February 19, 2009
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Good Vibrations—the Earth's Music
When an earthquake occurs, why do we feel the ground shake? The shaking we feel is from seismic waves radiating away from the earthquake. These waves are similar to waves radiating away from a stone thrown into water. They are the Earth's way of dissipating energy released during an earthquake.
Dissipation of earthquake energy is an example of the continuous process of converting energy from one form to another. In this case, elastic potential energy (in the form of underground stresses built up within the earth) is suddenly released during an earthquake and converted to kinetic energy (the energy an object has due to its motion).
The same energy conversion happens when the string of a guitar or `ukulele is plucked. As the player pulls on a string, its elastic potential energy increases. When the string is released, that energy radiates as kinetic energy in the form of a vibrating string.
The string vibrates at a frequency (number of vibrations per second) that we can hear—and call music. Frequency, known as "pitch" in music, is measured in cycles per second, or Hertz (Hz). Humans can hear a wide range of frequencies, varying from about 50 Hz (low bass) up to 14,000-20,000 Hz (high pitch).
Around the globe, earthquakes happen continually, so seismic waves are constantly travelling throughout the earth. In essence, the earth is continually making music, but for the most part, this music is at a frequency below our hearing range. Most small, local earthquakes release most of their energy between 1-15 Hz. Larger earthquakes can produce significant energy down to much lower frequencies.
While small earthquakes tend to dissipate their energy like ripples radiating away from a stone thrown into water, large earthquakes can actually vibrate the whole earth at once. These normal modes of vibration are often described as the earth ringing like a bell.
Because most seismic energy is below our hearing range, and because it is difficult for seismic waves to travel from the earth to the atmosphere, we do not often hear the energy radiated from earthquakes. This is not always the case with volcanoes. Volcanoes can release energy directly to the atmosphere, sometimes explosively, and radiate energy at a range of frequencies.
The new eruptive vent at Kilauea's summit has produced strong energy between 0.5-2.0 Hz. This energy includes seismic energy recorded by local seismic instruments, as well as energy radiated directly to the atmosphere, which is recorded with nearby infrasound microphones.
While this energy is below our hearing range, the vent has also produced audible sounds, including hissing and jet-engine-like sounds. These noises are also a form of energy dissipating from the volcano, but they are more closely related to the sound produced by an instrument, such as a trumpet, rather than the sound produced by the vibrating strings of an `ukulele.
When pressurized air is forced through a constricted opening (the player's lips), the vibration produces music. The pitch of the music is controlled both by the frequency of the vibration and by the shape and length of the brass tubing on the trumpet.
On the volcano, pressurized gases are being forced through tight constrictions within the vent. The frequency of the resulting vibrations is controlled by both the amount of pressure and by the shape and size of the vent.
At a place like Kilauea volcano, the continual process of energy dissipation allows for a unique multi-sensory experience in which we can see, hear, and feel the Earth's music.
Kīlauea Volcano continues to be active. A vent in Halema`uma`u Crater is emitting elevated amounts of sulfur dioxide gas and producing 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 Hawai`i Volcanoes National Park, during kona wind periods. The small, crusted lava lake more than 100 m (100 yards) below the vent rim, visible only with a thermal camera, has been replaced by a small, weakly spattering cone.
Pu`u `Ō`ō also continues to produce significant amounts of sulfur dioxide. Trade winds tend to pool these emissions along the West Hawai`i coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo.
Lava erupting from the Thanksgiving Eve Breakout (TEB) vent at the eastern base of Pu`u `O`o continues to flow to the ocean at Waikupanaha through a well-established lava tube. A delta collapse on Tuesday, February 17, scattered fist-sized rocks up to 275 yards inland from the ocean entry. Breakouts from a western branch of the lava tube were active and reached the ocean by Wednesday at the long-buried archeological site of Poupou-Kauka within the National Park. The Waha`ula entry, farther to the west, has been inactive now for a couple of weeks.
Be aware that active lava deltas can collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions. The Waikupanaha delta collapsed several other times over the last several months, with three of the collapses also resulting in rock blasts. The largest tossed television-sized rocks up onto the sea-cliff and threw fist-sized rocks more than 200 yards inland.
Do not approach the ocean entry or 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. Call Hawai`i County Civil Defense at 961-8093 for viewing hours.
Mauna Loa is not erupting. Two earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano, combined with slow eastward slippage of its east flank.
March and April of 2009 are the 25-year anniversary of the most recent eruption of Mauna Loa and HVO staff would like to remind the interested public that the volcano is still active and will erupt again. The first events are a pair of public "After Dark in the Park" lectures in Hawai`i Volcanoes National Park on March 17 and 24. On March 28, we plan to hold an informational forum on the UHH campus at UCB100 in conjunction with our colleagues at the University of Hawai`i at Hilo (UHH) and Hawai`i County Civil Defense. Then, on April 4, we will be at the Konawaena Intermediate School cafeteria. Come join us in March and April to learn more about Mauna Loa-its history and current status.
Three earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.4 earthquake occurred at 5:07 a.m., H.s.t, on Saturday, February 14, 2009, and was located 17 km (10 miles) northwest and offshore of Kailua at a depth of 9 km (6 miles). A magnitude-2.2 earthquake occurred at 5:22 a.m. later that morning and was located 12 km (8 miles) southeast of Mauna Loa summit at a depth of 48 km (30 miles). Another magnitude-2.2 earthquake occurred at 9:45 a.m. on Monday, February 16, and was located 3 km (2 miles) southeast of Pahala at a depth of 5 km (3 miles).
Visit our Web site (http://hvo.wr.usgs.gov) for daily Kīlauea eruption updates, a summary of volcanic events over the past year, 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.
Updated: March 13, 2009 (pnf)