Each year, Hawaii County residents likely feel dozens of earthquakes. They might readily agree that earthquakes are indeed part of living on the Big Island.
But residents throughout the state, recently nudged by three small earthquakes, were gently reminded they, too, live with earthquakes.
During the early morning of March 28, a magnitude-3.6 earthquake struck beneath the isthmus connecting West and East Maui. Four days later, late on the night of March 31, an M4.2 earthquake, centered roughly 88 km (55 mi) east of Hana, Maui, was felt throughout the state. Three days later, the morning of April 5, an 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 who felt them submitted reports at the U.S. Geological Survey “Did You Feel It?” web page (http://earthquake.usgs.gov/data/dyfi/).
Typically, an earthquake’s location provides the first clues as to its cause.
The recent Maui and Oahu earthquakes — recorded statewide by seismographic networks of the USGS Hawaiian Volcano Observatory and National Oceanic and Atmospheric Administration 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 Kilauea or Mauna Loa, the flexing, or bending, of the Earth’s lithosphere (the crust and uppermost rigid layer of the mantle) because of the weight of the islands produces forces that result in occasional earthquakes. Lithospheric flexure occurs through 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 disprove.
We lack sufficiently detailed information and understanding of conditions at these depths beneath the Hawaiian Islands 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 such as Maui and Honolulu counties. In general terms, areas where large earthquakes 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, an M6.9 earthquake occurred northeast of Maui. Among its extreme effects on Maui, Wyss and Koyanagi reported cracked water catchment and oil tanks and broken water pipelines. Buildings were also damaged on Maui, as well as on Oahu, Kauai and the Big Island. It’s not possible to say if the M4.2 earthquake March 31 ruptured the same fault as in 1938, but an M6.9 earthquake near Maui today would result in far greater losses.
Wyss and Koyanagi also list two noteworthy earthquakes west of Lanai: an M6.9 earthquake in February 1871 and an 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 M2.8 earthquake occurred on the same fault as the 1871 and 1948 earthquakes. But again, an M6.9 earthquake would result in substantially greater impacts in Honolulu and Maui counties today.
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 throughout the entire state when a large earthquake strikes. Resources such as the Great Hawaii ShakeOut website (www.shakeout.org) and the USGS Earthquake Hazards Program (http://earthquake.gov) offer useful information about earthquake awareness and preparedness.
Volcano activity update
Kilauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 31 and 58 m (102–190 ft) below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, mapping April 12 showed scattered lava flow activity within about 5.7 km (3.5 mi) of Pu‘u ‘O‘o. These flows were not threatening nearby communities.
Mauna Loa is not erupting. Seismicity remains elevated above long-term background levels, but no significant changes were recorded during the past week. GPS measurements show continued deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone of Mauna Loa, with inflation recently occurring mainly in the southwestern part of the magma storage complex.
No earthquakes were reported felt in the past week.
Visit the HVO website (http://hvo.wr.usgs.gov) for past Volcano Watch articles, Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos, recent earthquakes info, and more; call for summary updates at 808-967-8862 (Kilauea) or 808-967-8866 (Mauna Loa); email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey`s Hawaiian Volcano Observatory.
