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1.
—The test that Kagan and Jackson (1991, 1995) applied to the seismic gap hypothesis did not bring us closer to understanding the generation of large earthquakes. On the contrary, it led some to the conclusion that the rebound theory of earthquake generation should be rejected. We disagree with this point of view and argue that a global test of the simplified gap hypothesis cannot be done because it cannot account for differences in the slip history of fault segments and tectonic differences between separate plate boundaries. Kagan and Jackson did show, however, that the original gap hypothesis was oversimplified and should be refined. We propose that consideration of all the facts, including slip history and seismicity patterns in the Andreanof Islands, show that the concept of seismic gaps and the elastic rebound theory are correct for that segment of the plate boundary. The coseismic slip in the M w 8.7 earthquake that broke this plate boundary segment in 1957 was only 2 m, as published before the repeat earthquake of 1986 (M w 8), and thus, using a plate convergence rate of 7.3 cm/year, the return time in this cycle was expected to be less than 30 years, unless substantial aseismic creep occurs. This supports the time predictable model of mainshock recurrence. In addition, Kisslinger et al. (1985) and Kisslinger (1986) noticed a seismic quiescence in the subsequent source volume before the 1986 earthquake and attempted to predict it. The specific parameters he estimated were not entirely correct although his interpretation of the observed quiescence as a precursor was. We conclude that the 1986, M w 8, Andreanof earthquake was not an example that disproves the seismic gap hypothesis. On the contrary, it shows that the hypothesis that plate motions reload plate boundaries after most of the elastic energy is released in great ruptures was correct in this case. This suggests that great earthquakes occur preferably in mature gaps. We believe the testing of the seismic gap hypothesis by algorithm on a global scale is an example that illustrates that overly simplified tests can lead to erroneous conclusions. To make progress in the actual understanding of the physics of the process of great earthquake ruptures, one must consider all the facts known for case histories.  相似文献   

2.
The 9 March 1957 Aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded—1200 km. However, due to a lack of high-quality seismic data, the actual source parameters for this earthquake have been poorly determined. We have examined all the available waveform data to determine the seismic moment, rupture area, and slip distribution. These data include body, surface and tsunami waves. Using body waves, we have estimated the duration of significant moment release as 4 min. From surface wave analysis, we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50–100×1020 Nm. Using the tsunami waveforms, we estimated the source area of the 1957 tsunami by backward propagation. The tsunami source area is smaller than the aftershock zone and is about 850 km long. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. We also inverted the tsunami waveforms for the slip distribution. Slip on the 1957 rupture zone was highest in the western half near the epicenter. Little slip occurred in the eastern half. The moment is estimated as 88×1020 Nm, orM w =8.6, making it the seventh largest earthquake during the period 1900 to 1993. We also compare the 1957 earthquake to the 1986 Andreanof Islands earthquake, which occurred within a segment of the 1957 rupture area. The 1986 earthquake represents a rerupturing of the major 1957 asperity.  相似文献   

3.
An interpretation of the parameters of earthquake sources is proposed for the two large earthquakes in the Rat Islands of February 4, 1965 (M W = 8.7), and November 17, 2003 (M W = 7.7–7.8), based on the analysis of focal mechanisms, the manifestation of aftershocks, and the specific features of the geological structure of the island slope of the Rat Islands. The source of the earthquake of 1965 is a reverse fault of longitudinal strike, with a length of ～350 km. It is located in the lower part of the Aleutian Terrace and probably is genetically connected with the development of the Rat submarine ridge. The westward boundary of the earthquake source is determined by the Heck Canyon structures, and the eastward boundary is determined by the end of Rat Ridge in the region of λ ～ 179°E–179.5°E. The source of the earthquake of 2003 is a steep E-W reverse fault extending for about 100 km. It is located in the eastern part of the Rat Islands, higher on the slope than the source of the earthquake of 1965. The westward end of the earthquake source is determined by Rat Canyon structures, and the eastward end is an abrupt change in isobaths in the region of λ ～ 179°E. According to the aftershock hypocenters, the depth of occurrence of the reverse fault could reach ～60 km. According to our interpretation, on the southern slope of the Rat and Near islands, there is a complex system of seismogenic faults that is caused by tectonic development of different structural elements. The dominant types of faults here are reverse faults, as in other island arcs. During earthquakes, reverse faults oriented along the island arc and also faults that intersect it exhibit themselves. The reverse faults of northeastern strike that intersect the arc characterize the type of tectonic motions in a series of canyons of the western part of the Aleutian Islands.  相似文献   

