October 15th Earthquake FAQs
Two damaging earthquakes struck the northwest side of Hawai'i Island early on Sunday morning, October 15, 2006. The first was a magnitude-6.7 that occurred at 7:07 AM HST and was located 20 km northeast of the Kona airport at a depth of 38 km. Seven minutes later, a second earthquake, assigned a magnitude-6.0, struck 44 km north of the Kona airport at a depth of 20 km. While the two events only 7 minutes apart, the difference in depths means that the M6.0 may not be an aftershock of the M6.7 and that they are independent quakes.
This is a helicorder seismogram displaying vertical ground velocities at the summit of Hualalai volcano south of the earthquake epicenters. Each line represents 10 minutes and time increases from left to right. Each line continues on the line below. The seismogram starts at 2 am and ends at 2 pm HST on October 15, 2006.
The top 26 lines show typical ground vibrations before the earthquake. The M6.7 earthquake immediately goes off scale. The M6.0 earthquake arrives on the line below. Each of the signals that resemble Christmas trees turned horizontal are aftershocks.
| large image |
Over 80 aftershocks with magnitudes greater than 1.7 were recorded in the first 24 hours after the quake. The largest was a magnitude 4.2 that occurred at 10:35 AM HST on October 15. Like the second earthquake, preliminary locations for most of the aftershocks placed them at depths less than 20 km.
These earthquakes were felt statewide but most strongly in the North Kona and Kohala areas (http://pasadena.wr.usgs.gov/shake/hi/). The shaking was strong enough to cause power generators to trip offline in Hawai'i, Maui, and O'ahu counties. Damage was reported mostly on the west side of Hawai'i island but also on Maui and O'ahu. There were no reported fatalities.
The first earthquake was the largest recorded in Hawai'i Island since the M6.7 under the east flank of Mauna Loa on November 16, 1983. The only two larger earthquakes were a M7.2 that occurred beneath Kilauea's South Flank on November 29, 1975 and an estimated M7.9 that occurred beneath the southeast flank of Mauna Loa on April 2, 1868.
The most recent earthquakes were unusual for this area of Hawai'i Island. Prior to October 15, a M4.8 quake on May 14, 1982 was the largest of thirty-one earthquakes with a magnitude greater than 4.0 that have been recorded by our modern seismic network since its inception in 1960.
The earthquakes probably reflect the earth's response to loading by the islands. Preliminary analyses show a possible east-west slip plane and a similar orientation to the clustering of aftershocks. Further examination of all available data should shed some light on this most interesting and destructive seismic event.
The earthquake had no discernable effect on the ongoing eruption of Kilauea. An overflight a few hours after the earthquake showed no new collapse at the eruption site, either at the ocean entries or at Pu`u `O`o cone.
Small landslides were reported in pit craters on Hualalai. The camera at the summit of Mauna Loa was briefly enveloped in the dust cloud from a rockslide on the north wall of Moku`aweoweo caldera.