M7.6 Great Swan Island Earthquake

Overview

At approximately 10:51PM, January 9th, 2018 Atlantic Standard Time (AST), a Magnitude 7.6 Earthquake occurred 44 Kilometers East of Great Swan Island, Honduras. This earthquake was felt from as far south as Panama to as far north as the Gulf Coast states of the United States of America with generally moderate to light shaking felt by most.

At 10:58PM, January 9th, 2018, the National Tsunami Warning Center (NTWC) issued its first tsunami advisory for Puerto Rico, the U.S. Virgin and British Virgin Islands. A tsunami advisory is issued when a tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, expected, or occurring. The threat may continue for several hours after initial arrival, but significant inundation is not expected for areas under an advisory.

Tsunami advisories, watches and warnings are only issued for U.S. territories and dependencies by the National Tsunami Warning Center and Pacific Tsunami Warning Center For all other countries in the Caribbean and Pacific, Tsunami threat messages are issued.

Pacific Tsunami Warning Center & U.S. National Tsunami Warning Center’s Areas of Responsibility (Credit: NWS)

At 10:57PM, January 9th, 2018, the Pacific Tsunami Warning Center (PTWC) issued its first threat message. The PTWC, ran by the U.S. National Weather Service, is the current agency responsible for issuing guidance to emergency personnel and governments in the Caribbean regarding possible tsunamis. Hazardous tsunami waves were possible from the earthquake for coasts within 1000 Kilometers from the epicenter. The Cayman Islands, Jamaica, Mexico, Honduras, Cuba, Belize, San Andres (Belize), Costa Rica, Panama, Nicaragua and Guatemala’s Caribbean coastlines were all under threat.

At 11:23PM, January 9th ,2018, the PTWC narrowed the tsunami threat area to Belize, Cuba, Honduras, Mexico, the Cayman Islands and Jamaica.

At 12:08AM, January 10th, 2018, the PTWC refined the threat area even further to Honduras and Belize. Tsunami waves had already occurred at Carrie Bow Cay, Belize (0.02 m) and George Town, Cayman Islands (0.19 m).

The Caribbean Sea, along the Belize coastline receded significantly as the tsunami threat was ongoing. While no damaging tsunami occurred, it is never save to be along the coastline when under a tsunami threat.

At 12:14AM, January 10th ,2018, the NTWC cancelled the tsunami advisory for Puerto Rico, U.S. and British Virgin Islands with no impacts to the advisory area.

At 12:48AM, January 10th, 2018, the PTWC issued its final threat message stating that the tsunami threat from this earthquake had passed, although fluctuations in sea levels may occur for the next several hours. This was observed along coastal areas in Belize, Guatemala and Honduras where the sea receded as much as 4 meters from average positions.

At no point were any of the Eastern Caribbean Islands, including Trinidad and Tobago, under a tsunami threat, advisory, watch or warning.

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M7.6 Earthquake

This M7.6 Earthquake, 44 Kilometers East of Great Swan Island, Honduras was strike-slip (nearly vertical faults where blocks of rock/plates move horizontally) by nature. It occurred along the Swan Fault, which is a transform plate boundary (plates slide past one another) between the North American and Caribbean Plate. At the location of this earthquake, the North America plate moves to the west-southwest with respect to the Caribbean plate at a rate of approximately 19 mm/yr. Strike-slip-faulting events of the size of the January 10, 2018, earthquake are typically about 140 x 20 km (length x width).

Diagram of a Strike Slip Fault – Mostly horizontal motion occurs at this fault boundary

Nine other earthquakes of M 6 or larger have occurred within 400 km of the January 10, 2018 event over the preceding century. Previous strong earthquakes along the North America-Caribbean plate boundary in this region include the destructive M 7.5 Guatemala earthquake of February 4, 1976, which resulted in more than 23,000 fatalities. The 1976 earthquake occurred on the Motagua fault, a segment of the plate boundary that lies in southern Guatemala, about 650 km west-southwest of the hypocenter of the January 10, 2018, event. In May 2009, a M 7.3 earthquake occurred along the Swan transform fault approximately 300 km west of the January 10, 2018 event. The 2009 earthquake (which was much closer to land than the 2018 event) resulted in 7 fatalities, 40 injuries and 130 buildings being damaged or destroyed.

