摘要:

Recent experimental and computational studies show that “islands” (unmixed regions that do not interact with surrounding regions) are ubiquitous features in chaotically advecting fluids. Such islands quite naturally account for the geochemically inferred coexistence of apparently distinct, long‐lived geochemical heterogeneity with relatively homogeneous regions of an actively convecting mantle. These results also indicate that mixing patterns—the set of islands and folds characterizing the large‐scale material advection— are sensitive to small variations in the rheology of the fluid. Therefore, interpretation of numerical simulations of mantle transport and mixing is less straightforward than currently supposed. Computational studies of analytic flow solutions with systematically introduced and controlled errors indicate that mantle simulations are unlikely to accurately compute individual trajectories for even moderate time, but that trajectory ensembles can be accurately computed for long time. Significantly, computations also indicate that mixing and transport results may not evolve smoothly with increased rheologi

ISSN:0094-8276    

DOI:10.1029/95GL00056

年代:1995

数据来源: WILEY

摘要:

Momentum and energy equations are solved simultaneously for an incompressible viscous fluid in order to model the changes in the shape of a subducted slab when active convergence between the subducting and overriding plates comes to the end and slab pull becomes the dominant tectonic mechanism. This model can be applied to the Tyrrhenian domain, where it has been suggested that active convergence terminated about 7–9 Myr ago. During the active phase the angle of immersion of the slab at intermediate depths between 100 and 270 km is small, about 45°–50°, and large, about 80°–90°, at depths greater than 300 km. The phase of passive gravitational sinking is characterized by a substantial modification in the shape of the slab, with a large angle of immersion of 70° now at intermediate depths, decreasing to 50° in proximity of the tip of the slab. When the shape of the modelled slab is compared with the seismogenic portion of the subducted Ionian lithosphere in the Tyrrhenian, our results are consistent with subduction driven by slab pull and with cessation of active convergence between 7–9 Myr b

ISSN:0094-8276    

DOI:10.1029/95GL00635

年代:1995

数据来源: WILEY

摘要:

The Departement of Mines and Geology has been monitoring the seismicity of the Central Himalayas of Nepal since 1985. Intense microseismicity and frequent medium‐size earthquakes (mL<4) tend to cluster beneath the topographic front of the Higher Himalaya. This 10–20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data. Both microseismic and geodetic data indicate strain accumulation on a mid‐crustal ramp that had been previously inferred from geological and geophysical evidence. This ramp connects a flat decollement under the Lesser and Sub‐Himalaya with a deeper decollement under the Higher Himalaya, and probably acts as a geometric asperity where strain and stress build up during the interseismic period. The large Himalayan earthquakes could nucleate there and probably activate the whole flat‐and‐ramp system up to the blind thrusts of the S

ISSN:0094-8276    

DOI:10.1029/94GL02971

年代:1995

数据来源: WILEY

摘要:

Previous monitoring and modeling of crustal deformation across the northern Cascadia margin at Vancouver Is. has provided strong evidence that the subduction thrust fault is locked and may generate future great earthquakes. The recent establishment of the Western Canada Deformation Array (WCDA), a network of continuous GPS trackers in southwestern British Columbia, provides a new tool for monitoring crustal strain and thereby helps to assess the earthquake hazard in this region. 17 months of continuous data from the three longest running WCDA sites (Penticton, Victoria, Holberg) indicate: 1.) A 7 mm/yr easterly motion of Victoria relative to Penticton. Victoria is located in the forearc at the southeastern end of Vancouver Is., 230 km from trench axis, whereas Penticton is located behind the arc, 520 km from the trench axis, and assumed fixed on the stable North American plate; 2.) A 3 mm/yr northwesterly motion and 10 mm/yr uplift (with a large uncertainty) of Holberg relative to Penticton. Holberg is located on the northern‐most part of Vancouver Is., 50 km from the trench and near the plate triple junction; and 3.) The presence of probably seasonal variations in the apparent relative positions of the two coastal sites with respect to Penticton. The landward motion of Victoria agrees with the deformation predicted by elastic dislocation models of the interseismic period for a great subduction‐thrust earthquake as constrained by previous deformation data. Although not as well resolved, the motion of Holberg, roughly parallel to the margin, is significantly different and is inconsistent with simple subduction models. The non‐linear variations in measured position are dominated by an annual period probably resulting from seasonal biases in the precise orbit estimates or in the modeling of tropospheric delays or tidal ef

