What makes low-frequency earthquakes low frequency?

Low-frequency earthquakes are repetitive seismic events that occur downdip of the seismogenic megathrust where slow aseismic fault slip dominates the tectonic budget. These tiny events, distinct from regular earthquakes, have the potential to provide in-situ constraints on the fault rheology where slow slip occurs, but are hard to study due to their small signal amplitudes. We take advantage of the unique geometry of seismicity in the Nankai subduction zone, where low-frequency earthquake sandwich regular earthquakes, to study the source signature of low-frequency earthquakes and the local seismic structure. We isolate two 10 km-radius depth columns, one where low-frequency earthquakes periodically occur and one where they do not. Taking the collocated regular earthquakes below and above the subducting plate interface, we develop a three-step cluster-based approach to correct for local site effects, extract the accurate Empirical Attenuation functions, and apply them to the correction of Low-frequency earthquakes. The falloff of corrected displacement spectra inversely obeys the 𝝎-square hypothesis with a maximum possible corner frequency of f_c at ~2 - 3Hz. The local 1-D vertical structure inverted from earthquake travel times suggests a distinct difference in the ratios of seismic attenuation and velocity. Assuming seismic waves go through similar paths within depth column, the local attenuation is insufficient to generate specific low-frequency spectral content that differs from fast earthquakes. Instead, our results support a relative contrast of Q over depth with a higher Q at shallow depth above the zone of low-frequency earthquakes. This high Q layer may serve as an impermeable layer and produce an environment with enhanced pore-fluid pressure and heterogeneous frictional characteristics different from the zone with regular earthquakes. This particular condition favors low-frequency earthquakes and generates distinct nucleation procedures or/and rupture processes of low-frequency earthquakes from regular earthquakes.