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1.
High-resolution magnetotelluric (MT) studies of the San Andreas fault (SAF) near Hollister, CA have imaged a zone of high fluid content flanking the San Andreas fault and extending to midcrustal depths. This zone, extending northeastward to the Calaveras fault, is imaged as several focused regions of high conductivity, believed to be the expression of tectonically bound fluid pockets separated by northeast dipping, impermeable fault seals. Furthermore, the spatial relationship between this zone and local seismicity suggests that where present, fluids inhibit seismicity within the upper crust (0–4 km). The correlation of coincident seismic and electromagnetic tomography models is used to sharply delineate geologic and tectonic boundaries. These studies show that the San Andreas fault plane is vertical below 2 km depth, bounding the southwest edge of the imaged fault-zone conductor (FZC). Thus, in the region of study, the San Andreas fault acts both as a conduit for along-strike fluid flow and a barrier for fluid flow across the fault. Combined with previous work, these results suggest that the geologic setting of the San Andreas fault gives rise to the observed distribution of fluids in and surrounding the fault, as well as the observed along-strike variation in seismicity.  相似文献   

2.
Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.  相似文献   

3.
GPS-derived velocities (1993–2002) in northwestern California show that processes other than subduction are in part accountable for observed upper-plate contraction north of the Mendocino triple junction (MTJ) region. After removing the component of elastic strain accumulation due to the Cascadia subduction zone from the station velocities, two additional processes account for accumulated strain in northern California. The first is the westward convergence of the Sierra Nevada–Great Valley (SNGV) block toward the coast and the second is the north–northwest impingement of the San Andreas fault system from the south on the northern California coastal region in the vicinity of Humboldt Bay. Sierra Nevada–Great Valley block motion is northwest toward the coast, convergent with the more northerly, north–northwest San Andreas transform fault-parallel motion. In addition to the westward-converging Sierra Nevada–Great Valley block, San Andreas transform-parallel shortening also occurs in the Humboldt Bay region. Approximately 22 mm/yr of distributed Pacific–SNGV motion is observed inland of Cape Mendocino across the northern projections of the Maacama and Bartlett Springs fault zones but station velocities decrease rapidly north of Cape Mendocino. The resultant 6–10 mm/yr of San Andreas fault-parallel shortening occurs above the southern edge of the subducted Gorda plate and at the latitude of Humboldt Bay. Part of the San Andreas fault-parallel shortening may be due to the viscous coupling of the southern edge of the Gorda plate to overlying North American plate. We conclude that significant portions of the upper-plate contraction observed north of the MTJ region are not solely a result of subduction of the Gorda plate but also a consequence of impingement of the western edge of the Sierra Nevada–Great Valley block and growth of the northernmost segments of the San Andreas fault system.  相似文献   

4.
Many bends or step-overs along strike–slip faults may evolve by propagation of the strike–slip fault on one side of the structure and progressive shut-off of the strike–slip fault on the other side. In such a process, new transverse structures form, and the bend or step-over region migrates with respect to materials that were once affected by it. This process is the progressive asymmetric development of a strike–slip duplex. Consequences of this type of step-over evolution include: (1) the amount of structural relief in the restraining step-over or bend region is less than expected; (2) pull-apart basin deposits are left outside of the active basin; and (3) local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes. This type of evolution of step-overs and bends may be common along the dextral San Andreas fault system of California; we present evidence at different scales for the evolution of bends and step-overs along this fault system. Examples of pull-apart basin deposits related to migrating releasing (right) bends or step-overs are the Plio-Pleistocene Merced Formation (tens of km along strike), the Pleistocene Olema Creek Formation (several km along strike) along the San Andreas fault in the San Francisco Bay area, and an inverted colluvial graben exposed in a paleoseismic trench across the Miller Creek fault (meters to tens of meters along strike) in the eastern San Francisco Bay area. Examples of migrating restraining bends or step-overs include the transfer of slip from the Calaveras to Hayward fault, and the Greenville to the Concord fault (ten km or more along strike), the offshore San Gregorio fold and thrust belt (40 km along strike), and the progressive transfer of slip from the eastern faults of the San Andreas system to the migrating Mendocino triple junction (over 150 km along strike). Similar 4D evolution may characterize the evolution of other regions in the world, including the Dead Sea pull-apart, the Gulf of Paria pull-apart basin of northern Venezuela, and the Hanmer and Dagg basins of New Zealand.  相似文献   

