Global & Disaster Medicine

Archive for the ‘Earthquake’ Category

Mexico: How tough are the building codes?

NY Times

“…..Tighter building codes, better construction materials and a robust public awareness surely played a role in limiting the carnage this time around. Fewer than 300 people died and about 40 buildings collapsed, while nearly 4,000 buildings were declared severely damaged and are likely to be uninhabitable, officials have said.

But what spared this metropolitan area of 21 million was, at least in part, luck.

The 1985 earthquake was 30 times more powerful than the one on Tuesday. It toppled apartment and office towers, killing more than 10,000 people.

Tuesday’s earthquake, while centered closer to the capital, struck hardest at smaller, less populated buildings, taking fewer lives….

In a 2016 study of a random sample of 150 buildings constructed after 2004, when the new codes were adopted, Mr. Reinoso found that many failed to meet city standards. In many cases, the buildings reviewed did not even have enough necessary paperwork to conduct a full assessment.

As it often goes in Mexico, it is not the law that is problematic, but rather the implementation………”


A magnitude 5.7 earthquake struck beneath the Pacific Ocean off the Northern California coast .

LA Times

ShakeMap Intensity image

 


9/21/1999: A 7.1M earthquake in Taiwan kills thousands, causes billions of dollars in damages and leaves an estimated 100,000 homeless

History Channel

 


