Probabilistic Evaluation of Fault Sources Based on Paleoseismic Evidence From Mass‐Transport Deposits: The Example of Aysén Fjord, Chile

Abstract

Contemporaneous mass‐transport deposits (MTDs) recorded in lake and fjord sediments provide evidence of past seismic shaking. However, because they are usually not connected to a fault rupture, assessment of the earthquake source remains difficult. Based on observed coseismic mass wasting and associated seismic shaking, previous studies assigned minimum intensities required to trigger them. Attempts to infer their earthquake source relied on methods developed to estimate the location and magnitude of historical earthquakes using intensity prediction equations, but considered these thresholds as actual intensities. Here we develop a probabilistic method to infer the most likely earthquake sources from the spatial distribution (or absence) of MTDs. This approach simultaneously allows the triggering intensity to exceed the assumed threshold and takes into account intensity prediction equation uncertainties, two shortcomings of existing methods. Additionally, we consider known active faults rather than a grid of possible epicenters. We apply this method to Aysén Fjord (southern Chile), which is intersected by the Liquiñe‐Ofqui Fault Zone. In 2007, an MW = 6.2 earthquake hit the fjord with intensities of VIII+, causing major landslides entering the fjord. Seismic‐reflection profiles show that its sedimentary fill contains nine prehistoric MTD levels. Following a sensitivity analysis, application of the method to the MTD record allows identifying the most likely fault sections and magnitude range for most events, confirming that they are mainly attributed to crustal earthquakes on the Liquiñe‐Ofqui Fault Zone. We conclude that the method has good potential to constrain the size and location of paleoearthquakes for which only shaking evidence is available.

Seismo‐Turbidites in Aysén Fjord (Southern Chile) Reveal a Complex Pattern of Rupture Modes Along the 1960 Megathrust Earthquake Segment

“…As a consequence, all seismo-turbidites that consist solely of deltaic material and are not associated with onshore landslides, that is, all except SL-G, SL-F, SL-D, and the youngest EM3 turbidite, are considered associated to megathrust earthquakes, bringing the total number of megathrust-earthquake related turbidites to 24. This is in agreement with previous research that already linked events SL-G, SL-F, and SL-D to crustal earthquakes and SL-EF and SL-DE to megathrust earthquakes (Vanneste et al, 2018;Wils et al, 2018). However, it is important to note that some far-field crustal (such as the 1927 earthquake) or intraslab earthquakes may have a similar effect in terms of seismic shaking on the fjord as megathrust earthquakes, resulting in the same turbidite signal in the sediment core.…”

“…Previous research in Aysén Fjord (Van Daele et al, 2013;Vanneste et al, 2018;Wils et al, 2018) has shown that its sediments are a valuable archive for paleoseismological research, with a particular focus on crustal earthquakes due to its location on the LOFZ (Figure 1b). Traces of the 2007 Aysén earthquake were identified on high-resolution bathymetric and seismic data covering the inner part of the fjord, together with nine older seismic-stratigraphic levels (termed SL-A, SL-B, SL-C, SL-CD, SL-D, SL-DE, SL-EF, SL-F, and SL-G) characterized by the presence of seismically triggered MTDs related to fjord wall collapses and/or subaqueous landslides (Van Daele et al, 2013;Wils et al, 2018).…”

“…Vanneste et al, 2018). Both of the deposits that are possibly resulting from a megathrust earthquake (SL-EF and SL-DE) are characterized by this type of deltaic turbidite, as well as small-scale offshore MTDs (seismic intensity ≥ VI½; Vanneste et al, 2018) near the vertical resolution limit of the reflection-seismic data (Wils et al, 2018). More megathrust-earthquake-related deposits might thus be present in the fjord infill that remain undetected on the seismic profiles but could potentially be revealed by the presence of turbidites in the sediment core.…”

“…Age ranges for these MTDs were provided by a 21 m long, radiocarbon‐dated sediment core (MD07‐3117, collected 2 months before the 2007 earthquake) through correlation of the mapped MTD‐levels with turbidites in the core (Wils et al, 2018). Forward modeling of seismic shaking intensities to match observed MTD distributions by considering the threshold values required to trigger different MTD types allowed to confirm that the majority of these MTDs are generated by crustal earthquakes on the LOFZ but can in some cases (i.e., for SL‐EF and SL‐DE) also result from megathrust earthquakes as no definite LOFZ source can be identified (Vanneste et al, 2018). Additionally, a megathrust‐related origin for SL‐EF was suggested by Wils et al (2018) as well by considering its MTD sizes, distribution and age overlap with a megathrust earthquake between AD ~ −50 and 50 known from another paleoseismic site (Kempf et al, 2017).…”

Nearly Instantaneous Tsunamis Following the Mw 7.5 2018 Palu Earthquake

“…Deltas, formed from unconsolidated material, are very sensitive to failure by an external trigger such as seismic shaking (Girardclos et al, 2007;Hasiotis et al, 2006;Hilbe & Anselmetti, 2014;Leithold et al, 2018). Furthermore, the potential for delta failures to generate significant tsunamis during strong shaking events (Vanneste et al, 2018) has been documented at sites with diverse climatic conditions, such as Alaska (USA; Field et al, 1982;Parsons et al, 2014) and Haiti (Hornbach et al, 2010).…”