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Earthquakes are scientifically defined as the sudden release of strain (or displacement of rock) in the earth's crust, resulting in waves of shaking that radiate outward from the earthquake source. They may result from crustal strain, volcanism, landslides or the collapse of caverns. Earthquakes can occur underwater or on land. Earthquakes can affect hundreds of thousands of square miles. Their intensity ranges from very minor (shaking not detected by humans without instruments) to very violent (catastrophic in nature). Damages follow this intensity ranging from minor to catastrophic. Earthquakes also occur without warning, resulting in deaths and injuries.
There are approximately twelve major plates and several dozen more minor plates on the earth’s crust. Plates are regions of the crust that continually move over the mantle. Areas where these plates meet, grind past each other, dive under each other, or spread apart, are called plate boundaries. Most earthquakes are caused by the release of stresses accumulated due to the sudden displacement of rock along opposing plates in the Earth's crust. The areas bordering the Pacific Plate, also known as the "Pacific Ring of Fire", are at a particularly high risk since most of the largest earthquake events of the last century have occurred in the region.
While earthquakes typically occur along plate boundaries, they can affect hundreds of thousands of square miles, causing damage to property (measured in the tens of billions of dollars), resulting in loss of life and injury to hundreds of thousands of persons, and disrupting the social and economic functioning of the affected area. The point where an earthquake starts is termed the focus or hypocenter and may be many miles to several hundred miles deep within the earth. The point at the surface directly above the focus is called the earthquake’s epicenter.
Most property damage and earthquake-related deaths are caused by the failure and collapse of structures due to ground shaking. The level of damage depends upon the amplitude and duration of the shaking, which are directly related to the earthquake size, distance from the fault, site, and regional geology. Other damaging earthquake effects include landslides, the down-slope movement of soil and rock (mountain regions and along hillsides), and liquefaction, in which ground soil loses the ability to resist shear and flows much like quicksand. In the case of liquefaction, anything relying on the substrata for support can shift, tilt, rupture, or collapse.
Earthquakes are measured in terms of their magnitude and intensity.
Magnitude Scales: Earthquake magnitude is measured using the Richter Scale or Moment Magnitude Scale. Beginning in 2002, the USGS began using Moment Magnitude as the preferred measure of magnitude for all USGS earthquakes greater than magnitude 3.5. This was primarily due to the fact the Richter Scale has an upper bound, so large earthquakes were difficult to measure. Moment Magnitude also has a scale, but no instrument is used to measure it. Instead, factors such as the distance the earthquake travels, the area of the fault, and land that was displaced (also known as “slip”) are used to measure moment magnitude. At the bottom of the scale, <3.5, earthquakes are unlikely to be felt by the public; damage starts around 5.4-6.0 for poorly designed buildings, and major damages above 6.0. Extensive damage is expected around 7.0 and greater, even in cities with modern building codes.
Intensity Scales: Intensity is most commonly measured using the Modified Mercalli Intensity (MMI) Scale based on direct and indirect measurements of seismic effects. The scale levels are typically described using roman numerals, ranging from “I” corresponding to imperceptible (instrumental) events to “XII” for catastrophic (total destruction).
Liquefaction: Liquefaction is the phenomenon that occurs when the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading.100 Essentially, the soil acts like a fluid, similar to wet sand near the beach, resulting in ground failure. Liquefaction causes several types of ground failure: lateral spread/flow failures and loss of bearing strength. Lateral spread develops on gentle slopes and entails the sidelong movement of large masses of soil as an underlying layer liquefies. Similarly, flow failures also displace large masses of soil laterally, generally on slopes greater than three degrees. Loss of bearing strength results when the soil supporting structures liquefies and causes structures to collapse. Liquefaction can also cause increased lateral pressure on retaining walls when the soil behind a wall liquefies, causing failure or displacement.
Hawai’i experiences thousands of earthquakes every year, typically caused by eruptive processes within active volcanoes. Shaking from earthquakes associated with volcanic activity are typically too small to cause damage. In Maui County, these small earthquakes, often referred to as swarms, are located around the Haleakalā volcano in eastern Maui. However, a major earthquake event could impact the entire county. Fault locations and earthquake risk areas also help define location. Notable faults within Maui County include the West Maui Fault, East Molokaʻi Fault, and an extensive fault system on Lānaʻi. Earthquakes occurring on other Hawaiian Islands, such as the Big Island of Hawai’i, or in the Pacific Ocean, have the potential to impact Maui County.
Previous research by the NOAA Coastal Service Center identified high liquefaction susceptibility areas in Maui, including the community planning areas of West Maui (from Lahaina to Nāpili), Kīhei-Mākena, and Wailuku-Kahului.
Use the RAMP application to view earthquake hazard and liquefaction zones across Maui County.
According to the NOAA Significant Earthquake Database, 13 significant earthquakes have been recorded in the Hawaiian Islands since 1850, most of which occurred on the big Island of Hawai’I, just south of Maui County. In addition to events reported by the NOAA Significant Earthquakes Database, the earthquakes described below were noted in the 2015 Maui County Hazard Mitigation Plan Update as having a significant impact on Maui County.
1871 Lānaʻi Earthquake: The Lānaʻi earthquake of 1871 was one of the most significant earthquakes to have affected Maui County. From the geographic distribution of its effects, its magnitude is estimated to have been 7.0 and its epicenter to be near the south coast of the Island of Lānaʻi.
1938 Maui Earthquake: The January 22, 1938 Magnitude-6.9 earthquake had an epicenter about 12 miles northeast of Keʻanae Point in East Maui. The Island of Maui suffered more damage than any other Hawaiian Island, though there were few injuries and no mortalities. Damage on Molokaʻi and Lānaʻi was small and resulted from a few ground cracks. No tsunami accompanied the shock.
