共查询到20条相似文献,搜索用时 31 毫秒
1.
Hill DP Reasenberg PA Michael A Arabaz WJ Beroza G Brumbaugh D Brune JN Castro R Davis S Depolo D Ellsworth WL Gomberg J Harmsen S House L Jackson SM Johnston MJ Jones L Keller R Malone S Munguia L Nava S Pechmann JC Sanford A Simpson RW Smith RB Stark M Stickney M Vidal A Walter S Wong V Zollweg J 《Science (New York, N.Y.)》1993,260(5114):1617-1623
The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma). 相似文献
2.
Microseisms: mode structure and sources 总被引:3,自引:0,他引:3
Frequency-wave number spectra of microseisms were obtained by use of a set of short-period and long-period seismometers at LASA (Large Aperture Seismic Array, Montana). At times of relatively high microseismic activity short-period (shorter than 5 seconds) microseisms consist of both body waves and higher-mode surface waves. From the phase velocity and direction of body waves, source areas were determined, coinciding with low-pressure regions on the weather map. At longer periods, microseisms consist of fundamental- mode Rayleigh and Love waves, the former being dominant. Most microseismic energy arrives at LASA from the northeast and the west. 相似文献
3.
Long-range correlations in the diffuse seismic coda 总被引:9,自引:0,他引:9
The late seismic coda may contain coherent information about the elastic response of Earth. We computed the correlations of the seismic codas of 101 distant earthquakes recorded at stations that were tens of kilometers apart. By stacking cross-correlation functions of codas, we found a low-frequency coherent part in the diffuse field. The extracted pulses have the polarization characteristics and group velocities expected for Rayleigh and Love waves. The set of cross-correlations has the symmetries of the surface-wave part of the Green tensor. This seismological example shows that diffuse waves produced by distant sources are sufficient to retrieve direct waves between two perfectly located points of observation. Because it relies on general properties of diffuse waves, this result has potential applications in other fields. 相似文献
4.
The phase velocities of Rayleigh waves increase more rapidly with distance from the East Pacific Rise (EPR) axis than is predicted by models of conductive cooling of the lithosphere. Low velocities near the axis are probably caused by partial melt at depths of 20 to 70 kilometers in a zone several hundred kilometers wide. The lowest velocities are offset to the west of the EPR. Wave propagation is anisotropic; the fast direction is approximately perpendicular to the ridge, parallel to the spreading direction. Anisotropy increases from a minimum near the axis to 3 percent or more on the flanks. 相似文献
5.
A model for a seismic computerized alert network 总被引:1,自引:0,他引:1
Heaton TH 《Science (New York, N.Y.)》1985,228(4702):987-990
In large earthquakes, damaging ground motions may occur at large epicentral distances. Because of the relatively slow speed of seismic waves, it is possible to construct a system to provide short-term warning (as much as several tens of seconds) of imminent strong ground motions from major earthquakes. Automated safety responses could be triggered by users after receiving estimates of the arrival time and strength of shaking expected at an individual site. Although warning times are likely to be short for areas greatly damaged by relatively numerous earthquakes of moderate size, large areas that experience very strong shaking during great earthquakes would receive longer warning times. 相似文献
6.
Hirose H Asano Y Obara K Kimura T Matsuzawa T Tanaka S Maeda T 《Science (New York, N.Y.)》2010,330(6010):1502
We identified a strong temporal correlation between three distinct types of slow earthquakes distributed over 100 kilometers along the dip of the subducting oceanic plate at the western margin of the Nankai megathrust rupture zone, southwest Japan. In 2003 and 2010, shallow very-low-frequency earthquakes near the Nankai trough as well as nonvolcanic tremor at depths of 30 to 40 kilometers were triggered by the acceleration of a long-term slow slip event in between. This correlation suggests that the slow slip might extend along-dip between the source areas of deeper and shallower slow earthquakes and thus could modulate the stress buildup on the adjacent megathrust rupture zone. 相似文献
7.
We have discovered nonvolcanic tremor activity (i.e., long-duration seismic signals with no clear P or S waves) within a transform plate boundary zone along the San Andreas Fault near Cholame, California, the inferred epicentral region of the 1857 Fort Tejon earthquake (moment magnitude approximately 7.8). The tremors occur between 20 to 40 kilometers' depth, below the seismogenic zone (the upper approximately 15 kilometers of Earth's crust where earthquakes occur), and their activity rates may correlate with variations in local earthquake activity. 相似文献
8.
