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1.
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
2.
The caustic of SKP is found at an epicentral distance ΔC = 129.5° for surface foci and at ΔC = 128.9° for foci at 400 km depth, by means of amplitude-distance graphs based upon short-period time-domain measurements. These results are essentially confirmed by long-period time-domain measurements of SKP as well as by frequency-domain studies, even though the spectra are less accurate for such determinations. The average period of SKP is from short-period records, significantly different from the corresponding PKP-period of 1.00 ± 0.31 sec. Likewise, the long-period averages of SKP = 10.8 ± 4.5 sec and of PKP = 7.7 ± 3.0 sec are significantly different from each other. A travel-time table of SKP1 is deduced, covering the epicentral distance range of 130–143° and the focal depth range of 100–700 km. All results are based on measurements on seismograms of the Swedish network of stations, deriving almost exclusively from earthquakes in the southwest Pacific area. 相似文献
3.
We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (M
w=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at 500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years. 相似文献
4.
The paper presents the current state of magnitude estimation for Koyna earthquakes exceeding magnitude 3.0. We estimate coda duration magnitude from analogue seismograms recorded on the short period vertical (SPZ) seismometer at Hyderabad seismic observatory HYB and determine moment magnitude using very broad-band (VBB) data from the Geoscope station (HYB)and short period digital data from the local seismic network of NationalGeophysical Research Institute (NGRI) around the Koyna and Warna reservoirs.Firstly, the seismograms of 97 Koyna earthquakes exceeding magnitude 4.0 havebeen used to establish a new empirical coda duration magnitude scale which includes the higher order terms of log10, where is the coda length in seconds. Four background noise levels (1, 2, 6 and 10 mm) areconsidered to estimate the coda duration. We found that the duration magnitudes for 1 mm background level are more stable than those for 2, 6 and 10 mm. The new coda duration magnitude (Mdnew) scale, for 1 mmlevel, is:Mdnew = –0.594 + 2.04 log10 – 0.0435 (log10)2The estimated Mdnew are compatible with the reported MS values of the NGRI observatory and the mb values of the United States Geological Survey (USGS). These magnitudes can be obtained within the standard deviation of ± 0.26 units of MS (NGRI). A new relatively homogeneous catalog for Koyna earthquakes of Mdnew 4.0 is obtained. The momentmagnitudes for 34 Koyna-Warna events of Mdnew ranging from 3.0 to 5.4 have been estimated using two techniques. The first utilizes amplitudes of band-pass filtered (between 15 and 30 sec) velocity traces of moderate Koyna-Warna earthquakes of MW} 4.4 to 5.4, we abbreviate the magnitude using MA. The second is based on the S-wave spectrum of short period seismograms of local earthquakes (MW < 3.8). Moment magnitudes estimated by spectral analysis mainly depend on the estimation of event's long-period spectral level and appears to saturate for moderate Koyna-Warnaearthquakes (MW > 3.8). We recommend the use of both techniques whenever possible. The estimated moment magnitudes and Mdnew show an almost linear relationship with a standard deviation of ± 0.05. 相似文献
5.
Gerhard Müller Abdel Hafiz Mula Søren Gregersen 《Physics of the Earth and Planetary Interiors》1977,14(1):30-40
The amplitudes of the core reflection PcP are sensitive to the wave velocities and densities in the neighborhood of the core-mantle boundary (CMB). We study the amplitude ratio of the long-period phases PcP and P from two South American deep-focus earthquakes with favorable fault-plane solution, depth and magnitude, as recorded by WWNSS and CSN stations in North America.Comparison is made with long-period PcP/P amplitude ratios, derived from theoretical seismograms for a variety of CMB models. Models from previous studies, which were mainly derived from short-period PcP observations and which are characterized by discrete layers above the CMB, are almost all inconsistent with the long-period data. The data also discriminate against low nonzero S velocities below the CMB. Simple first-order-discontinuity models of the CMB, for instance according to the Jeffreys-Bullen earth model or according to recent models based mainly on free oscillations, explain the data reasonably well.Model improvements are attempted by varying the P-velocity gradient above the CMB. The best amplitude fit is obtained for a rather strong decrease in P velocity with depth in this zone which, however, gives no acceptable traveltime fit for PcP. The scatter in body-wave amplitudes is considerable even for long-period waves and may prevent the correct assessment of that part of the amplitude variation of a phase with distance that is due to the variation of velocities and densities with depth alone. 相似文献
6.
