Coast Earth East Quake

Scientists study earthquake triggers in Pacific ocean

Nearly 1,500 meters (almost one mile) of sediment cores collected from the ocean floor off the coast of Costa Rica reveal detailed records of some two million years of tectonic activity along a seismic plate boundary.

The scientific drilling vessel JOIDES Resolution retrieved the samples during a recent month-long Integrated Ocean Drilling Program (IODP) Costa Rica Seismogenesis Project (CRISP) Expedition.

The expedition and IODP are funded in part by the National Science Foundation (NSF).

"It's critical to understand how subduction zone earthquakes and tsunamis originate--especially in light of recent events in Japan ," says Rodey Batiza of NSF's Division of Ocean Sciences. "The results of this expedition will also help us learn more about our own such zone off the Pacific Northwest."

Participating scientists aim to use the samples to better understand the processes that control the triggering of large earthquakes at subduction zones, where one plate slides beneath another.

"We know that there are different factors that contribute to seismic activity--these include rock type and composition, temperature differences and how water moves within the Earth's crust," explained co-chief scientist Paola Vannucchi of the University of Florence in Italy, who led the expedition with co-chief scientist Kohtaro Ujiie of the University of Tsukuba in Japan.

Vannucchi added, "but what we don't fully understand is how these factors interact with one another and if one may be more important than another in leading up to different magnitudes of earthquakes.

"This expedition provided us with crucial samples for answering these fundamental questions."

More than 80 percent of global earthquakes above magnitude 8.0 occur along subduction zones.

The Pacific Ocean is famous for these boundaries, known as convergent margins, which are found along the coasts of the East Pacific from Alaska to Patagonia, New Zealand, Tonga and Marianas--all the way to Japan and the Aleutians.

A view of the rig floor of the scientific drilling vessel JOIDES Resolution. Credit: IODP

In a preliminary report published this month, CRISP scientists say that they have found evidence for a strong subsidence, or sinking, of the Costa Rica margin combined with a large volume of sediment discharged from the continent and accumulated in the last two million years .

Magnitude 3.5 - EASTERN MONTANA - Earthquake Hazards Program

Most of North America east of the Rocky Mountains has infrequent earthquakes. Here and there earthquakes are more numerous, for example in the New Madrid seismic zone centered on southeastern Missouri, in the Charlevoix-Kamouraska seismic zone of eastern Quebec, in New England, in the New York - Philadelphia - Wilmington urban corridor, and elsewhere. However, most of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S. states have never reported a damaging earthquake. The earthquakes that do occur strike anywhere at irregular intervals.

Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

FAULTS Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most of the region's bedrock was formed as several generations of mountains rose and were eroded down again over the last billion or so years.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. All parts of this vast region are far from the nearest plate boundaries, which, for the U.S., are to the east in the center of the Atlantic Ocean, to the south in the Caribbean Sea, and to the west in California and offshore from Washington and Oregon. The region is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even most of the known faults are poorly located at earthquake depths. Accordingly, few earthquakes east of the Rockies can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake. In most areas east of the Rockies, the best guide to earthquake hazards is the earthquakes themselves.

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