# Volcano Watch — "Slow" earthquake expected this week on the Big Island

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After the jolting wake-up call of October 15, 2006, most of us in the islands gained first-hand experience with what a large earthquake feels like.

In January 2005, a silent ("aseismic") earthquake struck Hawaii's Kīlauea Volcano. The red arrows correspond to the motions of individual GPS stations (black triangles) during that two-day event, which triggered a swarm of very small seismic earthquakes (black circles). Scientists want to find out if silent events in other parts of the world also generate tiny earthquakes, information that one day may prove useful in seismic hazard forecasting. Credit: Emily Desmarais/Stanford University.

(Public domain.)

The paired Kiholo Bay and Mahukona earthquakes caused an estimated 200 million dollars in damage and frayed the nerves of island residents. Unfortunately, there is no way to know when earthquakes like those of last fall will strike. Earthquake prediction remains an elusive objective for scientists, and there is no indication that the goal will be reached soon.

However, a different class of earthquakes, popularly referred to as “slow earthquakes,” has been successfully predicted. In fact, a slow earthquake has been predicted to occur on the south flank of Kīlauea Volcano this very week!

An earthquake occurs as two masses of rock within the earth move against each other along a fault. This motion usually takes place in a matter of seconds, producing the shaking that most of us associate with earthquakes. But what happens if the motion along the fault occurs slowly, over the course of a few days? These “slow” earthquakes do not generate perceptible shaking, and can only be detected by sensitive seismometers and surveying equipment.

In the last decade, slow earthquakes have been recognized all over the world, including Japan, Mexico, and the Pacific Northwest of the USA and Canada. Curiously, some of these areas have slow earthquakes that recur on a regular timescale. In the Pacific Northwest, for example, slow earthquakes occur every 14 months (give or take a few weeks). Such periodicity is unusual in nature, and it is unclear why a repetitive cycle of slow fault slip exists. Scientists have taken advantage of the periodicity by establishing temporary networks of seismic and continuous surveying stations to record ground motion and small earthquakes associated with the events. By studying slow earthquakes, it may be possible to gain insights into how and why the more destructive variety of earthquakes occur, the first step on the road to predicting such earthquakes.

In 2002, scientists from the Hawaiian Volcano Observatory used ground motion data from a network of continuously operating Global Positioning System stations to identify a slow earthquake that occurred in 2000 at Kīlauea, the first time a slow earthquake had been documented at a volcano. Since that time, scientists at HVO, Stanford University, and the University of Hawaii have worked together to identify several more slow earthquakes, the most recent of which occurred in January 2005. The 2005 event occurred over a period of about 2 days, during which time the south flank of Kīlauea moved seaward by roughly 3 centimeters (1 inch). If the fault had slipped suddenly over the course of a few seconds, the motion would have generated a magnitude-6 earthquake!

Scientists from the University of Hawaii further noted that the slow earthquakes at Kīlauea repeated at regular intervals, every 774 days (give or take a week), to be exact. Given that the last slow earthquake occurred on January 26, 2005, the next event should occur within a week of March 17, 2007.

With knowledge that a slow earthquake was likely to occur in mid-March and with permission from the Hawaii Volcanoes National Park, the U.S. Geological Survey, Stanford, UH, and the University of Wisconsin worked together during the first few months of 2007 to deploy a large network of seismometers, GPS receivers, and tiltmeters, supplementing the equipment that was already in place on the south flank of Kīlauea Volcano. The large dataset that will result from the focused deployment of new and existing equipment will aid in the understanding of how and why slow earthquakes occur, and will provide new insights into the subsurface structure of Kīlauea Volcano. This information is critical for assessments of volcanic and earthquake hazards.

As of this writing on Thursday, March 22, the predicted slow earthquake has yet to occur. Perhaps it is running a little late. Perhaps the periodicity was upset by some other unrelated volcanic or seismic event. Or perhaps by the time you read this during the weekend, the slow fault slip will have taken place (pretty much on schedule!). Even if the event does not happen as predicted, a slow earthquake will almost certainly occur at Kīlauea soon, based on the fact that at least 7 such events have been documented since 1998. When that day comes, the new equipment ensures that we will not miss the opportunity to learn more about how and why these interesting events take place, and what role they play in the evolution of the Big Island.

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### Volcano Activity Update

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Puu Oo to the ocean. About 1 km south of Puu Oo, the Campout flow branches off from the PKK tube. The PKK tube appears to no longer be active below the top of Pulama pali, but is feeding a persistent breakout that is often visible after dark streaming down the face of the pali. The Campout tube, on the other hand, is the source for the dominant ocean entry at Kamokuna, as well as for a much smaller ocean entry at East Laeapuki. Both ocean entries are located inside Hawaii Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube, in addition to the PKK tube, have been seen on the slope of Pulama pali and on the coastal plain. A branch of the Campout tube below the pali continues to host minor surface flows inland from the sea cliff at East Laeapuki and on the lava delta below.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

One earthquake beneath Hawaii Island was reported felt within the past week. A magnitude-2.2 earthquake occurred at 8:20 a.m. H.s.t. on Wednesday, March 21, and was located 6 km (4 miles) west-southwest of Waimea at a depth of 32 km (20 miles). It is the most recent of nearly 500 aftershocks from the October 15, magnitude-6.7, earthquake.

Mauna Loa is not erupting. Ten earthquakes were located beneath the volcano during the week ending March 21. They occurred at a depth of 40 km (23 miles) or more, and are the long-period type, with magnitudes less than 3. Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.