Residents of Hawaii are no strangers to earthquakes. Those of us on the Big Island, especially, are used to feeling several earthquakes every year. But did you feel the magnitude-5.5 earthquake on Memorial Day?
Residents of Hawaii are no strangers to earthquakes. Those of us on the Big Island, especially, are used to feeling several earthquakes every year. But did you feel the magnitude-5.5 earthquake on Memorial Day?
Probably not. That’s because the event that started on Monday was a “slow earthquake.”
Most earthquakes are caused by motion along faults, when patches of rock slide against each another. This motion usually occurs over the course of a few seconds for small-magnitude events or several minutes in the case of large earthquakes. These rapid motions generate seismic waves, which can travel great distances, and, when strong enough, damage buildings, roads and other infrastructure.
Slow earthquakes, on the other hand, occur over the course of several days. The events are still caused by slip along faults, like regular earthquakes, but the motion happens so slowly that no strong shaking is generated — thus, the name “slow” earthquakes.
The existence of slow earthquakes has been known for decades.
Some of the first well-documented events were detected with sensitive surface deformation-monitoring instruments in the 1990s along the San Andreas Fault in California. Starting in 1999, data from Global Positioning System (GPS) stations helped to identify slow earthquakes in Japan, Mexico, Alaska, Costa Rica and the Pacific Northwest of the United States and Canada.
In many of these regions, slow earthquakes occur repeatedly and some are even periodic. For example, in the Pacific Northwest, they occur every 14 months (give or take a few weeks).
Although first detected by GPS, we now recognize that slow earthquakes are often accompanied by seismic tremor, probably caused by the “noise” of fault motion. Therefore, monitoring both seismic and deformation changes has made it comparatively easy to detect slow earthquakes.
In Hawaii, slow earthquakes occur on a large fault about 6 miles below Kilauea’s south flank, at the boundary between the volcano and the underlying ocean crust — the same fault that caused the 1975 magnitude-7.7 Kalapana earthquake. Slow earthquakes take place along this fault about once every 26 to 30 months. The last Kilauea slow earthquake occurred during Feb. 1-3, 2010 — 28 months ago — so we were due for another one.
Early on Memorial Day, scientists at the Hawaiian Volcano Observatory (HVO) noticed a flurry of small (less than magnitude-3.5) earthquakes about 3 to 6 miles beneath Kilauea’s south flank, just north of Halape. Past slow earthquakes have been accompanied by seismic activity in the same area, so the Memorial Day swarm was a clue that the expected slow earthquake might be happening. Indeed, deformation measurements from tilt and GPS stations on the coast soon confirmed that a slow earthquake was under way.
During the past week, Kilauea’s south flank slid about 1.5 inches toward the sea — motion that would have resulted in a roughly 5.5-magnitude earthquake, had it happened all at once.
What is not yet confirmed about Kilauea’s slow earthquakes is whether or not seismic tremor accompanies the events. Fortunately, scientists from the University of Wisconsin worked with HVO to deploy an array of seismometers on Kilauea’s south flank in anticipation of the event. Now that the slow earthquake has happened, the data analysis can begin.
Slow earthquakes may not seem like significant events, since they don’t have an obvious impact on our daily lives (unlike large earthquakes or volcanic eruptions), but they represent a form of Earth’s behavior that was unknown until just a few years ago. We hope that by better understanding slow earthquakes, we’ll learn more about earthquake cycles and, eventually, how large, damaging earthquakes, such as the 1975 Kalapana event, are initiated.
The 2012 Memorial Day slow earthquake is a potential watershed event to better understand Kilauea, and earthquake processes in general. Stay tuned to this column for details on what we learn from this fascinating event!
Kilauea update
A lava lake within the Halema‘uma‘u Overlook vent during the past week resulted in night-time glow that was visible from the Jaggar Museum overlook. The lake, which through much of May has been about 200-260 feet below the floor of Halema‘uma‘u Crater and visible by HVO‘s webcam, dropped to a comparatively low level, roughly 390-460 feet below the crater floor, last weekend. By mid-week, the level had recovered, and the lava lake was again about 200 feet below the Halema‘uma‘u Crater floor.
Two earthquakes were reported felt beneath Hawaii Island this past week. A magnitude-3.5 earthquake occurred on Saturday, May 26, at 2:11 a.m., 8 miles west of Pahala at a depth of 4 miles. A magnitude-3.3 earthquake occurred on Tuesday at 8:21 p.m. 9 miles south of Volcano at a depth of 7 miles.
Visit the HVO website (http://hvo.wr.usgs.gov) for detailed Kilauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey‘s Hawaiian Volcano Observatory.