4.
Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones   总被引：1，自引：0，他引：1  
—We classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface (typical interplate events), earthquakes at the outer rise, within the subducting slab or overlying crust (intraplate events), and "tsunami earthquakes" that generate considerably larger tsunamis than expected from seismic waves. The depth range of a typical interplate earthquake source is 10–40km, controlled by temperature and other geological parameters. The slip distribution varies both with depth and along-strike. Recent examples show very different temporal change of slip distribution in the Aleutians and the Japan trench. The tsunamigenic coseismic slip of the 1957 Aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long. This asperity ruptured again in the 1986 Andreanof Islands and 1996 Delarof Islands earthquakes. By contrast, the source of the 1994 Sanriku-oki earthquake corresponds to the low slip region of the previous interplate event, the 1968 Tokachi-oki earthquake. Tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes; tsunami hazard assessments must include such events. Similarity in macroseismic data from two southern Kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights. Most moment release of tsunami earthquakes occurs in a narrow region near the trench, and the concentrated slip is responsible for the large tsunami. Numerical modeling of the 1996 Peru earthquake confirms this model, which has been proposed for other tsunami earthquakes, including 1896 Sanriku, 1946 Aleutian and 1992 Nicaragua.  相似文献   

5.
An interpretation of the type, size, and location of the source of the Aleutian earthquake on April 1, 1946, which was characterized by the highest intensity (I = 4), is proposed. The earthquake source is a subvertical reverse fault striking along the island arc and dipping at an angle of 85° toward the deep-sea trench. The reverse fault is located in the lower part of the island slope, within the eastern termination of the Aleutian terrace. The western end of the reverse fault is located in the area of the Krenitsyn Islands (λ ∼ 165°W), where the pattern of isobaths changes, and an abrupt widening of the shelf part of the Fox Islands takes place. Large (M S ∼ 7) shocks, preceding the 1946 earthquake, occurred here in 1940, 1942, and 1944. Structural inhomogeneities in the island slope in the area of the Sanak Islands (λ ∼ 162°W) determine the eastern edge of the source-reverse fault, whose length within the specified boundaries is about 200 km. The mean magnitude of the earthquake corresponding to such a source is ∼8.3. According to the regular relation between the rupture length and the mean movement, the vertical displacement of the ocean floor in the source region could attain 5–6 m. A significant vertical displacement of the ocean floor over its large length (∼200 km) was responsible for the high tsunamigenic ability of this earthquake. A favorable combination in the source area of the topographic and other conditions necessary for the tsunami formation could additionally contribute to an increase in the intensity of the tsunami. The earthquake of April 1, 1946, in the Fox Islands, as well as the tsunamigenic earthquakes of March 9, 1957, in the Andreanof Islands and February 4, 1965, in the Rat Islands, does not belong to the class of “slow” earthquakes.  相似文献   

6.
—The 12 November 1996 M w 7.7 Peru subduction zone earthquake occurred off the coast of southern Peru, near the intersection of the South American trench and the highest topographical point of the subducting Nazca Ridge. We model the broadband teleseismic P-waveforms from stations in the Global Seismic Network to constrain the source characteristics of this subduction zone earthquake. We have analyzed the vertical component P-waves for this earthquake to constrain the depth, source complexity, seismic moment and rupture characteristics. The seismic moment determined from the nondiffracted P-waves is 3–5 × 1020 N·m, corresponding to a moment magnitude M w of 7.6–7.7. The source time function for the 1996 Peru event has three pulses of seismic moment release with a total duration of approximately 45–50 seconds. The largest moment release occurs at approximately 35–40 seconds and is located ～90km southeast of the rupture initiation. Approximately 70% of the seismic moment was released in the third pulse.¶We find that the 1996 event reruptured part of the rupture area of the previous event in 1942. The location of the 1996 earthquake corresponds to a region along the Peru coast with the highest uplift rates of marine terraces. This suggests that the uplift may be due to repeated earthquakes such as the 1996 and 1942 events.  相似文献   

