Volumetric Strain Changes Research Articles

A widespread simultaneous increase of the microearthquake seismicity and minor crustal movements in and around the assumed rupture zone of the Tokai earthquake

Data from an extensive routine network and a high-density temporary seismic observation using the ocean bottom seismometer in and around Suruga Bay, Japan, showed that a simultaneous increase of microearthquake seismicity occurred from mid-July 1984 (Ukawa et al., 1988). The area extended over a region of about 60 km (NS) × 80 km (EW), and is nearly included in the assumed fault zone of the presumed Tokai earthquake. The analyses of the bore-hole ground tilt and volumetric strain data revealed that an anomalous small ground tilt and volumetric strain change occurred at the time of the seismic activity with a duration of about a month. Data of geodetic measurements, groundwater, and radon content were examined with the result that many observational items showed small anomalies during that period. The observed crustal movement was compared with that of slip models of the Philippine Sea plate around Suruga Bay, suggesting that some type of episodic aseismic subducting motion occurred and, thus, caused a crustal movement in the overlying continental plate in the very region of the future fault zone.

A theoretical investigation of the sliding crack model of dilatancy

Laboratory measurements of volumetric strain and velocity in Westerly granite are analyzed to determine the physical nature of microcracks in the rock. The commonly used factional ‘sliding crack’ model of dilatancy is examined in detail. The characteristics of an ensemble of sliding cracks are determined from the characteristics of the individual sliding surfaces. It is found that sliding cracks which open with a stick slip motion cannot account for the observed ‘memory’ of the peak stress in cycling experiments. A system of stably sliding cracks would have the required memory but would also produce a very long dead band at the beginning of sample unloading during which no change in inelastic volumetric strain could take place. An analysis of the elastic constants at the beginning of unloading shows that this dead band is not observed in laboratory experiments. This analysis shows that the sliding crack is an unlikely mechanism for dilatancy. An alternative is developed in the following paper.

Gravity changes over the East-Izu uplift, Japan.

Precise gravity surveys have been repeated twenty times at 36 stations on the Izu Peninsula since December 1976. A free-air gradient gravity decrease occurred concurrently with a dome-like uplift centering at Hiekawa Pass, while a Bouguer gradient gravity increase was observed in association with a coseismic crustal subsidence accompanied by the 1978 Izu-Oshima-Kinkai earthquake. Growth and migration of the coswarm uplift around Cape Kawanazaki were successfully detected as temporal gravity changes during the period from 1978 to 1980. At a benchmark located nearby a borehole volume-strainmeter station, we found an interesting fact that gravity changes are proportional to changes in volumetric strain with a rate of 1.4×10-6/μgal.