Volcano Watch: Stressed out: Hawaiian volcanoes are heavy

Many people living in the Hawaiian Islands are accustomed to feeling occasional earthquakes since the State of Hawaii is one of the most seismically active locations in the United States. Unlike some other earthquake-prone places in the U.S., for example California, where the earthquakes are related to tectonic plates sliding past each other, our earthquakes are related to volcanoes.

The three main causes for earthquakes here in Hawaii are: 1.) movement of magma under active volcanoes; 2.) sliding of volcanoes’ flanks along the surface that separates the ancient oceanic crust and overlying volcanoes; and 3.) bending or flexing of the Earth’s crust and upper mantle in response to the weight of the overlying volcanoes.

On July 5, 2021, a moderate magnitude-5.2 earthquake occurred approximately three miles off the north shore of Hawaii Island near Waipio Valley at 17 miles (27 kilometers) below sea level. Shaking related to this earthquake was felt as far away as Kauai. Two days later, another earthquake, this time a magnitude-4.2, occurred approximately 45 miles west-northwest of Kalaoa, on the west side of the Island of Hawaii. Both earthquakes were likely related to stress caused by the enormous weight of the Hawaiian volcanoes on the underlying crust and mantle.

What do we mean by ‘stress caused by the weight of the volcanoes’? As Hawaiian volcanoes erupt and grow, they add more and more weight to the Earth’s surface.

This causes the Pacific Plate to flex downward, much like the bending caused by heavy books on an overloaded bookshelf.

Some weight may just make the shelf bow, but a lot of weight may cause the shelf to start to splinter or break. Those breaks are similar to what happens in tectonic plates if they bend too much.

The magnitude-5.2 and 4.2 earthquakes a couple of weeks ago are just two instances of earthquakes due to bending of the Pacific Plate. Earthquakes caused by this flexure can be quite large — some even greater than magnitude-6. Some additional past Hawaiian flexure earthquakes include the magnitude-6.8 Lanai earthquake on February 19, 1871; magnitude-6.8 Maui earthquake on January 22, 1938; magnitude-5.2 Oahu earthquake on June 28, 1948; magnitude-6.2 Honomu earthquake on April 26, 1973; magnitude-6.7 Kiholo Bay and magnitude-6.1 Mahukona earthquakes on October 15, 2006; and magnitude-4.6 Hawaiian Ocean View earthquake on February 7, 2019.

These earthquakes are deep, typically 25–40 km, within the uppermost brittle mantle underneath volcanoes and oceanic crust. As you can imagine, measuring stress building up within the Earth’s crust and upper mantle can be challenging.

One type of measurement that scientists at the USGS Hawaiian Volcano Observatory use to monitor and study motions from stress accumulation is Global Positioning System data. GPS instruments receive signals from satellites and use the transmitted information to calculate how a specific point on the Earth’s surface moves throughout time.

HVO operates a network of over 65 scientific-grade GPS stations on the Island of Hawaii, which continuously measure their positions to a fraction of an inch. These stations are used to monitor and track extremely small movements at the Earth’s surface, including some displacements related to plate flexure stress.

To improve the scientific understanding of plate bending in the Hawaiian Islands, Dr. Jeff Freymueller, a researcher in the Department of Earth and Environmental Sciences at Michigan State University, in collaboration with University of Hawaii at Manoa, is in the process of installing approximately 7 new GPS sites on the Island of Hawaii, Molokai, and Lanai.

These new sites are strategically placed to capture movement associated with plate bending and to test whether certain motions result from ongoing plate bending or from the dynamics of very deep magmatic systems beneath the volcanoes.

The first three GPS sites for this project were installed in May 2021, and the others will be installed in the coming months. Collecting years of data from these stations should help scientists to better understand these tiny but important motions of the Pacific Plate and how they relate to these deep, moderate-to-large flexure earthquakes in Hawaii.

Volcano activity updates

Kilauea is not erupting. Its USGS Volcano Alert level is at ADVISORY (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kilauea updates are issued weekly.

No surface activity at Kilauea Volcano has been observed by field crews or webcam images since May 23, 2021.

Seismicity has slowly increased in recent weeks in the summit region, with continued gradual summit inflation over the past several months and one deflation-inflation cycle over the past week. Sulfur dioxide emission rates remain slightly elevated. It is possible that the Halema‘uma‘u vent could resume eruption or that Kilauea is entering a longer period of quiescence prior to the next eruption. For more information on current monitoring of Kilauea, see https://www.usgs.gov/volcanoes/kilauea/monitoring.

Mauna Loa is not erupting and remains at Volcano Alert Level ADVISORY.

This alert level does not mean that an eruption is imminent or that progression to an eruption from the current level of unrest is certain. Mauna Loa updates are issued weekly.

This past week, about 38 small-magnitude earthquakes were recorded below Mauna Loa. Global Positioning System measurements show no major deformation in the summit region over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone remain stable. Webcams show no changes to the landscape.

For more information on current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There were 2 events with 3 or more felt reports in the Hawaiian Islands during the past week: a M2.9 earthquake 5 km (3 mi) SSW of Volcano at 0 km (0 mi) depth on July 21 at 6:53 p.m. HST and a M3.4 earthquake 9 km (5 mi) SSW of Volcano at 25 km (15 mi) depth on July 21 at 4:11 p.m. HST.

HVO continues to closely monitor both Kilauea and Mauna Loa for any signs of increased activity.

Please visit HVO’s website for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.