On 4 January 1920, a magnitude 6.4 earthquake occurred in the eastern Trans‐Mexican volcanic belt (TMVB) to the southwest of the city of Jalapa, Mexico. Structural damage was extensive in towns near the epicentral area. The more damaging phenomena, however, were mudflows in the high valleys that covered several villages. It was estimated that approximately 700 people died under the mud avalanches triggered by the earthquake on the steep slopes of the river valleys. The moderate‐sized 1920 Jalapa earthquake is the second deadliest earthquake in the history of Mexico and the only one in which extensive mudflows triggered by the earthquake were reported. Several active faults mapped in the central part of the Mexican volcanic belt have produced earthquakes as large as 7.2. In the eastern portion of the belt, where the Jalapa earthquake took place, few faults have been mapped. Therefore, there is doubt about whether the 1920 earthquake was a shallow, crustal earthquake or whether it occurred within the subducted Cocos plate, as with other earthquakes located to the south of the TMVB. In what constitutes probably the first aftershock study after a destructive earthquake, the Mexican Seismological Service installed an 80‐kg, Wiechert vertical seismograph in the city of Jalapa after the earthquake. The S–P times of three aftershocks indicate that they occurred in the epicentral area, confirming it as a shallow crustal earthquake. This pioneering aftershock study helps confirm that crustal seismicity extends throughout the TMVB, where 40% of the population of Mexico lives.