Volcano Watch — Continued Rumblings of the 2006 Kiholo Bay Earthquake

Residents of and visitors to the island of Hawai`i are reminded of the earthquake in this and other ways. Some, like the replacement bridge on the Mamalahoa Highway (Route 19) in Pa`auilo are quite visible.

For the most part, the October 2006 earthquake experiences are memories. We were fortunate that our community was not forced to endure more widespread and more devastating consequence due to the earthquake.

At the same time, as residents of an earthquake-prone region, we know that future large earthquakes are expected. The principal means of mitigating the effects of large earthquakes include developing and adopting appropriate building codes and use of appropriate earthquake-resistant design and building practice, as well as establishing community and personal earthquake response plans.

While current scientific capabilities do not afford the means to precisely predict the time, location, and magnitude of future large earthquakes, we are able to forecast the effects of future large earthquakes as the probabilities of strong earthquake shaking. The U. S. Geological Survey (USGS) features this information online, with explanations, as probabilistic seismic hazards maps at http://earthquake.usgs.gov/research/hazmaps/.

As is the case for any large earthquake, the 2006 Kiholo Bay earthquake sequence (including main and after shocks) provided important observations and data that will fuel research toward a better understanding of earthquakes and their effects. For Kiholo Bay, such data were recorded by a set of instruments installed and maintained by the USGS National Strong Motion Project (NSMP). Beginning in the year 2000 and only now recently completed, the NSMP has upgraded all of its strong motion instrumentation, some of which recorded on film in 2006, to current operational (digital) standards.

The NSMP instruments are referred to as "strong motion accelerographs" that record the strongest shaking expected from earthquakes without exceeding the maximum working range of the instruments. There are two-dozen NSMP instruments on the island of Hawai`i, and a few more on O`ahu and in Maui County.

The maximum shaking from the October 15 M6.7 mainshock was not recorded by the NSMP instrument nearest the earthquake epicenter as expected. Instead, the strongest shaking was recorded at the Waimea Fire Station (more than 32 km or 20 miles away), and the overall pattern of strong motion data suggested significant variations in response due to soil and geological conditions beneath the individual instrument locations.

Data collected from the NSMP sites since 2006 have been compiled into a new map of strong motion site conditions for the Big Island that was presented earlier this month at the Fall 2008 Meeting of the American Geophysical Union (AGU). While an earlier version of the map showed much of the island to be classified as "rock" sites, the recent work suggests that much of Hawai`i Island should be considered soft rock or very dense soil. Shaking at soft rock or very dense soil sites would be amplified over shaking at hard rock sites. The differences must be incorporated into updated seismic hazard maps of Hawai`i to properly estimate future strong earthquake shaking.

Also presented at the AGU meeting was an HVO study of the rupture process of the Kiholo Bay M6.7 mainshock. This study also used NSMP recordings from the Kiholo Bay sequence, including the M5.0 aftershock that occurred on Thanksgiving Day, 2006.

The October 15, 2006 M6.7 Kiholo Bay earthquake occurred on a deep fault, approximately 39 km (24 miles) below sea level. The slippage that caused the earthquake started at its hypocenter and continued over an area of the fault roughly 30 km X 20 km (18 miles X 12 miles) in size, in a westward direction away from the island at a speed of 3.5 km/s (2.2 miles/s or 7.900 miles/hr). Maximum slippage was more than 1 m (3.3 ft).

Such studies will contribute to a better understanding of large earthquakes and how their effects are distributed across Hawai`i.

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Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is erupting elevated amounts of sulfur dioxide gas and very small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the south part of Kīlauea caldera and produced occasional air quality alerts in more distant areas, such as Pahala and communities adjacent to Hawai`i Volcanoes National Park, during kona wind periods.

Pu`u `Ō`ō continues to produce sulfur dioxide at even higher rates than the vent in Halema`uma`u Crater. Trade winds tend to pool these emissions along the West Hawai`i coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

Lava erupting from the Thanksgiving Eve Breakout (TEB) vent at the eastern base of Pu`u `Ō`ō continues to flow to the ocean at Waikupanaha through a well-established lava tube. Breakouts from the lava tube were active in the abandoned Royal Gardens subdivision and on the coastal plain in the past week. Active portions of the flow on the coastal plain were within 100 yards of the National Park boundary, as they have been during the last several weeks. Ocean entry activity has fluctuated in the past week, due to a deflation-inflation cycle that began on Sunday, Dec. 21. These cycles normally cause changes in lava supply to the flow field that can last a few days.

Be aware that active lava deltas can collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions. The Waikupanaha delta has collapsed many times over the last several months, with three of the collapses resulting in rock blasts that tossed television-sized rocks up onto the sea-cliff and threw fist-sized rocks more than 200 yards inland.

Do not approach the ocean entry or venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Call Hawaii County Civil Defense at 961-8093 for viewing hours.

Mauna Loa is not erupting. Two earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano, combined with slow eastward slippage of its east flank.

No earthquakes beneath Hawai`i Island were reported felt within the past week.

The staff of the Hawaiian Volcano Observatory wishes you a Happy Holiday Season. Visit our Web site for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov. skip past bottom navigational bar