5.
Aseismic slip or fault creep is occurring on many faults in California. Although the creep rates are generally less than 10 mm/yr in most regions, the maximum observed rate along the San Andreas fault between San Juan Bautista and Gold Hill in central California exceeds 30 mm/yr. Changes in slip rates along a 162 km segment of the San Andreas fault in this region have occurred at approximately the same time at up to nine alinement array sites. Rates of creep on the fault near the epicenters of moderate earthquakes (ML 4–6) vary for periods of several years, decreasing before the main shocks and increasing thereafter, in agreement with prior observations based on creepmeter results. The change of surface slip rate is most pronounced within the epicentral region defined by aftershocks, but records from sites at distances up to 100 km show similar variations. Additionally, some variations in rate, also apparently consistent among many sites, have a less obvious relation with seismic activity and have usually taken place over shorter periods. Not all sites exhibit a significant variation in rate at the time of a regional change, and the amplitudes of the change at nearby sites are not consistently related. The time intervals between measurements at the nine array sites during a given period have not always been short with respect to the intervals between surveys at one site; hence, uneven sampling intervals may bias the results slightly. Anomalies in creep rates thus far observed, therefore, have not been demonstrably consistent precursors to moderate earthquakes; and in the cases when an earthquake has followed a long period change of rate, the anomaly has not specified time, place, or magnitude with a high degree of certainty. The consistency of rate changes may represent a large scale phenomenon that occurs along much of the San Andreas transform plate boundary.  相似文献   

6.
Deformed marine terraces and alluvial deposits record Quaternary crustal deformation along segments of a major, seismically active branch of the San Andreas fault which extends 190 km SSE roughly parallel to the California coastline from Bolinas Lagoon to the Point Sur area. Most of this complex fault zone lies offshore (mapped by others using acoustical techniques), but a 4-km segment (Seal Cove fault) near Half Moon Bay and a 26-km segment (San Gregorio fault) between San Gregorio and Point Ano Nuevo lie onshore.At Half Moon Bay, right-lateral slip and N—S horizontal compression are expressed by a broad, synclinal warp in the first (lowest: 125 ka?) and second marine terraces on the NE side of the Seal Cove fault. This structure plunges to the west at an oblique angle into the fault plane. Linear, joint0controlled stream courses draining the coastal uplands are deflected toward the topographic depression along the synclinal axis where they emerge from the hills to cross the lowest terrace. Streams crossing the downwarped part of this terrace adjacent to Half Moon Bay are depositing alluvial fans, whereas streams crossing the uplifted southern limb of the syncline southwest of the bay are deeply incised. Minimum crustal shortening across this syncline parallel to the fault is 0.7% over the past 125 ka, based on deformation of the shoreline angle of the first terrace.Between San Gregorio and Point Ano Nuevo the entire fault zone is 2.5–3.0 km wide and has three primary traces or zones of faulting consisting of numerous en-echelon and anastomozing secondary fault traces. Lateral discontinuities and variable deformation of well-preserved marine terrace sequences help define major structural blocks and document differential motions in this area and south to Santa Cruz. Vertical displacement occurs on all of the fault traces, but is small compared to horizontal displacement. Some blocks within the fault zone are intensely faulted and steeply tilted. One major block 0.8 km wide east of Point Ano Nuevo is downdropped as much as 20 m between two primary traces to form a graben presently filling with Holocene deposits. Where exposed in the sea cliff, these deposits are folded into a vertical attitude adjacent to the fault plane forming the south-west margin of the graben. Near Point Ano Nuevo sedimentary deposits and fault rubble beneath a secondary high-angle reverse fault record three and possibly six distinct offset events in the past 125 ka.The three primary fault traces offset in a right-lateral sense the shoreline angles of the two lowest terraces east of Point Ano Nuevo. The rates of displacement on the three traces are similar. The average rate of horizontal offset across the entire zone is between 0.63 and 1.30 cm/yr, based on an amino-acid age estimate of 125 ka for the first terrace, and a reasonable guess of 200–400 ka for the second terrace. Rates of this magnitude make up a significant part of the deficit between long-term relative plate motions (estimated by others to be about 6 cm/yr) and present displacement rates along other parts of the San Andreas fault system (about 3.2 cm/yr).Northwestward tilt and convergence of six marine terraces northeast of Ano Nuevo (southwest side of the fault zone) indicate continuous gentle warping associated with right-lateral displacement since early or middle Pleistocene time. Minimum local crustal shortening of this block parallel to the fault is 0.2% based on tilt of the highest terrace. Five major, evenly spaced terraces southeast of Ano Nuevo on the southwest flank of Mt. Ben Lomond (northeast side of the fault zone) rise to an elevation of 240 m, indicating relatively constant uplift (about 0.19 m/ka and southwestward tilt since Early or Middle Pleistocene time (Bradley and Griggs, 1976).  相似文献   

