Shaking levels at a site may be increased, or amplified, by focusing of seismic energy caused by the geometry of the sediment velocity structure, such as basin subsurface topography, or by surface topography.
When you throw a pebble in a pond, it makes waves on the surface that move out from the place where the pebble entered the water. The waves are largest where they are formed and gradually get smaller as they move away. This decrease in size, or amplitude, of the waves is called attenuation. Seismic waves also become attenuated as they move away from the earthquake source.
Directivity is an effect of a fault rupturing whereby earthquake ground motion in the direction of rupture propagation is more severe than that in other directions from the earthquake source.
Elastic rebound is what happens to the crustal material on either side of a fault during an earthquake. The idea is that a fault is stuck until the strain accumulated in the rock on either side of the fault has overcome the friction making it stick. The rock becomes distorted, or bent, but holds its position until the earthquake occurs, and the rock snaps back into an unstrained position, releasing energy that produces seismic waves.
A horst is an upthrown block lying between two steep-angled fault blocks. A graben is a down-dropped block of the earth's crust resulting from extension, or pulling, of the crust.
A process by which water-saturated sediment temporarily loses strength and acts as a fluid, like when you wiggle your toes in the wet sand near the water at the beach. This effect can be caused by earthquake shaking.
Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.
A thrust fault is a reverse fault with a dip of 45 or less. This animation shows a reverse fault.
An asperity is an area on a fault that is stuck. The earthquake rupture usually begins at an asperity.
A thrust fault that does not rupture all the way up to the surface so there is no evidence of it on the ground. It is buried under the uppermost layers of rock in the crust.
A divergent boundary is where two adjacent tectonic plates are moving away from each other.
Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all mainshocks have foreshocks. Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 fault lengths distance from the mainshock fault. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.
The shortest path between two points on the surface of a sphere lies along a great circle.
Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse.
The shadow zone is the area of the earth from angular distances of 104 to 140 degrees that, for a given earthquake, does not receive any direct P waves. The shadow zone results from S waves (not shown in animation) being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.
The wavefront is the instantaneous boundary between the seismic waves in the earth material, and the material that the seismic energy has not yet reached.