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1.
A destructive earthquake of magnitude Mw = 6.8 hit the region of Boumerdes and Algiers (Algeria) on May 21, 2003. This is among the strongest seismic events of the mediterranean region and the most important event in the capital Algiers since 1716. It caused a widespread damage in the epicentral region, claimed 2271 human lives, injured 10000, about 20000 housing units affected and left about 160000 homeless. The main shock was felt about 250 km far from the epicenter and triggered sea waves of 1–3 m in amplitude in Balearic islands (Spain). Based on field observations and press report an intensity IX (MSK scale) is attributed to the epicentral area. The main shock was followed by many aftershocks among them several are of magnitude greater than 5.0, which added panic to inhabitants. The main shock triggered ground deformation, particularly liquefaction whose features are in different forms and sizes and caused damage and collapse of roads. The focal mechanism determined by worldwide institutions yield a pure reverse faulting with a compressional axis striking NE-SW. The epicenter is located offshore about 7 km from the Boumerdes-Dellys coast. Field observations show 0.7 m of coseismic uplift of shoreline between Boudouaou and Dellys. This uplift is about a half of the extracted coseismic slip from the seismic moment. On the other hand there is no clear surface break onshore, confirming hence, that the causative active fault is offshore. However, the rupture may propagate onshore to the SE near the Boudouaou region where ground cracks showing reverse faulting are observed a long a corridor of about 1 km wide. These fissures may correspond to a diffuse coseismic deformation.  相似文献   

2.
The Pollino Range is the southernmost segment of the Southern Apennines at the boundary with the Calabrian Arc. While several strong earthquakes (magnitude 6.5–7.0) have occurred in nearby regions, the Pollino area has no known historical record of seismic events of magnitude > 5. We carried out an aerial photograph interpretation and a field survey of the Pollino fault (the major Quaternary normal fault of the area) in order to characterize geologically the seismic potential of this structure. We dug two sets of trenches across fault scarps within the apecies of latest Pleistocene to Holocene alluvial fans at the Masseria Quercia Marina (MQM) and Grotta Carbone (GC) sites, in the central segment of the southern Pollino Range front. At both sites we identified two surface faulting events affecting the alluvial fan deposits and two overlying colluvial units of historical age. The penultimate event produced a vertical offset of 80–90 cm at GC and 50–60 cm at MQM; while the last event produced a vertical offset of 40–50 cm at GC and few centimeters of offset at MQM. Detailed geomorphological field observations suggest that the two historical earthquakes reactivated the entire length of the Masseria Marzano-Civita segment of the Pollino fault (rupture length about 18 km). For events in this range of rupture length and vertical displacement, comparison with surface faulting earthquakes in the Apennines (and abroad) indicates a magnitude of 6.5–7.0. Therefore, the maximum potential earthquake and the seismic hazard of the Pollino area are significantly larger than that suggested by the available historical seismic catalogue.  相似文献   

3.
This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 (M = 5.9), the Gubbio 1984 (M = 5.2) and the Colfiorito 1997 (Mmax = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Colfiorito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north–west (e.g. Gubbio, 6–7 km) to the south–east (e.g. Norcia, 11–12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well.  相似文献   

4.
The December 26, 2003 Mw 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. QuickBird panchromatic and multispectral satellite imagery with 61 cm and 2.4 m ground resolution, respectively provide new insights into the surface rupturing process associated with this earthquake. The results indicate that this earthquake produced a 2–5 km-wide surface rupture zone with a complex geometric pattern. A 10-km-long surface rupture zone developed along the pre-existing Bam fault trace. Two additional surface rupture zones, each 2–5 km long, are oblique to the pre-existing Bam fault in angles of 20–35°. An analysis of geometric and geomorphic features also shows that movement on the Bam fault is mainly right-lateral motion with some compressional component. This interpretation is consistent with field investigations, analysis of aftershocks as well as teleseismic inversion. Therefore, we suggest that the 2003 Bam earthquake occurred on the Bam fault, and that the surface ruptures oblique to the Bam fault are caused by secondary faulting such as synthetic shears (Reidel shears). Our fault model for the Bam earthquake provides a new tectonic scenario for explaining complex surface deformations associated with the Bam earthquake.  相似文献   

