Volcano Watch — A POHA of a different sort

The Saddle Road to Pohakuloa Training Area (PTA) might seem noticeably more worn this month. For the past three weeks, two field engineers from the U. S. Geological Survey's Albuquerque Seismological Laboratory (ASL) have been on temporary assignment to Hilo and to the Hawaiian Volcano Observatory (HVO) to complete the installation of a new seismographic station at the Army base. We greatly appreciate the support and assistance of the staff at Pohakuloa Training Area, in particular the Public Works group. Without their help, the completion of this project truly would not have been possible.

Using what they refer to as a "separated station" configuration, the Albuquerque engineers have been driving back and forth between HVO and PTA, to install not only the seismic sensor and electronics at PTA, but also the data processing unit and Internet access to data from this station at HVO. The station is designated by the code name POHA.

The sensor installation is by no means commonplace. There are 6 seismic sensors providing signals to the remote data logger. Three of these sensors are very broad band devices, capable of registering without distortion seismic signals or ground motions whose frequencies of oscillation range from 0.01 to 30 cycles per second. This represents a huge increase over our principal HVO instrumentation, which is narrowly focused on recording small volcanic earthquakes whose characteristic frequencies range from 5 to 10 cycles per second.

In addition, the broad band sensors are installed at the bottom of a 100-meter-deep hole. This is done to get away from the ambient ground vibrations at the ground surface and thus afford greater overall recording sensitivity. Being in Hawai'i, in the middle of the Pacific plate, these sensors will add valuable data for the study of teleseismic or distant earthquakes.

The broad band sensors are complemented by three "strong motion" sensors. As the term implies, these sensors are built to respond only to very vigorous shaking, most typically generated by nearby large earthquakes. All of these signals are transmitted digitally from PTA to HVO over telephone lines, and then to ASL via the Internet. The data processing system installed at HVO features a tape drive, as the full collection of data from PTA will be archived at ASL, together with data from the global network of digital seismographic stations.

Such an undertaking, including the ongoing maintenance of the station and data processing systems, is not insignificant. The principal impetus for the POHA installation came from the Incorporated Research Institutions for Seismology (IRIS). Over 100 universities and research groups make up the IRIS consortium which is supported by the National Science Foundation. IRIS, in partnership with the USGS, has built up the global digital seismographic network to promote research and understanding of the whole Earth using seismology. At a price approaching $500,000 per station, not many individual institutions, including HVO, could afford to build and maintain stations like POHA or operate a network of such stations.

As we collect the data from POHA, we will look anxiously to see what this new view of earthquakes, originating not only from distant regions but also from our volcanoes, has to offer. With such a capable station added to the network of seismographs that we already have in place, it is expected that additional research will be focused on Hawai'i and its earthquakes and volcanoes.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a tube to the southeast in the direction of the seacoast. Breakouts from the tube feed a series of perched lava ponds above Pulama pali. A broad `a`a flow originates from the lowest perched pond, and as of November 26, two lobes of the `a`a flow have made it down to the base of the pali at the 150 meter (500 ft) elevation. The distal end of the flow is 3 km (1.8 mi) from the ocean.

There were no earthquakes reported felt during the week ending on November 18.