Locating Earthquakes and Measuring Magnitude

Vocabulary flashcards covering seismic wave intervals, epicenter triangulation, the Richter scale, and the seismic moment magnitude scale.

S minus P travel time

The difference between the arrival time of the first S wave and the earlier first P wave on a seismogram, which increases with distance from the earthquake.

Three seismic stations

The minimum number of seismic stations required to pinpoint the exact epicenter of an earthquake using overlapping distance circles.

Richter scale

An early logarithmic scale devised in the 1930s by Charles Richter and colleagues to measure earthquake severity and energy release based on S-wave seismogram amplitude.

Amplitude of pen deflection

The deviation of seismic waves from a straight baseline of zero ground motion, measured in millimeters for the maximum S wave.

Standardized distance

The reference distance of $100\text{ kilometers}$ at which the base-10 logarithm of S-wave amplitude is standardized for the Richter scale.

Nomogram

A diagram containing three calibrated scales—distance/S-P interval on the left, Richter magnitude in the center, and amplitude on the right—used to calculate magnitude by connecting points with a straight line segment.

$$M_L$$

The abbreviation denoting local magnitude on the Richter scale, originally developed for moderately strong and weaker earthquakes in California studied close to their source.

Saturation

The leveling-off effect where ground shaking amplitude fails to increase proportionally with energy release during very large earthquakes.

Seismic moment magnitude scale

A magnitude scale that models total energy release by estimating seismic moment, matching the Richter scale up to magnitude $7$ before providing more accurate readings for larger events.

Seismic moment

The product of three factors multiplied together: the shear strength of rock, the area of rupture, and the average displacement across the fault.

Shear strength of rock

The amount of shear stress rock tolerates before breaking; treated as a constant in seismic moment calculations because large earthquakes occur in strong igneous rock.

Area of rupture

The total surface area of rock broken or torn during an earthquake, which varies from event to event.

Average displacement

The mean distance of rock slip across the entire tear region, which reaches its maximum near the focus or hypocenter.

Energy scaling factor

The factor of $10 \times \sqrt{10}$ (approximately $32$) by which energy release increases for each single whole-number step on the moment magnitude scale.

Haiti earthquake (January 2010)

A magnitude $7$ earthquake whose shallow focus and epicenter located directly in Port-au-Prince caused catastrophic loss of life.

Japan earthquake (March 2011)

A magnitude $9$ earthquake that caused a major tsunami and shut down a nuclear power plant, releasing $1000$ times more energy than the 2010 Haiti earthquake.