Abstract

At depths less than 4 km in crystalline crustal rocks, water‐filled fractures can be held open by hydrostatic pressure, by extensional tectonic stress and by asperities. Earthquakes in this environment exhibit cubic spectral decay above a single corner frequency, a stress drop down to 2 orders of magnitude lower than typical crustal earthquakes, and higher than normal b‐values. These properties are indicators of low stress and low strength. Hence, both natural and induced earthquakes in areas of open fractures can be triggered by any process that further decreases the strength of the fracture, most commonly processes that cause short‐term increases in fluid pressure. These earthquakes are often spatially and temporally associated with reservoir impoundment. Numerical models of stress surrounding an asperity on a fracture in a gravitating elastic media demonstrate that normal or strike‐slip earthquakes in fluid‐saturated low‐stress environments and the destruction of asperities during faulting can reduce the fracture volume and increase the fluid pressure in the fracture. As a result, the increased fluid pressure would tend to migrate to surrounding and shallower fractures and create a positive feedback condition that can trigger additional earthquakes. When this positive feedback dominates over other natural processes, foreshock activity can increase exponentially leading up to the main event(s) and hypocenters can migrate beyond and above the location of the initial triggering event. The fluid‐induced near‐surface positive feedback mechanism is a form of self‐organized criticality (SOC) and at shallow depths is superimposed on other processes related to a relaxation of elastic stresses.

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