M 6.1 – 281km ESE of Kamaishi, Japan

ShakeMap Intensity image

USGS:  The North America plate, Pacific plate, Philippine Sea plate, and Eurasia plate all influence the tectonic setting of Japan, Taiwan, and the surrounding area. Some authors divide the edges of these plates into several microplates that together take up the overall relative motions between the larger tectonic blocks, including the Okhotsk microplate in northern Japan, the Okinawa microplate in southern Japan, the Yangzee microplate in the area of the East China Sea, and the Amur microplate in the area of the Sea of Japan.
The seafloor expression of the boundary between the Pacific and North America plates lies 300 km off the east coasts of Hokkaido and Honshu at the Kuril-Kamchatka and Japan trenches. The subduction of the Pacific plate beneath the North America plate, at rates of 83-90 mm/yr, generates abundant seismicity, predominantly as a result of interplate slip along the interface between the plates. The 1958 M 8.4 Etorofu, 1963 M 8.6 Kuril, 2003 M 8.3 Tokachi-Oki, and the 2011 M 9.0 Tohoku earthquakes all exemplify such megathrust seismicity. The 1933 M 8.4 Sanriku-Oki earthquake and the 1994 M 8.3 Shikotan earthquake are examples of intraplate seismicity, caused by deformation within the lithosphere of the subducting Pacific plate (Sanriku-Oki) and of the overriding North America plate (Shikotan), respectively.
At the southern terminus of the Japan Trench the intersection of the Pacific, North America, and Philippine Sea plates forms the Boso Triple Junction, the only example of a trench-trench-trench intersection in the world. South of the triple junction the Pacific plate subducts beneath the Philippine Sea plate at the Izu-Ogasawara trench, at rates of 45-56 mm/yr. This margin is noteworthy because of the steep dip of the subducting Pacific plate (70° or greater below depths of 50 km depth), and because of its heterogeneous seismicity; few earthquakes above M 7 occur at shallow depths, yet many occur below 400 km. The lack of large shallow megathrust earthquakes may be a result of weak coupling at the plate interface, or simply a reflection of an incomplete earthquake catalog with respect to the length of typical seismic cycles.
The northernmost section of the Philippine Sea plate shares a 350 km boundary with the North America plate that runs approximately east-west from the Boso Triple Junction towards the Izu Peninsula. This short boundary is dominated by the subduction of the Philippine Sea plate beneath Japan along the Sagami Trough, but also includes small sections of transform motion.
The subduction of the Philippine Sea plate under the Eurasia plate begins at the Suruga Trough, immediately southwest of the Izu peninsula. In the northern Tōkai, Tonankai and Nankai sections of this subduction zone, historical data indicate M 8+ earthquake recurrence intervals of 100-150 years. The Tonankai and Nankai sections last ruptured in M 8.1 earthquakes in 1944 and 1946, respectively, while the Tōkai section last broke in 1854. In the 1980’s studies began to forecast the imminence of a large earthquake in the Tōkai region, and warned of its potential impact on the cities of Tokyo and Yokohama (the two largest cities in Japan); to date, the expected event has not occurred.
The boundary between the Philippine Sea and Eurasia plates continues south and southwestwards from the Suruga Trough, extending 2000 km along the Nankai and Ryukyu trenches before reaching the island of Taiwan. Along the Ryukyu Trench, the Philippine Sea plate exhibits trench normal subduction at rates increasing from 48 mm/yr in the northeast to 65 mm/yr in the southwest. Convergence and the associated back-arc deformation west of the oceanic trench creates the Ryukyu Islands and the Okinawa Trough. The largest historic event observed along this subduction zone was the M 8.1 Kikai Island earthquake in 1911.
In the vicinity of Taiwan the structure of the Philippine Sea: Eurasia plate boundary and the associated pattern of seismicity becomes more complex. 400 km east of Taiwan a clockwise rotation in the trend of the margin (from NE-SW to E-W), paired with an increase in subduction obliquity creates a section of the plate boundary that exhibits dextral transform and oblique thrusting motions. South of Taiwan the polarity of subduction flips; the Eurasia plate subducts beneath the Philippine Sea plate. Debate surrounds contrasting models of the plate boundary position between the zones of oppositely verging subduction, and the boundary’s relation to patterns of seismicity. Many studies propose that crustal thickening causes the majority of regional seismicity, while others attribute seismicity to deformation associated with subduction. Another resolution proposes a tear in the Philippine Sea plate and a complex assortment of subduction, transform, and collisional motion. All the models concede that seismicity around the island of Taiwan is anomalously shallow, with few earthquakes deeper than 70km.
While there are no instances of an earthquake M>8 in the modern record, Taiwan and its surrounding region have experienced eight M>7.5 events between 1900 and 2014. The dominance of shallow M<8 earthquakes suggests fairly weak plate boundary coupling, with most earthquakes caused by internal plate deformation. The 1935 M 7.1 Hsinchu-Taichung earthquake and the 1999 M 7.6 Chi-Chi Earthquake both exemplify the shallow continental crust thrust faulting that dominates regional seismicity across the island. A major tectonic feature of the island is the Longitudinal Valley Fault, which ruptures frequently in small, shallow earthquakes. In 1951, the Longitudinal Valley Fault hosted twelve M≥6 events known as the Hualien-Taitung earthquake sequence.
Large earthquakes in the vicinity of Japan and Taiwan have been both destructive and deadly. The regions high population density makes shallow earthquakes especially dangerous. Since 1900 there have been 13 earthquakes (9 in Japan, 4 in Taiwan) that have each caused over 1000 fatalities, leading to a total of nearly 200,000 earthquake related deaths. In January 1995 an earthquake that ruptured a southern branch of the Japan Median Tectonic Line near the city of Kobe (population 1.5 million) killed over 5000 people. The 1923 Kanto earthquake shook both Yokohama (population 500,000, at that time) and Tokyo (population 2.1 million), killing 142,000 people. The earthquake also started fires that burned down 90% of the buildings in Yokohama and 40% of the buildings in Tokyo. Most recently, the M9.0 Tohoku earthquake, which ruptured a 400 km stretch of the subduction zone plate boundary east of Honshu, and the tsunami it generated caused over 20,000 fatalities.

Image result for japan tectonic plates


Mexico’s search for survivors: More than 50 survivors have been plucked from several disaster sites since Tuesday afternoon’s 7.1-magnitude quake.

Reuters

“…..At least 237 others have died and 1,900 were injured……”

 

 


At least 216 people died in Mexico City and in the states of Puebla, Mexico and Morelos, Mexico

CNN

“…..22 bodies were found in the debris of an elementary school in Mexico City that collapsed due to the earthquake. At least 30 children were still missing Tuesday night……”

 


September 19, 1985: An 8.1M earthquake strikes Mexico City and leaves 10,000 people dead, 30,000 injured and thousands more homeless.

