Reverse-Polarity Earthquakes at Intermediate Depth in North Chile and Bolivia: Observations of a Rare Phenomenon

To date, there are only a few well-documented examples of reverse-polarity earthquakes worldwide, especially at intermediate depth. These are pairs or families of very closely located events, where one or more members show flipped polarities, indicating nearly reversed earthquake mechanisms. We identify four intermediate-depth reverse-polarity earthquake families in a catalog of originally 3153 repeating earthquake sequences for the North Chilean subduction zone from 2006 to 2024. A detailed analysis of the largest group, which consists of 38 events and contains five reverse events, is presented. Relative event relocations, cluster properties, and interevent times suggest a very small active volume of the order of only a few hundred meters, partially with overlap of the rupture area, and a weak upward migration of seismicity over several years. As uncertainties remain, we discuss several possible models to explain the observations. We propose that they may reflect transient processes linked to dehydration embrittlement and fluid pulses.