Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Sources/Usage: Public Domain. View Media Details

The most recent notable felt earthquake happened on Saturday night, July 6, at 8:47 p.m HST. The magnitude (M) 4.1 earthquake was on Kīlauea’s south flank at a depth of about 7 km (4.4 miles) below sea level. This event produced a handful of aftershocks, including three above M2 that occurred within ten minutes of the M4.1. 

Earthquakes that occur on Kīlauea’s south flank typically happen on either the Hilina fault system or the fault called the “décollement.” The steep faults of the Hilina fault system are easy to visualize as they appear on the surface as steep pali (cliffs) along the southeast coast of the Island of Hawaiʻi. These steep faults continue through the subsurface and can produce large earthquakes as rocks along the nearly vertical faults slip against each other. 

The décollement, or detachment fault, sits beneath the Hilina fault system. This fault is nearly horizontal beneath Kīlauea's south flank at the interface between the island and the ocean floor. This interface can produce larger events and, according to seismologists at the USGS Hawaiian Volcano Observatory (HVO), was the likely source of Saturday’s M4.1 based on the depth and motion. 

Slip along the décollement can be produced as a combination of gravity and changes in pressure occurring in the volcano that sits above. In the past 50 years, there have been three décollement earthquakes above M6 on Kīlauea's south flank. 

The most recent was the M6.9 earthquake that occurred on May 4, 2018. This earthquake was caused by the magmatic intrusion in Kīlauea’s East Rift Zone, which led to the 2018 eruption in the lower East Rift Zone.

The décollement also produced a M6.2 earthquake in 1989. This event caused injuries, destroyed or damaged houses in Puna District, caused landslides that blocked roads, and generated a small local tsunami.

The most destructive of the three events was in 1975, and it was the largest earthquake in Hawaii since 1868. A M7.7 on the décollement fault beneath Kīlauea caused several meters (yards) of horizontal and vertical movement along faults in the summit and south flank regions. The earthquakes caused building and road damage, along with a tsunami that resulted in two local fatalities. 

Within this greater context, Saturday’s M4.1 was only a minor slip along Kīlauea’s décollement, but its widely felt shaking serves as a reminder of the potential for this region to produce damaging and widely felt earthquakes. More than 700 people reported feeling the recent M4.1, spanning the Island of Hawaiʻi and even as far as Maui and Lanaʻi.

As residents of a very shaky island chain, the USGS "Did you feel it?" website is a phenomenal resource that citizens and scientists alike can use to report how we individually feel earthquakes. 

While the magnitude of an earthquake is the size derived from data collected by the network of seismic instruments, the intensity of an earthquake is a measure of shaking derived from the network of people reporting how they felt it. Based on the felt reports, “Community Internet Intensity Maps” or CIIMS are generated in near real-time and help us understand how different types of earthquakes can impact different regions in Hawaii. 

The USGS Fact Sheet “Did You Feel It? Citizens contribute to Earthquake Science” describes the importance of CIIMs: "...as a result of work by the U.S. Geological Survey and with the cooperation of various regional seismic networks, people who experience an earthquake can go online and share information about its effects to help create a map of shaking intensities and damage...CIIMs contribute greatly toward the quick assessment of the scope of an earthquake emergency and provide valuable data for earthquake research." 

The next time you feel an earthquake, first ensure that you and your surroundings are safe. Then, if you would like to support the science happening in Hawaii, please fill out your felt report.  Mahalo to everyone who reports feeling earthquakes in Hawaii; your reports help us understand impacts of earthquakes in our dynamic environment.