Modelling high-frequency seismograms at ocean bottom seismometers: effects of heterogeneous structures on source parameter estimation for small offshore earthquakes and shallow low-frequency tremors

Abstract

Summary The source characteristics of offshore seismic events, especially regular (or fast) and slow earthquakes, can provide key information on their source physics and frictional conditions at the plate boundary. Due to strong three-dimensional heterogeneities in offshore regions, such as those relating to seawater, accretionary prism, and small-scale velocity heterogeneity, conventional methods using a one-dimensional Earth model may misestimate source parameters such as the duration and radiation energy. Estimations could become severe inaccuracies for small offshore seismic events because high-frequency (> 1 Hz) seismograms, which are strongly affected by three-dimensional heterogeneities, are only available for analysis because of their signal-to-noise ratio. To investigate the effects of offshore heterogeneities on source parameter estimation for small seismic events, we analysed both observed and simulated high-frequency seismograms southeast off the Kii Peninsula, Japan, in the Nankai subduction zone. Numerical simulations of seismic wave propagation using a three-dimensional velocity structure model clarified the effects of each heterogeneity. Comparisons between observations and model simulations demonstrated that the thick low-velocity accretionary prism has significant effects on high-frequency seismic wave propagation. Especially for shallow low-frequency tremors occurring at depths just below the accretionary prism toe, seismogram durations are significantly broader than an assumed source duration, even for stations with epicentral distances of approximately 10 km. Spindle-shape seismogram envelopes were observed even at such close stations. Our results suggest that incorporating three-dimensional heterogeneities is necessary for practical estimation of source parameters for small offshore events.

Sedimentary Structure Derived From Multi‐Mode Ambient Noise Tomography With Dense OBS Network at the Japan Trench

“…Such a high‐resolution S‐wave velocity structure of seafloor sediments will enhance seismic waveform modeling of body waves at high frequencies, at generally greater than 0.1 Hz, for studying the deeper structures (e.g., Harmon et al., 2007) or seismic source properties beneath the ocean (e.g., Nakano et al., 2015; Takemura et al., 2020). This is because the velocity contrast across the acoustic basement strongly affects the waveform amplitude.…”

“…Ocean bottom seismometer (OBS) networks of both temporary and permanent deployment have been recently developed along subduction zones (e.g., Cascadia Initiative: Toomey et al., 2014; DONET: Kaneda et al., 2015; S‐net: Kanazawa et al., 2016). High‐frequency waveform data from OBSs, most of which were obtained at frequencies higher than 0.1 Hz, can be used in receiver function analysis for evaluating shallow subduction structures (e.g., Akuhara & Mochizuki, 2015; Reeves et al., 2015) or waveform modeling to determine seismic source properties (e.g., Nakano et al., 2015; Takemura et al., 2020). Such studies help us to better understand the lithology, deformation, and earthquake processes of subduction zones.…”

Changes of Event Size Distribution During Episodes of Shallow Tectonic Tremor, Nankai Trough

“…We first determined tremor source locations by the envelope correlation method (Ide, 2010;Obara, 2002). We used the daily average of these locations (Figure S1) for energy estimations because of their large scatter, which may be due to strong heterogeneities in velocity structures in the accretionary prism (Takemura et al, 2020). We next computed the energy rate waveforms at the source from three-component seismograms that were band-pass filtered between 2 and 8 Hz and corrected for the site amplification factors given by Yabe et al (2021).…”

Seismic energy radiation and along-strike heterogeneities of shallow tectonic tremors at the Nankai Trough and Japan Trench

“…The seismic energy rate was then calculated by dividing the estimated Ei by the duration of the tremors defined above. Takemura et al (2020) showed that the estimated durations of shallow tremors described above could be overestimated. An accretionary prism with a low seismic velocity is present in the Nankai Trough.…”