Earthquakes can give you that sinking feeling

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Most of us who were in the islands on October 15, 2006, are familiar with ground motion associated with earthquakes. On that day, two quakes, both above magnitude-6, shook the east side of Hawai`i Island and were felt throughout the state. In addition to the obvious shaking, the fault slip that causes earthquakes often also causes permanent deformation of the Earth's surface.

Permanent motion due to earthquakes can be measured using a variety of techniques, including the Global Positioning System (GPS, which uses satellites to pinpoint the location of a receiver on the ground) and radar interferometry (InSAR, which computes surface changes using satellite data).

The largest and most rapid surface motion is caused by strong, shallow earthquakes, like the 2004 magnitude-9 Sumatra-Andaman earthquake in Indonesia. It caused a tsunami that affected the entire Indian Ocean and resulted in the deaths of about 230,000 people. During that event, the sea floor above the fault that caused the earthquake moved by at least 15 meters (49 feet) in a matter of seconds.

Earthquakes in Hawai`i also cause permanent deformation of the surface. During the few moments of the 1975 magnitude 7.2 earthquake on the south flank of Kīlauea Volcano, Halape, on Kīlauea's southern coast, moved over 8 meters (24 feet) seaward.

Given these examples, one might expect that the October 15, 2006, earthquakes also caused major ground motions in addition to violent shaking. GPS and InSAR results, however, show that the most significant permanent regional change of Hawai`i Island was only about 1 centimeter (less than half an inch). The small amount of motion is a result of the offshore locations, great depths (both quakes were below 20 km, or 12 miles), and relatively small magnitudes of the earthquakes.

InSAR measurements of Hawai`i Island, however, did reveal a type of deformation that had never been seen before. InSAR data covering the October 15, 2006, earthquakes show three small-scale patches of subsidence, where regions about 2-4 km across sank by 6-10 centimeters (2-4 inches). One of these regions includes Kiholo Bay, near the epicenter of the first earthquake; the second region is along the coast immediately makai of the Kona airport at Keahole Point; the third region is high on the southwest flank of Mauna Loa.

Understanding why the sinking occurred is difficult without additional information, which is not available (there were no instruments on the ground in the locations that subsided). Nevertheless, perhaps the most likely cause involves a well-known earthquake effect: liquefaction.

When loose ground - for example, a thick deposit of unpacked soil - is shaken during a quake, the ground can liquefy and subside. Liquefaction is a common problem in many earthquake-prone areas, including San Francisco, California, where ground that has been reclaimed from the bay suffered severe shaking during the 1989 Loma Prieta earthquake.

Liquefaction seems an unlikely cause of the October 15, 2006, subsidence patches, given that the ground composition in areas that subsided is solid lava – exactly the opposite of the type of ground prone to liquefaction. The material beneath the lava, however, may be composed of loose sand (the subsidence patches near the coast) or ash deposits (the subsidence patch on Mauna Loa). Liquefaction of these buried deposits could cause the lavas above to sink in response as support is removed from below.

Small-scale subsidence probably occurs during many earthquakes, both in Hawai`i and on the mainland. The lack of major broad-scale permanent deformation during the October 15, 2006, earthquakes, combined with Hawai`i's unique surface geology, made it possible to see these small patches, which might otherwise have been missed. While interesting, these unique ground motions fortunately do not represent a hazard that will affect those of us who live in Hawai`i. Instead, they are an example of what can be learned through careful observation of the Earth and may prove important to understanding earthquake effects, both in Hawai`i and elsewhere around the world.

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Volcano Activity Update

Kīlauea summit and Pu`u `O`o continued to deflate. Seismic tremor levels continued to be low. Earthquakes were located beneath Halema`uma`u Crater, the south flank area, and the lower southwest rift zone.

The July 21 eruption remains active. The erupting lava flows through an open lava channel and into a lava pond about 2/3 miles from the vent. In the past week, the lava channel has partially drained from its previous full and frequent overflowing status, exposing high channel walls. The slow-moving `a`a flow, fed by lava seeping from a rupture at the base of the pond wall and heading toward Pu`u Kia`i, had stagnated.

For the last several days, most of the lava is accumulating in the area just north of Kupaianaha shield. Frequent, small pahoehoe overflows from the channel and pond have caused them to become perched up to 50-60 feet above the surrounding lava surface. Upward growth of the channel and pond has decreased the slope of the lava channel to nearly horizontal. This has led to even more overflows, including small overflows right at the vent itself.

As of Thursday, September 20, fissure D continues to supply a lava channel about 1 km (0.6 miles) long. The channel filled, and continued to rise until it burst at the east margin, sending a new `a`a flow toward the northeast yesterday. The flow is confined to previous flows from this eruption. Another overflow is supplying an advancing `a`a flow along the southern margin of earlier flows.

At Pu`u `O`o, no incandescence has been seen on the Webcam at night for the last few weeks. The heavy fume coming from Pu`u `O`o completely obscures any view into the crater. As has been seen in years past, Pu`u `O`o could be acting as temporary storage for lava that passes beneath the cone on its way to the erupting fissure. There have also been a number of collapses in Pu`u `O`o crater since late August, and cracks on the north rim and south flank of the cone seem to be widening.

Vent areas are hazardous. Access to the eruption site, in the Pu`u Kahauale`a Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).

No earthquakes beneath Hawai`i Island were reported felt within the past week.

Mauna Loa is not erupting. No earthquakes were located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.