Okal, E. A., and C. R. Bina, The deep earthquakes of 1921-1922 in northern Peru, Physics of the Earth and Planetary Interiors, 87, 33-54, doi:10.1016/0031-9201(94)90020-5, 1994.
Abstract. We present a detailed analysis of the 1921 and 1922 deep events in Northern Peru, which we identify as the only reliably located earthquakes in the vicinity of the great 1970 deep Colombian shock. All other reported seismicity in a three-dimensional radius of 550 km centered on the 1970 hypocenter is erroneous, and can be relocated to other seismic provinces. The 1921-1922 earthquakes relocate approximately 230 km to the south of the 1970 event, at depths of 630 km and 660 km respectively. A combination of centroid moment tensor inversion and forward modeling using ray theory and reflectivity synthetics yields focal mechanisms indicating down-dip compression and seismic moments of 1.2e+27 dyn cm and 6.e+27 dyn cm, respecetively. The 1921, 1922 and 1970 events take place on a piece of subducted Farallon plate aged approximately 55 Ma, i.e., anomalously young to support deep seismicity. We suggest that this unusual seismic activity may be due to a local heterogeneity on the slab, whose nature, however, remains unknown.Copyright © 1994 Elsevier Science B.V.