Intermediate-period Rayleigh and Love waves propagating across Tibet indicate marked radial anisotropy within the middle-to-lower crust, consistent with a thinning of the middle crust by about 30%. The anisotropy is largest in the western part of the plateau, where moment tensors of earthquakes indicate active crustal thinning. The preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy. The middle-to-lower crust of Tibet appears to have thinned more than the upper crust, consistent with deformation of a mechanically weak layer that flows as if confined to a channel. 相似文献
9.
Stress triggering of the 1994 m = 6.7 northridge, california, earthquake by its predecessors 总被引:1,自引:0,他引:1
A model of stress transfer implies that earthquakes in 1933 and 1952 increased the Coulomb stress toward failure at the site of the 1971 San Fernando earthquake. The 1971 earthquake in turn raised stress and produced aftershocks at the site of the 1987 Whittier Narrows and 1994 Northridge ruptures. The Northridge main shock raised stress in areas where its aftershocks and surface faulting occurred. Together, the earthquakes with moment magnitude M >/= 6 near Los Angeles since 1933 have stressed parts of the Oak Ridge, Sierra Madre, Santa Monica Mountains, Elysian Park, and Newport-lnglewood faults by more than 1 bar. Although too small to cause earthquakes, these stress changes can trigger events if the crust is already near failure or advance future earthquake occurrence if it is not. 相似文献
10.
Two years of local earthquake, temperature, and rainfall data taken near a tiltmeter site were used in a study of the numerical relation between these phenomena and the recorded tilt response. A least-squares shaping and predictive error filter approach was used. The relations were ranked in part according to the root mean square (r.m.s.) error of fit across the entire sample space. The tilt data with an annual range of tilt of approximately 10 microradians were fitted to the combined weather data of temperature and rainfall with a 0.75-microradian r.m.s. error. The best fit of earthquakes to these same tilt data is the subclass of events with magnitude (M) > 2.5 within 30 kilometers of the tilt site. The filter that mapped earthquakes to tilt yielded a 1.03-microradian r.m.s. error. The most unusual tilt anomaly over the entire 2-year period has the best fit of rainfall to the data for any single month of the entire data set. This unusual anomaly was the basis of an erroneously predicted earthquake (M approximately 5). These data indicate that if there are premonitory earthquake signals, they are buried in local meteorlogical noise. Separating an earthquake anomaly from the response to surface phenomena becomes more difficult as the earthquake anomaly lead time approaches the rise time of the soil to weather and seasonal variations. 相似文献
11.
Earthquakes with non--double-couple mechanisms 总被引:2,自引:0,他引:2
Frohlich C 《Science (New York, N.Y.)》1994,264(5160):804-809
Seismological observations confirm that the pattern of seismic waves from some earthquakes cannot be produced by slip along a planar fault surface. More than one physical mechanism is required to explain the observed varieties of these non-double-couple earthquakes. The simplest explanation is that some earthquakes are complex, with stress released on two or more suitably oriented, nonparallel fault surfaces. However, some shallow earthquakes in volcanic and geothermal areas require other explanations. Current research focuses on whether fault complexity explains most observed non-double-couple earthquakes and to what extent ordinary earthquakes have non-double-couple components. 相似文献
12.
Residuals for P-wave traveltimes at a seismnograph station near Bear Valley, California, for small, precisely located local earthquakes at distances of 20 to 70 kilometers show a sharp increase of nearly 0.3 second about 2 months before a magnitude 5.0 earthquake that occurred within a few kilometers of the station. This indicates that velocity changes observed elsewhere premonitory to earthquakes, possibly related to dilatancy, occur along the central section of the San Andreas fault system. 相似文献
13.
Sanders CO 《Science (New York, N.Y.)》1993,260(5110):973-976
Two lines of evidence suggest that large earthquakes that occur on either the San Jacinto fault zone (SJFZ) or the San Andreas fault zone (SAFZ) may be triggered by large earthquakes that occur on the other. First, the great 1857 Fort Tejon earthquake in the SAFZ seems to have triggered a progressive sequence of earthquakes in the SJFZ. These earthquakes occurred at times and locations that are consistent with triggering by a strain pulse that propagated southeastward at a rate of 1.7 kilometers per year along the SJFZ after the 1857 earthquake. Second, the similarity in average recurrence intervals in the SJFZ (about 150 years) and in the Mojave segment of the SAFZ (132 years) suggests that large earthquakes in the northern SJFZ may stimulate the relatively frequent major earthquakes on the Mojave segment. Analysis of historic earthquake occurrence in the SJFZ suggests little likelihood of extended quiescence between earthquake sequences. 相似文献
14.
