No abstract

Polarization anomalies in seismic shear wavetrains, diagnostic of propagation through anisotropic media, have now been observed in dilatancy zones in seismic regions. Stress-induced dilatancy will open cracks with preferred orientations, which will be effectively anisotropic to shortperiod seismic waves. The polarization anomalies are due to the shear waves splitting, in propagation through anisotropic media, into components with different polarizations and different velocities. This writes characteristic signatures into the shear wavetrains. The paper examines ways in which the differential shear-wave anisotropy (the delay between the split shear-waves) varies with direction by plotting stereograms of the relative delays, and their polarizations, for possible dilatancy symmetry-systems. It seems likely, that if sufficient observations of these anomalies can be obtained at each stage of the dilatancy episode, it w i l l be possible to estimate the symmetry directions of the dilatancy and the geometry of the stress-induced crack-system, as well as monitoring the progress of the dilatancy episode.

The azimuthal variations of P wave velocity measured in U.S. Geological Survey (USGS) seismic refraction experiments around Mount Hood, Oregon, are modeled by propagation through the effective anisotropy of parallel vertical cracks. The three refractors at 1, 3.7, and 8.5 km depth show different behavior. The shallow refractor at 1 km displays P wave velocity variations typical of the parallel water‐filled vertical microcracks found throughout much of the upper continental crust. The arrivals from the refractor at 3.7 km display velocity variations consistent with either dry cracks or, more probably, with cracks filled with supercritical water, as one interpretation of the heat flow around Mount Hood suggests that temperatures at 3.7 km would be above the critical temperature for water at in situ pressures. The velocity variations at the deepest refractor at 8.5 km depth are poorly constrained but again suggest liquid‐filled cracks. The possibility of high temperatures suggests that, at this depth, the cracks could be thin skins of melt along grain boundaries. The vertical cracks in all three refractors strike in the northwest to southeast quadrants, which is consistent with the compressional quadrants in the mechanisms of local earthquakes. Shear‐wave splitting reported elsewhere indicates that there are stress‐aligned water‐filled microcracks (extensive‐dilatancy anisotropy or EDA) throughout at least the brittle upper 10 or 20 km of the continental crust. The high heat flow in the High Cascades may confine the range of crack behavior, usually spread over the whole thickness of the crust, to the top 10 km beneath Mount Hood. Thus the P wave velocity variations beneath Mount Hood may indicate that EDA cracks occur throughout the whole of the continental crust. The large number of shot‐to‐recorder travel times measured by the USGS allows extensive‐dilatancy anisotropy to be recognized from the analysis of P wave velocity variations for the first time.

The paper examines P-wave propagation in anisotropic solids, and demonstrates the effect of anisotropy on the polarizations of quasi P-waves. The deviation of the polarization of the quasi P-wave from the propagation vector may be significant, but is in almost the same direction as the deviation of the group-velocity vector. Since the group-velocity, or energy propagation, vector follows seismic ray paths, the apparent deviation is the difference of the polarization and group-velocity deviations and is small. Consequently, it may be easily overlooked, hidden by noise, or attributed to the effects of inhomogeneity .

The file of Turkish seismicity developed by Kandilli Observatory, Istanbul, for earthquakes to 1970 is extended here up to 1978 using 1SC and PDE data. Entries into this file are maintained on the surface wave magnitude scale M,, and conversion of body wave magnitude mb t o M , has been carried out where necessary using a formula derived for Turkish earthquakes. Completeness analysis suggests that magnitudes M,> 4.5 may be used for statistical evaluation of seismic risk. This file is analysed by a range o f methods to provide a suite of risk forecasts.Forecasting results from least squares and maximum likelihood estimates of the whole process Gutenberg-Richter cumulative frequency law of earthquake magnitude occurrence, and from the part process of Gumbel's first extreme value distribution, all show small systematic differences in forecasts, but all three methods lead to magnitude forecasts which fall well within the range of standard deviation. However, these forecasts are obtained by subdividing Turkish seismicity in a cellular manner, and many of these cells show curvature of the earthquake frequency magnitude distribution curve concave with respect t o the origin: Gumbel's third asymptotic distribution of extreme values is chosen as an appropriate statistical description. Approximate upper bounds o t o earthquake surface wave magnitude occurrence are evaluated and estimates of largest magnitudes expected over an interval of 7 5 y r are forecast with uncertainties. Values of w are asymptotic, uncertain, and theoretically correspond t o infinite return periods. Strain energy release diagrams are then invoked t o estimate empirically the large magnitude M , which is equivalent to the total strain energy which may be accumulated in a region. This equivalent magnitude M , is consistently less than w and there is a finite 'waiting time', typically ranging from about 15 to 7 0 yr, during which the energy equivalent to M3 may be accumulated. 476 P. W. Burton et al.Combination of the Gumbel I11 earthquake occurrence statistics for each cell with an inferred intensity attenuation law leads to a suite of perceptibility curves which give the probability of perceiving specific intensity levels from each possible earthquake magnitude up to the local upper bound magnitude w. This family of curves generated for an individual cell is seen t o be nested, and shows a peak probability for each intensity level which typically occurs at similar magnitudes defined as the 'most perceptible' earthquake. These 'most perceptible' earthquakes show values which range from an M, o f about 5.5 t o 7.5 for regional seismicity cells in Turkey; local values may be used as a criterion for choosing engineering design time histories.The seismic risk parameters from the cellular analysis of seismicity are interpreted as contoured seismic risk maps. These maps show that contoured values of the maximum strain energy earthquake M3 usually exceed the 75 yr earthquake by about one-half magnitude for similar geographic locations, but there is overall s...

Cracks are a very common feature of crustal rocks, and their behavior and properties under various conditions are of growing importance to a variety of applications, including investigations of oil and water resources in cracked reservoirs, extraction of geothermal heat, and earthquake prediction by the seismic effects of dilatancy. This paper discusses mapping cracks by measurements of velocity‐anisotropy in underground refractors. The technique is demonstrated by interpreting the velocity measurements of Bamford and Nunn (1979) in shallow fractured limestone. The estimated crack distributions are found to be consistent with rose‐diagrams at neighboring outcrops, and the degree of saturation of the cracks has a major effect on velocity variations. Elastic constants can be derived for an equivalent purely elastic solid which has the same velocity variations as the cracked solid. This allows synthetic seismograms to be calculated for propagation through cracked structures, which demonstrates that use of polarization anomalies in three‐component recordings is a sensitive technique for investigating cracked solids.

Exact modelling of the earthquake location process in regions of dilatancy-anisotropy shows that failure to take account of the velocity anisotropy in the determination of local hypocentres can result in erroneous and misleading locations. In particular, the locations can indicate spurious migrations of foci from the true epicentral positions and the true depths of foci. These spurious locations may indicate planes of hypocentres deviating from the true fault plane. Observations of such phenomena have been noted several times in the literature. However, once the anisotropic model is known, a simple location program, incorporating the anisotropic velocity-variations, permits accurate location of local earthquakes using Pand first S-wave arrival times.

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