  Long-term slow-slip events (LSSEs) usually occur on the deep, shallow parts
of subducting plates and have substantial relation with adjacent megathrust
fault motion. Conventional techniques of quantifying slow earthquake frictional
features show that these features may be indicative of predictive seismic
motion; however, quantifying high-accuracy uncertainty of the frictional fields
has not yet been achieved. We therefore propose a method of uncertainty
quantification for spatially inhomogeneous frictional features from slip motion
on an LSSE fault--megathrust fault complex in southwestern Japan. By combining
a fault motion model that mimics slow-slip motion and a variational data
assimilation (DA) technique using a second-order adjoint method, we have
succeeded in quantifying the spatial distribution of the uncertainty of the
frictional features. Further, evaluation of the spatial distribution in
high-resolution reveals the correlation between the dynamics of the slow-slip
motion and the important components of the frictional features, which is
valuable information for observational DA design. Findings from this study are
expected to advance the theoretical foundation of applied seismic motion
prediction techniques using slow-slip frictional features as stress meters for
megaquakes, as well as improve understanding of the relationship between the
slow-slip motion and frictional parameters of a fault.
