Caltech researchers used advanced imaging techniques to investigate increased seismic activity in California's Long Valley Crater, a dormant supervolcano. Their results suggest that the region is not on the verge of a major eruption, but is experiencing seismic activity due to cooling processes.
Since the 1980s, researchers have observed distinct periods of instability in an area east of California's Sierra Nevada Mountains, characterized by swarms of earthquakes and periods during which the ground swelled and rose by nearly half an inch per year. The activity is concerning because the area, known as the Long Valley Caldera, sits atop a massive dormant volcano. Long Valley Crater was formed 760,000 years ago in a violent eruption that spewed 650 cubic kilometers of ash into the air - enough ash to cover the entire Los Angeles area with a layer of sediment 1 kilometer thick.
Breakthrough imaging reveals the inside story
Volcanic activity has become increasingly frequent over the past few decades. What is the reason behind this? Could it be that the area is preparing for another outbreak? Or is the increased activity actually a sign that the risk of a large eruption is decreasing?
To answer these questions, Caltech researchers have created the most detailed image yet of the subsurface of Changgu Crater, which reaches depths of up to 10 kilometers into the Earth's crust. These high-resolution images reveal the structure of the Earth beneath the crater and suggest that the recent seismic activity is caused by fluids and gases released as the area cools and subsides.
The Changgu Caldera is the site of a large-scale super volcanic eruption 760,000 years ago. What happened under the surface of the Changgu Caldera? A new study using seismic waves to image the region's underground environment has found that the magma chamber is cooling. Still, gases and liquids released when magma crystallizes can cause earthquakes. Source: E.Biondi
This research was conducted in the laboratory of geophysics professor Zhongwen Zhan (Ph.D. ’14). A paper describing the research was published Oct. 18 in the journal Science Advances.
"We don't think the area is preparing for another supervolcano eruption, but the cooling process may release enough gases and liquids to trigger earthquakes and small eruptions. For example, in May 1980, four magnitude 6 earthquakes occurred in the area," Zhan Zhongwen said.
High-resolution images show that the volcano's magma chamber is covered by a hard cap of crystalline rock, which forms when liquid magma cools and solidifies.
Innovative imaging technology
To create images of the subsurface, researchers infer what the underground environment looks like by measuring seismic waves produced by earthquakes. Earthquakes produce two types of seismic waves: primary waves (P waves) and secondary waves (S waves). The two types of seismic waves travel at different speeds in different materials - seismic waves travel slowly in elastic materials like liquids but very quickly in rigid materials like rocks. By using seismometers at different locations it is possible to measure the differences in time of the waves and determine the properties of the material they pass through - how elastic or stiff it is. In this way, researchers can create a picture of the underground environment.
earthquake source
There are dozens of seismometers throughout the Dongshan area, but the technology used by the researchers uses fiber optic cables (like those that provide the Internet) to conduct seismic measurements, a process called distributed acoustic sensing (DAS). The 100 kilometers of cable used to image the Changgu caldera is equivalent to 10,000 single-component seismometers. Over a year and a half, the team used the cable to measure more than 2,000 seismic events, most of which were too small for people to feel. Machine learning algorithms process these measurements and generate images.
This study is the first to use DAS to produce such in-depth, high-resolution images. Previous local tomography study images were either limited to shallow subsurface environments about 5 kilometers deep or covered larger areas at lower resolutions.
"This is the first demonstration of how DAS can change our understanding of Earth's crustal dynamics," said Ettore Biondi, DAS scientist at Caltech and first author of the paper. "We are excited to apply similar techniques to other areas where we are curious about subsurface environments."
Next, the team plans to use a 200-kilometer-long cable to image deeper into the Earth's crust, perhaps 15 to 20 kilometers deep, where the crater's magma chamber - its "beating heart" - is cooling.