On October 10, 2021, a magnitude-6.2 earthquake near the southern coast of the Big Island shook residents and visitors across the state of Hawaiʻi.
By Liliane M.L. Burkhard, Lauren A. Ward, Helen A. Janiszewski, Ph.D., and Bridget R. Smith-Konter, Ph.D., University of Hawaiʻi at Mānoa, and Jonathan R. Weiss, Ph.D., NOAA/NWS Pacific Tsunami Warning Center
Citation: Burkhard, L.M.L, Ward, L.A., Janiszewski, H.A., Smith-Konter, B.R., Weiss, J.R., 2021, Hawaiʻi offshore quake related to tectonic plate bending, Temblor, http://doi.org/10.32858/temblor.215
When a magnitude-6.2 earthquake struck offshore of the southern tip of the Big Island of Hawaiʻi last week, shaking was felt across the Hawaiian chain, including in Honolulu on the island of O‘ahu, some 160 miles (260 kilometers) away.
The earthquake occurred at a depth of 22 miles (35 kilometers). According to the U.S. Geological Survey (USGS), it resulted from simultaneous horizontal and vertical motion along a reverse fault below Earth’s surface. This type of “oblique reverse faulting” happens when rocks are both sheared and compressed.
Big Island earthquakes are often triggered by magma flow under Kīlauea or Mauna Loa, the two active volcanoes on the island. These shallow quakes are generally small, and most are imperceptible by all but sensitive seismic instruments. Swarms of these earthquakes sometimes precede an eruption.
However, two other types of earthquakes also occur in Hawaiʻi. These are caused by either faulting within the flanks of the active volcanoes or the bending (“flexure”) of the Earth’s crust and upper mantle due to the weight of the islands (Watts and Ten-Brink, 1989; Wessel, 1993; Klein, 2016). Volcano flank and flexure-related earthquakes can be quite large.
Big Hawaiian flank quakes
The largest Hawaiian temblor in the past 150 years was the 1975 magnitude-7.7 Kalapana earthquake, which was a flank event that struck along the southeastern shore of the Big Island. The event moved the south flank of Kīlauea volcano southward (e.g., Swanson et al., 1976; Lipman et al., 1985; Nettles et al., 2004; Owen and Burgmann, 2006; Chen et al., 2019) and triggered a local tsunami, which caused two fatalities. It caused about $4.1 million in property damage.
In 2018, a similar type of flank event occurred nearby at a depth of 3.6 miles (5.8 kilometers). This earthquake was related to new lava vent formation in the region during the 2018 eruption (Neal et al., 2019) and likely resulted from slip on a large fault that separates the volcano from the underlying oceanic crust (Lay et al., 2018; Chen et al., 2019).
Plate flexure earthquakes are not uncommon
Earthquakes related to flexure occur at depths ranging from 10 to 35 miles (15 to 60 kilometers), and are not unusual in Hawai‘i. Last week’s event was likely caused by flexure of the plate. Similar to the way that a diving board flexes when a person stands on the end, the crust flexes under the weight of the islands. Of the Hawaiian Islands, the Big Island is currently pushing the plate downward with the greatest force and is subsiding at around 0.1 inches (2.5 millimeters) per year. In contrast, Oʻahu is slowly being lifted upwards because it sits on top of a bulge that forms in response to the flexure. Flexure-related earthquakes occur when stresses related to the bending are suddenly released when a fault slips.
What makes last week’s earthquake particularly interesting is its large magnitude. Most of these deep earthquakes are less than about magnitude-5.0 (Klein, 2016). Only four flexure-related earthquakes of magnitude-6.0 and larger have been recorded since the damaging magnitude-6.2 Honomu earthquake in 1973, which struck north of Hilo and caused over $5.6 million in damage. The largest recorded flexure earthquake was the offshore 2006 magnitude-6.7 Kīholo Bay event, which produced a magnitude-6.0 aftershock (McGovern, 2007).
Fortunately, despite the widespread shaking that occurred, last week’s earthquake did not generate a tsunami. To date, the earthquake has been followed by at least 30 aftershocks, the largest of which was a magnitude-4.2 that struck five minutes after the main quake. Neither the mainshock nor any aftershocks appear to have had any effect on the ongoing eruption at Kīlauea.
This event is also likely unrelated to an ongoing deep seismic swarm in the Pāhala region on the Big Island’s south-central coast, located about 30 miles (50 kilometers) to the northeast. The Pāhala activity is likely related to an increase in mantle-derived magma driving volcanic and seismic unrest at Kīlauea, Mauna Loa and the offshore Kamaʻehuakanaloa (formerly known as Lōʻihi) volcano (Burgess and Roman, 2021).
Although major damage on the islands is unlikely based on the low level of shaking that occurred, Hawaiʻi County Civil Defense recommends residents inspect their homes. This earthquake serves as a reminder that visitors and residents throughout Hawaiʻi should be prepared for a variety of geohazards including earthquakes, tsunamis and volcanic eruptions.
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