文献   

6.
Assuming a relation of “b” to stress state, the possibility of globe-wide stress variation and transmission was investigated. The NOAA earthquake data file served to determine the temporal change in “b” of log N = a − bM from 1963 to 1975.Periods of six to eight years are observed in the b-values (stress pattern) for most circum-Pacific areas (South America, Tonga, Kermadec, New Hebrides, Kamchatka and Eastern Aleutians).In the Kurils, fore- and aftershock sequences of large earthquakes seem to mask any characteristic global pattern that might exist. These sequences exhibit low b-values (high stress) through the time of foreshocks and early stages of aftershocks, followed by rapid increase in b-values (decrease in stress).Use of a worldwide earthquake data file clearly yields less resolution of the temporal “b” variation than the use of local network studies published by other authors.Incidental to the study, 1124 earthquakes of the NOAA data file yield the Ms − mb relations: Ms = 1.16mb − 0.835 for 4.5 mb 6 and: log10Ms = 0.1432mb − 0.0629 formb > 6 with correlative coefficients of 0.994 and 0.992 respectively.  相似文献   

7.
Tokutaro Hatori 《GeoJournal》1996,38(3):313-319
The regional characteristics of tsunami magnitudes in the SE Asia region are discussed in relation to earthquake magnitudes during the period from 1960 to 1994. Tsunami magnitudes on the Imamura-Iida scale are investigated by the author's method (Hatori 1979, 1986) using the data of inundation heights near the source area and tide-gauge records observed in Japan. The magnitude values of the Taiwan tsunamis showed relatively to be small. On the contrary, the magnitudes of tsunamis in the vicinities of the Philippines and Indonesia exceed more than 1–2 grade (tsunami heights: 2–5 times) compared to earthquakes with similar size on the circum-Pacific zone. The relation between tsunami magnitude, m, and earthquake magnitude, M s, is expressed as m = 2.66 M s– 17.5 for these regions. For example, the magnitudes for the 1976 Mindanao tsunami (M s= 7.8, 3702 deaths) and the 1992 Flores tsunami (M s= 7.5, 1713 deaths) were determined to be m = 3 and m = 2.5, respectively. The focal depth of tsunamigenic earthquakes is shallower thand< 36 km, and the detectively of tsunamis is small for deep earthquakes being d > 40 km. For future tsunamis, it is indispensable to take precautions against shallow earthquakes having the magnitudes M s> 6.5.  相似文献   

8.
The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes (mb > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc.Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.  相似文献   

9.
The earthquake hazard in Jordan and its vicinity is assessed on the basis of probabilistic methods. For this purpose, an updated earthquake catalog is compiled which covers the period between AD 1–1989. The earthquakes lie between latitudes 27.0°-35.5° N and longitudes 32.0°-39.0° E. Thirteen seismic zones are defined on a regional seismic and tectonic map presented for the area. Point-source and line-source models are used. The seismic hazard parameters, namely, theb-parameter (of the Gutenberg-Richter relation),m 1 (the upper bound magnitude), and 4 (the annual rate of occurrence of earthquakes with local magnitudeM L 4.0) are calculated for each zone. The results of the seismic hazard assessment are displayed as iso-acceleration contours expected to be exceeded during typical economic life times of structures, i.e. 50 and 100 years. For each model, two seismic hazard maps are derived. In order to determine the importance of the South-eastern Mediterranean zone and the north part of the Red Sea zone from a seismic hazard point of view for Jordan, one seismic hazard map which corresponds to 50 years' economic life for every model, excluding the seismicity of these zones, is derived.  相似文献   

10.
Empirical mb, Ms and Mo data are used to develop an average spectral scaling relation for plate-margin earthquakes. Using equations based upon a rectangular, bilateral dislocation model with uniform rupture velocity, the spectra give values of fault rupture length and width, static stress drop and average fault displacement as a function of mb, Ms and Mo. Compared to mid-plate earthquakes of the same seismic moment, the large average plate-margin earthquake has a bigger rupture length, rupture area and average fault displacement and a smaller rupture width and static stress drop.  相似文献   

11.
Spatial and temporal analysis of global seismological data 1964–2005 reveals a distinct teleseismic earthquake activity producing a columnar-like formation in the continental wedge between the Krakatau volcano at the surface and the subducting slab of the Indo-Australian plate. These earthquakes occur continuously in time, are in the body-wave (m b) magnitude range 4.5–5.3 and in the depth range 1–100 km. The Krakatau earthquake cluster is vertical and elongated in the azimuth N30°E, suggesting existence of a deep-rooted fault zone cutting the Sunda Strait in the SSW-NNE direction. Possible continuation of the fault zone in the SW direction was activated by an intensive 2002/2003 aftershock sequence, elongated in the azimuth of N55°E. Beneath the Krakatau earthquake cluster, an aseismic gap exists in the Wadati-Benioff zone of the subducting plate at the depths 100–120 km. We interpret this aseismic gap as a consequence of partial melting inhibiting stress concentration necessary to generate stronger earthquakes, whereas the numerous earthquakes observed in the overlying lithospheric wedge beneath the volcano probably reflect magma ascent in the recent plumbing system of the Krakatau volcano. Focal depth of the deepest events (~100 km) of the Krakatau cluster constrains the location of the primary magma generation to greater depths. The ascending magmatic fluids stress fault segments within the Sunda Strait fault zone and change their friction parameters inducing the observed tectonic earthquakes beneath Krakatau.  相似文献   

