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Time-lapse HD movie of Kilauea Caldera from HVO

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Hawaiian Volcano Observatory’s Recent Earthquakes Web page from Tuesday, July 31, with earthquake shown as colored circles, according to their times of occurrence.

Tuesday morning, July 31, started as a typical office day at the Hawaiian Volcano Observatory (HVO). Beginning at 10:25 a.m., a number of text messages and emails arrived in rapid succession. It was time to take a closer look.

The messages all came from ENS, or Earthquake Notification Service. The U.S. Geological Survey (USGS) offers this service to subscribers who wish to automatically receive messages after earthquakes of specific interest to them. (https://sslearthquake.usgs.gov/ens/%5D )

The earthquakes flagged in this series of messages all occurred beneath Kilauea volcano’s south flank. More specifically, they were concentrated beneath a section of Hilina Pali some 13 km (8 miles) south-southeast of the volcano’s summit. We have focused on earthquakes in this region recently because of their apparent relationship to slow-slip events (SSE) that occur beneath Kilauea’s south flank. Was another SSE starting? What else might be going on?

It was easy to spot Tuesday’s earthquakes on HVO’s earthquake Web page (http://hvo.wr.usgs.gov/seismic/volcweb/earthquakes/). Other small clusters of earthquakes also appeared, extending from a few miles due west of Kilauea’s summit crater, into the summit caldera, and along Kilauea’s upper east rift zone. To be sure, these are familiar earthquake source regions to us, and we have a general understanding of what causes them and what their relationships to other processes are.

The earthquakes just west of Kilauea’s summit appear to have occurred along the downward extensions of faults comprising the easternmost part of the Ka`oiki fault system which lies between Kilauea and Mauna Loa Volcanoes. These are “normal” faults where the “hanging” wall block moves downward relative to the “footwall.”

Along the Ka`oiki faults, the southeastern, Kilauea side of the faults has moved downward relative to the northwestern, Mauna Loa side. This is a natural part of the volcanoes’ evolution, as their flanks slump seaward due to gravity.

Earthquakes clustering west of Kilauea summit, in what we have called Namakanipaio or Ka`oiki Pali, sequences and swarms, are also consistent with slumping. Earthquake swarms here have preceded intrusions of magma from Kilauea’s summit into the east rift zone during the continuing Pu`u `O`o eruption. Movements along these faults pulse neighboring parts of the volcano and have, at times, led to repeating patterns of earthquake and intrusive activity.

Earthquakes occurring beneath Kilauea’s summit caldera are related to magma accumulation and movement. Changes within the magma system stress the surrounding areas that produce earthquakes on nearby faults and cracks and within magma conduits in the caldera.

Along Kilauea’s upper east rift zone—delineated by the “Chain of Craters” in Hawai`i Volcanoes National Park between the summit caldera and Mauna Ulu—earthquake swarms are directly associated with magma movement. Such swarms have coincided with other HVO observations and measurements to confirm their relationship to magma intrusions and eruptions. Tuesday’s upper east rift zone earthquakes were not swarm-like, but rather, continued a trend of persistent, but low-level activity.

Since Tuesday, we have arrived at several conclusions. While eruptions continue in Halema`uma`u at Kilauea’s summit and at Pu`u `O`o in its east rift zone, there was no large intrusion of magma from the summit into the rift zone. Such an intrusion, as might have been expected after seeing clustered earthquakes near Ka`oiki Pali, would have been heralded by more intense upper rift zone earthquakes. The Ka`oiki Pali earthquakes could be related to the earthquakes in Kilauea caldera.

The recent Hilina Pali earthquakes do not appear to be signaling the start of another Kilauea SSE. The most recent SSE occurred two months ago, and the next one would not be expected for roughly two years. There was a brief show of similar Hilina Pali events there in early July. Tuesday’s earthquakes are plausibly very similar to those seen in July, but their connection to SSEs is not clear.

Kilauea continues to erupt. Earthquakes continue in familiar locations but subsequent events do not always follow the same patterns. Meanwhile we’re back to a typical day at HVO, keeping up with the incoming data and observations studying the different patterns and processes, and where they fit—or don’t fit—together.

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Time-lapse HD movie of Pu‘u ‘O‘o Crater