7.
玉树地震序列重新定位及其地震构造研究   总被引：3，自引：0，他引：3  
对玉树地震序列自2010年4月11日至9月15日由台网记录到的1 832个地震采用双差地震定位法进行重新定位,获得了1 670个地震重新定位的震源参数。重新定位后的震源深度主要分布在15 km以内。重新定位后的Ms 7.1级主震发生在无地表破裂段,余震活动向两侧破裂扩展。余震沿地表破裂带基本呈线性分布,剖面上显示为近垂直的结构面,在北西端无地表破裂出露处,出现近垂直于断裂方向较宽的北东向地震密集带。震源机制解显示的主压应力方向斜交地表破裂带,地表破裂与震源破裂都表现为纯左旋走滑的错动性质,而在北西端主压应力方向偏转为近垂直于断裂带的方向,此处较宽的北东向地震密集带可能由近东西与南北两个方向的共轭破裂所组成。余震的后期活动与发展并不局限于主震形成的破裂带内,更多的受局部应力调整被触发而产生新的破裂。  相似文献   

8.
In the southern South–North Seismic Zone, China, seismic activity in the Yingjiang area of western Yunnan increased from December 2010, and eventually a destructive earthquake of Ms5.9 occurred near Yingjiang town on 10 March 2011. The focal mechanism and hypocenter location of the mainshock suggest that the Dayingjiang Fault was the site of the mainshock rupture. However, most of foreshocks and all aftershocks recorded by a portable seismic array located close to the mainshock occurred along the N–S-striking Sudian Fault, indicating that this fault had an important influence on these shocks. Coulomb stress calculations show that three strong(magnitude ≥5.0) earthquakes that occurred in the study region in 2008 increased the coulomb stress along the plane parallel to the Dayingjiang Fault. This supports the Dayingjiang Fault, and not the Sudian Fault, as the seismogenic fault of the 2011 Ms5.9 Yingjiang earthquake. The strong earthquakes in 2008 also increased the Coulomb stress at depths of ≤5 km along the entire Sudian Fault, and by doing so increased the shallow seismic activity along the fault. This explains why the foreshocks and aftershocks of the 2011 Yingjiang earthquake were located mostly on the Sudian Fault where it cuts the shallow crust. The earthquakes at the intersection of the Sudian and Dayingjiang faults are distributed mainly along a belt that dips to the southeast at ~40°, suggesting that the Dayingjiang Fault in the mainshock area also dips to the southeast at ~40°.  相似文献   

9.
The San Andreas Fault zone in central California accommodates tectonic strain by stable slip and microseismic activity. We study microstructural controls of strength and deformation in the fault using core samples provided by the San Andreas Fault Observatory at Depth (SAFOD) including gouge corresponding to presently active shearing intervals in the main borehole. The methods of study include high-resolution optical and electron microscopy, X-ray fluorescence mapping, X-ray powder diffraction, energy dispersive X-ray spectroscopy, white light interferometry, and image processing.The fault zone at the SAFOD site consists of a strongly deformed and foliated core zone that includes 2–3 m thick active shear zones, surrounded by less deformed rocks. Results suggest deformation and foliation of the core zone outside the active shear zones by alternating cataclasis and pressure solution mechanisms. The active shear zones, considered zones of large-scale shear localization, appear to be associated with an abundance of weak phases including smectite clays, serpentinite alteration products, and amorphous material. We suggest that deformation along the active shear zones is by a granular-type flow mechanism that involves frictional sliding of microlithons along phyllosilicate-rich Riedel shear surfaces as well as stress-driven diffusive mass transfer. The microstructural data may be interpreted to suggest that deformation in the active shear zones is strongly displacement-weakening. The fault creeps because the velocity strengthening weak gouge in the active shear zones is being sheared without strong restrengthening mechanisms such as cementation or fracture sealing. Possible mechanisms for the observed microseismicity in the creeping segment of the SAF include local high fluid pressure build-ups, hard asperity development by fracture-and-seal cycles, and stress build-up due to slip zone undulations.  相似文献   

