# Volcano Watch — 2004's earthquakes in review

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Along with the rest of the world, we at the U.S. Geological Survey's Hawaiʻian Volcano Observatory continue to follow the aftermath of the great Indian Ocean earthquake and tsunami of December 26, 2004.

Among the valuable lessons learned, hopefully, is the recognition that not only monitoring and warning systems, but also appropriate emergency response preparations, plans, and education are critical to public safety and mitigation against the effects of natural hazards.

In our own region of interest, HVO monitors those processes that pose volcanic, earthquake, and tsunami hazards to our communities. As we have described numerous times in our Volcano Watch reports, a large fraction of our monitoring effort is dedicated to cataloging and tracking earthquake activity in Hawai'i.

Through 2004, we reviewed over 13,000 earthquakes. Three of these earthquakes were greater than magnitude 4. They occurred beneath Kīlauea on February 5 (magnitude 4.1), October 11 (magnitude 4.0), and October 12 (magnitude 4.5). Seventy-nine others were felt by residents of Hawai'i County, but they were still small enough to be considered only microearthquakes. None of these earthquakes caused any damage.

Each year HVO compiles and posts a summary of Hawaiʻian earthquake activity. For these reporting purposes, the summary is generally complete for all earthquakes of magnitudes 1.5 or greater. Our 2004 summary contains 4,228 earthquakes. The remainder did not meet our summary reporting criteria.

The principal earthquake source regions are the active volcanoes Kīlauea, Mauna Loa, and Loihi. Less concentrated seismicity is located elsewhere beneath the island.

Beneath the flanks of the volcanoes, the seismicity helps delineate the structures which deform in response to magmatic and tectonic forces. Recent history shows that these are the areas which have also produced large earthquakes, and some of our research at HVO is exploring how the microseismicity patterns might relate to larger earthquakes.

In 2004 we also recorded significant numbers of volcanic earthquakes referred to as LP - or long period - earthquakes beneath the summit caldera regions of both Kīlauea and Mauna Loa. The LP earthquakes feature a dominant frequency of oscillation of 3 to 5 Hertz (cycles per second) on our seismograph records. They are indicators of magma pressure and magma transport.

Following a small flurry of earthquakes and an abrupt change in the measured surface deformation patterns on Mauna Loa in early- to mid-2002, we have noted and reported unprecedented deep LP seismicity beneath Mauna Loa that began in July 2004. The number of deep Mauna Loa LP earthquakes in the HVO summary now approaches 2,000.

The deformation patterns measured since 2002 continue. Moreover, augmented monitoring suggests that the rates of deformation have possibly increased in recent months. The volcano continues to command our monitoring and research attention.

Our descriptions and interpretations of seismicity begin fundamentally with earthquake locations. Traditional earthquake location methods depend on measuring the times when seismic waves first arrive at our recording stations. Because the LP earthquake waveforms generally feature gradual or emergent arrivals, it is often difficult to identify the wave onset times. Thus our LP locations determined via the traditional procedures are subject to larger uncertainties, and our interpretations are correspondingly less specific.

Besides our traditional earthquake location techniques, we also have more sophisticated procedures to improve upon the precision of our locations. We have successfully applied these to datasets comprised of tectonic and LP earthquakes from Kīlauea - our volcano laboratory, so to speak - and developed new views of earthquakes here.

We have begun to apply these newer techniques to the Mauna Loa deep LPs. Time will tell how successful we will be at determining their locations more precisely. From this will follow our ability to describe specific causes for these earthquakes, how they may or may not relate to possible sources of the measured surface deformations, and, somewhat optimistically, Mauna Loa's intentions.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. Spatter cones in the crater of Puu Oo glow brightly on clear nights but have not produced any lava flows for several months. The MLK vent area, at the southwest base of the cone, intermittently erupts small Pāhoehoe flows that stack up close to the vent.

The PKK flow continues to host substantial breakouts from above the top of Pulama pali to the coastal plain. Lava is not entering the ocean. As of January 12, lava flows were active on the coastal plain, about 400 m (435 yd) inland of the shore at Laeapuki. The area of breakouts is about 3.2 km (2 mi) from the end of the pavement on Chain of Craters Road in Hawaii Volcanoes National Park. Expect a 2-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs.

During the week ending January 13, four earthquakes were felt on our Big Island. The first was a magnitude-3.6 quake which occurred at 8:16 pm on January 12 and was located between Waikii and Mauna Kea summit at a depth of 20 km (13 miles). It was felt in much of the northern half of the island. The other three occurred at 8:44 p.m., 10:58 p.m. and 2:03 a.m. (the next day) were magnitude-2.4 earthquakes located in the same approximate area at a depth of 16-18 km (10-11 miles). They were felt in Kalaoa, Ahualoa, and Waimea.

Mauna Loa is not erupting. The summit region continues to inflate. Since July 2004, the rate of inflation and number of deep earthquakes has increased. Weekly earthquake counts have varied from 5 to over 150. During the week ending January 5, only 8 earthquakes were recorded beneath the summit area. This is the same as the past week. Nearly all are 30 km (18 mi) or more deep and are the long-period type, with magnitudes less than 3.