5.
Paleoseismology, the study of past earthquakes based on their geological record in the stratigraphy and landscape, is a successful newly developing field of research. The application of fault trench studies in volcanic environments is one of the youngest branches of paleoseismology. In this paper, we present the results of the first exploratory trenches excavated at Mt. Etna in Sicily, the largest European volcano. Modern surface faulting at Etna is a very well known feature, which poses significant hazard to the local community, both in terms of ground displacement of essential lifelines and ground shacking from frequent damaging earthquakes. However, while the geomorphology and the seismicity of the active fault in the Etna region consistently show very high rates of tectonic activity, the Holocene cumulative throw and slip-rates, along with the nature (coseismic vs. creeping fault slip), dimension and timing of the displacement events, are still poorly constrained. For this purpose, we selected as a sample area the Moscarello fault, one of the most outstanding segment of the Timpe system of active normal faults in the volcano’s lower eastern flank. Displaced landforms and volcanic units at the Fondo Macchia basin, in the central sector of this fault, indicate some hundreds of meters of vertical offset in the last ca. 80 kyr, with a long-term slip-rate substantially higher than 1.5–2.0 mm/yr. According to the historical sources and instrumental observations, the Moscarello fault ruptured four times in the last 150 years during shallow (H < 5 km) and moderate magnitude (M < 4.8) earthquakes. These events were associated with severe damage in a narrow epicentral area (macroseismic intensities up to the IX–X grade of the MSK scale) and extensive surface faulting (end-to-end rupture length up to 6 km, vertical offsets up to 90 cm). This clearly indicates very high modern rates of deformation along this fault. We conducted trench investigations at the Fondo Macchia site, in a point where eyewitnesses observed ca. 20 cm of coseismic vertical displacement after the April 21, 1971, Ms=3.7, earthquake. The excavated sections provided direct stratigraphic evidence for a vertical slip-rate of 1.4–2.7 mm/yr in the last ca. 6 kyr. This should be regarded as a minimum slip-rate for the central section of the fault. We explored a single scarp at a single site, while we know from recent historical observations that several parallel scarps may rupture coseismically at Fondo Macchia. Thus, the relevant deformation rate documented for the modern period might be likely extended back in the past to a time-span of some thousands of years at least. As expected, for such a volcanic environment, the activity rates of the Moscarello fault are also significantly higher than for the Apennines normal faults, typically showing slip-rates lower than 1 mm/yr. The agriculturally reworked trench hangingwall stratigraphy did not allow to recognize individual displacement events. Nevertheless, the sedimentary structures observed in the trench footwall strongly suggest that, as for the last 150–200 years of detailed historical record, fault behavior at Fondo Macchia is governed by coseismic surface displacement rather than fault creep. This research confirms that paleoseismology techniques can be effectively applied also in active volcanic environments, typically characterized by rheology and, consequently, seismicity and fault dynamics very different from those of other tectonic environments in which paleoseismology has been firstly developed and is today extensively applied.  相似文献   

6.
A long sequence of earthquakes causing few casualties and considerable damage in a wide zone struck Central Italy starting on September 26, 1997. Theearthquakes are characterized by normal faulting mechanism, with a NE-SW(anti-Apenninic direction) tension axis. In this paper we analyze the accelerometric recordings collected by the accelerograph stations belonging to the National Accelerograph Network. About 10 stations were triggered by the mainshocks of the sequence. In particular, a small size foreshock and the two mainshocks that occurred on September,26 (00:33(GMT) MW = 5.7 and 09:40 MW = 6.0) have been recorded by two digital 3-C accelerometers located at near source distances (within 30 km from the faults). These records are relevant to investigate the detail of therupture kinematics, due to the close epicentral distance and azimuthallocation relative to the fault orientation and geometry. Using a trial and error approach we modeled the source mechanism through the fit of the arrival times, the apparent source time duration, the main polarization features and the entire waveforms of the recorded signals, in order to get some insight on the rupture evolution, the location of the fracture origin point and the fault geometry. Based on this fault kinematic model, inferences on fault slip distribution are obtained by modeling the S acceleration waveform, comparing the ray theory synthetics with 1–5 Hz band filtered ground velocity records.The final model shows that the seismic ruptures occurred along two adjacent,sub-parallel, low angle dipping normal faults. Ruptures bothnucleated from the fault bottom and propagated up-dip, showing differentrupture velocity and length. The presence of a transfer zone (barrier)can be suggested by the mainshocks rupture evolution. This transfer zonehas probably controlled the amplitude increase of local stressreleased by the first rupture at its NW edge which triggered about 9 hourslater the second rupture. The inferred model was used to compute the predictedground acceleration in the near source range, using a hybridstatistical-deterministic approach.A similar trial and error method has been also applied to the October 14, 199715:23 earthquake (MW = 5.6). The inferred kinematic model indicates a rupture nucleating from the faultbottom and propagating up-dip, toward the SE direction. Thus the three mainshocks ruptured distinct fault segments, adjacent and slightly offsetfrom one to another.  相似文献   