10.
On March 10 and September 13, 2007 two earthquakes with moment magnitudes 3.66 and 3.94, respectively, occurred in the eastern part of the United Arab Emirates (UAE). The two events were widely felt in the northern Emirates and Oman and were accompanied by a few aftershocks. Ground motions from these events were well recorded by the broadband stations of Dubai (UAE) and Oman seismological networks and provide an excellent opportunity to study the tectonic process and present day stress field acting in this area. In this study, we report the focal mechanisms of the two main shocks by two methods: first motion polarities and regional waveform moment tensor inversion. Our results indicate nearly pure normal faulting mechanisms with a slight strike slip component. We associated the fault plane trending NNE–SSW with a suggested fault along the extension of the faults bounded Bani Hamid area. The seismicity distribution between two earthquake sequences reveals a noticeable gap that may be a site of a future event. The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated from displacement spectra. The moment magnitudes were very consistent with waveform inversion. The recent deployment of seismic networks in Dubai and Oman reveals tectonic activity in the northern Oman Mountains that was previously unknown. Continued observation and analysis will allow for characterization of seismicity and assessment of seismic hazard in the region.  相似文献   

11.
The results of deep seismic profiling through Southern Sakhalin, the southern Sea of Okhotsk, and the Southern Kuril Islands allowed the identification of deep fault zones, the hypocenter locations, the features of the stress state, and the types of seismic dislocations at the earthquake sources. The east side of the fault was upthrown relative to the west side beneath the southern part of the Tatar Strait and Sakhalin Island and led, as a result of multiple thrusting events along the fault over the geologic history, to the rise and 5-8 km displacement of the seismic boundaries. The true uplift of the Greater Kuril arc block was determined using the focal mechanism solutions. The seismoctectonics and present-day dynamics of the crustal blocks were estimated using a detailed joint analysis of the position of the structural boundaries at the seismic section and the seismotectonic movements according to the earthquake focal mechanisms.  相似文献   

12.
Uranium-series dating of corals overlying the undeformed Punta Coyote gravels indicates that the underlying La Paz fault zone has been relatively inactive in this part of the Baja California peninsula during the last 140,000 years, and possibly for a significantly longer period. However, Holocene seismic activities along extensions of the fault zone north of Cabo San Lucas suggest potential seismic hazards for the city of La Paz (population 200,000), which lies about 6 km from the fault.  相似文献   

13.
Deciphering the internal structure of large fault zones is fundamental if a proper understanding is to be gained of their mechanical, hydrological and seismological properties. To this end, new detailed mapping and microstructural observations of the excellently exposed Carboneras fault zone in southeastern Spain have been used to elucidate both the internal arrangement of fault products and their likely mechanical properties. The fault is a 40 km offset strike-slip fault, which constitutes part of the Africa–Iberia plate boundary. The zone of faulting is 1 km in width not including the associated damage zone surrounding the fault. It is composed of continuous strands of phyllosilicate-rich fault gouge that bound lenses of variably broken-up protolith. This arrangement provides a number of fluid flow and fluid sealing possibilities within the fault zone. The gouge strands exhibit distributed deformation and are inferred to have strain hardening and/or velocity hardening characteristics. Also included in the fault zone are blocks of dolomite that contain thin (<1 cm thick) fault planes inferred to have been produced by strain weakening/velocity weakening behaviour. These fault planes have a predominantly R1 Riedel shear orientation and are arranged in an en echelon pattern. A conceptual model of this type of wide fault zone is proposed which contrasts with previous narrow fault zone models. The observed structural and inferred mechanical characteristics of the Carboneras fault zone are compared to seismological observations of the San Andreas fault around Parkfield, CA. Similarities suggest that the Carboneras fault structure may be a useful analogue for this portion of the San Andreas fault at depth.  相似文献   

