Learn more about the pahala deep earthquakes

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(BIVN) – This week’s Volcano Watch article, written by scientists at the US Geological Survey’s Hawaiian Volcano Observatory, updates what is known about the ongoing deep Pahala earthquake swarm.

From the Volcano Watch article:

The city of Pahala is located in the southeastern part of the island of Hawaii, above the most seismically active region of the state. In the past five years, the average weekly number of earthquakes has increased from about 60 to 600 earthquakes per week.

These frequent earthquakes occur in the depths of the Pahala region at depths of 20 to 40 kilometers (12 to 25 miles). Larger events are experienced by residents living within Pahala and across the island on a regular basis.

In a 2015 study, scientists at the USGS identified and classified earthquakes in this region into two main groups.

A more persistent seismic tremor, a type of signal often used to track magma movement within a volcanic system, was identified at a depth of about 40 km (25 miles), both inside and outside the Pahala region. This activity has been interpreted as the migration of magma from the depths of the Hawaiian hotspot to shallower depths.

Above this deep zone of seismic activity, a zone of volcanic-tectonic (VT) earthquakes (indicating rock breaking) is observed extending from this earthquake zone into the depths of the Kīlauea region. Based on the linear trend of this seismic activity, USGS scientists have suggested that this distribution of earthquakes represents an underground magma migration pathway from storage below Pahala toward the summit of Kīlauea. However, the evidence supporting this hypothesis was limited to a single series of VT events.

A recently released study by seismologists at the California Institute of Technology (CalTech) has shed new light on the processes occurring in the depths of Pahala. These scientists used newly developed machine learning (computer systems that analyze large data sets and identify key patterns in the data) to identify a large number of extremely small earthquakes (microquakes) occurring below the Pahala region that had not previously been cataloged by the Hawaiian Volcano Observatory. Such micro-earthquakes are too small to be determined using the automated methods normally used to detect earthquakes.

Many of these events are VT earthquakes located within the deep Pāhala seismic zone previously identified by USGS scientists. This large number of newly identified VT events define what appears to be a region of horizontally layered magma storage deep below Pāhala, called the sill complex.

Further supporting this idea, long-term earthquakes (which are indicative of the movement of fluids such as magma) have been seen to occur within these sills. Together, these observations show that the deep earthquakes below Pāhala are consistent with the migration and storage of magma within this threshold complex.

The CalTech study also identified a limited number of VT events extending from the Threshold complex to the magma storage area below Kīlauea’s summit. The authors suggested that this distribution of seismicity may represent a potential magma pathway from the deep Pahala Threshold complex to the magma storage reservoir at Kīlauea. However, more evidence is needed to support or refute this hypothesis.

The Hawaii Volcano Observatory (HVO) is conducting two large seismic experiments in the area to better understand the seismic and volcanic activity in the Pāhala and Kīlauea regions. In the summer of 2022, 86 temporary seismic instruments will be deployed throughout Pahala to record earthquakes occurring within this region for a period of three months. In the summer of 2023, it is proposed that approximately 1,600 temporary seismic instruments be deployed across the summit of Kīlauea to record seismic signals for a period of six weeks.

Dense temporary deployments will record seismic activity across the Kīlauea and Pāhala regions more accurately than the network of permanent HVO instruments. HVO seismologists will analyze this seismic data to understand whether magma stored within the active Pāhala Threshold complex is connected, and thus a source of magma for Kīlauea volcano.

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