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

Jessica Johnson and Ben McLeod in front of active lava flows on the coastal plain of Kilauea Volcano. Photo courtesy of USGS/HVO

The Hawaiian Volcano Observatory is saying goodbye to two good friends: Jessica Johnson and Ben McLeod. Jess and Ben arrived in Hawai`i in October 2011, when Jess accepted an offer to be a post-doctoral research fellow at HVO through the University of Hawai`i, Hilo.

Before coming to Hawai`i, Jess and Ben lived in Wellington, New Zealand—Ben’s home, and where Jess (originally from England) completed a Ph.D. in geophysics at Victoria University of Wellington. As part of her Ph.D. work, Jess specialized in seismic anisotropy—a way of detecting whether seismic waves travel through the Earth faster in one direction than another. A change in this directionality can signal an important change in volcanic activity. She quickly became one of the world’s experts in the field.

Upon arriving in Hawai`i, Jess analyzed over 2,000 earthquakes that occurred between 2007 and 2011. She found that the orientation of “fast” seismic waves changed at about the start of Kilauea’s summit eruption in 2008. Such “fast” directions are usually interpreted to be a sign of a structural grain in the Earth (due to aligned cracks, for instance) or of stress imposed by an inflating magma chamber.

Neither of these hypotheses fits the changes that Jess measured at Kilauea. The “fast” directions she detected in 2008 were perpendicular to the structural grain of Kilauea Caldera, and they occurred at the time the magma chamber beneath the caldera was deflating. So what caused the changes?

Jess hypothesized that increased gas emissions during the start of the summit eruption—a time of the highest gas emission levels ever recorded at Kilauea—might be the culprit. In early 2008, volcanic gas probably filled much of the pore space in the lava flows that make up the caldera floor. As gas pressure increased, the pore space expanded, causing cracks aligned with the magma chamber to open and change the orientation of the fast seismic waves. Over time, this gas pressure gradually dissipated, causing the seismic signature to return to normal by 2009. This unique conclusion can be applied to volcanoes elsewhere around the world—especially in remote locations that might have seismic, but not gas, monitoring. Jess’s results were published this year in the prestigious journal Nature Communications.

While Jess studied seismic waves, Ben provided tremendous assistance to the seismic monitoring group at the Observatory. HVO depends on accurate locations for earthquakes in order to interpret whether or not changes in seismic activity indicate an impending change in eruptive activity. This requires that most earthquakes be located by hand—a time-consuming process that requires careful analysis of seismic waves recorded by stations across the Island of Hawai`i. Ben’s work focused mostly on precisely locating earthquakes that accompanied Kilauea lava outbreaks in 2011, and the catalog he helped to form will be invaluable to better understanding those eruptions.

Ben and Jess also contributed to aspects of HVO’s and UHH’s missions beyond their scientific work. They frequently helped with field operations, provided tours of HVO facilities to visitors, gave talks at community events, and even taught classes, both to UHH students and as part of international volcanology courses. They truly became part of the HVO family during their two years on the island.

Jess and Ben also took advantage of their tropical surroundings. Accomplished SCUBA divers, they explored the underwater environment around the state. They also spent weekends hiking and camping, especially with other staff and volunteers at HVO. They even found time to get engaged! Now they are on their way to England, where Jess will begin a new position at the University of Bristol—one of the top universities for volcanology in the world—while Ben completes a degree in geography (something he did “on the side” while helping out at HVO).

HVO and UHH will surely miss our friends as they move on to new positions on the other side of the globe, but we are richer for having known them both and are grateful for their contributions in their all-too-brief stay in Hawai`i. Aloha, Jess and Ben! Thank you for being such wonderful colleagues and friends. We hope to see you back in Hawai`i again soon!