Earthquakes in Hawai‘i—an underappreciated but serious hazard

U.S. GEOLOGICAL SURVEY—REDUCING THE RISK FROM VOLCANO HAZARDS

he State of Hawaii has a history of damaging earthquakes. Earthquakes in the State are primarily the result of active volcanism and related geologic processes. It is not a question of “if” a devastating quake will strike Hawaiÿi but rather “when.” Tsunamis generated by both distant and local quakes are also an associated threat and have caused many deaths in the State. The U.S. Geological Survey (USGS) and its cooperators monitor seismic activity in the State and are providing crucial information needed to help better prepare emergency managers and residents of Hawaiÿi for the quakes that are certain to strike in the future.

Each year, thousands of earthquakes occur in the State of Hawaii. As in other seismically active regions around the world, the vast majority of these are microearthquakes. Earthquakes in the State are detected by sensitive seismometers operated by the Hawaiÿi Integrated Seismic Network. Sometimes the earthquakes are large enough to be felt by people. Less frequently, strong quakes occur like those that struck the northwest coast of the Island of Hawaiÿi on the morning of October 15, 2006—a magnitude 6.7 quake at 7:07 a.m. and a magnitude 6.0 quake at 7:14 a.m. These earthquakes damaged roads and structures on Hawaiÿi and Maui, the islands nearest the quakes’ epicenters.

Earthquakes and Volcanoes

Earthquakes in the Hawaiian Islands chain are closely linked to active volcanism. Most quakes occur closest to Kïlauea and Mauna Loa Volcanoes, along the southeastern coastline of the Island of Hawaiÿi, extending inland beneath the flanks of these volcanoes. Earthquake activity is also evident at Löÿihi seamount, the State’s youngest volcano, about 22 miles (~35 km) southeast of the Island of Hawaiÿi and still some 3,000 feet (1,000 m) beneath the surface of the Pacific Ocean. Rates of earthquake activity decrease progressively to the northwest, toward the older islands in the Hawaiian Islands chain.

Where an earthquake is located typically offers a basic understanding of its underlying cause. Earthquake patterns showing events closely spaced in time with points of origin, or “hypocenters,” clustered within the summit caldera and rift zone regions of Hawaiÿi’s active volcanoes indicate magma (molten rock) movement within the volcanoes. Away from active volcanic centers, on faults buried within and beneath the flanks of the volcanoes, quakes occur in response to stress changes resulting from magma movement and adjustments to the geologic structure of an island. In these regions, quakes occur on, and help delineate, active fault structures and geometries within volcanic edifices. On the Island of Hawaiÿi, one such prominent fault structure lies at a depth of roughly 6 miles (~9 km). This is thought to represent the interface between ancient oceanic crust and the overlying, much younger volcanic edifice. Most of the State’s large and damaging historically recorded quakes are thought to have occurred along this interface.

Another type of earthquake reflects the flexing or bending of the Earth’s crust and upper mantle, or “lithosphere,” beneath the weight of the Hawaiian Islands. Lithospheric earthquakes occur deep beneath the Islands and are more common beneath Hawaiÿi and Maui, the geologically youngest islands. Some of these earthquakes, like the October 15, 2006, quakes, can be large and damaging. Such quakes are caused by the weight of the islands rather than by active

Large Earthquakes in Hawai‘i, 1823–2010

volcanism. Similar powerful quakes could occur beneath older Hawaiian Islands and closer to the State’s larger and more densely concentrated populations like those on Oÿahu.

Measuring Earthquake Intensity

Most effects of earthquakes felt or observed by people are the result of shaking produced by seismic waves. In the United States, these effects or the “intensity” of a quake are compared using the Modified Mercalli Intensity

Pacific Ocean

(MMI) scale. The effects of a quake on the Earth’s surface, humans, objects of nature, and manmade structures are ranked on a scale of I through XII; for example, I means not felt and XII means total destruction. Generally the values are higher closest to the quake’s epicenter but will vary depending on local geology. Historically, the most extreme earthquake effects in Hawaiÿi were recorded from the April 1868 Great Kaÿu earthquake, but this quake predated instrumental seismic monitoring in the State. Using historical accounts of this earthquake, intensities of VII to XII were inferred to have occurred on the Island of Hawaiÿi, leading the USGS scientists to estimate the quake’s magnitude at about 7.9. The earthquake produced tsunamis and landslides and caused stone buildings to collapse, killing a total of 77 people.

Earthquakes and Tsunamis

Hawaiians know that their islands have been struck by tsunamis produced by distant earthquakes, such as the 1960 magnitude 9.5 Chilean quake that generated a tsunami, which killed 61 people in Hilo, and the March 2011 magnitude 9.0 quake in Japan. However, many people are possibly unaware that large earthquakes in Hawai‘i, like the 1868 Great Ka‘u quake, have also produced tsunamis with devastating local impacts. The maximum height of the tsunami produced by the 1868 earthquake was approximately 45 feet (14 m) on the southeast coast of the Island of Hawai‘i, and 46 people were killed by the waves. More recently, the 1975 magnitude 7.2 Kalapana earthquake, also along the southeast coast of the Island of Hawai‘i, produced a tsunami with maximum height of roughly 45 feet (14 m), killing two people.

