摘要:

This study examines the three‐dimensional velocity structure in a 60‐ by 80‐km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shotParrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three‐dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across‐fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4‐km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low‐velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20‐km‐long high‐velocity slice is imaged northeast of the SAF near Gold Hill. Its along‐fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right‐lateral strike‐slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and re

ISSN:0148-0227    

DOI:10.1029/93JB01029

年代:1993

数据来源: WILEY

摘要:

The April 22, 1991, Valle de la Estrella, Costa Rica earthquake (Ms= 7.6) was a back‐arc thrusting event associated with the underthrusting of the Caribbean plate beneath Central America. A network of three PASSCAL‐type, portable instruments was deployed to monitor the aftershock activity in southern Costa Rica 2 to 6 weeks after the main shock. The waveforms recorded on three‐component midperiod seismometers were used to recover source information for 15 small aftershocks (magnitudes between 3.2 and 4.4) with a linear moment tensor inversion method. We conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery. The longer‐period waveforms, in general, are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data. The earlier part of the seismic wave‐forms has proven to be the most important carrier of the source information. A gross crustal model can be used to describe the structure for the source study. The small changes in the waveform character resulting from the mislocation of the events, or inexact Green's functions generated from the oversimplified crustal model, do not prohibit us from the recovery of the source orientation at local distances. In contrast, the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information. Our focal mechanisms are generally in good agreement withPwave first‐motion fault plane solutions determined from a local short‐period network. The aftershocks show a clear spatial segmentation based on focal mechanism type. Most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock. In contrast, a cluster of aftershocks northwest of the main shock showed dominantly left‐lateral, strike‐slip motion on a northeasterly striking nodal plane. This suggests that a diffuse deformation zone exists in central Costa Rica and is characterized by left‐lateral strike‐slip motion. This diffuse, transcurrent deformation zone coincides with several geologic and geophysical features, and perhaps is a result of the slower subduction rate of the buoyant Cocos Ridge, than its adjacent segments along the Middle America Trench (MAT). The diffuse transcurrent boundary may intersect with the North Panama Deformed Belt (NPDB) near limon, Costa Rica, and is very likely a plate boundary for the

ISSN:0148-0227    

DOI:10.1029/93JB01557

年代:1993

数据来源: WILEY

摘要:

Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central Tien Shan are used to determine one‐ and three‐dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one‐dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three‐dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km‐thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk‐Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk‐Kul is overthrusting the surrounding ranges. The low‐velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, withPvelocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso‐Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low‐velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surr

ISSN:0148-0227    

DOI:10.1029/93JB01560

年代:1993

数据来源: WILEY

摘要:

We analyzed teleseismicPandSwaves using a multiple‐event deconvolution method to investigate the source process of the Spitak, Armenia, earthquake of December 7, 1988. Teleseismic long‐period body waves exhibit complex waveforms, significantly more complex than those normally seen for an event of this size. We identified two groups of subevents. One is a group of strike‐slip events during the first 20 s. The other is a dip‐slip event initiating at about 30 s after the initial rupture. The tensor sum of all the subevents is essentially strike‐slip (strike=302°, dip=59°, rake=143°) with a scalar moment of 1.47×1019N m. The seismic moment of the dip‐slip event (strike=89°, dip=60°, rake=88°) is 5.6 ×1018N m, nearly 40% of the total, and the moment release rate is slower than the other subevents with a strike‐slip mechanism. We call this event a slow after‐slip event. The mechanism of the after‐sup event is consistent with the present‐day deformation pattern in the Caucasus where north‐south compression is predominant due to the continental collision between the Arabian plate and the Russian Platform. This result suggests that in a region where both strike slip motion and crustal shortening are taking place, like southern California, a large strike‐slip event may accompany large thrust events and vice versa. In order to determine the total coseismic deformation pattern, it is essential to resolve these multiple event

ISSN:0148-0227    

DOI:10.1029/93JB01568

年代:1993

数据来源: WILEY

摘要:

Two seismic wave attenuation factors, scattering attenuation (Qs−1) and intrinsic absorption (Qi−1), can be determined from estimates of two parameters, the reciprocal of extinction length Le−1≡ (Qs−1+ Qi−1) ·ω/v and seismic albedo B0≡ Qs−1/ (Qs−1+ Qi−1), where ω and v are angular frequency and seismic wave velocity, respectively. These parameters are measured using the multiple lapse time window analysis method over frequency bands of 1–2, 2–4, and 4–8 Hz from 16 stations located throughout Japan. In the analysis, time‐integrated seismic wave energy from earthquakes having hypocentral distances less than 120 km and focal depths less than 40 km are compared with simulations of multiple isotropic scattering based on the assumption of spatial uniformity of Qs−1and Qi−1. The modified coda normalization method is used to correct for sources and site effects, where the spatially non uniform distribution of the coda wave energy is considered. The Le−1estimates do not have a distinct frequency dependence, but the Be estimates do: 0.2–0.7 for 1–2 Hz and 0.1–0.3 for 4–8 Hz. The results mean that Qs−1is similar to Qi−1for 1–2 Hz but is 3–5 times smaller for 4–8 Hz and that the frequency dependence of Qs−1is larger than Qi−1. Analyses using different components of motion and different window lengths give nearly the same results. Coda attenuation (Qc−1) estimates based on the single scattering approximation are nearly equal to intrinsic absorption

