ISSN:0148-0227    

DOI:10.1029/94JB00091

年代:1994

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/94JB00092

年代:1994

数据来源: WILEY

摘要:

A theoretical study of the propagation of a plane wave in a material with nonlinear response is presented. We start with the wave equation for an isotropic, homogeneous, elastic solid with cubic anharmonicity in the moduli, accounting for attenuation by introducing complex linear and nonlinear moduli. A hierarchy of equations, ordered in powers of the displacement field, is developed. Using a Green function technique, we solve this set of equations systematically for the displacement field at distancexfrom the source. We examine the influence of propagation distance, source frequency spectrum, source displacement amplitude, attenuation, and nonlinear coefficient on the spectrum of a propagating wave. The displacement field for various source functions is calculated using parameters typical of rocks.

ISSN:0148-0227    

DOI:10.1029/93JB02974

年代:1994

数据来源: WILEY

摘要:

Seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stressing history control the timing of earthquakes. The model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate‐ and state‐dependent fault properties. This yields a general state‐variable constitutive formulation for rate of earthquake production resulting from an applied stressing history. To illustrate and test the model some characteristics of seismicity following a stress step have been explored. It is proposed that various features of earthquake clustering arise from sensitivity of nucleation times to the stress changes induced by prior earthquakes. The model gives the characteristic Omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate. Earthquake data appear to support a model prediction that aftershock duration, defined as the time for rates to return to the back‐ground seismicity rate, is proportional to mainshock recurrence time. Observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial dependence of stress changes of the first event of the pair and stress‐sensitive time‐dependent nucleation. Applications of the constitutive formulation are not restricted to the simple stress step models investigated here. It may be applied to stressing histories of arbitrary complexity. The apparent success at modeling clustering phenomena suggests the possibility of using the formulation to estimate short‐ to intermediate‐term earthquake probabilities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in

ISSN:0148-0227    

DOI:10.1029/93JB02581

年代:1994

数据来源: WILEY

摘要:

PandSwave amplitude data from digital, three‐component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three‐dimensional attenuation structure in the upper crust of Long Valley caldera, California. Differential attenuation (∂Q−1) values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios. The spectral ratios are inverted for ∂Q−1using the LSQR algorithm. Zones of high, positivePwave ∂Q−1(greaterPwave attenuation) are observed at a depth of 4–5 km beneath the east flank of Mammoth Mountain and at 6–8 km beneath the southwestern edge of the resurgent dome. A region of positiveSwave ∂Q−1is seen at 7–8 km beneath the southern edge of the resurgent dome, whereas at 6–7 km, theSwave results are near average. The region of positive S wave ∂Q−1at 7–8 km beneath the resurgent dome may indicate the roof of a magmatic system. The positivePwave ∂Q−1at 4–5 km beneath Mammoth Mountain and 6–8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water. These zones may represent some of the source regi

ISSN:0148-0227    

DOI:10.1029/93JB03405

年代:1994

数据来源: WILEY

摘要:

We show that resonances in near‐surface layers (path effects), or a combination of near‐surface resonances and source resonances (source effects), could be responsible for the sharp spectral peaks found in the records of gas‐piston activity and volcanic tremor recorded near Puu Oo crater on Kilauea Volcano, Hawaii. Two types of sources are found to be compatible with observations and can not be distinguished using our data. In one model, impulsive, explosion point sources are embedded in the Puu Oo structure. In the other model, impulsive, point sources of pressure excite the resonances of a fluid‐filled crack embedded in that structure. Both models require shallow sources (z<100 m) and display a strong dependence of the radiated spectrum on source depth over a depth range comparable to the overall thickness of the surficial layers. Based on these results, it may be possible to track changes in source depth through temporal changes in observed spectra, a potentially useful tool for monitoring volcanic activity at this site. We estimate the depth and spatial extent of volcanic tremor and gas‐piston activity using two dense arrays with respective apertures of 800 and 120 m located near Puu Oo. Measurements of slowness (ray parameter) and azimuth as a function of time clearly indicate that the sources of volcanic tremor and gas‐piston activity are located beneath or in close proximity to the Puu Oo crater at depths shallower than approximately 1 km. Based on slowness and particle motion analyses we find that the records of volcanic tremor and gas‐piston events at Puu Oo consist of a complex combination of body and

ISSN:0148-0227    

DOI:10.1029/93JB02639

年代:1994

数据来源: WILEY

摘要:

Travel time inversion and amplitude modeling of a 350‐km Lithoprobe seismic refraction/wide‐angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the Omineca Belt of the Canadian Cordillera. The upper crust to 12–18 km depth has velocities from 5.6 to 6.2 km s−1, and two shear zones, the Monashee Décollement and Gwillim Creek Shear Zone, are imaged by the wide‐angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Décollement may be related to separate rock origins. Prominent reflections define the boundaries of a low‐velocity midcrustal layer from 10–15 km to 20–25 km depth with velocities less than 6.1 km s−1. The low velocities of the midcrust, associated with high electrical conductivities and high heat flow, may be considered as support for the hypothesis of fluids in the Cordilleran crust, though other possibilities, such as the effect of high temperatures on rock velocities are possible. In the lower crust velocities range from 6.4–6.5 km s−1at the top of the lower crust to 6.6–6.8 km s−1at its base. The Moho is very clearly defined by the refraction/wide‐angle reflection data and has a gentle southerly dip. Crustal thicknesses are 35–37 km. A thin crust‐mantle transition zone of 1–2 km thickness in which velocities vary between 7.6 and 7.7 km s−1is consistent with coincident reflection data. Upper mantle velocities range from 7.9 to 8.1 km s−1with indications from the data of upper mantle layering. In comparison with neighboring regions, the Omineca Belt has an anomalously thin crust, low crustal velocities, and a low‐velocity upper mantle, similar only to the Basin and Range province. The velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method. The characteristics of a thin crust and lithosphere, along with low velocities from midcrust to mantle suggests that both the Basin and Range and the southern Canadian Cordillera are currently bei