History Channel


7.1M Earthquake Strikes Mexico, Killing Over 100

ShakeMap Intensity image

Population Exposure Map

Tectonic Summary

The September 19, 2017, M 7.1 earthquake in Central Mexico occurred as the result of normal faulting at a depth of approximately 50 km. Focal mechanism solutions indicate that the earthquake occurred on a moderately dipping fault, striking either to the southeast, or to the northwest. The event is near, but not directly on, the plate boundary between the Cocos and North America plates in the region. At the location of this event, the Cocos plate converges with North America at a rate of approximately 76 mm/yr, in a northeast direction. The Cocos plate begins its subduction beneath Central America at the Middle America Trench, about 300 km to the southwest of this earthquake. The location, depth, and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event, within the subducting Cocos slab, rather than on the shallower megathrust plate boundary interface.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal-faulting events of the size of the September 19th, 2017 earthquake are typically about 50×20 km (length x width).

Over the preceding century, the region within 250 km of the hypocenter of the September 19th, 2017 earthquake has experienced 19 other M 6.5+ earthquakes. Most occurred near the subduction zone interface at the Pacific coast, to the south of the September 19 event. The largest was a M 7.6 earthquake in July 1957, in the Guerrero region, which caused between to 50-160 fatalities, and many more injuries. In June 1999, a M 7.0 at 70 km depth, just to the southeast of the September 19, 2017 earthquake, caused 14 fatalities, around 200 injuries, and considerable damage in the city of Puebla (MMI VIII).

Today is the anniversary of the devastating 1985 M 8.0 Michoacan earthquake, which caused extensive damage to Mexico City and the surrounding region. That event occurred as the result of thrust faulting on the plate interface between the Cocos and North America plates, about 450 km to the west of the September 19, 2017 earthquake. Today’s earthquake also occurs 12 days after a M 8.1 earthquake offshore of Chiapas, in southern Mexico. The epicenter of the M 8.1 event is located about 650 km to the southeast of today’s quake. That earthquake also occurred as the result of normal faulting within the subducting Cocos Plate, at a depth of 50-70 km.


Plate Tectonics & The Ring of Fire

National Park Service

The Earth is made up of roughly a dozen major plates and several minor plates. These plates are constantly moving, some as fast as 15 centimeters a year

FAULT TYPES

 

SUBDUCTION ZONE:  At a convergent plate boundary the older, more dense crust will sink, or “subduct” under the other. There can be a Normal-Angle Subduction and a Low-Angle Subduction.

 

ISLAND ARC SETTING:  An island-arc setting includes a chain of offshore, island volcanoes above a subducting plate. The back-arc is located opposite the trench and subducting plate, behind the chain of volcanoes called an “island arc”.

 

HOTSPOTS:  This graphic shows the evolution of a chain of islands over stationary mantle plume (hotspot) within Earth’s crust.

 

PACIFIC OCEAN HOTSPOTS:  Tectonic setting of the Pacific Plate with selected hotspots indicated. The “kink” between the Emperor Seamounts and Hawaiian Islands chain shows how the direction of plate motion changed while the Hawaiian hotspot remained stationary.


The Mexican earthquake, which had a magnitude of 8.2 and struck shortly before midnight on Thursday, was felt by tens of millions of people in Mexico and in Guatemala and resulted in at least 60 deaths.

NY Times

USGS

Tectonic Summary

The September 8th, 2017, M 8.1 earthquake offshore Chiapas, Mexico, occurred as the result of normal faulting at an intermediate depth. Focal mechanism solutions for the earthquake indicate slip occurred on either a fault dipping very shallowly towards the southwest, or on steeply dipping fault striking NW-SE. At the location of this event, the Cocos plate converges with North America at a rate of approximately 76 mm/yr, in a northeast direction. The Cocos plate begins its subduction beneath Central America at the Middle America Trench, just over 100 km to the southwest of this earthquake. The location, depth, and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event, within the subducting Cocos slab, rather than on the shallower megathrust plate boundary interface.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal-faulting events of the size of the September 8th, 2017 earthquake are typically about 200×50 km (length x width).

Over the preceding century, the region within 250 km of the hypocenter of the September 8th, 2017 earthquake has experienced 8 other M 7+ earthquakes. Most occurred in the subduction zone to the southeast of the September 8 event, near the Mexico-Guatemala border, and none were larger than M 7.5. The largest, a M 7.4 thrust faulting earthquake offshore Guatemala in November 2012, resulted in at least 48 fatalities and over 150 injuries, and significant damage near the coast.

 

 


Categories

Recent Posts

Archives

Admin