Aftershocks continue six months after Kiholo Bay earthquake

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Last week marked the six-month anniversary of the October 15th Kiholo Bay and Mahukona earthquakes.

These large, destructive earthquakes and their aftershocks kept the west side of the Big Island rocking for months. Although not as frequent, aftershocks are still occurring in the vicinity of the initial quakes.

Since the morning of October 15th, there have been about 500 aftershocks, with the vast majority occurring within the first two months after the big quakes. The largest was a magnitude 5.0 on Thanksgiving Day. Currently, we are locating about three earthquakes per week in the north Kona and Kohala areas. Most are too small to be felt, but occasionally have magnitudes of about 3. The normal background level of seismicity in this area is about three earthquakes per month. The decay in the rate of aftershocks follows a similar pattern to that observed after numerous large earthquakes all over the world.

Coincident with the six-month anniversary of the earthquakes, hundreds of scientists from around the world gathered on the Big Island last week for the annual meeting of the Seismological Society of America (SSA).

The meeting took place in Waikoloa, just a few miles from the epicenter of the larger of the two quakes, the Kiholo Bay magnitude-6.7 temblor.

The meeting was divided into sessions that grouped together presentations in similarly focused areas of seismological research. The session topics ranged from studies of earthquakes that had been observed extremely close to the epicenter to studies of earthquakes on other planets.

One of the sessions was devoted to the October 15th earthquakes and was convened by HVO seismologist Paul Okubo.

The presentations in this session ranged from research on the causes of the earthquakes to reports from structural and geotechnical engineers on the damage caused by the quakes.

One of the conclusions of a study of structural vulnerabilities was that homes elevated by posts and piers suffered significantly more damage than those on cement slabs.

Fred Klein, a former HVO seismologist who currently works for the U.S. Geological Survey in California, presented research that reveals a consistent pattern in the catalog of large, deep earthquakes on the Big Island, such as the Kiholo Bay quake.

The pattern of stress release from this type of earthquake indicates that they are caused by the immense weight of the Big Island volcanoes flexing, or bending, the oceanic crust and the upper mantle beneath the island.

The study also shows that this was not an isolated incident, and there have been many earthquakes caused by the same process in the past. As the island continues to grow with the addition of new lava flows, it is inevitable that these earthquakes will continue to occur. Unfortunately, predicting when the stresses will accumulate to the point of earthquake rupture is not possible.

Although the Big Island is subject to tsunami-generating earthquakes, these very deep earthquakes do not produce large tsunami. However, the shaking can be felt for great distances and can cause significant damage on land, as island residents know first-hand from the October quakes.

The scope of the session was exceptionally broad for a scientific conference. In addition to scientists sharing their seismological research, the participants included representatives of governmental agencies responsible for rapid response following the earthquake. Mayor Harry Kim emphasized the need to not only study earth processes and monitor natural hazards, but also to effectively communicate results to emergency managers.

The session concluded with a panel discussion including members of scientific, civil and governmental agencies. All of the panel members expressed their commitment to maintaining open communication lines and working together to ensure everyone's safety during future earthquakes and other natural disasters in Hawai`i.

An additional highlight of the meeting was a lunch-time address by the director of the U.S. Geological Survey. He spoke of the productive partnership between the USGS and the Seismological Society of America and how the science of seismology has made, and continues to make, great discoveries that enhance hazard assessment and benefit public safety.

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Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The summit caldera has been expanding, indicating inflation, since the beginning of 2007. The number of earthquakes located in the summit area is at low levels (usually fewer than 10 per day are large enough to locate). There have been local concentrations of earthquakes immediately south of the summit caldera.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. There have been several small lava flows in the last few months from vents on the southwest flank of Pu`u `O`o, and from a hornito at the head of the PKK lava tube. Lava continues to flow through the PKK lava tube, though, and is feeding a persistent breakout that is often visible after dark, streaming down the face of the pali. About 1 km south of Pu`u `O`o, the Campout tube branches off from the PKK tube. The Campout tube carries lava to the ocean at Kamokuna located inside Hawai`i Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube have been seen on the slope of Pulama pali and on the coastal plain. An eastern branch of the Campout tube continues to send lava northeastward along the base of the pali, where it is burning trees at the base of the Royal Gardens Subdivision. A western branch of the Campout tube hosts surface flows inland from the sea cliff at East Lae`apuki.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

Three earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-2.9 earthquake occurred at 10:14 p.m. H.s.t. on Saturday, April 14, and was located 7 km (4 miles) southeast of Waimea at a depth of 14 km (9 miles). A magnitude-3.2 aftershock occurred at 9:55 a.m. on Sunday, April 15, and was located 36 km southwest of Kawaihae at a depth of 43 km (26 miles). A magnitude-2.7 earthquake occurred at 4:17 p.m. on Monday, April 16, and was located 6 km (3 miles) southwest of Pu`u `O`o Crater at a depth of 9 km (6 miles).

Mauna Loa is not erupting. Two earthquakes were located beneath the summit. Extension of distances between locations spanning the summit, indicating inflation, continues at steady slow rates.