We conducted moment tensor inversion and studied source rupture process for M
S=7.9 earthquake occurred in the border area of China, Russia and Mongolia on September 27 2003, by using digital teleseismic
P-wave seismograms recorded by long-period seismograph stations of the global seismic network. Considering the aftershock
distribution and the tectonic settings around the epicentral area, we propose that the M
S=7.9 earthquake occurred on a fault plane with the strike of 127°, the dip of 79° and the rake of 171°. The rupture process
inversion result of M
S=7.9 earthquake shows that the total rupture duration is about 37 s, the scalar moment tensor is M
0=0.97×1020 N·m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated
is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust
under the earth surface. The maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed
that when the rupture propagated towards northwest and closed to the area around the M
S=7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. The consistence
of spatial distribution of slip on the fault plane with the distribution of aftershocks also supports that the rupture is
a heterogeneous process owing to the presence of barriers. 相似文献
7.
—A finite-source rupture model of the July 30, 1995, M w = 8.1 Antofagasta (Northern Chile) subduction earthquake is developed using body and surface waves that span periods from 20 to 290s. A long-period (150–290s) surface-wave spectral inversion technique is applied to estimate the average finite-fault source properties. Deconvolutions of broadband body waves using theoretical Green’s functions, and deconvolutions of broadband fundamental mode surface waves using empirical Green’s functions provided by a large aftershock, yield effective source time functions containing periods from 20 to 200s for many directivity parameters. The source time functions are used in an inverse radon transform to image a one-dimensional spatial model of the moment rate history. The event produced a predominantly unilateral southward rupture, yielding strong directivity effects on all seismic waves with periods less than a few hundred seconds. The aftershock information, spectral analysis, and moment rate distribution indicate a rupture length of 180–200km, with the largest slip concentrated in the first 120km, a rupture azimuth of 205°± 10° along the Chilean coastline, and a rupture duration of 60–68s with a corresponding average rupture velocity of 3.0–3.2km/s. The overall rupture character is quite smooth, accentuating the directivity effects and reducing the shaking intensity, however there are three regions with enhanced moment rate distributed along the rupture zone near the epicenter, 50 to 80km south of the epicenter, and 110 to 140km south of the epicenter. 相似文献
8.
Complexity of rupture propagation has an important bearing on the state of stress along the earthquake fault plane and on the prediction of strong ground motion in the near-field. By studying far-field body waveforms recorded by WWSSN long-period seismograms it has been possible to investigate the degree of complexity of several Turkish earthquakes. The results, which are obtained by matching synthetic P waveforms to observed data indicate that the July 22, 1967 Mudurnu Valley earthquake (Ms = 7.1) is a complex event which can be explained by the superposition of elementary sources with variable amplitudes and source time sequence history. In this regard, it is very similar to the February 4, 1976 Guatemala earthquake (Ms = 7.5). A comparison of these two events indicates that their source-time series ranges from 5 to ca. 20 s and, regardless of the total moment of the earthquake, the moment of the individual events is bounded at around 5 × 1026 dyn cm. The November 24, 1976 E. Turkey earthquake (Ms = 7.3), on the other hand, has a complexity which cannot be explained by such a simple model; in this respect, it may be more similar to the Tangshan, China, earthquake and as such, may involve significant thrust, normal or other complications to its faulting mechanism than the strike-slip mechanism of the P-wave first-motion data. The source time history for the 1967 Mudurnu Valley event is used to illustrate its significance in modeling strong ground motion in the near field. The complex source-time series of the 1967 event predicts greater amplitudes (2.5 larger) in strong ground motion than a uniform model scaled to the same size for a station 20 km from the fault. Such complexity is clearly important in understanding what strong ground motion to expect in the near-field of these and other continental strike-slip faults such as the San Andreas. 相似文献
9.
