Earthquake-induced Groundwater and Gas Changes |
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Authors: | Chi-Yu King Wei Zhang Zhaocheng Zhang |
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Institution: | (1) Earthquake Prediction Research, Inc, 381 Hawthorne Ave, Los Altos, CA, 94022, U.S.A;(2) Center for Analysis and Prediction, China Seismological Bureau, Beijing, 100036, China |
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Abstract: | Active faults are commonly associated with spatially anomalously high concentrations of soil gases such as carbon dioxide
and Rn, suggesting that they are crustal discontinuities with a relatively high vertical permeability through which crustal
and subcrustal gases may preferably escape towards the earth's surface. Many earthquake-related hydrologic and geochemical
temporal changes have been recorded, mostly along active faults especially at fault intersections, since the 1960s. The reality
of such changes is gradually ascertained and their features well delineated and fairly understood. Some coseismic changes
recorded in ``near field' are rather consistent with poroelastic dislocation models of earthquake sources, whereas others
are attributable to near-surface permeability enhancement. In addition, coseismic (and postseismic) changes were recorded
for many moderate to large earthquakes at certain relatively few ``sensitive sites' at epicentral distances too large (larger
for larger earthquakes, up to 1000 km or more for magnitude 8) to be explained by the poroelastic models. They are probably
triggered by seismic shaking. The sensitivity of different sites can be greatly different, even when separated only by meters.
The sensitive sites are usually located on or near active faults, especially their intersections and bends, and characterized
by some near-critical hydrologic or geochemical condition (e.g., permeability that can be greatly increased by a relatively
small seismic shaking or stress increase). Coseismic changes recorded for different earthquakes at a sensitive site are usually
similar, regardless of the earthquakes' location and focal mechanism. The sensitivity of a sensitive site may change with
time. Also pre-earthquake changes were observed hours to years before some destructive earthquakes at certain sensitive sites,
some at large epicentral distances, although these changes are relatively few and less certain. Both long-distance coseismic
and preseismic changes call for more realistic models than simple elastic dislocation for explanation. Such models should
take into consideration the heterogeneity of the crust where stress is concentrated at certain weak points (sensitive sites)
along active faults such that the stress condition is near a critical level prior to the occurrence of the corresponding earthquakes.
To explain the preseismic changes, the models should also assume a broad-scaled episodically increasing strain field. |
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