Kīlauea Report: Intrusion Quiets Volcano

Following the previous week of frequent earthquakes and two intrusion pulses beneath Kīlauea’s south caldera, the past week has seen the volcano transition to a phase of relatively quiet ground adjustments. The first pulse of intrusion, between August 23rd and 25th, was dominated by a 478-earthquake swarm, after which the activity had waned to only 13 events within the next 36 hours. The second pulse began with another 167 earthquakes at a peak rate of 24 per hour, similar to the 28 per hour during the first pulse but diminishing much more quickly. In the ensuing days, the lessened seismicity expanded down the Southwest Rift up to 8 km, or 5 miles, away from the summit, suggesting growth of the underground magma body. Initial USGS modeling based on recent InSAR suggests a similarly broad area of uplift of up to 18 cm or 7 in, with a GPS station above the zone of seismicity on the eastern margin showing the ground rising roughly 10 cm or 4 in during the week-long event. Localized ground tilt marked the onset of the second pulse ahead of the earthquakes on the evening of August 26th, and continued until the afternoon of August 30th when it slowed or ceased. GPS readings show both sides of the summit caldera were forced north by the intrusion, with larger movement closer by in the south, resulting in a net contraction of the cross-caldera distance. Since then, monitors suggest continued, albeit slower, ground movement around the south caldera intrusion, but enough to still produce deviations in the changing distance across the caldera from the longer-term trend. The latest USGS Hawaiian Volcano Observatory weekly update summarized that “earthquake rates and ground deformation in this area have remained near pre-intrusion levels” with no other significant changes. A relatively sparse 75 small-magnitude earthquakes were recorded in the area during the past week, down significantly from 732 the previous week. Further, seismicity has remained low elsewhere around the summit including the Upper East Rift or East Rift Connector, suggesting relief from the magmatic pressure that had been building for several months right up to the latest intrusion, still visible in the earthquake plots for the past month. Despite all this action south of the 2020 eruption site at Halemaʻumaʻu, the shallower magmatic system that fed it seems remarkably unaffected, at least thus far. While commensurate tilt changes at Uēkahuna were reported early on by the USGS and might be correlated to the signals at Sandhill, their magnitude is miniscule in comparison, and exhibit a smaller range of change than other background processes registering earlier in the month, with no significant change since then. Gas measurements and ambient readings remain in the low background range, despite a slight rise to 115 tonnes per day on September 2nd. An anomalous ambient gas reading on September 1st at station HRPKE, near Puʻukoaʻe on the Southwest Rift, has been confirmed as station maintenance. According to the USGS, “currently, there are no indications of the Halemaʻumaʻu vent resuming eruption.” In summary, after building up and relieving pressure, all underground, the volcano shows less signs of stress for now, while still adjusting. As for what comes next, we return to the usual wait and see, having tracked this latest cycle from the build-up following the end of the summit eruption in May to its peak with the intrusion at the end of August, and now into a quiet September. -- Join our weekly live video review of Kīlauea and Maunaloa volcanoes, at 5pm Hawaiʻi time Thursdays! To support our productions please like, share and subscribe! Mahalo! #Kilauea2021 [USGS Image Caption: View to the southwest from HVO station HRPKE, showing Pu‘ukoa‘e on Kīlauea's Southwest Rift Zone in the background. Pu‘ukoa‘e formed during an eruption more than 200 years ago; lava flows from the December 1974 eruption of Kīlauea's Southwest Rift Zone—which are visible in the foreground—flowed southwest towards Pu‘ukoa‘e, with one lava flow from the eruption stopping at Pu‘ukoa‘e. USGS photo by P. Nadeau.]