The potential of using soft-sediment deformation structures for quantitatively reconstructing paleo-seismic shaking intensity: progress and prospect

Quantifying the magnitude of an earthquake is very important for long-term and medium-term earthquake prediction, post-earthquake emergency rescue and seismic hazard assessment. Paleo-seismology is the investigation of past earthquakes in the geological record, in particular their location, timing and size. Uncertainties remain in the paleo-earthquake magnitudes determined by traditional surface rupture parameters, especially because most seismic events do not result in surface ruptures or are of less than 0.3 m ( M = ~ 6–6.8). To address the problem of magnitude evaluation of earthquakes that did not reveal major dislocations, this paper deals with the methods used to determine the seismic shaking intensity based on the types and forms of soft-sediment deformation structures, including maximum liquefaction distance, thickness of disturbed layer, empirical formulae, and thickness of rapidly deposited sand layer. Then we discuss and analyze these methods in terms of their theoretical basis, advantages and disadvantages, accuracy, applicability and problems. We chose two case studies: first, a typical seismics-related deposit (liquefied layer and disrupted layer) represented by a seismite in the late-Pleistocene Lake Lisan section near Masada in the Dead Sea Basin; and second, the liquefied diapir triggered by an earthquake in the late-Quaternary lacustrine sediments at Luobozhai in the upper reaches of the Minjiang River, East Tibet. The five methods listed above are employed to determine earthquake magnitudes associated with the seismics-related deposit and liquefied diapir, yielding magnitudes of 5.5–6.5 and 6.0–7.0, respectively. The combination of the five methods, provided a new and relatively convenient method for determining seismic shaking, especially in lacustrine sediments. This study can serve as a valid reference for comparing methods of calculating the magnitude of a paleo-earthquake based on surface rupture parameters, and provides a better understanding of the long-term seismic activity and risk in tectonically active regions.