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1.
A detailed analysis of the 35 yr of seismicity between 1962 and 1997 using a gridding technique shows that the M 7, Spitak earthquake of 1988 December 7 was preceded by a quiescence anomaly that started at approximately 1984±0.5, and lasted about 5±0.5 yr, up to the main shock. This quiescence anomaly had a radius of about 20±3 km, estimated from circular areas with 75 per cent rate decrease, centred at the point of maximum significance of the anomaly. The quiescence was clearly present in the aftershock volume during the 5 yr before the 1988 main shock, but its statistically strongest expression was located 30 km NW of the epicentre. This anomaly fulfills the association rules between precursory quiescence anomalies and main shocks, even for a tight definition, and is therefore proposed as a case of precursory quiescence. The largest value of the standard deviate Z , found by random selection of samples by gridding, was Z =14 for a time window of T w =3 yr, using a sample size of N =300 events. This makes this anomaly the strongest observed so far, and it is the first documented in an environment of continental collision. There are no false alarms exceeding in significance the precursor. The Armenian earthquake catalogue used for this study had 4600 earthquakes with M ≥ M min =2.2 in the area bounded by 39.5° to 42°N/42.5° to 47°E. From the point of view of homogeneous reporting this is the best catalogue we have analysed so far. The limits of the data used and the density of the grid are dictated by the data, and have no influence on the results. The choice of free parameters does not influence the results significantly within the following limits: 100≤ N ≤500, 2≤ T w ≤7, 2.2≤ M min ≤2.8. 相似文献
2.
Max Wyss Malte Westerhaus Hans Berckhemer Ref'an Ates 《Geophysical Journal International》1995,123(1):117-124
The seismicity rate in the Mudurnu Valley of Turkey was studied using an earthquake catalogue that reports events homogeneously down to magnitude 2.3 for the years 1985–1989, and covers the area between latitudes 40.2° and 41.0°N, and longitudes 30.0° and 31.5°E. During this period the only two main shocks, M = 4.0 and M = 4.3, occurred on 1988 September 6 and 1988 December 9 within about 30km of each other. A highly significant seismic quiescence is evident in the area surrounding these main shocks, while the seismicity rate in the rest of the area covered by the catalogue remains constant. the quiescence becomes more pronounced the smaller the area around the main shocks that is studied. the smallest areas that can be studied contain about 60 earthquakes and have dimensions of approximately 25km on each side. the decreases in seismicity rates are 50–80 per cent depending on the volume and period used for defining the quiescence. the quiescence started in 1988 January and lasted about seven months, with approximately 4.5 months of normal activity separating it from the main shock of December. the precursor time of 12 months for an M = 4.3 main shock is similar to those observed in California. It is concluded that it is possible to resolve precursory quiescence before moderate and large earthquakes in the Mudurnu area with the existing seismograph network. 相似文献
3.
By means of the region–time–length (RTL) algorithm, which is widely used for investigating the precursory seismicity changes in China, Italy, Japan, Russia and Turkey, we examine the precursory seismic activity occurred prior to the 1999, M w = 7.6, Chi-Chi earthquake around its epicentre. Based on our calculation of the RTL values, the epicentral area has been found to strongly exhibit the signature of anomalous activity, associated with the seismic quiescence and activation, before the main shock. Also proposed in this study is a helpful method for determining two important parameters used in the RTL analysis, the characteristic time and distance. Such method will largely reduce the ambiguity in the original RTL algorithm. 相似文献
4.
Hideo Aochi Marc Cushing Oona Scotti Catherine Berge-Thierry 《Geophysical Journal International》2006,165(2):436-446
The Middle Durance fault system, southeastern France, is a slow active fault that produced moderate-size historical seismic events and shows evidence of at least one M w ≳ 6.5 event in the last 29 000 yr. Based on dynamic rupture simulation, we propose earthquake scenarios that are constrained by knowledge of both the tectonic stress field and of the 3-D geometry of the Durance fault system. We simulate dynamic rupture interaction among several fault segmentations of different strikes, dips and rakes, using a 3-D boundary integral equation method. 50 combinations of reasonable stress field orientations, stress field amplitudes and hypocentre locations are tested. The probability of different rupture evolutions is then computed. Each segment ruptures mainly as a single event (44 per cent of the 50 simulations test in this paper). However, the probability that an event triggers simultaneously along three segments is high (26 per cent), leading to a potential rupture length of 45 km. Finally, 2 per cent of the simulations occur along four adjacent segments, producing the greatest total rupture length of 55 km. The simulation results show that the southernmost segment is most easily ruptured (40 per cent), because of its favourable orientation with respect to the tectonic stress and of its favourable location for interaction with the other segments. South-bound unilateral propagation is slightly preferable (41 per cent), compared to north-bound unilateral and bilateral propagation modes. Although, these rupture scenarios cannot be directly translated into probabilities of occurrence, they do provide a better insight as to which rupture scenarios are more likely, an important element to better estimate near-field strong ground motion and seismic hazard. 相似文献
5.
