Unexpected far-field hydrological response to a great earthquake
Abstract
Permeability, an important crustal property that controls transport processes in the shallow crust, is known to change during large earthquakes. Analysis of the tidal response of groundwater in wells has been increasingly used to detect such changes, but often multiple interpretations of the changes of the tidal response are possible and the suggested interpretation may be tentative. Here we use the water level data from a groundwater well in North China before and after the far-field great 2011 Mw 9.1 Tohoku earthquake to illustrate the use of the barometric response as a method to resolve this uncertainty. The analyzed tidal response of water level in this well shows abrupt coseismic increases of both phase shift and amplitude ratio, which appears constant in the four years after the earthquake until the water-level record was disrupted by well cleaning. Based on the available classical models for tidal response of water level in wells, these changes may be interpreted either by a large increase in the horizontal permeability of a confined aquifer or an occurrence in the vertical permeability of an unconfined aquifer. In order to differentiate between these two interpretations we analyze the water-level response to barometric pressure before and after the Tohoku earthquake. The result shows that the aquifer was confined both before and after the earthquake. Thus the changes of the tidal response in this well are due to a coseismic increase in the horizontal permeability. Interpretation of the changes with a model for confined aquifer yields a coseismic change in the poroelastic property and the change stays constant 4 years after the earthquake. Such changes are unexpected in the far field of an earthquake because previous studies suggest that these can occur only in the near field of earthquakes. The study implies that re-examination may be needed of the magnitude-distance relation for water-level response in the far field of great earthquakes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S51D0370Z
- Keywords:
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- 9805 Instruments useful in three or more fields;
- GENERAL OR MISCELLANEOUSDE: 9820 Techniques applicable in three or more fields;
- GENERAL OR MISCELLANEOUSDE: 1895 Instruments and techniques: monitoring;
- HYDROLOGYDE: 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS