Subaqueous Paleoseismology: Fresh perspectives on sedimentary response to regional tectonics

Sharp changes in lithology and increases in grain size and sedimentation rate of sedimentary sequences from tectonically active basins are often used to indicate regional neotectonic activity. However, these conventional methods have been challenged by others who argue that the sedimentary evidence used to infer tectonism could be climatically induced. Therefore, some forms of independent evidence or sedimentary criteria are required to discriminate between these two alternatives.

Seismites, sedimentary units preserved in subaqueous stratigraphic sequences that are caused by seismic shaking, are reliable indicators of regional tectonic activity. Subaqueous paleoseismology, can extend the record of strong earthquakes and augment the understanding of fault zone tectonic activity by studying seismites preserved in subaqueous sedimentary sequences. Here, we use the Dead Sea Basin (Middle East) and the Qaidam Basin (NE Tibet) as examples to further understand regional neotectonic activity from the perspectives of subaqueous paleoseismology.

The Dead Sea Basin is the deepest and largest continental tectonic structure in the world. In situ folded layers and intraclast breccia layer in the ICDP Core 5017-1 that recovered from the Dead Sea depocenter are identified as earthquake indicators, based on their resemblance to the lake outcrop observations of seismites that are known to be earthquake-induced. Based on the Kelvin-Helmholtz instability, we model the ground acceleration needed to produce each seismite by using the physical properties of the Dead Sea deposits. We invert acceleration for earthquake magnitude by considering regional earthquake ground motion attenuation, fault geometry, and other constraints.

Based on the magnitude constraints, we develop a 220 kyr-long record of Mw ≥7 earthquakes. The record shows a clustered earthquake recurrence pattern and a group-fault temporal clustering model, and reveals an unexpectedly high seismicity rate on a slow-slipping (~5 mm/yr) plate boundary. We also propose a new approach to establish the seismic origin of prehistoric turbidites that involves analyzing in situ deformation that underlies each turbidite. Moreover, our sedimentological data validate a long-lasting hypothesis that soft-sediment deformation in the Dead Sea formed at the sediment-water interface.

The Qaidam Basin is the largest topographic depression on the Tibetan Plateau that was formed by the ongoing India-Asia collision. The northeastward growth of Tibet formed a series of sub-parallel NW-SE-trending folds over a distance of ~300 km in the western Qaidam Basin. A long core was drilled in the basin on the crest of one such fold, the Jianshan Anticline. Sedimentological analysis reveals micro-faults, soft-sediment deformation, slumps, and detachment surfaces preserved in the core, which we interpret as paleoearthquake indicators. The core records five seismite clusters during 3.6-2.7 Ma. This suggests that the rate of tectonic strain accommodated by the folds/thrusts in the region varies in time and thus reveals episodic local deformation. During the clusters, regional deformation is concentrated more in the fold-and-thrust system than along regional major strike-slip faults.

This kind of research provides a fresh perspective for understanding regional tectonism by linking paleoseismic events and recurrence patterns with regional deformation, and can expand the ability of paleoseismology to understand the history of regional tectonics.