# 2005: Another relatively quiet year for Hawaiian earthquakes

Release Date:

It has become somewhat of a New Year's habit, in a January Volcano Watch report prepared by the U. S. Geological Survey's Hawaiian Volcano Observatory (HVO), that we briefly summarize the previous year's earthquakes in Hawai'i. This week, we also wish to bring to your attention a slightly new look to our web reporting of recent earthquakes.

Seismograph

(Public domain.)

Our description usually begins by saying, "During the previous calendar year, we located [this many] earthquakes." During 2005, HVO seismic data analysts processed a total of 7,426 earthquakes from data generated by the HVO seismic network.

This number represents the number of times that the HVO automated computer procedures detected seismic signals from any part of the HVO network and that subsequent interactive review by an HVO seismic data analyst confirmed the source of the signal as an earthquake. Thousands of additional triggers were recognized as noise or possibly even small earthquakes so poorly recorded that the data were simply passed over for lack of sufficient data.

Out of this number, a total of 4,360 - basically the largest of these 7,426 events - were studied in greater detail, and their locations were recalculated and entered into an annual compilation of Hawaiian seismicity. The largest of these earthquakes, magnitude-5 earthquakes on May 13, July 15, and July 17, occurred offshore of Hawai'i. The May 13 earthquake occurred in the vicinity of Lo'ihi Volcano, approximately 35 km (22 miles) from the southeastern Ka'u coastline. While widely felt, these earthquakes caused no damage reported to HVO.

While not the largest of earthquakes in 2005, it could be argued that the most significant earthquake was a magnitude-4.5 earthquake that occurred on November 29, 2005, at 10:26 PM. This earthquake occurred on the south flank of Kīlauea Volcano, near Kalapana.

Occurring on the 30th anniversary of the 1975 magnitude-7.2 Kalapana earthquake and tsunami, this smaller Kalapana earthquake gently reminded us of the ever-present hazard posed by large earthquakes that are a very important part of active volcanic processes in Hawai'i. All small earthquakes that we feel should remind us of this constant threat.

The 1975 Kalapana earthquake was preceded by a moderate magnitude-5.7 foreshock, occurring some 72 minutes before the M7.2 mainshock, and a number of smaller microearthquakes near Kalapana. Given the length of time between the 1975 earthquake and now, and the computer-generated location posted on the HVO web pages for this recent magnitude-4.5, the reminder from this small earthquake on this November 29, 2005, might have seemed more vivid. The 2005 "anniversary earthquake" could have been a foreshock to a larger Kalapana earthquake, possibly similar to the 1975 earthquake.

Last year's November 29 earthquake, luckily, was not a foreshock, but it is not necessarily the case that Kalapana's next damaging, tsunamigenic earthquake will be preceded by a foreshock. There are other possibly precursory behaviors that could be observed, but our level of understanding of earthquake behaviors is not yet at the point where we can confidently predict in detail what will actually occur. Thus, continued monitoring and study of earthquakes remain high priorities for hazard mitigation.

Surfers to the HVO web pages have probably already noticed a modification within the HVO "Recent Earthquakes in Hawai'i" pages (http://hvo.wr.usgs.gov/earthquakes/new/). Implemented in December 2005, this change features a somewhat expanded presentation of earthquake seismograms for the posted earthquakes.

From these pages, it is possible to get a view not only of the closer HVO stations, but now, also of a more distant collection of seismic stations that record the earthquakes. Among other things, studying these pages will show, to a greater extent than in our earlier web page implementation, how seismic waves traverse the island, how earthquakes will appear differently at our different stations, and how earthquakes of different magnitudes appear on the HVO seismic network.

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### Volcano Activity Update

Eruptive activity at Puu Oo also continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava is still flowing through the PKK lava tube from its source on the flank of Puu Oo to the ocean, with scattered surface flows breaking out of the tube. In the past week, flows were active intermittently about 1 kilometer (0.6 miles) downslope of Puu Oo, as well as on the steep slopes of Pulama pali down to about the 400-ft elevation. Surface flows on the pali are visible at night (weather permitting) from the end of Chain of Craters Road.

As of January 12, lava is entering the ocean at East Laeapuki, in Hawaii Volcanoes National Park. The active lava bench continues to regrow following the major collapse of November 28, 2005. Access to the ocean entry and the surrounding area remains closed due to significant hazards. If you visit the flow field at the coast, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

During the week ending January 12, 4 earthquakes were felt on Hawaii Island. At 1:17 p.m. on January 7, a magnitude-3.1 earthquake located 19 km (12 miles) north-northwest of Naalehu occurred at a depth of 0.8 km (0.5 miles); this earthquake was felt widely across the southern part of the island. A second earthquake on January 7 at 3:56 p.m. occurred 6 km (4 miles) southeast of Puu Oo at a depth of 39 km (24 miles); it was felt from Volcano to Hilo and in Waimea. A magnitude-3.8 earthquake on January 8 at 1:13 p.m. located 34 km (21 miles) west of Captain Cook at a depth of 40 km (25 miles) was felt locally on the west side of the island. At 7:25 p.m. on January 9, a magnitude 2.4 earthquake located 4 km (2 miles) south-southwest of Captain Cook occurred at a depth of 14 km (8.5 miles); it was felt in Captain Cook and Honaunau.

Mauna Loa is not erupting. During the past week, two earthquakes were located beneath the volcano. Inflation continues but at a slightly slower rate since early October 2005.