(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
In the early hours of Monday, Feb. 1 a swarm of microearthquakes began to occur on the south flank of Kilauea 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 all 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 Kilauea’s 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.”
The term “slow earthquakes” describes a relatively recently identified process that was first documented for volcanoes in Hawaii and is also recognized at other locations. The Hawaii examples have all been located on Kilauea 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 16 feet per year — about 1.2 inches over two days.
This rate of ground motion is hundreds of times faster than the rate of the persistent slow slip of Kilauea’s south flank, which has been occurring at rates of about 3 inches/yr in 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 Monday-Tuesday had occurred all at once, it would have been a magnitude 5+ earthquake.
In the past 13 years, the eastern part of Kilauea’s 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 two most recent ones.
Each slow earthquake was accompanied by a swarm of microearthquakes located in north-south streaks (why they piqued 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.
“Slow earthquakes,” which we have only just begun to study, appear to be a benign way to relieve volcano stress.
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