摘要:

A tomographic inversion procedure is described and applied to a synthetic three‐dimensional (3‐D) seismic refraction data set, demonstrating that tomography is capable of determining a densely sampled velocity model with large velocity contrasts. Forward and inverse modeling procedures are chosen to minimize the computational costs of the inversion. Parameterizing the linearized inversion using functions defined along the ray paths, simple backprojection with zero pixel size is shown to exactly solve the linear problem, producing the smallest model for the slowness perturbation. For small grid cells, simple backprojection closely approximates the exact solution and is a sufficient solution for an iterative nonlinear inversion. This eliminates the need to store or solve a large system of linear equations. Accurate first arrival travel times are rapidly computed using a finite difference algorithm. Forward modeling between each simple backprojection allows the procedure to correctly account for the locations of the rays. This becomes more important as the spatial resolution of the model is improved. The computational efficiency of the entire nonlinear procedure allows the model to be densely sampled, providing a spatially well‐resolved 3‐D tomographic image. The synthetic refraction survey is designed to be similar to a published 3‐D survey over the East Pacific Rise. Tests based on this example and others show that 3‐D tomography is capable of inverting a large travel time data set for detailed earth structure with large lateral velocity variations and is stable in the presence of

ISSN:0148-0227    

DOI:10.1029/92JB00235

年代:1992

数据来源: WILEY

摘要:

The shortest path method [Moser, 1991] for the calculation of seismic ray paths and travel times along them can be applied directly in the hypocenter location method proposed byTarantola and Valette[1982]. It uses the analogy between seismic rays in the Earth and shortest paths in networks to construct first arrival times from one point to all other points of a three‐dimensional grid simultaneously in a fast, robust way, in Earth models of arbitrary complexity. Doing this for all stations of a seismic array, one can find the hypocenter location by minimizing the difference between the observed and the calculated travel times at the stations over the three‐dimensional grid. The concept of probability density functions allows then for a fully nonlinear examination of the uncertainties in the hypocenter location, due to uncertainties in the travel time data, numerical errors in the calculated travel times and, to a limited extent, incomplete knowledge about the Earth model. The result is a three‐dimensional contour map of regions of equal confidence for the earthquake location. The method becomes especially attractive when more than one event recorded by the same array is studied, because the calculation of the travel times, which is relatively the most time consuming operation, has to be done only once. The method is applied on the location of an event that occurred on January 18, 1989, in I

ISSN:0148-0227    

DOI:10.1029/91JB03176

年代:1992

数据来源: WILEY

摘要:

A velocity model of the inner core boundary (ICB) region is developed from broadband wave‐form modeling. The data consist of long period and short period record sections gathered from the U.S. arrays, WWSSN, and LRSM. Deep events beneath Indonesia were used; these sampled the ICB structure beneath the North Pacific. The Cagniard‐de Hoop technique was used to generate the synthetics, which allowed considerable flexibility in forward modeling these individual PKP phases. Our preferred model was developed from modifications of the Preliminary Reference Earth Model (PREM). It contains a relatively low velocity gradient above the ICB, a sharp jump at the ICB of about 0.78 km/s, and a relatively large gradient underneath. These features are constrained largely by the differential travel times and amplitude ratios of PKIKP and PKiKP of the short period section. Further modification of PREM in the fluid core was required to explain the long‐period observations associated with PKP waveforms. Our modeling results suggest a low‐velocity gradient at the bottom (roughly 400 km) of the fluid core. A lowQzone at the top of the inner core is required to fit the relative amplitudes of PKIKP phases similar to earlier studies. This model indicates possible inhomogeneity above the ICB, as suggested by other

ISSN:0148-0227    

DOI:10.1029/92JB00330

年代:1992

数据来源: WILEY

摘要:

