Temporal changes in shear wave splitting during an earthquake swarm in Arkansas

Abstract

Shear wave splitting has been on the free surface. Outside the shear wave observed in seismograms of two aftershock and one window, the incident polarizations are distorted foreshock sequence recorded over 12 days by a local network of nine three-component digital seismometers during the 1982 Enola, Arkansas, earthquake swarm. Polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field. The time delay between split shear waves appears to increase before, and to decrease at or after the time of each main shock.This behavior is similar to observations before and after a M= 6.0 event near the Anza seismic gap, southern California, and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes.There are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction. Booth, 1985].The time delay, which is due to the velocity difference between the split shear waves, depends on the direction of propagation and the path length through the cracked rock. It is also sensitive to the crack density, crack identified on almost all suitable ray paths, in a dimensions, aspect ratio, pore fluid content and wide variety of tectonic regimes (see Crampin [1987a] for a review, and elsewhere in this issue for more recent examples).By "suitable ray paths", we mean those ray paths which are incident at the Earth's surface within the shear wave window, which is the locus of propagation paths which subtend an incidence angle less than the critical angle, sin-•(Vs/Vp), at a receiver other parameters of the cracked rock.If the observed shear wave splitting is caused by stress-aligned EDA cracks, then changes in the stress field before an impending earthquake will alter the most compliant feature of the rock mass -the EDA crack distribution -and thus modify the characteristic behavior of the split shear waves and, in particular, their polarizations and the time delay between them. Crampin [1987b] has suggested that most earthquake precursors, and the often irregular behavior of precursors, can be explained by indirect effects of temporal variations in the characteristics of EDA cracks, and their anisotropy.

Shear-Wave Splitting in a Critical Crust: the Next Step

“…The approach of fracture criticality, and the consequent earthquake, could be recognized by monitoring shear-wave splitting as above small earthquakes. Such changes have been observed (with hindsight) by Crampin et al (1990Crampin et al ( , 1991, Booth et al (1990), Liu et al (1997), and Gao et al (1998). More controlled investigations could be made by cross-hole observations between deviated boreholes (Crampin and Zatsepin, 1997b).…”

Time-domain Pure-state Polarization Analysis of Surface Waves Traversing California

“…The effects of the earth's lateral heterogeneity on the propagation of surface waves have been studied extensively (e.g., MCGARR, 1969;CAPON, 1970;BUNGUM and CAPON, 1974;PATTON, 1980;LAY and KANAMORI, 1985;JOBERT and JOBERT, 1983;WOODHOUSE and WONG, 1986;LERNER-LAM and PARK, 1989;LEVSHIN et al, 1992). Surface wave amplitude and travel-time anomalies are often associated with polarization anomalies and involve horizontal refraction caused by lateral heterogeneities.…”

“…However, for a laterally heterogeneous and anisotropic medium like the earth, the particle motion usually involves interference of direct, refracted, reflected, and scattered body and surface waves that are not constrained to a plane, resulting in complex polarization and energy transport off the great-circle. This complexity has motivated the development of sophisticated polarization analysis techniques to constrain the effects of heterogeneity and anisotropy on seismic wave propagation, including path deflection effects (e.g., BOOTH et al, 1991;CRAMPIN, 1970CRAMPIN, , 1975CRAMPIN et al, 1990CRAMPIN et al, , 1991KIRKWOOD and CRAMPIN, 1981;PARK et al, 1987;SILVER and CHAN 1988;VINNIK et al, 1989;LASKE and MASTERS, 1996;VIG and MITCHELL, 1990;ASTER et al, 1990ASTER et al, , 1991LEVSHIN et al, 1992).…”

Changes of Strain before Earthquakes: The Possibility of Routine Monitoring of Both Long-Term and Short-Term Precursors.

“…The APE model suggests that the precursory sequences before earthquakes observed by , Booth et al (1990), Liu (1995), and Gao et al (1997) are monitoring the build-up of strain in the crust. Observations of shear waves, if analyzed at the time, would have provided a measure of the amount of the stored strain, and hence the potential magnitude of the eventual earthquake by the duration of the increase in time delays.…”