03/30/2011

African and Arabian seismicity maps unveiled

Anne M Stark, LLNL, (925) 422-9799, stark8@llnl.gov



Path map of surface wave dispersion measurements,shown for
50-second surface waves. Yellow circles indicate events, red
triangles indicate stations, and blue lines indicate paths.

Not much is known about the seismic structure of Africa and Arabia. But with measurements from seismic stations around the region, the Lab's Michael Pasyanos and a Penn State colleague have produced high-resolution surface wave maps.

"And from there, we can invert these maps to determine the velocity structure of the region," said Pasyanos, of the Atmospheric, Earth and Energy Division.

In addition to being an information boon to a broad community of Earth scientists, the findings also are of  fundamental importance to the Lab's (and the nation's) effort in seismic-based nuclear explosion monitoring in this region of the world.

The seismic structure of Africa and Arabia has not been well examined, partly because there is a sparse distribution of seismic stations in the region and large areas of these tectonic plates are aseismic (areas without much seismicity), which limits the ability to study the region.

Surface waves, as their name indicates, travel along the surface of the earth. Even if both the earthquake and recording stations are outside of the study region, the waves propagate through, providing information on these areas. Surface waves are sensitive to the velocity structure at different depths.

"So a short period surface wave (10 seconds or so) might be sensitive to the top few kilometers of the earth's crust like sedimentary basins, but a long period surface wave (100 seconds) is sensitive to the top 100 or so kilometers, which extends beyond the crust into the earth's mantle," Pasyanos said.


Group speeds (in km/s) for 20 second surface waves.
Slow speeds are indicated in red and fast velocities
in blue.

The earth's mantle is a highly viscous layer between the crust and the outer core and is about 2,890 kilometers thick.

Pasyanos and Penn State University colleague Andy Nyblade, who serves as co-director of AfricaArray, a public-private partnership supporting training and research in earth, atmospheric and space sciences in Africa (LLNL participates as a partner), were able to measure surface wave velocities with periods shorter than 35 seconds, allowing them to examine shallower features than in previous studies of the whole continent.

One result was a crustal thickness map of Africa. Main features include crustal thickness increase under the West Africa, Congo and Kalahari cratons (the old, stable part of the continental lithosphere, which is made up of the crust and a portion of the upper mantle). Other features include crustal thinning in the Benue Trough, Red Sea, and East, Central and West African rifts, where the underlying mantle is hot and slow.

"Under cratons, we found thick lithosphere and fast upper mantle velocities, while under rifts we found thinned lithosphere and slower upper mantle velocities," Pasyanos said.

The research published in the journal, Tectonophysics, entitled "A Top to Bottom Lithospheric Study of Africa and Arabia," has been selected as one of the top 50 most cited articles out of the approximately 1,300 papers published in the journal from January 2006-February 2011. The paper appeared online Aug. 22, 2007.









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