# Volcano Watch — Volcanoes get that sinking feeling from large earthquakes

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Observers of natural phenomena have long known that large earthquakes can trigger volcanic eruptions. For example, Charles Darwin reported that at least three volcanoes in Chile erupted after a massive earthquake there in 1835. Here in Hawai‘i, the magnitude-7.7 earthquake in 1975 triggered an eruption in Kīlauea’s summit caldera.

Not all large earthquakes trigger eruptions, however. Last week, many news outlets spotlighted research on the interaction between recent massive earthquakes and neighboring volcanoes in Chile and Japan that did not erupt in the aftermath of the quakes.

Instead, these volcanoes, within 200 km (125 miles) of the epicenters, sank by up to 15 cm (6 inches) during the earthquakes.

This unexpected effect was discovered in two unrelated studies that used satellite radar data (InSAR) to analyze the deformation of Earth's surface caused by the 2011 magnitude-9.0 Tohoku-Oki earthquake in Japan and the 2010 magnitude-8.8 Maule earthquake in Chile.

These large earthquakes, called megathrusts, were generated in subduction zones, where one tectonic plate is thrust beneath another as the plates converge. These zones generate the largest earthquakes on Earth. Megathrust earthquakes release the strain that has accumulated around a locked fault for decades or centuries as a result of steadily converging plate motion.

A ubiquitous feature of subduction zones is an arc of volcanoes located on the overriding plate and inland of the plate collision zone. It is within these volcanic arcs that the anomalous subsidence occurred. The patterns of subsidence were remarkably similar in the two regions—large elliptical areas up to 15 by 30 km (10 by 20 miles) and elongated in a direction parallel to the volcanic arc.

The exact cause of the subsidence is not known, but the researchers propose two different mechanisms to explain their observations.

Scientists studying the phenomena in Chile think that the most likely cause was a release of mineral-rich water heated by magma beneath the volcanoes, or geothermal fluids. They propose that the shaking from the earthquake opened pathways for geothermal fluids that were trapped underground to flow to the surface, causing the ground to sink.

In Japan, scientists point to the fact that the hot magma and host rocks associated with magma reservoirs beneath active volcanoes create zones that are weak compared with the surrounding crust. These weak zones would sink farther than the surrounding crust as the whole area subsided following the earthquake. The existence of these magma reservoirs is supported by measurements of high thermal gradients, high temperature geothermal fluids, and low velocities of passing seismic waves. The researchers simulated the effect in a computer model and found that the simulated deformation matched the observed signal quite well.

The scientists in Japan had the advantage of continuously recording GPS instruments in the volcanic arc, so they were able to see that the subsidence happened instantaneously during the Tohoku-Oki earthquake. This implies that, at least in Japan, it was not a mechanism that requires time to produce subsidence, such as increased fluid flow.

This result raises the exciting prospect of using observations of surface displacement during large earthquakes to identify anomalous areas in the Earth's crust. For example, on Hawai‘i Island, the 2006 Kīholo Bay earthquakes caused some areas of subsidence on the west coast. This may point to inhomogeneous properties of the rock in these areas.

It is likely that the effect of large earthquakes producing discrete pockets of anomalous subsidence in volcanic, as well as other areas, is wide spread. As our techniques for measuring the changing surface of the Earth become more and more accurate and accessible, the potential for learning about the structure and processes below the surface increases as well. And while we don’t yet fully understand the implications of the subsidence of volcanic areas for their eruption potential, there is no doubt that it influences volcano hazard, making this an important area of study for those of us who live on and near active volcanoes.

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### Volcano Activity Update

A lava lake within the Halemaumau Overlook vent produced nighttime glow that was visible from the Jaggar Museum overlook and via HVO's Webcam during the past week. The lava lake level fluctuated slightly over the past week, in concert with deflation and inflation of the summit.

On Kīlauea's East Rift Zone, breakouts from the Peace Day tube remain active on the coastal plain. Small ocean entries are active on both sides of the Hawaii Volcanoes National Park boundary. The Kahaualea 2 flow, fed from a spatter cone on the northeast edge of the Puu Oo crater, continues to advance slowly along the edge of the forest north of Puu Oo, burning vegetation. The front of the Kahaualea 2 flow this past week was roughly 3 km (about 2 miles) north of Puu Oo.

There were no earthquakes reported felt in the past week across the Hawaiian Islands.