And You Thought You Were Standing Still

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People who live in areas with high earthquake activity are familiar with the sensation of the earth shaking beneath their feet during an earthquake. But few realize that the ground beneath our feet is never truly at rest and is continually vibrating. This is known as background noise, background tremor, or-a term that seismologists like to use-"microseism".

Microseismic energy is present all over the globe but is lowest at the center of large continents. This is because most microseism arises from energy transferred from ocean waves into the earth's crust as a type of seismic wave known as a surface wave.

Microseism has two main components: one that oscillates about every 16 seconds that is generated by waves breaking on coastlines, and a second, generally much stronger component, that oscillates about every 8 seconds that is generated by the periodic variation of sea bottom pressure due to standing wave components (waves with predominantly vertical motion) of ocean waves.

Studies of seismic data over the past 30 years indicate that microseism levels can nearly double during northern Pacific winter storms. Similar observations have been made for southern hemisphere storms during the austral winter, as well as other phenomena, such as the 1997-1998 El Nino event.

When seismologists refer to the background noise level, they are usually talking about the average microseism level for that location, but other sources can contribute to the background noise as well, such as cultural noise (cars, buses, generators), meteorological noise (wind and rain), and other natural sources, such as rivers and flowing lava.

The seismic network at the Hawaiian Volcano Observatory is predominantly used to locate earthquakes, but it is also continuously monitoring the ground shaking or seismic tremor levels.

In late 2007, tremor levels near the summit of Kīlauea began to slowly rise above their normal background levels. By early 2008, tremor levels had risen to nearly 5 times background levels, indicating that magma and magmatic gases were moving about vigorously beneath Kīlauea's summit.

Tremor levels at Kīlauea's summit have remained high throughout the explosive activity at the summit, which began on March 19, reaching a maximum of about 7 times background levels in early April.

Temperature measurements (using remote temperature sensors and thermal cameras) of the vent opening within Halema`uma`u have also shown a similar pattern, increasing after the March 19 event, reaching a maximum in early April, then decreasing slowly through mid-June. Observations of spatter and audible sounds from the vent also peaked in late March to early April.

While tremor at the beginning of 2008 was characterized by relatively steady, high values, the current Kīlauea summit tremor shows an alternating pattern of relatively low tremor for a minute or two, followed by an emergent burst of tremor lasting a minute or more. This type of tremor is known as episodic and has been occurring, on average, every two-and-a-half minutes since mid-April.

Vent temperature measurements correlate with the bursts of episodic tremor as well, with higher temperatures recorded during the tremor bursts. Analysis of night shot video also indicates that the glow from the vent is stronger during the tremor bursts.

Those who have visited the summit recently and gazed at the plume may have noticed that the plume exhibits a "puffing" behavior. Analysis of sped-up plume video indicates that this puffing occurs, on average, every two seconds and correlates with another characteristic of current summit tremor, the dominant frequency (one cycle every 2 seconds, or .5 Hz).

Looking at seismic tremor is a valuable tool for signaling to us when activity is picking up underground, and it is good to remember that tremor level measurements are relative to the ever-present background level of earth shaking beneath our feet.

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Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is erupting elevated amounts of sulfur dioxide gas and very small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the south part of Kīlauea caldera and produced occasional air quality alerts in more distant areas, such as Pahala and communities adjacent to Hawaii Volcanoes National Park during kona wind periods.

Pu`u `O`o continues to produce sulfur dioxide at even higher rates than the vent in Halema`uma`u Crater. Trade winds tend to pool these emissions along the West Hawai`i coast. Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.

The new gas vent observed on May 23 inside Pu`u `O`o has remained active, with no observed change. Lava from the 2007 Thanksgiving Eve Breakout (TEB) flow, erupting from fissure D of the July 21 eruption, continues to flow through what remains of Royal Gardens and across the coastal plain to the ocean in well-established lava tubes. Over the past week, the Waikupanaha ocean entry has remained active, with occasional small explosions and a moderate-size plume.

Be aware that lava deltas could collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions, as have been seen lately. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Check the County of Hawaii Civil Defense Website or call 961-8093 for viewing hours.

Mauna Loa is not erupting. One earthquake was located beneath the summit. Continuing extension between locations spanning the summit indicates slow inflation of the volcano.

Four earthquakes beneath Hawai`i Island were reported felt within the past week; two of these occurred in the same region as the damaging magnitude-6.7 earthquake on October 15, 2006. A magnitude-1.9 earthquake occurred at 7:31 a.m., H.s.t., on Sunday, June 15, 2008, and was located 10 km (6 miles) northwest of Pahala at a depth of 10 km (6 miles). A magnitude-3.8 earthquake occurred at 1:55 p.m. on Tuesday, June 17, and was located 22 km (14 miles) west of Waikoloa Village at a depth of 41 km (26 miles). A magnitude-2.5 earthquake occurred at 4:49 a.m. on Wednesday, June 18, and was located 13 km (8 miles) southeast of Mauna Loa summit at a depth of 9 km (6 miles). A magnitude-2.9 earthquake occurred at 3:43 p.m. on Wednesday, June 18, and was located 7 km (4 miles) southwest of Waikoloa Village at a depth of 38 km (24 miles).

Visit our Website for daily Kīlauea eruption updates and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808)967-8862. Questions can be emailed to askHVO@usgs.gov. skip past bottom navigational bar