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...

Subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates. The cracks tend to grow parallel to the maximum compressive stress so that, when this stress is aligned over a large enough region, the cracks will also be aligned and possess effective seismic anisotropy. It is suggested that such subcritical crack growth produces extensive-dilatancy anisotropy (EDA) throughout earthquake preparation zones. This process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones, and provides, in the shear-wave splitting which has been observed in several seismic regions, a possible technique for monitoring the build-up of stress before earthquakes.

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The behaviour of shear-waves is of great importance in identifying and investigating seismic anisotropy in the Earth. However, shear wavetrains recorded at the Earth's surface do not always reflect the motion at depth, introducing practical problems of interpretation. Shear wavetrains incident on the surface of an isotropic half-space at angles less than critical (about 35") are broadly preserved, but at greater angles substantial distortions can occur. For stations situated close to the source, as in local earthquake studies, the local SP phase, a radially polarized precursor to S, may occur. The behaviour at the surface of an anisotropic half-space is further complicated by the divergence of phase and energy propagation vectors. All of these complications suggest that detailed seismogram modelling is essential to any study of shear wave propagation in the Earth, and in particular to investigations of anisotropy-induced shear-wave splitting.

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This paper is a response to comments by Aster et al. the following arguments in paper A arc also invalid and the [1990] (hereafter referred to as paper A) about temporal results of papers 1 and 2 still stand. variations in shear wave splitting by Peacock et al. [1988] 2. DURATION OF LINEAR SHEAR WAVE MOTION and Crampin et al. [1990]. Paper A analyzes shear wave arrivals at the Anza Seismic Network, southern California, The automatic technique developed by paper A identifies with an automatic technique to obtain a measure of the the largest eigenvalue of the variance tensor, which it duration of the linearity of the initial shear wave particle claims measures the duration and orientation of the motion. Paper A claims that this linearity is an objective linearity: the linear motion of the initial split shear wave measure of the time delay between split shear waves and arrival on three-component digital seismograms. Figure la does not display the temporal variations at station KNW is a modified version of Figure 10b of paper A showing the before and after the North Palm Springs (NPS) earthquake variation with time of the estimates of linearity interval at of July 8, 1986, reported by Peacock et al. [1988] Anza station KNW. Figure lb is a similar plot of Figure (hereafter referred to as paper 1) and Crampin et al. 3d from paper 2. This is the data set which papers 1 and 2 [1990] (hcrcaher referred to as paper 2). suggest displayed temporal variations in shear wave splitting. Paper A also claims to identify doublets (earthquakes Both diagrams arc plotted for arrivals within the shear wave with similar seismograms from similar loci), which are window making angles of more than 14.5ø to the vertical widely spaced in time, to further demonstrate temporal plane parallel to the polarizations of the leading split shear stability along the ray paths. In addition, paper A claims wave. Papers 1 and 2 interpret the splitting as the result of that the linearity it measures is correlated with earthquake propagation through distributions of stress-aligned vertical moment. On the basis of these three principal results, fluid-filled cracks, microcracks, and preferentially oriented paper A makes a wide ranging examination of shear wave pore space known as extensive dilatancy anisotropy or EDA splitting at Anza and of papers 1 and 2. It concludes that [Crampin, 1985]. The polarizations of the leading split although the data do display shear wave splitting, as shear waves through such EDA-cracks arc parallel to the recognized at Anza by papers 1 and 2 (and at many other strike of the cracks. This means that the 14.5 ø is the angle sites worldwide [Crampin, 1987a]), there is no evidence of the ray paths make with the faces of the cracks in the temporal variations correlated with the occurrence of the distributions of parallel vertical EDA-cracks. Paper 1 NPS earthqua. showed that this angle 14.5 ø separates the shear wave Paper A is a careful and rigorous analysis. However, window [Booth and Crampin, 1985] into areas in which the wc dem...

We have simulated exchange of inner-sphere and bulk water molecules for different sizes of Al3+(aq) clusters, Al(H2O)63+ + nH2O for n = 0, 1, 6, or 12, with ab initio and molecular dynamics simulations, in order to understand how robust the ab initio method is for identifying hydrolytic reaction pathways of particular importance to geochemistry. In contrast to many interfacial reactions, this particular elementary reaction is particularly simple and well-constrained by experiment. Nevertheless, we find that a rich array of parallel reaction pathways depend sensitively on the details of the solvation sphere and structure and that larger clusters are not necessarily better. Inner-sphere water exchange in Al3+(aq) may occur through two Langford-Gray dissociative pathways, one in which the incoming and outgoing waters are cis, the other in which they are trans to one another. A large majority of exchanges in the molecular dynamics simulations occurred via the trans mechanism, in contrast to the predictions of the ab initio method. In Al(H2O)63+ + H2O, the cis mechanism has a transition state of 84.3 kJ/mol, which is in good agreement with previous experimental and ab initio results, while the trans mechanism has only a saddle point with two negative frequencies, not a transition state, at 89.7 kJ/mol. In addition to the exchange mechanisms, dissociation pathways could be identified that were considerably lower in energy than experiment and varied considerably between 60 and 100 kJ/mol, depending on the particular geometry and cluster size, with no clear relation between the two. Ab initio calculations using large clusters with full second coordination spheres (n = 12) were unable to find dissociation or exchange transition states because the network of hydrogen bonds in the second coordination sphere was too rigid to accommodate the outgoing inner-sphere water. Our results indicate that caution should surround ab initio simulation of complicated dynamic processes such as hydrolysis, ion exchange, and interfacial reactions that involve several steps. Dynamic methods of simulation need to accompany static methods such as ab initio calculation, and it is best to consider simulated pathways as hypotheses to be tested experimentally rather than definitive properties of the reaction.

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