摘要:

Although far‐field body waves and surface waves are widely recognized, the improvements in broadband networks now allow the measurement of “near‐field” deformation of large earthquakes at great distances. Near‐field motions have been recognized previously only in theory and in close‐in recordings of earthquakes. We show examples from two recent events. For the large deep event on June 9, 1994 in Bolivia, there is a clear offset after the arrival of the P wave that has the amplitude expected for the near‐field term. In the shallow September 2, 1992 Nicaragua earthquake, the very long‐period motion observed between the P and S waves has roughly the amplitude expected from near‐field terms. Such near‐field terms are insensitive to earth structure, but supply information on long‐period source processes, and their observation begins to close the gap between long‐period

ISSN:0094-8276    

DOI:10.1029/94GL02893

年代:1995

数据来源: WILEY

摘要:

Measurements ofSKSsplitting parameters have been made at stations of the Canadian National Seismograph Network for events over the period 1993–mid1994. Here we report on results obtained from western stations. In the northern Cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently. Splitting parameters in the southern Cordillera, however, are oriented approximately perpendicular to the strike of major tectonic features, notably the Cascadia subduction zone. This contrast in the character of fast polarization directions between the southern and northern Cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern B.C. coast to transform north of the triple junction. One plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the Juan de Fuca ridge, subduction along the coast and associated back‐arc asthenospheric flow beneath the B.C. inter

ISSN:0094-8276    

DOI:10.1029/94GL02789

年代:1995

数据来源: WILEY

摘要:

The fault geometry and slip distribution of the Hokkaido Nansei‐oki, Japan, earthquake of July 12, 1993 are estimated using seismic wave, tsunami, and geodetic data. The Moment Tensor Rate Function inversion from P waves shows one nodal plane shallowly dipping to the west and the other nodal plane steeply dipping to the east. The best depth is estimated as 10–15 km. The source time history consists of an initial pulse with a duration of 10 s and moment release of 2 × 1020Nm, followed by a complex rupture for at least 40 s. The Centroid Moment Tensor (CMT) solution shows one nodal plane shallowly dipping to the east and the other steeply dipping to the west. The overall seismic moment is estimated as 5.5 × 1020Nm (Mw7.8). The joint inversion of geodetic data on Okushiri Island and tsunami waveforms in Japan and Korea shows that the largest slip, about 6 m, occurred at a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost fault, dipping to the east, is about 2 m. The slips on the southern faults, dipping steeply to the west, are more than 3 m. Total seismic moment of 4.9 × 1020Nm, estimated from the slip distribution, is similar to the estimate from CMT inv

ISSN:0094-8276    

DOI:10.1029/94GL02787

年代:1995

数据来源: WILEY

摘要:

In this paper we outline a procedure we use for routine moment‐tensor analysis of regional data from broadband seismic stations in northwestern North America and apply it to the moment magnitude 5.5, March, 1993, Scotts Mills, Oregon, earthquake. The results compare favorably with those obtained from teleseismic data. We found that the earthquake occurred at a depth of 13–15 km and had a mechanism with approximately equal amounts of reverse and right‐lateral strike‐slip components. The estimated stress drop of 40 bar is average on a world‐wide basis, supporting the view that the rather large damage was caused primarily by poor construction and not by exceptional properties of the source. The Scotts Mills earthquake is most likely related to the Mt. Angel Fault. This fault is a part of the Gales Creek‐Mt. Angel structural lineament (GCMAL) extending about 150 km across the Willamette Valley. At present data are not sufficient to estimate the likelihood of an earthquake involving the entire GCMAL, but given its length an earthquake of magnitude 7 is conceivable. The results of this study, together with investigations of other earthquakes, suggest that sparse broadband networks can be used efficiently for determining source parameters of earthquakes of magnitude greater than 4.0 in regions with infrequent

ISSN:0094-8276    

DOI:10.1029/94GL02760

年代:1995

数据来源: WILEY

摘要:

