Revisiting Hawaii's October earthquake

Everybody remembers the shaking. The October 15, 2006 magnitude-6.7 earthquake will also be remembered for the blackouts, damage, and/or closing of a few local icons like the Mauna Kea Beach Hotel, the Hulihe`e Palace, and the Kalahikiola Church in Kohala. But the 15-20 seconds of strong shaking is an experience that will not be forgotten. The shaking is, of course, what caused the damage. Most people are familiar with the magnitude of an earthquake, and some know that earthquake magnitude is related to the logarithm of the earthquake’s energy. But we don’t experience the earthquake energy directly — we experience the ground shaking.

For any given earthquake, the intensity of shaking depends on how far you are from the epicenter and what kind of material you are on. If you were on the same type of ground, the shaking would be more intense closer to the quake’s epicenter. However, thick soils amplify earthquake shaking. For example, just 0.5 m (1.5 feet) of ash or soil can double the shaking that would be felt on a barren lava flow at the same distance from the earthquake epicenter.

How do we measure the intensity of shaking? Almost all of HVO’s seismic instruments are designed to detect earthquakes with magnitudes up to about 4. These instruments are very sensitive, and we generally locate them well away from roads and surf to avoid noise caused by ocean waves, cars, and people. Stronger earthquakes drive our instruments off-scale, and we lose the kind of data that would help us estimate shaking intensity.

The U.S. Geological Survey has installed specialized instruments, called Strong-Motion recorders, inside buildings at more than 30 locations in Hawai`i. These instruments are designed to stay on scale even during the strongest earthquakes. Strong-Motion recorders measure the acceleration of the ground, whereas the sensors deployed in our regular network measure ground velocity. As you may remember from high school physics, acceleration is the time-rate of change of velocity, and force is equal to mass times acceleration. Not only do the strong motion recorders stay on scale, they also measure the very thing we need to estimate the magnitude of shaking forces — acceleration.

For easy comparison, these values can be scaled to an acceleration we are all familiar with — the acceleration of gravity. For example, a shaking intensity or peak ground acceleration (PGA) of .1g is an acceleration equal to 1/10th that of gravity.

The largest values of acceleration were recorded for the magnitude-6.7 earthquake. As expected, the accelerations measured for each earthquake were higher for sites on ash or soil. The PGA measured on hard lava at `Anaeho`omalu was .18g, even though that location was almost directly the earthquake. The largest PGA measured anywhere was 1.0g at the Waimea Fire Station, more than 33 km (21 miles) away from the epicenter but on thick soil. The Waimea firemen were shaken by forces 5 times stronger than guests staying in hotels right over the earthquake.

A similar discrepancy can be seen when looking at the measurements around Hilo at sites that are all about the same distance from the earthquake’s center. Acceleration values measured just north of the Wailuku River (on Mauna Kea ash) were 3-5 times what was measured in Hilo itself (on hard lava). Again, ash and soil amplified earthquake shaking.

To make these numbers more real, compare them to accelerations most of us experience in one direction. Every time you take off in a commercial airliner, you are experiencing accelerations of around 0.1g. This was similar to the PGA experienced by those in Puna, Ka`u, South Hilo, and Maui County. A passenger in a car that accelerates from 0 to 60 mph in 10 seconds experiences 0.25g. This level of PGA was experienced by people in the Humu`ula Saddle area, and north and south Kona (except at Kona Hospital). Only northern Hamakua, Waimea, Kohala and Kealakekua (Kona Hospital) experienced stronger shaking.

Imagine these forces alternately exerted on you in opposite directions, and you can relive the October 15th earthquake!

Activity Update

This past week, activity levels at the summit of Kilauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day are large enough to locate).

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 km south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawai`i Volcanoes National Park.

A third entry, fed by an offshoot of the Campout flow, has been active since December 26. It is located at Kamokuna, about midway between the two older entries. In the last week, intermittent breakouts from the Campout tube have continued on the slope of Pulama pali and on the coastal plain.

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.

There were two earthquakes beneath Hawai`i Island reported felt within the past week. A magnitude-2.0 earthquake occurred at 10:55 p.m. H.s.t on Friday, January 5, and was located 4 km (2 miles) east of Honaunau at a depth of 10 km (6 miles). A magnitude-2.2 earthquake occurred at 10:36 p.m. on Wednesday, January 10, and was located 9 km (5 miles) northeast of Waiki`i at a depth of 12 km (8 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano’s summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by with permission.

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