7.
The recent 1997 Umbria-Marche, Central Italy, earthquake sequence allowed us to model recorded ground motions using a method developed by Beresnev and Atkinson [Bull Seism Soc Am 87 (1997) 67–84; Seism Res Lett, 69 (1998) 27–32; Bull Seism Soc Am 88 (1998) 1392–1401]. The method generalizes the stochastic ground-motion simulation technique, developed for point sources, to the case of finite faults. It subdivides the fault plane into subfaults and assumes each subfault to be a point source with a ω2 spectrum. Geometric spreading and regional anelastic attenuation are included in the model. The data include horizontal acceleration recordings from the SSN and ENEL databases of the 1997 Umbria-Marche events on 26 September, at 00:33 GMT, with Mw=5.7, and at 09:40 GMT, with Mw=6.0; and on 14 October at 15:23 GMT, with Mw=5.6. The strong motion simulations are performed using model parameters based on the results of previous studies, and adjusting the subfault size to calibrate the simulation model against recorded ground motions. Local site response is considered to account for observed amplification effects at specific recording sites (e.g. Nocera Umbra). A good agreement is found between the simulated response spectra and the recorded data, concluding that this method reproduces the salient ground-motion characteristics at different distances and azimuths.  相似文献   

8.
This article presents equations for the estimation of vertical strong ground motions caused by shallow crustal earthquakes with magnitudes Mw 5 and distance to the surface projection of the fault less than 100km. These equations were derived by weighted regression analysis, used to remove observed magnitude-dependent variance, on a set of 595 strong-motion records recorded in Europe and the Middle East. Coefficients are included to model the effect of local site effects and faulting mechanism on the observed ground motions. The equations include coefficients to model the observed magnitude-dependent decay rate. The main findings of this study are that: short-period ground motions from small and moderate magnitude earthquakes decay faster than the commonly assumed 1/r, the average effect of differing faulting mechanisms is similar to that observed for horizontal motions and is not large and corresponds to factors between 0.7 (normal and odd) and 1.4 (thrust) with respect to strike-slip motions and that the average long-period amplification caused by soft soil deposits is about 2.1 over those on rock sites.  相似文献   

9.
We calculated focal mechanisms for 30 of the strongest events (1.5 M L 3.3) in distinct subregions of Vogtland/Western Bohemia between 1990 and 1998. Our investigations are concerned with events of the swarms near Bad Elster (1991), Haingrün (1991), Nový Kostel (1994 and 1997) and Zwickau (1998), two events from a group of earthquakes near Klingenthal (1997) and eight single events. Seismograms were provided by the digital station networks of the Geophysical Observatory of the University of Munich, the Technical University of Freiberg, the Academy of Sciences of the Czech Republic in Prague, the Masaryk University in Brno and some nearby stations of the German Regional Seismic Network (GRSN). To calculate focal mechanisms two inversion methods were applied. The inferred focal mechanisms do not show a simple, uniform pattern of seismic dislocation. All possible dislocation types – strike-slip, normal and thrust faulting - are represented. The prevailing mechanisms are normal and strike-slip faulting. Considerable differences in the fault plane solutions are noted for the individual subregions as well as in some cases among the individual events of a single swarm. For the Nový Kostel area we succeeded to resolve a change in the orientations of the nodal planes for the two successive swarms of December 1994 and January 1997. Besides this we also observe a change in the mechanisms, namely from strike-slip and normal faulting (December 1994) to strike-slip and thrust faulting (January 1997). Based on the inferred focal mechanisms the stress field was estimated. It turned out, that the dominating stress field in the region Vogtland/Western Bohemia does not substantially differ from the known stress field of West and Central Europe, being characterized by a SE-NW direction of the maximum compressive horizontal stress. We conclude that the seismicity in the Vogtland/Western Bohemia region is not predominantly caused by an independent local stress field, but rather controlled by the dominating stress regime in Central Europe.  相似文献   

