Deep Earthquakes Near Iñapari, Peru: Insights into Subduction Zone Dynamics

On November 24, 2015, at 22:50 local time, a magnitude 7.6 earthquake struck 185 km west-northwest of Iñapari, Peru, at a depth of 620.5 km. A second magnitude 7.6 event occurred the same day 155 km west-northwest of Iñapari. Both events originated within the subducted Nazca Plate beneath the South American Plate.

The Iñapari region lies in southeastern Peru’s Madre de Dios department, near the borders with Brazil and Bolivia. Although surface geology consists of Amazonian sedimentary basins and tropical lowland terrain, the seismicity reflects processes occurring hundreds of kilometers below. The Nazca Plate descends eastward beneath the continent along the Peru-Chile trench, forming a steeply dipping Wadati-Benioff zone that extends into the mantle transition zone. Earthquakes at depths exceeding 600 km result from phase transitions and dehydration embrittlement within the cold slab interior.

Peru’s subduction margin has produced numerous deep-focus events throughout recorded history. Notable examples include the 1922 and 1963 earthquakes near the Peru-Brazil border, both exceeding magnitude 7.0 at depths greater than 500 km. These events demonstrate that the slab remains seismically active well below the volcanic arc, which in this segment of the Andes lies more than 1,000 km to the west.

The 2015 pair of earthquakes occurred within a region of relatively sparse deep seismicity since 2000. Their proximity—separated by roughly 53 km—suggests rupture along distinct segments of the same slab volume. Focal mechanisms typically show downdip compression, consistent with resistance encountered by the slab as it approaches the 660 km discontinuity.

Because of their great depth, the events produced minimal surface shaking in Iñapari and surrounding communities. No significant damage or casualties were reported, underscoring the attenuating effect of thick overlying mantle and crust on high-frequency seismic waves.

Ongoing monitoring by regional and global networks continues to refine the geometry of the subducted slab in this area. Improved three-dimensional velocity models reveal a continuous, though locally contorted, seismic zone that records the long-term history of Nazca Plate subduction beneath South America.

References

• U.S. Geological Survey Earthquake Catalog (event parameters for 24 November 2015)
• National Earthquake Information Center, USGS
• Regional tectonic summaries published by the Incorporated Research Institutions for Seismology (IRIS)