Subdaily Slow Fault Slip Dynamics Captured by Low‐Frequency Earthquakes

Abstract

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.

Laboratory Hydrofractures as Analogs to Tectonic Tremors

“…We present tremor observations during an example slow slip event in Cascadia in Figure 5. Despite a general trend of tremors migrating southward in that specific event, we also observe intermittent occurrence of tremor, punctuated by pauses with no radiation interpreted as temporary stalling of tremor-initiating slow slip (Frank & Brodsky, 2019;Frank et al, 2018;Jolivet & Frank, 2020;Mouchon et al, 2023;Rousset et al, 2019), and large variability in centroid locations of the tremors occurring in 2-hr intervals. Aspects of these observations mirror our lab-based findings, albeit with greater complexity, and may suggest the potential reopening of sealed or healed fault valves (e.g., backward ruptures).…”

“…However, recent geological investigations (Condit & French, 2022;Mindaleva et al, 2020;Schmidt & Platt, 2022;Ujiie et al, 2018) and numerical simulations (Cruz-Atienza et al, 2018;Farge et al, 2021) reveal that fluid propagation can potentially generate seismic behaviors similar to tectonic tremor via a combined mechanism of shear and tensile deformations. While we focus only on the tensile mechanisms due to our experimental limitations, our results suggest previous studies of tremor intermittency that attribute it exclusively to sporadic fault locking and stalled tremor-initiating slow shear slip (Frank & Brodsky, 2019;Frank et al, 2018;Jolivet & Frank, 2020;Mouchon et al, 2023;Rousset et al, 2019) should also consider a role for variable pore pressures that may cause tensile fracturing and sealing. Additionally, in the model we propose in Figure 4, the cracking and sealing of fractures actually modify the potential for shear slip and thus tremor cannot be considered strictly as a result of the overall driving slow slip event, but instead as a facilitator of shear slip.…”

“…The observed coexistence of slow slip and tectonic tremor has led seismologists to use tremor, or its constituent low‐frequency earthquakes (LFEs), as markers of slow earthquakes. Their timing can help detect and locate slow‐slip events (Frank, 2016; Frank et al., 2014; Mouchon et al., 2023; Wech & Creager, 2011), estimate their size (Frank & Brodsky, 2019; Passarelli et al., 2021), and infer the mechanical and stress state on the fault (Bürgmann, 2018; Hulbert et al., 2020; Nishikawa et al., 2019). Despite the ample observations of tectonic tremors, their source mechanisms and relation to slow slip remain uncertain.…”

Imaging the Spatiotemporal Evolution of Plate Coupling With Interferometric Radar (InSAR) in the Hikurangi Subduction Zone

“…Our results demonstrate that the interseismic phase is not stationary due to the interplay of multiple slow slip cycles superimposed on the long‐term kinematic coupling that is likely associated with major earthquakes (Jolivet & Frank, 2020). This highlights that any estimate of plate coupling derived from a given time period is a snapshot of a continuously evolving plate interface (Frank, 2016; Mouchon et al., 2023). We note that the observed agreement between our 10‐year model, which most closely represents the long‐term interseismic phase, and a previously published interseismic coupling model suggests a relative stability of plate coupling over the past several decades.…”

“…The discovery of slow slip along subduction zones more than two decades ago (Dragert et al., 2001) has upended this simple conceptual model of a stationary (e.g., constant slip deficit rate) interseismic phase in these tectonic settings (e.g., Frank, 2016; Maubant et al., 2022; Mouchon et al., 2023; Saux et al., 2022). Geodetic observations across many tectonic plate boundaries have demonstrated how these transient slip events, which do not radiate seismic waves, can episodically release as much accumulated tectonic stress as major earthquakes (>M7) (e.g., Graham et al., 2016; Wallace, 2020).…”