M 7.2 - Nippes, Haiti

Aftershock information

• More earthquakes than usual (aftershocks) will continue to occur in the southern peninsula of Haiti. The aftershocks will decrease in frequency in the days, months, and years after the magnitude 7.2 main shock. 
• During the first weeks there are likely to be hundreds of aftershocks large enough to be felt by people in this area.
• Larger aftershocks could cause additional damage, especially in weakened structures. Several magnitude 5 and larger aftershocks have occurred already, and it's likely that there will be more earthquakes of this size over the next week, month, and beyond. 
• There is a very small chance that this earthquake could be followed by another earthquake larger than 7.2. About 1 in 20 mainshocks, like the 7.2, are followed by a similar-sized or larger quake, within the first week. This chance goes down with time however, large aftershocks can occur even months after the mainshock. 
• Scientists continue to monitor the situation and gather more information about this aftershock sequence. 

Advice on what to do during an earthquake varies depending on the country. For example, evacuation is not recommended within the United States. According to Geohazards International, if you currently live in Haiti and are inside your house when you feel an earthquake, this is what to do:

• If you are inside and can easily get out, evacuate to a safe open place covering your head and your neck. Head to an open space where walls and electric poles cannot fall on you.   
• If you can’t evacuate, drop where you are, cover your head and neck with one arm and get under a sturdy table, and then hold on to the table legs until the shaking stops.
• Stay away from landslide areas and hillsides with cracks, as aftershocks can cause new landslides and existing landslides to move again.

Enfòmasyon sou Replik (Aftershock Information Translated)

• Plis tranbleman tè (replik) ap kontinye rive nan zòn Sid Peyi Dayiti a. Frekans Replik yo ap diminye nan jou yo, mwa yo, ak ane yo apre prensipal tranbleman tè mayitid 7.2 a. 
• Pandan premye semèn yo gen anpil chans pou gen dè santèn replik byen fò ke moun ki nan zòn sa yo kapab santi yo.
• Pi gwo replik ka lakòz plis domaj, espesyalman nan estrikti ki fèb. Genyen plizyè replik mayitid 5 ak lòt pi gwo pase deja, e li posib pou genyen plis tranbleman tè nan nivo sa a nan semen ak mwa kap vini yo, e menm apre.
• Gen yo Pwobabilite ki trè ba pou ta genyen yon tranbleman de tè ki pi gwo pase sa ki te 7.2 a. Apeprè 1 sou 20 sekous prensipal , tankou 7.2 a, genyen yon sekous menm gwosè oswa pi gwo ki pase aprè li nan premye semèn nan. Chans sa a diminye ak tan sepandan, gwo replik ka rive menm kèk mwa apre sekous prensipal la. 
• Syantis yo kontinye kontwole sitiyasyon an epi ranmase plis enfòmasyon sou sekans replik sa a. 

Konsèy sou kisa ou dwe fè pandan yon tranbleman tè varye selon peyi a. Pa egzanp, yo pa rekòmande evakyasyon nan Etazini. Daprè GeoHazards International, si wap viv an Ayiti kounye a epi ou anndan kay ou lè ou santi yon tranbleman tè, me kisa pouw fè:

• Si ou andedan epi ou ka soti fasilman, evakye ale nan yon espas ouvè ki an sekirite pandan ou ap kouvri tèt ak kou ou. Ale nan yon espas ouvè kote mi yo ak poto elektrik pa ka tonbe sou ou.   
• Si ou pa kapab evakye, Koupi kote ou ye a, kouvri tèt ou ak kou ou ak yon men epi antre anba yon tab solid, epi kenbe pye tab la jiskaske sekous la sispann.
• Rete lwen zòn glisman teren ak fant mòn, paske replik yo ka lakòz nouvo glisman tèren ak glisman teren ki deja egziste yo kapab deplase ankò.

Tectonic Summary

The August 14, 2021 M 7.2 Haiti earthquake occurred as the result of oblique reverse motion along the Enriquillo-Plantain Garden fault zone, ~125 km west of the Haitian capital Port-au-Prince. The earthquake occurred at shallow depths on either a reverse fault striking west and dipping to the north with a component of left-lateral slip, or a fault striking southeast and dipping to the southwest with a component of right-lateral slip. At the location of the earthquake, the local plate boundary is dominated by left-lateral strike slip motion and compression. The plate boundary in this location accommodates eastward, left-lateral motion of the Caribbean plate relative to the North America plate. Within this context, the earthquake likely occurred on the east-west striking, north dipping fault plane with a component of left-lateral slip.

On January 12, 2010, an M 7.0 earthquake struck the same peninsula of Haiti and was located ~75 km east of the August 2021 earthquake. The 2010 earthquake caused substantial damage in the city of Port-au-Prince and the surrounding regions where damage from the earthquake and subsequent cascading hazards caused over 200,000 fatalities. The August 2021 earthquake likely occurred within the same fault system as the January 2010 earthquake; however, the 2010 earthquake occurred on a blind thrust fault and not directly on the main plate-boundary fault.

The location and focal mechanism solutions of the August 2021 earthquake are consistent with the event resulting from primarily reverse faulting with a component of left-lateral strike slip faulting on the Enriquillo-Plantain Garden fault zone (EPGFZ). Overall, the EPGFZ accommodates about 7 mm/yr of motion, nearly half the total oblique convergence between the Caribbean and North America plates (~20 mm/yr). Haiti occupies the western part of the island of Hispaniola, one of the Greater Antilles Islands, situated between Puerto Rico and Cuba. At the location of the August 2021 earthquake, motion between the Caribbean and North America plates is partitioned between two major east-west-trending, strike slip fault systems—the Septentrional fault system in northern Haiti and the EPGFZ in southern Haiti.

The EPGFZ had produced a series of major earthquakes in both instrumental and historical time periods. In addition to the 2010 M 7 earthquake that devastated Port-au-Prince, the EPGFZ is the likely source of historical large earthquakes in 1860, 1770, and 1751, though none of these has been confirmed in the field as associated with this fault. The sequence of events possibly associated with the Enriquillo fault in 1751–1860 are as follows:

October 18, 1751: a major earthquake caused heavy destruction in the Gulf of Azua (the eastern end of the Enriquillo fault); this earthquake also generated a tsunami. It is unclear if the rupture occurred on the Muertos reverse belt or on the eastern end of the Enriquillo fault.

November 21, 1751: a major earthquake destroyed Port-au-Prince but was centered to the east of the city on the Plaine du Cul-de-Sac.

June 3, 1770: a major earthquake destroyed Port-au-Prince again and appeared to be centered west of the city. As a result of the 1751 and 1770 earthquakes and minor earthquakes that occurred between them, local authorities required building with wood and forbade building with masonry.

April 8, 1860: a major earthquake occurred farther west of the 2010 earthquake and was accompanied by a tsunami.

An M6.9 earthquake in Alaska (an aftershock to a previous M8.2 earthquake on July 29, 2021) preceded the Haiti earthquake by ~31 minutes. Despite the timing coincidence between these two earthquakes, the large distance between these two events makes a causal relationship unlikely.