R. S. Spratt 《Geophysical Journal International》1982,71(1):173-186
Summary. The 1973 Hawaii earthquake occurred north of Hilo, at a depth of 40 to 50km. The location was beneath the east flank of Mauna Kea, a volcano dormant historically, but active within the last 4000 yr. Aftershocks were restricted to a depth of 55–35km. The event and its aftershock sequence are located in an area not normally associated with the seismicity of the Mauna Loa and Kilauea calderas. The earthquake was a double event, the epicentres trending NE-SW. The events were of similar size and faulting mechanism. The fault plane solutions obtained by seismic waveform analysis are a strike-slip fault striking EW and dipping 55° S, the auxiliary plane a NS vertical plane with a faulting plunge of 35°. The axis of maximum compressive stress is aligned with the direction of the gravity gradient associated with the island of Hawaii. The fault plane striking EW parallels a surface feature, the Mauna Kea east rift zone. The earthquakes were clearly not associated with volcanic activity normally associated with Mauna Loa and Kilauea and may indicate a deep seated prelude to a resumption of activity at Mauna Kea. 相似文献
6.
A geomagnetic precursor to the 1979 Carlisle earthquake 总被引：3，自引：0，他引：3
David Beamish 《Geophysical Journal International》1982,68(2):531-543
Summary. A study of horizontal field transfer functions, in the period range 10–104 s, has been made for the 2 yr period preceding the 1979 Carlisle earthquake (magnitude 5). The study using two sites, local to and remote from the earthquake epicentre, reveals that a precursory effect is observed at short periods (<102 s) some 35 km from the main epicentre. The results indicate a change from two-dimensional geolectric anisotropy to a more conductive three-dimensional geoelectric configuration during the period immediately preceding the earthquake. 相似文献
7.
Double-planed deep seismic zone and upper-mantle structure in the Northeastern Japan Arc 总被引：6，自引：0，他引：6
Summary. The ScSp wave converted from the ScS wave at the boundary between the descending lithospheric slab and the mantle above it was clearly observed from a nearby deep earthquake with magnitude 7.7 at some stations of the seismic network of Tohoku University which covers the Tohoku District, the northeastern part of Honshu, Japan. By applying the three-dimensional seismic-ray tracing method, the location of this boundary was determined from the difference in arrival time between the ScS and ScSp waves. The result shows that the upper boundary of the descending slab lies exactly on the upper plane of the double-planed deep seismic zone found in the Northeastern Japan Arc.
There is an additional evidence that the boundary is located on the upper plane of the double-planed deep seismic zone. The hypocentre distribution of intermediate-depth earthquakes located by the small-scale seismic-array observation is extremely different from that obtained by the relatively large-scale seismic network. The discrepancy in the distribution of hypocentres of the same earthquake independently located is well explained by the inclined lithospheric slab model derived from the difference in arrival time between the ScS and ScSp waves.
The earthquakes with reverse faulting or with down-dip compressional stresses occur at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down-dip extensional stresses also occur in a very narrow zone from 30 to 40 km below the dipping boundary in the depth range from 50 to about 200 km, and these shocks form the lower plane of the double-planed deep seismic zone. 相似文献
There is an additional evidence that the boundary is located on the upper plane of the double-planed deep seismic zone. The hypocentre distribution of intermediate-depth earthquakes located by the small-scale seismic-array observation is extremely different from that obtained by the relatively large-scale seismic network. The discrepancy in the distribution of hypocentres of the same earthquake independently located is well explained by the inclined lithospheric slab model derived from the difference in arrival time between the ScS and ScSp waves.
The earthquakes with reverse faulting or with down-dip compressional stresses occur at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down-dip extensional stresses also occur in a very narrow zone from 30 to 40 km below the dipping boundary in the depth range from 50 to about 200 km, and these shocks form the lower plane of the double-planed deep seismic zone. 相似文献
8.
Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s−1 . The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 106 km3 of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma. 相似文献
9.
A structural model for the seismicity of the Arudy (1980) epicentral area (Western Pyrenees, France)
Noalwenn Dubos-Sallée Bertrand Nivière Pierre Lacan Yves Hervouët 《Geophysical Journal International》2007,171(1):259-270
The Western Pyrenees presents a diffuse and moderate ( M ≤ 5.7) instrumental seismicity. It nevertheless historically suffered from strong earthquakes (I = IX MSK). The seismic sources of these events are not yet clearly identified. We focus on the Arudy (1980) epicentral area ( M = 5.1) and propose here the reactivation of early Cretaceous normal faults of the Iberian margin as a potential source. The late Cretaceous inversion of this basin, first in a left-lateral strike-slip mode and then in a more frontal convergence, resulted in a pop-up geometry. This flower structure attests of the presence of a deep crustal discontinuity.
The present-day geodynamic arrangement suggests that this accident is reactivated in a right lateral mode. This reactivation leads to a strain partitioning between the deep discontinuity that accommodates the lateral component of the motion and shallow thrusts, rooted on this discontinuity. These thrusts accommodate the shortening component of the strain. The distribution of the instrumental seismicity fits well the structural model of the Arudy basin. Whatever the compressive regional context, the structural behaviour of the system explains too the extensive stress tensor determined for the Arudy crisis if we interpret it in terms of strain ellipsoid. Indeed numerical modelling has shown that this concomitant activity of strike-slip and thrust faulting results in an extensive component that can rise 50 per cent of the finite strain.
We identify too a 25–30 km long potential seismic source for the Arudy area. The size of the structure and its potential reactivation in a strike-slip mode suggest that a maximum earthquake magnitude of ∼6.5 could be expected. The extrapolation of this model at the scale of the Western Pyrenees allows to propose other potential sources for major regional historical earthquakes. 相似文献
The present-day geodynamic arrangement suggests that this accident is reactivated in a right lateral mode. This reactivation leads to a strain partitioning between the deep discontinuity that accommodates the lateral component of the motion and shallow thrusts, rooted on this discontinuity. These thrusts accommodate the shortening component of the strain. The distribution of the instrumental seismicity fits well the structural model of the Arudy basin. Whatever the compressive regional context, the structural behaviour of the system explains too the extensive stress tensor determined for the Arudy crisis if we interpret it in terms of strain ellipsoid. Indeed numerical modelling has shown that this concomitant activity of strike-slip and thrust faulting results in an extensive component that can rise 50 per cent of the finite strain.
We identify too a 25–30 km long potential seismic source for the Arudy area. The size of the structure and its potential reactivation in a strike-slip mode suggest that a maximum earthquake magnitude of ∼6.5 could be expected. The extrapolation of this model at the scale of the Western Pyrenees allows to propose other potential sources for major regional historical earthquakes. 相似文献
10.
Mammoth Mountain is a seismically active volcano 200 000 to 50 000 years old, situated on the southwestern rim of Long Valley caldera, California. Since 1989 it has shown evidence of unrest in the form of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (Pitt & Hill 1994), increased output of magmatic 3 He (Sorey et al. 1993) and the emission of about 500 tonnes day −1 of CO2 (Farrar et al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which has killed trees and poses a threat to human safety. Local-earthquake tomography shows that in mid-1989 areas of subsequent tree-kill were underlain by extensive regions where the ratio of the compressional and shear elastic-wave speeds VP /VS was about 9 per cent lower than in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic areas (Julian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) indicate that VP /VS is sensitive to pore-fluid compressibility, through its effect on VP . The observed VP /VS anomaly is probably caused directly by CO2 , and seismic VP /VS tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity. 相似文献
11.