G. Müller 《Physics of the Earth and Planetary Interiors》1975,10(1):70-73
Short-period precursors to PKIKP are still sometimes explained as reflections from and refractions below discontinuities in the lower part of the outer core. The most recent core model with such discontinuities, published by Bertrand and Clowes (1974), is investigated with respect to the corresponding long-period precursors. Theoretical seismograms show that they would be strong enough to be observed. Long-period WWNSS seismograms for an earthquake in the New Hebrides, however, do not show them. Therefore, the discontinuities of Bertrand and Clowes' model are not real. It is concluded that the P-velocity depth function in the outer core is smooth down to the innercore boundary. 相似文献
10.
Georgios N. Stavrakakis Akis-G. Tselentis John Drakopoulos 《Pure and Applied Geophysics》1986,124(6):1039-1050
Teleseismic long-period P waves from the June 20, 1978, Thessaloniki (N. Greece) earthquake (M
s=6.4) were modeled in an attempt to extract information about asperities or barriers on the fault plane. The analysis is based on the inversion method of complex P waves developed by Kikuchi and Kanamori (1982). A far-field source time function with a rise time of 2 sec and a process time of 5 sec is inferred, corresponding to a source dimension of about 10 km when a rupture velocity of 2 km/sec is assumed.The source depth of this shock, estimated by matching synthetic seismograms to observations, is found to be 8 km. The sum of the seismic moments of the individual subevents amounts to 3.3×1025 dyn-cm. 相似文献
11.
Source Characteristics of the 12 November 1996 Mw 7.7 Peru Subduction Zone Earthquake 总被引：1，自引：0，他引：1
—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
12.
Prantik Mandal R. K. Chadha N. Kumar I. P. Raju C. Satyamurty 《Pure and Applied Geophysics》2007,164(10):1963-1983
During the last six years, National Geophysical Research Institute, Hyderabad has established a semi-permanent seismological
network of 5–8 broadband seismographs and 10–20 accelerographs in the Kachchh seismic zone, Gujarat with a prime objective
to monitor the continued aftershock activity of the 2001 Mw 7.7 Bhuj mainshock. The reliable and accurate broadband data for the 8 October Mw 7.6 2005 Kashmir earthquake and its aftershocks from this network as well as Hyderabad Geoscope station enabled us to estimate
the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the Peninsular India.
Firstly, we measure Rayleigh-and Love-wave group velocity dispersion curves in the period range of 8 to 35 sec and invert
these curves to estimate the crustal and upper mantle structure below the western part of Peninsular India. Our best model
suggests a two-layered crust: The upper crust is 13.8 km thick with a shear velocity (Vs) of 3.2 km/s; the corresponding values
for the lower crust are 24.9 km and 3.7 km/sec. The shear velocity for the upper mantle is found to be 4.65 km/sec. Based
on this structure, we perform a moment tensor (MT) inversion of the bandpass (0.05–0.02 Hz) filtered seismograms of the Kashmir
earthquake. The best fit is obtained for a source located at a depth of 30 km, with a seismic moment, Mo, of 1.6 × 1027 dyne-cm, and a focal mechanism with strike 19.5°, dip 42°, and rake 167°. The long-period magnitude (MA ~ Mw) of this earthquake is estimated to be 7.31. An analysis of well-developed sPn and sSn regional crustal phases from the bandpassed
(0.02–0.25 Hz) seismograms of this earthquake at four stations in Kachchh suggests a focal depth of 30.8 km. 相似文献
13.