The St. Elias, Alaska earthquake of 28 February, 1979 (Ms7.2) is reanalyzed using broadband teleseismic body waves and long‐period surface waves because of unresolved questions about its depth, focal mechanism, seismic moment, and location in a seismic gap. Teleseismic waveforms are simultaneously inverted to determine the source mechanism, seismic moment, rupture history and centroid depth. These data are well modeled with a point source propagating in the ESE direction with an average kinematic rupture velocity of 2.5 km/s. The best‐fitting source mechanism indicates underthrusting on a NE‐dipping plane. The mainshock depth of 24 km and the depth of aftershocks determined from inversions are consistent with locations on the gently dipping main thrust of the Pacific‐North American plate boundary. These depths are substantially different from those of earlier body wave studies and regional seismic network aftershock depth determinations but are in accord with the Harvard Centroid‐Moment Tensor and International Seismological Centre determinations. The seismic moment determined from body waves is 9.4×1019N‐m (Mw7.3). The spatial and temporal distribution of moment release indicates that the St. Elias earthquake was a complex rupture consisting of two distinct subevents within 38 s of the initial onset, followed by low moment release during the next 34 s. Earlier studies indicated an unusual amount of surface wave energy at very long periods (>200 s) that led some workers to suggest that St. Elias was a “slow” earthquake. Our broadband modeling does not require more than 34 s of additional moment release after the first two subevents. Moreover, we are able to match the phase and amplitude of 200‐s Love and Rayleigh waves with a thrust fault point source of moment 1.3×1020N‐m (Mw7.4) located at the body wave centroid. The moment difference is not discernible with body waves for moment evenly distributed over 72 s. Thus, the St. Elias earthquake is not slow with respect to 200‐s surface waves but is complex with regard to the broadband body waves. Upper plate structure apparently controlled the gross characteristics of rupture. The rupture direction parallels mapped upper plate faults. Rupture propagated unilaterally to the ESE, with little initial moment release, as a shallow, north‐dipping thrust that later changed to more steeply NE dipping with a large right‐lateral strike‐slip component. The locations and source mechanisms of these subevents and locations of aftershocks define a shallow dipping surface at the eastern edge of the Pacific plate. Moreover, the component of strike‐slip motion increases with time in the mainshock implying that the transition to strike‐slip faulting occurs along the plate interface. The estimated nucleation point of the second subevent coincides with a large concentration of aftershocks interpreted as representing a barrier to continuous rupture associated with the northern‐most boundary of the Yakutat terrane. Joint relocation of aftershocks suggests that the main plate boundary may be offset vertically by 5–10 km as a result of this structure. The southern part of the aftershock zone, while containing many aftershocks, appears not to have ruptured coseismically, but may have failed later by aseismic creep as seen in geodetic measurements. Faults associated with the Malaspina fault system (the onshore extension of the Aleutian trench) appear to be the surface expression of the underthrusting plate boundary; however, upper plate deformation is widespread because of the collision of the Yakutat terrane. The convergence direction may explain the lack of a highly active Wadati‐Benioff zone downdip of the St. Elias zone. The neotectonic deformation of the Chugach‐St. Elias mountains is probably related to collision and subduction of the Yakutat terrane: A terrane in the process of accreting and subducting will cause considerable upper plate deformation over a wide zone. Once subduction of a terrane has beg

ISSN:0148-0227    

DOI:10.1029/92JB00131

年代:1992

数据来源: WILEY

摘要:

An earthquake swarm at the eastern offshore region of Ito, the Izu peninsula, started on June 30, 1989, and became more active on and after July 4. The largest earthquake among the swarm (MJMA= 5.5) occurred at 1109 JST (0209 UT) on July 9, 1989, and a second significant event followed 45 s later. A remarkable volcanic tremor started on July 11, and a submarine eruption occurred just above the source region of the largest earthquake on July 13. In this paper we present a new inversion method to derive the source time function of rupture process. The source time function is divided into several sections following an automatic algorithm and is interpolated with a cubic spline function. The parameters, which are the values and first derivatives at knots of the spline function, are determined by minimizing a squared error between the observed and synthetic seismograms. To demonstrate the performance of this method, some numerical experiments (simulations) are carried out on near‐field seismic records. Applying this method to analysis of the two large earthquakes, the detailed rupture processes are obtained. A heterogeneous crustal structure near the volcanic area affects the rupture processes of these earthquakes. The large stress drop due to the largest earthquake just beneath the submarine eruption point would assist a magma ascending and may have triggered the eruption 4 days after these earthquake

