What happens when the earthquake ‘lights’ go out?
As we enter the 2012 holiday season, Kilauea Volcano’s continuing eruptions in Halema‘uma‘u at the summit and at Pu‘u ‘O‘o on the middle east rift zone keep providing us at the U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO) with much to study and understand. HVO puts a great deal of effort into tracking earthquakes.
Not only do we want to understand the hazards posed by infrequent large earthquakes, like those that have damaged our communities in 2006 most recently, we also want to know what the many small earthquakes tell us about processes taking place within the volcanoes and what might happen next. Through time, we look for earthquake patterns — areas where earthquakes most frequently occur — to help build a working model of Kilauea.
When a more-than-usual number of earthquakes occur in one of these areas, the earthquakes “light up” our computer monitors and sometimes suggest the possibility of imminent changes in the eruption. Recently, HVO has been closely following earthquake activity beneath Kilauea’s upper east rift zone, between the summit caldera and Pauahi Crater.
This section underlies the upper part of the Chain of Craters Road in Hawaii Volcanoes National Park, where dots on the map indicate the locations of individual pit craters Lua Manu, Puhimau, Koko‘olau, Hi‘iaka and Pauahi.
As mentioned in the last several Volcano Watch articles, increased numbers of earthquakes beneath this section of Chain of Craters Road have reflected increased magma pressure along the upper east rift zone. Swarms of earthquakes in that area have also preceded earlier eruptions. In early October, HVO noted a clear increase in the number of upper east rift zone earthquakes.
The earthquake rate increased through the month, reaching peaks of more than 40 earthquakes per day and 150 earthquakes per week. These numbers are the largest for any time period during 2012. They exceeded even those for Kilauea’s eruptive episodes in 2007 and 2011, although it should be noted that the seismicity for some of those episodes was focused farther downrift. As we monitored the concentrations of earthquakes beneath the upper east rift zone in October 2012, HVO revisited possible eruption scenarios. Field surveys were conducted and a temporary seismic station was installed essentially on top of the earthquake cluster.
But, soon after Nov. 1, the number of earthquakes dropped quite abruptly. Our earthquake maps were no longer “lit” by the brightly colored symbols we use to indicate recent earthquakes. The relatively sudden decrease in upper east rift zone earthquakes in early November — and overall low levels of earthquake activity elsewhere beneath the island — was somewhat unexpected.
The absence of earthquakes, both where activity had been so high and in areas that typically light up with earthquakes during rapid magma movement, was striking and, ironically, unsettling.
Changes in earthquake rates are related to changes in Coulomb stresses, the stresses that cause earthquakes. Changes in the Coulomb stress only increases or decreases the abundance of earthquake activity in a region, but does not necessarily produce larger earthquakes. The earthquakes cluster at subsurface locations where the stress changes are happening.
Observations of the rising lava lake within the Halema‘uma‘u vent at Kilauea’s summit, tilt measurements of ground surface deformation around the summit region, and the large number of earthquakes along Kilauea’s upper east rift zone in October all support the interpretation that pressure had increased within the summit and upper east rift zone due to the addition of magma.
The subsequent drop in earthquake activity, reflecting a decrease in Coulomb stress along the upper east rift zone, suggests that the additional magma has moved to different parts of the volcano. If this is the case, it is not clear where the magma has gone. It could be that stress changes related to the magma movement were not large enough to produce more earthquakes, or it could be too early for changes in earthquake activity in these areas to be seen.
So, we continue to carefully watch HVO’s volcano monitoring data streams and to be mindful of the range of possible events that could happen next on the volcano.
Kilauea activity update
A lava lake within the Halema‘uma‘u Overlook vent produced night-time glow that was visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. Since achieving an unusually high level during October, the lava lake has dropped to a deeper level within the Overlook crater.
Over the past week, the lava level was relatively steady, but rose briefly during a few short-lived rise-fall cycles. On Kilauea’s east rift zone, surface lava flows are still slowly moving across the coastal plain. As of Tuesday, Nov. 20, the flow front had reached to within 575 feet of the ocean and was advancing steadily, straddling the Hawaii Volcanoes National Park boundary.
Within the Pu‘u ‘O‘o crater, the northeastern pit still holds a small lava lake, and occasional small flows were erupted from a few openings near the southeastern edge of the crater. One earthquake beneath Hawai‘i Island was reported felt in the past week. On Nov. 18 at 6:26 a.m., a magnitude-3.2 earthquake occurred 11 miles northwest of Naalehu at a depth of 8 5 miles.
Visit the HVO website (http://hvo.wr.usgs.gov) for detailed Kilauea, Mauna Loa, and Hualalai activity updates, recent volcano photos, recent earthquakes, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey‘s Hawaiian Volcano Observatory.
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