10.
Strong site effects were observed during the two M W 5.7 and M W 6.0 main shocks of the Colfiorito seismic crisis which occured on September 26, 1997 in Umbria-Marche (Central Italy).The most obvious indications of these effects are the dramatic differences in damage shown by buildings of similar construction in neighboring villages.Such observations were specifically made in the Verchiano valley in the fault area, 15 km south of Colfiorito where the Verchiano village and the Colle and Camino hamlets were heavily damaged (MCS intensity IX-X) since the first main shock of 1997/09/26,while in contrast, the Curasci village located 2 km eastwards remains almost intact.In order to study the anomalous ground motion amplifications in this area, an array of 11, 3-components seismo- and accelero-meters was set up during the 1997/10/20-24 period, extending from the western side of the valley, up to the top of Mount San Salvatore, going accross the Colle and Curasci hamlets.During the experiment, 67 aftershocks enlightened the valley from the Colfiorito (10 km north) and the Sellano (6 km south) active swarms.Seismic refraction experiments were conducted at the same time in the 500 m wide, 1500 m long Verchiano valley in order to determine the thickness and main characteristics of the alluvial infilling.The main results are: (i) compared to the valley side ground motion, and for all the events, recordings in the central part of the valley (piana di Verchiano) show relative amplification of 10 with a clear lengthening of the seismogram duration by a factor of 2 – (ii) broad band relative amplification of 6–8 is also clearly identified at the top of the Mount San Salvatore overhanging the valley – (iii) any of the site effect measurements done explains by itself the strongly contrasted damage observed at Colle and Curasci: i.e. the modification of the near-field radiation pattern by interaction with the free heterogeneous surface may have induced local shadow zones that saved Curasci.  相似文献   

11.
Maps of faults (geologically defined source zones) may portray seismic hazards in a wide range of completeness depending on which types of faults are shown. Three fault terms — active, capable, and potential — are used in a variety of ways for different reasons or applications. Nevertheless, to be useful for seismic-hazards analysis, fault maps should encompass a time interval that includes several earthquake cycles. For example, if the common recurrence in an area is 20,000–50,000 years, then maps should include faults that are 50,000–100,000 years old (two to five typical earthquake cycles), thus allowing for temporal variability in slip rate and recurrence intervals. Conversely, in more active areas such as plate boundaries, maps showing faults that are <10,000 years old should include those with at least 2 to as many as 20 paleoearthquakes. For the International Lithosphere Programs’ Task Group II-2 Project on Major Active Faults of the World our maps and database will show five age categories and four slip rate categories that allow one to select differing time spans and activity rates for seismic-hazard analysis depending on tectonic regime. The maps are accompanied by a database that describes evidence for Quaternary faulting, geomorphic expression, and paleoseismic parameters (slip rate, recurrence interval and time of most recent surface faulting). These maps and databases provide an inventory of faults that would be defined as active, capable, and potentially active for seismic-hazard assessments.  相似文献   

