Kaoiki aftershocks recorded on a rotating drum seismograph at Desert station located within a few miles of the epicentral area of the Hawaii earthquake of November 16, 1983. USGS Photo

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

From 1912 to 1950, HVO used mechanical seismometers, called tromometers (derived from “tremble”), that recorded the vibrations sensed by a long, suspended boom. Imagine how an unlatched gate would swing back and forth in response to an earthquake, and you have a basic idea how these early instruments worked. Recordings were made as the end of the boom scratched a line on a smoke-coated paper on a cylindrical rotating drum. The instruments were quite large, consisting of a vertical post about 1.5 m (5 ft) high, to which a 1-m (3-ft) or longer boom was attached on one end and suspended by a wire and damped with a weight exceeding 100 pounds on its free end.

These mechanical tromometers had many limitations. The booms were easily dismantled by large earthquakes and had to be reassembled before operation could resume. They had low sensitivity and the HVO tromometers were able to detect earthquakes of only magnitude-3.0 or greater.

Hawaii Volcanoes National Park. Joel Swartz, Hank Joesting, and Gordon Macdonald at the entrance to the seismograph station near the end of the Mauna Loa truck trail, at an elevation of approximately 7,000 feet. 1950. USGS Photo

In 1950, HVO had put an electromagnetic seismograph into operation as the first step in an upgrade of the network. The hiring of Jerry Eaton, a new seismology Ph.D. from University of California at Berkeley, was the second step. He was the seventh seismologist to be stationed at Kilauea but the first who was familiar with the new electromagnetic seismographs that were destined to replace the mechanical tromometers.

The electromagnetic seismographs were based on a solenoid, a cylindrical coil of wire within which a metallic cylindrical core could move. Oscillation of the metal core within the wire coil during an earthquake would generate an electrical current in the wire which could be recorded in several ways. The electromagnetic seismograph had many benefits: they were much smaller, lighter, and their signals could be amplified to be 1,000 times more sensitive than the mechanical tromometers.

The electromagnetic seismograph could also be tuned to better detect the smaller earthquakes associated with Hawaiian volcanism. Eaton experimented with a few changes, using seismographs installed in his basement. HVO machinists used Eaton’s designs to modify an existing electromagnetic seismograph, dubbed HVO-1. When Jerry put an HVO-1 seismograph into operation alongside the older electromagnetic seismograph, “it recorded between 5 and 10 times as many earthquakes.” The network of electromagnetic seismometers was able to record 100 times more earthquakes and allowed the identification of families of earthquakes as they occurred through space and time.

But Jerry wasn’t done. He saw the value of monitoring ground tilt to forecast volcanic eruptions, but also saw the limitations in HVO’s 1950 methods, using the wanderings of the mechanical tromometer booms for the measurement.

Adapting a Japanese design, he and the HVO machinists built water-tube tiltmeters out of artillery-shell casings and garden hose. When completed and installed, they tracked the tromometer tilt very well, validating the concept. After setting up a small network of these tiltmeters to complement the expanding seismic network at Kilauea summit, Eaton was able to locate and track the subsurface magma injections, along with earthquakes, that led to the 1959 eruption of Kilauea Iki.

Jerry Eaton

In the 8 years Eaton was at HVO, he reinvigorated seismic and geodetic monitoring and, for his efforts, was able to propose the first comprehensive model for Kilauea’s eruption mechanism. Next week, the final Volcano Watch article in this series will complete the story of HVO’s first 100 years.

This week’s Volcano Awareness Month activities include a presentation about Mauna Loa in the Hawai`i Volcanoes National Park’s Visitor Center on January 24, a talk about HVO’s history of earthquake monitoring during the past 100 years at the University of Hawai`i at Hilo on January 26, and daily guided hikes in Hawai`i Volcanoes National Park. HVO scientists will also give a talk about Kilauea and Mauna Loa volcanoes at Lyman Museum on January 23 and will provide a display about, and be available to answer questions about, Hawaiian volcanoes at the annual Onizuka Science Day at UH-Hilo on January 28. Details about these activities are available at hvo.wr.usgs.gov or by calling 808-967-8844.