Beghin C Lebreton JP Maehlum BN Troim J Ingsoy P Mimacrchau JL 《Science (New York, N.Y.)》1984,225(4658):188-191
The effects of electron beam emissions from Spacelab were recorded with onboard diagnostic instruments. The variation of the Spacelab-shuttle potential with respect to the ambient plasma near the scientific air lock was investigated. Data on the waves and instabilities triggered by the electron beams are discussed. Within the electron gyrofrequency and electron plasma frequency range, strong signals were detected by both electric and magnetic antennas during the beam emissions. The frequencies of the emitted waves were compared to the characteristic plasma frequencies to enable mode identification. 相似文献
15.
Engdahl ER 《Science (New York, N.Y.)》1968,161(3838):263-264
Seismic waves reflected as many as four times within Earth's outer core are routinely recorded from large earthquakes. Observations of these waves are confined to rays near grazing incidence on the core-mantle boundary, in agreement with theoretical expectation. Minor adjustments to outer-core velocities may be necessary to account for certain of these arrivals that are not predicted by present core models. A change of 10 kilometers or more in the currently accepted core radius, 3473 kilometers, is not corroborated by the new data. 相似文献
16.
Surface-wave magnitudes (M(8)) of 4.1 and 3.2 are determined for two events in southern Algeria. The corresponding body-wave magnitudes (m(b)) are 5.8 and 4.9, respectively. The surface waves from these events are much smaller than would be expected from most earthquakes of comparable body-wave magnitudes, which fact suggests that these waves were generated by underground explosions. 相似文献
17.
A destructive earthquake that occurred in 1886 near Charleston, South Carolina, was associated with widespread liquefaction of shallow sand structures and their extravasation to the surface. Several seismically induced paleoliquefaction structures preserved within the shallow sediments in the meizoseismal area of the 1886 event were identified. Field evidence and radiocarbon dates suggest that at least two earthquakes of magnitudes greater than 6.2 preceded the 1886 event in the past 3000 to 3700 years. The evidence yielded an initial estimate of about 1500 to 1800 years for the maximum recurrence of destructive, intraplate earthquakes in the Charleston region. 相似文献
18.
Brownell PH 《Science (New York, N.Y.)》1977,197(4302):479-482
Loose sand conducts compressional and surface (Rayleigh) waves at relatively low velocities (95 to 120 meters per second and 40 to 50 meters per second, respectively) compared to other natural substrates. For frequencies between 1 and 5 kilohertz, the specific attenuation factor, Q, for sand is 18. Compound slit sensilla on basitarsal leg segments of sand-dwelling scorpions respond to surface waves generated by movements of insects as far as 50 centimeters away, and tarsal sensory hairs respond to higher-frequency (mostly compressional-wave) components of the signal. 相似文献
19.
Between November 1976 and November 1977 a swarm of small earthquakes (local magnitude = 3) occurred on or near the San Andreas fault near Palmdale, California. This swarm was the first observed along this section of the San Andreas since cataloging of instrumental data began in 1932. The activity followed partial subsidence of the 35-centimeter vertical crustal uplift known as the Palmdale bulge along this "locked" section of the San Andreas, which last broke in the great (surface-wave magnitude = 8(1/4)+) 1857 Fort Tejon earthquake. The swarm events exhibit characteristics previously observed for some foreshock sequences, such as tight clustering of hypocenters and time-dependent rotations of stress axes inferred from focal mechanisms. However, because of our present lack of understanding of the processes that precede earthquake faulting, the implications of the swarm for future large earthquakes on the San Andreas fault are unknown. 相似文献
20.
San andreas fault zone head waves near parkfield, california 总被引:1,自引:0,他引:1
Microearthquake seismograms from the borehole seismic network on the San Andreas fault near Parkfield, California, provide three lines of evidence that first P arrivals are "head" waves refracted along the cross-fault material contrast. First, the travel time difference between these arrivals and secondary phases identified as direct P waves scales linearly with the source-receiver distance. Second, these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct P arrivals. Third, the first motion polarities of the emergent arrivals are reversed from those of the direct P waves as predicted by the theory of fault zone head waves for slip on the San Andreas fault. The presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms. The fault zone head waves indicate that the velocity contrast across the San Andreas fault near Parkfield is approximately 4 percent. Further studies of these waves may provide a way of assessing changes in the physical state of the fault system. 相似文献