To understand the ground-motion contribution by multiple-mode surface-wave arrivals, the surface-wave theory required for predicting ground-motion time histories is discussed. The adequacy of the theory in accounting for observed earthquake ground motion is tested by comparing theoretically predicted long-period seismograms with real seismograms for two earthquakes in the central United States. The agreement between the predicted and observed maximum vertical component Lg ground velocities and accelerations in the 2- to 3-sec period range is excellent over a distance range of 100 to 2,000 km. 相似文献
14.
Prantik Mandal R. K. Chadha N. Kumar I. P. Raju C. Satyamurty 《Pure and Applied Geophysics》2007,164(11):2235-2254
During the last six years, the National Geophysical Research Institute, Hyderabad has established a semi-permanent seismological
network of 5 broadband seismographs and 10 accelerographs in the Kachchh seismic zone, Gujarat, with the prime objective to
monitor the continued aftershock activity of the 2001 Mw7.7 Bhuj mainshock. The reliable and accurate broadband data for the Mw 7.6 (8 Oct., 2005) Kashmir earthquake and its aftershocks from this network, as well as from the Hyderabad Geoscope station,
enabled us to estimate the group velocity dispersion characteristics and the one-dimensional regional shear-velocity structure
of peninsular India. Firstly, we measure Rayleigh- and Love-wave group velocity dispersion curves in the range of 8 to 35
sec and invert these curves to estimate the crustal and upper mantle structure below the western part of peninsular India.
Our best model suggests a two-layered crust: The upper crust is 13.8-km thick with a shear velocity (Vs) of 3.2 km/s; the
corresponding values for the lower crust are 24.9 km and 3.7 km/sec. The shear velocity for the upper mantle is found to be
4.65 km/sec. Based on this structure, we perform a moment tensor (MT) inversion of the bandpass (0.05–0.02 Hz) filtered seismograms
of the Kashmir earthquake. The best fit is obtained for a source located at a depth of 30 km, with a seismic moment, Mo, of
1.6 × 1027 dyne-cm, and a focal mechanism with strike 19.5°, dip 42°, and rake 167°. The long-period magnitude (MA ~ Mw) of this earthquake is estimated to be 7.31. An analysis of well-developed sPn and sSn regional crustal phases from the bandpassed
(0.02–0.25 Hz) seismograms of this earthquake at four stations in Kachchh suggests a focal depth of 30.8 km. 相似文献
15.
An earthquake of M
S=7.4 occurred in Mani, Xizang (Tibet), China on November 8, 1997. The moment tensor of this earthquake was inverted using
the long period body waveform data from China Digital Seismograph Network (CDSN). The apparent source time functions (ASTFs)
were retrieved from P and S waves, respectively, using the deconvolution technique in frequency domain, and the tempo-spatial
rupture process on the fault plane was imaged by inverting the azimuth dependent ASTFs from different stations. The result
of the moment tensor inversion indicates that the P and T axes of earthquake-generating stress field were nearly horizontal, with the P axis in the NNE direction (29°), the T axis in the SEE direction (122°) and that the NEE-SWW striking nodal plane and NNW-SSE striking nodal plane are mainly left-lateral
and right-lateral strike-slip, respectively; that this earthquake had a scalar seismic moment of 3.4×1020 N·m, and a moment magnitude of M
W=7.6. Taking the aftershock distribution into account, we proposed that the earthquake rupture occurred in the fault plane
with the strike of 250°, the dip of 88° and the rake of 19°. On the basis of the result of the moment tensor inversion, the
theoretical seismograms were synthesized, and then the ASTFs were retrieved by deconvoving the synthetic seismograms from
the observed seismograms. The ASTFs retrieved from the P and S waves of different stations identically suggested that this
earthquake was of a simple time history, whose ASTF can be approximated with a sine function with the half period of about
10 s. Inverting the azimuth dependent ASTFs from P and S waveforms led to the image showing the tempo-spatial distribution
of the rupture on the fault plane. From the "remembering" snap-shots, the rupture initiated at the western end of the fault,
and then propagated eastward and downward, indicating an overall unilateral rupture. However, the slip distribution is non-uniform,
being made up of three sub-areas, one in the western end, about 10 km deep ("western area"); another about 55 km away from
the western end and about 35 km deep ("eastern area"); the third about 30 km away from the western end and around 40 km deep
("central area"). The total rupture area was around 70 km long and 60 km wide. From the "forgetting" snap-shots, the rupturing
appeared quite complex, with the slip occurring in different position at different time, and the earthquake being of the characteristics
of "healing pulse". Another point we have to stress is that the locations in which the rupture initiated and terminated were
not where the main rupture took place. Eventually, the static slip distribution was calculated, and the largest slip values
of the three sub-areas were 956 cm, 743 cm and 1 060 cm, for the western, eastern and central areas, respectively. From the
slip distribution, the rupture mainly distributed in the fault about 70 km eastern to the epicenter; from the aftershock distribution,
however, the aftershocks were very sparse in the west to the epicenter while densely clustered in the east to the epicenter.
