Volcano Watch — Hawai'i Earthquake Machine Rumbles On

Last year we received reports of earthquake effects (what we refer to as earthquake "felt reports") for 78 earthquakes. Because of their relatively low magnitudes, many of these earthquakes affected only small, localized areas and were often described in these reports as little jolts.

The largest earthquake of the past year occurred on August 26 at 8:24 p.m. Hawaiian Standard Time. This magnitude-5.0 earthquake was located beneath Kīlauea volcano's south flank at a depth of 10 km (6.2 miles).

The vast majority of earthquakes-consistent with previously recorded patterns of earthquake behavior-occurred beneath the southeastern portion of Hawai'i Island. They are related to the active volcanoes Kīlauea, Mauna Loa, and Lo'ihi submarine volcano offshore to the southeast. Based on details of earthquake location or timing, we understand that many of these earthquakes are related to magma transport within the volcanoes. The others reflect structural adjustments that are not thought to be related directly to magma movement but, rather, to the complex interaction among volcanic processes.

The rest of the year's earthquakes, including the 2 next largest-a magnitude-4 on January 6 at 4:37 p.m. and a magnitude-4.1 on July 8 at 9:36 a.m.-were apparently not related to the active volcanoes (for example, we located the July 8 earthquake over 100 km (62 miles) southwest of South Point.) More likely, they are associated with the enormous mass of the island atop the Earth's crust. As the Earth's crust adjusts to this load, earthquakes like these occasionally occur.

While we can describe earthquake locations and other aspects of their behavior in fine detail, we are still trying to understand some important and basic aspects of earthquakes.

The rate or frequency of earthquakes, for example, has been relatively steady at roughly 1,700 earthquakes of magnitude 1.5 or larger per year over the past 5 years. Our annual earthquake catalogs contain 1,386 earthquakes of magnitude 1.5 or larger for 1999, 2,067 for the year 2000, 1,459 for the year 2001, and 1,686 for the year 2002. This makes sense, as most of the earthquakes are related to our active volcanoes, and the volcanic activity has been relatively steady through this period.

We are looking more closely, however, at specific earthquake source regions where there is a great deal more variability in the rate of earthquake activity.

With USGS and other colleagues, we are trying to understand the variations in the rates at which earthquakes are produced in different regions of Hawai'i Island. We are initially focusing on Kīlauea, given the data and observations collected from Hawai'i's natural volcanological laboratory at HVO.

To describe earthquake rates, we use a mathematical model developed from laboratory studies of earthquake faulting. In simplest terms, these are measurements of forces required to push two pieces of rock past each other, under controlled laboratory conditions.

Through the measurements and subsequent mathematical and computer modeling, we recognize that the rate at which earthquakes occur can be described and, more importantly, calculated in terms of an earthquake-generating stress. We found that changes in earthquake rate are related to changes in earthquake-generating (or "Coulomb") stress. We can convert our observed changes in rates of earthquake occurrence into changes in Coulomb stress.

We have divided Kīlauea into a number of blocks, or cells. From the earthquake catalog, we have calculated stress changes in each cell and made maps of the changes resulting from the 1977 and 1983 intrusions and eruptions in Kīlauea's east rift zone.

With a model consistent with stress changes calculated from earthquake rates, we can account for other measurements made during these events. Immediately adjacent to the rift zone, the intrusions and eruptions produce small decreases in stress. At greater distances from the rift zone, their net effect is a stress increase as the rift zone expands and slip occurs on a deeper fault underlying Kīlauea's south flank.

We expect that we will modify and adjust our models of earthquake and volcanic behavior as we observe and learn more about how our volcanoes, and other volcanoes in general, work. Meanwhile, the island of Hawai'i's earthquake machine rumbles on.

Volcano Activity Update

Eruptive activity at the Pu`u `O`o vent of Kīlauea Volcano continued unabated during the past week. Most lava flows have been along the rootless shield complex along the Mother's Day lava tube. Vents within the crater of Pu`u `O`o remain incandescent. No active flows are on Pulama pali or the coastal flat below Paliuli. No lava is entering the ocean.

One earthquake was felt on the island during the week ending early February 19. A magnitude 3.0 earthquake took place at 6:55 p.m. on February 16, located 11 km (7 miles) northeast of Captain Cook. It was felt at Captain Cook and Kealakekua.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with no earthquakes located in the summit area during the last 7 days.