2006 Kiholo Bay and Mahukona Earthquakes: The most recent earthquakes to strike Hawai’i were the Kiholo Bay and Mahukona earthquakes of October 15, 2006, centered in the Pacific Ocean west of the Island of Hawai’i. The Magnitude-6.7 Kiholo Bay earthquake occurred at 7:07 a.m., followed by the Magnitude-6.0 Mahukona earthquake at 7:14 a.m. Although the Mahukona earthquake was the smaller of the two in magnitude, its intensity of was reportedly equal to or greater than that of the Kiholo Bay earthquake in some areas due to its shallower depth. On the Island of Maui, the earthquakes induced several landslides and rockfalls along Piilani Highway (Highway 31) on the southeastern coast. About 500 residents were cut off in the Manawainui area due to an incipient rockfall hazard and due to closure of a bridge damaged by erosion at Pahihi. This event resulted in a disaster declaration for Maui County, with estimated damages for the county at $28.1 million.
One way to measure the extent of an earthquake is peak ground acceleration. USGS peak ground acceleration (PGA) measures the intensity of an earthquake. It is the probability that ground motion will reach a certain level during an earthquake (the fastest measured change in speed, for a particle at ground level that is moving horizontally due to an earthquake). PGA is expressed as g (the acceleration of gravity, equivalent to g-force), where a higher level means higher shaking. It is frequently stated as “x percent probability of exceedance in C years,” For data stating PGA (%g) as “10 percent probability of exceedance in 50 years,” a map would indicate that there is a 10 percent probability of reaching that level of shaking (%g) in 50 years. (It is meant to show the upper bounds of possible shaking).
Given an input of 2% probability of exceedance in 50 years, most of Maui County, including the Island of Maui and part of eastern Molokaʻi, have a PGA value of 33%g (strong shaking - damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built structures). The remainder of the county, including most of Molokaʻi and all of Lānaʻi, have an estimated PGA value of 17%g (moderate shaking - felt by all, many frightened; some heavy furniture moved; a few instances of fallen plaster; damage is slight).
Several active faults are within Maui County. Further, the Island of Hawai’i, south of Maui County, is known for having seismic activity. The probability of future major earthquake events impacting Maui County was determined using historic occurrence information. Twenty-five significant (above a 6.0 magnitude) events have been reported over a 170-year reporting period, resulting in a significant earthquake every 6-7 years, on average. Considering the number of major historic earthquakes occurring in or near Maui County, the probability assigned to the entire county for a significant earthquake event is “possible” (1 to 10 percent annual probability). Due to the ability of earthquakes to have a regional impact, all community planning areas in Maui County were assigned the same probability.
It can be assumed that all existing and future buildings, infrastructure, and populations are at risk to the earthquake hazard. There are several factors that impact vulnerability to earthquake in Maui County.
An earthquake could result in deaths, injuries, property damage, environmental damage and disruption of normal services and business activities. The effects could be aggravated by collateral emergencies such as fires, flooding, tsunamis, landslides, hazardous material spills, utility disruptions, and transportation emergencies. Aftershocks to major earthquakes could also be large enough to cause damage.
Buildings and Infrastructure: All buildings, including critical facilities, are at risk to earthquakes, but certain structures may be at a higher risk. Any structures not built to the Hawai’i State Building Code are at higher risk for damage or collapse. In addition, certain building types, such as unreinforced masonry and concrete, are at a higher risk of damage. Softer soils may collapse or slide during an earthquake, and soils subject to liquefaction may cause buildings to sink or foundations to fail. All infrastructure within Maui County is also considered at risk to earthquakes, including all pipes, roads, bridges, dams, water and wastewater treatment facilities, and utility poles. During earthquakes, underground infrastructure, such as water and sewer systems and natural gas pipelines, are especially vulnerable. Damaged or impassable roads may cut off certain populations from vital resources. In addition, in the event that a dam is damaged during an earthquake, there is potential for dam failure. Further, infrastructure in areas with liquefaction-prone soils may sink (e.g., drinking water pipes) or buried infrastructure may float to the surface (e.g., manholes or other infrastructure containing air). Secondary impacts from an earthquake, such as rock falls or fires, may also damage infrastructure. Much of Maui County’s infrastructure was built in the 1970s and may not be seismically constructed or retrofitted to withstand strong earthquakes. The County of Maui 2020 HMP used model earthquake scenarios to estimate structural damage for two scenario earthquakes; refer to the HMP document for additional information, including loss and damage estimates.
Health and Safety: It can be assumed that all existing and future populations are at risk to the earthquake hazard. Injuries are possible if earthquake shaking causes items to fall off shelves or walls, or if shaking is severe enough to cause structural damage. Damages to structures or infrastructure could have impacts on the population. For instance, down power lines could result in power outages, or a failed dam may result in localized flooding. Populations living in housing built before 1950, and prior to the enactment of modern building codes, are more susceptible to damages and losses as a result of an earthquake. Earthquakes typically occur with little to no warning. Therefore, evacuations are unlikely for an earthquake event, but individuals should take cover under a heavy, sturdy object (such as a desk or table) in the event of an earthquake. Earthquakes that are strong enough to damage infrastructure may have public health impacts, such as contaminated water supply, fires from natural gas leaks, or prolonged power outages (which can especially impact public health when combined with extreme temperatures.
Economic Impacts: Economic impacts of an earthquake in Maui County include losses from damaged buildings and infrastructure, building contents (e.g., goods falling off shelves in grocery stores), and disruption of services, such as loss of power and water service). Loss of service could cause prolonged business interruptions and reduced tourism for significant events.