ISSN:0148-0227    

DOI:10.1029/93JB03108

年代:1994

数据来源: WILEY

摘要:

A large (Mw=7.1) deep earthquake (606 km) occurred beneath Sakhalin Island, about 200 km away from the nearest well‐defined Wadati‐Benioff zone in southwestern Kurile. Moment tensors obtained in various frequency bands using the teleseismic data show that the Sakhalin event produced double‐couple radiation over a very broadband frequency range, and the P waveforms appear simple at most teleseismic stations (Δ=30–90°). On the other hand, many broadband seismometers successfully recorded the seismic waves at regional distances (Δ=3–25°) in Japan, showing two significant arrivals in the P wave seismograms whose time interval is about 3 s. The two P wave arrivals have the same polarity, the same apparent velocity, and similar amplitudes, which suggests that the two arrivals are the result of two sub‐events in the source process of the Sakhalin event. Analysis of the regional and teleseismic waveform data suggests an anomalously rapid slip propagation in the source process of the Sakhalin deep event. The difference in P waveform between the regional and teleseismic stations can be explained by having a long distance (>14–15 km) and short time interval (<1.5–2 s) between the two subevents, which implies that the apparent rupture velocity is larger than the S wave velocity unless the two subevents occurred simultaneously. The observation that the source duration of the Sakhalin event is shorter than those of other deep events of the same size supports the notion of rapid slip‐propagation f

ISSN:0148-0227    

DOI:10.1029/93JB02992

年代:1994

数据来源: WILEY

摘要:

Long‐period multiple‐ScSwaveforms from intermediate‐ and deep‐focus earthquakes surrounding China, recorded by the Chinese Digital Seismograph Network, are used to estimate the multiple‐ScSattenuation operator and differential travel times using both frequency (phase equalization and stacking) and time domain (waveform inversion) techniques. Theoretical considerations, augmented by synthetic testing, indicate that for continental regions the frequency domain approach yields a more stable estimate of whole mantle attenuation (QScS) than the time domain approach, which is better for whole mantle travel time (τScS). When teamed together, the two afford unprecedented success with continental paths. The results, 16 path‐averaged estimates of QScSand τScSconstitute a significant addition to the catalog of multiple‐ScSstudies of subcontinental mantle and provide sufficiently dense coverage to invert “path” estimates of attenuation and travel time for “regional” quantities of direct tectonic relevance. Across the frequency band of 10 to 50 mHz we obtain high (>215) values of regionalizedQScSτScSthe Tarim platform and the fold systems of northeastern China and low (<170) values elsewhere on the continent, defining a pattern that is well correlated with present‐day levels of tectonic activity. The corresponding two‐way travel time residuals with respect to the Jeffreys‐Bullen (J‐B) tables, ΔτScS(= τScS‐ 935.7 s), vary from −1.3±1.4 to +0.8±4.3 s and are typical of other “fast” continental regions. UnlikeQScS, ΔτScSvaries little from region to region and is correlated poorly with tectonic activity, indicating thatQScSis much more sensitive to changes in the current tectonic environment. Offshore, ΔτScSfor the Sea of Japan is +2.7±3.6 s and for the combined East China and Philippine seas region is +9.6±2.7 s. The latter, very large residual likely reflects the low velocities associated with back arc spreading. Throughout the study region, ΔτScScorrelates well with the residuals predicted by recent upper mantle tomographic models, although the observed variance e

ISSN:0148-0227    

DOI:10.1029/93JB03106

年代:1994

数据来源: WILEY

摘要:

We describe structural data from a 2000 km N‐S dextral strike‐slip zone extending from northern Sakhalin to the southeast corner of the Japan Sea. Satellite images, field data, and focal mechanisms of earthquakes in Sakhalin are included in the interpretation. Since Miocene time the deformation in Sakhalin has been taken up by N‐S dextral strike‐slip faults with a reverse component and associated en échelon folds. Narrow en échelon Neogene basins were formed along strike‐sup faults and were later folded in a second stage of deformation. We propose a model of basin formation along extension al faults delimitating dominos between two major strike‐slip faults, and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime, basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening. The association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in Sakhalin. We present fault set measurements undertaken in Noto Peninsula and Yatsuo Basin at the southern end of the Sakhalin‐East Japan Sea strike‐slip zone. Early and middle Miocene formations recorded the same transtensional regime as observed along the west coast of NE Honshu. During the early and middle Miocene the strike‐slip regime was transpressional to the north in Sakhalin and Hokkaido, and transtensional to the south along the west coast of NE Honshu as far as Noto Peninsula and Yatsuo basin. Dextral motion accommodated the opening of the Japan Sea as a pull‐apart basin, with the Tsushima fault to the west. The opening of the Japan Sea ceased at the end of the middle Miocene when transtension started to change to E‐W compression in the Japan arc. Subduction of the Japan Sea lithosphere under the Japan arc started 1.8 Ma ago. The evolution of the stress regime from transtensional to compressional in the southern part of the strike‐slip zone is related to the inception of the subduction of the young Philippine Sea Plate lithosphere under the Japan arc during the late Miocene. Subduction related extension is a necessary condition for the opening of the Japan Sea. Two possible mechanisms can account for dextral shear in this area: (1) counterclockwise rotation of crustal blocks due to the collision of India with Asia, (2) extrusion of the Okhotsk Sea block squeezed between the North A

ISSN:0148-0227    

DOI:10.1029/93JB02026

年代:1994

数据来源: WILEY