Abstract
Fluid anomalies were often considered as possible precursors before earthquakes. However, fluid properties at the surface can change for a variety of reasons, including environmental changes near the surface, the response of the superficial fluid system to loads associated with the mechanical nucleation of earthquake fractures, or as a result of transients in fluid flow from the depths. A key problem is to understand the origin of the anomaly and to distinguish between different causes. We present a new approach to monitor geochemical and geophysical fluid properties along a vertical profile in a set of drillings from a depth of a few hundred metres to the surface. This setup can provide hints on the origin of temporal variations, as the migration direction and speed of properties can be measured. In addition, potential admixtures of fluids from a deep crustal or mantle origin with meteoric fluids can be better quantified. A prototype of a multi-level gas monitoring system comprising flow and pressure probes, as well as monitoring of fluid-geochemical properties and stable isotopes is being implemented in a mofette field with massive CO2 (up to 97 tons per day) degassing. The mofette is believed a gas emission site where CO2 ascends through crustal-scale conduits from as deep as the upper mantle, and may therefore provide a natural window to ongoing magmatic processes at mantle depth. Fluids from three adjacent boreholes - 30 m, 70 m, and 230 m deep - will be continuously monitored at high sampling rates.
Highlights
The majority of earthquake precursor studies follow a simple scheme: identify an anomaly in a timeseries and relate it one-on-one to a selected earthquake (see anomaly-earthquake compilation in Cicerone et al (2009))
Hatuda measured the radon concentration of soil air from 0.6, 1, and 2 m depth once a day for more than 2 years and noted about an earthquake-related anomaly “the deeper the sampling site was, the greater proportionately was the increase in concentration, an instance opposite to the case where meteorological influences are at work”
The center of the noise anomaly at a depth of 100 m was located slightly east of the old well. Extrapolating this trend to greater depth indicated that the fluid channel is hit at a depth of about 200 m at the location of F3
Summary
The majority of earthquake precursor studies follow a simple scheme: identify an anomaly in a timeseries (often defined as values above 2, 3, or 4 standard deviations) and relate it one-on-one to a selected earthquake (see anomaly-earthquake compilation in Cicerone et al (2009)). The idea is to identify strain transients in the crust, to understand the groundwater response to crustal deformation related to episodic slow-slip events (Itaba et al, 2010) We adapt this idea, adding fluid geochemical composition and CO2 isotopic signatures to the online monitoring of geophysical parameters. Hatuda measured the radon concentration of soil air from 0.6, 1, and 2 m depth once a day for more than 2 years and noted about an earthquake-related anomaly “the deeper the sampling site was, the greater proportionately was the increase in concentration, an instance opposite to the case where meteorological influences are at work”. F3 instrumentation will be completed similar to F2, if technically feasible
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