ISSN:0148-0227    

DOI:10.1029/93JB00347

年代:1993

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/93JB00234

年代:1993

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/93JB00702

年代:1993

数据来源: WILEY

摘要:

We present a comprehensive set of Sr, Nd, and O isotope data and trace element concentrations from tholeiitic and alkaline lavas of the neovolcanic zones of Iceland (picrites, olivine and quartz tholeiites, transitional and alkali basalts, differentiated rocks). Variations in the oxygen isotope results allow us to distinguish two groups. The first, which comprises quartz tholeiites and more differentiated rocks usually associated with central volcanoes, has low δ18O values (+5 to +1‰) resulting from interaction with the hydrothermally altered Icelandic crust. The second group, which contains picrites, olivine tholeiites, and alkali basalts, has normal mantle oxygen isotopic compositions (δ18O = +5 to +6‰) which are thought to represent those of the mantle source. Nd isotopic compositions vary greatly, from143Nd/144Nd = 0.51314 in picrites to 0.51295 in alkali basalts. To produce such a variation for rocks with the chemical compositions of Icelandic volcanics (147Sm/144Nd = 0.12=0.28) requires>200 m.y., a period that greatly exceeds the maximum age of Icelandic crust. Previous models, in which the Sr isotopic variations were explained in terms of evolution of crustal reservoirs, are invalidated, and mantle reservoirs with different Nd and Sr isotopic compositions are indicated. The Iceland data define a linear array in the Sr‐Nd isotope diagram which overlaps both mid‐ocean ridge basalt and oceanic island basalt fields and indicates mixing between depleted and enriched end‐members. Alkali basalts come preferentially from an isotopically and chemically enriched component of the Iceland plume, and picrites come from a more refractory, more depleted portion. Positive Sr, Rb, and Ba anomalies are present in picrites and other lavas with low trace element contents. These anomalies are not correlated with isotopic differences but are nevertheless believed to result from interaction between the parent magmas of these rocks and altered Icelandic crust. This indicates that even the most primitive Icelandic lavas have been contaminated with some crust

ISSN:0148-0227    

DOI:10.1029/93JB01093

年代:1993

数据来源: WILEY

摘要:

Following the episodes of inflation of the resurgent dome associated with the May 1980 earthquake sequence (fourM6 earthquakes) and the January 1983 earthquake swarm (twoM5.2 events), 7 years of frequently repeated two‐color geodimeter measurements spanning the Long Valley caldera document gradually decreasing extensional strain rates from 5 ppm/yr in mid‐1983, when the measurements began, to near zero in mid‐1989. The corresponding seismic activity within the caldera persisted at a low rate of fewer than 10M≥ 1.2 earthquakes per week from 1985 through November 1989 with no events exceedingM3.0. Early October 1989 marked a change in activity when measurements of the two‐color geodimeter network showed a significant increase in extensional strain rate (9 ppm/yr) across the caldera. The seismic activity began exceeding 10M≥ 1.2 per week in early December 1989 and rapidly increased to a sustained level of tens ofM≥ 1.2 per week with bursts having hundreds of events per day. Many events exceededM3.0 and the largest event wasM≈ 4. The 1989–1991 inflation episode is the first time that we have sufficient geodetic measurements in Long Valley to define the temporal relation between onset of an inflation episode and onset of brittle failure (earthquake swarm within the caldera). Here, the onset of deformation preceded the onset of increased earthquake activity by more than 2 months. The seismicity rate began to decrease in mid‐July 1990, consistent with a gradually slowing of extension across the caldera as measured by the two‐color geodimeter. The recent episode of inflation can be modeled by a single Mogi point source located about 7 km beneath the center of the resurgent dome. In contrast, the deformation pattern observed between mid‐1983 and mid‐1989 is best reproduced by fault slip in the south moat, inflation at 6.5 km depth near Casa Diablo Hot Springs and inflation beneath the resurgent dome. It appears that the 7‐km source beneath the resurgent dome that was active for the earlier episodes is the primary source for the more recent episode. The model used to satisfy the line length observations predicts 7.5 cm of uplift along leveling route along highway 395 from mid‐1983 to mid‐1989 and an additional 11 cm through the end of 1991. To comp are with the energy release from seismicity, the modeled inflation from late 1989 through the end of 1991 has a moment that is a factor of 40 more than the cumulative seismic moment from earthquakes located within the caldera from the same period. Thus the recent inflation episode represents a significant portion of the observed geodetic deformation with onl

ISSN:0148-0227    

DOI:10.1029/93JB00558

年代:1993

数据来源: WILEY

摘要:

The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in3He/4He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5RA. Potential sources of the3He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a3He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7RA), the persistence of relatively high values (>5RA) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased3He/4He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in3He/4He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of3He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with M

ISSN:0148-0227    

DOI:10.1029/93JB00703

年代:1993

数据来源: WILEY