URI Graduate School of Oceanography student Jiahang Li and faculty member Dr. Yang Shen are currently off the coast of the Island of Hawaii as part of a “rapid response” to the ongoing eruption of the Kilauea volcano and accompanying earthquakes and tremors. As the pair works with other researchers to deploy instruments on the ocean bottom, Jiahang will provide updates throughout the week-long cruise.
The R/V KOK kept heading east off the Island of Hawaii. On July 10th, we started the deployment of the linear ocean bottom seismometer (OBS) array across the lower part of the Kilauea south flank. As the volcanic island grows, it overlays on the oceanic crust formed at the mid-ocean “spreading center” called the East Pacific Rise. As the Pacific plate moves to the west towards Hawaii, it has accumulated a layer of sediment over millions of years. Under the weight of the volcanic island, the plate bends, creating a depression beneath the island and the immediate surrounding seafloor, and a collection of volcanic debris within that depression. The sediment and volcanic debris are mechanically weak and have low seismic wave speeds. This low wave speed layer can “trap” waves generated by earthquakes near the layer.
The linear array of OBSs is designed to record the trapped waves beneath the Big Island, which is likely the rupture surface of the May 4th magnitude 6.9 earthquake, as well as some other historical large Hawaiian earthquakes. The low wave-speed of this layer comes from pelagic sediments and volcanic debris, which are soft and less bonded than the rocks above and below. The linear array is placed between the section of the south flank that has an abundant number of earthquakes and the section that released stress in slow motion (aseismically) every two to three years previously and has experienced few aftershocks. By using earthquakes from different directions, we may be able to determine the differences in physical properties between the two sections. The deployment started at early morning around 5:30 am and wasn’t finished until 11:00 pm. During the night, we kept mapping the seafloor at speed of about 4 knots.
On July 11th, we deployed an OBS at our last deepwater site, then by that afternoon we arrived back at the first deployment site around the lava ocean entry point. At a distance, we could see smoke rising from the vent of the volcano all the way up to the sky. As we got closer, we could see white vapor escaped the boiling water, where the lava meets the sea. Since the water depth near the ocean entry is shallow, the time it takes for OBSs to sink to the ocean bottom is relatively short, so we finished deploying the last three OBSs before sunset.
With the OBS deployments complete, we started mapping the bathymetry with acoustic reflection. We also collected some water samples near the ocean entry point for biologists to test the unique biological and chemical responses to the flow of lava into the ocean.
At nightfall, we watched the lava entering the ocean with constant explosions near the surface, which sound like a low-pitched drum. The OBSs we had just dropped into the ocean were recording these sounds at the same moment! When the smoke was blown away by the wind, we could even see the lava splashed from the vent of the volcano. We were all surprised that the volcano is still so active after the initial eruption two months ago.
The cruise will finally come to its end at Hilo on July 13. We have had a great trip on this ship so far and seen the most amazing display of nature’s power that formed the Big Island and the Hawaiian-Emperor seamount chain over geological times. In two months, the data from our OBS deployments will be collected by another cruise, and we hope to use this data to better understand the local geological structure and physical properties in the fault zone.