Crustal model of the Bransfield Rift, West Antarctica, from detailed OBS refraction experiments 总被引：2，自引：0，他引：2
M. Grad H. Shiobara T. Janik A. Guterch H. Shimamura 《Geophysical Journal International》1997,130(2):506-518
The first detailed deep seismic refraction study in the Bransfield Strait, West Antarctica, using sensitive OBSs (ocean bottom seismographs) was carried out successfully during the Antarctic summer of 1990/1991. The experiment focused on the deep crustal structure beneath the axis of the Bransfield Rift. Seismic profile DSS-20 was located exactly in the Bransfield Trough, which is suspected to be a young rift system. Along the profile, five OBSs were deployed at spacings of 50-70 km. 51 shots were fired along the 310 km profile. This paper gives the first presentation of the results. A detailed model of the crustal structure was obtained by modelling the observed traveltimes and amplitudes using a 2-D ray-tracing technique. The uppermost (sedimentary?) cover, with velocities of 2.0-5.5 km s−1 , reaches a depth of up to 8 km. Below this, a complex with velocities of 6.4-6.8 km s−1 is observed. The presence of a high-velocity body, with V p = 7.3-7.7 km s−1 , was detected in the 14-32 km depth range in the central part of the profile. These inhomogeneities can be interpreted as a stage of back-arc spreading and stretching of the continental crust, coinciding with the Deception-Bridgeman volcanic line. Velocities of 8.1 km s−1 , characteristic of the Moho, are observed along the profile at a depth of 30-32 km. 相似文献
12.
Detailed seismic stratigraphic analysis of 2D seismic data over the Faroe‐Shetland Escarpment has identified 13 seismic reflection units that record lava‐fed delta deposition during discrete periods of volcanism. Deposition was dominated by progradation, during which the time shoreline migrated a maximum distance of ～44 km in an ESE direction. Localised collapse of the delta front followed the end of progradation, as a decrease in volcanic activity left the delta unstable. Comparison with modern lava‐fed delta systems on Hawaii suggests that syn‐volcanic subsidence is a potential mechanism for apparent relative sea level rise and creation of new accommodation space during lava‐fed delta deposition. After the main phase of progradation, retrogradation of the delta occurred during a basinwide syn‐volcanic relative sea level rise where the shoreline migrated a maximum distance of ～75 km in a NNW direction. This rise in relative sea level was of the order of 175–200 m, and was followed by the progradation of smaller, perched lava‐fed deltas into the newly created accommodation space. Active delta deposition and the emplacement of lava flows feeding the delta front lasted ～2600 years, although the total duration of the lava‐fed delta system, including pauses between eruptions, may have been much longer. 相似文献
13.
Long-period (30 days1 year) electromagnetic sounding and the electrical conductivity of the lower mantle beneath Europe 总被引：5，自引：0，他引：5
Nils Olsen 《Geophysical Journal International》1999,138(1):179-187
The C -response connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic variations and forms the basis for deriving the conductivitydepth profile of the Earth. Time-series of daily mean values at 42 observatories typically with 50 years of data are used to estimate C -responses for periods between 1 month and 1 yr. The Z : Y method is applied, which means that the vertical component is taken locally whereas the horizontal components are used globally by expansion in a series of spherical harmonics.
In combination with results from previous analyses, the method yields consistent results for European observatories in the entire period range from a few hours to 1 yr, corresponding to penetration depths between 300 and 1800 km.
1-D conductivity models derived from these results show an increase in conductivity with depth z to about 2 S m-1 at z =800 km, and almost constant conductivity between z =800 and z =2000 km with values of 310 S m-1 , in good agreement with laboratory measurements of mantle material. Below 2000 km the conductivity is poorly resolved. However, the best-fitting models indicate a further increase in conductivity to values between 50 and 150 S m-1 . 相似文献
In combination with results from previous analyses, the method yields consistent results for European observatories in the entire period range from a few hours to 1 yr, corresponding to penetration depths between 300 and 1800 km.
1-D conductivity models derived from these results show an increase in conductivity with depth z to about 2 S m
14.