ISSN:0094-8276    

DOI:10.1029/95GL00469

年代:1995

数据来源: WILEY

摘要:

Frequent measurements since mid‐1981 of the distances from a geodetic monument located about 100 km south‐southeast of San Francisco to three monuments 30 to 40 km distant provide an unusually complete record of the deformation before and after two nearby earthquakes, the 1984 Morgan Hill (ML= 6.2) and 1989 Loma Prieta (Ms= 7.1) earthquakes. Except possibly for the first few months postseismic, the extension rates indicated by these measurements appear to be steady over the four or five years both preceding and following those earthquakes. However, the preseismic and postseismic rates differ significantly for at least one of the baselines measured for each earthquake. The data over the four to five year postseismic records available are not adequate to demonstrate whether the postseismic rates are relaxing back to the preseismic ra

ISSN:0094-8276    

DOI:10.1029/95GL00084

年代:1995

数据来源: WILEY

摘要:

A detailed 3‐DP‐wave velocity structure of the crust in the epicentral area of the 17 January, 1994 Northridge earthquake is determined by using 104,709 arrival times from 1673 Northridge aftershocks and 2948 other local earthquakes. A test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3‐D model are accurate to about 2 km. We found that regions with high aftershock activity are generally associated with fasterP‐wave velocities. The velocity is high around the main south‐dipping fault of the 1994 Northridge earthquake and the north‐dipping fault of the 1971 San Fernando earthquake. A linear distribution of strike‐slip aftershocks was found along a NE‐SW boundary between high‐velocity and low‐velocity structures. To the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low‐velocity area, while to the east aftershocks with thrust mechanisms occurred in a high‐velocity area. These observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the Transverse Ranges. A better understanding of these features is important for long‐term seismic hazard assessmen

ISSN:0094-8276    

DOI:10.1029/94GL03222

年代:1995

数据来源: WILEY

摘要:

Measurements of SKS phase splitting were obtained from nineteen seismic stations in southern California. The fast polarization directions are 53° at the southern end of the Great Valley, 82±8° in the western Transverse Ranges and northern Peninsular Ranges, 95±4° in Mojave Desert, and 70° on San Clemente Island. The splitting time ranges from 0.8 to 1.8 seconds, which is consistent with an anisotropic layer of 100 to 200 km thick for 4% aniso

ISSN:0094-8276    

DOI:10.1029/95GL00487

年代:1995

数据来源: WILEY

摘要:

The interpretation of long term radon measure‐ments in a spring located in Vogtland shows a significant relationship to the seismicity of a region close to the sampling point. A statistical check was performed in order to identify the anomaly‐generating area. The cause for this relation is emission of mantle originated gases (including CO2) influenced by tectonic processes in the cr

ISSN:0094-8276    

DOI:10.1029/94GL03074

年代:1995

数据来源: WILEY

摘要:

Multiplets, i.e. events with similar waveforms, are selected from shallow earthquakes recorded on Merapi volcano (Indonesia) before the eruption of February 2nd, 1992. Two multiplet families are found with their sources close to the summit. Their seismograms are analyzed using the Moving Window Cross Spectrum technique which measures the precise time delay between seismic phases in the entire seismogram. For both families of multiplets, a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption: coda waves are becoming progressively faster (up to 1.2 per cent) as the time interval to the eruption shortens. This observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano. The increase in velocity started in May 1991 and was observed until September 1991, 4 months before the eruption. This velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks.

ISSN:0094-8276    

DOI:10.1029/95GL00302

年代:1995

数据来源: WILEY

摘要:

Scale invariant or self‐similar properties have been demonstrated for a wide range of geophysical processes and rock properties. Evidence from well logs and inferences from aeromagnetic field power spectra suggest that crustal susceptibility also shows this behaviour with a power spectrum proportional to the −4 power of the spatial frequency. We examine two large susceptibility measurement data sets and find that they both support the scaling hypothe

ISSN:0094-8276    

DOI:10.1029/95GL00486

年代:1995

数据来源: WILEY