Volcano Watch — Farewell to legacy seismic systems—welcome to ARRA upgrades in Kīlauea Caldera

In late 1994, in a collaborative study on the physics of magma transport with Bernard Chouet and Phil Dawson, USGS colleagues in Menlo Park, California, we installed a number of "broadband" seismometers in Kīlauea Caldera to record earthquakes and seismic tremor over a wide range of seismic wave frequencies.

HVO's traditional "short period" systems were designed to principally record and locate very small earthquakes. Broadband recordings more fully reflect earthquake and tremor source processes. The difference is analogous to watching color, instead of black-and-white, TV.

Broadband digital seismic field systems existed in 1994 but were quite expensive, and the components of digital radio telemetry were not nearly as accessible as they are today. In a truly pioneering manner, USGS engineer Gray Jensen designed and built digital telemetry systems (DSTs) around fax modem chips with low cost, low power consumption, and field worthiness.

Established telemetered seismic data acquisition systems in 1994 were typically built around equipment that converted seismic signals, transmitted via FM radio methods, to digital format on minicomputers at a data-collection/processing hub back at the office.

The newer DST network strategy was to digitize the signals in the field and transmit those values to the data-processing hub. A suitable computer system was needed to collect and subsequently process the incoming data from the DSTs.

Jensen's colleague, USGS seismologist Willie Lee, provided that piece of the puzzle with an adaptation to seismic acquisition and processing systems that he had developed for PC-clone microcomputers. These systems had proven themselves during the USGS response to the 1991 Mount Pinatubo eruption crisis, recording seismic data acquired via traditional telemetry systems. Subsequently, an interface was developed between DSTs and the PC-clone systems.

The plan in 1994 was to test the broadband/DST/PC-clone systems at HVO for, at most, a year, and then to relocate the equipment to Alaska. But the unprecedented opportunity to conduct a large project at Kīlauea, in partnership with dozens of Japanese seismologists bringing several hundred temporary seismic stations to the volcano, drove the decision to continue the DST test into 1996.

Fate then revealed itself. In February 1996, after removal of the Japanese equipment had begun, a vigorous swarm of earthquakes struck Kīlauea, reflecting magma intrusion into the volcano's summit caldera. In fact, a Japanese scientist working on the temporary stations in the field had to be evacuated via helicopter for fear of a sudden outbreak.

In addition to numerous volcanic microearthquakes heralding the magma intrusion, an unusual record of very-long-period (VLP) signals was obtained from the DST instruments. With very low frequencies of oscillation, these signals would not have been seen in the "black and white" view provided by traditional short-period seismic systems.

Detailed analyses of the VLP oscillations, which result from magma containing both gas and liquid phases moving within conduits and through constrictions, led to new insights into physical processes occurring within Kīlauea. Thus, the plan to move the instruments to Alaska was abandoned, the original seismometers were replaced with permanent units, and the DSTs and old PC-clone remained in operation until this month.

Continued study and analysis of VLP signals have allowed details of Kīlauea Caldera's magmatic conduit structure to be further articulated. Chouet and Dawson have extended their work to other volcanoes, and other research teams have also undertaken the study of VLPs.

The ARRA upgrades to the DST systems are most welcome, because they will allow us to continue recording and studying VLP and other seismic activity within Kīlauea Caldera. As a tribute to Jensen and Lee, we will strive to further understand magma transport at Kīlauea using contemporary, high-fidelity digital instruments and computer platforms that simplify operations and maintenance.

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Volcano Activity Update

The lava lake within the Halema‘uma‘u Overlook vent dropped for a few days last weekend, in tandem with a brief deflation of Kīlauea's summit. Since then, it has returned to its previous level and has been relatively steady. Heating of the vent walls above the lava lake continues to cause loud popping and cracking noises, sometimes audible from the Jaggar Museum Overlook.

After pausing during last weekend's deflation, uplift of the crater floor at Pu‘u ‘Ō‘ō has resumed. This slow rise is likely caused by shallow injection of magma from the lava lake in the crater into the layered flows that make up the crater floor. Small flows continue to seep from areas outside the lava lake, but all activity is confined within the Pu‘u ‘Ō‘ō crater.

No earthquakes beneath Hawai‘i Island were reported felt this past week.