长江三角洲城市群区域滨海湿地利用时空变化特征 总被引：7，自引：1，他引：6
应用遥感和地理信息系统技术,以研究区遥感影像为主要数据源,研究了1990～2000年和2000～2005年间长江三角洲城市群区域围海造地行为下滨海湿地利用的时空演变特征。围海造地在不同城市化发展时期分别为农业用地和建设用地扩张所需。以海岸线为基准,通过分别向陆地和向海洋方向进行滨海湿地分区,每个方向设立7个缓冲带,靠近海岸线的前6个缓冲带宽1km,第7个缓冲带宽10km。结果表明,1990～2000年期间,围海造地以农业用地扩张为主,其平均年扩张速率为48.9km2/a,明显大于建设用地的平均年扩张速率(28.2km2/a);而2000～2005年期间,建设用地成为围海造地的主要目的,建设用地平均年扩张速率大幅增大至91.8km2/a,而农业用地平均年扩张率仅为4.2km2/a。在空间分布上,1990～2000年期间,农业用地平均年扩张百分率最大值出现在距海岸线5km的陆地缓冲区;但在2000～2005年期间,各陆地缓冲区农业用地面积在减少,其平均年扩张百分率为-1.3%/a;而建设用地在各缓冲区的平均年扩张百分率由1990～2000年期间的0.62%/a增大为2000～2005年期间的2.66%/a。对长江三角洲滨海湿地土地流转驱动力的分析表明,城市化是建设用地扩张的重要驱动力;一些天然滨海湿地直接转变为建设用地,两个时期建设用地净扩张区域相同,即距海岸线1～3km和>6km陆地缓冲带;还有一些天然滨海湿地,如滩涂,被用于水产养殖,而转变为人工湿地,在1990～2000年期间,农业用地的扩张区域是距海岸线4～6km的陆地缓冲带,在2000～2005年期间,农业用地的扩张区域是1～3km的近海缓冲带。 相似文献
15.
Veronique Louvel Jérôme Dyment Jean-Claude Sibuet 《Geophysical Journal International》1997,128(1):188-196
The deep seismic reflection profile Western Approaches Margin (WAM) cuts across the Goban Spur continental margin, located southwest of Ireland- This non-volcanic margin is characterized by a few tilted blocks parallel to the margin. A volcanic sill has been emplaced on the westernmost tilted block. The shape of the eastern part of this sill is known from seismic data, but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth. Forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the ocean-continent transition. The volcanic body thickens towards the ocean, and seems to be in direct contact with the oceanic crust. In the contact zone, the volcanic body and the oceanic magnetic layer display approximately the same thickness. The oceanic magnetic layer is anomalously thick immediately west of the volcanic body, and gradually thins to reach more typical values 40 km further to the west. The volcanic sill would therefore represent the very first formation of oceanic crust, just before or at the continental break-up. The ocean-continent transition is limited to a zone 15 km wide. The continental magnetic layer seems to thin gradually oceanwards, as does the continental crust, but no simple relation is observed between their respective thinnings. 相似文献
16.
Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering 总被引：54，自引：0，他引：54
Ross S. Stein Aykut A. Barka James H. Dieterich 《Geophysical Journal International》1997,128(3):594-604
10 M ≥ 6.7 earthquakes ruptured 1000 km of the North Anatolian fault (Turkey) during 1939–1992, providing an unsurpassed opportunity to study how one large shock sets up the next. We use the mapped surface slip and fault geometry to infer the transfer of stress throughout the sequence. Calculations of the change in Coulomb failure stress reveal that nine out of 10 ruptures were brought closer to failure by the preceding shocks, typically by 1–10 bar, equivalent to 3–30 years of secular stressing. We translate the calculated stress changes into earthquake probability gains using an earthquake-nucleation constitutive relation, which includes both permanent and transient effects of the sudden stress changes. The transient effects of the stress changes dominate during the mean 10 yr period between triggering and subsequent rupturing shocks in the Anatolia sequence. The stress changes result in an average three-fold gain in the net earthquake probability during the decade after each event. Stress is calculated to be high today at several isolated sites along the fault. During the next 30 years, we estimate a 15 per cent probability of a M ≥ 6.7 earthquake east of the major eastern centre of Ercinzan, and a 12 per cent probability for a large event south of the major western port city of Izmit. Such stress-based probability calculations may thus be useful to assess and update earthquake hazards elsewhere. 相似文献
17.
Expanding spread profile at the northern Jan Mayen Ridge 总被引：1，自引：0，他引：1
An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex. 相似文献
18.
M. Cocco J. Pacheco S. K. Singh F. Courboulex 《Geophysical Journal International》1997,131(1):135-145
An analysis of the Zihuatanejo, Mexico, earthquake of 1994 December 10 ( M = 6.6), based on teleseismic and near-source data, shows that it was a normal-faulting, intermediate-depth ( H = 50 ± 5 km) event. It was located about 30 km inland, within the subducted Cocos plate. The preferred fault plane has an azimuth of 130°, a dip of 79° and a rake of −86°. The rupture consisted of two subevents which were separated in time by about 2 s, with the second subevent occurring downdip of the first. The measured stress drop was relatively high, requiring a Δσ of about a kilobar to explain the high-frequency level of the near-source spectra. A rough estimate of the thickness of the seismogenic part of the oceanic lithosphere below Zihuatanejo, based on the depth and the rupture extent of this event, is 40 km.
