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Volcanowatch

July 22, 2010

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


More rattling from last Sunday's small O`ahu earthquake

Plume-Lithosphere Undersea Melt Experiment deployment map

The bits of dust kicked up by last Sunday's magnitude-3.6 earthquake south of O`ahu have settled. The earthquake occurred at 3:27 p.m. Hawai`i standard time (01:27h 19-JUL-2010 UTC), and its location was determined to be 26 km (16 miles) southeast of downtown Honolulu at a depth of 32 km (20 miles). This and other information related to earthquake are available on, and linked from, the Hawaiian Volcano Observatory's (HVO) web site (http://hvo.wr.usgs.gov), as well as on other U. S. Geological Survey (USGS) web sites, for example http://earthquake.gov .

Sunday's earthquake was widely felt on O`ahu and was reported felt from as far away as Kihei, Maui. It was a not large earthquake, and it would be unusual for an earthquake of this magnitude to cause any damage.

This earthquake reminds us that, while much of Hawai`i's earthquake activity is closely linked to our active volcanoes on the island of Hawai`i, earthquakes do occur beneath other parts of the state. When such earthquakes do happen, they impact Hawai`i's larger population centers on Maui and on O`ahu.

For a number of years, the USGS has collected reports of earthquake effects from the general public via our "Did You Feel It?" web pages (http://earthquake.usgs.gov/earthquakes/dyfi/). This information is valuable and we appreciate everyone's contribution to this site.

For Sunday's O`ahu earthquake, there were 875 reports contributed to the "Did You Feel It?" site. Other earthquakes of comparable magnitudes receive quite different numbers of reports, reflecting population densities where they are located. For example, a magnitude-3.8 earthquake beneath Pahala on April 10, 2010, had 130 contributions, and a magnitude-3.6 earthquake beneath Hawai`i Volcanoes National Park on June 29, 2010, had 24 responses. At another extreme, there were over 21,000 "Did You Feel It?" reports following a magnitude-3.6 earthquake on July 10, 2010, in the Washington, D.C. metropolitan area.

Larger earthquakes would be tied to correspondingly larger numbers of reports and expose larger numbers of people to the damaging effects.

Through the last two centuries, O`ahu has sustained infrequent earthquake damage. The two largest earthquakes occurring within roughly 100 km (60 mi) of Honolulu are an estimated magnitude-4.8 earthquake in 1948 and an estimated magnitude-7 in 1871. Without adequate instrumental recordings, it is difficult to pinpoint their locations, as well as their magnitudes, but the best interpretations place the 1871 earthquake off the western coast of Lana`i. Shaking from this earthquake is reported to have damaged every building on the campus of Punahou School. It is thought that the magnitude 4.8 1948 earthquake occurred somewhat closer to O`ahu.

Much of the ability to understand the causes of earthquakes comes from robust estimates of their hypocenters, or locations. Even with increased numbers of seismographs operating in the Hawaiian Islands now, it remains a challenge to precisely locate offshore earthquakes and to subsequently associate them with causative faults or other structures. In general, we associate earthquakes in Hawai`i, away from the active volcanic centers, with adjustments in the Earth's uppermost mantle to the weight of the Hawaiian Island chain.

A research project called Plume-Lithosphere Undersea Melt Experiment, or PLUME (http://igppweb.ucsd.edu/~gabi/plume.html#experiment), began in 2005 with the purpose of resolving larger scale and deeper structures in the mantle around Hawai`i that might account for these non-Hawai`i-Island earthquakes. Among the principal investigators of this project was Cecily Wolfe, a professor of geophysics at the University of Hawaii at Manoa. From 2005 to 2007, dozens of seismographic stations were deployed, both on the seafloor and on land. Research with these unique data has led to provocative results relating to large-scale Earth processes and imaging of the mantle hot spot that drives all Hawaiian volcanoes.

In addition, these instruments recorded numerous Hawaiian earthquakes. Wolfe and her student have also analyzed these earthquakes and are currently interpreting the earthquake distributions. Their findings will no doubt shed important light on earthquakes beneath O`ahu and the other Hawaiian Islands.

Kīlauea Activity Update

Over the past week surface flows were active in two areas on the coastal plain. As of Thursday, July 22, surface flows had crossed over a segment of road at the end of Highway 130 and reached about 700 m (770 yards) below the road. The flows are advancing south toward the higher topography of the Hakuma horst and continue to fill in the area around Highway 137. Flows were also active approximately 1 km (0.6 miles) above Highway 130. Repeated deflation/inflation (DI) events at Kilauea's summit, ongoing for the last several days, may cause the surface flows on the coastal plain to stall and restart repeatedly.

At Kīlauea's summit, a circulating lava pond deep in the collapse pit within the floor of Halema`uma`u Crater was visible via the Webcam throughout the past week. The baseline lava level was punctuated sporadically by short-lived lava-level increases. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Three earthquakes beneath Hawai`i Island were reported felt during the past week. A magnitude-2.8 earthquake occurred at 7:34 a.m. on Sunday, July 18, 2010, H.s.t., and was located 11 km (7 mi) northwest of Ka`ena Point at a depth of 9 km (6 mi). A magnitude-3.6 earthquake occurred at 3:27 p.m. on Sunday, July 18, and was located 19 km southeast of Diamond Head, O`ahu, at a depth of 32 km (20 mi). A magnitude-2.3 earthquake occurred at 2:59 a.m. on Tuesday, July 20, and was located 2 km (1 mi) southeast of Kilauea summit at a depth of 15 km (10 mi).

Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov.

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Updated: July 25, 2010 (pnf)