Did you feel that earthquake? Probably not …

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 began Monday was a “slow earthquake.”

Most earthquakes are caused by motion along faults, when patches of rock slide against each other. 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 “slow” moniker.

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 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 six 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 between February 1 and 3, 2010 — 28 months ago — so we were due for another one.

Early on Memorial Day, scientists at the Hawaiian Volcano Observatory noticed a flurry of small — less than 3.5 magnitude — earthquakes about three to six 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 a slow earthquake was under way.

During the week, Kilauea’s south flank slid about 11⁄2 inches toward the sea — motion that would have resulted in a roughly magnitude 5.5 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 significant, since they don’t have the same obvious impact on our daily lives as 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 activity update

A lava lake within the Halemaumau Overlook vent resulted in a nighttime glow visible from the Jaggar Museum overlook. The lake, which, through much of May, has been about 200 to 260 feet below the floor of Halemaumau Crater and visible by HVO’s webcam, dropped to a comparatively low level, roughly 390 to 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 Halemaumau Crater floor.

On Kilauea’s east rift zone, surface lava flows on the pali and coastal plain stagnated last weekend. Activity started again early in the week, and breakouts from the tube sent new flows pouring down the Pulama pali. Lava reached the coastal flats and advanced several hundred yards out from the base of the pali by mid-week. There was no ocean entry as of Thursday, and, based on recent patterns, the surface flows are not expected to reach the ocean any time soon.

Two earthquakes were reported felt beneath Hawaii Island this past week. A magnitude 3.5 earthquake occurred at 2:11 a.m. May 26 and was located eight miles west of Pahala at a depth of four miles. A magnitude 3.3 earthquake occurred at 8:21 p.m. Tuesday and was located nine miles south of Volcano at a depth of seven miles.

Visit 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.