ISSN:0148-0227    

DOI:10.1029/92JB00097

年代:1992

数据来源: WILEY

摘要:

For 57 fault plane solutions of earthquakes with magnitudeML≥3.0 located west of the southwest rift zone of Mauna Loa volcano in Hawaii, which occurred between 1972 and 1988, the dominant focal mechanisms (44 events) are decollement type with one nodal plane nearly horizontal (dip ≤ 30°). The average slip vector of the upper crust on the decollement plane points in an azimuth of 260° toward the ocean, away from Mauna Loa's southwest rift zone at an angle of about 150° with respect to the NE‐SW oriented rift. Two other types of focal mechanisms are present: normal faults (four events) and strike‐slip faults with normal component (nine events). The orientation of the principal stresses was derived by minimizing the sum of the misfits between the theoretical and observed fault geometry for each focal mechanism. After subdividing the data set into three different regions we found that the stress tensor is not homogeneous in west Hawaii. The orientation for the stress tensor is best resolved in the area located between latitudes 19.27°N and 19.4°N and longitudes 155.7° and 155.9° W where the greatest principal stress directions within the 95% confidence limits are nearly vertical with some spreading to the west. Their plunge varies between 53° and 85°. The intermediate and least principal stresses are mostly horizontal and have similar magnitudes. Their plunge varies between 1° and 36°. The area between latitudes 19.4° and 19.6°N and longitudes 155.7° and 156°W is characterized partly by near‐vertical and partly by east‐west oriented greatest principal stresses. The least principal stresses are approximately horizontal and have magnitudes similar to the intermediate principal stresses. In the two areas where the stress tensor was well resolved, the principal strain directions differed from the principal stress directions by 30° in the plunge, suggesting that the faulting in western Hawaii takes place on a weak plane, but this result could not be established at the 95% confidence level. The tectonic model proposed for west Hawaii is similar to the model for the Kalapana region. The strain is accumulated through magmatic intrusions in the southwest rift zone of Mauna Loa, and the earthquakes occur along a zone of weakness composed of oceanic sediment at about 10 km depth. The west flank of Mauna Loa slips in the d

ISSN:0148-0227    

DOI:10.1029/91JB02709

年代:1992

数据来源: WILEY

摘要:

A new method recently developed by Hoshiba et al. (1991) was used to separate the effects of scattering Q−1and intrinsic Q−1from an analysis of the S wave and its coda in Hawaii, Long Valley, and central California. Unlike the method of Wu [1985], which involves integration of the entire S wave energy, the new method relies on the integration of the S wave energy for three successive time windows as a function of hypocentral distance. Using the fundamental separability of source, site, and path effects for coda waves, we normalized the energy in each window for many events recorded at many stations to a common site and source. We plotted the geometric spreading‐corrected normalized energy as a function of hypocentral distance. The data for all three time windows were then simultaneously fit to Monte Carlo simulations assuming isotropic body wave scattering in a medium of randomly and uniformly distributed scatterers and uniform intrinsic Q−1. In general, for frequencies less than or equal to 6.0 Hz, scattering Q−1was greater than intrinsic Q−1, whereas above 6.0 Hz the opposite was true. Model fitting was quite good for frequencies greater than or equal to 6.0 Hz at all distances, despite the model's simplicity. The small range in energy values for any particular time window demonstrates that the site effect can be effectively stripped away using the coda method. Though the model fitting generally worked for 1.5 and 3.0 Hz, the model has difficulty in fitting the whole distance range simultaneously, especially at short distances. Despite the poor fit at low frequency, the results generally support that in all three regions the scattering Q−1is strongly frequency dependent, decreasing proportional to frequency or faster, whereas intrinsic Q−1is considerably less frequency dependent. This suggests that the scale length of heterogeneity responsible for scattering is at least comparable to the wavelength for the lowest frequencies studied, of the order of a few kilometers. The lithosphere studied in all three regions can be characterized as a random medium with velocity fluctuation characterized by exponential or Gaussian autocorrelation functions which predict scattering.Q−1decreasing proportional to frequency or faster. For all frequencies the observed coda Q−1is intermediate between the total Q−1and expected coda Q−1in contrast with theoretical results for an idealized case of uniform distribution of scatterers and homogeneous absorption which predict that coda Q−1should be close to the intrinsic Q−1. We will discuss possible

