Volcano Watch — Earthquakes provide warnings of volcanic activity

Seismic monitoring is perhaps the most commonly used and well-known method for tracking volcanic activity—and for good reason. The very movement of magma beneath the surface causes earthquakes, both due to breaking of rock from magma pressure and from the fluid motion itself. Tracking the number, magnitude, depth, and type of earthquakes at a volcano provides an indication of where magma is located and whether or not it is rising toward the surface.

While simple in concept, however, volcano seismology is much more complicated in practice. For example, earthquakes associated with volcanic activity come in many different styles, depending on the characteristics of the earthquake waves. Volcano-tectonic (VT) earthquakes have high rates of shaking, while long-period (LP) and very-long period (VLP) earthquakes have lower rates of shaking. These earthquake types are analogous to sound waves. Very short-period acoustic waves, like chaotic radio static, are like VT earthquakes, whereas the longer-period oscillations in pitch produced by a fire engine's siren exemplify LP and VLP earthquakes.

VT, LP, and VLP earthquakes also suggest different sources. VT seismicity is often associated with breaking rock, which might occur at the front of a propagating magmatic intrusion. In contrast, LP and VLP earthquakes are thought to reflect fluid oscillations in subsurface magma and gas.

In addition to discrete earthquakes, volcanoes also produce continuous, low-level seismic tremor, which can be considered the background noise of a volcano. Tremor tends to increase before an eruption and decrease during periods of no eruption and is, therefore, a useful indicator of the level of activity of a given volcano.

Locating tremor and earthquake sources is critical for mapping a volcano's magma plumbing system and assessing volcanic activity. For example, tremor, LP, and VLP earthquakes have been used to map magma storage areas beneath Kīlauea's summit, while migration of VT events is commonly used to track magma migration along Kīlauea's east rift zone, and even magma ascent prior to eruptions.

A network of seismometers capable of measuring very small levels of ground motion is required to locate an earthquake. Like earthquakes themselves, seismometers come in many varieties, each of which is sensitive to different types of ground shaking. Short-period seismometers are the least expensive and are excellent at detecting VT earthquakes. Broadband seismometers are more expensive but can detect LP and VLP earthquakes.

Kīlauea and Mauna Loa were the first volcanoes in the United States to be monitored by a modern seismic network, which was installed in the 1950s and continues to be expanded today. Thanks to funding from the American Recovery and Reinvestment Act (ARRA), HVO's seismometers are currently being upgraded to all digital instruments with a mix of short-period and broadband sensors.

Using the wealth of seismic data recorded by HVO, seismologists have learned much about the structure of Hawai‘i's volcanoes. For example, a persistent source of VLP and LP earthquakes and tremor occurs about 1 km (0.6 mi) beneath Kīlauea's summit, indicating the presence of a shallow magma reservoir.

Next week, in the final part of our series on volcano monitoring methods, we will describe how volcanologists use motion of the Earth's surface to assess where magma is located beneath the ground and whether or not it is likely to erupt.

Until then, you may want to attend some of this week's Volcano Awareness Month activities, which include a presentation about the 1911 observations of Kīlauea by volcanologist Frank Perret in the Hawai‘i Volcanoes National Park Visitor Center on January 25; a talk about volcanic gas emissions and vog at the Konawaena High School cafeteria on January 27; a talk about the link between volcanoes and early Hawaiian stone tools at the University of Hawai‘i at Hilo on January 28, and daily National Park programs.

Details about these and other Volcano Awareness Month activities is available at http://hvo.wr.usgs.gov or by calling 808-967-8844.

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Volcano Activity Update

Over the past week, lava flows have been active on the pali and coastal plain in two distinct lobes west of Kalapana. The easternmost of the active flow lobes destroyed a local resident's home on January 13 and continues to advance slowly to the southeast near the end of Highway 130. The western lobe is no longer entering the ocean. At Pu‘u ‘Ō‘ō, lava flows continue to erupt from a spatter cone in the north-central part of the crater floor, slowly filling it.

At Kīlauea's summit, the circulating lava lake in the collapse pit deep within the floor of Halema‘uma‘u Crater has been visible via Webcam throughout the past week. Overnight on January 17-18, several large portions of the vent walls, including a 12 m (40 ft) wide piece of the Halema‘uma‘u crater floor, collapsed into the lava lake. The largest collapse caused a small explosion that littered the rim of Halema‘uma‘u Crater directly above the vent with spatter up to 30 cm (1 ft) across. The circulation pattern of the lava lake was interrupted by the explosion, and only a few short-lived increases in the height of the lava surface have occurred since. A small DI event, the first of 2011, began on January 19 and ended on the following day. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Three earthquakes beneath the Hawaiian Islands were reported felt during the past week. A magnitude-3.4 earthquake occurred at 11:27 a.m. (HST) on Saturday, January 15, 2011 and was located 7 km (4 miles) southeast of Waiki‘i at a depth of 15 km (9 miles). A magnitude-3.5 earthquake occurred at 11:11 p.m. on Monday, January 17, and was located 2 km (1 mile) southwest of Kīlauea summit at a depth of 1 km (1 mile). A magnitude-3.2 earthquake occurred at 5:36 a.m. on Wednesday, January 19, and was located 13 km (8 miles) southwest of Mauna Loa summit at a depth of 13 km (8 miles).