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Powerful M6.9 earthquake hits Guatemala – Mexico border region


A powerful earthquake registered by the USGS as M6.9 hit Guatemala – Mexico border region at 07:29 UTC (01:29 local time) on June 14, 2017. The agency is reporting a depth of 111.5 km (69.2 miles). EMSC is reporting M6.8 at a depth of 105 km (65 miles). USGS issued aYellow alert level for shaking-related fatalities and economic losses. The quake occurred under Tajumulco volcano.

According to the USGS, the epicenter was located 8.6 km (5.3 miles) SW of Tajumulco (population 4 329), 12.6 km (7.9 miles) NE of San Pablo (population 13 668), 15.7 km (9.7 miles) WNW of San Marcos (population 25 088), 18.4 km (11.4 miles) WNW of San Pedro Sacatepéquez (population 40 021), and 18.4 km (11.4 miles) NE of Malacatán (population 14 923), Guatemala.

There are about 6 443 428 people living within 100 km (62 miles). 4.9 million are living within 75 km (46 miles), 2.5 million within 50 km (31 miles), 420 000 within 20 km (12.4 miles), 110 000 within 10 km (6.2 miles), 24 000 within 5 km (3.1 miles) and about 4 200 within 2 km (1.2 miles).

Based on all available data, there is no tsunami threat from this earthquake, PTWC said.

Some 997 000 people are estimated to have felt Strong (VI), and 8.5 million Moderate (V) shaking on the Modified Mercalli Intensity scale.

Residents said the earthquake was felt across the region and woke up many people.

At least 5 casualties, direct and indirect, have been reported. One woman in the city of San Marcos was killed ba a falling wall. Another person, a homeless man, was killed in the town of San Sebastian Retalhueleu after he was struck by the collapse of part of a church. Three women in different departments died from heart attacks attributed to fright caused by the earthquake. At least 14 people have been injured.

Guatemala earthquake June 14, 2017

The USGS issued a Yellow alert for shaking-related fatalities and economic losses. Some casualties and damage are possible and the impact should be relatively localized. Past yellow alerts have required a local or regional level response.

Estimated economic losses are less than 1% of GDP of Guatemala.

Overall, the population in this region resides in structures that are a mix of vulnerable and earthquake resistant construction. The predominant vulnerable building types are mud wall and adobe block with concrete bond beam construction.

Recent earthquakes in this area have caused secondary hazards such as landslides that might have contributed to losses.

Estimated population exposure to earthquake shaking

Guatemala - Mexico border region earthquake - Estimated population exposureGuatemala - Mexico border region earthquake - Estimated population exposure - Table

Selected cities exposed

Guatemala - Mexico border region earthquake - Selected cities exposed

Under Tajumulco volcano

The quake occurred below Tajumulco stratovolcano. Tajumulco is Guatemala's highest peak and the highest volcano in Central America. This volcano has had several unconfirmed reports of historical eruptions. Sapper (1917) considered it to have erupted during historical time, but without accurate dates. The volcano was reported to eject many rocks, destroying houses on October 24, 1765, but this may have been a rock avalanche. Juarros reported some eruptions before 1808, and there are unlikely reports of eruptions in 1821 (or 1822), 1863, and 1893.

Tajumulco volcano earthquake - June 14, 2017

Regional seismicity

Guatemala - Mexico regional seismicity


Tectonic summary

The June 14, 2017 M 6.9 earthquake northeast of San Pablo, Guatemala, occurred as the result of normal faulting at an intermediate depth, approximately 100 km beneath the surface of coastal Guatemala. The focal mechanism solution indicates the earthquake occurred on either a shallowly dipping normal fault striking southeast, or on a steeply dipping normal fault striking northwest. At the location of the earthquake, the Cocos plate converges with the North America plate at a rate of approximately 79 mm/yr, subducting beneath North America lithosphere at the Middle America Trench, 200 km to the southwest of this earthquake. The mechanism, location, and depth of the June 14th event indicate that the earthquake occurred within the subducting Cocos plate, rather than on the shallow thrust interface between the two plates.

Central America is very seismically active, and the region within 250 km of the June 14, 2017 event has experienced 34 other M 6.5+ earthquakes over the preceding century. Most occurred on or near the shallow plate interface in this region. Only two such earthquakes have occurred at intermediate depths, both to the northwest of the June 14, 2017 earthquake beneath southern Mexico – a M 6.9 earthquake at a depth of 165 km in March 1994, and a M 6.6 event at a depth of 85 km in December 2015. The latter event caused two fatalities, and landslides near the coast.

The June 14, 2017 event also follows a series of shallow earthquakes on or near the subduction thrust interface about 150 km to the southwest. Since late May 2017, 16 earthquakes of M 4.1 and larger have occurred there, including a M 5.5 event on June 10th with a thrust faulting mechanism, and M 5.4 and M 5.5 thrust faulting earthquakes on June 14th in the hour prior to the M 6.9 event.

Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed "intermediate-depth" earthquakes. Intermediate-depth earthquakes represent deformation within subducted lithosphere rather than at the shallow plate interfaces between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters. Earthquakes have been reliably located to depths of just over 200 km in this region. (USGS)

Seismotectonics of the Caribbean region and vicinity

Extensive diversity and complexity of tectonic regimes characterizes the perimeter of the Caribbean plate, involving no fewer than four major plates (North America, South America, Nazca, and Cocos). Inclined zones of deep earthquakes (Wadati-Benioff zones), ocean trenches, and arcs of volcanoes clearly indicate subduction of oceanic lithosphere along the Central American and Atlantic Ocean margins of the Caribbean plate, while crustal seismicity in Guatemala, northern Venezuela, and the Cayman Ridge and Cayman Trench indicate transform fault and pull-apart basin tectonics.

