Tectonic leakage of fault bounded aquifers subject to non-isothermal recharge: a mechanism generating thermal precursors to seismic events

• Published in: Physics of the earth and planetary interiors Vol. 126; no. 3; pp. 163 - 177
• Main Author: Hamza, Valiya M
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2001
• Subjects: Analytical models; Earthquake prediction; Tectonic leakage; Thermal precursors; Analytical models; Earthquake prediction; Tectonic leakage; Thermal precursors
• ISSN: 0031-9201; 1872-7395
• DOI: 10.1016/S0031-9201(01)00253-9

The thermal consequences of flow transients generated by deformation induced leakage at fault zones intersecting confined aquifers have been examined using a simple analytical model. The recharge of the aquifer is assumed to take place by vertical infiltration through the confining layers. The energy equation relevant in this case has been derived by evaluating enthalpy fluxes associated with lateral and vertical components of fluid flows as well as that due to volume changes in the permeable layer, generated by deformation induced alterations in the hydraulic head. Results of numerical simulations indicate that thermal transients arise during the stress build-up period when the infiltrating fluids mix with those in the aquifer, but relatively rapid return to equilibrium conditions occur as soon as the leakage is cut-off. The model is thus capable of reproducing the characteristic features of thermal precursors found in observational records. According to model results, the growth of thermal perturbation as well as the time for attaining steady-state conditions is proportional to the recharge rate. The magnitude of the perturbation is related to the thermal gradients in the confining layers but permeability contrast between the confining layer and the aquifer also plays a significant role. The model results also indicate that precursory signals are likely to be significant only in areas close to the fault plane where tectonic leakage has a direct influence on the hydraulic head of the aquifer. If the fault plane is sensitive to deformation the tectonic leakage rate may undergo substantial changes in response to changing stress patterns and this in turn may easily lead to abrupt and substantial changes in the local thermal regime. Some of the transient thermal anomalies observed in records of temperatures in boreholes in tectonically active areas have characteristics similar to those predicted by the ‘deformation induced leakage’ model. Examples are presented illustrating model fits to thermal transients, identified as precursors to seismic events, in Firjusa (Turkmenistan), Didiweli (Georgia) and Izu peninsula (Japan). The possibility of obtaining complementary information on processes responsible for fluid flow during pre-seismic periods is pointed out. It appears that the occurrence of thermal anomalies associated with small-scale tectonic deformation may not be uncommon. Its detection depends on availability of suitable boreholes and of the facilities for monitoring of temperatures using high precision sensors over periods comparable to local recurrence periods of earthquakes.