Seismic streaks signal silent slip to the sea

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In the early hours of Monday, February 1, 2010, , a swarm of microearthquakes began to occur on the south flank of Kīlauea volcano.

The swarm started slowly, with only a few earthquakes per hour, and reached a peak of almost a dozen earthquakes per hour on that afternoon. By Tuesday afternoon, approximately 40 hours later, it was over. In total there were 85-90 microearthquakes in the swarm, many of them too small for computing a high quality location.

The earthquakes occurred in the central region of Kīlauea south flank, and were in a relatively narrow north-south streak, extending southward from Poliokeawe Pali. This region of the south flank has seen flurries of earthquake activity in the past, and these flurries have often been associated with a phenomenon known as a slow earthquake.

Slow earthquakes are a relatively recently identified process that was first documented in Hawai`i and then quickly recognized at other locations. The Hawai`i examples have all been located on Kīlauea Volcano's south flank. A slow earthquake is almost identical to regular earthquakes: the island moves in response to sudden slip on a fault. The only difference is that instead of lasting just a few seconds, the event lasts for many hours. These are often also called "silent earthquakes", because the slip does not excite seismic waves, thus there is no ground shaking. However, the earth movement can trigger many small regular earthquakes like those recorded this week.

Careful analysis of data by Hawaiian Volcano Observatory scientists indicate that between Sunday evening and Tuesday afternoon, the south flank indeed slipped southward. Stations that monitor ground motion include instruments that sense even very slight changes in ground tilt, and Global Positioning System (GPS) instruments that measure three-dimensional changes in position. Preliminary results from these stations suggest that the south flank moved at speeds up 5m per year--about 3 cm over 2 days.

This rate of ground motion is hundreds of times faster than the rate of the persistent slow slip of Kīlauea south flank, which has been occurring at rates of about 7 cm/yr (3 inches/yr) in the at least the last few decades. However, it still about a trillion times slower than the ground motion that occurs in a typical earthquake, where the full amount of slip happens within a few to tens of seconds. If the south flank slip that occurred on Mon-Tue had occurred at all at once, it would have been a magnitude 5+ earthquake.

In the past 13 years, the eastern part of Kīlauea south flank (Poliokeawe to Kalapana) has experienced at least six slow and five "fast" earthquakes with equivalent magnitudes of 5 or greater. The time interval between slow earthquakes has been increasing from about 750 days to almost 960 days between the most recent two.

Each of the slow earthquakes was accompanied by a swarm of microearthquakes located in north-south streaks, hence the peak in our interest when they occurred in the same pattern this past week. Close study of the slow earthquake and swarm sequences revealed that the swarms actually followed the slow earthquakes .The microearthquakes were a response to the stress perturbations caused near the slow earthquake.

Whereas nearly instantaneous movement on these faults has been responsible for damaging earthquakes (most recently in 1975 and 1989), flank movement is now also understood to occur in almost unnoticed slow jerks as it did earlier this week. While study of slow earthquakes has just begun, it appears to be a benign way to relieve volcano stress.

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Volcano Activity Update

Surface flows have been active above and within the Royal Gardens subdivision. By midweek, lava had crossed through the western part of Royal Gardens and had reached the base of the pali. A deflation/inflation cycle at Kīlauea's summit, however, started on Tuesday, and may cause these flows to slow or stop. Surface flows in the same general area will likely be renewed when the volcano re-inflates.

At Kīlauea's summit, the lava surface deep within the collapse pit, inset within the floor of Halema`uma`u Crater, was sporadically visible via webcam. The deflation phase of the deflation/inflation cycle caused the lava to retreat to a deeper level, but it will likely rise again after the volcano begins to inflate. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Two earthquakes beneath Hawai`i Island were reported felt during the past week. A magnitude-3.5 earthquake occurred at 9:06 p.m. on Monday, February 1, 2010, H.s.t., and was located 1 km (1 miles) northwest of Pahala at a depth of 37 km (23 miles). A magnitude-2.7 earthquake occurred at 4:13 p.m. on Tuesday, February 2, and was located 26 km (16 miles) northwest of Kailua-Kona at a depth of 36 km (22 miles). On Wednesday morning, several Kona residents reported feeling vibrations that were not associated with an earthquake, but which may have been produced by a meteorite, according to the Infrasound Laboratory of the University of Hawai`i.