Seismologist Deborah Kilb was wading through California earthquake records from the past four decades when she noticed something odd — a series of deep earthquakes that had occurred under the Sierra Nevada at a depth where Earth’s crust would typically be too hot and high pressure for seismic activity.
“In Northern California usually the (earthquake) data goes down to about 10 kilometers (6 miles). In Southern California, they’ll go down a little bit deeper into 18 kilometers (11 miles),” said Kilb, a researcher at the Scripps Institution of Oceanography at the University of California, San Diego, referring to the depths at which earthquakes usually occur in those regions.
But the earthquakes she found taking place near the central region of the mountain range were up to twice as deep — and appear to be ongoing.
“The fact that we see some seismicity that’s below 20 kilometers (12.4 miles) — like 20 kilometers to 40 kilometers (25 miles) — is very odd,” Kilb said. “It’s not something you would typically see in crustal earthquakes.” CONTINUE READING