The 2015 Illapel Earthquake Sequence and Chilean Subduction Zone Seismicity

The Coquimbo Region of central Chile lies above the convergent boundary where the Nazca Plate subducts beneath the South American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent megathrust earthquakes and places the area within the seismically active Andean margin. On 16 September 2015, two significant events occurred near Illapel: a magnitude 7.0 earthquake located 25 km west-northwest of the city at a depth of 28.4 km, followed closely by a magnitude 8.3 mainshock centered 48 km west of Illapel. Both events originated within the subduction interface, consistent with the region's characteristic thrust-faulting regime. Central Chile has a well-documented history of large earthquakes. Notable prior events include the 2010 Maule earthquake (M8.8) to the south and the great 1960 Valdivia earthquake (M9.5) farther south, both generated by the same plate-boundary system. The Illapel sequence fits this pattern of recurring megathrust activity, with the 2015 mainshock rupturing a segment of the plate interface that had accumulated strain since earlier 20th-century events. The proximity of the two 2015 shocks—separated by only 23 km—illustrates how foreshocks or early aftershocks can cluster near the main rupture zone. Geological studies of the margin highlight the role of subduction segmentation, where variations in plate coupling and sediment input influence rupture extent. The 2015 sequence released substantial seismic energy but did not propagate into adjacent locked patches, leaving potential for future events. Post-event analyses confirm that afterslip and viscoelastic relaxation continue to influence regional deformation years afterward. The events underscore Chile's ongoing seismic hazard. Modern monitoring networks operated by Chilean and international agencies provide rapid characterization of such sequences, supporting improved building codes and tsunami preparedness along the coast.

References

USGS Earthquake Catalog (event parameters as provided).
Global CMT Project focal mechanism solutions for 2015 Illapel sequence.
Servicio Nacional de Geología y Minería (SERNAGEOMIN) regional tectonic reports.