Volcano Watch — Did you feel that earthquake? Probably not…

Probably not. That's because the event that started on May 28, 2012, 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 Hawai‘i, slow earthquakes occur on a large fault about 10 km (6 mi) below Kīlauea'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 26–30 months. The last Kīlauea slow earthquake occurred during February 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 5–10 km (3–6 mi) beneath Kīlauea's south flank, just north of Halapē. 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 underway.

During the past week, Kīlauea's south flank slid about 4 cm (1.5 in) towards 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 Kīlauea'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 Kīlauea'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 Kīlauea, and earthquake processes in general. Stay tuned to this column for details on what we learn from this fascinating event!

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Volcano Activity 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 60–80 m (200–260 ft) below the floor of Halema‘uma‘u Crater and visible by HVO's Webcam, dropped to a comparatively low level, roughly 120–140 m (390–460 ft) below the crater floor, last weekend. By mid-week, the level had recovered, and the lava lake was again about 60 m (200 ft) below the Halema‘uma‘u Crater floor.

On Kīlauea'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 meters (yards) out from the base of the pali by mid-week. There was no ocean entry as of Thursday, May 31, and, based on recent patterns, the surface flows are not expected to reach the ocean any time soon.

Two earthquakes were reported felt beneath Hawai‘i Island this past week. A magnitude-3.5 earthquake occurred on Saturday, May 26, at 2:11 a.m., HST, and was located 13 km (8 mi) west of Pāhala at a depth of 7 km (4 mi). A magnitude-3.3 earthquake occurred on Tuesday, May 29, at 8:21 p.m., HST, and was located 14 km (9 mi) south of Volcano at a depth of 11 km (7 mi).