The legacy of the 7.7 magnitude Kalapana earthquake, 50 years on

(BIVN) – This week's USGS Hawaii Volcano Observatory Volcano Watch article takes a look at the 7.7 magnitude Kalapana earthquake that occurred 50 years ago today.

This week's article was written by HVO geophysicist Ingrid Johansson:

The 1975 Kalapana earthquake brought about major changes in the lives of people affected by deaths and damage, as well as major changes in our understanding of Kilauea's southern flank.

Scientists at the USGS Hawaiian Volcano Observatory (HVO) have been collecting data and forming a hypothesis that Kīlauea's southern flank could produce a large earthquake soon. The 1975 earthquake occurred on November 29, while their findings were awaiting print.

Previous large earthquakes, such as the 7.9 magnitude Great Kau earthquake of 1868, have been attributed to a nearly horizontal fault (décollement) at the boundary between the original oceanic crust and erupting lava from Mauna Loa and Kilauea. It is deepest below the middle of Hawaii Island and becomes slightly shallower towards shore.

USGS: “Road damage in Hawaii Volcanoes National Park caused by the magnitude 7.7 earthquake of November 29, 1975.” (USGS photo)

Seismic and deformation data for the Kalapana earthquake indicated that initial movements were also on the fault fault, but the 1975 rupture was more complex. Vertical displacements have also been observed along the top of several worn cliffs on the south side of Kīlauea. This pali is the surface expression of the Helena fault system. This indicates that both the Helena Fault System and the Helena Detachment Fault slipped during the 1975 earthquake, leading scientists to wonder whether the Helena Fault System might connect directly to the Helena Detachment Fault.

Additional data collection in the past 50 years has shown that this scenario is not the case. One key piece of evidence is from a series of marine seismic reflection profiles published in 2000, a technique that takes images inside the crust, similar to an MRI.

Reflection profiles showed that the head bends in the Helena Fault System bend deeply and do not continue deep enough to intersect the thoracic region. This means that the Helena Fault blocks are smaller than they would be if they were connected to the separation layer and that the two structures are not directly connected.

USGS: “The view from the GPS station continues on the south side of Kilauea, looking over Helena and Huli Pali to the ocean. This device was installed in 1996 to monitor the seaward movement of the south side.” (USGS photo by K. Kamibayashi)

After the 1975 earthquake, there was greater emphasis on monitoring the movement of the south side. The HVO and the University of Hawaii at Manoa installed continuous GPS instruments on the south side of Kilauea that recorded the constant seaward motion of the south side at a rate of about 5 cm (2 in) per year. They also showed that wing motion was not always constant. Every 2-3 years, the wing accelerates for 2-3 days, in a phenomenon called a slow glide event. These events released fault stress energy equivalent to a magnitude 6 earthquake, but they did so too slowly to generate any harmful vibrations.

Slow slip events have been observed on faults around the world, including the San Andreas Fault and the Cascadian Subduction Zone. Observing slow-slip events at Kīlauea was key to showing that their occurrence is not limited to plate boundaries, but is a fundamental consequence of velocity-enhancing fault friction that can occur wherever such frictional conditions exist.

The 2018 magnitude 6.9 earthquake provided further insights into the 1975 quake. There was no evidence of movement on the Balice during this most recent earthquake. Given what we've learned about the structure of the South Side in the past 50 years, that's likely because the 2018 quake was smaller than the 1975 quake. Strong shaking during the 1975 quake may have been what set the Helena fault system in motion, a process called dynamic stimulation.

However, even in the event of a large earthquake, there is a limit to how far the Helena Fault Zone can move. Seismic reflection files published in 2000 showed that the fault fault creates an area of ​​raised crust (“toe”) as it approaches the surface. This finger acts as a brace on the flank, preventing large movements.

Our improved understanding of how the south side of Kīlauea behaves is part of the legacy of the 1975 earthquake, as is our appreciation of the magnitudes of earthquakes that can be generated on the fault line. Being well prepared for earthquakes is always a good idea. Information about how to prepare your home and family for earthquakes can be found at (ready.gov/earthquakes).

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