Time Variations in Teleseismic Residuals Prior to the Magnitude 5.1 Bear Valley Earthquake of February 24, 1972

Cramer, Chris H.; Kovach, Robert L.

doi:10.1007/978-3-0348-5534-1_24

Cited by 3 publications

(1 citation statement)

References 20 publications

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“…NERSESOV et al (1971) reported velocity anomalies based on ray paths within the upper 10km of the crust at Garm. For the 1972 earthquake at Bear Valley, Cal., a number of studies (BAKUN et al, 1973;JOHNSON, 1973, 1974;ROBINSON et al, 1974;CRAMER and KOVACH, 1975) indicate that an anomalous zone was highly localized and confined to the upper 5 or 10 km of the crust. ANDERSON and WHITCOMB (1973) imply that the upper 20km of the crust was dilatant prior to the San Fernando earthquake.…”

Section: Field Evidencementioning

confidence: 99%

Depth Constraints on Dilatancy Induced Velocity Anomalies

Winkler

Nur

1977

J,Phys,Earth

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depth. High pore pressures require larger porosity changes to achieve undersaturation, but when undersaturation does occur, the velocity anomaly will be slightly larger.Some of the approximations we have made deserve a closer look. Crack aspect ratio has been shown to have a significant effect on wave velocities in rock (WALSH, 1969;NUR and SIMMONS, 1969;O'CONNELL and BUDIANSKY, 1974). While aspect ratio will not affect the occurrence of undersaturation, it will affect the magnitude of the velocity anomalies. We looked into this by reducing our constant aspect ratio porosity increase went from 0.67 at 1km depth to 0.44 at 10km, so the effect of a lower aspect ratio increases with depth (compare to Fig. 4A, line (2)). However, as HADLY (1976) points out, average aspect ratio should increase with depth in the crust as confining pressure closes flat cracks and leaves round pores relatively unchanged. Aspect ratios increasing with depth will have the net effect of increasing the anomalies at shallow depth in Fig. 4 and reducing the anomalies at greater depth.In our crustal models we have not included pore porosity which takes the form of large aspect ratio voids in rock. has estimated pore porosity to be on the order of 0.001 for many rocks and independent of depth. Our results would not be significantly altered if we had considered this pore porosity. At shallow depths the low aspect ratio crack porosity, of comparable magnitude (or greater), will have a much greater effect on velocity than the high aspect ratio pore porosity. It will slightly increase the dilatant strain required for undersaturation. At greater depths the pore porosity will have a greater relative effect on velocities because the net porosity increase is significant, but the high aspect ratios should keep the effect small. However, this additional porosity at depth will significantly increase the dilatant porosity increase required for undersaturation. Therefore, while the affects of pore porosity are somewhat variable with depth, it appears that the net effect will reinforce the results arrived at above, namely, that the potential for velocity anomalies decreases with depth.While our assumption of a hydrostatic pore pressure gradient seems reasonable, it conflicts somewhat with the conclusions of HANKS (1974, BOOKER (1975) and ANDERSON and WHITCOMB (1975), who suggest that substantially lower pore pressures may exist at depth. If this is so, then the pore fluid at depth will be closer to the liquid-vapor transition pressure than we have assumed, and undersaturation will be achieved with smaller porosity increases than we have calculated. The smaller porosity increases will create correspondingly smaller velocity anomalies and so we again arrive at the conclusion that the largest velocity anomalies will be found at shallow depths (assuming undersaturation occurs everywhere). It is, of course, possible to conceive of pore pressure and dilatant strain distributions that will cause undersaturation only at great depths. However, this would severely l...

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Section: Field Evidencementioning

confidence: 99%

Depth Constraints on Dilatancy Induced Velocity Anomalies

Winkler

Nur

1977

J,Phys,Earth

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P-wave travel-time variations before the August 1, 1975 Oroville, California earthquake

Cramer

Bufe

Morrison³

1977

Bulletin of the Seismological Society of America

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On August 1, 1975, a magnitude 5.9 (mb) (BRK, M = 5.7), normal dip-slip earthquake occurred 10 km south of Oroville, California. P arrivals for teleseismic and regional sources at the few seismographs in the area have been carefully timed to an accuracy of ±0.02 sec and the relative residual technique has been applied to these data. The data cover the period from August 1968 through March 1976. A significant delay of about 0.1 sec in travel-time residuals for Russian nuclear blasts was observed over a 3-year period preceding the Oroville earthquake at station ORV 10 km north of the epicenter. A 0.1-sec delay in travel-time residuals for U.S. nuclear blasts occurred after the Oroville event at station MGL, 40 km north of the main shock's epicenter. P arrivals from deep Tonga-Fiji earthquakes have also been analyzed but reveal no systematic time variations beyond ±0.05 sec from their mean values. P arrivals from moderate-size earthquakes along the San Andreas fault system in central California proved to be an unsatisfactory source of data because of ambiguities created by multiple P-phase arrivals and the emergent nature of the arrivals. The sparse station coverage does not allow adequate delineation of the extent and character of the anomalous P velocity zones, but the data do provide some limitations. The postearthquake travel-time delay at MGL may be precursory to a future earthquake or may only be related to the redistribution of stress in the Oroville area.

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Temporal stability of P-velocity anisotropy before earthquakes in central California

Steppe

Bakun

Bufe

1977

Bulletin of the Seismological Society of America

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An examination of P-wave travel-time residuals from small earthquakes (source events) located near three larger earthquakes (4 ≦ M ≦ 5) that occurred on the San Andreas fault, near Bear Valley in central California, shows no temporal variations in the residuals extending over broad azimuth ranges (Δφ > ∼40°). Such variations could have resulted from changes in horizontal velocity anisotropy precursory to the larger events. The examination also shows (1) numerous azimuthal variations in the residuals within narrow azimuth bands (Δφ < ∼30°), apparently due to spatial heterogeneity of crustal velocity, (2) a dependence of residual on magnitude for a few stations but not for most stations, and (3) trends of residual versus source event focal depth for about one-third of the 75 source region-station pairs examined. Residuals in these cases typically change by 0.1 sec, and occasionally by 0.2 to 0.3 sec, over the 2- to 12-km focal depth range sampled. The trends vary from station to station in a complex manner. The assumption that traveltime changes are reflected in the residuals is tested by modifying arrival times from some source events according to assumed forms for the travel-time change, relocating those events, and comparing the resulting residuals with those from the unmodified data.

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Time Variations in Teleseismic Residuals Prior to the Magnitude 5.1 Bear Valley Earthquake of February 24, 1972

Cited by 3 publications

References 20 publications

Depth Constraints on Dilatancy Induced Velocity Anomalies

Depth Constraints on Dilatancy Induced Velocity Anomalies

P-wave travel-time variations before the August 1, 1975 Oroville, California earthquake

Temporal stability of P-velocity anisotropy before earthquakes in central California

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