Categorized | Featured, Sci-Tech, Volcano

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

This map shows seismicity (white circles) and deformation from GPS stations (black arrows) that occurred at Kilauea during the week of May 28, 2012.  GPS arrow length indicates the magnitude of motion according to the scale in the lower right, and earthquake circle size indicates the earthquake magnitude according to the key in the upper right.  In just a few days, the coast moved by up to 4 cm (1.5 in) towards the sea, and a swarm of small earthquakes (less than M3.5) occurred—hallmarks of a slow earthquake! Map courtesy of USGS/HVO

This map shows seismicity (white circles) and deformation from GPS stations (black arrows) that occurred at Kilauea during the week of May 28, 2012. GPS arrow length indicates the magnitude of motion according to the scale in the lower right, and earthquake circle size indicates the earthquake magnitude according to the key in the upper right. In just a few days, the coast moved by up to 4 cm (1.5 in) towards the sea, and a swarm of small earthquakes (less than M3.5) occurred— hallmarks of a slow earthquake! Map courtesy of USGS/HVO

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Residents of Hawai`i 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 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 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 26–30 months. The last Kilauea 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 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 underway.

During the past week, Kilauea’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 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!

One Response to “Volcano Watch: Did you feel that earthquake? Probably not…”

  1. dutchsinse says:

    so now we’re calling earthquake swarms “slow earthquakes”… LOL! epic..

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