Measurements of Wave-Induced Attenuation in Saturated Metapelite and the Band-Limitation of Low-Frequency Earthquakes

The most common explanation for the depletion of high frequency waves that defines low-frequency earthquakes (LFEs) and very low-frequency earthquakes (VLFEs) is that fault rupture and slip are slower than typical earthquakes. However, it is difficult to rule out the possibility that the high frequency waves are produced during slip, but attenuated near the LFE source. One reason this hypothesis has been poorly tested is that there are no measurements of attenuation on the relevant rocks. We present the results of forced oscillation experiments that measure the frequency-dependent attenuation of a chlorite-rich metapelitic schist, a lithology found along the subduction plate boundary where LFEs and VLFEs have been documented. Experiments were run on dry and water-saturated schist at effective pressures of 2-10 MPa and at frequencies of 2 x 10(-5)-30 Hz. We find that pore fluids and low effective pressure result in the attenuation of high frequencies. The frequency-dependent attenuation is consistent with the concomitant operation of two wave-induced fluid flow mechanisms, squirt flow, and patchy saturation. When the effects of these mechanisms are extrapolated to geologic conditions using rock physics models, our results predict that attenuation is capable of completely diminishing the frequencies depleted in LFEs and VLFEs. Therefore, LFEs and VLFEs may not necessarily record slow fault slip, but possibly the presence of high fluid pressure.