As part of its monitoring activities, the ANSS includes a national Backbone network, the National Earthquake Information Center (NEIC), the National Strong Motion Project, and 15 regional seismic networks operated by USGS and its partners.
When earthquakes strike, ANSS delivers real-time information, providing situational awareness for emergency-response personnel. In regions with sufficient seismic stations, that information includes –within minutes– a ShakeMap showing the distribution of potentially damaging ground shaking, information used to target post-earthquake response efforts. When fully implemented, ANSS will provide such dense station coverage for all at-risk urban areas. Information from ANSS is a key input to the USGS National Seismic Hazard Maps, which help communities in earthquake-prone regions develop safer building practices.
An aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to known laws. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.
Most large earthquakes are followed by additional earthquakes, called aftershocks, which make up an aftershock sequence. While most aftershocks are smaller than the mainshock, they can still be damaging or deadly. A small fraction of earthquakes are followed by a larger earthquake, in which case the first earthquake is referred to as a foreshock. For example, the 2011 M9.1 Japan earthquake and tsunami was preceded by a M7.3 foreshock two days before. When the M7.3 earthquake first occurred, it was called the mainshock, and then when the M9.1 earthquake occurred, that larger earthquake became the mainshock.
The Cascadia Subduction Zone (CSZ) "megathrust" fault is a 1,000 km long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino California. It separates the Juan de Fuca and North America plates. New Juan de Fuca plate is created offshore along the Juan de Fuca ridge. The Juan de Fuca plate moves toward, and eventually is shoved beneath, the continent (North American plate).
An earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period of time. The length of time used to define the swarm itself varies, but may be of the order of days, months, or even years. Such an energy release is different from what happens commonly when a major earthquake (mainshock) is followed by a series of aftershocks: in earthquake swarms, no single earthquake in the sequence is obviously the mainshock. In particular, a cluster of aftershocks occurring after a mainshock is not a swarm