Along the northern margin of the Caribbean plate, the North America plate moves westwards with respect to the Caribbean plate at a velocity of approximately 20 mm/yr. Motion is accommodated along several major transform faults that extend eastward from Isla de Roatan to Haiti, including the Swan Island Fault and the Oriente Fault. These faults represent the southern and northern boundaries of the Cayman Trench. Further east, from the Dominican Republic to the Island of Barbuda, relative motion between the North America plate and the Caribbean plate becomes increasingly complex and is partially accommodated by nearly arc-parallel subduction of the North America plate beneath the Caribbean plate. This results in the formation of the deep Puerto Rico Trench and a zone of intermediate focus earthquakes (70-300 km depth) within the subducted slab. Although the Puerto Rico subduction zone is thought to be capable of generating a megathrust earthquake, there have been no such events in the past century. The last probable interplate (thrust fault) event here occurred on May 2, 1787 and was widely felt throughout the island with documented destruction across the entire northern coast, including Arecibo and San Juan. Since 1900, the two largest earthquakes to occur in this region were the August 4, 1946 M8.0 Samana earthquake in northeastern Hispaniola and the July 29, 1943 M7.6 Mona Passage earthquake, both of which were shallow thrust fault earthquakes. A significant portion of the motion between the North America plate and the Caribbean plate in this region is accommodated by a series of left-lateral strike-slip faults that bisect the island of Hispaniola, notably the Septentrional Fault in the north and the Enriquillo-Plantain Garden Fault in the south. Activity adjacent to the Enriquillo-Plantain Garden Fault system is best documented by the devastating January 12, 2010 M7.0 Haiti strike-slip earthquake, its associated aftershocks and a comparable earthquake in 1770.

Moving east and south, the plate boundary curves around Puerto Rico and the northern Lesser Antilles where the plate motion vector of the Caribbean plate relative to the North and South America plates is less oblique, resulting in active island-arc tectonics. Here, the North and South America plates subduct towards the west beneath the Caribbean plate along the Lesser Antilles Trench at rates of approximately 20 mm/yr. As a result of this subduction, there exists both intermediate focus earthquakes within the subducted plates and a chain of active volcanoes along the island arc. Although the Lesser Antilles is considered one of the most seismically active regions in the Caribbean, few of these events have been greater than M7.0 over the past century. The island of Guadeloupe was the site of one of the largest megathrust earthquakes to occur in this region on February 8, 1843, with a suggested magnitude greater than 8.0. The largest recent intermediate-depth earthquake to occur along the Lesser Antilles arc was the November 29, 2007 M7.4 Martinique earthquake northwest of Fort-De-France.

The southern Caribbean plate boundary with the South America plate strikes east-west across Trinidad and western Venezuela at a relative rate of approximately 20 mm/yr. This boundary is characterized by major transform faults, including the Central Range Fault and the Boconó-San Sebastian-El Pilar Faults, and shallow seismicity. Since 1900, the largest earthquakes to occur in this region were the October 29, 1900 M7.7 Caracas earthquake, and the July 29, 1967 M6.5 earthquake near this same region. Further to the west, a broad zone of compressive deformation trends southwestward across western Venezuela and central Colombia. The plate boundary is not well defined across northwestern South America, but deformation transitions from being dominated by Caribbean/South America convergence in the east to Nazca/South America convergence in the west. The transition zone between subduction on the eastern and western margins of the Caribbean plate is characterized by diffuse seismicity involving low- to intermediate-magnitude (M<6.0) earthquakes of shallow to intermediate depth.

The plate boundary offshore of Colombia is also characterized by convergence, where the Nazca plate subducts beneath South America towards the east at a rate of approximately 65 mm/yr. The January 31, 1906 M8.5 earthquake occurred on the shallowly dipping megathrust interface of this plate boundary segment. Along the western coast of Central America, the Cocos plate subducts towards the east beneath the Caribbean plate at the Middle America Trench. Convergence rates vary between 72-81 mm/yr, decreasing towards the north. This subduction results in relatively high rates of seismicity and a chain of numerous active volcanoes; intermediate-focus earthquakes occur within the subducted Cocos plate to depths of nearly 300 km. Since 1900, there have been many moderately sized intermediate-depth earthquakes in this region, including the September 7, 1915 M7.4 El Salvador and the October 5, 1950 M7.8 Costa Rica events.

The boundary between the Cocos and Nazca plates is characterized by a series of north-south trending transform faults and east-west trending spreading centers. The largest and most seismically active of these transform boundaries is the Panama Fracture Zone. The Panama Fracture Zone terminates in the south at the Galapagos rift zone and in the north at the Middle America trench, where it forms part of the Cocos-Nazca-Caribbean triple junction. Earthquakes along the Panama Fracture Zone are generally shallow, low- to intermediate in magnitude (M<7.2) and are characteristically right-lateral strike-slip faulting earthquakes. Since 1900, the largest earthquake to occur along the Panama Fracture Zone was the July 26, 1962 M7.2 earthquake. (USGS)

References for the Panama Fracture Zone: 
Molnar, P., and Sykes, L. R., 1969, Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity: Geological Society of America Bulletin, v. 80, p. 1639-1684.

Featured image credit: USGS


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