What Controls the Remobilization and Deformation of Surficial Sediment
by Seismic Shaking? Linking Lacustrine Slope Stratigraphy to Great
Earthquakes in South-Central Chile
Abstract
Remobilization and deformation of surficial subaqueous slope sediments
create turbidites and soft sediment deformation structures (SSDS), which
are common features in many depositional records. Paleoseismic studies
have used seismically-induced turbidites and SSDS preserved in
sedimentary sequences to reconstruct recurrence patterns and—in some
cases—allow quantifying rupture location and magnitude of past
earthquakes. However, our understanding of earthquake-triggered
remobilization and deformation lacks studies targeting where these
processes take place, the subaqueous slope, and involving direct
comparison of sedimentary fingerprint with well-documented historical
earthquakes. Here we investigate the sedimentary imprint of six
megathrust earthquakes in 17 slope sediment cores from two Chilean
lakes, Riñihue and Calafquén, and link it to magnitude, seismic
intensity, peak ground acceleration (PGA) and Arias Intensity (Ia).
Centimeter-scale stratigraphic gaps—caused by remobilization of
surficial slope sediment—were identified using high-resolution
multi-proxy core correlation of slope to basin cores and six types of
SSDS using high-resolution 3D X-ray computed tomography data.
Centimeter-scale gaps occur at the studied sites when Ia and moment
magnitude (Mw) exceed 3.85 m/s and 8.8, respectively. Total
remobilization depth correlates best with Ia and is highest in both
lakes for the strongest earthquakes (Mw ~9.5). In lake
Riñihue, SSDS thickness and type correlates best with PGA providing
first field-based evidence of progressive SSDS development with
increasing PGA for SSDS caused by Kelvin-Helmholtz instability (KHI).
Stratigraphic gaps occur on slope angles of ≥2.3°, whereas deformation
already occurs from slope angle 0.2°. The thickness of both
stratigraphic gaps and SSDS increases with slope angle suggesting that
increased slope angle—and thereby gravitational shear
stress—promotes both remobilization and deformation. Seismic shaking
is the dominant trigger for remobilization and deformation at our
studied lakes. We propose that long duration and low frequency content
of seismic shaking facilitates surficial remobilization, whereas ground
motion amplitude controls KHI-related SSDS development.