Earthquake

A transient release of stored mechanical energy in the solid planet, often associated with fault motion. Plate tectonics is a major terrestrial driver, but earthquakes can also occur without an active global plate system.

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.

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.

The time history of a specified earthquake source quantity, commonly moment rate. It differs from recorded ground velocity, which also contains radiation, propagation, instrument response and rupture directivity. A measured pulse width requires those assumptions before it is called a rupture duration.

Azimuth-dependent compression or stretching of observed source radiation because later-rupturing patches have different source-to-receiver travel times. A unilateral rupture generally yields shorter apparent duration in its forward direction. Pulse width also contains local slip rise time, instrument response and path effects, so directivity requires controlled comparisons.

The speed at which the rupture front advances along a fault plane. It is distinct from the local particle slip velocity and the propagating seismic wave speeds. Length equals $V_{r}T$ only for the stated approximately constant-speed propagation geometry.

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.

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 proposed mechanism by which reactions releasing fluid promote brittle failure or instability at depth. A spatial association with dehydration alone does not demonstrate causation.

An elastic rebound model describes slow loading of a locked fault, storage of elastic strain energy, and sudden slip with stress reduction when the fault fails. The surrounding rock partly relaxes while retaining a permanent displacement across the fault. It does not by itself predict the failure time, rupture extent or frictional dynamics, and continuous creep need not follow this ideal cycle.

An earthquake centroid is an effective average location of seismic moment release. It differs from the hypocenter, where rupture starts, and can depend on the source representation and observations. An associated centroid time similarly averages release rather than marking its first instant.

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 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.

A fault plane is an idealized planar surface across which rock has slipped. A curved fault can be described by local fault planes. Its orientation is specified by fault strike and fault dip, and the slip direction within it by fault rake.

The fault rake specifies the slip direction within a fault plane. With an along-strike unit vector $\mathbf{s}$ and a down-dip unit vector $\mathbf{d}$, one explicit convention is $\mathbf{b}/b=\cos \lambda \mathbf{s}-\sin \lambda \mathbf{d}$. Positive $90^{\circ }$ then denotes up-dip slip. A convention must accompany a signed fault rake.

The fault dip is the angle between a fault plane and a horizontal plane, measured perpendicular to fault strike towards the descending side. It lies between $0$ and $90^{\circ }$.

The fault strike is the azimuth of the intersection of a fault plane with a horizontal plane. A right-hand convention chooses the along-strike direction for which the fault dips to the observer's right.

The relative displacement across a fault surface. Its vector and the fault normal determine an ideal double-couple seismic moment; a finite rupture can have spatially and temporally variable slip.

The observed separation of an originally correlated marker across a fault. Its value depends on marker geometry and measurement direction; apparent map separation is not automatically the full fault slip vector. Matching channel reaches, beds or dated surfaces supplies candidate piercing constraints.

Displacement across a fault per unit time, with the component and averaging interval specified. A long-term geological average and a short-term geodetic rate need not coincide.

A marker that starts with zero offset and accumulates displacement at a constant fault slip rate $v$ has age $t=d/v$. More generally $d={\int }_0^{t}v(s)ds$. The inferred age belongs to the establishment, abandonment or last resetting of that particular marker. A channel incised into an alluvial fan can be younger than the sediment it cuts. Uncertain channel correlation, erosion, distributed slip and time-variable rates require corresponding age uncertainty; a numerical ratio alone does not date a fan’s deposition.

The horizontal dip-direction component of displacement across a fault. The displayed expression assumes pure dip slip, with magnitude $D$ and fault dip $\delta$.

The vertical component of displacement between correlated markers across a fault. For pure dip slip of magnitude $D$ at fault dip $\delta$, the displayed formula applies. Erosion and ground slope can make surface fault scarp height different from throw.

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.