From temblor: “Deep M=6.9 earthquake in Guatemala possibly preceded by foreshock sequence”

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temblor

June 14, 2017
David Jacobson

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Guatemala City, approximately 160 km from the epicenter of today’s M=6.9 earthquake experienced moderate shaking. While there are only reports of moderate damage throughout the country, it was very widely felt. (Picture from: imgix.net)

At 1:29 a.m. local time, a M=6.9 earthquake struck western Guatemala near the border with Mexico. Despite the quake’s deep depth (94 km according to the USGS) it was widely felt throughout Central America, including in Guatemala City 160 km away, which is home to 3.3 million people. According to officials in Guatemala, landslides were triggered, blocking highways, and houses were moderately damaged. Fortunately, the region around the epicenter, which, based on the USGS ShakeMap experienced strong shaking, is mountainous and sparsely populated and there is only one known injury. Across the border in Mexico, minor damage was also sustained.

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This Temblor map shows the location of today’s M=6.9 earthquake in western Guatemala. Also included in this map is the location of the M=7.5 earthquake in 1976 that killed nearly 23,000 people. The thick red line around this epicenter is the fault rupture area.

This part of Central America is highly seismically active due to a combination of relative plate motions. Off the southern coast, the Cocos Plate is subducting beneath both the North American and Caribbean plates. Additionally, left-lateral strike slip motion is also present in much of eastern and central Guatemala due to the active plate boundary between the North American and Caribbean plates. This means that Guatemala sits on what is known as a triple junction, where three tectonic plates meet.

As a result, this region has experienced large, damaging earthquakes, including a M=7.5 earthquake in 1976 in eastern Guatemala that killed nearly 23,000 people and left more than 75,000 injured (Olcese et al., 1977). That devastating quake occurred at a depth of 5 km, and was on the Motagua Fault, which is a pure left-lateral strike-slip fault. In comparison, today’s quake was almost purely extensional and occurred at the much greater depth of 94 km (according to the USGS). While the location and depth of the earthquake suggests that the rupture occurred near the subducting slab, its extensional nature indicates that it may have been caused by localized steepening of the slab.

While today’s earthquake and the deadly one in 1976 were vastly different in their nature and impact on the region, they highlight an important earthquake characteristic: depth. Even had today’s earthquake been a M=7.5, like the one in 1976, it would not have caused as much damage, because it occurred at a depth 19 times greater. This is because significant energy is lost, resulting in less shaking at the surface. In the side-by-side figures below, USGS ShakeMaps are shown for today’s earthquake, and the one in 1976. From these, one can see that while violent shaking was felt in the 1976 quake, today’s shaking only reached strong levels.

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These USGS ShakeMaps show shaking in today’s M=6.9 earthquake (left) and the M=7.5 earthquake in 1976 (right). What is clear in these figures is that shaking levels were significantly higher in the 1976 earthquake. This was due to the fact that the magnitude was greater, and because it occurred at a shallow depth.

Another aspect of this quake which deserves extra attention is that in the nine hours prior to the M=6.9 mainshock, there were five smaller magnitude earthquakes, ranging from M=4.4 to M=5.6, just offshore. Two of these occurred within an hour of the M=6.9, including one just 6 minutes prior. While not necessarily indicative of foreshock events, given they were over 100 km away, the rate at which they occurred is substantially higher than the normal background rate. Therefore, if they were foreshocks, they could indicate that a larger creep event took place at the Middle America Trench. While creep events do not necessarily precede large earthquakes, if this area along the subduction zone were to rupture, a M+7.5 earthquake could result. This would generate severe shaking along the Central American coastline, and could trigger a tsunami. Therefore, these smaller events that preceded the mainshock deserve attention.

Because of the complex tectonic environment on which Honduras, and the rest of Central America sits, large earthquakes should be expected. Using the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, we can see what the likely earthquake magnitude in your lifetime is anywhere on earth. While today’s quake is deeper than GEAR takes into account, we can still see in the map below that for much of central, western, and southern Guatemala, M=6.75+ earthquakes should be expected. Because of this, residents of the small country should be prepared for, and understand the regional earthquake hazards.

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This Temblor map shows the Global Earthquake Activity Rate (GEAR) model for much of Central America. While today’s earthquake is deeper than is considered by GEAR, it does show that M=6.75+ earthquakes are likely to occur in your lifetime in much of central, western, and southern Guatemala. The GEAR model uses global strain rates and seismicity since 1977 to produce an earthquake forecast.

References
USGS
European-Mediterranean Seismological Centre
Associated Press
A. F. Espinosa, The Guatemalan earthquake of February 4, 1976, a preliminary report, Professional Paper 1002, 1976

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Quake-Catcher Network

The Quake-Catcher Network is a collaborative initiative for developing the world’s largest, low-cost strong-motion seismic network by utilizing sensors in and attached to internet-connected computers. With your help, the Quake-Catcher Network can provide better understanding of earthquakes, give early warning to schools, emergency response systems, and others. The Quake-Catcher Network also provides educational software designed to help teach about earthquakes and earthquake hazards.

After almost eight years at Stanford, and a year at CalTech, the QCN project is moving to the University of Southern California Dept. of Earth Sciences. QCN will be sponsored by the Incorporated Research Institutions for Seismology (IRIS) and the Southern California Earthquake Center (SCEC).

The Quake-Catcher Network is a distributed computing network that links volunteer hosted computers into a real-time motion sensing network. QCN is one of many scientific computing projects that runs on the world-renowned distributed computing platform Berkeley Open Infrastructure for Network Computing (BOINC).

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The volunteer computers monitor vibrational sensors called MEMS accelerometers, and digitally transmit “triggers” to QCN’s servers whenever strong new motions are observed. QCN’s servers sift through these signals, and determine which ones represent earthquakes, and which ones represent cultural noise (like doors slamming, or trucks driving by).

There are two categories of sensors used by QCN: 1) internal mobile device sensors, and 2) external USB sensors.

Mobile Devices: MEMS sensors are often included in laptops, games, cell phones, and other electronic devices for hardware protection, navigation, and game control. When these devices are still and connected to QCN, QCN software monitors the internal accelerometer for strong new shaking. Unfortunately, these devices are rarely secured to the floor, so they may bounce around when a large earthquake occurs. While this is less than ideal for characterizing the regional ground shaking, many such sensors can still provide useful information about earthquake locations and magnitudes.

USB Sensors: MEMS sensors can be mounted to the floor and connected to a desktop computer via a USB cable. These sensors have several advantages over mobile device sensors. 1) By mounting them to the floor, they measure more reliable shaking than mobile devices. 2) These sensors typically have lower noise and better resolution of 3D motion. 3) Desktops are often left on and do not move. 4) The USB sensor is physically removed from the game, phone, or laptop, so human interaction with the device doesn’t reduce the sensors’ performance. 5) USB sensors can be aligned to North, so we know what direction the horizontal “X” and “Y” axes correspond to.

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Earthquake safety is a responsibility shared by billions worldwide. The Quake-Catcher Network (QCN) provides software so that individuals can join together to improve earthquake monitoring, earthquake awareness, and the science of earthquakes. The Quake-Catcher Network (QCN) links existing networked laptops and desktops in hopes to form the worlds largest strong-motion seismic network.

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