Subdaily Slow Fault Slip Dynamics Captured by Low-Frequency Earthquakes

Geodetic positioning is the geophysical record of reference for slow slip events, but typical daily solutions limit studies of the evolution of slow slip to its long-term dynamics. Accompanying seismic low-frequency earthquakes located precisely in time and space provide an opportunity to image slow slip dynamics at subdaily time scales. Here we show that a high-resolution time history of low-frequency earthquake fault slip alone can reproduce the geodetic record of slow slip that we observe to be dominated by subdaily fault slip dynamics. However, a simple linear model cannot accommodate the complex dynamics present throughout the slow slip cycle, and an analysis of different phases of the slow slip cycle shows that the ratio of geodetic to seismic fault slip varies as a function of time. This suggests that the low-frequency earthquake source region saturates as slow slip grows in moment and area. We propose that rheological heterogeneities at the plate boundary associated with low-frequency earthquakes do not play a significant role in the slow slip rupture process, thus implying that their activity is incidental to the driving aseismic slip.Plain Language Summary Slow slip events can be observed in many subduction zones where they play an important role in the earthquake cycle. Decades after their discovery, slow slip events are now captured routinely in geodetic datasets with slip dynamics occurring over a broad range of time scales. Using high-time resolution seismological observations together with the geodetic record allows us to go beyond the coarse daily GNSS sampling rate to image slow slip dynamics at short time scales. Here we use the temporal evolution of seismic slip produced by low-frequency earthquakes to study the subdaily dynamics of a slow slip event cycle, reproducing the geodetic record of slow slip using only seismological observations. We develop a simple model where long-term loading is in competition with the intermittent release of stress tied to the seismic slip of low-frequency earthquakes. We show the full slow slip cycle is driven by bursts of slip at subdaily time scales that low-frequency earthquake events witness only in their immediate source region. This result implies that the low-frequency earthquake rupture process is incidental to slow fault slip and does not play a major role in the slow slip cycle.