Volcano Watch – New research sheds light on recent Pāhala earthquake swarms

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Volcano Watch is a weekly article and activity update written by scientists and affiliates of the US Geological Survey Hawaiian Volcano Observatory.

The increase in earthquakes in recent years, which is currently at a historically high rate, is of interest to residents and scientists alike.

These earthquakes occur under our volcanoes and even under the base of the underlying oceanic crust, in the upper mantle. This part of the mantle is called the lithospheric mantle because it is cold enough to rupture and create earthquakes. The deeper mantle is too hot and too plastic to generate earthquakes and tends to sink like putty or clay.

Historically, earthquakes in the lithospheric mantle, like those under Pāhala and elsewhere in Hawaii, occur in response to the weight of the island bending the lithospheric mantle down. The magnitude 6.9 earthquake in Kīholo Bay in 2006 was a prime example of this type of “bending” earthquake.

Several ideas have been put forward about the cause of earthquakes deep beneath Pāhala and the southeastern coast of Kaʻū since their first observation in the late 1960s. The initial hypothesis was that these earthquakes occurred on planes of fault and are linked to the flexion of the lithospheric mantle due to the weight of the overlying volcanoes. However, the deep Pāhala earthquakes differ from other lithospheric earthquakes under Hawaii in the large number of earthquakes, especially since 2015.

Other authors have suggested that the region deep beneath Pāhala was a pathway for magma from the upper mantle plume below the island of Hawaii to the Kīlauea volcano. They hypothesized that the number of earthquakes increased when the magma was supplied to Kīlauea, although there is no direct evidence to support a movement of the magma and that it requires a still unidentified conduit to move the magma over great distances horizontally to pass under Kīlauea and Mauna Loa.

Scientists recently examined the characteristics of the strong seismicity that began under Pāhala in 2015. They found that as the number of earthquakes began to increase, many earthquakes occurred on steep faults as well as on faults historically interpreted as flat. The number of earthquakes increased again in 2019, as did the variety of faults that produce them.

The overall load of volcanoes on the lithosphere would not change significantly in a few years, so the variety of fault orientations requires a more complex answer to explain the large number of earthquakes and their change in orientation. The Pāhala region is located at the structural link between three volcanoes – Kīlauea, Mauna Loa and Lō’ihi. However, there is insufficient information to make a link between the accumulation of stresses in this region and the generation of these seismic swarms.

Instead, scientists speculated that magma could escape from the mantle plume and migrate into the Pāhala region and stall at depth. The inferred magma could pressurize the region, causing an increase in the number and variety of earthquakes occurring under Pāhala. While there is no direct evidence that magma is accumulating, it is an interesting hypothesis that warrants further investigation.

The Pāhala earthquake swarm in its current state appears to be a unique event that has never been observed before in Hawai’i, although the historic record for seismographs dates back just over 50 years. Further research is planned and involves placing a dense network of tiny temporary seismometers at Ka’ū to visualize what’s going on deep below Pāhala. If you get a call from a USGS Hawaiian Volcanoes Observatory or a University of Hawaii scientist looking to place a small instrument on your land next year, engage with them and do what you can to help expand our understanding of this complex system by supporting this important and interesting volcanic research.

Map and graph showing earthquakes at a depth of 20 to 40 km (12 to 25 miles) below the island of Hawaii over the past week. Most of the earthquakes at this depth were clustered under the southern edge of the island near the town of Pāhala (blue dots). USGS chart.

(Public domain.)

Volcano activity updates

The Kīlauea volcano is erupting. Its USGS volcano alert level is at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are released daily.

Lava activity is confined to Halema’uma’u with lava erupting from a vent on the northwest side of the crater. Laser range finder measurements this morning, May 20, indicate that the lava in the western (active) part of the lake is 229m (751ft) deep, with most of the lava lake solidifying on the surface. The summit inclinometers have recorded minor changes over the past 24 hours. The sulfur dioxide emission rates measured on May 18 were 100 t / d. Seismicity remains stable, with high tremors. For the most recent information on the eruption, see https://www.usgs.gov/volcanoes/kilauea/current-eruption.

Mauna Loa is not erupting and remains at the AVIS volcano alert level. 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 released weekly.

Last week, around 113 low-magnitude earthquakes were recorded under Mauna Loa; most of these occurred below the summit and above elevations at depths less than 8 kilometers (about 5 miles). Global Positioning System (GPS) measurements show low rates of deformation in the summit region over the past week. The gas concentrations and the temperatures of the fumaroles at the summit and at Sulfur Cone in the southwest rift zone remain stable. The webcams do not show any change in the landscape. For more information on the current monitoring of Mauna Loa volcano, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There have been 2 events with 3 or more felt reports in the Hawaiian Islands in the past week: an M3.2 earthquake 4 km (2 mi) west of Pāhala 33 km (20 mi) deep on May 19 at 3:18 a.m. HST and an M2.8 earthquake 1 km (0 mi) southwest of Pāhala at a depth of 33 km (20 mi) on May 17 at 2:41 p.m. HST.

The HVO continues to closely monitor both the ongoing eruption of Kīlauea and Mauna Loa for any signs of increased activity.

Please visit the HVO website for previous articles on Volcano Watch, Kīlauea and Mauna Loa updates, volcano photos, maps, information on recent earthquakes, and more. Email your questions to [email protected]

Volcano Watch is a weekly article and activity update written by scientists and affiliates of the US Geological Survey Hawaiian Volcano Observatory.



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