Recently Zeng et al. (1994) proposed a composite source model for convolution with synthetic Green's functions for computing strong ground motion due to a complex rupture process of a large earthquake. The composite source consists of sub‐events with a distribution of sizes, distributed randomly on the fault. In this paper, we extend our discussion of the source by allowing subevent rake to vary within the positive strain quadrant and by including the effect of the random lateral heterogeneity of the earth by adding scattered waves into the Green's function computed from a layered elastic earth model. Both of these variations contribute towards equalizing different components of the high frequency waveform train. The random scattering has the stronger effect on increasing the amplitudes of the nodal components. The early part of these scattered coda waves also tends to elongate the strong shaking of the earthquak

ISSN:0094-8276    

DOI:10.1029/94GL02798

年代:1995

数据来源: WILEY

摘要:

The melting curve between ϵ and liquid iron (100GPa

ISSN:0094-8276    

DOI:10.1029/94GL02892

年代:1995

数据来源: WILEY

摘要:

As a first step to establish the law governing shear failure of typical crustal materials in the brittle‐plastic transition regime under lithospheric conditions, and thereby to properly estimate a depth profile of lithospheric strength in quantitative terms, the effects of the normal stress σnacross the fault surfaces and ambient temperatureTon the shear failure strength of dry Westerly granite in the brittle to brittle‐plastic transition regimes are evaluated quantitatively, using experimental data published by earlier authors. The empirical law proposed can predict the shear failure strength at strain rates of 10−4‐10−5/S under any (σn,T) conditions in the brittle to brittle‐plastic trans

ISSN:0094-8276    

DOI:10.1029/94GL02791

年代:1995

数据来源: WILEY

摘要:

The crustal seismic activity in southern Italy during the last three centuries has been analyzed in order to confirm or disprove the evidence of a northward migration of earthquake epicenters from Calabria towards southern Apennines and Irpinia that we have found in a previous work. The results suggest such a migration to be statistically significant at a velocity of about 13 km/yr. This, in turns, stands for the existence of stress pulses propagating northward along the Apennine axis.

ISSN:0094-8276    

DOI:10.1029/94GL02792

年代:1995

数据来源: WILEY

摘要:

Extrusion driven collision is controlled by strike‐slip faults of opposite senses whereas subduction related oblique convergence is accommodated by margin parallel faults of identical senses. Northern Andes geodynamics has been interpreted either as resulting from the obliquity of convergence between Nazca and South American plates or from the collision of the Carnegie Ridge with Ecuadorian margin. The sense of the Cauca‐Patia and Romeral Fault System (CPRFS) has been interpreted in two opposite manners: right or left‐lateral agreeing with oblique or collisional patterns, respectively. We analyze arguments which favor, South to 4°N, a right‐lateral motion along the CPRFS that is consistent with obliquity driven geodynamics. However, North of 5°N, the CPRFS is actually left‐lateral. These opposite motions between the northern and southern part of the CPRFS have to be accommodated around 5°N by N‐S stretching and/or shortening. Existing data support N‐S shortening around 5°N, this buttressing could be due to the collision between the Panama

ISSN:0094-8276    

DOI:10.1029/94GL02837

年代:1995

数据来源: WILEY

摘要:

GPS measurements spanning ∼3 years have been used to determine velocities for 7 sites on the Australian, Pacific and Antarctic plates. The site velocities agree with both plate model predictions and other space geodetic techniques. We find no evidence for internal deformation of the interior of the Australian plate. Wellington, New Zealand, located in the Australian‐Pacific plate boundary zone, moves 20±5 mm/yr west‐southwest relative to the Australian plate. Its velocity lies midway between the predicted velocities of the two plates. Relative Euler vectors for the Australia‐Antarctica and Pacific‐Antarctica plates agree within one standard deviation with the NUVEL‐1A

ISSN:0094-8276    

DOI:10.1029/94GL02788

年代:1995

数据来源: WILEY