Long Valley Caldera: Dormant Supervolcano Seismic Activities, No Impending Eruptions but Just Cooling Down - California

Although the dormant supervolcano is producing seismic activity, geologists stated no eruptions are imminent and that the Long Valley caldera in California is simply cooling down.

The Long Valley Caldera and the Supervolcano Underneath

Since the 1980s, researchers have monitored a region in California's Eastern Sierra Nevada mountains exhibiting periodic unrest. This area, the Long Valley Caldera, experiences earthquake swarms and ground inflation at nearly half an inch annually. The concern arises because this caldera rests atop a massive dormant supervolcano. Seven hundred and sixty thousand years ago, it experienced a violent eruption, ejecting 650 cubic kilometers of ash, enough to blanket Los Angeles in a 1-kilometer-thick layer.

A "supervolcano" refers to a volcano with a Magnitude 8 eruption on the Volcano Explosivity Index (VEI), releasing over 1,000 cubic kilometers of material. In the early 2000s, the term "supereruption" emerged to describe VEI 8 eruptions. These events expel so much magma that they create a circular collapse feature known as a caldera above the emptied magma storage area.

Long Valley in eastern California is one example of a caldera-forming volcano that has produced extraordinarily big pyroclastic eruptions in the last 2 million years.

High-Resolution Underground Imaging

Caltech researchers created highly-resolution underground images of the Long Valley Caldera, revealing structures up to 10 kilometers deep. These images suggest recent seismic activity results from cooling and settling processes releasing fluids and gases, not a supervolcanic eruption. While not anticipating such an eruption, the cooling process may trigger earthquakes and small eruptions, as seen in May 1980 when four magnitude 6 earthquakes occurred.

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The images also show a crystallized rock lid covering the magma chamber, formed as liquid magma cools and solidifies. This research sheds light on the caldera's subsurface dynamics and potential seismic risks.

Distributed Acoustic Sensing: From Seismic Waves To Photos

Researchers create underground images by inferring subsurface conditions from seismic wave measurements. Earthquakes produce primary (P-waves) and secondary (S-waves) seismic waves, each traveling at different speeds through materials. By analyzing wave arrival times from seismometers at various locations, they determine material characteristics. This allows them to visualize the subsurface environment.

In the Eastern Sierra region, a unique technique employs fiber optic cables, like those for the internet, for seismic measurements through distributed acoustic sensing (DAS). A 100-kilometer cable used to image the Long Valley Caldera is equivalent to 10,000 single-component seismometers. Over 1.5 years, the cable recorded over 2,000 seismic events, most imperceptible to humans. A machine learning algorithm processed this data, resulting in high-resolution images.

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This study marks the first instance of creating deep, high-resolution images using DAS. Previous tomography studies were limited to shallow depths or provided lower resolution over larger areas. Ettore Biondi, a DAS scientist at Caltech and the paper's lead author highlights how DAS can revolutionize our understanding of crustal dynamics. The team is eager to apply this technology to other regions to explore their subsurface environments.

Deeper Investigation

The crew will then utilize a 200-kilometer cable to photograph the area where the caldera's cooling magma chamber, which is also known as its "beating heart", is located, which is 15 to 20 kilometers beneath the surface of the Earth.