Earthquakes: Ground Truth About the Shaking Earth

Earthquakes are the only major natural hazard that strikes without any forecast warning - there are no earthquake watches or warnings in the traditional meteorological sense. For the more than 143 million Americans living in areas of significant seismic hazard, preparedness based on scientific understanding is the only reliable risk reduction strategy.

The Science of Earthquakes

Earthquakes result from the sudden release of energy stored in the Earth's crust, typically through the rupture or slip along a fault - a fracture or zone of fractures between blocks of rock. The Earth's outer shell is divided into tectonic plates that move continuously, and most seismic activity occurs along or near the boundaries of these plates. The point within the Earth where rupture begins is called the hypocenter or focus; the point on the Earth's surface directly above it is the epicenter.

When a fault ruptures, it radiates seismic waves outward in all directions. Different wave types travel at different speeds and interact differently with surface materials: P-waves (primary/compressional) arrive first and cause the initial shaking; S-waves (secondary/shear) arrive next and cause stronger lateral movement; surface waves travel along the Earth's surface and are responsible for most of the damage associated with large earthquakes.

Measuring Earthquake Magnitude and Intensity

Magnitude describes the energy released by an earthquake at its source and is measured on a logarithmic scale - a magnitude 7.0 earthquake releases approximately 32 times more energy than a magnitude 6.0. The Moment Magnitude Scale (Mw) is the most scientifically rigorous modern measure. Intensity, by contrast, describes the local strength of shaking at a specific location. The Modified Mercalli Intensity (MMI) scale ranges from imperceptible (MMI I) to catastrophic widespread destruction (MMI XII).

Seismic Hazard in the United States

While the Pacific Coast is widely recognized as earthquake country - California's San Andreas Fault system, the Cascadia Subduction Zone in the Pacific Northwest, and the seismically active Aleutian Arc in Alaska - significant seismic hazard exists across much of the United States. The New Madrid Seismic Zone in the central Mississippi River Valley is capable of generating very large earthquakes. The Wasatch Front in Utah, areas of South Carolina, and parts of the intermountain West all face meaningful seismic risk. Additionally, induced seismicity from wastewater injection associated with oil and gas production has dramatically increased earthquake frequency in states including Oklahoma, Texas, and Kansas.

Cascadia Warning: The Cascadia Subduction Zone, extending 600 miles off the coasts of northern California, Oregon, and Washington, is capable of generating a magnitude 9.0+ megathrust earthquake - the largest type of earthquake on Earth. Such an event would cause devastation across the Pacific Northwest and generate a regional tsunami.

Earthquake Preparedness: Drop, Cover, and Hold On

Unlike most other hazards, earthquake preparedness must be completed before the event because shaking typically begins with no warning (though Earthquake Early Warning systems like ShakeAlert are now operational in California, Oregon, and Washington, providing seconds to tens of seconds of warning). The recommended response when shaking begins is Drop, Cover, and Hold On:

• DROP to your hands and knees immediately.
• Take COVER under a sturdy desk or table if one is nearby, or against an interior wall away from windows. Protect your head and neck with your arms.
• HOLD ON to your shelter until shaking stops, as earthquakes often have multiple strong wave sequences.

Post-earthquake, be aware of aftershocks, gas leaks (leave the building and don't return if you smell gas), and structural damage. In coastal areas, move to high ground immediately if a large earthquake caused strong, prolonged shaking - tsunami waves can arrive within minutes.