Seismic triggering of landslides and turbidity currents offshore Portugal

Abstract

[1] Sediments in deep water basins often include turbidites that record sediment input from adjacent continental margins. In seismically active areas, where turbidity currents are triggered by earthquakes, the basinal turbidite sequence may thus contain a record of palaeoseismicity, which can be used to infer the frequency of earthquakes affecting the margins of the basin. This is particularly useful where large earthquakes have a recurrence interval than is greater than the historical record. However, turbidity currents can be triggered by several processes, and it is often difficult to trace individual turbidites to their precise source areas and to assign a definite trigger to a particular turbidite. Here, we demonstrate that turbidites emplaced at $6600 and $8300 Cal yr BP in the Tagus Abyssal Plain, off Portugal, correlate with erosional hiatuses in two submarine canyons on the continental margin. The turbidites are sourced from simultaneous landsliding in both canyons, requiring regional triggers interpreted as earthquakes. An earthquake recurrence interval for the continental margin of $4000 years is estimated by extrapolation to deeper turbidites in the basin sequence. However, the example of the 1755 earthquake, which caused widespread devastation in southwest Iberia, shows that palaeoseismic interpretations must be made with caution. The 1755 earthquake had a magnitude >8.5 and yet the associated turbidite in the abyssal plain is typically $5 cm thick, while older turbidites can be >1 m thick. Given the large 1755 earthquake magnitude, the difference in turbidite thickness is unlikely to be related to the relative size of triggering earthquakes. Instead, we suggest that the offshore location of the 1755 earthquake, coupled with low sedimentation rates during the Holocene, may have limited the size of the associated turbidite.

Timing and frequency of large submarine landslides: implications for understanding triggers and future geohazard

“…If such a link between sea level and landslide frequency exists it is too weak to be detected using the available global data base. It is possible that controlling factors vary between different geographical areas, and their role is therefore hidden Stigall and Dugan, 2010;Masson et al, 2011). These hypotheses are 20 poorly tested, and even less is known about the effect of other preconditioning 21 factors such as fluid flow focussing in the slope (Dugan, 2012).…”

Impact of sea-level rise on earthquake and landslide triggering offshore the Alentejo margin (SW Iberia)

“…Based on the turbidite frequency, Gràcia et al (2010) proposed a recurrence interval of approximately 2,000 years for the large earthquakes. Masson et al (2011) presented further compelling evidence of seismic triggering of landslides and turbidity currents along the SW Iberian margin. Their study demonstrated that turbidites emplaced at 6,600 and 8,300 cal yr BP on the TAP correlate with erosional hiatuses in two submarine canyons on the continental margin.…”

“…There is extensive evidence of mass movements associated with these faults, as demonstrated throughout this work. Several studies in the area have considered landslides and turbidite records as proxies of paleoseismic activity (Gràcia et al 2010;Masson et al 2011). Recently, deep drilling during IODP Expedition 339 at Site U1391 revealed the existence of several debris flow events interbedded within the sedimentary succession of the Alentejo margin (Hernández-Molina et al 2016).…”

“…The main objectives of this study are to (1) model the plate bending due to sea-level rise since the LGM in order to explore the possible relation between identified slope failures and sea level-induced fault ruptures, and (2) evaluate the Coulomb stresses due to sea-level rise on specific target faults, in the context of a broader search for preconditioning factors and trigger mechanisms of Holocene slope failure and mass movement. Gulf of Cadiz G1-G4 MPF area Vizcaino et al (2006) Gràcia et al (2010 TAP Masson et al (2011) Pro et al (2013). Colored dots Locations of sediment cores on HAP (Gràcia et al 2010) and TAP (Masson et al 2011), and of gravity cores in the MPF area (Vizcaino et al 2006) Earthquakes and landslides Regarding seismic activity, the area offshore SW Iberia is dominated by low-to moderate-magnitude events (Mw≤6.0), although large and destructive earthquakes have nucleated here (Fig.…”

“…Gulf of Cadiz G1-G4 MPF area Vizcaino et al (2006) Gràcia et al (2010 TAP Masson et al (2011) Pro et al (2013). Colored dots Locations of sediment cores on HAP (Gràcia et al 2010) and TAP (Masson et al 2011), and of gravity cores in the MPF area (Vizcaino et al 2006) Earthquakes and landslides Regarding seismic activity, the area offshore SW Iberia is dominated by low-to moderate-magnitude events (Mw≤6.0), although large and destructive earthquakes have nucleated here (Fig. 1).…”