  Anchorage  | |
| UTC time | 2018-01-23 09:31:40 |
|---|---|
| ISC event | 611653794 |
| USGS-ANSS | ComCat |
| Local date | January 23, 2018 |
| Local time | 00:31:42Alaska Standard Time |
| Magnitude | 7.9Mww[1] |
| Depth | 25 km (15.5 mi) |
| Epicenter | 56°02′46″N149°04′23″W / 56.046°N 149.073°W /56.046; -149.073 |
| Type | Strike-slip |
| Areas affected | United States Canada |
| Max. intensity | MMI V (Moderate)[2] |
| Tsunami | Yes |
| Aftershocks | 564 (As of March 1, 2018)[3] |
| Casualties | None |
On January 23, 2018, at 00:31AKST, anearthquake occurred in theGulf of Alaska nearKodiak Island. The earthquake, measured at 7.9 on theMw scale, was approximately 280 kilometers (170 mi) southeast ofKodiak and happened at a depth of 25 kilometers (16 mi).[2]
It was initially measured as a M 8.2 event, but later downgraded by theUnited States Geological Survey.[4] The earthquake was felt throughout most of southern Alaska, including the major cities ofAnchorage andFairbanks, and parts of neighboringBritish Columbia.[5][6]
The earthquake promptedtsunami warnings and advisories for Alaska,British Columbia,Washington,Oregon,California, andHawaii. Residents in low-lying areas along the Gulf of Alaska and in British Columbia were evacuated to shelters and higher ground. ThePacific Tsunami Warning Center later cancelled most of the alerts within four hours of the earthquake, due to the apparent lack of tsunami.[4][7] The highest recorded waves after the event measured at under 8.3 inches (21 cm) above tide level on Kodiak Island.[8]
According to theUnited States Geological Survey, the earthquake occurred 175 miles (282 km) southeast ofKodiak, Alaska at 12:31 AMlocal time (AKST).[2] 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 andmercalli intensity, as its epicentre was hundreds of kilometres offshore. This was corroborated by witnesses in Anchorage.[9][7] There was no immediate damage reported.[7] The earthquake woke people inAnchorage, 350 miles (560 km) from the epicenter, and was felt as far asFairbanks to the northeast andBritish Columbia to the southeast.[5][6] The earthquake'sS waves arrived in Anchorage within one minute, and reached Fairbanks within three minutes.[10]
The earthquake was astrike-slip event that occurred within thePacific plate. Theepicenter was to the south of theAleutian Trench, where the Pacific platesubducts beneath theNorth American plate. The region has produced twelve large earthquakes since 1900, including severalmegathrust earthquakes.[2][11]
The first majoraftershocks occurred 20 minutes after the earthquake. The strongest aftershock measured 5.5 on the moment magnitude scale, while most measured 4–5.[7][12] Within two days, the United States Geological Survey recorded over 50 aftershocks.[13] More than 560 aftershocks have been recorded well into March.
Communities on Kodiak Island and other coastal areas on the Gulf of Alaska andCook Inlet were evacuated after warning sirens were activated. Hundreds gathered in local schools, which were opened as evacuation shelters, in Kodiak,[14]Seward,[6] andHomer.[15] Evacuees seeking higher ground filled local roads and the parking lots of supermarkets,[7] while locals reported large numbers of evacuees parking near awind farm on Pillar Mountain near Kodiak.[5] An unrelated power outage at theAlaska Earthquake Center in Fairbanks caused some alerts to be delayed, while its website was overwhelmed by the number of users.[6] TheNational Weather Service sent emergency alerts to cell phones in Alaska, reading, "Emergency Alert. Tsunami danger on the coast. Go to high ground or move inland."[16]
The National Weather Service andPacific Tsunami Warning Center initially issued a tsunami watch for Hawaii and the entire West Coast, but cancelled them after less than two hours.[17] In theSan Francisco Bay Area, residents within three blocks of the Pacific coast and five blocks of theSan Francisco Bay were warned by the city's Department of Emergency Management to be ready for evacuation.[9]
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.[18]