12.
We have relocated 259 inland earthquakes in northern Egypt using the double-difference hypocenter technique. Among this dataset we are able to determine source mechanisms of 200 events using P-wave polarities and amplitude ratios as well. The studied earthquakes have been recorded by the Egyptian National Seismological Network from October 1997 to December 2006 with local magnitude (ML) varies between 1.5 and 5.0. Three earthquake dislocations have been defined namely: Dahshour, southeast Beni-Suef, and Cairo-Suez district. Earthquake activities tend to occur in clusters along the first dislocation (Dahshour) however, relatively scattered along the second (southeast Beni-Suef) and the third (Cairo-Suez district) dislocations. At Dahshour dislocation three distinct clusters have been distinguished. Source mechanism solutions of Dahshour earthquakes displayed normal faulting with a strike-slip component to strike-slip faulting with a minor normal dip-slip component. Most of earthquake focal mechanism orientations are varying from NE-SW to NW-SE. The fault plane solutions of Beni-Suef earthquakes represented normal faulting with a strike-slip component. If the NNW-SSE striking plane has been chosen to be the actual fault plane, some solutions would indicated normal faulting with a sinistral strike-slip motion and other reflect normal faulting with a dextral strike-slip component. The fault plane solutions of Cairo-Suez district earthquakes are compatible with E-W to ENE-WSW striking normal fault with a dextral strike-slip motion.  相似文献   

13.
In this study we analyse coseismic GPS displacements and DInSAR data to constrain a dislocation model for the three largest earthquakes of the 1997 Umbria-Marche seismic sequence. The first two events, which occurred on September 26 at 00:33 GMT (Mw 5.7) and 09:40 GMT (Mw 6.0) respectively, are investigated using both GPS displacements and DInSAR interferograms. We discuss and compare the results of previous studies which separately modeled a smaller subset of geodetic data. We provide a dislocation model for these two earthquakes which fits well both GPS and DInSAR data and agrees with the results of seismological and geological investigations. The first event consists of a unilateral rupture towards the southeast with a uniform dislocation. The strike, rake and dip angles are those resulting from the CMT solution. The second event consists of an unilateral rupture towards the northwest and a variable slip distribution on the fault plane. The strike and the rake are consistent with the CMT solution, but the dip angle has been slightly modified to improve the simultaneous fit of GPS and DInSAR data. While the second rupture (09:40 GMT) arrived very close to the surface, the fit to geodetic data shows that the first rupture (00:33 GMT) is deeper (2 km), despite the more evident surface geological effects. The analysis of new SAR interferograms allows the identification of a 5–6 cm additional displacement caused by the October 3 (Mw 5.2) and 6 (Mw 5.4) seismic events.We use data from a new DInSAR interferogram to model the displacement field of the Sellano earthquake of October 14, 1997. For this event significant GPS measurements were not available. We tested two different fault plane geometries: a blind, planar fault (top depth = 2.4 km), and a curved (listric) fault reaching the surface. The two models provide a generally similar fit to the data, and show that most of the slip was released at depths greater than 2.4 km along a gently dipping (40°–45°) fault surface. They also show that a unilateral rupture does not allow fitting the interferometric fringes since there is evident surface deformation to the northwest of the hypocenter. Moreover, we suggest that the concentration of high residuals in the southern part of our uniform slip model may in fact indicate a certain slip variability in this area.We conclude that, despite the moderate magnitudes and the lack of significant surface faulting, the space geodetic data allowed to constrain dislocation models giving new insights in the rupture process of the three largest events of the sequence.  相似文献   

14.
王辉  谷一山 《地震学报》1991,13(3):344-353
1988年11月6日,在云南省西南部的澜沧-耿马断裂带上发生了两次大于7级的地震.地震造成的严重破坏和人员伤亡主要是由于极震区内抗震性能极差的毛石房、砖柱土坯房的大量倒塌所致.澜沧地震的震中基本烈度可达Ⅸ度,耿马地震极震区烈度达Ⅹ度.澜沧地震构造活动的地表证据主要是出现在极震区内的张性地裂缝带和小断层陡坎.地裂缝带和小断坎主要出现在四条相对连续的北北西走向的狭窄地带内,其长度从几百 m 到6km 不等.澜沧地震地表破裂带长约35km,宽约3km,最大垂直位移量和最大右旋水平位移量分别为1.5m 和1.4m.耿马地震地表断裂活动的明显证据包括一系列北北西走向的地裂缝带和一段长约5km的地震断层陡坎.耿马地震地表破裂带长约24km,其最大垂直位移3.5m,最大右旋水平位移为3m.两次地震形成的地表破裂带均具有右旋-正断层性质.本文讨论了这两次地震的 度分布及地表破裂特征.   相似文献   

