by William Frank (MIT)
Dec 8, 2023
What makes low-frequency earthquakes low frequency
With the increasing density of geophysical observations across subduction zones, the earthquake science community (represented by Faulting and Earthquake Cycles in the SZ4D initiative) has been able to explore the broad spectrum of fault motion in subduction zones to an unprecedented degree in the past two decades. From ruptures that are too slow to radiate seismic waves to earthquakes that lack the typical high-frequency radiation of regular seismicity, we have discovered many new modes of fault slip that all play a role in the devastating megathrust earthquake cycle. Without in situ observations of these strange slip events, it is challenging to infer what conditions on the subducting plate boundary are responsible for these different modes of fault motion. To tackle one small piece of this puzzle, we recently investigated what makes atypical low-frequency earthquakes low frequency (Wang et al., Science Advances, 2023).
As implied by their name, low-frequency earthquakes have a distinct spectral signature compared to typical earthquakes, lacking the high-frequency radiation that we would expect for their size (typically less than magnitude 2). Tiny seismic events with little consequence for the overall tectonic budget, low-frequency earthquakes are almost always observed to accompany major slow slip events that can aseismically release as much energy as magnitude 7 earthquakes. Understanding what is responsible for the unique waveforms of low-frequency earthquakes can thus shed light on the physical conditions responsible for slow slip phenomena.
Prior research (e.g., Bostock et al., JGR, 2017) has proposed two end-member explanations for the low frequency of these earthquakes: either near-source structure attenuates all of the high frequencies as the seismic waves travel from their source to the surface, or the low-frequency earthquake rupture is much slower than that of a typical earthquake. Leveraging a distinct geometry of earthquakes that sandwich the low-frequency earthquake source region in Nankai subduction zone offshore Japan, we were able to isolate the contribution of the Earth’s structure to the seismic signature of low-frequency earthquakes. We demonstrated that while crustal structure does indeed strongly distort the low-frequency earthquake signal, it cannot explain alone their unique spectral shape.
The above figure shows that once we account for crustal attenuation, the low-frequency earthquake source looks like a typical earthquake-like pulse, albeit much longer in duration. We suggest that the presence of pressurized fluids within the low-frequency earthquake source region along the plate interface play an important role in warping what would be a regular earthquake into one whose rupture plays out over time scales at least an order of magnitude slower than expected. This unique rupture process provides hints about the rheology of the broader fault zone that hosts major slow slip events, which regularly transfer tectonic stress onto the neighboring megathrust that is building towards the next major earthquake.
Qing-Yu Wang et al. ,What makes low-frequency earthquakes low frequency.Sci. Adv.9, eadh3688(2023). DOI:10.1126/sciadv.adh3688
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