Historical 1942 Ecuador and 1942 Peru subduction earthquakes, and earthquake cycles along Colombia-Ecuador and Peru subduction segments

Two large shallow earthquakes occurred in 1942 along the South American subduction zone in close proximity to subducting oceanic ridges. The 14 May event occurred near the subducting Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern Peru near the southern flank of the subducting Nazca ridge. Source parameters for these two historic events have been determined using long-period P waveforms, P-wave first motions, intensities and local tsunami data. We have analyzed the P waves for these two earthquakes to constrain the focal mechanism, depth, source complexity and seismic moment. Modeling of the P waveform for both events yields a range of acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca plate beneath the South American plate. The source time function for the 1942 Ecuador event has one simple pulse of moment release with a duration of 22 seconds, suggesting that most of the moment release occurred near the epicenter. The seismic moment determined from the P waves is 6-8 × 10²⁰N·m, corresponding to a moment magnitude of 7.8-7.9. The reported location of the maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershocks are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic moment is released in three distinct pulses over approximately 74 seconds; the largest moment release occurs 32 seconds after rupture initiation. The seismic moment as determined from the P waves for the 1942 Peru event is 10-25 × 10²⁰N m, corresponding to a moment magnitude of 7.9-8.2. Aftershock locations reported by the ISS occur over a broad area surrounding the main shock. The reported locations of the maximum intensities (IX) are concentrated south of the epicenter, suggesting that at least part of the rupture was to the south. We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments of the South American subduction zone. We find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant.