The odds of Hilo experiencing a damaging earthquake have gone up while Honolulu’s chances remain about the same in the latest Hawaii earthquake hazard model by the U.S. Geological Survey.
The updated Seismic Hazard Model for the State of Hawaii, published Wednesday in the science journal Earthquake Spectra, indicates the entire island chain remains vulnerable to damage from an earthquake.
The model shows a 90% chance that the 345,000 people on the islands of Hawaii and Maui could experience damaging levels of shaking during the next 100 years, while a lower but still significant chance of damaging shaking — from 50% to 75% — was pinned to urban Honolulu and much of East and Windward Oahu.
The model, last updated over 20 years ago, is the first to be completed for the latest version of the U.S. National Seismic Hazard Model, which is designed to inform building codes, planning documents, insurance rates and other public decisions. All 50 states are expected to get a similar hazard update.
“This study incorporates new data, models, and methods that were not available 20 years ago,” Mark Petersen, USGS research geophysicist and lead author, said in an email.
Petersen said the past two decades produced an abundance of data from a handful
of sizable earthquakes and volcanic eruptions.
“This has improved our understanding of where earthquakes occur, how often they occur, the sizes of large earthquakes, and how strong the ground will shake,” he said.
The study includes maps, new earthquake catalogs, assessments of activity on active faults using GPS measurements and evaluations of seismic data to define the ground-shaking levels expected from earthquakes across the islands.
As one might expect, ground shaking is forecast to be highest near the active volcanoes of Kilauea and Mauna Loa, where magma action pushes the crust outward toward the ocean along a nearly horizontal fault about
6 miles beneath the surface.
Large earthquakes occurred in this area in 1868, 1975, 2018 and Oct. 10 of this year, when a magnitude-6.2 quake rocked the Big Island.
The 2018 earthquake heralded the initial phases of the destructive Puna eruption and was followed by numerous
collapses of Kilauea’s summit crater. Petersen said the collapses offered data that helped define the shaking levels predicted by the model.
Petersen said levels of ground-shaking on the southernmost islands are comparable to portions of coastal California, including the San Francisco and Los Angeles areas, which are notorious for their damaging earthquakes.
Predicted shaking levels are gradually lower the farther up the island chain you go and the farther you are from the magma source that lies beneath Hawaii
island.
Earthquakes in the region farther from the volcanoes are thought to be caused by the bending of the earth’s crust due to the weight of overlying volcanoes or from nearby oceanic fracture zones, according to scientists.
But Peterson said large earthquakes cannot be ruled out for the western half of the state, which includes Oahu. “They may just occur at lower rates,” he said.
The updated model is based on more data than ever before, offering several hundred maps and localized forecasts that account for the effects of subsurface structures, rocks and soils and whether ground-shaking will be amplified by the geology.
While the model for Hilo forecasts a higher probability of shaking, the model for the Honolulu area is quite similar to the one created over two decades ago.
Rhett Butler, geophysicist with the University of Hawaii’s Hawaii Institute of Geophysics and Planetology, said he was surprised by that finding, especially after conducting a study last year of the 1871 Lanai earthquake and discovering its size was underestimated.
Butler said he examined as much evidence as he could find, including the damage reported on Oahu at the time, and upgraded the probable magnitude to 7.5 from about 7.0. That makes it the third-largest earthquake in Hawaii history and the largest historic earthquake northwest of the Big Island.
Oahu College, the former name of Punahou School, endured substantial damage from the temblor — although most of it probably wouldn’t have happened today with the stricter building codes, he said.
Butler, a member of the Hawaii Earthquake and Tsunami Advisory Committee, said that while he was surprised Honolulu’s ground-
shaking forecast remained the same, he understands why.
“You have to look at the bigger picture,” he said, noting that the model considered the geology and its effect on ground-shaking, among other things.
While the level of earthquake activity is still considered high in Hawaii, it appears to have declined in comparison with the past century — a fact that some scientists have attributed to fewer eruptions of Mauna Loa.
But could that change? Could the world’s largest active volcano — last heard from in 1984 — rumble back to life and make damaging earthquakes even more common in Hawaii?
Petersen said the possibility is considered in the new model, with forecasts of earthquake activity both reverting to previous levels and continuing at the current level.
Mauna Loa has indeed shown signs of restlessness. Three years ago the U.S. Geological Survey’s Hawaiian Volcano Observatory changed the volcano’s alert level from “normal” to “advisory” following a steady increase in earthquakes and ground-swelling. “Advisory” means the volcano is showing signs of elevated unrest compared with the usual long-term background levels.
Researchers at the University of Miami in Florida published a paper this summer suggesting Mauna Loa is nearing its next eruption and that a magnitude-6 earthquake or greater could set it off.
Well, there was a 6.2-magnitude earthquake — in October — but the resting giant was unmoved.
Instead, Kilauea stole the show when it roared again Sept. 29. Since then Hawaii’s youngest volcano has oozed an estimated 10 billion gallons of lava into Halemaumau Crater, according to HVO. As of Friday it continued to erupt from a single vent in the crater’s western wall.
Petersen said the USGS has been wanting to update the Hawaii hazard model for some time, but it didn’t have the resources to make it happen sooner.
”The model improves the knowledge of shaking so engineers can improve designs of both short and tall buildings,” he said. “We need buildings to be built strong enough or flexible enough to resist the shaking without collapsing.”
The model will not only enhance building codes and seismic safety, he said, but potentially save resources by preventing overbuilding in areas where earthquakes are less likely.
