Quantified sensitivity of small lake sediments to record historic earthquakes: Implications for paleoseismology

Abstract

Seismic hazard assessment is a critical but challenging issue for modern societies. A key parameter to be estimated is the recurrence interval of damaging earthquakes. This requires the establishment of earthquake records long enough to be relevant, i.e., far longer than historical observations. We study how lake sediments can be used for this purpose and explore conditions that enable lake sediments to record earthquakes. This was achieved (i) through the compilation of eight lake-sediment sequences from the European Alps to reconstruct chronicles of mass movement deposits and (ii) through the comparison of these chronicles with the well-documented earthquake history. This allowed 24 occurrences of mass movements to be identified, of which 21 were most probably triggered by an earthquake. However, the number of earthquake-induced deposits varies between lakes of a same region, suggesting variable thresholds of the lake sequences to record earthquake shaking. These thresholds have been quantified by linking the mass movement occurrences in a single lake to both intensity and distance of the triggering earthquakes. This method offers a quantitative approach to estimate locations and intensities of past earthquake epicenters. Finally, we explored which lake characteristics could explain the various sensitivities. Our results suggest that sedimentation rate should be larger than 0.5 mm yr À1 so that a given lake records earthquakes in moderately active seismotectonic regions. We also postulate that an increasing sedimentation rate may imply an increasing sensitivity to earthquake shaking. Hence, further paleoseismological studies should control carefully that no significant change in sedimentation rates occurs within a record, which could falsify the assessment of earthquake recurrence intervals.

A multi-centennial record of past floods and earthquakes in Valle d'Aosta, Mediterranean Italian Alps

“…formation of concentrated density flows of suspended sediments during a subaquatic mass movement (e.g. Mulder and Cochonnat, 1996;Arnaud et al, 2002;Wilhelm et al, 2016b). The HB characteristics are very similar to deposits previously described by many studies (e.g.…”

“…Van Daele et al, 2015;Wilhelm et al, 2016b). Here changes of lake level can be excluded because the water level of Lago Inferiore is well controlled by a bedrock outlet.…”

“…According to several studies providing a comprehensive overview of event layers (e.g. Mulder and Cochonat, 1996;Gani, 2004;Van Daele et al, 2015;Wilhelm et al, 2016b), the 77 beds represent short-term depositional events and they correspond to 74 graded beds (GBs), 1 matrix-supported bed (MSB), 1 homogeneous bed (HB) and 1 deformed layer (Fig. 3).…”

“…In addition to the chronological agreement, the potentially recorded earth-quakes are also expected to be the strongest and/or the closest to the lake, as those are expected to have generated the largest ground motions in the lake area. To take into account this second parameter, the seismic intensity of each historical earthquake in the lake area was estimated in first order by using the following equation from Wilhelm et al (2016b):…”

“…Reconstruction of past earthquake magnitudes and location is approached by comparing regional records of seismic-induced deposits (e.g. Strasser et al, 2006;Wilhelm et al, 2016b) or through the deposit's spatial extent and thickness (Howarth et al, 2014;Moernaut et al, 2014).…”