abstract

A four-component quartz tube strainmeter with capacitance transducers has been installed in a surface site on the small coral island of Tongatapu in Tonga. Analysis of 30 days of data indicates that temperature and pressure effects are small, but that rainfall may have a significant effect. Semidiurnal lunar tidal strains with peak-to-peak amplitudes larger than 1.0 × 10−7 are observed on all components, as are semidiurnal areal strains of 2.0 × 10−7 and higher. These results are not completely explained by conventional ocean loading models, but a simple argument shows that these strains are consistent with recent reports of anomalously large tidal amplitudes in well levels on coral island aquifers. The tidal strains exhibit a “saw-tooth” shape, for the strain curve approaches compressional minima with steeper slopes than compressional maxima. This indicates a nonlinear response to the lunar driving foce, which apparently is due to loading by nonlinear tides in a lagoon near the strain site. A long-period horizontal seismometer was equipped with displacement transducers to record as a tiltmeter and was placed in a surface vault near the strainmeter site. Meaningful tidal tilts could not be measured until the surface vault was covered with 60 cm of soil, and even then the tiltmeter was affected by thermal changes with periods longer than a few days. The tiltmeter was not affected by rainfall. Surface sites on tropical islands seem practical for strainmeters and tiltmeters at seismic and tidal frequencies, but future researchers attempting to measure tectonic strains or tilts in island arcs might wish to consider simultaneous monitoring of ocean tidal levels and water well levels along with temperature, pressure, and rainfall.

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