15.
A first tentative comparison between the structural framework related to the active tectonics and the long-term seismicity of the Umbria–Marche Apennines (affected by the 1997 seismic sequence) has provided some insight for discussing the seismotectonic characteristics of the area. This Apennine sector is affected by 15 to 20-km-long active fault systems, consisting of minor fault-segments. Each of these fault-segments may be responsible for earthquakes characterised by magnitudes ranging between 5.5 and 6.0 (such as those occurred in 1599, 1730, 1838, 1859, 1979). However, the occurrence of one large-magnitude event (1703, Ms = 6.7) and of seismic sequences (1747–1751; 1997–1998) indicate that an entire fault system may be activated suddenly (at least in the southern part of the investigated area) or during seismic crises which may last many months. The comparison between the active faulting framework and the long-term seismicity also indicates that no significant earthquakes may be related to the Mt. Vettore Fault System since 1000 AD.  相似文献   

16.
The Tafilalt region, located at the eastern end of the Anti-Atlas chain in Morocco, was shaken on 23 and 30 October 1992 by two moderate earthquakes of magnitude mb ∼ 5 and intensity ∼ VI MSK64, which caused two deaths and great damage in the area between Erfoud and Rissani. The review of data available on the seismic crisis allowed us to improve the knowledge on the macroseismic, instrumental and source parameters of the earthquakes. The main results of the present study are: (1) location of the epicentres with the help of data from a close portable network allowed us to propose new epicentral coordinates at 31.361° N, 4.182° W (23 October) and 31.286° N, 4.347° W (30 October); both events have focal depths of 2 km; (2) the shock of 30 October was followed by a series of 305 aftershocks, most of which were located west of Rissani; the 61 best-constrained events had focal depths of 5 to 19 km and magnitudes 0.7 to 3; (3) the largest damage was located in an area between the two epicentres within the Tafilalt valley and was probably amplified by site effects due to the proximity of the water table within the Quaternary sediments; (4) focal mechanisms of the main events correspond to strike-slip faulting with fault planes oriented N–S (left lateral) and E–W (right lateral); the only mechanism available for the aftershocks also corresponds to strike-slip faulting; (5) spectral analysis shows that the scalar seismic moment (Mo) of the first event is slightly larger than the second; the corresponding values of Mw are 5.1 and 5.0, respectively; (6) the dimensions of the faults for a circular fault model are 7.7 ± 1.4 and 7.4 ± 1.2 km, respectively; the average displacement is 4 cm for the first event and 3.7 cm for the second; the stress drop is 0.4 and 0.3 MPa, respectively, in agreement with standard values; (7) the Coulomb Stress test performed for both earthquakes suggests a relationship between both events only when the used location is at the limit of the horizontal uncertainty; (8) finally, the occurrence of these shocks suggests that the Anti-Atlas is undergoing tectonic deformation in addition to thermal uplift as suggested by recent publications.  相似文献   

17.
A study of soil–structure–fluid interaction (SSFI) of a lock system subjected to harmonic seismic excitation is presented. The water contained lock is embedded in layered soils supported by a half-space bedrock. The ground excitation is prescribed at the soil–bedrock interface. The response is numerically obtained through a hybrid boundary element (BEM) finite element method (FEM) formulation. The semi-infinite soil and the fluid are modeled by the BEM and the lock is modeled by the FEM. The equilibrium equation for the lock system is obtained by enforcing compatibility and equilibrium conditions at the fluid–structure, soil–structure and soil–layer interfaces under conditions of plane strain. To the authors’ knowledge this is the first study of a lock system that considers the effects of dynamic soil–fluid–structure interaction through a BEM–FEM methodology. A numerical example and parametric studies are presented to examine the effects of the presence of water, lock stiffness, and lock embedment on the response.  相似文献   

