Earthquake basics: faults, magnitude, and where US risk is highest

What causes an earthquake.

In layman’s terms an earthquake occurs when two blocks of the earth slip. This slip surface is called a fault or fault plane.

Classification of the type of fault uses the angle of the fault with respect to the surface (Dip) and the direction of slip along the fault. Faults moving along the direction of the dip plane are dip-slip faults and can be either normal or reverse, depending on their motion. Faults that move horizontally are called strike-slip faults and can be either right or left lateral. Some faults have both dip-slip and strike-slip motion and are called oblique-slip faults.

Where the earthquake originates below the surface is known as the hypocenter and the location directly above on the surface is known as the epicenter.

Foreshocks, mainshocks and aftershocks.

Earthquakes generally occur in a sequence, first are small shocks known as foreshocks, then the main, largest earthquake is the mainshock and then the mainshock is followed by aftershocks, which are smaller earthquakes, some still can be significant.

For large earthquakes the aftershocks can occur for weeks or even years after the mainshock.

Measuring earthquakes.

The size of an earthquake is defined by a magnitude scale and the scales are logarithmic, for example a magnitude 5 earthquake shakes the ground 10 times more than a magnitude 4 and around 32 times the energy released.¹ For every minor increase of 0.1 in magnitude, the energy release increases by about 1.5 times.²

One common scale is the Moment Magnitude (Mw) which shows how much energy is released. Magnitude is a measure of size and intensity is the measure of shaking at a location. The US uses the Modified Mercalli Intensity (MMI).

The magnitudes versus number of earthquakes follows a logarithmic distribution known as the Gutenberg-Richter relationship. This relationship holds worldwide as well as for individual regions.

Seismologists have three metrics to understand the behavior of buildings in an earthquake.

• Peak Ground Acceleration (PGA)
• Peak Ground Velocity (PGV)
• Peak Spectral Acceleration (PSA)

The largest US earthquakes.

The largest earthquake to affect the US in the last 10 years was the 2021 Chignik earthquake in Alaska, and the largest since records was the 1964 Great Alaska earthquake (Prince William Sound), again in Alaska.³⁴

For the contiguous 48 states the largest in the last ten years was the 2019 Ridgecrest earthquake sequence, CA and the largest recorded was either the 1857 Fort Tejon earthquake or the 1906 San Francisco earthquake, both magnitude 7.9 and on the San Andreas Fault in CA.⁵⁶⁷

Where damaging earthquakes are most likely.

There are 16 states with a relatively high likelihood of experiencing damaging ground shaking, defined by having historically experienced M6.0+ earthquakes:⁸⁹

1. Alaska,
2. Arkansas,
3. California,
4. Hawaii,
5. Idaho,
6. Illinois,
7. Kentucky,
8. Missouri,
9. Montana,
10. Nevada,
11. Oregon,
12. South Carolina,
13. Tennessee,
14. Utah,
15. Washington, and
16. Wyoming.¹⁰

California

According to the US Geological Survey (USGS), California has a greater than 99% chance of experiencing a magnitude 6.7 or larger earthquake statewide over a 30-year period.¹¹

Within the next 30 years the probability for the two main regions is:¹²¹³

Los Angeles

• 60% earthquake magnitude 6.7¹⁴
• 46% earthquake magnitude 7¹⁵¹⁶
• 31% earthquake magnitude 7.5¹⁷¹⁸

San Francisco Bay Area

• 72% earthquake magnitude 6.7¹⁹²⁰
• 51% an earthquake magnitude 7²¹²²
• 20% an earthquake magnitude 7.5²³²⁴