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July 29, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Here today, gone to Maui

All of sudden, there's an earthquake...or was it an earthquake? Where was it? How big was it? Does it signal the start of an eruption or magma shifting its position within the volcano? Is there a possibility of a tsunami produced by this earthquake?

The U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) routinely reports on earthquakes occurring in Hawai'i. We have cataloged or processed nearly a quarter of a million earthquakes since HVO was established. We post earthquake information on our web site, and many scientists use our seismographic data and earthquake catalogs for basic and applied research. How is this done?

At the very heart of our volcano monitoring program lies our telemetered seismographic network. Ground movement registered by our earthquake sensors, or seismometers, is continuously transmitted to HVO in real time over a network of FM radios from our 65 seismographic stations on the island of Hawai'i. The signals carried over the radio network are fed into our computer systems which are automatically recording and trying to recognize earthquakes. To get the radio signals around the mountains of Hawai'i Island, we must channel many of these through radio repeaters, sometimes as many as 3 or 4 for data from one station.

There are literally thousands of components and pieces that comprise the seismographic field network. Our electronics staff at HVO is responsible for installing, maintaining, and upgrading the capabilities of our field network. Because we are interested in even the very small earthquakes that occur within the volcanoes, we must operate our instruments at very high sensitivities. This requires that our stations be located in remote areas where our signals will not be complicated or overshadowed by ground motions produced by non-earthquake or non-volcanic sources such as artillery fire, vehicular traffic or even human footsteps. At these sites, we have to provide our own power and infrastructure. In addition, at some sites, merely fighting back the forest is a big part of the job.

This summer, our three ETs (electronics technicians) have been even busier than normal. Recent geologic field investigations have suggested a different eruptive history of Haleakala than previously recognized. Thus we are installing a small network of seismic stations on the island of Maui to monitor its seismic activity as it might relate to Maui volcanism.

Rather than record those data remotely and periodically retrieve them, our plan is to build the Maui network onto our existing telemetered network. To receive the Maui signals at HVO, we have upgraded, or are in the process of upgrading, radio sites at HVO, Mauna Kea, Hualalai, and Kohala. We have also begun our field work on Maui, and it is our plan to complete these installations and begin continuous seismic monitoring of Maui by summer's end.

Laying in this required additional infrastructure, in addition to setting up a continuous operation on another island, has proven to be a large and complex task. As in many previous summers, we are fortunate to have the help of students from Hawai`i Community College/UH-Hilo in this field effort. These students come to us as participants in the Student Temporary Employment Program of our parent Department of the Interior. While providing us with the much-needed technical assistance to carry out projects like the Maui project, this program provides students with valuable experience in, and exposure to, working in technical jobs.

This small network will also provide data fundamental to understanding earthquakes and earthquake processes beneath Maui as part of the central Hawaiian Island chain. After Hawai'i County, Maui County is the next greatest earthquake risk in the State. Long-time Maui residents might recall the magnitude 6.9 earthquake on January 22, 1938. As we often use information about small earthquakes to draw inferences about the behavior of large earthquakes, our small network on Maui will help us begin to frame ideas quantitatively about earthquake hazards facing Maui County.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes from the vent to the sea. Lava is entering the ocean near Kamokuna and enlarging the bench. Occasional phreatic activity was observed at the entry. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

Three earthquakes were reported felt during the week ending on July 29, 1999. Residents of Hilo felt an earthquake at 1:37 p.m. on Tuesday, July 27. The magnitude-3.5 earthquake was located 14 km (8.4 mi) southeast of the summit of Kilauea Volcano at a depth of 6.6 km (3.9 mi). At 3:12 p.m. the same day, residents of Hilo, Hakalau, Mt. View and Glenwood reported feeling an earthquake. The magnitude-3.7 temblor was located 20 km (12 mi) southeast of the summit of Kilauea Volcano at a depth of 5.26 km (3.2 mi). The third earthquake was felt by residents of Hilo and Papaikou at 9:04 p.m. on Wednesday, July 28. The magnitude-3.4 earthquake was located in the same area as the earthquakes on Tuesday but slightly deeper at 7.75 km (4.65 mi).

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Updated: 13 Aug 1999