18.
The static displacement field of the Athens 1999 earthquake has been numerically modeled by a BEM method and analysed from SAR interferometry images with compatible results: (a) for a fault that reaches the surface the subsidence field coincides with the hangingwall domain of the Fili neotectonic normal fault with maximum amplitude, d max, 5.5–7 cm, which is consistent with the possibly co-seismic displacement of 6–10 cm observed in the field, the average fault dislocation of 5–8 cm found by the application of circular source models, and the displacement up to 6 cm predicted by empirical relations between magnitude and displacement; the field of uplift covers the footwall domain of the fault with d max1.5 cm;d gradually decreases with distance from the fault at a gradient of 0.4 cm/km, (b) for a blind fault d max is only 1.8 and 0.3 cm in the hangingwall and footwall, respectively, and the decay gradient becomes 0.15 cm/km, (c) the total deformation area is 15 km × 15 km and the Fili fault, with a preferred mean dip of 60°, constitutes the natural boundary between the subsidence and uplift areas. The macroseismic field pattern is similar with that of the static ground deformation. The majority of intensity values VI (MM and EMS-98 scales), are distributed within the hangingwall of the Fili fault, while the highest intensities (VIII and IX) concentrate very close to the Fili fault within its hangingwall domain. A gradual decrease of the intensities with the distance from the Fili fault is evident. Because of the similarity between the intensity distribution pattern and the static ground deformation pattern, we make the hypothesis that the latter predicts well enough the main characteristics of the former although the ground displacement is dominated by relatively low frequency as compared to the ground acceleration.  相似文献   

19.
An earthquake of magnitude Ms = 6.0 (CSEM, Strasbourg) occurred at Constantine (Algeria) on 27 October 1985. This seismic event is the strongest felt in the Tellian Atlas since the El Asnam seismic crisis of October 10, 1980. A team from the Centre de Recherche d'Astronomie, d'Astrophysique et de Géophysique (CRAAG, Algeria), utilising 8 portable stations, registered the activity a few days after the main shock. The aftershocks follow a N045° direction, and show the existence of three ruptured segments. Cross sections display a remarkable vertical fault plane and suggest asperities in the rupture process. Surface breaks were found affecting Quaternary deposits. The principal segment is about 3.8 km long showing “enéchelon” cracks with left-lateral displacement while the main direction of the rupture is N055°. Although the vertical motion is small, the northwestern block shows a normal component of the main surface faulting, while the left-lateral displacement is about 10 cm. The strike-slip focal mechanism solution determined from the global seismic network and field observations are in good agreement.  相似文献   

20.
This paper aims to review the main scientific achievements which were obtained in the first phase (1997–2003) of the Global Geodynamics Project (GGP) consisting of a worldwide network of superconducting gravimeters (SG) of about 20 instruments. We show that the low noise levels reached by these instruments in various frequency bands allow us either to investigate new signals of very small amplitude or to better determine other signals previously seen. We first report new results in the long-period seismic band with special emphasis on the detection of the 2S1 normal mode and the splitting of the fundamental spheroidal mode 2S0 after the magnitude 8.4 Peru earthquake in 2001. We also discuss briefly the ‘hum’, which consists of a sequence of fundamental normal modes existing between 2 and 7 mHz even in the lack of any seismic excitation, and was first discovered on the Syowa (in Antarctica) instrument in 1998. We will comment on the search for the Slichter mode 1S1 of degree 1 which is associated with a translational motion of the inner core inside the liquid core. Atmospheric effects are reviewed from the local to the global scale and the improvement due to pressure loading computations on residual gravity signals is shown. An interesting study exhibiting the gravity consequence due to sudden rainfall and vertical mass motion in the atmosphere (without ground pressure change) is presented. The precision of the SGs leads to some convincing results in the tidal domain, concerning the fluid core resonance effect (free core nutation (FCN)) on diurnal tides or various loading effects (linear, non-linear) from the oceans. In particular, SGs gravity measurements are shown to be useful validating tools for ocean tides, especially if they are small and/or confined to coastal regions. The low instrumental drift of the SGs also permits to investigate non-tidal effects in time-varying gravity, especially of annual periodicity. Hydrology has also a signature which can be seen in SG measurements as shown by several recent studies. At even lower frequency, there is the Chandler motion of 435-day period which leads to observable gravity changes at the Earth's surface. We finally report on the progress done in the last years in the problem of calibrating/validating space satellite data with SG surface gravity measurements.  相似文献   

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