14.
The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20–25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L. , and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427–434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by  100 m. Tests using “virtual earthquakes” (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically.  相似文献   

15.
The present paper is a continuation of the previous work on modeling the local stress field induced by the San Andreas fault system (Nikonov et al., 1975). This system has been simulated on plane elastic models made of optically sensitive material, the models being under homogeneous uniaxial compression. The photoelastic method has been used to study the redistribution of τmax around the fault system with sides closed under compression.Three main features emerge in the kinematics of fault-system modeling. The first is a peculiar distortion of an originally rectangular grid, reflecting right-lateral movements on the San Andreas fault. This is especially noticeable in its central part. The second is the appearance and spreading of tear breaks near the ends of the zone nearly normal to the strike of the ends of the master fault. The third feature is separation of fault wings in certain sections of the San Andreas fault in the model. All these features are in general correspondence with the phenomena actually observed in the San Andreas fault system.  相似文献   

16.
Abstract: There are two co-seismic faults which developed when the Wenchuan earthquake happened. One occurred along the active fault zone in the central Longmen Mts. and the other in the front of Longmen Mts. The length of which is more than 270 km and about 80 km respectively. The co-seismic fault shows a reverse flexure belt with strike of N45°–60°E in the ground, which caused uplift at its northwest side and subsidence at the southeast. The fault face dips to the northwest with a dip angle ranging from 50° to 60°. The vertical offset of the co-seismic fault ranges 2.5–3.0 m along the Yingxiu-Beichuan co-seismic fault, and 1.5–1.1 m along the Doujiangyan-Hanwang fault. Movement of the co-seismic fault presents obvious segmented features along the active fault zone in central Longmen Mts. For instance, in the section from Yingxiu to Leigu town, thrust without evident slip occurred; while from Beichuan to Qingchuan, thrust and dextral strike-slip take place. Main movement along the front Longmen Mts. shows thrust without slip and segmented features. The area of earthquake intensity more than IX degree and the distribution of secondary geological hazards occurred along the hanging wall of co-seismic faults, and were consistent with the area of aftershock, and its width is less than 40km from co-seismic faults in the hanging wall. The secondary geological hazards, collapses, landslides, debris flows et al., concentrated in the hanging wall of co-seismic fault within 0–20 km from co-seismic fault.  相似文献   

17.
郯庐断裂带是中国东部板内一条规模最大的强构造变形带与地震活动断裂带,其断裂结构与历史地震活动性具明显的分段活动性。文中通过沿郯庐断裂带中南段的历史地震活动性、精定位背景地震活动性与震源机制解分析,讨论了断裂带的深部几何结构与现今活动习性。现今地震活动在中段主要沿1668年郯城MS 8地震破裂带线性分布,线性条带在泗洪-诸城间延伸约340 km长,为1668年地震长期缓慢衰减的余震序列活动。大震地表破裂遗迹与精定位地震分布都揭示出郯庐断裂带中段的两条全新世活动断裂昌邑-大店断裂与安丘-莒县断裂以右阶斜列的形式共同参与了1668年郯城MS 8地震破裂。精定位震源剖面刻画出两条断裂结构面呈高角度相背而倾,其中昌邑-大店断裂倾向SE,安丘-莒县断裂倾向NW,两条断裂在深部没有合并汇聚。余震活动所代表的1668年地震震源破裂带是郯庐断裂带中现今尚未闭锁的安全段落,对应于高b值段。而未发生破裂的安丘以北段,小震活动不活跃,b值低,现今可能已成为应力积累的闭锁段。震源机制解揭示的断裂应力状态在中段以NE向主压应力为主,表现为右旋走滑活动性质,且存在少量正断分量,南段转为以NEE至近EW向为主,存在少量的逆冲分量。在中段与南段的转折处宿迁-嘉山段,主压应力方向垂直断裂带走向呈NWW向,反映出局部以挤压为主的应力特征,其中泗洪-嘉山段也是历史地震未破裂段,现今小震活动不活跃,因此该段可能更易于应力积累。精定位小震活动在郯庐断裂与北西向断裂相交汇处聚集,反映出北西向断裂的新活动性,以及郯庐断裂带现今的逆冲作用。在断裂带南端,精定位背景地震活动沿与其相交汇的襄樊-广济断裂带东段呈北西向线性分布,表明了该段的现今活动性。  相似文献   