7.
Maximum earthquake size varies considerably amongst the subduction zones. This has been interpreted as a variation in the seismic coupling, which is presumably related to the mechanical conditions of the fault zone. The rupture process of a great earthquake indicates the distribution of strong (asperities) and weak regions of the fault. The rupture process of three great earthquakes (1963 Kurile Islands, MW = 8.5; 1965 Rat Islands, MW = 8.7; 1964 Alaska, MW = 9.2) are studied by using WWSSN stations in the core shadow zone. Diffraction around the core attenuates the P-wave amplitudes such that on-scale long-period P-waves are recorded. There are striking differences between the seismograms of the great earthquakes; the Alaskan earthquake has the largest amplitude and a very long-period nature, while the Kurile Islands earthquake appears to be a sequence of magnitude 7.5 events.The source time functions are deconvolved from the observed records. The Kurile Islands rupture process is characterized by the breaking of asperities with a length scale of 40–60 km, and for the Alaskan earthquake the dominant length scale in the epicentral region is 140–200 km. The variation of length scale and MW suggests that larger asperities cause larger earthquakes. The source time function of the 1979 Colombia earthquake (MW = 8.3) is also deconvolved. This earthquake is characterized by a single asperity of length scale 100–120 km, which is consistent with the above pattern, as the Colombia subduction zone was previously ruptured by a great (MW = 8.8) earthquake in 1906.The main result is that maximum earthquake size is related to the asperity distribution on the fault. The subduction zones with the largest earthquakes have very large asperities (e.g. the Alaskan earthquake), while the zones with the smaller great earthquakes (e.g. Kurile Islands) have smaller scattered asperities.  相似文献   

8.
—By rupturing more than half of the shallow subduction interface of the Nazca Ridge, the great November 12, 1996 Peruvian earthquake contradicts the hypothesis that oceanic ridges subduct aseismically. The mainshock’s rupture has a length of about 200 km and has an average slip of about 1.4 m. Its moment is 1.5 × 1028 dyne-cm and the corresponding M w is 8.0. The mainshock registered three major episodes of moment release as shown by a finite fault inversion of teleseismically recorded broadband body waves. About 55% of the mainshock’s total moment release occurred south of the Nazca Ridge, and the remaining moment release occurred at the southern half of the subduction interface of the Nazca Ridge. The rupture south of the Nazca Ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth. Because the axis of the Nazca Ridge is at a high angle to the plate convergence direction, the subducting Nazca Ridge has a large southwards component of motion, 5 cm/yr parallel to the coast. The 900–1200 m relief of the southwards sweeping Nazca Ridge is interpreted to act as a "rigid indenter," causing the greatest coupling south of the ridge’s leading edge and leading to the large observed slip. The mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inverts local and teleseismic data. Most aftershocks were within the outline of the Nazca Ridge. A three-month delayed aftershock cluster occurred at the northern part of the subducting Nazca Ridge. Aftershocks were notably lacking at the zone of greatest moment release, to the south of the Nazca Ridge. However, a lone foreshock at the southern end of this zone, some 140 km downstrike of the mainshock’s epicenter, implies that conditions existed for rupture into that zone. The 1996 earthquake ruptured much of the inferred source zone of the M w 7.9–8.2 earthquake of 1942, although the latter was a slightly larger earthquake. The rupture zone of the 1996 earthquake is immediately north of the seismic gap left by the great earthquakes (M w ～8.8–9.1) of 1868 and 1877. The M w 8.0 Antofagasta earthquake of 1995 occurred at the southern end of this great seismic gap. The M w 8.2 deep-focus Bolivian earthquake of 1994 occurred directly downdip of the 1868 portion of that gap. The recent occurrence of three significant earthquakes on the periphery of the great seismic gap of the 1868 and 1877 events, among other factors, may signal an increased seismic potential for that zone.  相似文献   