USGS Tectonic Summary of the M7.6 Great Swan Island Earthquake & Tsunami.
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Tsunami

As stated above, this earthquake occurred along a transform boundary or strike slip fault. While earthquakes along these boundaries typically don’t produce tsunamis due to little or no vertical motion (disrupting the above water column to generate a tsunami), large strike-slip earthquakes do, but not in the traditional manner.

Diagram of tsunamigenesis along a strike-slip boundary.

With large strike-slip earthquakes, particularly in areas where there is irregular bathymetry (terrain of the sea floor), these horizontal displacements resulting from an earthquake use the undulating terrain analogous to paddles. These terrain changes such as seamounts displace the water column in the direction of movement, which triggers localized tsunamis.

Bathymetry (underwater terrain) of the Cayman Trough, showing high variability with numerous sea mounts.

Tsunamis generated by this mechanism are highly dispersive. This is the reason why the Eastern Caribbean, including Trinidad and Tobago, was not under any threat and 2) why tsunami effects only occurred along coasts bordering the immediate vicinity of the earthquake.

Thankfully, the tsunami generated by this earthquake, resulted in no damages or loss of life. In fact, observations were only recorded in 3 locations with tidal gauge data and one in Guatemala with eyewitness accounts.

  • Roatán Island, Honduras: 0.4 m
  • George Town, Cayman Islands: 0.19 m
  • Carrie Bow Cay, Belize: 0.02 m
  • Izabal, Guatemala: Sea levels dropped 3-4 meters below normal levels. No destructive waves reported.
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Earthquake’s Effect on Nearby Volcanoes

Remarkably, the M7.6 Great Swan Island earthquake dynamically triggered the seismic activity at Tenorio Volcano, Costa Rica. Approximately 5 minutes after the M7.6 earthquake, a M4.7 earthquake occurred at 10:56PM (AST) or 8:56PM (CST – Costa Rica Local Time), 9th January, 2018. This seismic activity has been ongoing at Tenorio Volcano with earthquake occurring as large as M5.3.

More Information: National Seismological Network of Costa Rica

Historical Seismicity & Tsunamis.

The Caribbean has a long history with tsunamigenic events, both within and outside of the Caribbean Sea. Using NOAA’s National Centers for Environmental Information (NCEI)’s Global Historical Tsunami Database, the M7.6 Great Swan Island Earthquake that occurred on 9th January 2018 became the 102nd tsunamigenic event that occurred within the Caribbean Basin.

Historical Seismicity Since 2150 B.C. within the Caribbean Basin.

Note that this number is not a fixed number. Paleotsunami research continues worldwide and historical tsunamis can always be discovered, altering this ranking. Very large tsunamis, such as the M9.5 May 22nd 1960 Chilean Earthquake & Tsunami as well as the 2004 Indian Ocean Earthquake and Tsunami, had global impacts where tsunami observations were recorded in the Caribbean Basin. These teletsunami observations would make the M7.6 Great Swan Island Earthquake the 114th tsunami recorded in the Caribbean Basin.

Historical Seismicity Since 2150 B.C. within the Caribbean Basin.

This M7.6 earthquake is definitely the strongest quake in the 21st century within the Caribbean Basin. It was proceeded by the M7.4 Martinique Earthquake that occurred in 2007. Overall, there are three magnitude 7.6 earthquakes tied for the 7th largest magnitude earthquakes in the Caribbean Basin. These include the Great Swan Island earthquake of 2018, and two other M7.6 earthquakes, 1822 and 1991, both near Costa Rica. The strongest earthquake on record in the Caribbean Basin, according to the NCEI‘s Global Significant Earthquake Database (2150 B.C. to present), is the Magnitude 8.4 (estimated) earthquake between Isla La Orchila and Macuto, Venezuela. This event was felt in Port of Spain, Trinidad.

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Sources

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