An analysis of coseismic tilt changes from an array in central California

Abstract

Local earthquakes with magnitudes of >•2.5 and within 20-50 km of tiltmeters along the San Andreas fault typically generate offsets in tilt, tilt seismograms, and impulsive tilt behavior at the time of the earthquake. The amplitudes and azimuths of the coseismic tilt offsets from local earthquakes observed at a small array of four instruments approximately 30 km south of Hollister, California, are compared to the amplitudes and azimuths predicted by a rectangular source, elastic half-space dislocation model. Almost all observed coseismic offset amplitudes are 1-3 orders of magnitude larger than the predicted amplitudes. The predicted offset azimuths also are not in agreement with the observed azimuths. There is neither a uniform method of scaling the predicted amplitudes nor a constant rotation that may be applied to the predicted azimuths that will consistently reproduce the observed offsets. Errors in hypocenter location and fault plane orientation are not sufficient to explain the discrepancies between observations and predictions. Similar results occur for teleseismic earthquakes. A lack of agreement in the observed offset amplitudes across the array indicates that tilt changes are triggered at or near the instrument site by the passage of seismic waves. No significant agreement was obtained between the direction of coseismic tilts and either the secular tilt trends or local geologic features. Triggered movement on near-surface cracks, fractures, and minor faults appears the most viable physical explanation for the observed offsets. Shortbase-line near-surface tiltmeters appear inadequate for measuring tilt displacement fields generated by local earthquakes. Deep borehole installations appear necessary for this measurement. Coseismic tilt transients appear to be caused by seismically induced water table perturbations near the tiltmeter site.

Tilt offset associated with local seismicity: the Mt. Etna January 9, 2001 seismic swarm.

“…A residual tilt offset associated with seismic events is assumed to represent the change produced by the fault dislocation [5]. Analyses of co-seismic variations on bore-hole instruments (e.g.…”

“…a dike) can dominate the deformation. Analyses of co-seismic variations in different areas have highlighted how continuous borehole tilt data may also comprise seismic shaking effects, such as movements on cracks and fractures near the instrument site, instability of instruments or translational ground acceleration caused by passing waves [1,5,7,8] that cause larger variations [9][10][11]. Nonetheless, these effects are not present on a long-base device [1,12,13].…”

“…Faulting causes tilt changes with amplitudes related to source-station distance and earthquake magnitude [5]; they are generally characterized by impulsive variations with simultaneous recordings at different stations in correspondence with the earthquake. Changes are generally small for shallow (0-5 km depth) local earthquakes at Mt.…”

“…These permanent offsets are assumed to represent the change produced by the fault dislocation in a tectonic context (e.g. [5,6]) while in an active volcanic setting, the tensile component (e.g. a dike) can dominate the deformation.…”

Applications of Geodesy to Geodynamics an International Symposium

“…More GEODYNAMIC PROBLEMS X Length Scale -Km careful attention to leveling observation, computation, and adjustment should also payoff. Point measurements-strain, tilt, and gravity-appear to have greater inherent difficulties of sorting out regional from local effects McHugh & Johnston, 1977;Lambert & Beaumont, 1977]. For a while at least, seismicity itself will remain the leading precursory data type: see, e.g., Shimazaki [1978] and Ishida & Kanamori [1978] for studies on two different scales.…”