Qualla: 1990-91 Alto Mayo Earthquakes https://qualla.com/1990-91-alto-mayo-earthquakes en-us © 2026 Bendyline Mon, 20 Jul 2026 10:21:08 GMT Qualla serial false Qualla support@bendyline.com yes https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/hero-small.webp Qualla: 1990-91 Alto Mayo Earthquakes https://qualla.com/1990-91-alto-mayo-earthquakes 1990-91 Alto Mayo Earthquakes: Introduction https://qualla.com/1990-91-alto-mayo-earthquakes/ Photo credit Matador3020, CC BY-SA 3.0. The third one was the worst, but by then the people of Alto Mayo already knew what was coming. On May 30, 1990, a magnitude 6.5 earthquake had flattened parts of Rioja and Moyobamba and killed 135 people. Eleven months later, on April 4 and 5, 1991, the ground shook twice more in the same valley. By the time the dust settled, at least 189 people were dead, 15,000 were homeless, and the rural Department of San Martín had become a case study in what happens when seismically active mountains are built on with adobe walls.

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]]> Thu, 16 Apr 2026 00:00:00 GMT https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/6qcj-1990-91-alto-mayo-earthquakes-intro.mp3 0:06 1 full 1990-91 Alto Mayo Earthquakes: Where the Andes Bend Inland https://qualla.com/1990-91-alto-mayo-earthquakes/ Photo credit Mikenorton, CC BY-SA 3.0. 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 eas...

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Image: Mikenorton | CC BY-SA 3.0

]]> Thu, 16 Apr 2026 00:00:00 GMT https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/6qcj-1990-91-alto-mayo-earthquakes-where-the-andes-bend-inland.mp3 0:06 2 full 1990-91 Alto Mayo Earthquakes: May 30, 1990 https://qualla.com/1990-91-alto-mayo-earthquakes/ Photo credit Mikenorton, CC BY-SA 3.0. 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 un...

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Image: Mikenorton | CC BY-SA 3.0

]]> Thu, 16 Apr 2026 00:00:00 GMT https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/6qcj-1990-91-alto-mayo-earthquakes-may-30-1990.mp3 0:06 3 full 1990-91 Alto Mayo Earthquakes: April 1991: Two Earthquakes in a Day https://qualla.com/1990-91-alto-mayo-earthquakes/ Photo credit Mikenorton, CC BY-SA 3.0. 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...

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]]> Thu, 16 Apr 2026 00:00:00 GMT https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/6qcj-1990-91-alto-mayo-earthquakes-april-1991-two-earthquakes-in-a-day.mp3 0:06 4 full 1990-91 Alto Mayo Earthquakes: Why Adobe Failed https://qualla.com/1990-91-alto-mayo-earthquakes/ Photo credit Mikenorton, CC BY-SA 3.0. 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 shak...

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]]> Thu, 16 Apr 2026 00:00:00 GMT https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/6qcj-1990-91-alto-mayo-earthquakes-why-adobe-failed.mp3 0:06 5 full 1990-91 Alto Mayo Earthquakes: The Sequence That Did Not End https://qualla.com/1990-91-alto-mayo-earthquakes/ Photo credit Mikenorton, CC BY-SA 3.0. 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 decade...

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]]> Thu, 16 Apr 2026 00:00:00 GMT https://qualla.com/_m/6/q/c/j/1990-91-alto-mayo-earthquakes-wp/6qcj-1990-91-alto-mayo-earthquakes-the-sequence-that-did-not-end.mp3 0:06 6 full