We analyze shear wave polarizations from local earthquakes recorded by the Anza network in southern California, using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival (the linearity interval). Initial shear wave particle motions are strongly aligned at four stations, a feature that is not predicted by focal mechanisms. The particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations, a result supported by the clear shear wave splitting seen in a borehole recording near one of the Anza stations. These results are consistent with an earlier analysis of these data by Peacock et al. [1988]. However, our analysis does not support claims by Crampin et al. [1990] that shear wave splitting delay times at station KNW exhibit temporal variations which can be correlated with the occurrence of the North Palm Springs earthquake (ML=5.6) of July 8, 1986. Automatically determined linearity intervals scatter widely from 0.02 to 0.15 s and exhibit no clear temporal trends. We find a correlation between earthquake moment and the linearity interval, possibly a result of longer effective source time functions for the larger events. The inability to identify a distinct slow quasi‐shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion, even in the very early shear wave coda. Analysis of earthquake clusters with similar waveforms recorded at KNW shows that seismic Green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events. If shear wave splitting is causing the observed delay times between horizontal components, the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5–10%.

At periods greater than 1000 seconds, Earth's seismic free oscillations have anomalously large amplitude when referenced to the Harvard Centroid Moment Tensor fault mechanism, which is estimated from 300- to 500-second surface waves. By using more realistic rupture models on a steeper fault derived from seismic body and surface waves, we approximated free oscillation amplitudes with a seismic moment (6.5 x 10(22) Newton.meters) that corresponds to a moment magnitude of 9.15. With a rupture duration of 600 seconds, the fault-rupture models represent seismic observations adequately but underpredict geodetic displacements that argue for slow fault motion beneath the Nicobar and Andaman islands.

The thermoelastic effects of a traveling wave of temperature on the surface of an infinite homogeneous elastic half space are examined. The horizontal and shear strain and the tilt are principally caused by tractions in the thermal boundary layer, and they decay vertically with the scale of the horizontal wavelength of the applied temperature wave. The vertical strain is larger at the surface by the ratio of this wavelength to the thermal boundary thickness, but below the boundary layer it behaves like the other components. The burial of instruments at practical depths is unlikely to reduce significantly the thermoelastic effects except on the vertical strain.

The rupture characteristics of several ML ∼ 3 earthquakes near Anza, California, are determined from time domain analysis of path‐corrected, displacement pulses. These pulses are corrected for propagation effects (apparent attenuation, scattering, site resonance) using the waveforms of adjacent small events (ML ≤ 2.1) as empirical Green's functions. The earthquakes studied occurred during two swarm sequences near Cahuilla, in an area characterized by swarm activity at shallow depth (< 5 km). The data consist of seismograms recorded by the Anza seismic network, a high dynamic range local array with digital telemetry. The waveforms of adjacent small events (ML ≤ 2.1) are deconvolved from those of the ML ∼ 3 events to yield the source time functions of the larger earthquakes. The ML ∼ 3 earthquakes display a variety of rupture modes, similar to those reported for large shocks but over much shorter time scales and distances. Events with single and multiple ruptures are observed. Relative locations of the subevents are obtained from azimuthal differences in the time separation of pulses in the deconvolved displacement waveforms. Some events (as small as ML 2.4) display unilateral rupture propagation, as manifested by azimuthal variations in the pulse widths of the deconvolved displacement pulses. A tomographic back projection technique is applied to the deconvolved displacement pulses to image the slip velocity on the fault as a function of time after nucleation and distance along the fault, using a one‐dimensional fault model. The tomographic inversion for one of the events (ML 2.4) reveals a unilateral rupture with rupture velocity about equal to the shear wave velocity (Vs) and fault length of 300 m. A similar inversion for an ML 3.7 event resolves two subevents separated by about 200 m and yields an average rupture velocity of 0.8Vs. These rupture velocities are similar to those reported for large earthquakes, indicating that rupture velocity is independent of seismic moment down to 3.1 × 1019 dyn cm, the smallest event studied. There are clear variations in pulse width for neighboring events with similar seismic moments, implying significant differences in static stress drops. Factor of five variations in stress drop (11–61 bars) are calculated for events within 200 m of each other. Thus the stress and/or strength along this fault zone varies considerably over distances of hundreds of meters.

We use records from the Project IDA modified LaCoste gravimeters to investigate ground noise at. frequencies from 1 to 10 mHz. At most sites the level between 2 and 10 mHz is nearly flat and close to 2 × 10−18 m2 s−3 Much higher values which are observed at island and coastal stations are due to loading by waves trapped along the shore. Data from the superconducting gravimeter at Piñon Flat Geophysical Observatory show that the noise power increases as ƒ−2.7 for frequencies between 1 and 0.001 mHz.

Numerical solutions for the earth tide on a model earth with elastic dilatant crustal inclusions indicate that up to 60 per cent changes in the tilt and strain tides result from a 15 per cent reduction of V,, the seismic p velocity, in the inclusion. The fractional changes in the tilt and strain tide amplitudes are proportional to the changes in Poisson's ratio and inverse areal bulk modulus of the inclusion and are not proportional to the Vp/Vs ratio. Further, detectable changes in the tidal amplitude will occur to a distance of 1 5 times the typical dimension of the dilatant inclusion. Monitoring the earth tide is therefore suggested as a sensitive and continuous method of earthquake prediction if such dilatancy precedes earthquakes. The time dependence of the tidal signal, due to dilatancy, will be the same as that of the V,/V, ratio if the dilatant material is elastic. A search of the tidal strain data from the laser strain meters at the Piiion Flat Geophysical Observatory reveals no evidence of anomalous changes in the tidal signal during the past three years. The limits of detection for changes in the tidal admittance are +2 per cent for a 696 hr averaging period.

An array of shallow borehole tiltmeters has been operated at Piñon Flat Observatory since early in 1977. The data from this array are examined for coherence between the individual instruments and compared with the corresponding data from three 732‐m laser strainmeters. In general, there is no significant coherence between the signals from the tiltmeters outside the microseismic and tidal bands even though they are spaced as closely as 10 m apart. Comparisons with the strain records show that the observed tilt noise power exceeds the strain noise power by 25–40 dB over the band from 10−6 to 1 Hz. Analysis of the coherence estimates establishes the necessity for an unacceptably large (≃1000) array of instruments in order to determine the common tectonic signals. The observed secular tilt rates are from 40 to 450 times the observed secular strain rates for the same period. The theoretical noise power limit of the tilt transducer renders the instrument incapable of recording background noise at a quiet site over the band from 10−4 to 10−1 Hz.

We have measured the power spectrum of the earth strain fluctuations over 10 decades in frequency from 10−8 to 102 Hz using data from three strain observatories. Although the strain meters were widely separated and of different design, they produced records whose power spectra are in close agreement with each other. We find that the composite power spectrum shows an approximate inverse square dependence on frequency over the entire band investigated.