(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

The bits of dust kicked up by last Sunday’s magnitude-3.6 earthquake south of Oahu have settled. The earthquake occurred at 3:27 p.m. 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 Oahu 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 Hawaii’s earthquake activity is closely linked to our active volcanoes on the island of Hawaii, earthquakes do occur beneath other parts of the state. When such earthquakes do happen, they impact Hawaii’s larger population centers on Maui and on Oahu.

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 Oahu 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 Hawaii 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, Oahu 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 Oahu.

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 Hawaii, 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 Hawaii that might account for these non-Hawaii-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 Oahu and the other Hawaiian Islands.