12.
We investigate spatial clustering of 2414 aftershocks along the Izmit Mw = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake Mw = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere–Düzce (KDC). We determine the earthquake frequency–magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event (Mw = 7.2) after the Izmit Mw 7.4 mainshock.  相似文献   

13.
In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km2. We located more than 2000 seismic events ranging from 0.7 to 4.5 ML by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated.Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc.At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.  相似文献   

14.
Strong tectonic earthquakes within the crust always occur on already existing faults, and they have the property of a shear rupture. Such earthquakes with surface-wave magnitudes M < 7 obviously have a geometric similarity. Because of this similarity and the validity of the Gutenberg and Richter's energy—magnitude relation, the expression M = 2 log10 L + const., with L = focal length, is valid.The expression LmaxL* for the maximum focal length, is also valid if L* is the length of the rectilinear extent of the seismic line on which the maximum earthquake occurs. The bounds of L* may be given by sharp bends and/or by traversing deep faults. Thus the maximum imaginable earthquake on a seismic line with the length L* has the magnitude Mmax = 2 log10 L* + const.For the investigated region — the Alps and adjacent areas — from the data of recent and historical strong earthquakes, it follows that Mmax = 2 log10 L* + 1.7, if L* is measured in kilometres. These limiting values lie in the centre-field of the magnitude range for maximum earthquakes, published by Shebalin in 1970. By the aid of this equation it is also possible to assess the upper limiting value of the accompanying maximum scale intensity.  相似文献   

15.
Summary The crystal structure of scotlandite —a=4.505(2),b=5.333(2),c=6.405(6) Å, =106.24(3)o; space groupP21/m; cell content 2 {PbSO3} — was determined from singlecrystal X-ray diffractometer data. Scotlandite is isotypic with molybdomenite, PbSeO3. Lead is coordinated to nine oxygen atoms with Pb-Oav=2.75 Å, and possibly further to one sulphur atom with Pb–S=3.46 Å. The average S–O distance in the pyramidal SO3 group is 1.52 Å. The structural relationships to cerussite, PbCO3, are discussed.
Die Kristallstruktur des Scotlandits, PbSO3
Zusammenfassung Die Kristallstruktur des Scotlandits —a=4,505(2),b=5,333(2),c=6,405(6) Å, =106,24(3)o; RaumgruppeP21/m; Zellinhalt 2 {PbSO3} — wurde aus Einkristall-Röntgendiffraktometerdaten bestimmt. Scotlandit ist mit Molybdomenit, PbSeO3, isotyp. Blei wird von neun Sauerstoffatomen mit Pb–Oav=2,75 Å und möglicherweise zusätzlich von einem Schwefelatom mit Pb–S=3,46 Å koordiniert. Der durchschnittliche S–O-Abstand in der pyramidalen SO3-Gruppe mißt 1,52 Å. Die strukturellen Beziehungen zu Cerussit, PbCO3, werden diskutiert.

With 2 Figures  相似文献   

16.
Past changes in phytoplankton assemblages in Lake Baikal over the last 4.5 Ma, both in population and composition, are inferred from the downcore profiles of the relatively stable chlorophyll derivatives steryl esters of pyropheophorbides a and b (steryl chlorine esters; SCEs) in the 0–200 m section of the BDP-98 drill core, supplemented by the data on biogenic silica (BSi) and total organic carbon (TOC) contents. SCEs-a and -b dominate among sedimentary chlorophyll derivatives in the BDP-98 sediments except for the upper few meters, indicating their high stability during diagenetic alteration of sediments. The depth (age) profiles of SCEs-a are consistent with BSi and TOC profiles and are interpreted as reflecting primary productivity of the lake in the past. Baikal proxies reveal close correlation with marine oxygen isotope records (MIS stratigraphy). These observations confirm that climate change in the northern hemisphere has been a primary factor controlling the total phytoplankton productivity in Lake Baikal during the last several million years.Among SCEs-a, C30 (dinostanol)-SCE-a, a marker of dinoflagellates was identified by GC–MS analysis. SCE-b, a marker of green algae, was identified by its UV–vis spectrum. The ratio of C30-SCE-a to total SCEs-a (TSCEs-a) was higher during 4.5–4.2 and 1.7–1.3 Ma, suggesting that dinoflagellates proliferated preferentially in those periods. The early Pleistocene maximum of this ratio corresponds to the broad minimum of diatom abundance previously suggested to have recorded a prolonged regional cooling. An abrupt increase in the SCE-b/TSCEs-a ratio was observed at 2.5–2.6 Ma, indicating that green algae containing chlorophyll b have proliferated in Lake Baikal during this period. This interval has also been suggested to contain evidence for a significant regional cooling based on minima of diatom abundance and BSi in sediments. The depth profile of C27Δ5 (cholesterol)-SCE-a relative to TSCEs-a showed a trend similar to that of BSi, suggesting that C27Δ5-SCE-a/TSCEs-a ratio is a potential marker of diatoms in Lake Baikal.Certain mismatches between the Lake Baikal profiles of biological indicators and the marine oxygen isotope records, as well as the slight temporal offsets between different Lake Baikal biological marker signals suggest that the regional component of climatic and/or lacustrine environmental changes also have played a role in determining the composition of the Lake Baikal Plio-Pleistocene phytoplankton assemblage.  相似文献   

