Figure 1. Locations and names of three ocean-bottom seismographs (OBS) used to locate events and NOAA-PMEL's bottom-pressure recorder (BPR). All located earthquakes are shown as green dots. Purple star indicates location of water-column anomaly as recorded during OBS deployments. Lower right figure shows outline of 1998 flow (Embley et al., 1999) compared with location of 1994 seismicity. Lower-left figure shows location of Axial volcano (AS is Axial Seamount) on Juan de Fuca Ridge, off coast of northwestern United States.

Several measurements of vertical ground motion at Piñon Flat Observatory, California, indicate the overall weakness and instability of the Earth's weathered surface with respect to the underlying rock. Cumulative long‐period motions of order 0.5 mm per year dominate these records, though smaller elastic deformations caused by precipitation loading, atmospheric loading, and tidal strains are evident at higher frequencies; all of these help to characterize the near‐surface material. The long‐period records suggest that near‐surface weathering is the dominant influence on monument motion, at least at this site on crystalline rock in a semi‐arid environment. Rainfall loading gives an average vertical modulus of 2.6 GPa for the material in the uppermost 26 m of the ground, compared with 88 GPa for granite under moderate confining stress; atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours. Earth tide records show Poisson's ratio to be 0.09 in contrast to the normative range of 0.2–0.25, establishing that horizontal strains couple only weakly into vertical ones, so that vertical strains near the surface are a poor measure of areal strain. The form of the ground‐surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument‐positioning error budget that increases with time. Because of ground instability, and the generally small rate of crustal tectonic motions, deeply emplaced monuments will be needed for observational programs designed to detect short‐term changes in crustal deformation over baselengths of order 10 km and less.

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Piñon Flat. Small‐scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short‐baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small‐scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross‐coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.

Measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth's crust. Existing techniques, capable of resolving the signals of interest (strain rates of 10 n∈/yr), are mainly observatory based. The limitation in the baselength of these instruments (∼1 km) results in a requirement that the reference monuments be exceptionally stable (10 μm/yr). However, records from Piñon Flat Observatory, California, show that the actual horizontal displacements for massive near surface monuments, emplaced in competent, weathered granite, are of the order of 50 μm/yr. The low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small. Until high‐accuracy geodetic techniques are developed, sophisticated monuments (or monument monitoring devices) will be necessary to record faithfully continuous crustal deformation.

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