Abstract

Earthquake-induced submarine slope destabilization is known to cause debris flows and turbidity currents, but the hydrodynamic processes associated with these events remain poorly understood. Records are scarce and this notably limits our ability to interpret marine paleoseismological sedimentary records. An instrumented frame comprising a pressure recorder and a Doppler recording current meter deployed at the seafloor in the Sea of Marmara Central Basin recorded consequences of a MW = 5.8 earthquake occurring Sept 26, 2019 and of a Mw = 4.7 foreshock two days before. The smaller event caused sediment resuspension but no strong current. The larger event triggered a complex response involving a mud flow and turbidity currents with variable velocities and orientations, which may result from multiple slope failures. A long delay of 10 hours is observed between the earthquake and the passing of the strongest turbidity current. The distance travelled by the sediment particles during the event is estimated to several kilometres, which could account for a local deposit on a sediment fan at the outlet of a canyon, but not for the covering of the whole basin floor. We show that after a moderate earthquake, delayed turbidity current initiation may occur, possibly by ignition of a cloud of resuspended sediment. Some caution is thus required when tying seismoturbidites with earthquakes of historical importance. However, the horizontal extent of the deposits should remain indicative of the size of the earthquake.

Highlights

  • Triggering of mass flows and turbidity currents by earthquakes is a hazard that can damage infrastructure at the seafloor (Heezen et al, 1954) and may enhance coseismic tsunami generation (Okal and Synolakis, 2001; Synolakis et al, 2002; Hebert et al, 2005; Ozeren et al, 2010)

  • Data obtained with a seafloor device located at the oultet of a canyon in the Central Basin in the Sea of Marmara bring some insight on how earthquakes scale with their hydrodynamic consequences

  • As the current velocity remained small, it can be concluded that this cloud did not evolve into a self-sustained turbidity current (Parker, 1982)

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Summary

Introduction

Triggering of mass flows and turbidity currents by earthquakes is a hazard that can damage infrastructure at the seafloor (Heezen et al, 1954) and may enhance coseismic tsunami generation (Okal and Synolakis, 2001; Synolakis et al, 2002; Hebert et al, 2005; Ozeren et al, 2010). In the Mediterranean region, the threshold is reportedly around Mw = 5 In spite of this high regional variability, turbidite deposits in several seismically active zones have been used successfully as paleoseimological event markers (Goldfinger et al, 2003, 2012; McHugh et al, 2014; Ikehara et al, 2016). This requires distinguishing between seismoturbidites, caused by earthquakes and related mass wasting events, from those resulting from other processes (e.g. floods, storms, sediment loading). The instrument (BPR) was deployed on a depositional fan at the base of slope and canyon outlet that differ in character from the hemipelagite / turbidite-homogenite sequence in the basin

Context and data collection
Pressure and tilt records
Current records and possible causes of tilting
Acoustic backscatter record
Temperature record
Inferred sequence of events
Discussion and conclusion
Full Text
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