9.
The new scale Mt of tsunami magnitude is a reliable measure of the seismic moment of a tsunamigenic earthquake as well as the overall strength of a tsunami source. This Mt scale was originally defined by Abe (1979) in terms of maximum tsunami amplitudes at large distances from the source. A method is developed whereby it is possible to determine Mt at small distances on the basis of the regional tsunami data obtained at 30 tide stations in Japan. The relation between log H, maximum amplitude (m) and log Δ, a distance of not less than 100 km away from the source (km) is found to be linear, with a slope close to 1.0. Using three tsunamigenic earthquakes with known moment magnitudes Mw, for calibration, the relation, Mt = log H + log Δ + D, is obtained, where D is 5.80 for single-amplitude (crest or trough) data and 5.55 for double-amplitude (crest-to-trough) data. Using a number of tsunami amplitude data, Mt is assigned to 80 tsunamigenic earthquakes that occurred in the northwestern Pacific, mostly in Japan, during the period from 1894 to 1981. The Mt values are found to be essentially equivalent to Mw for 25 events with known Mw. The 1952 Kamchatka earthquake has the largest Mt, 9.0. Of all the 80 events listed, at least seven unusual earthquakes which generated disproportionately-large tsunamis for their surface-wave magnitude Ms are identified from the relation. From the viewpoint of tsunami hazard reduction, the present results provide a quantitative basis for predicting maximum tsunami amplitudes at a particular site.  相似文献   

10.
Many authors have proposed that the study of seismicity rates is an appropriate technique for evaluating how close a seismic gap may be to rupture. We designed an algorithm for identification of patterns of significant seismic quiescence by using the definition of seismic quiescence proposed by Schreider (1990). This algorithm shows the area of quiescence where an earthquake of great magnitude may probably occur. We have applied our algorithm to the earthquake catalog on the Mexican Pacific coast located between 14 and 21 degrees of North latitude and 94 and 106 degrees West longitude; with depths less than or equal to 60 km and magnitude greater than or equal to 4.3, which occurred from January, 1965 until December, 2014. We have found significant patterns of seismic quietude before the earthquakes of Oaxaca (November 1978, Mw = 7.8), Petatlán (March 1979, Mw = 7.6), Michoacán (September 1985, Mw = 8.0, and Mw = 7.6) and Colima (October 1995, Mw = 8.0). Fortunately, in this century earthquakes of great magnitude have not occurred in Mexico. However, we have identified well-defined seismic quiescences in the Guerrero seismic-gap, which are apparently correlated with the occurrence of silent earthquakes in 2002, 2006 and 2010 recently discovered by GPS technology.  相似文献   

11.
《Geofísica Internacional》2013,52(2):173-196
An analysis of local and regional data produced by the shallow, thrust Ometepec-Pinotepa Nacional earthquake (Mw 7.5) of 20 March 2012 shows that it nucleated at 16.254°N 98.531°W, about 5 km offshore at a depth of about 20 km. During the first 4 seconds the slip was relatively small. It was followed by rupture of two patches with large slip, one updip of the hypocenter to the SE and the other downdip to the north. Total rupture area, estimated from inversion of near-source strong-motion recordings, is ~25 km × 60 km. The earthquake was followed by an exceptionally large number of aftershocks. The aftershock area overlaps with that of the 1982 doublet (Mw 7.0, 6.9). However, the seismic moment of the 2012 earthquake is ~3 times the sum of the moments of the doublet, indicating that the gross rupture characteristics of the two earthquake episodes differ. The small-slip area near the hypocenter and large-slip areas of the two patches are characterized by relatively small aftershock activity. A striking, intense, linear NE alignment of the aftershocks is clearly seen. The radiated energy to seismic moment ratios, (Es/M0), of five earthquakes in the region reveal that they are an order of magnitude smaller for near-trench earthquakes than those that occur further downdip (e.g., 2012 and the 1995 Copala earthquakes). The near-trench earthquakes are known to produce low Amax. The available information suggests that the plate interface in the region can be divided in three domains. (1) From the trench to a distance of about 35 km downdip. In this domain M~6 to 7 earthquakes with low values of (Es/M0) occur. These events generate large number of aftershocks. It is not known whether the remaining area on this part of the interface slips aseismically (stable sliding) or is partially locked. (2) From 35 to 100 km from the trench. This domain is seismically coupled where stick-slip sliding occurs, generating large earthquakes. Part of the area is probably conditionally stable. (3) From 100 to 200 km from the trench. In this domain slow slip events (SSE) and nonvolcanic tremors (NVT) have been reported.The earthquake caused severe damage in and near the towns of Ometepec and Pinotepa Nacional. The PGA exceeded 1 g at a soft site in the epicentral region. Observed PGAs on hard sites as a function of distance are in reasonable agreement with the expected ones from ground motion prediction equations derived using data from Mexican interplate earthquakes. The earthquake was strongly felt in Mexico City. PGA at CU, a hard site in the city, was 12 gal. Strong-motion recordings in the city since 1985 demonstrate that PGAs during the 2012 earthquake were not exceptional, and that similar motion occurs about once in three years.  相似文献   

