Earth shakes every 26 seconds and scientists don’t know why

Like a lot of strange happenings, it was first noticed in the 1960s: a small seismic pulse, large enough to register on seismological instruments but small enough to go otherwise unnoticed, occurring every 26 seconds. Jack Oliver, a researcher at the Lamont-Doherty Geological Observatory, documented the “microseism” and sussed out that it was emanating from somewhere “in the southern or equatorial Atlantic Ocean.” Not until 2005 was it determined that the pulse’s true origin was in the Gulf of Guinea, just off Africa’s western coast, but to this day, scientists still don’t know something just as important — why it’s happening in the first place.

There are theories, of course, ranging from volcanic activity to waves, but still no consensus. There does happen to be a volcano on the island of São Tomé in the Gulf of Guinea near the pulse’s origin point, not to mention another microseism linked to the volcano Mount Aso in Japan, which has made that particular explanation more popular in recent years. Though there’s no way of knowing when (or even if) we’ll learn the why of this phenomenon, one thing’s for sure: better a microseism than a macroseism.

California isn’t the most earthquake-prone state

That would be Alaska, which isn’t just the most earthquake-prone state in the country — it’s one of the most seismically active areas in the world, with 11% of all earthquakes occurring there. That’s because Alaska is part of the Ring of Fire, a nearly 25,000-mile-long area along the Pacific Ocean, characterized by volcanic and seismic activity. The second-largest earthquake ever recorded (a staggering 9.2 on the Richter scale) took place in the Prince William Sound region there on March 27, 1964, lasting about 4.5 minutes and causing a tsunami that was felt as far away as California. Beyond that, three of the eight largest recorded earthquakes in the world have also been in Alaska, as were seven of the 10 largest in America. It has experienced an average of one magnitude 7 to 8 earthquake every year since 1900 and one “great” earthquake (magnitude 8 or higher) every 13 years.

EARTHQUAKE RISK IN ALASKA

Building collapses during 1964 quake in Anchorage

Scientists have long recognized that Alaska has more earthquakes than any other region of the United States and is, in fact, one of the most seismically active areas of the world. The second largest earthquake ever recorded shook the heart of southern Alaska on March 27th, 1964, with a magnitude of 9.2 (see photos below). The 1964 earthquake was slightly larger than the magnitude 9.0 Sumatra-Andaman Islands earthquake that devastated northern Sumatra in December 2004 and generated a tsunami that killed more than 280,000 people. The largest on-land earthquake in North America in almost 150 years occurred on the Denali fault in central Alaska on November 3rd, 2002, with a magnitude of 7.9.

It is not possible to predict the time and location of the next big earthquake, but the active geology of Alaska guarantees that major damaging earthquakes will continue to occur. Scientists have estimated where large earthquakes are most likely to occur, and the probable levels of ground shaking to be expected throughout the state (see maps below). With this information, as well as information on soil properties and landslide potential, it is possible to estimate earthquake risks in any given area. It is also possible to estimate the potential for earthquakes to generate tsunamis, and to model the extent to which tsunamis will inundate coastal areas.

Alaska has changed significantly since the damaging 1964 earthquake, and the population has more than doubled. Many new buildings are designed to withstand intense shaking; some older buildings have been reinforced, and development has been discouraged in some particularly hazardous areas.

Destruction of homes in the Turnagain Heights subdivision of Anchorage during the 1964 earthquake.

Despite these precautions, and because practices to reduce vulnerability to earthquakes and tsunamis are not applied consistently in regions of high risk, future earthquakes may still cause life-threatening damage to buildings, cause items within buildings to be dangerously tossed about, and disrupt the basic utilities and critical facilities that we take for granted.

All recorded earthquakes in Alaska from 1898 to the present

The Federal Emergency Management Agency1 estimates that with the present infrastructure and policies, Alaska will have the second highest average annualized earthquake-loss ratio (ratio of average annual losses to infrastructure) in the country. Reducing those losses requires public commitment to earthquake-conscious siting, design, and construction. The Seismic Hazards Safety Commission is committed to addressing these issues. Earthquake-risk mitigation measures developed by similar boards in other states have prevented hundreds of millions of dollars in losses and significant reductions in casualties when compared to other seismically active areas of the world that do not implement effective mitigation measures. The Loma Prieta (1989), Northridge (1994) and Nisqually (2001) earthquakes caused comparatively low losses as a result of mitigation measures implemented in those areas. Many of these measures were recommended by the states' seismic safety commissions.

Probabilistic ground-acceleration map for Alaska. Colors depict the peak acceleration that has a 10 percent probability of being exceeded in a 50-year period.

Alaska Earthquake Statistics

• Alaska is home of the the second largest earthquake ever recorded (1964 Great Alaska Earthquake, magnitude 9.2)

• Alaska has 11 percent of the world's recorded earthquakes

• Three of the eight largest earthquakes in the world were in Alaska

• Seven of the ten largest earthquakes in the United States were in Alaska

Since 1900, Alaska has had an average of:

• One "great" earthquake (magnitude 8 or larger) earthquake every 13 years

• One magnitude 7 to 8 earthquake every year

• Six magnitude 6 to 7 earthquakes per year

• Forty five magnitude 5 to 6 earthquakes per year

• Three hundred twenty magnitude 4 to 5 earthquakes per year

• An average of a 1,000 earthquakes are located in Alaska each month

1. HAZUS 99 Estimated Annualized Earthquake Losses for the United States, Federal Emergency Management Agency Report 66, September 2000.