Earthquake Odds: A Global Perspective

A 4.4 magnitude earthquake rattled the Los Angeles area this morning, striking at 4:04 am. No immediate injuries were reported. This quake follows a 6.6 magnitude quake that hit Tokyo on March 14.

This latest series of quakes started with Haiti’s catastrophic earthquake (magnitude 7.0) on January 12. Then an 8.8 monster rocked Chile on February 27, just one day after a 7.0 temblor near Japan’s Ryukyu Islands. Less than a week later Taiwan suffered its own 6.2 earthquake on March 4, followed four days later by a 6.1 quake in eastern Turkey.

This cluster may seem frightening, but to seismologists it’s business as usual. According to the US Geological Survey (USGS), between 2000 and 2009 the world averaged 1.3 earthquakes annually in the 8.0 to 9.9 range, 13.1 quakes measuring 7.0 to 7.9, and 144.5 events with magnitudes between 6.0 and 6.9 (any quake measuring 6.0 or higher is likely to cause damage in populated areas). The odds of an earthquake with a magnitude of 8.0 or higher occurring in a year are 1 in 1.62.

Earthquake risks are higher than average in Chile, Japan, and Taiwan, all of which are located on the “Ring of Fire”—an arc around the Pacific Ocean where tectonic plates rub against each other. The area is named for its many active volcanoes, which erupt at openings in the Earth’s crust, but it is also one of the most seismically active zones on Earth. Some of the most deadly earthquakes in history have occurred around the Ring of Fire, including the 2004 Indonesian quake (magnitude 9.1), which triggered a tsunami that killed nearly 228,000 people, the Great Tokyo Earthquake of 1923 (7.9), which killed 142,800 people, and a 1970 earthquake in Peru (7.9) that caused 70,000 deaths. The largest earthquake ever recorded in the world (9.5) occurred in Chile in 1960, and the biggest ever in the United States (9.2) struck Alaska in 1964.

Haiti also sits on a plate boundary and has an active seismic history. At least a dozen earthquakes measuring 7.0 or higher have occurred near Hispaniola (the island that Haiti shares with the Dominican Republic), Puerto Rico, and the US Virgin Islands in the past 500 years, including two that destroyed the Haitian city of Port-au-Prince in 1751 and again in 1770.

Although earthquakes are common worldwide, they don’t always generate massive casualties. Chile’s earthquake last month was some 500 times stronger than Haiti’s, but estimated deaths in Chile number in the hundreds, while more than 220,000 people are believed to have been killed in Haiti. The key difference is that Chile had required strict building codes for several decades, while Haiti had few such standards, and many of its people lived in marginal conditions when the January quake struck. Poorly enforced building codes were also blamed for many of the 87,000-plus deaths that occurred in China’s Sichuan Province when it suffered a 7.9 earthquake in 2008.

Both Chile and Haiti are at serious risk from aftershocks—lesser quakes that occur in the same area as Earth’s crust adjusts to the main quake’s impacts. From February 27 through March 5, the USGS detected 172 aftershocks measuring 5.0 or higher in Chile, with 11 of them measuring at least 6.0. USGS estimated in late February that the odds of an aftershock measuring 5.0 or higher in Haiti were 1 in 1.81 for the next 30 days and 1 in 1.25 over the next 90 days. Aftershocks usually decrease over time, but Haiti has 1 in 4 odds of an aftershock measuring at least 6.0 through February 2011. That means more stress for survivors, but it’s also a reason to rebuild Haitian towns to higher standards.

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