12.
This work generalizes the results of tomographic imaging performed by the authors for epicentral zones. Seismic events in North Africa (the M w = 5.8 earthquake of 1985 near the town of Constantine), eastern Anatolia (the Erzincan M w = 6.7 earthquake of 1992), the Lesser and Greater Caucasus (the 1988 Spitak M w = 6.8 and the 1991 Racha M w = 7.0 earthquakes), and northern Sakhalin (the 1995 Neftegorsk M w = 7.1 earthquake) are examined. It is shown how various morphokinematic types of active faults differ in the resulting tomographic images at various depths. A classification of tomographic images of strong earthquake source zones is proposed in accordance with the rank of their generating faults. The sources of the Spitak, Racha, and Erzincan earthquakes are confined to large boundary faults separating tectonic zones. Lower velocity bands are revealed in the tomographic images, and low velocity “pockets” 1–2 km or somewhat more in width penetrating to a depth of up to 15 km are observed near the fault zones. The Constantine and Neftegorsk earthquakes were generated by faults of a lower rank. The source zones of these events are imaged tomographically as narrow gradient zones.  相似文献   

13.
主要研究2009年7月24日西藏尼玛西南MS5.6地震的基本参数、地震序列特征、震源参数、发震构造等;利用震中附近600km范围内台站测定参数研究地震的震源机制解,与哈佛大学给出的震源机制解较一致,且与通过现场考察的发震断层走向具有一致性。研究认为本次地震发生在冈底斯山—拉萨块体内部,断裂为NNW向,主要受张应力作用产生左旋走滑正断层活动。此外还分析了震前地震学条带异常特征,结束表明,震前1年出现NW向条带非常显著,研究结论为该地区今后地震预测提供科学依据。  相似文献   

14.
The size of major tsunamigenic earthquakes which occurred in the Japan Sea is quantified on the basis of seismic and tsunamigenic source parameters. The tsunami magnitude Mt is determined from the instrumental tsunami-wave amplitudes. The Mt values thus obtained are on average 0.2 units larger than the values of moment magnitude Mw, though the Mt scale has originally been adjusted to agree with Mw. Moreover, the volume of displaced water at the source is on average 2.3 times as large as that for the Pacific events with a comparable Mw. Nevertheless, the observed height of the sea-level disturbance at the source is found consistent with the amount of crustal deformation computed for the seismic fault models. These results indicate that the tsunami source potential itself is large for Mw in comparison with the Pacific events. The large source potential is explained in terms of the effective difference both in the rigidity of the source medium and in the geometry of the fault motion. For the Japan Sea events, the Mt scale still provides the physical measure of the tsunami potential, and Mt minus 0.2 corresponds to Mw. This predicts that the maximum amplitude of tsunami waves from Japan Sea earthquakes is at least two times as large as that from Pacific earthquakes with a comparable Mw.  相似文献   

15.
The Hsingtai, China earthquakes of March 1966 were a series of destructive earthquakes associated with the Shu-lu graben. Five strong shocks of Ms ≥ 6 occurred within a period of less than a month, the largest of which was Ms 7.2. Body and surface waves over the period range from several to 100 s have been modeled for the four largest events using synthetic seismograms in the time domain and spectral analysis in the frequency domain. Data from ground deformation, local geology, regional seismic network, and teleseismic joint epicenter determination have also been used to constrain the source model and the rupture process.The fault mechanism of the Hsingtai sequence was mainly strike-slip with a small component of normal dip-slip. The strikes of the four largest shocks range from ～ N26° to 30°E, approximately along strike of the major faults of the Shu-lu graben and the aftershock distribution. The source mechanisms can be explained with a NNW-SSE extensional stress and a NEE-SWW compressional stress acting in the area. The major shocks all had focal depths ～ 10 km.The four largest shocks in the sequence were characterized by a relatively simple and smooth dislocation time history. The durations of the far-field source time functions ranged from 3.5 to 5 s, while the rise times were all ～ 1 s. The seismic moments of the four largest earthquakes ranged from 1.43 × 1025 to 1.51 × 1026 dyne cm?1. The fault sizes of the four events were very close. Assuming circular faults, the diameters of the four events were determined to be between 10 and 14 km. Stress drops varied from ～ 52 to 194 bars. A trend of increasing stress drop with earthquake size was observed.A survey of stress drop determinations for 15 major intraplate earthquakes shows that on the average the magnitude of stress drop of oceanic intraplate earthquakes and passive continental margin events is higher (～ 200 to several hundred bars) than that of continental intraplate earthquakes (～ 100 bars or less).  相似文献   

