LEVELING SURVEY OF KOA`E FAULT SYSTEM AFTER THE 2018 KILAUEA ERUPTION

The Koa`e fault system is located south of the Kīlauea volcano summit between the southwest and east rift zones of Kīlauea volcano. Six line-level survey stations lie across the Koa`e fault system. These stations were established by USGS-HVO in the 1990’s, and were monitored periodically over the last 35 years. Overall widening of the fault system at a rate of 1-2 centimeters per year has been documented within the fault system, with individual faults moving at rates averaging a few millimeters per year. After the 2018 eruption of Kīlauea, we remeasured vertical changes along these lines and complementary crack stations. New data from these stations establish a post-2018 eruption baseline for future deformation studies.

The 2018 Kīlauea eruption was accompanied by an M6.9 earthquake and 62 collapse events (M5.0-M5.3) at Kīlauea summit from May to August 2018. This caused deformational change in the Koa`e fault system, an area already affected by southward creep and volcanic and seismic events. Leveling data collected from 1998-2015 were compared to the data collected during our post-2018 eruption field surveys. Our surveys used the same methodology as historical USGS-HVO surveys. Line-leveling of these stations in 2019 revealed significant vertical change across the Kulanaokuaiki Pali, the southernmost pali in the Koa`e fault system. Vertical displacement varied across the level line, and ranged from 2.2 cm - 18.5 cm, a significantly higher rate of change than the average annual movement.

Long-term creeping along the Koa`e fault system is augmented by abrupt high-deformation events such as those that occurred in 2018 along with intrusions in the neighboring east rift zone in 1965 and 1973. Kīlauea volcano’s southern flank undergoes significant deformation due to gravitational failure linked dynamically to magmatic intrusions. Our surveys provide a quantitative measure of this interaction along a well-defined zone of failure.