Forty historical documents have been found which describe a previously unknown very large earthquake on 22 July 1816. This event occurred along the strike-slip boundary between the Caribbean and North American plates in Guatemala and southern Mexico. Modified Mercalli intensities are estimated from these accounts, and a rough isoseismal map is constructed. The damage pattern indicates that the causative fault was the left-lateral Chixoy-Polochic fault for which no damaging earthquake has previously been reported. Damage of Modified Mercalli intensity VII or greater covered an area of at least 13,000 km2, extending over 340 km from Alta Verapaz province in Guatemala westward to San Cristobal Ias Casas, in Chiapas, Mexico. The area of intensity VII indicates a moment of 1 × 1028 dyne-cm, while the estimated length of the intensity VII isoseismal indicates a more likely figure of 3.5 × 1027 dyne-cm, or an equivalent magnitude (Mw) of 712 to 734. The reported aftershock sequence is compatible with an earthquake in this magnitude range. These data demonstrate that the Chixoy-Polochic fault is currently an active part of the Caribbean-North American plate boundary and is capable of producing very large earthquakes.

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