VUME Upper Mantle of the Earth

Earthquake Swarms and Storms.

Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, may be on the order of days, weeks, or months. They are differentiated from earthquakes succeeded by a series of aftershocks by the observation that no single earthquake in the sequence is obviously the main shock. Earthquake swarms are pervasive at volcanoes, but have seldom been studied systematically. Most swarms that are described in the literature are those that occurred in association with eruptions; indeed, earthquake swarms are the most reliable method of forecasting eruptions. For the purpose of this report, a swarm is defined as many earthquakes of the same size occurring in a small volume. Swarms are different in these two ways from a mainshock-aftershock sequence or a foreshock-mainshock-aftershock sequence. Swarms are especially common in volcanic areas.

An earthquake storm is a theory about earthquakes, where one triggers a series of other large earthquakes—along the same plate boundary—as the stress transfers along the fault system. This is similar to the idea of aftershocks, with the exception that they take place years apart.

Remotely Triggered Earthquakes

Remotely triggered earthquakes theory postulated that large earthquakes can have an influence outside of the immediate aftershock zone, and actually activate other earthquakes at considerable distance. The further one gets from the initiating earthquake in both space and time, the more controversial is the association. Remotely triggered earthquakes have now been observed commonly in geothermal and volcanic regions. Analysis of historic earthquake sequences reveals that remotely triggered earthquakes occur commonly in intraplate crust as well. Because neither abundant geothermal/volcanic fluids nor weak faults are expected to exist in intraplate crust, this provides evidence that at least some faults in intraplate regions are critically stressed. The locations of observed remotely triggered earthquakes suggest that intraplate crust is not critically stressed everywhere, but rather in certain regions where long-lived stress concentrations persist.