2018 Alaska Islands earthquake ShakeMap3

On January 23, 2018, at 00:31 AKST, an

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

occurred in the

Gulf of Alaska The Gulf of Alaska (Tlingit: ''Yéil T'ooch’'') is an arm of the Pacific Ocean defined by the curve of the southern coast of Alaska, stretching from the Alaska Peninsula and Kodiak Island in the west to the Alexander Archipelago in the east ...

near

Kodiak Island Kodiak Island ( Alutiiq: ''Qikertaq''), is a large island on the south coast of the U.S. state of Alaska, separated from the Alaska mainland by the Shelikof Strait. The largest island in the Kodiak Archipelago, Kodiak Island is the second la ...

. The earthquake, measured at 7.9 on the scale, was approximately southeast of Kodiak and happened at a depth of . It was initially measured as a M 8.2 event, but later downgraded by the

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

. The earthquake was felt throughout most of southern Alaska, including the major cities of Anchorage and Fairbanks, and parts of neighboring

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

. The earthquake prompted

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...

warnings and advisories for Alaska,

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...

Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

, and

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...

. Residents in low-lying areas along the Gulf of Alaska and in British Columbia were evacuated to shelters and higher ground. The

Pacific Tsunami Warning Center The Pacific Tsunami Warning Center (PTWC) is one of two tsunami warning centers that are operated by NOAA in the United States. Headquartered on Ford Island, HI, the PTWC is part of an international tsunami warning system (TWS) program and s ...

later cancelled most of the alerts within four hours of the earthquake, due to the apparent lack of tsunami. The highest recorded waves after the event measured at under above tide level on Kodiak Island.

Earthquake

According to the

, the earthquake occurred southeast of

Kodiak, Alaska Kodiak ( Alutiiq: , russian: Кадьяк), formerly Paul's Harbor, is the main city and one of seven communities on Kodiak Island in Kodiak Island Borough, Alaska. All commercial transportation between the island's communities and the outside ...

at 12:31 AM local time (AKST). Witnesses to the earthquake itself reported that it was very long in duration, feeling like a "slow roller," but it was not a violent earthquake despite its magnitude and

mercalli intensity The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

, as its epicentre was hundreds of kilometres offshore. This was corroborated by witnesses in Anchorage. There was no immediate damage reported. The earthquake woke people in Anchorage, from the epicenter, and was felt as far as Fairbanks to the northeast and

to the southeast. The earthquake's

S-wave __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...

s arrived in Anchorage within one minute, and reached Fairbanks within three minutes.

Tectonic setting

The earthquake was a strike-slip event that occurred within the Pacific Plate. The

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

was to the south of the

Aleutian Trench The Aleutian Trench (or Aleutian Trough) is an oceanic trench along a convergent plate boundary which runs along the southern coastline of Alaska and the Aleutian islands. The trench extends for from a triple junction in the west with the Ulak ...

, where the Pacific Plate subducts beneath the

North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacif ...

. The region has produced twelve large earthquakes since 1900, including several

megathrust earthquake Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthqu ...

Aftershocks

The first major aftershocks occurred 20 minutes after the earthquake. The strongest aftershock measured 5.5 on the moment magnitude scale, while most measured 4–5. Within two days, the United States Geological Survey recorded over 50 aftershocks. More than 560 aftershocks have been recorded well into March.

Alerts and evacuations

Communities on Kodiak Island and other coastal areas on the Gulf of Alaska and

Cook Inlet Cook Inlet ( tfn, Tikahtnu; Sugpiaq: ''Cungaaciq'') stretches from the Gulf of Alaska to Anchorage in south-central Alaska. Cook Inlet branches into the Knik Arm and Turnagain Arm at its northern end, almost surrounding Anchorage. On its so ...

were evacuated after warning sirens were activated. Hundreds gathered in local schools, which were opened as evacuation shelters, in Kodiak, Seward, and

Homer Homer (; grc, Ὅμηρος , ''Hómēros'') (born ) was a Greek poet who is credited as the author of the ''Iliad'' and the ''Odyssey'', two epic poems that are foundational works of ancient Greek literature. Homer is considered one of the ...

. Evacuees seeking higher ground filled local roads and the parking lots of supermarkets, while locals reported large numbers of evacuees parking near a

wind farm A wind farm or wind park, also called a wind power station or wind power plant, is a group of wind turbines in the same location used to produce electricity. Wind farms vary in size from a small number of turbines to several hundred wind turb ...

on Pillar Mountain near Kodiak. An unrelated power outage at the

Alaska Earthquake Center Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...

in Fairbanks caused some alerts to be delayed, while its website was overwhelmed by the number of users. The

National Weather Service The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...

sent emergency alerts to cell phones in Alaska, reading, "Emergency Alert. Tsunami danger on the coast. Go to high ground or move inland." The National Weather Service and

initially issued a tsunami watch for Hawaii and the entire West Coast, but cancelled them after less than two hours. In the

San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area Go ...

, residents within three blocks of the Pacific coast and five blocks of the San Francisco Bay were warned by the city's Department of Emergency Management to be ready for evacuation. Some aspects of the warning systems in place were delayed or failed to issue a warning at all, prompting multiple reviews of the warning infrastructure.

References

External links