Red dots show updated locations and magnitudes of three earthquakes that occurred on or around the Islands of O‘ahu and Maui in late March and early April 2016, as revised by seismologists at the U.S. Geological Survey’s Hawaiian Volcano Observatory. The blue dots on and near the Island of Hawai‘i show the locations of small (most were less than magnitude-3) earthquakes that occurred between March 28 and April 14, 2016. USGS map.

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

Each year, Hawai‘i County residents likely feel dozens of earthquakes. They might readily agree that earthquakes are indeed part of living on the Island of Hawai‘i. But residents across the State of Hawaii, recently nudged by three small earthquakes, were gently reminded that they, too, live with earthquakes.

Early morning on March 28, 2016, a magnitude-3.6 (M3.6) earthquake struck beneath the isthmus connecting West and East Maui. Four days later, late on the night of March 31, a M4.2 earthquake, centered roughly 88 km (55 mi) east of Hana, Maui, was felt across the State. Three days later, on the morning of April 5, a M2.8 earthquake, centered about 5 km (3 mi) off Diamond Head, shook parts of Honolulu. While all three were considered small earthquakes, they were large enough that people felt them and submitted reports at the U.S. Geological Survey (USGS) “Did You Feel It?” webpage (http://earthquake.usgs.gov/data/dyfi/).

Typically, an earthquake’s location provides the first clues as to its cause. The recent Maui and O‘ahu earthquakes—recorded State-wide by seismographic networks of the USGS Hawaiian Volcano Observatory (HVO) and NOAA Pacific Tsunami Warning Center—were all located at depths between about 18 and 36 km (11–22 mi).

At such depths, and away from centers of volcanic activity, such as Kīlauea or Mauna Loa on Hawai‘i Island, the flexing, or bending, of the Earth’s lithosphere (the crust and uppermost rigid layer of the mantle) due to the weight of the islands produces forces that result in occasional earthquakes. Lithospheric flexure occurs over geologic time, with associated earthquakes occurring much less frequently, compared to earthquakes more directly linked to active Hawaiian volcanism. Volcanic processes, which can rapidly change conditions within active volcanoes, generate many thousands of earthquakes recorded by HVO each year.

When relatively infrequent earthquakes occur within a week of one another, it’s reasonable to wonder whether they might be closely linked. Their connections are possibly more difficult to prove than to disprove. We lack sufficiently detailed information and understanding of conditions at these depths beneath Hawaii to explain why one small earthquake would lead to another, islands apart.

Awareness of relevant earthquake history and experiences is essential, especially when earthquakes occur and are felt in relatively inactive regions like the Counties of Maui and Honolulu. In general terms, areas where large earthquakes have occurred in the past are expected to continue to experience large earthquakes in the future.

USGS Bulletin 2006, Isoseismal Maps, Macroseismic Epicenters, and Estimated Magnitudes of Historical Earthquakes in the Hawaiian Islands, serves as the authoritative reference for large earthquakes in Hawaii between 1823 and 1992. Authors Max Wyss and Bob Koyanagi, former University of Alaska at Fairbanks professor and HVO seismologist, respectively, based their assessments of past earthquakes on historical documents, along with insights derived from modern geology and seismology.

In January 1938, a M6.9 earthquake occurred northeast of Maui. Among its extreme effects on Maui, Wyss and Koyanagi report cracked water catchment and oil tanks and broken water pipelines. Buildings were also damaged on Maui, as well as on O‘ahu, Kaua‘i, and Hawai‘i. It’s not possible to say if the M4.2 earthquake on March 31, 2016, ruptured the same fault as in 1938, but a M6.9 earthquake near Maui today would result in far greater losses now than then.

Wyss and Koyanagi also list two noteworthy earthquakes west of Lāna‘i: a M6.9 earthquake in February 1871 and a M4.8 earthquake in September 1948. Damage from these earthquakes occurred in Honolulu, as well as in Maui County. Because of the difficulties in precisely locating these two older earthquakes, it is uncertain whether the April 5, 2016, M2.8 earthquake occurred on the same fault as the 1871 and 1948 earthquakes. But again, a M6.9 earthquake would result in substantially greater impacts in Honolulu and Maui Counties today than in 1871.

Predicting the precise location, time, and magnitude of a future earthquake is still not possible. Small earthquakes, however, remind us that damaging earthquakes are part of Hawaii’s history, and we must understand how to prepare and respond effectively across the entire State when a large earthquake strikes. Resources like the Great Hawai‘i ShakeOut website (http://www.shakeout.org) and the USGS Earthquake Hazards Program (http://earthquake.gov) offer useful information on earthquake awareness and preparedness.