Volcano Watch — Earthquake location laboratory

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One of the basic objectives for seismographic networks is monitoring and maintaining records of earthquakes and other seismic sources. At the Hawaiian Volcano Observatory (HVO), our monitoring focuses on the active volcanoes. Each year, they give us not only thousands of earthquakes, but also special volcanic seismic sources related to magma transport.

For each earthquake or seismic event, our first task is to calculate its location. Once the location of the source is available, other earthquake parameters can be derived and, in turn, used to develop and improve our models of how the volcanoes work. At a very simple level, knowing where an earthquake is located also provides basic clues as to why the earthquake occurred.

Our traditional earthquake-location techniques are based on precisely measuring the times when seismic waves arrive at the numerous stations that we monitor on the island of Hawai'i. These times are collected and compared - presuming some prior knowledge of the speeds at which the waves travel from the earthquakes to our stations - to determine the earthquake location.

The task of locating tectonic earthquakes is relatively straightforward, as the wave arrivals are generally clearly recognizable. Tectonic earthquakes are among the ways that the Earth releases energy stored while crustal plates move, mountains uplift, or islands subside. We have implemented computer procedures that automatically measure arrival times, locate the earthquakes, and, if successful, post information to the Internet.

In addition to our numerous tectonic earthquakes, we also have volcanic earthquakes and sustained volcanic tremor that occur, to our best understanding, when there are changes in how and where magma moves through the volcanoes. As with tectonic earthquakes, we look for wave arrivals to determine the earthquake and tremor locations.

Unlike tectonic earthquakes, however, the volcano-related seismic signals tend to have a dominant frequency of oscillation and their wave onsets or first arrivals are emergent, that is, the ground shaking that we record begins gradually. We refer to these as long-period, or LP, signals and, correspondingly, LP earthquakes. LP signals are common to both basaltic and explosive volcanic systems.

The emergent signals from LPs make them more difficult to locate with our standard tools. Recently, we have begun to apply different techniques to the location of volcanic LP earthquakes and tremor. These calculations are being spearheaded by Dr. Jean Battaglia, who has been working at HVO and the Center for the Study of Active Volcanoes at the University of Hawai'i at Hilo after finishing his Ph.D. studies at Reunion Island and Piton de la Fournaise Volcano.

Rather than using just the arrival times to determine the locations, we are trying to use the entire volcanic earthquake signal. Using wave shapes and wave amplitudes, the calculations seek coherence and correlation among the seismic waves recorded on our network for collections of LP events.

What results is a somewhat different picture of volcanic LP sources from our earlier views. The LP locations based on wave amplitude distributions lie beneath Kilauea's summit caldera, but they appear to cluster more tightly and also separately from the locations determined from only their first arrivals. Precisely determined locations, using the similarity or correlation of wave shapes from collections of LPs, describe an elongate vertical source volume within this cluster.

If the LPs reflect the excitation of fluid-driven oscillations, as theory suggests, then these early results point to a feature that is repeatedly sensitive to changes in magma transport beneath Kilauea's caldera. We may be able to focus on specific areas whose activity may herald eruptions or changes in eruptive behavior. Much more work needs to be done in our LP location laboratory, but we hope these applications and results will improve our ability to recognize and understand indicators and mechanisms of unrest in any volcanic system.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava extruded sporadically from several vents in the crater of Pu`u `O`o as well as from Puka Nui, a small crater at the southwest base of the cone. The two long flows reported last week to be emanating from the "rootless" shields are slowly moving forward. The lower flow has reached the base of Pulama pali and is burning vegetation near the bottom of Royal Gardens. The second flow has entered the uppermost portion of the kipuka containing what remains of Royal Gardens and poses a threat to the subdivision. There are no ocean entries.

Two earthquakes were reported felt during the week ending on May 9. Residents of Waimea, Honoka`a, Pa`auilo, and Papa`aloa felt an earthquake at 10:45 p.m. on May 6. The magnitude-2.9 temblor was located 16 km (10 mi) northeast of Honoka`a at a depth of 3.7 km (2.2 mi). Another magnitude-2.9 earthquake was felt in Kaimu and the summit area of Kilauea Volcano at 12:42 p.m. on May 7. The earthquake was located 5 km (3 mi) southeast of Pu`u `O`o at a depth of 4.45 km (2.7 mi).