Volcanoes get that sinking feeling from large earthquakes


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. In Hawaii, a magnitude-7.7 earthquake in 1975 triggered an eruption in Kilauea’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 125 miles of the epicenters, sank by up to 6 inches during the earthquakes.

This unexpected effect was discovered in two unrelated studies that used satellite radar data 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 they 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 10-by-20 miles and elongated in a direction parallel to the volcanic arc.

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

Scientists studying the phenomena in Chile think 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 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 researchers simulated the effect in a computer model and found that the simulated deformation matched the observed signal. 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 result raises the prospect of using observations of surface displacement during large earthquakes to identify anomalous areas in the Earth’s crust. For example, on Hawaii Island, the 2006 Kiholo 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 the effect of large earthquakes producing discrete pockets of anomalous subsidence in volcanic, as well as other areas, is widespread. As our techniques for measuring the changing surface of the Earth become more accurate and accessible, the potential for learning about the structure and processes below the surface increases. 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.

Kilauea activity update

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

On Kilauea’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 two miles north of Puu Oo. There were no earthquakes reported felt in the past week across the Hawaiian Islands.

Visit hvo.wr.usgs.gov for Kilauea, Mauna Loa, and Hualalai activity updates, recent volcano photos, recent earthquakes, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists who work with the U.S. Geological Survey’s Hawaiian Volcano Observatory.