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An earthquake occurring in the region affected by an earlier event during its subsequent relaxation and stress redistribution. A spatial aftershock pattern can help identify a fault, but not every aftershock must lie on the main rupture plane.

British geological survey organization publishing geological and earthquake observations and explanatory material. www.bgs.ac.uk

A wedge-shaped slope deposit, commonly thickest beside a scarp and thinning away from it. When combined with faulting, burial and cross-cutting relations it can record a past surface-rupturing earthquake; a wedge alone does not prove a separate event.

Differences between arrival times of nearby earthquakes at a common station largely cancel common propagation errors. Comparing observed and predicted differences constrains relative source separation and origin-time differences. Closely similar waveforms permit precise differential picks by cross-correlation. Absolute position still needs an accurate model or external anchor.

The change in stress during an earthquake, often inferred through a source model. It is not a direct measurement of the absolute pre-event stress.

The epicenter is the projection of an earthquake's hypocenter onto the surface directly above it.

Surface distance from an earthquake epicenter to a receiver. It differs from hypocentral distance when the source has nonzero depth.

An earthquake source with spatial extent and a history of fault slip, rather than a single instantaneous point. Local slip rise time, advance of the rupture front and radiation travel times jointly control observed pulse shapes.

A description of earthquake source geometry, often strike, dip and rake for a double-couple shear fault. First-motion polarities constrain the radiation sign pattern, with a fundamental interchange ambiguity between fault and auxiliary planes.

A relatively down-dropped block bounded by opposing normal faults. A small secondary graben can form within the distributed surface rupture zone of one earthquake.

A seismology researcher associated with deep earthquake distributions, the Wadati-Benioff zone and seismograph development.

The hypocenter is the point where an earthquake rupture starts, rather than necessarily the centroid of its eventual slip distribution.

Distance from an earthquake hypocenter to a receiver. In a uniform velocity model it is the straight ray-path length; in layered material an interphase delay instead constrains a model-dependent path.

The institutional Earthquake Hazards Program page explains how disrupted young layers and unbroken overlying cover identify event horizons, and how ages above and below the affected horizon bracket a past earthquake. It distinguishes primary fault displacement from secondary shaking evidence.

A seismology researcher associated with the recognition of deep earthquakes and the Wadati-Benioff zone.

Downslope movement of geological material under gravity. An earthquake can trigger a landslide without its rupture surface coinciding with the causative tectonic fault.

A large earthquake involving slip on the shallow-dipping plate interface of a subduction zone.

Suppression of relative slip on part of a megathrust during an interval of plate convergence, producing a slip deficit and elastic deformation. GNSS, leveling and offshore geodesy constrain it through a fault-geometry and deformation model. A locking fraction is therefore an inferred kinematic quantity; it is not a direct measurement of friction or the magnitude of the next earthquake.

A logarithmic measure of earthquake size based on seismic moment. A commonly used SI relation is $M_w=\frac{2}{3}[{\log }_{10}(M_0/(\mathrm{N}\mathrm{m}))-9.1]$. Because seismic moment integrates fault slip over the fault plane and includes the shear modulus, a peak slip value cannot replace the area integral.

Official explanation of megathrust earthquakes, crustal earthquakes and deep intraslab events in the Pacific Northwest.