Identification of Micro-cracks to Define Nucleation Stage of Earthquake Occurrence
Abstract
Earthquakes are understood to be fracture of crustal rocks or stick slip of neighboring blocks. Preshocks are known to occur generally in the rupture preparatory stage of natural earthquake. And, microcracks are detected in the nucleation stage of accelerated deformation in almost all laboratory experiments. Preshock activity has been found to be less regular to be used for confident predictive purpose. Previous efforts to detect micro-cracks related with natural earthquakes have not succeeded to find particular phenomena to identify the nucleation stage. Here we show that a deep underground electric field measurement using special antenna could detect micro-cracks appearing in the nucleation stage of the Tohoku Earthquake. The measurement method has been proved to be robust to the natural and man-made noises. Whole large pulse-like variations were grouped into three by frequency and waveforms, and found to characterize different phases in the preparatory and after shock processes. Those variations are grouped by time constants: A -type with duration of several to several ten minutes just the same as the pulse-like signals detected previously at the time of volcanic eruption and seismic swarms, B of several hundred Herz, and C of several kiloherz. The B- type variations are sub-grouped into three by waveform: type B-1 of similar to the GUV except in the stepped decay in the relaxation phase after sharp rise, type B-2 of wave packet similar to acoustic emission, and type B-3 of superposition of B-1 and B-2 Different types of electric waveforms are suggested to correspond to crack rupture modes of tensile and shear, and to be generated by confined water movement through the electro-kinetic effects on the ground of previous investigations on laboratory experiments. The second kind of cracks (B-type) is suggested to define the nucleation period: activity increases sharply before the earthquake following the modified Ohmori's law (Figure 1). Field monitoring of micro-crack activity is shown to be practically possible to investigate the nucleation stage providing a break-through for the short term prediction method.;
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMNH41B1607F
- Keywords:
-
- 0694 ELECTROMAGNETICS / Instruments and techniques;
- 1835 HYDROLOGY / Hydrogeophysics;
- 7209 SEISMOLOGY / Earthquake dynamics;
- 4315 NATURAL HAZARDS / Monitoring;
- forecasting;
- prediction