At 9:23 a.m. Feb. 12, a magnitude-4.1 earthquake occurred beneath Kilauea Volcano’s south flank. But this is probably not news to many “Volcano Watch” readers. Shaking from the earthquake was felt throughout the Big Island, with reports to the USGS “Did you feel it?” website (http://earthquake.usgs.gov/earthquakes/dyfi) from as far away as Captain Cook and Holualoa.
Felt earthquakes are not unusual. Kilauea’s south flank is one of the most seismically active areas in the United States, and Hawaii Island residents are accustomed to feeling the occasional shake. However, Friday’s earthquake occurred on a fault that also has produced large and damaging events in past years, so it serves as a reminder that we should be prepared for stronger shaking in the future.
The faults responsible for the majority of Kilauea south flank earthquakes are members of the Hilina Fault System. This system includes steep faults that form the cliffs lining the island’s southeast coast, of which the Hilina and Holei Pali are spectacular examples. Underneath these faults is another, and more uncommon, type of fault called a décollement. Analysis of Friday’s earthquake by USGS Hawaiian Volcano Observatory seismologists indicates it likely occurred on this unique structure.
Décollement, or detachment fault, refers to a nearly flat-lying fault often completely buried underground. At Kilauea, a décollement exists at the interface between the original seafloor and the overlying volcano. Sliding along this fault is driven partly by magma intruding into Kilauea’s East Rift Zone, which puts pressure on the south flank of the volcano and pushes it seaward. Fault slip is also driven by gravity. Repeated eruptions oversteepen the slopes of Kilauea, contributing to their instability and encouraging them to slide away from the island.
Data from HVO’s continuous GPS monitoring network shows that most of the time, Kilauea south flank motion occurs at a steady rate of 6 cm (2.5 in) per year. This indicates stable sliding on the fault, referred to as creep, which accommodates motion without the shaking that accompanies earthquakes. In this way, creep is a “safe” form of fault motion.
However, Kilauea’s south flank décollement doesn’t only creep. It also can suddenly lurch forward in a matter of seconds, producing felt earthquakes.
While the steep faults responsible for Hilina, Holei, and other pali produce the majority of earthquakes on Kilauea’s south flank, the décollement is responsible for the strongest quakes.
In 1989, slip on the décollement produced a magnitude-6.1 earthquake that injured five people, destroyed five houses and was felt throughout the Big Island. The strongest shaking was centered in the island’s lower Puna District, an area that has since seen rapid population growth.
The 1975 magnitude-7.7 Kalapana earthquake was even more destructive. At the time, there were few structures near the epicenter, but severe shaking occurred throughout the Puna District and in Hilo, which experienced heavy damage, including bending of walls at Hilo Hospital. The earthquake also caused the coastline to suddenly drop by up to 3.5 m (11 ft), generating a tsunami that resulted in the two fatalities associated with this event.
We live on an earthquake-prone island and each small event (for example, Friday’s M-4.1 earthquake) is like a ping on our cellphones, reminding us “you’ve got earthquakes!” We should take these reminders as opportunities to get ourselves, our families and our homes prepared for Hawaii’s next large event.
Before an earthquake, be sure objects and furniture that could present falling hazards are well-secured to a shelf or wall. During an earthquake, drop, cover and hold on until the shaking stops. Afterward, check for injuries, be careful of broken glass and debris and carefully inspect your surroundings for hazardous conditions including fires and damaged structures.
For more information about earthquake awareness and preparedness, visit these websites: USGS Earthquake Hazards Program (http://earthquake.usgs.gov/learn/preparedness.php) and Great Hawai‘i ShakeOut (http://shakeout.org/hawaii/).
Volcano activity updates
Kilauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 29 and 40 m (95–131 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, scattered lava flow activity remains within about 6 km (4 mi) of Pu‘u ‘O‘o and is not currently threatening nearby communities.
Mauna Loa is not erupting. Seismicity remains elevated above long-term background levels, with earthquakes continuing to occur mostly beneath the volcano’s upper Southwest Rift Zone and southern caldera region at depths of less than 5 km (3 mi). GPS measurements show continued deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone of Mauna Loa, with inflation recently occurring mainly in the southwestern part of the magma storage complex.
Two earthquakes were reported felt on the Big Island in the past week.
The magnitude-4.1 tremor Feb. 12 occurred 7.7 km (4.8 mi) west of Kalapana at a depth of 8.3 km (5.1 mi). At 3:42 a.m. Feb. 14, a magnitude-2.7 earthquake occurred 12.4 km (7.7 mi) west of Kailua-Kona at a depth of 39.5 km (24.5 mi).
Visit the HVO website (http://hvo.wr.usgs.gov) for past Volcano Watch articles, Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos, recent earthquakes info, and more; call for summary updates at 808-967-8862 (Kilauea) or 808-967-8866 (Mauna Loa); email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey`s Hawaiian Volcano Observatory.
