Earthquake Prediction: Variation of Seismic Velocities before the San Francisco Earthquake

Whitcomb, James H.; Garmany, Jan; Anderson, Don L.

doi:10.1126/science.180.4086.632

Cited by 328 publications

(111 citation statements)

References 9 publications

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“…Saoarensky [1968] and ,4ggarwal et al [1973] suggested that the main effect was due to an increase in V• during the anomalous period. However, the San Fernando study [Whitcomb et al, 1973] showed that the V•,/V• anomaly was due to a large decrease in Vp and a smaller decrease in V,. This observation combined' with the large spatial extent and time scale of the velocity anomaly for the San Fernando event seemed to give support to the dilatancy-diffusion model as formulated by Nur [1972], Whitcomb et al [1973], and Anderson and Whitcomb [1973a, b].…”

mentioning

confidence: 95%

Time-dependent seismology

Anderson¹,

Whitcomb²

1975

J. Geophys. Res.

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The time variation of crustal velocities in tectonic regions is most reasonably attributed to stressinduced variations in crack porosity. The decrease in V•,/Vs before earthquakes is due primarily to a large decrease in Vp. This supports the Nur dilatancy hypothesis but not the effective stress hypothesis. New data from the San Fernando region verify the Vp drop, show that this drop cannot be entirely due to source depth effects, and give strong support to the explanation of material property, or path effect, rather than source effect variations. Calculations show that the crack-widening model works even for midcrustal depths in saturated rock. Narrow cracks of low aspect ratio are required to satisfy the velocity and uplift constraints. The recovery of velocity prior to fracture can be due to fluid flow or crack closure. The t • L •' relation does not require diffusion. Diffusion of groundwater or crack closure leads to increased pore pressure and rock weakening. Observations of gravity, conductivity, and crustal distortions along with velocities should narrow the choice of models. The crust in regions of thrust tectonics is probably always dilatant to some degree. The aftershock region is smaller than tl•e anomalous velocity region, which in turn must be smaller than the dilatant region. A simple relationship is derived for the relative sizes of the anomalous and aftershock regions.

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mentioning

confidence: 95%

Time-dependent seismology

Anderson¹,

Whitcomb²

1975

J. Geophys. Res.

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“…It is thought recently that positive rock dilat;mcy associated with microcrack opening could explain the reported seismic variation prior to earthquakes [Nur, 1972;Whitcomb et al, 1973;Anderson and Whitcomb, 1973;Aggarwal et al, 1973], and as a consequence, rock dilatancy is considered to be of central importance in the understanding of precursory phenomena associated with earthquakes. When it is viewed in this light, Scholz et al [1973] describe the phenomenon of positive dilatancy in rocks as a physical basis for such possible earthquake precursors as change in seismic velocity ratio, electrical resistivity, radon emission, and geodetic measurements, Beaumont and Berger [1974] computed the effect of dilatancy on the tidal response of the crust, and Whitcomb [1975] examined 3495 ltu AND LIVANOS: BULGING IN UNIAXIAl.LY COMPRESSED GRANITE quantitatively the dependence of vertical geodetic (including leveling, tilt, and geometric measurement relative to a celestial frame of reference), and gravity measurements in relation to dilatancy density change.…”

Section: Introductionmentioning

confidence: 99%

Dilatancy and precursory bulging along incipient fracture zones in uniaxially compressed westerly granite

Liu¹,

Livanos²

1976

J. Geophys. Res.

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The existence of precursory bulge along an incipient fracture zone in a uniaxially compressed Westerly granite sample has been investigated by two optical methods. The first method is the method of slit diffraction. The cross section of a cylindrical rock sample is monitored by bringing two straightedges next to the rock sample to form two slits (each slit being formed between one straightedge and one side of the rock sample) and by illuminating alternately the two slits with a collimated laser beam. The FraunhoFer diffraction pattern is recorded on film in the direction perpendicular to the straightedge and can be interpreted as the absolute value squared of the one-dimensional spatial Fourier transform of the slit under certain conditions, thereby providing a simple method of magnification of the rock surface geometry. The conditions under which the Fraunhofer diffraction pattern can be interpreted as the absolute value squared of a one-dimensional Fourier transform are related to the radius of the rock sample, width of the slit, position of the recording film plane, and nature of deformation of the rock surface and are presented in this paper. The films are digitized by a microdensitometer. The data are analyzed by digital filtering and interpolation techniques to give a strain resolution of 10-•. During a test with the slit diffraction method, strain inhomogeneities in terms of local bulges indicative of incipient failure zones were found to develop at -92% of the uniaxial compressive strength, and their propagation is traced at 2.66-s intervals until failure. Local strains in the incipient failure zones are of the order of 10-• before failure takes place. Because of the large amplitude of the strain inhomogeneity prior to failure recorded by the slit diffraction method, we then tried the faster method of recording without magnification by a motion picture camera. In the second test a precursory bulge in the middle of the sample first appeared at -3.75 s prior to failure at a load of >99.7% of the uniaxial compressive strength. The bulge developed rapidly in successive frames until eventually a failure plane passed through this sharp bulge. The results from both tests demonstrate the formation of a concentrated weak zone as a result of the interaction and coalescence among the microcracks in the final stage of the test, which then develop into fracture zones. The bulging is the result of accentuated deformation in the weak zone because of its reduced deformation moduli. It is considered that the local bulge and orientation of the fracture zones in the first test were controlled by the stress concentration at the sample-load block interface, whereas those in the second test were controlled mainly by the inhomogeneity in the material properties within the sample. The precursor times of both tests do not fit into the empirical relationship between precursor time and fault dimension as derived from earthquakes and mine rock bursts. The precursor times of these tests are too long by 3 orders of magnitude in compa...

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“…This phenomenon, which is also known as dilatancy-diffusion process (BRACE and ORANGE, 1968), is believed responsible for the observed seismic velocity anomalies preceding the earthquakes (NUR, 1972;WHITCOMB et al, 1973;SCHOLZ et al, 1973). Actual drops in resistivity have been measured preceding earthquakes BARSUKOV (1972) in the Garm region of the U. S. S. R. and by MAZZELLA and MoRRISON (1974) in central California.…”

Section: Introductionmentioning

confidence: 99%

“…Based on observed seismic velocity anomalies (WHITCOMB et al, 1974;KANAMORI and HADLEY, 1975) we have chosen three sites in and around the Los Angeles basin. We are very limited in usable sites because of high cultural noise.…”

Section: Introductionmentioning

confidence: 99%

Monitoring of time dependent electrical resistivity by magnetotellurics.

Reddy

Phillips

Whitcomb

et al. 1976

J. geomagn. geoelec

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A feasibility study is being made to monitor possible electrical resistivity changes preceding the earthquakes. Numerical calculations, based on dilatancy hypothesis, indicate that the resistivity changes as much as 60 percent can be possible at some of the Southern California sites. Preliminary magnetotelluric measurements in the eight month period indicate that changes in the resistivities greater than 10 percent can be monitored by magnetotelluric measurements. The observed scatter in the magnetotelluric measurements are within the random errors of the estimates, and no major earthquake activity is reported in the vicinity of these sites, during this period.

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Earthquake Prediction: Variation of Seismic Velocities before the San Francisco Earthquake

Cited by 328 publications

References 9 publications

Time-dependent seismology

Time-dependent seismology

Dilatancy and precursory bulging along incipient fracture zones in uniaxially compressed westerly granite

Monitoring of time dependent electrical resistivity by magnetotellurics.

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