16.
The refinement of the accuracy and resolution of the monthly global gravity field models from the GRACE satellite mission, together with the accumulation of more than a decade-long series of these models, enabled us to reveal the processes that occur in the regions of large (Mw≥8) earthquakes that have not been studied previously. The previous research into the time variations of the gravity field in the regions of the giant earthquakes, such as the seismic catastrophes in Sumatra (2004) and Chile (2010), and the Tohoku mega earthquake in Japan (2011), covered the coseismic gravity jump followed by the long postseismic changes reaching almost the same amplitude. The coseismic gravity jumps resulting from the lower-magnitude events are almost unnoticeable. However, we have established a long steady growth of gravity anomalies after a number of such earthquakes. For instance, in the regions of the subduction earthquakes, the growth of the positive gravity anomaly above the oceanic trench was revealed after two events with magnitudes Mw=8.5 in the Sumatra region (the Nias earthquake of March 2005 and the Bengkulu event of September 2007 near the southern termination of Sumatra Island), after the earthquake with Mw=8.5 on Hokkaido in September 2007, a doublet Simushir earthquake with the magnitudes Mw = 8.3 and 8.1 in the Kuriles in November 2006 and January 2007, and after the earthquake off the Samoa Island in September 2009 (Mw=8.1). The steady changes in the gravity field have also been recorded after the earthquake in the Sichuan region (May 2008, Mw = 8.0) and after the doublet event with magnitudes 8.6 and 8.2, which occurred in the Wharton Basin of the Indian Ocean on April 11, 2012. The detailed analysis of the growth of the positive anomaly in gravity after the Simushir earthquake of November 2006 is presented. The growth started a few months after the event synchronously with the seismic activation on the downdip extension of the coseismically ruptured fault plane zone. The data demonstrating the increasing depth of the aftershocks since March 2007 and the approximately simultaneous change in the direction and average velocity of the horizontal surface displacements at the sites of the regional GPS network indicate that this earthquake induced postseismic displacements in a huge area extending to depths below 100 km. The total displacement since the beginning of the growth of the gravity anomaly up to July 2012 is estimated at 3.0 m in the upper part of the plate’s contact and 1.5 m in the lower part up to a depth of 100 km. With allowance for the size of the region captured by the deformations, the released total energy is equivalent to the earthquake with the magnitude Mw = 8.5. In our opinion, the growth of the gravity anomaly in these regions indicates a large-scale aseismic creep over the areas much more extensive than the source zone of the earthquake. These processes have not been previously revealed by the ground-based techniques. Hence, the time series of the GRACE gravity models are an important source of the new data about the locations and evolution of the locked segments of the subduction zones and their seismic potential.  相似文献   

17.
For evaluating the parameters of the vibrations of the Earth’s surface in the case of strong earthquakes, which are possible in the future, the regular patterns of the emission and propagation of seismic waves in the North Caucasus regions are investigated. The regional parameters of emission and propagation of seismic waves are evaluated by solution of the inverse problems of stochastic modeling of the accelerograms of the earthquakes, recorded by the seismic station in Sochi. The horizontal components of the strongest earthquakes (M w ～ 3.9?5.6), that occurred in 2002–2006 within a radius of ～300 km from the seismic station, with source depths up to 60 km are modeled. For calculations of accelerograms, estimates of the quality are used, obtained earlier for this region in the form: Q(f) ～ 80 ～ f 0.9. The parameter settings are carried out, which determine the shapes of the source spectra, the amplification of the seismic waves in the Earth’s crust, the weakening of the waves at high frequencies (κ), the parameters that determine the shape and duration of accelerograms, etc. Sufficiently good agreement of the calculated and recorded accelerograms is obtained, the regional characteristics of emission and propagation of seismic waves, which can be used for prediction of the parameters of strong motions in the North Caucasus, are evaluated; however, in the future these characteristics should be studied in more detail.  相似文献   

