Hilina Fault System, Large Earthquakes Linked

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 decollement fault that has no visible surface expression but was responsible for the May 4, 2018, magnitude-6.9 earthquake. Photo courtesy of Ingrid Johanson

The pali (cliffs) of Kīlauea’s south flank are some of the volcano’s most striking features. Reaching up to 1,500 feet high, they stand out against the otherwise gentle slopes of Hawai’i’s most active shield volcano. These pali, including the Hilina Pali, the Holei Pali and others, are often referred to as faults, and are together called the Hilina fault system. The faults form head-scarps—the above-ground evidence for a large landslide called the Hilina slump.

Offshore seismic reflection surveys—similar to doing an ultrasound on the earth—show rock and rubble layers consistent with the Hilina slump extending to about 2.5 miles below the surface, and its leading edge to be about 13 miles offshore. While the size of the slump and the possibility of catastrophic landslide can be alarming, the seismic reflection data also show that there is good news.

The good news is courtesy of the decollement fault below Kīlauea’s south flank. The decollement fault is a nearly horizontal fault at the boundary of the original ocean floor and overlying volcanic rocks. It lies deeper below the surface than the Hilina slump, but although they are right on top of each other, current evidence shows that the two are not connected. Instead, the Hilina slump rides on top of the portion of the island moving with the decollement.

It was the decollement fault that was responsible for last summer’s magnitude-6.9 earthquake and for the magnitude-7.7 Kalapana earthquake in 1975. Slow movement on this fault is also responsible for moving the south flank steadily seaward and for episodic slow slip events that nudge the flank an additional inch or so farther every three to four years.

When the decollement fault slips, the over-riding rocks move seaward and slightly up. Over many thousands of years, the slight upward motion has caused a ridge to form in the ocean floor where the fault approaches the surface. In 2003, scientists from Rice University concluded that this ridge acts as a backstop, making it more difficult for the large Hilina slump to slide. This is similar to how a bookend prevents books from sliding down a shelf.

In this way, continued slip on the decollement helps prevent catastrophic landslides. However, the decollement slip also produces large earthquakes that can affect the slump. In 1975, two faults in the Hilina fault system moved by over a yard during the magnitude-7.7 Kalapana earthquake. The magnitude-6.9 earthquake in 2018 resulted in a lot of questions from the public about whether it might trigger slump motion. However, in the days that followed, there was no evidence for widespread motion of the Hilina slump.

, mostly by less than a half-inch. The only location with substantial movement during the earthquake was a short, 1.5 mile segment of a previously unmapped fault near the earthquake’s epicenter. Furthermore, data from the 17 years prior to the magnitude-6.9 earthquake showed no resolvable motion of the Hilina fault during that time period.

Despite triggering motion on the pali, no catastrophic failure of the slump occurred in either 1975 or 2018. Plus, in 2018, motion is attributed to only the shallowest part of the slump, not the entire structure. This assures us that some motion can occur at the surface, without that motion continuing deeper in the slump and generating widespread failure.

Taken all together, these observations suggest that movement on the Hilina fault system occurs only very occasionally and perhaps only together with the very largest earthquakes, like the 1975 Kalapana earthquake. The Hilina fault system certainly plays a role in Kīlauea’s complex dynamics and understanding that role is important in order to know the full range of hazards on the volcano. The USGS Hawaiian Volcano Observatory continues to monitor motion on Kīlauea’s south flank to better determine the area’s hazard potential.

Photo caption: 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 decollement fault that has no visible surface expression but was responsible for the May 4, 2018, magnitude-6.9 earthquake. Photo courtesy of Ingrid Johanson.

Volcano Activity Updates

Kῑlauea Volcano is not erupting and its USGS Volcano Alert level remains at NORMAL. Reflecting this level, HVO is now issuing monthly updates for Kīlauea. For definitions of the alert levels, see https://volcanoes.usgs.gov/vhp/about_alerts.html.

Monitoring data for deformation have shown no significant changes in Kīlauea activity over the past week. Rates of seismicity across the volcano remain low. Sulfur dioxide emission rates are low at the summit and below detection limits at Puʻu ʻŌʻō and the Lower East Rift Zone (LERZ).

At or near the 2018 LERZ eruptive fissures, elevated ground temperatures and minor releases of gas—steam, tiny amounts of hydrogen sulfide and carbon dioxide—persist. These are typical post-eruption conditions and are expected to be long-term, as they were after the 1955 LERZ eruption.

The water level at the bottom of Halema‘uma‘u continues to slowly rise. HVO is monitoring the pond closely. Under the current conditions, its presence in the crater has not increased the risk to public safety.

Hazards remain at the LERZ and summit of Kīlauea. Residents and visitors near the 2018 fissures, lava flows and summit collapse area should heed Hawai‘i County Civil Defense and Hawaiʻi Volcanoes National Park closures and warnings. The 2018 lava flows are primarily on private property, and people are asked to be respectful and not enter or park on private property.

Mauna Loa is not erupting. Its USGS Volcano Alert level remains at ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption is certain. A similar increase in activity occurred between 2014 and 2018 and no eruption occurred.

This past week, approximately 60 small-magnitude earthquakes—all less than M2.5—occurred beneath the upper elevations of Mauna Loa. Deformation measurements show continued summit inflation, suggestive of recharge of the volcano’s shallow magma storage system. No significant changes in volcanic gas release on the Southwest Rift Zone were measured, and fumarole temperatures there and at the summit remain unchanged.

Mauna Loa updates are issued weekly. For more info on the status of the volcano, visit https://volcanoes.usgs.gov/volcanoes/mauna_loa/status.html.

Two earthquakes with three or more felt reports occurred in Hawai‘i this past week: a magnitude-3.1 quake four miles southwest of Kailua-Kona at a six-mile depth on Aug. 22, 2019, at 7:54 p.m. HST, and a magnitude-4.2 quake 35 miles southeast of Pāhala at a depth of 29 miles, also on Aug. 22 at 4:33 a.m. HST.

HVO continues to closely monitor both Kīlauea and Mauna Loa for any signs of increased activity.

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