Rake-changing repeaters : a new class of co-localised similar earthquakes with both correlated and anti-correlated waveforms

Repetitive rupture of the same asperity has been documented in various tectonic settings through the observation of highly correlated waveforms from co-located earthquakes. These events, known as repeaters, are of particular interest because they provide key constraints on small-scale fault processes and can be used to investigate slip rates on faults, aseismic deformation, earthquake nucleation... In fewer cases, highly similar earthquakes exhibiting polarity inversions at all recording stations have been identified. These so-called anti-repeaters are thought to rupture the same asperity with an inverted focal mechanism and are associated with specific driving processes. To our knowledge, no intermediate case involving rupture of the same asperity with a change in slip direction (rake) has been reported, despite the fact that such a scenario is theoretically plausible.

In this study, we search for such events within the aftershock sequence of the M7.1 Miyagi-Oki earthquake (26 May 2003), in the northeastern Japan subduction zone. This intermediate-depth intraslab sequence (70 km depth) exhibits a high seismicity rate and is well recorded by a dense seismic network. Using data from the 17 closest three-component broadband stations, we compute waveform coherence, cross-correlation and anti-correlation (flipped traces) for both P and S waves of close event pairs. We identify 40 pairs of highly similar earthquakes displaying polarity inversions at several (but not all) stations. After performing relative hypocenter relocation using correlation-derived time delays, we retain ten co-located pairs with high-quality waveform similarity and polarity inversion. By comparing measured amplitude ratios with synthetic radiation pattern ratios, we invert the rake change for each event pair.

At this stage, the physical mechanism responsible for this newly identified class of similar earthquakes, that we name ‘rake-changing repeaters’, is uncertain. A change in rake between successive ruptures of the same asperity likely reflects a highly localised modification of the stress field. It could be driven by transient pore-fluid pressure variations or stress perturbations induced with nearby moderate slip. The identification of rake-changing repeaters opens new perspectives for investigating the complexity of local faulting processes at depth, and complements existing insights from repeaters and anti-repeaters.