18.
Crustal deformation by the M w 9.0 megathrust Tohoku earthquake causes the extension over a wide region of the Japanese mainland. In addition, a triggered M w 5.9 East Shizuoka earthquake on March 15 occurred beneath the south flank, just above the magma system of Mount Fuji. To access whether these earthquakes might trigger the eruption, we calculated the stress and pressure changes below Mount Fuji. Among the three plausible mechanisms of earthquake–volcano interactions, we calculate the static stress change around volcano using finite element method, based on the seismic fault models of Tohoku and East Shizuoka earthquakes. Both Japanese mainland and Mount Fuji region are modeled by seismic tomography result, and the topographic effect is also included. The differential stress given to Mount Fuji magma reservoir, which is assumed to be located to be in the hypocentral area of deep long period earthquakes at the depth of 15 km, is estimated to be the order of about 0.001–0.01 and 0.1–1 MPa at the boundary region between magma reservoir and surrounding medium. This pressure change is about 0.2 % of the lithostatic pressure (367.5 MPa at 15 km depth), but is enough to trigger an eruptions in case the magma is ready to erupt. For Mount Fuji, there is no evidence so far that these earthquakes and crustal deformations did reactivate the volcano, considering the seismicity of deep long period earthquakes.  相似文献   

19.
According to S.A. Fedotov’s long-term earthquake forecast, the Middle Kuril Is. has long (since 1965) been a likely location for the next M ≥ 7.7 earthquake, i.e., a seismic gap. The present study integrates seismological, geological, and geophysical data to assess the earthquake potential of the gap prior to November 15, 2006. Seismological data were used to carry out a comparative analysis of 3D seismic energy density for three zones of the Kuril region. The density for the Middle Kuril Is. turned out to be twice as small as that for the North Kuril Is. and nearly six times as small as that for the South Kurils. Various parameters of the seismic process for the Kuril region have been estimated in quantitative terms. It is shown that the rate of completely reported (M ≥ 6) earthquakes occurring down to 70 km depth in the Middle Kuril Is. is approximately three times as small as that for the entire Kuril arc. Increased heat flow was recorded there (up to 100 mW/m2). The top of the high conductivity layer is shallower (at a depth of 100 km). The trends of major faults and other seismotectonic features have been taken into account. Based on these data (prior to November 15, 2006), the previous conclusion about the low seismic activity of the Middle Kuril Is. was corroborated. Two great earthquakes occurred in the region on November 15, 2006 (M w = 8.3) and January 13, 2007 (M w = 8.1) with subsequent tsunami waves. The erroneous inference as to low seismic activity was related to the fact that the seismic cycle in the Middle Kuril Is. may be as long as 150–200 years. We come to the conclusion that an analysis of the level of seismic activity for the region should start with the construction of standardized recurrence curves and determining the magnitude of the maximum possible earthquake.  相似文献   

20.
The deep-focus Sea of Okhotsk earthquake that occurred on May 24, 2013 (h = 630 km, M w = 8.3) was accompanied by anomalous effects that were unknown previously. A combined analysis of published data concerning the source rupture evolution and some features of the deep structure provided an explanation of some anomalous effects, such as the large number of aftershocks and the low level of ground shaking in the epicentral area. However, GPS observations revealed high coseismic vertical displacements in the area. The seafloor uplift in the Sea of Okhotsk and the adjacent coasts was 3–12 mm, peaking at the approximate center of the sea, while Kamchatka and the North Kuril Islands subsided by 3–18 mm, peaking at the Apacha station 190 km east of the earthquake epicenter. These maximum estimates are 1.2–1.8 times the analogous values (10 mm) for the Chile mega-earthquake of May 20, 1960 (M w ~ 9.5). It is known that the large distances at which ground shaking is felt during deep-focus earthquakes are due to the fact that the body waves travel through the high-Q lower mantle. However, this does not explain the paradox of the present earthquake in the Sea of Okhotsk, viz., a constant intensity of shaking (two grades) in the range of epicentral distances between 1300 and 9500 km. The explanation requires consideration of the earth’s free oscillations excited by the earthquake.  相似文献   

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