This event and the Oaxaca earthquake of 1931 January 15 ( M = 7.8) are the two significant normal-faulting, intermediate-depth shocks whose epicentres are closest to the coast. Both of these earthquakes were preceded by several large to great shallow, low-angle thrust earthquakes, occurring updip. The observations in other subduction zones show just the opposite: normal-faulting events precede, not succeed, updip, thrust shocks. Indeed, the thrust events, soon after their occurrence, are expected to cause compression in the slab, thus inhibiting the occurrence of normal-faulting events. To explain the occurrence of the Zihuatanejo earthquake, we note that the Cocos plate, after an initial shallow-angle subduction, unbends and becomes subhorizontal. In the region of the unbending, the bottom of the slab is in horizontal extension. We speculate that the large updip seismic slip during shallow, low-angle thrust events increases the buckling of the slab, resulting in an incremental tensional stress at the bottom of the slab and causing normal-faulting earthquakes. This explanation may also hold for the 1931 Oaxaca event. 相似文献
This event and the Oaxaca earthquake of 1931 January 15 ( M = 7.8) are the two significant normal-faulting, intermediate-depth shocks whose epicentres are closest to the coast. Both of these earthquakes were preceded by several large to great shallow, low-angle thrust earthquakes, occurring updip. The observations in other subduction zones show just the opposite: normal-faulting events precede, not succeed, updip, thrust shocks. Indeed, the thrust events, soon after their occurrence, are expected to cause compression in the slab, thus inhibiting the occurrence of normal-faulting events. To explain the occurrence of the Zihuatanejo earthquake, we note that the Cocos plate, after an initial shallow-angle subduction, unbends and becomes subhorizontal. In the region of the unbending, the bottom of the slab is in horizontal extension. We speculate that the large updip seismic slip during shallow, low-angle thrust events increases the buckling of the slab, resulting in an incremental tensional stress at the bottom of the slab and causing normal-faulting earthquakes. This explanation may also hold for the 1931 Oaxaca event. 相似文献
19.
K. Groß U. Micksch TIPTEQ Research Group Seismics Team 《Geophysical Journal International》2008,172(2):565-571
We describe results of an active-source seismology experiment across the Chilean subduction zone at 38.2°S. The seismic sections clearly show the subducted Nazca plate with varying reflectivity. Below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2–5 km thick, highly reflective region, which we interpret as the subduction channel, that is, a zone of subducted material with a velocity gradient with respect to the upper and lower plate. Further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 Valdivia hypocentre. The plate interface itself can be traced further down to depths of 50–60 km below the Central Valley. We observe strong reflectivity at the plate interface as well as in the continental mantle wedge. The sections also show a segmented forearc crust in the overriding South American plate. Major features in the accretionary wedge, such as the Lanalhue fault zone, can be identified. At the eastern end of the profile a bright west-dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc. 相似文献
20.
Candidate precursors: pulse-like geoelectric signals possibly related to recent seismic activity in Japan 总被引：1，自引：0，他引：1
Yuji Enomoto Akito Tsutsumi Yukio Fujinawa Minoru Kasahara Hiroshi Hashimoto 《Geophysical Journal International》1997,131(3):485-494
Telemetric network observations of pulse-like geoelectric charge signals using a vertical dipole buried under the ground were undertaken at various observation sites over a wide area in Japan from April 1996. From continuous records of the signals during the six months following that, we attempted to select anomalous signals, possibly related to seismic electric activity. Special attention was paid to the elimination of noise resulting from industrial and meteorological electric activity, comparison with other electromagnetic signals in the VLF band and the relation between the precursor period and the distance from the eventual main shock that occurred in Japan. Four candidate precursor electric signals, which were not contaminated by industrial and meteorological electric activity, were then selected, of which the second appeared before the Akita-ken Nairiku-nanbu earthquake swarm, for which the maximum M = 5.9 occurred on 1996 August 11, and the third and fourth before the Chiba-ken Toho-oki earthquake, M = 6.6, on 1996 September 11. A tentative qualitative model explaining why the candidate precursory signal is related to stress building up before an earthquake is presented in terms of the electrification of gases released from fracturing rocks immediately prior to the main shock. 相似文献