17.
The focal mechanisms for 86 selected earthquakes (3.0 mb 5.5) located in central Alaska have been investigated from P-wave first motions; the data were gathered by local seismic networks. The results show a depth-dependent characteristic to the fault-plane solutions. For earthquakes having focal depths shallower than 60–70 km, the focal mechanisms indicate either strike-slip or normal faults, while for earthquakes with foci at intermediate depths the focal mechanisms correspond to thrust faults. The nature of the seismicity indicates the hinge line of the Pacific lithospheric plate under the study area to be striking N17°E from Cook Inlet towards interior Alaska. The comparison of the focal mechanisms with the seismicity shows that the strike-slip and normal faults are the predominant processes of stress release along the shallow section of the plate. The earthquakes with intermediate foci systematically occur along the inclined section of the plate. If the gently dipping nodal planes for these earthquakes are chosen as the fault planes, the focal mechanisms correspond to underthrust motions at the foci. In these, the slip vectors are oriented either to the west or north with the resultant being in the N30°W direction. The tension axes for the underthrust solutions are also found to be parallel to the local dip of the plate, indicating that the subducted plate in interior Alaska is undergoing gravitational sinking.  相似文献   

18.
Seismic hazard of Egypt   总被引：1，自引：0，他引：1  
Earthquake hazard parameters such as maximum expected magnitude,M max, annual activity rate,, andb value of the Gutenberg-Richter relation have been evaluated for two regions of Egypt. The applied maximum likelihood method permits the combination of both historical and instrumental data. The catalogue used covers earthquakes with magnitude 3 from the time interval 320–1987. The uncertainties in magnitude estimates and threshold of completeness were taken into account. The hazard parameter determination is performed for two study areas. The first area, Gulf of Suez, has higher seismicity level than the second, all other active zones in Egypt.b-values of 1.2 ± 0.1 and 1.0 ± 0.1 are obtained for the two areas, respectively. The number of annually expected earthquakes with magnitude 3 is much larger in the Gulf of Suez, 39 ± 2 than in the other areas, 6.1 ± 0.5. The maximum expected magnitude is calculated to be 6.5 ± 0.4 for a time span of 209 years for the Gulf of Suez and 6.1 ± 0.3 for a time span of 1667 years for the remaining active areas in Egypt. Respective periods of 10 and 20 years were reported for earthquakes of magnitude 5.0 for the two subareas.  相似文献   

19.
Some 455 events (mb ⩾ 4.5) in the Indo-Myanmar subduction zone are compiled using the ISC/EHB/NEIC catalogues (1964–2011) for a systematic study of seismic precursors, b-value and swarm activity. Temporal variation of b-value is studied using the maximum likelihood method beside CUSUM algorithm. The b-values vary from 0.95 to 1.4 for the deeper (depth ⩾60 km) earthquakes, and from 0.85 to 1.3 for the shallower (depth <60 km) earthquakes. A sudden drop in the b-value, from 1.4 to 0.9, prior to the occurrence of larger earthquake(s) at the deeper depth is observed. It is also noted that the CUSUM gradient reversed before the occurrence of larger earthquakes. We further examined the seismicity pattern for the period 1988–1995 within a radius of 150 km around the epicentre (latitude: 24.96°N; longitude: 95.30°E) of a deeper event M 6.3 of May 6, 1995 in this subduction zone. A precursory swarm during January 1989 to July 1992 and quiescence during August 1992 to April 1995 are identified before this large earthquake. These observations are encouraging to monitor seismic precursors for the deeper events in this subduction zone.  相似文献   

20.
Earthquake hazard maps for Syria are presented in this paper. The Peak Ground Acceleration (PGA) and the Modified Mercalli Intensity (MMI) on bedrock, both with 90% probability of not being exceeded during a life time of 50, 100 and 200 years, respectively are developed. The probabilistic PGA and MMI values are evaluated assuming linear sources (faults) as potential sources of future earthquakes. A new attenuation relationship for this region is developed. Ten distinctive faults of potential earthquakes are identified in and around Syria. The pertinent parameters of each fault, such as theb-parameter in the Gutenberg-Richter formula, the annual rate 4 and the upper bound magnitudem 1 are determined from two sets of seismic data: the historical earthquakes and the instrumentally recorded earthquake data (AD 1900–1992). The seismic hazard maps developed are intended for preliminary analysis of new designs and seismic check of existing civil engineering structures.  相似文献   

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