18.
The most recent seismic profiles in southern Italy show the existence of a sudden thinning of the crust at the boundary between the chain and the Tyrrhenian margin. The abrupt change in thickness can be followed along a zone which has the same geometry in plan as the Calabrian arc. By considering these data and the gravimetric anomalies, it is possible to associate this crustal anomaly with a deep-seated shear zone which determines a crustal shortening of about 40–60 km which can only be a consequence of post-Tortonian tectonics. The surface equivalents of the deep-seated shear zone can be recognized, from north to south, in the alignment of the intra-Apenninic basins (Vallo di Diano, San Arcangelo, Potenza), in the Crati-Mesima graben and in the Mount Kumeta-Alcantara fault zone.The distribution of the seismicity and its connection with the surface structures shows that the largest earthquakes occur along the deep shear-zone. In particular, shocks with the highest magnitude and the longest recurrence intervals are located in those areas where the deep shear zone is at an angle of about 90° with the direction of maximum shortening (Crati-Mesima graben). The fault zones nearly parallel to the regional compression axis (e.g., Mount Kumeta-Alcantara fault zone) are characterized by earthquakes of lower magnitude. Taking into account the neotectonic evolution of the regional structures, as well as the orientation of the stress field and its connection with the deep-seated shear zone, it is possible to distinguish the following seismotectonic zones: Upper Crati-Mesima graben, transverse throughs, Mount Pollino-Mount Raparo fault zone, external Ionian area, San Arcangelo basin-zone of the external flysch, Sicani Mountains, Mount Kumeta-Alcantara fault zone, Caltanissetta basin, and Iblean plateau-Bradanic trough-Murge ridge. The definition of the geometry of the shear zone at depth is one of the most important, but still unresolved problems.  相似文献   

19.
The Coyote Lake basalt, located near the intersection of the Hayward and Calaveras faults in central California, contains spinel peridotite xenoliths from the mantle beneath the San Andreas fault system. Six upper mantle xenoliths were studied in detail by a combination of petrologic techniques. Temperature estimates, obtained from three two-pyroxene geothermometers and the Al-in-orthopyroxene geothermometer, indicate that the xenoliths equilibrated at 970–1100 °C. A thermal model was used to estimate the corresponding depth of equilibration for these xenoliths, resulting in depths between 38 and 43 km. The lattice preferred orientation of olivine measured in five of the xenolith samples show strong point distributions of olivine crystallographic axes suggesting that fabrics formed under high-temperature conditions. Calculated seismic anisotropy values indicate an average shear wave anisotropy of 6%, higher than the anisotropy calculated from xenoliths from other tectonic environments. Using this value, the anisotropic layer responsible for fault-parallel shear wave splitting in central California is less than 100 km thick. The strong fabric preserved in the xenoliths suggests that a mantle shear zone exists below the Calaveras fault to a depth of at least 40 km, and combining xenolith petrofabrics with shear wave splitting studies helps distinguish between different models for deformation at depth beneath the San Andrea fault system.  相似文献   

20.
In estimating the likelihood of an earthquake hazard for a seismically active region, information on the geometry of the potential source is important in quantifying the seismic hazard. The damage from an earthquake varies spatially and is governed by the fault geometry and lithology. As earthquake damage is amplified by guided seismic waves along fault zones, it is important to delineate the disposition of the fault zones by precisely determined hypocentral parameters. We used the double difference (DD) algorithm to relocate earthquakes in the Koyna-Warna seismic zone (KWSZ) region, with the P- and S-wave catalog data from relative arrival time pairs constituting the input. A significant improvement in the hypocentral estimates was achieved, with the epicentral errors <30 m and focal depth errors <75 m i.e. errors have been significantly reduced by an order of magnitude from the parameters determined by HYPO71. The earthquake activity defines three different fault segments. The seismogenic volume is shallower in the south by 3 km, with seismicity in the north extending to a depth of 11 km while in the south the deepest seismicity observed is at a depth of 8 km. By resolving the structure of seismicity in greater detail, we address the salient issues related to the seismotectonics of this region.  相似文献   

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