Volcano watch: New research sheds light on recent Pahala earthquake swarms

From the United States Geological Survey, Hawaii Volcano Observatory scientists and its affiliates.

April 5, 2021, Coloya. Public domain.

Many people in Kau have noticed a swarm of earthquakes over the past few years. These earthquakes occur in an area 18-25 miles below Pahala and stretch south about 6 miles offshore. Some of the largest earthquakes from this area were felt across the island of Hawaii.

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

These earthquakes occur beneath volcanoes and even below the base of the underlying ocean crust, in the upper mantle. This part of the mantle is called the lithosphere mantle because it is cool enough to be torn apart and create earthquakes. The deeper mantle is very hot and plastic to generate earthquakes and tends to flow like mud or paste.

Historically, earthquakes in the rocky mantle, such as those below Pahala and elsewhere in Hawaii, have occurred in response to the weight of the island that the lithosphere mantle is bent down. The Kohulo Bay earthquake 6.9 on the Richter scale of 2006 was a great example of this type of “flexion” earthquake.

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Numerous ideas have been put forward as to the cause of deep earthquakes under Pahala and the southeast coast of Kai since they were first observed in the late 1960s. The initial hypothesis was that these earthquakes were occurring at flat fault levels and associated with the bending of the lithosphere mantle due to the weight of the volcanoes above. However, deep earthquakes in Pahala differ from other lithosphere earthquakes under Hawaii due to the sheer number of earthquakes, especially since 2015.

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Other authors have suggested that the deep area beneath Pāhala was a passage of magma from the upper mantle plume below Hawai’i Island to Kīlauea Volcano. They hypothesized that the number of earthquakes increased when magma was supplied to Kīlauea, although there is no direct evidence to support any movement of magma and requires a hitherto unknown channel to move magma for large distances horizontally to reach down Kīlauea and Mauna Loa.

Scientists recently examined the characteristics of high earthquakes that started under Pāhala in 2015. They found that as the number of earthquakes increased, many earthquakes occurred on steep faults in addition to those that have been historically interpreted as flat earthquakes. The number of earthquakes increased again in 2019, and so did the variety of faults that resulted from them.

The total load of volcanoes on the lithosphere will not change significantly within years, so the diversity of fault trends requires a more complex answer to explain the large number of earthquakes and their changing direction. The Pāhala region lies at the structural link between three volcanoes – Kīlauea, Mauna Loa and Lō’ihi. However, there is insufficient information to link the accumulation of stress in this region to the formation of these earthquake swarms.

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Instead, the scientists hypothesized that some magma might leak from the mantle plume and migrate to the Pahala region and stop in depth. The inferred magma could compress the area, resulting in an increase in the number and variety of earthquakes occurring beneath the Pāhala. Although there is no direct evidence of magma accumulation, it is an interesting hypothesis that requires further investigation.

Map and plot showing earthquakes 20-40 km (12-25 miles) deep under the island of Hawaii over the past week. Most of the earthquakes at this depth were collected below the southern edge of the island near Pahala City (blue dots). USGS drawing. Public domain.

The Bahala earthquake swarm in its current state appears to be a unique event previously not observed in Hawaii, although the historical record of seismometers is just over 50 years old. Additional research is planned and includes placing a dense set of small temporary seismometers at Ka ‘Ka to depict what is going on in the depths of the Pāhala. If you get a call from the USGS Hawaiian Volcano Observatory or a University of Hawaii scientist looking to put a small instrument on your ground next year, please engage with them and do what you can to help broaden our understanding of this complex system by supporting this important and interesting volcanic research.

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

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