1920 Xalapa earthquake
The 1920 Xalapa earthquake rocked the gulf coast of Mexico on January 4, causing major damage in the states of Veracruz and Puebla. The epicenter was located somewhere in mountainous region of the eastern Trans-Mexican Volcanic Belt which triggered massive mudflows and landslides which claimed anywhere between 700 to 4,000 lives. The event produced extreme ground motions reaching causing severe ground effects. The epicentral region of this earthquake was allocated XI ("Extreme") on the Modified Mercalli intensity scale. It would remain the deadliest earthquake in Mexico until the 1985 Mexico City earthquake. Shaking was felt as  far as Toluca in the State of Mexico and throughout the central region of the country. Tectonic setting. The west coast of Mexico is dominated by a subduction zone where the Rivera and Cocos Plates converge with the North American Plate at a rate of 3–9 cm/yr, varying in locations. At the interface of the subduction zone, earthquakes occur occasionally with magnitudes reaching greater than 8.0. These events are capable of producing tsunamis that devastate coastal communities. At the same time, the subducting plates are still seismically active. Faults within the underthrusted slab which accommodate extension as it flexes  and dives into the earth's interior can produce intermediate-depth earthquakes that are just as destructive. Intraplate faults in the downgoing slab at the outer trench swell can also generate tsunamis. Trans-Mexican Volcanic Belt. This volcanic range stretches from the country's Pacific Coast to the Gulf of Mexico for more than 1,000 km. It does not run parallel to the Middle America Trench where the plates are subducting due to the difference in subduction angle. Along the southern coast of Mexico, subduction is flat-slab whereas in the northern area, the plates dive steeply. For that reason , the volcanic arc migrate away from the trench in the south. Earthquakes in the volcanic belt may be a result of an active crustal extension within the North American Plate. Seismicity of the volcanic belt is usually low, and rarely do these earthquakes reach magnitudes of 5.0 or greater. Earthquake. The source of the earthquake has been the subject of debate and controversy. It had a focal mechanism corresponding to normal faulting with some sense of strike-slip, as seen in other earthquakes in the area. A 25-km-long fault under the volcanic belt was consistent with the damage  and shaking intensity patterns. Ground cracks and fissures thought to be surface ruptures were later correctly identified as slumping rather than faulting. In 1546, a shallow crustal earthquake similar to that in 1920 reportedly caused heavy damage in the affected area. The absence of visible surface ruptures has also led to geologists speculating if this was a blind rupture quake or an intermediate depth event. Magnitude. The magnitude of earthquake varies between catalogs with values 7.8 and 6.4 presented. Magnitude 7.8 has been considered far-fetched, and most likely a non-instrumental estimate, biased by reports of damage and intensities . Effects. The heaviest damage was in the vicinity between the volcanic cones of Cofre de Perote and Pico de Orizaba, and around the Sierra Madre Oriental where large landslides triggered by strong ground motions traveled down the Huitzilapa and Pescados river valleys. The flow traveled some 15 km, destroying the towns in the process. The settlements of Acuatlatipa, El Rincón, Mecatitla, and Pet-lacuacán were buried by mudflows, some of them were never rebuilt. Some 300 residents were lost under the flow, and in Barranca Grande, only 80 of the 400 villagers survived. In some places, the mudflow deposits were  up to 15 meters thick. It traveled for 80 km before entering the Gulf of Mexico. Churches in Chilchotla, Santiago Lafragua, and Patlanalán were totally destroyed. Little damage was done to the ground and homes as their building material was mainly wood thus structural failures did not claim any lives. In the village of Ayahualulco however, many of the buildings were constructed with bricks and adobe materials which were destroyed. The towns of Cosautlán and Teocelo although unaffected by the mudflows, lost over 150 inhabitants mostly from collapses. In the city of Xalapa, damage was less severe and churches and  buildings sustaining some damage. No collapses were reported. In the epicenter area, the seismic intensity was X, which was sufficient to trigger the devastating landslides. In Xalapa, the shaking intensity did not exceed IX ("Violent"). The towns of Chilchotla, Coatepec, Xico, and Teocelo felt the highest intensity at XI.