Signal Attenuation of Tectonic Tremor-like Signals due to Sediment in the Cascadia and Izu-Bonin-Mariana Subduction Zones

Abstract

Defined limitations of the locked state in subduction zones are necessary to understand the frictional characteristics of the subduction interface as well as the hazard associated with earthquakes and tsunamis. Slow slip events provide insight into which areas are locked by revealing the segments that can accommodate movement between large earthquakes and segments that cannot. Tectonic tremor, a seismic signal, is an indicator of these events and can be seen on seismometers. Cascadia and Izu-Bonin-Mariana are contrasting subduction zones and from their differences in seabed roughness and sediment thickness, we can better understand how tectonic tremors propagate. Both subduction zones have offshore plate boundaries where seismic stations are limited, which makes observation of tectonic signals difficult. Therefore, we search stations for tectonic tremor one at a time instead of triangulating the signal. Resulting signals are analyzed for any overlaps between stations to further validate the signal. Generalized seafloor transects are studied to see how seabed roughness and sediment thickness influence the number of overlapping signals. Regions with high sediment thickness tend to have fewer subsurface signals and overlapping signals. Regions with a smooth seabed tend to generate more water column signals from bottom currents, short duration events, and ship-noise. Cascadia, an area with a smooth seabed and high sediment thickness, has over 60 times the overall signals that Izu-Bonin-Mariana has. However, 99.89% are considered water column noise and 0.11% are overlapping tremor-like signals. Izu-Bonin-Mariana, an area with a rough seabed and thin sediment thickness, has fewer signals but less noise and more overlapping signals. With a deeper understanding of the influences on offshore signal propagation, we are able to have a fuller picture of the locked state. Limitations of the locked state enables accurate predictions of earthquakes and mitigation of the risk associated.