16 December 2013

Seismic activity near Samoan Islands hints at crust’s thickness

Posted by pweiss

A seismic monitoring station on an island in Samoa

A seismic monitoring station on an island in Samoa. Credit: Bob Greschke

Seismologists at James Madison University are analyzing 20 years worth of seismic data to create a map of the Earth’s crust and a possible mantle plume underneath the Samoan Islands.

When two plates slip past one another during an earthquake, waves of energy pulse deep into the Earth’s layers and up into the crust — the outermost skin of the planet. The behavior of earthquake waves changes when they encounter different materials in various chemical states. Because the structure of the Earth’s interior is too deep to study directly, researchers use the strength and direction of seismic activity detected at stations around the world to understand the composition of the Earth’s interior.

After analyzing seismic data collected by recording stations on the Samoan Islands, Jillian M. Browning, an undergraduate student at James Madison University, mapped the thickness of the outer crust beneath the islands. She found that the Samoan crust ranges from 14 to 22 kilometers thick, greater than the average thickness of Earth’s crust, which is 7 kilometers. This is normal for oceanic islands because they tend to have larger crusts.

But when Browning compared the new Samoan data to the much-studied Hawaiian island chain, she noted something unusual. In Hawaii, newer islands in the east are thicker than older islands in the west. Seismologists believe this is because the volcanic plume that rises from the mantle to create new islands is increasing over time. But in Samoa, Browning saw that the thickness gradient ran in the opposite direction. The older islands in the west were thicker than the younger islands in the east.

“We’re interested in ocean islands, like Hawaii and Samoa in general, because if they are formed by a plume from deep in the Earth. It’s a nice connection to see a lot of the things we can’t see,” Anna M. Courtier, a seismologist at James Madison University leading the study, said at a poster session Thursday at the American Geophysical Union’s Fall Meeting. “If the source of the plume comes from deep within the mantle, you’re tapping a different reservoir which might be a window into the deeper mantle,” she said.

Courtier theorized that volcanic production may be dwindling instead of increasing near Samoa, which the thinning crust suggests. She said the volcanic material may not be pushed up toward the crust as hard in Samoa as it is in Hawaii.

Finding similarities and differences across multiple oceanic island locations helps seismologists better understand the composition and structure of the Earth’s interior.

Next up, the team plans to further scrutinize how the mantle is changing from west to east. They hope to compare their data to other oceanic island locations using the same methods.

Julia Calderone is a science communication graduate student at UC Santa Cruz