Shear wave splitting above small earthquakes in the Anza seismic gap on the San Jacinto fault, southern California, displays the effective anisotropy of vertical fluid-filled microcracks.The delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence.These changes can be modeled by increasing the aspect ratio ("bowing") of stressoriented microcracks, which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid-filled microcracks. This is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake.Res., 85, 6113-6156, 1980.

High-resolution seismic experiments, employing arrays of closely spaced, four-component ocean-bottom seismic recorders, were conducted at a site off western Svalbard and a site on the northern margin of the Storegga slide, off Norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed. Data from P-waves and from S-waves generated by P-S conversion on reflection were inverted for P-and S-wave velocity (Vp and Vs), using 3D travel-time tomography, 2D ray-tracing inversion and 1D waveform inversion. At the NW Svalbard site, positive Vp anomalies above a sea-bottomsimulating reflector (BSR) indicate the presence of gas hydrate. A zone containing free gas up to 150-m thick, lying immediately beneath the BSR, is indicated by a large reduction in Vp without significant reduction in Vs. At the Storegga site, the lateral and vertical variation in Vp and Vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the BSR. Derivation of hydrate content from Vp and Vs was evaluated, using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host. The error in the average Vp of an interval of 20-m thickness is about 2.5%, at 95% confidence, and yields a resolution of hydrate concentration of about 3%, if hydrate forms a connected framework, or about 7%, if it is both pore-filling and framework-forming. At NW Svalbard, in a zone about 90-m thick above the BSR, a Biot-theory-based method predicts hydrate concentrations of up to 11% of pore space, and an effective-medium-based method predicts concentrations of up to 6%, if hydrate forms a connected framework, or 12%, if hydrate is both pore-filling and frameworkforming. At Storegga, hydrate concentrations of up to 10% or 20% were predicted, depending on the hydrate model, in a zone about 120-m thick above a BSR. With seismic techniques alone, we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer, not only because of the uncertainty in the layer-by-layer variation in lithology, but also because of the negative correlation in the errors of estimation of velocity between adjacent layers. In this investigation, an interval of about 20-m thickness (equivalent to between 2 and 5 layers in the model used for waveform inversion) was the smallest within which one could sensibly estimate the hydrate content. If lithological layering much thinner than 20-m thickness controls hydrate content, then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data.

Changes in shear wave splitting are observed at KNW before and after the M = 6 North Palm Springs earthquake of July 8, 1986. KNW is a station of the Anza seismic network monitoring the Anza seismic gap on the San Jacinto fault, southern California. The gradual increase in the delays between the split shear waves over 3 years at KNW, reported by Peacock et al. (1988), ended in June 1986. The further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the North Palm Springs earthquake, 33 km north of KNW. Peacock et al. demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress-aligned fluid-filled inclusions, and speculated that this increase might be the result of a build up of stress before an impending earthquake. The new data appear to confirm this speculation, but the temporal variations require a more complex interpretation, although they still suggest that the changes in shear wave splitting are caused by earthquake-induced stress changes to the fluid-filled inclusions throughout the rockmass. Central to our interpretation of temporal changes in shear wave splitting is the well established existence, throughout at least the uppermost 10 to 20 km of the crust, of small fluid-filled cracks, microcracks, and pores. The existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock. We term these distributions of inclusions extensive dilatancy anisotropy or EDA, and the individual inclusions EDA cracks because, although they may include a wide range of shapes, many of the seismic properties can be simulated by distributions of thin parallel cracks. We present a further 2 years of data that splitting) than seismograms displayed as timeindicate that the shear wave splitting recorded series, and all the measurements of shear wave at KN• appears to change before and after the splitting in this paper are taken from polariza-M = 6 North Palm Springs (NPS) earthquake of tion diagrams. Figure 2 shows rotated seismo-July 8, 1986. Figure 1 shows a map of the Anza grams and horizontal polarization diagrams of a seismic network and location of the NPS earth-number of events. The positions of the arrowquake. The temporal changes display different heads marking the arrival times of the split behavior along different ray paths that place shear waves are those read from polarization constraints on the possible cause of the changes. diagrams. Many of the polarization diagrams in We speculate on how .these variations may be Figure 2 and throughout the data set display interpreted in terms of stress-induced modific-classic patterns of shear wave splitting familiar ations to the cracked rockmass. from synthetic modelling [Crampin. 1978; Crampin and Booth, 1985], so that the analysis is Crampin, S., Seismic wave propagation through a cracked EH9 3LA, Scotland. solid: polarization as a possible dilatancy diag-J.B. Fletcher, U.S. Geological Survey, 345 Middlefield nostic, Geophys. •. R. /ts tr. Soc., 53, 46...

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