Images

A traveltime curve is a graph of arrival times, commonly P or S waves, recorded at different points as a function of distance from the seismic source. Seismic velocities within the earth can be computed from the slopes of the resulting curves. 

Turbidites (interbedded with mudstones/siltstones) from the Ross Sandstone Formation Turbidite system of Namurian age in County Clare, Western Ireland. The sandstone beds were formed in a deep basin by turbidites coming from a delta area.

A seismic refraction or seismic reflection line is a set of seismographs usually lined up along the earth's surface to record seismic waves generated by an explosion for the purpose of recording reflections and refractions of these waves from velocity discontinuities within the earth. The data collected can be used to infer the internal structure of the earth.

An illustration depicting an asperity (pink blob) on fault surface (yellow parallelogram). 

Diagram of dip. Dip is the angle that a planar geologic surface (for example, a fault) is inclined from the horizontal.

Strike-slip, normal, and reverse faults. A reverse fault with a small dip angle is called a thrust fault.

Photo of fault gouge. Fault gouge is crushed and ground-up rock produced by friction between the two sides when a fault moves.

Alluvium visible in Salt River, Arizona (Photo by Sharon Reynolds, Arizona State University)

Cartoon of half-space model. A half space is a mathematical model used to approximate the earth when performing some calculations in seismology. The model is much simpler than the real earth.

Plot showing 440 Hertz and 880 Hertz. Hertz (Hz) is a unit of measurement named in honor of Heinrich Hertz, the German physicist who in 1887 first produced electromagnetic waves. Expresses the frequency in cycles per second; 1 Hz = 1 cycle of rise and fall of a wave per second .

Photo of a leaning transmission tower being held up by lifelines. Lifelines are structures that are important or critical for a community to function, such as roadways, pipelines, powerlines, sewers, communications, and port facilities.

Sand ejected through a crack forming a series of sand boils along the railroad tracks adjacent to Deschutes Parkway in Olympia. 

Diagram of a locked fault showing the shortening and uplift at a coastline. A locked fault is a fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault (it is stuck).

Refraction is (1) the deflection, or bending, of the ray path of a seismic wave caused by its passage from one material to another having different elastic properties. (2) bending of a tsunami wave front owing to variations in the water depth along a coastline.

The residual is the difference between the measured and predicted values of some quantity.

A sand boil is sand and water that come out onto the ground surface during an earthquake as a result of liquefaction at shallow depth.

Slickensides are polished striated rock surfaces caused by one rock mass moving across another on a fault.

Subduction zone of the Juan de Fuca Plate and North America Plate, creating the Mt. St. Helens and Mt. Hood volcanoes.

The time history is the sequence of values of any time-varying quantity (such as a ground motion measurement) measured at a set of fixed times. Also termed time series.

Ripples on a pond are attenuated as they move away from the source. 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.

The strike is the trend or bearing, relative to north, of the line defined by the intersection of a planar geologic surface (for example, a fault or a bed) and a horizontal surface such as the ground.