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Where Earthquakes Are Most Likely in the USA

Where Earthquakes Are Most Likely in the USA

North America, like most continents, is formed by several lithospheric plates, with boundaries that are potentially seismically hazardous regions. These include:

  • The ancient North American platform, formed by the Greenland Shield, as well as several tectonic plates – the Midcontinent, Great Plains, Canadian, and Hudson Bay plates;
  • The Arctic epipaleozoic platform;
  • The Mexican-Appalachian formation;
  • The Meso-Cenozoic belt of the North American Cordillera.

The overall geophysics of the region are complemented by the Atlantic and Pacific oceanic platforms.

In the Atlantic, tectonics at the plate boundaries lead to the formation of rift zones and underwater mountains. The Pacific Ring of Fire, which includes the entire western American coastline, is an area of tectonic subduction and is the most seismically active region on our planet.

Where the Most Powerful Earthquakes Occur in the USA

It is traditionally believed that California is the most seismically dangerous region in the country. However, the state’s 39 million residents might be surprised to learn that this is not the case. The state of Alaska holds the lead not only in the number of earthquakes but also in their strength. The most powerful earthquake in the USA, with a magnitude of 9.2, occurred here.

Alaska

According to the United States Geological Survey (USGS), earthquakes are a common reality for Alaska. Eleven percent of the world’s earthquakes and 17.5 percent of seismic activity cases in the USA occur in Alaska. On average, there is one tremor of magnitude 7.0–8.0 per year, and one of magnitude 8.0 or higher approximately every 13 years. The active fault, known as the Alaska-Aleutian Megathrust, is the main culprit for seismic activity in this region. The low population density and the small number of buildings account for the lack of significant casualties and destruction in Alaska. With a land area of 732,000 square miles, it is home to 663,000 people. ### Great Alaskan Earthquake

The Great Alaskan Earthquake on March 27, 1964, was the second most powerful earthquake in human history (after the Valdivia earthquake). It was caused by tectonic processes resulting from the subduction of the Pacific Plate beneath the North American continental plate. The epicenter of the main 9.2 magnitude shock was located in the Prince William Sound. The earthquake triggered avalanches and landslides in the mountains, which destroyed railway and road communication in the region, as well as hydraulic, port structures, bridges, and residential buildings. The overall economic damage from the Great Alaskan Earthquake amounted to $500 million, and it led to the deaths of 9 people. A tsunami generated by the earthquake struck the coast and reached as far as Antarctica, claiming 131 lives.

California

According to the USGS, California has remained tectonically active for over 200 million years, since the breakup of the supercontinent Pangaea. The high frequency of earthquakes in the state is due to its location near the western edge of the North American lithospheric plate. The collision between the Pacific and North American plates has resulted in the formation of numerous faults in California, facilitating lateral movement between the plates. The most famous of these is the San Andreas Fault, which extends 1,276 km along the coast. This fault causes the region between the Wasatch Mountains in Utah and the Sierra Nevada to actively expand and stretch westward.

Some of the most powerful seismic events in California include:

  • The catastrophic 1994 Northridge earthquake with a magnitude of 6.7, resulting in 60 deaths and over 7,000 injuries. It damaged 40,000 buildings and left 20,000 people homeless.
  • The 1989 Northern California earthquake with a magnitude of 6.9, which claimed 63 lives and caused over $6 billion in infrastructure damage.
  • The 1971 San Fernando earthquake with a magnitude of 6.6, which took 65 lives and injured more than 2,000 people. Its epicenter was located in a sparsely populated area of the San Gabriel Mountains.

On December 4, 2024, an earthquake with a magnitude of 7.0 was recorded in California. The strongest tremors were felt in San Francisco and Eureka.

Hawaii

Thousands of earthquakes occur annually on the island of Hawaii. Most of them are so weak that they can only be detected with instruments. However, some are strong enough to be felt, and some cause significant damage, including ground subsidence, soil cracks, landslides, and massive tsunami waves. Most earthquakes in Hawaii are directly related to volcanic activity and are caused by the movement of magma beneath the earth’s surface. Earthquakes can occur before or during eruptions, as well as from subterranean magma movements that come close to the surface but do not erupt.

For instance, the eastern rift zone of Kilauea is constantly being torn apart by the influx of new magma, most of which is stored deep underground. Since the northern side of Kilauea is immobilized by the adjacent mass of Mauna Loa, the southern flank, facing the ocean, must push outward to make room for the additional magma. Periodically, Kilauea’s southern flank abruptly shifts seaward in response to this pressure, causing earthquakes. Most of these earthquakes are small, but there have been reports of several large seismic events.

In 1975, Kilauea’s southern flank was the site of the most powerful earthquake of the last century, with a magnitude of 7.2. The coastline of Kalapana subsided by 11 feet, triggering a massive tsunami that claimed the lives of several visitors to Hawaii Volcanoes National Park, destroyed homes in Punaluu, flooded fishing boats in Keauhou Bay, and damaged boats and piers in Hilo.

State of Washington

The source of earthquakes in the state of Washington is the Cascadia Subduction Zone, where the Juan de Fuca tectonic plate is subducting (diving) beneath the North American continental plate. Through constant monitoring of this seismically active zone, scientists have concluded that there is a 15-25% chance of an earthquake with a magnitude greater than 8.0 occurring in Cascadia within the next 50 years.

Eighty kilometers off the coast of Oregon, at the boundary of the Cascadia subduction zone, divers discovered a vent spewing hot fluid. A new fracture stretches 965 km from Northern California to the coast of Canada, and it is releasing water with a temperature significantly higher than that of sources in the subduction zone. The loss of this water, which acts to reduce the friction between tectonic plates, could soon lead to a series of powerful earthquakes with magnitudes of up to 9.0 or more, capable of devastating cities along the entire Pacific coast of North America.

Although it is impossible to predict exactly when an earthquake will occur, we can still take measures to mitigate damage and enhance safety by preparing in advance for the next shock. Proper assessment of hazards and seismic risks is the way to effectively prepare for the next major earthquake. After all, the question is not if it will happen, but when the catastrophe will strike.