It indicated that the Mani M
S=7.9 earthquake was resulted from the nearly eastward extension of the NEE-SWW to nearly E-W striking fault in the northwestern
Tibetan plateau.
Contribution No. 99FE2016, Institute of Geophysics, China Seismological Bureau.
This work is supported by SSTCC Climb Project 95-S-05 and NSFDYS 49725410. 相似文献
16.
An early detection of the presence of rupture directivity plays a major role in the correct estimation of ground motions and
risks associated to the earthquake occurrence. We present here a simple method for a fast detection of rupture directivity,
which may be additionally used to discriminate fault and auxiliary planes and have first estimations of important kinematic
source parameters, such as rupture length and rupture time. Our method is based on the inversion of amplitude spectra from
P-wave seismograms to derive the apparent duration at each station and on the successive modelling of its azimuthal behaviour.
Synthetic waveforms are built assuming a spatial point source approximation, and the finite apparent duration of the spatial
point source is interpreted in terms of rupture directivity. Since synthetic seismograms for a point source are calculated
very quickly, the presence of directivity may be detected within few seconds, once a focal mechanism has been derived. The
method is here first tested using synthetic datasets, both for linear and planar sources, and then successfully applied to
recent Mw 6.2–6.8 shallow earthquakes in Peloponnese, Greece. The method is suitable for automated application and may be
used to improve kinematic waveform modelling approaches. 相似文献
17.
We characterize the heterogeneous source slip model of intraslab earthquakes to compare source scaling properties with those of inland crustal and subduction-zone plate-boundary earthquakes. We extracted rupture area (S), total area of asperity (S a), average slip (D) and average slip on asperity (D a) of eleven intraslab earthquakes following the procedure proposed by Somerville et al. (Seism Res Lett 70:59?C80, 1999) and proposed the empirical scaling relationship formula of S, S a, and D for intraslab earthquakes. Under the same seismic moment, an intraslab earthquake has a smaller rupture area and total area of asperity, and smaller average slip than an inland crustal earthquake. The area ratio of asperity area and total rupture area of intraslab earthquakes are similar to those of inland crustal earthquakes. The strong motion generation area (SMGA) scaling of intraslab earthquakes appears self-similar, and those results support the idea the characterized source model of intraslab earthquakes can be modeled in a manner similar to that of inland crustal earthquakes. 相似文献
18.
