The Hilina Pali on Kīlauea Volcano’s south flank is visible evidence of the steep Hilina Fault System. Beneath this system lies the flat-lying décollement fault that has no visible surface expression, but has produced several large earthquakes in the past 200 years. Photo courtesy of Ingrid Johanson.

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

Epicenter of the 4.1 magnitude earthquake on February 12, 2016.

On Friday, February 12, at 9:23 a.m., HST, a magnitude-4.1 earthquake occurred beneath Kīlauea Volcano’s south flank. But this is probably not news to many “Volcano Watch” readers. Shaking from the earthquake was felt throughout the Island of Hawaiʻi, with reports to the USGS “Did you feel it?” website (http://earthquake.usgs.gov/earthquakes/dyfi) from as far away as Captain Cook and Hōlualoa on the west side of the island.

Felt earthquakes are not unusual. Kīlauea’s south flank is one of the most seismically active areas in the United States, and Hawai‘i Island residents are accustomed to feeling the occasional shake. However, Friday’s earthquake occurred on a fault that has also 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 Kīlauea south flank earthquakes are members of the Hilina Fault System. This system includes steep faults that form the cliffs lining Hawai‘i’s southeast coast, of which the Hilina and Hōlei 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 (HVO) seismologists indicates that it likely occurred on this unique structure.

“Décollement” or “detachment fault” refers to a nearly flat-lying fault that is often completely buried underground. At Kīlauea, 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 Kīlauea’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 Kīlauea, contributing to their instability and encouraging them to slide away from the island.

Waveform generated by the 4.1 magnitude temblor on February 12, 2016.

Data from HVO’s continuous GPS monitoring network shows that most of the time Kīlauea 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, Kīlauea’s south flank décollement doesn’t only creep. It can also suddenly lurch forward in a matter of seconds, producing felt earthquakes.

While the steep faults responsible for Hilina, Hōlei, and other pali produce the majority of earthquakes on Kīlauea’s south flank, the décollement is responsible for the strongest quakes.

In 1989, slip on the décollement produced a magnitude-6.1 earthquake, which injured five people, destroyed five houses, and was felt throughout the Island of Hawai‘i. 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 cell phones, 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 that 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 on earthquake awareness and preparedness, please visit these websites: USGS Earthquake Hazards Program (http://earthquake.usgs.gov/learn/preparedness.php) and Great Hawai‘i ShakeOut (http://shakeout.org/hawaii/)