** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.
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Solid earth sciences
Detection of repeating earthquakes and their application in characterizing slow fault slip
Naoki Uchida
Repeating earthquake, Interplate slip, Fault creep, Aseismic slip
Estimation of interplate coupling based on repeating earthquake data (b) and GPS data (a) (after Nomura et al., 2017).
Repeating earthquakes (repeaters) that rupture the same fault area (patch) are interpreted to be caused by repeated accumulation and release of stress on the seismic patch in a creeping area. This relationship between repeaters with fault creep can be exploited for tracking the fault creep (slow slip) based on the repeaters’ activity. In other words, the repeaters can be used as creepmeters embedded on a fault. To do this, it is fundamentally important to select earthquakes that definitely re-rupture in the same area. The selections are usually done based on waveform similarity or hypocenter location. In hypocenter-location based detection, the precision of the relative location compared with the dimension of earthquake sources is critical for confirming the co-location of the source area. On the other hand, waveform-similarity-based detection needs to use appropriate parameters including high enough frequency components to distinguish neighboring sources. Inter-event timing (recurrence interval) and/or the duration of a sequence’s activity are good diagnostic features for finding appropriate detection parameters and eliminating non-repeating events, which are important because an inappropriate selection leads to including triggered sequences that do not re-rupture the same area. Repeaters provide an independent estimation of creep from geodetic data and such estimations are mostly in good agreement when both kinds of data are available. Repeater data are especially useful in the deeper part of strike-slip faults and in near-trench areas of subduction zones where geodetic data’s resolution is usually limited. The repeaters also have an advantage with geodetic data analysis because they are not contaminated by viscoelastic deformation or poroelastic rebound which are prominent postseismic process for large earthquakes and occur outside of faults. On the other hand, the disadvantages of repeater analysis include their uneven spatial distribution and the uncertainty of the estimates of slip amount requiring a scaling relationship between earthquake size and slip. There are considerable variations in the inferred slip amounts from different relationships. Applications of repeater analysis illuminate the spatial distribution of interplate stable slip, after slip, and spontaneous and cyclic slow slip events that represent important components of interplate slip processes in addition to major earthquakes.
