Spatial Relationships Between Coseismic Slip, Aseismic Afterslip, and On-Fault Aftershock Density in Continental Earthquakes

Damaging aftershock sequences often exhibit considerable spatio-temporal complexity. The stress changes associated with coseismic slip and aseismic afterslip are commonly proposed to drive aftershock sequences, but few systematic studies exist and do not always support strong, universal driving relationships. To investigate the roles that these two sources of stress changes may play in driving aftershocks, we assess the spatio-temporal relationships between coseismic slip, afterslip, and on-fault (within 5 km) aftershock density following seven Mw6.0-7.6 continental-settings earthquakes, using available high-quality slip models and regional seismic data. From previous empirical work and frictional considerations, near the mainshock we expect coseismic slip and afterslip to be anti-correlated, and aftershocks to occur where coseismic slip is low/zero, near high slip gradients, and/or to migrate with afterslip. However, we find that spatial relationships between afterslip and coseismic slip, and between afterslip and aftershock density differ between earthquakes. Aftershock density correlates with coseismic slip following five of the earthquakes, and with total cumulative slip (coseismic slip + afterslip) following six: indicating that on-fault aftershock distributions may be approximated by total slip (at current resolutions). Additionally, we find that the gradients of coseismic slip and afterslip (proxies for new stress concentrations) do not clearly correlate with aftershock distributions and that the choice of spatial domain over which relationships are tested can affect results significantly. A possible explanation of these results is that fault zones contain considerable fine-scale structural and frictional heterogeneity. Nonetheless, the empirical evidence for frequently assumed relationships between coseismic slip, afterslip and on-fault aftershocks is mixed. The spatial distributions of aftershocks following major earthquakes can be complex and varied. We investigate how the locations of aftershocks on the main fault relate to the distributions of slip from the mainshock itself and from a gentle relaxation process that occurs on the fault in the following weeks, months, and years, called afterslip: we expect aftershocks to broadly occur where afterslip is and where mainshock slip is low/zero. We investigate seven interesting mainshocks that occur within comparable continental settings and show that spatial relationships between coseismic slip, afterslip, and aftershocks vary between these case studies. Aftershocks generally occur where slip of some kind (either mainshock slip or afterslip) has been modeled, which may be useful for estimating the broad distribution of on-fault aftershocks in the future. As mainshock slip, afterslip, and aftershocks often appear to be occurring in exactly the same regions (contradicting common assumptions), we propose that fault zones contain finer scale detail than can currently be resolved. We explore how coseismic slip and afterslip distributions may influence on-fault aftershock density following seven key continental earthquakesTotal modeled slip often correlates well with on-fault aftershock density thus could be used as an indicator of aftershock evolutionCoseismic slip, afterslip, and aftershocks commonly co-locate, contrary to expectations, possibly due to unresolved slip heterogeneity