ISSN:0148-0227    

DOI:10.1029/91JB03094

年代:1992

数据来源: WILEY

摘要:

Pwaves from 13 teleseismic events, well distributed in azimuth and recorded on a large short‐period network in southern California, show a positive correlation between travel time residuals and relative amplitudes. Slower arrivals are consistently larger than faster arrivals. Although there is a large amount of scatter, the data show an amplitude increase of about a factor of 2 for a 1‐s increase in travel time. The correlation can be seen in various subsets of the data: individual earthquakes, single stations recording 10 or more earthquakes and data averaged over all recorded events at each station. Because the variations in amplitude and travel time can be associated with surface geology, much of the data recorded at non bedrock sites can be explained in terms of a near‐surface layer of varying velocity. The travel time residual would then be directly proportional to the velocity in the layer and the amplitude proportional to the square root of the velocity contrast at the base of the layer. However, some stations show azimuthally varying travel time residuals that are attributed to upper mantle velocity structures. We use a two‐dimensional finite difference calculation to show that velocity anomalies in the upper mantle can also contribute to the observed correlation of travel time and am

ISSN:0148-0227    

DOI:10.1029/91JB02578

年代:1992

数据来源: WILEY

摘要:

This paper demonstrates how synthetic seismicity calculations which are based on the concept of fault segmentation and incorporate the physics of faulting through static dislocation theory can improve earthquake recurrence statistics and hone the probabilities of hazard. Compared to forecasts constructed from a handful of earthquake recurrence intervals, forecasts constructed from synthetic seismicity are more robust in that they embody regional seismicity information over several units of magnitude, they can extrapolate seismicity to higher magnitudes than have actually been observed, and they are formulated from a catalog which can be extended as long as needed to be statistically significant. Synthetic seismicity models can also be used to judge the stability of common rate estimates and the appropriateness of idealizations to the earthquake cycle. I find that estimates of fault slip rate are unbiased regardless of sampling duration, while estimates of earthquake recurrence time are strongly biased. Recurrence intervals estimated from seismicity samples less than about 10 times the actual recurrence interval will almost certainly be too short. For the Middle America Trench (MAT), it would take 200 and 400 years of monitoring to constrain slip and recurrence rates to ±10%. EventsM≥ 6 have as much as a 60% probability of recurrence within 5 years due to the clustering of small earthquakes in foreshocks and aftershocks. This probability drops to less than 15% forM≥ 7 events. Increasing gap time generally increases conditional probability of earthquake occurrence, but the effect is weak. For the MAT, the spread parameters of the best fitting lognormal or Weibull distributions (≈0.75) are much larger than the 0.21 intrinsic spread proposed in the Nishenko‐Buland hypothesis. Stress interaction between fault segments disrupts time or slip predictability and causes earthquake recurrence to be far more aperiodic than has been s

ISSN:0148-0227    

DOI:10.1029/92JB00236

年代:1992

数据来源: WILEY

摘要:

Multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors (BSRs) on the northern Cascadia subduction zone margin off Vancouver Island. The reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate. The shallow water depth of 1300 m and the 3600‐m‐long hydrophone array have allowed BSR amplitude‐versus‐offset and high‐resolution velocity analysis, as well as modelling of vertical incidence data. The results of all three types of analysis can be best explained by a 10 to 30‐m‐thick high‐velocity layer located immediately above the BSR about 300 m below the seafloor, having a sharp base and transitional top. In the layer, about one third of the sediment pore spaces must be filled with hydrate “ice”. There is no seismically detectable free gas beneath the BSRs. These results put important constraints on models for the distribution and forma

ISSN:0148-0227    

DOI:10.1029/92JB00234

年代:1992

数据来源: WILEY