Relative Afterslip Moment Does Not Correlate With Aftershock Productivity: Implications for the Relationship Between Afterslip and Aftershocks

Abstract

Aseismic afterslip has been proposed to drive aftershock sequences. Both afterslip moment and aftershock number broadly increase with mainshock size, but can vary beyond this scaling. We examine whether relative afterslip moment (afterslip moment/mainshock moment) correlates with several key aftershock sequence characteristics, including aftershock number and cumulative moment (both absolute and relative to mainshock size), seismicity rate change, b‐value, and Omori decay exponent. We select Mw ≥ 4.5 aftershocks for 41 tectonically varied mainshocks with available afterslip models. Against expectation, relative afterslip moment does not correlate with tested aftershock characteristics or background seismicity rate. Furthermore, adding afterslip moment to mainshock moment does not improve predictions of aftershock number. Our findings place useful empirical constraints on the link between afterslip and potentially damaging Mw ≥ 4.5 aftershocks and raise questions regarding the role afterslip plays in aftershock generation.

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

“…Estimates of relative aftershock productivity correspond to those calculated by Churchill et al. (2022b) or approximated subsequently using similar methods. Churchill et al.…”

“…Churchill et al. (2022b) defined relative aftershock productivity ( n rel ) by the number of aftershocks observed (above a given magnitude) divided by an expectation based on the Utsu‐Seki productivity scaling law (Utsu, 1970). They found values in the range ∼0.1–10 (where values ∼1 are considered typical) and that n rel does not correlate with $${M}_{\text{rel}}^{\text{aslip}}$$.…”

“…In a systematic analysis of 41 mainshocks, Churchill et al. (2022b) showed that relative afterslip moment $$\left({M}_{\text{rel}}^{\text{aslip}}\right)$$ does not correlate discernibly with key characteristics of aftershock sequences, including the productivity of M w ≥ 4.5 aftershocks. To reconcile case study evidence that supports strong links between afterslip and aftershocks with systematic testing that appears to contradict this, Churchill et al.…”

“…Cattania et al (2015) incorporated afterslip into Coulomb Rate-and-State models following the 2004 M w 6.0 Parkfield and the 2011 M w 9.1 Tohoku earthquakes, finding that it was also not the principal driver of the aftershocks and that secondary triggering (i.e., subsequent generations of aftershocks being triggered by the evolving stress state caused by earlier aftershocks) may play a more important role. In a systematic analysis of 41 mainshocks, Churchill et al (2022b) showed that relative afterslip moment 𝐴𝐴 ( 𝑀𝑀 aslip rel…”

Aftershocks following the 2011 Tohoku-Oki earthquake driven by both stress transfer and afterslip

“…Our results suggest that the Coulomb stress model and afterslip model should be combined in future studies to better understand aftershock activity. Furthermore, applying our method to the aftershock sequences of other large earthquakes and quantifying the afterslip-induced seismicity may provide new insights into the relationship between afterslip and aftershocks (Churchill et al 2022). Our method can be also applied to estimate the afterslip evolution if earthquake catalogs are available, even in areas where geodetic observation networks are sparse.…”