A1 Earthquakes and Seismic Sequences

Topic

An earthquake is the sudden release of elastic energy that has accumulated within a fault zone; the magnitude of this sudden release is characterized by the earthquake's seismic moment—a quantity proportional to the area of ​​the ruptured fault, the average displacement experienced by the fault, and the rigidity of the rock where the rupture occurred.

The number of earthquakes occurring in a given region follows a highly consistent empirical relationship between frequency and magnitude: the number of events decreases logarithmically as magnitude increases, such that for every additional unit of magnitude, there are approximately ten times fewer earthquakes than in the preceding magnitude unit.

The largest subduction earthquakes—with moment magnitudes exceeding 8.5 or even 9.0—are the most energetic of all; the rupture of the contact between the two tectonic plates can extend for hundreds of kilometers, generating coseismic displacements of tens of meters along that interface. The typical sequence of such a subduction earthquake begins with a series of foreshocks, followed by the main event itself, and concludes with a long sequence of aftershocks, the frequency of which declines over time according to an empirical relationship well-established in seismology.

Seismicity induced by fluid injection—whether in deep wells or reservoirs—reactivates pre-existing faults that had long been inactive: the injected fluid raises the water pressure within the rock pores, and this increased pore pressure reduces the effective pressure holding the fault firmly closed, meaning far less additional force is required for the fault to slip again.

The expected behavior of this induced seismicity is for it to migrate along the pore-pressure diffusion front as the injected fluid spreads through the rock, moving progressively further away from the injection point over time. Furthermore, when a major earthquake occurs, it redistributes mechanical stress to neighboring faults; this redistribution can either trigger or inhibit seismicity on those faults, depending on how each is oriented relative to the sudden release of elastic energy mentioned at the beginning of this description.

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