Since the 1980s, scientists have observed periods of significant unrest in a region within California’s eastern Sierra Nevada Mountains, characterized by earthquake swarms and ground swells, rising nearly half an inch per year during these events.
Caltech researchers have now produced detailed underground images of the Long Valley Caldera, diving up to 10 kilometers into the Earth’s crust.
The research, led by Professor Chongwen Zhan, was published in the journal Science Advances on October 18.
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“We don’t think the region is preparing for another massive volcanic eruption, but the cooling process may release enough gas and liquid to cause earthquakes and small explosions,” Zhan explains. “For example, in May 1980, four magnitude 6 earthquakes occurred in the area alone.”
High-resolution images show that the volcanic magma chamber is covered by a solid cover of crystallized rock, as a result of the cooling and solidification of liquid magma.
To create these underground images, researchers make inferences about the subsurface environment by measuring seismic waves generated by earthquakes.
The technology used by Zahn’s team uses fiber optic cables, similar to those used in Internet services, to perform seismic measurements using distributed acoustic sensing (DAS).
The 100 km cable they used to image the Long Valley Caldera area was equivalent to the span of 10,000 single-component seismometers. Over the course of 18 months, the team measured more than 2,000 seismic events, most of which were too small for people to feel.
This research represents the first time such deep, high-resolution images have been produced using DAS.
Previous images from local tomography studies were either confined only to the shallow subsurface environment at depths of about 5 km, or covered a larger area at lower resolution.
“This is one of the first pieces of evidence of how DAS is changing our understanding of crustal dynamics,” says Ettore Biondi, a DAS scientist at Caltech and first author of the study. “We are excited to apply similar technology to other areas where we are curious about the subsurface environment.”
The team’s future plans include using a 200-kilometre-long cable to delve deeper into the Earth’s crust, reaching depths of about 15 to 20 kilometres, where the caldera’s magma chamber, or “pulsating core,” cools.
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