Record seismic events over the last 220,000 years
Large earthquakes (magnitude ≥ 7) are among the most dangerous natural hazards on Earth. Our direct knowledge about the hazard potential for large earthquakes on most faults is limited as they usually have recurrence intervals longer than the time span of modern seismograph operation (< 100 years). Historic records in some regions can extend over the past few hundred years to perhaps two thousand years, but for slow slipping faults, even such long timespans may not cover multiple recurrence intervals of large earthquakes. The "subaquatic paleoseismology" discipline uses continuous, temporally high-resolution sediment sequences in the ocean and in lakes to extend records of strong shaking. Distinctive sediment deformations caused by earthquakes preserve "earthquake events", thus provide data to improve our understanding of the hazard potential where these data may be obtained.
The Dead Sea Fault, a left-lateral, > 1,000-km-long strike-slip plate boundary separating the African and Arabian plates, is one of the most famous earthquake-generating faults on Earth. The Dead Sea, the lowest place on Earth (-434 m), is situated on the central part of the fault. In 2010-2011 a 457 m-long sediment sequence, spanning the past 220,000 years, was obtained in the 300 m deep central Dead Sea by the International Continental Scientific Drilling Program (ICDP).
More than 400 earthquake-caused soft-sediment deformation structures were identified in this core. These structures are very similar to those softly deformed in other environments (e.g., air and ocean), suggesting a common mechanism, Kelvin Helmholtz instability. During earthquake shaking, the upper less-dense mud moves much faster than the lower denser mud (in the same direction), creating shear localized at the layer interface. These subtle structures are subsequently buried by new layers, resetting the system to record the next shaking event. The resulting sedimentary record in the center of the Dead Sea has recorded hundreds of earthquake shaking events with different shaking intensities over the past 220,000 years.
To recover shaking intensity from the recorded structures, Lu et al., ran a series of 2-D numerical simulations using the physical properties of the soft sediments at the bottom of the Dead Sea. Using the Kelvin-Helmholtz instability mechanism, Lu et al., modeled the ground acceleration needed to produce each deformed structure. These accelerations correspond to different levels of earthquake shaking intensity. These can be converted to earthquake magnitudes by considering earthquake ground motion attenuation in the region, fault geometry, and other limiting conditions. Lu et al., found that over the past 220,000 years large earthquakes occurred with recurrence times ranging from a few years to a few thousand years, with a mean of 1400 years. This mean recurrence time is significantly shorter than the previous estimate of 7 to 11 thousand years, thus revealing an unexpectedly high seismicity rate on a slow-slipping (< 5 mm/year) plate boundary. In addition, unlike the periodic recurrence of earthquakes on fast-slipping and geometrically simple strike-slip faults, e.g., the Wrightwood Section of the San Andreas Fault and Alpine Fault in New Zealand, our work confirms a clustered recurrence pattern for large earthquakes on the slow-slipping Dead Sea Fault.
This is the first attempt to apply a computational fluid dynamic modeling-based quantitative “fossil seismograph” to develop a large earthquake record. This is the longest large earthquake record of a major fault in the world. It is also calibrated to historic earthquakes, for which the Dead Sea area has a famously long span.
