Recent earthquake is a small piece to a much larger puzzle

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On April 2, 2011, at 11:11 a.m. HST, a magnitude-3.6 earthquake occurred 9 km (5 mi) southwest of Volcano Village. Following such an earthquake, the Hawaiian Volcano Observatory (HVO) and the United States Geological Survey (USGS) ask, "Did you feel it?"

For the April 2 earthquake, the answer to the above question was, "Evidently not," as we received no responses on the "Did You Feel It?" Web page.

So, the question to ask now is, "Why not?"

HVO seismic analysts who reviewed the data from this earthquake verified that the earthquake occurred in a part of Kīlauea volcano known as the Koa‘e fault system.

The Koa‘e system lies south of Kīlauea's summit caldera, extending southwest-to-northeast between Kīlauea's southwest and east rift zones. It is delineated by a series of cracks and mostly north-facing pali (cliffs). The most striking of these pali is possibly the one that visitors to Hawai‘i Volcanoes National Park cross as they drive along Hilina Pali Road.

The reason for the lack of "Did You Feel It?" responses may have been that the magnitude-3.6 earthquake was located at a depth of just 200 meters (650 ft) below the Earth's surface. This is unusually shallow for an earthquake occurring outside Kīlauea's summit caldera or rift zones.

Given its magnitude and depth, the earthquake would most likely have been felt only by people in very close proximity to the epicenter. Seismic energy released by the earthquake would have been severely dampened in the shallow crust.

Recent InSAR (Interferometric Synthetic Aperture Radar) results clearly show a signal from the April 2 earthquake. The north side of the Kulanaokuaiki pali (eastern Koa‘e fault zone) dropped relative to its south side by 1–2 centimeters (0.4–0.8 in) over a distance of 900 m (0.5 mi) along the pali. InSAR, an exciting and rapidly evolving volcano monitoring technology afforded by satellites orbiting around the Earth and sweeping its surface with radar signals, is described in more detail in another Volcano Watch article.

This week, HVO scientists resurveyed points along Kulanaokuaiki pali that were originally set up more than 40 years ago and measured repeatedly in the years since. They found that the pali had moved up relative to the north side by nearly 2 cm (0.8 in), confirming the InSAR interpretation.

While a magnitude-3.6 earthquake is usually large enough to be felt, it is somewhat unusual that it would lead to ground disruption large enough to be visible in an InSAR image. Evidently, the shallowness of the April 2 earthquake, in addition to improvements in HVO's InSAR capability, allowed the earthquake effects to be seen in the InSAR image, as well as in direct measurements of surface motions.

Because of its location amidst Kīlauea's volcanically active summit caldera, rift zones, and mobile south flank, the Koa‘e fault system has long been viewed as one of the most complex features of the volcano. For example, some scientists have suggested that the Koa‘e fault system overlies a large zone of magma accumulation or that it is a connector between the east and southwest rift zones, which forms the northern limit of Kīlauea's mobile south flank. Others have suggested that the Koa‘e faults provide evidence that a larger summit caldera complex may have existed in Kīlauea's earlier history.

While they may not solve large problems related to Kīlauea's history and evolution, individual magnitude-3.6 earthquakes and InSAR images add useful pieces of data to HVO's existing collections. More importantly, perhaps, they remind us of our unprecedented and improving capabilities to work on the wonderfully complex puzzles that Kīlauea presents.

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Volcano Activity Update

Lava continued to erupt within Pu‘u ‘Ō‘ō over the past week, feeding a small lava lake in the center of the crater. Small overflows onto the surrounding crater floor added to the levees that confine the lake, but lava remains about 85 m (280 ft) below the east rim of Pu‘u ‘Ō‘ō. No lava is erupting outside the crater.

A small lava lake was also present deep within the Halema‘uma‘u Overlook vent during the past week. At times, the lava level fluctuated gently over periods of tens of minutes to a few hours due to sporadic rise-and-fall cycles. Volcanic gas emissions remain elevated, resulting in relatively high concentrations of sulfur dioxide downwind.

Two earthquakes beneath Hawai‘i Island were reported felt this past week. A magnitude-3.3 earthquake occurred at 7:56 a.m. HST on Saturday, April 16, 2011, and was located 11 km (7 mi) southwest of Lō‘ihi Seamount at a depth of 13 km (7 mi). A magnitude-2.9 earthquake occurred at 8:19 p.m. on Monday, April 18, and was located 8 km (5 mi) southwest of Ho‘okena at a depth of 43 km (27 mi).