Tidal Tilt Research Articles

Strainmeter and tiltmeter measurements from the Tonga island arc

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.

地球潮汐による傾斜・ひずみと海洋潮汐について(第4報)

地球潮汐による傾斜・ひずみと海洋潮汐について(第4報)

A Discussion on the measurement and interpretation of changes of strain in the Earth - Some instruments and techniques for measurements of tidal tilt

The U.S. Geological Survey has operated continuously recording long-base liquid-level tiltmeters for several years at two locations in the San Francisco Bay Area. The meters are capable of resolving 10~8 rad and exhibit long-term drifts of about 1.5 urad/year. Since these instruments require elaborate vaults for their operation, we have developed a simple pendulum tiltmeter of a geometry suitable for operation in a shallow borehole for measurements at similar sensitivities. Techniques developed for emplacement of instruments in boreholes seem satisfactory, but early results indicate that such sites must be selected with great care and attention to local geology. Sample data presented include plots of output from the long-base instruments and of the pendulum.

A Discussion on the measurement and interpretation of changes of strain in the Earth - Methods for prediction and evaluation of tidal tilt data from borehole and observatory sites near active faults

Tilt has been measured continuously at tidal sensitivity in borehole and observatory sites in the San Francisco Bay region, California. These sites, within a few kilometres of a m ajor fault and the Pacific Ocean, are part of an extensive network measuring strain and microseismicity. Static response of the Presidio site to ocean loading at the M 2 frequency best fits a finite-element model with lower shear modulus in the San Andreas fault zone than in adjacent material at the same depth. The Presidio tilt data exhibit a secular trend less than 3 (rrad/year superim posed on local earthquake and meteorological effects. On two occasions earthquakes (M„ > 4.3) occurred within 55 km of a station and were preceded by anom alous tilt accumulating to 1 ^rad over several days with an accelerated rate of tilt a few hours before the events. The root-mean-square (r.m.s.) difference for two stations of 25 km apart for 700 h before and after one of these events was 5 x 10-8 and 2 x 10 ^ respectively . A similar r.m.s. difference was observed before and after a larger ( M b> 5) but more distant (180 km) earthquake from the same two stations. This latter event did not, however, exhibit the extreme linear slope (10~9 rad/h) of the two earlier earthquakes. Although such anomalies can be correlated with meteorological activity over short periods of time, they do not correlate for periods approaching one month. A transfer function derived during a period w hen there were no local earthquakes can be used for calculating tilt response to surface loading from telemetered meteorological and tilt data. These results can then be input for a prediction beyond the data, and the error in prediction monitored as a final output for instrument performance and potential earthquake hazard.

Tidal Tilt Anomalies

King and Bilham1 have made an interesting suggestion for a mechanism to explain the large number of published anomalous tidal tilt results. The proposed coupling between the tidal strain and the tidal tilt leads to tilt anomalies of the order of 8 msec for the M2 body tide and, for the loading tide, anomalies depending on the magnitude of the loading strain.

地球潮汐による傾斜・ひずみと海洋潮汐について(第2報)

地球潮汐による傾斜・ひずみと海洋潮汐について(第2報)

Seismology of the Moon and Implications on Internal Structure, Origin and Evolution

The objective of the passive seismic experiment is to measure vibrations of the lunar surface produced by all natural and artificial sources of seismic energy and to use these data to deduce the internal structure and constitution of the Moon, the nature of tectonic processes which may be active within the Moon, the rate of strain energy release for the lunar body, and the numbers and masses of meteroroids striking the lunar surface. The instrument used is also capable of measuring changes in gravity and tidal tilts which occur in its vicinity. To accomplish these objectives, seismic data must be combined with data from laboratory measurements of the physical and chemical properties of surface rocks, and many other geophysical and geochemical measurements. Thus far, we have had the opportunity to record data from two lunar seismic stations which were installed by the astronauts during Apollo missions 11 and 12. The combined recording time from these stations is presently over 9 months, but there was no overlap to permit recording the same event at two stations. Results from the analysis of these data have been presented by the seismic experiment team in five previous papers.

Theoretical tides on a rigid, spherical moon

Theoretical tidal tilts and changes in gravitational acceleration make possible the determination of gross physical properties of the moon