Off the Peruvian coast, the Nazca plate slides beneath the South American plate at the Peru-Chile Trench. That collision builds the Andes. It also flexes the continent inland, generating shallow earthquakes far from the subduction zone itself. The Alto Mayo valley sits in the eastern foothills of the northern Peruvian Andes, where a fold-and-thrust belt of imbricate faults catches the ongoing strain of the convergence above. These faults are steep at the surface and flatten with depth, classic geometry for a mountain front being shoved over the lowlands behind it. The 1990 and 1991 earthquakes occurred at relatively shallow depths within the South American plate itself, on two different faults running roughly parallel to the mountain front.

The first shock struck south of the Alto Mayo valley, rupturing the Pucatambo Fault. Magnitude 6.5. The maximum Modified Mercalli intensity in the area was VII, very strong, but in Rioja, Moyobamba, and Soritor the shaking was severe enough, intensity VIII, to liquefy the soil under towns. Liquefaction collapsed buildings outright. At least 9,277 homes were severely damaged, mostly the rammed-earth and adobe construction that defines rural Peruvian housing in the highlands. In Moyobamba alone, 35 people died. The toll climbed in the surrounding jungle communities of Rioja, Nuevo Cajamarca, and Soritor as rescuers reached more remote villages. President Alan García directed two air force aircraft to deliver 126 tons of relief supplies, but communications and power across the earthquake zone had collapsed and seventy-five people remained missing for days.

An aftershock of magnitude 5.5 on June 9, 1990, killed one more person and destroyed fourteen homes. Then the valley got eleven months of relative quiet. On April 4, 1991, a magnitude 6.4 earthquake struck the area as a foreshock, injuring fifteen people and damaging buildings in Nueva Cajamarca and Rioja. The next day, April 5, came the largest shock of the entire sequence: magnitude 6.9, this time on the Angaisa Fault to the north. It killed approximately fifty-three people and injured 252 others. Strike-slip and reverse faulting produced the rupture. Maximum intensity hit Moyobamba and Yantaló, both of which had already been damaged by the 1990 quake. For families in the valley, the second wave arrived before the first reconstruction had begun.

Most of the houses in Alto Mayo were built of rammed earth and adobe brick, materials that have served Andean villages for centuries. Adobe is cheap, locally sourced, and thermally efficient. It is also brittle and heavy, with no tensile strength to speak of. When the ground shakes, adobe walls crack along their seams, then fall outward, often pulling roofs down on whoever is sleeping inside. Soil liquefaction, where saturated alluvial deposits temporarily lose their bearing strength under shaking, took out entire blocks in Rioja, Moyobamba, and Soritor. Engineering studies after the earthquakes documented patterns that have since influenced reconstruction guidelines across the Andes, but adobe persists in rural areas because the alternatives, concrete or fired brick, cost money that subsistence farmers do not have.

Three large earthquakes in eleven months is unusual but not freakish for the eastern foothills of the Peruvian Andes. The same fold-and-thrust belt produced significant earthquakes near here in 2005, 2019, and 2021. Each shock releases strain that has been accumulating for decades or centuries beneath the mountain front, but the strain itself never stops. The Nazca plate keeps converging at roughly seven centimeters a year, the Andes keep rising, and the faults keep building toward the next rupture. For the survivors of Alto Mayo, the sequence of 1990 and 1991 reshaped the valley physically and demographically, and stands as a marker of what these mountains do on a regular basis when no one is paying attention.