Images

Acceleration, velocity, and displacement records. 

Displacement is the difference between the initial position of a reference point and any later position. The amount any point affected by an earthquake has moved from where it was before the earthquake.

First motion mechanism. On a seismogram, the first motion is the direction of ground motion as the P wave arrives at the seismometer. Upward ground motion indicates an expansion in the source region; downward motion indicates a contraction.

Small foreshocks on a seismic record. Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all mainshocks have foreshocks.

Photo of Zion Canyon located in Zion National Park in Utah, United States. (Image courtesy of Justin Gould)

The earthquakes (colored dots) in the middle of the continent are intraplate events. Those near the ocean boundary are at the boundary between two plates (interplate).

Diagram of left-lateral motion on fault. If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away. See also right-lateral. 

A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Ripples on a pond are attenuated as they move away from the source. Q is the inverse of attenuation. A high Q means a low attenuation.

Geologic time scale showing eras, periods, and notable events in terms of millions of years ago.

Secular refers to long-term changes that take place slowly and imperceptibly. Commonly used to describe changes in elevation, tilt, and stress or strain rates that are related to long-term tectonic deformation.

The source is the term for the released forces that generate acoustic or seismic waves, also called the earthquake source.

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.

Tsunamigenic is referring to those earthquakes, commonly along major subduction zone plate boundaries such as those bordering the Pacific Ocean, that can generate tsunamis.

YBP is an abbreviation for "years before present".

Formation of Earth: 4.6x10^9 YBP

Dinosaur extinction: 6.5x10^7 YBP

First hominids: 2x10^6 YBP

Last great ice age: 1x10^4 YBP

Declaration of Independence: 2x10^2 YBP

Establishment of UWB: 1x10 YBP

High, medium, and low amplitude. The amplitude is the size of the wiggles on an earthquake recording.

Seismicity of Hawaii: earthquakes M>5 from the 1840-1899, 1900-1959, 1960-2019 catalogs shown separately. While earthquake activity remains high today, it seems to have decreased over the past 60 years, which could be related to less volcanic activity at Mauna Loa. 

Figure 2.

Figure 1. Chance of minor or greater damaging earthquake shaking in the next 100 years. Population exposure estimates are rounded to the nearest 1000. 

USGS Geologists R. Sickler, J. Willard, and B. Philibosian observe as an excavator digs a pit used to investigate sediments uplifted by active tectonic activity in the San Joaquin – Sacramento Delta, California, November 29, 2021.

USGS infographic on the magnitude 5.8 earthquake that struck central Virginia August 23, 2011, east coast earthquake hazards and how the USGS is helping inform critical decisions to protect lives and property. 

Full Text Description:

10-YEAR ANNIVERSARY OF THE M5.8 EARTHQUAKE IN VIRGINIA
America’s Most Widely Felt Earthquake

Joanne Chan conducting a seismic survey in San Carlos, California, looking for traces of the Serra Fault.