Because of the proximity of source zones for earthquakes to low-lying

MODIFIED MERCALLI INTENSITIES FOR THE 1868 GREAT KA‘U EARTHQUAKE

II-III VII VIII V IX IV X+

Pacific Ocean

coastal regions in Hawaiÿi, there is little time to react and respond to such earthquakes and their associated tsunamis. The tsunami generated by the 1975 quake reached populated areas along that coastline in seconds and Hilo Harbor in less than 20 minutes. Emergency managers in the State are concerned about tsunamis generated by quakes along the western coast of the Island of Hawaiÿi. It has been estimated that a tsunami from such a quake would reach Honolulu in 35 minutes.

Seismic Monitoring in Hawai‘i

There are three principal groups that monitor earthquakes in the State of Hawaii—the U.S. Geological Survey (USGS) Hawaiian Volcano Observatory, the USGS National Strong Motion Project, and the Pacific Tsunami Warning Center (PTWC) of the National Oceanic and Atmospheric Administration (NOAA). Although each group has specific seismic monitoring objectives, improvements and upgrades to seismic instruments and data transmission technologies are presenting new opportunities for cooperatively operating seismic monitoring stations and integrating and coordinating data sharing, data handling, and earthquake reporting among these three groups. To meet the State’s seismic monitoring needs, the USGS and PTWC, in cooperation with the State of Hawaii Civil Defense and other organizations, have formed the Hawaiÿ i Integrated Seismic Network (HISN).

Using seismic data streams from the HISN and other sources, the USGS automatically generates and posts on the Internet the time, location, and magnitude of quakes in Hawaiÿi. The USGS also automatically posts enhanced earthquake information products, including near real-time earthquake analysis and post-earthquake decision-making tools. The data and related products are shared among USGS and NOAA partners for enhanced hazard warning capabilities. The information products are intended to assist with effective post-earthquake response and recovery efforts. Through online questionnaires, the USGS also collects reports of earthquake effects and earthquake damage from the general public. As quickly after a significant earthquake as possible, these are automatically compiled and posted to the Web in the form of maps that reflect the intensity and extent of a quake’s impact.

The Earthquake Hazard in Hawai‘i

Because of Hawaiÿi’s history of damaging earthquakes and its active volcanism, it is not a question of “if” another devastating quake will strike but rather “when.” Hawaiÿi’s recorded history of damaging quakes provides helpful clues about the impact of future large earthquakes.

Probabilistic seismic hazard maps quantify the shaking hazard posed by future earthquakes. The USGS has produced a new generation of seismic hazard maps, recently developing and applying a consistent methodology across the entire United States. Important input and participation in arriving at the final products came from local expertise and experience. The National Seismic Hazard Maps are intended to reflect the best available science in earthquake hazards estimation. As of 2000, the USGS seismic hazards maps have been incorporated into all U.S. model building codes.

Seismic hazards maps show levels of

Community Internet Intensity Map

earthquake ground motions computed for specified probability levels of exceedance, within specified timeframes. Often, these are stated as 10% probabilities that calculated levels of ground motion will be exceeded in a 50-year period. This is roughly equivalent to saying that these ground motions have a 1-in-475 chance of being exceeded in any given year.

Hawaiÿi seismic hazard maps, along with hazards maps for other regions, are available online at http://earthquake.usgs.gov/hazards/.The online Hawaiÿi maps show both 2% and 10% exceedance probabilities for a 50-year time interval. There is general agreement between the hazard maps and historical earthquake experience in Hawaiÿi. It is also worth noting that the entire southeast coast of the Island of Hawaiÿi is associated with 50-yearexceedance probabilities for ground motions of 100% g (g is the acceleration due to gravity or 980 cm/sec/sec), comparable to those associated with the maximum Modified Mercalli Intensity (MMI) values recorded from the 1868 Kaÿu earthquake. As incorporated in updated seismic provisions of building codes, the hazards maps will help mitigate against future earthquake losses as Hawaiÿi’s built environment expands and evolves. Hawaiÿi seismic hazards maps are expected to be updated, taking advantage of additional data from Hawaiÿi’s damaging earthquakes of October 2006, as well as data derived from the HISN.

The work of the USGS and its cooperators to monitor seismic activity in the State of Hawaii is providing crucial information needed to help better prepare emergency managers and residents of Hawaiÿi for the quakes that are certain to strike in the future. This effort is only part of the USGS hazards programs’ ongoing efforts to protect people’s lives and property, including in all of the volcanic regions of the United States.

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