The January 18, 2010, shallow earthquake in the Corinth Gulf, Greece (M w 5.3) generated unusually strong long-period waves (periods 4–8 s) between the P and S wave arrival. These periods, being significantly longer than the source duration, indicated a structural effect. The waves were observed in epicentral distances 40–250 km and were significant on radial and vertical component. None of existing velocity models of the studied region provided explanation of the waves. By inverting complete waveforms, we obtained an 1-D crustal model explaining the observation. The most significant feature of the best-fitting model (as well as the whole suite of models almost equally well fitting the waveforms) is a strong velocity step at depth about 4 km. In the obtained velocity model, the fast long-period wave was modeled by modal summation and identified as a superposition of several leaking modes. In this sense, the wave is qualitatively similar to P long or Pnl waves, which however are usually reported in larger epicentral distances. The main innovation of this paper is emphasis to smaller epicentral distances. We studied properties of the wave using synthetic seismograms. The wave has a normal dispersion. Azimuthal and distance dependence of the wave partially explains its presence at 46 stations of 70 examined. Depth dependence shows that the studied earthquake was very efficient in the excitation of these waves just due to its shallow centroid depth (4.5 km). 相似文献
19.
Based on digital teleseismic P-wave seismograms recorded by 28 long-period seismograph stations of the global seismic network, source process of the November 14, 2001 western Kunlun Mountain M S=8.1 (M W=7.8) earthquake is estimated by a new inversion method. The result shows that the earthquake is a very complex rupture event. The source rupture initiated at the hypocenter (35.95°N, 90.54°E, focal depth 10 km, by USGS NEIC), and propagated to the west at first. Then, in several minutes to a hundred minutes and over a large spatial range, several rupture growth points emerged in succession at the eastern end and in the central part of the finite fault. And then the source rupture propagated from these rupture growth points successively and, finally, stopped in the area within 50 km to the east of the centroid position (35.80°N, 92.91°E, focal depth 15 km, by Harvard CMT). The entire rupture lasted for 142 s, and the source process could be roughly separated into three stages: The first stage started at the 0 s and ended at the 52 s, lasting for 52 s and releasing approximately 24.4% of the total moment; The second stage started at the 55 s and ended at the 113 s, lasting for 58 s and releasing approximately 56.5% of the total moment; The third stage started at the 122 s and ended at the 142 s, lasting for 20 s and releasing approximately 19.1% of the total moment. The length of the ruptured fault plane is about 490 km. The maximum width of the ruptured fault plane is about 45 km. The rupture mainly occurred within 30 km in depth under the surface of the Earth. The average static slip in the underground rocky crust is about 1.2 m with the maximum static slip 3.6 m. The average static stress drop is about 5 MPa with the maximum static stress drop 18 MPa. The maximum static slip and the maximum stress drop occurred in an area within 50 km to the east of the centroid position. 相似文献
20.
—?We present results from a large-scale study of surface-wave group velocity dispersion across the Middle East, North Africa, southern Eurasia and the Mediterranean. Our database for the region is populated with seismic data from regional events recorded at permanent and portable broadband, three-component digital stations. We have measured the group velocity using a multiple narrow-band filter on deconvolved displacement data. Overall, we have examined more than 13,500 seismograms and made good quality dispersion measurements for 6817 Rayleigh- and 3806 Love-wave paths. We use a conjugate gradient method to perform a group-velocity tomography. Our current results include both Love- and Rayleigh-wave inversions across the region for periods from 10 to 60 seconds. Our findings indicate that short-period structure is sensitive to slow velocities associated with large sedimentary features such as the Mediterranean Sea and Persian Gulf. We find our long-period Rayleigh-wave inversion is sensitive to crustal thickness, such as fast velocities under the oceans and slow along the relatively thick Zagros Mts. and Turkish-Iranian Plateau. We also find slow upper mantle velocities along known rift systems. Accurate group velocity maps can be used to construct phase-matched filters along any given path. The filters can improve weak surface wave signals by compressing the dispersed signal. The signals can then be used to calculate regionally determined M S measurements, which we hope can be used to extend the threshold of m b :M S discriminants down to lower magnitude levels. Other applications include using the group velocities in the creation of a suitable background model for forming station calibration maps, and using the group velocities to model the velocity structure of the crust and upper mantle. 相似文献