NH5.6

Caldera lakes are prominent volcanic features that can pose an additional hazard due to water presence, such as tsunamis, lahars, or flooding by lake breakout. Many of these lakes are populated and occupied by infrastructure on their shore, such as hydroelectric facilities. Volcanogenic tsunamis are a lesser modelled hazard compared to their seismogenic relatives, and the understanding of wave-making potential from subaqueous explosive eruptions is poor due to practical limitations of volcanic observation. Prior studies utilised models of surface waves produced from analogous chemical and nuclear explosions; however, these are derived from dated naval research and require reassessment.

This study verifies a non-hydrostatic, vertically-Lagrangian multilayer method from the open-source software Basilisk against a laboratory flume experiment to assess suitability for modelling waves produced by variable size disturbances. This is then used to evaluate free-surface initial condition models of shallow water explosions on a U.S. Army submerged explosive series on generating waves in Mono Lake, California. On establishing fitness of the underlying models, these are applied to simulate hypothetical scenarios of submarine eruptions at Lake Taupō, New Zealand. Event locations and disturbance sizes are chosen corresponding to vent sites and magnitudes of eruptions during the Holocene. The initial disturbance is fitted to a function of estimated eruption energy. Vulnerable areas include small settlements across the eastern shore, Taupō township and the lake outflow control gates. After the initial tsunami waves, a seiche of lower amplitude is established in the hour following the event. Such eruptions in lakes may pose multiple simultaneous hazards with minimal arrival times (< 15 minutes for tsunami waves at Taupō); therefore the modelling of eruptive scenarios is a primary approach to inform local area hazard maps of submarine volcanism and raise preparedness for surrounding facilities and communities.

How to cite: Hayward, M., Whittaker, C., Lane, E., and Power, W.: Numerical simulations of tsunami generation in caldera lakes by subaqueous explosive volcanism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7080, https://doi.org/10.5194/egusphere-egu21-7080, 2021.

Lake Iznik (NW Turkey), is bordered by the middle strand of the North Anatolian Fault (MNAF), whose seismic activity is debated because of its quiescence during the instrumental period. In contrast, significant historical activity is documented by several chronicles over the last two millennia.

This study aims to get a new insight into its long-term seismicity and its tectonic setting. Lacustrine sediment cores reveal fourteen earthquake-induced turbidites since their ages correspond to seismic events during the past two millennia. Bathymetry and high-resolution seismic reflection data allow describing two hitherto unknown subaquatic active fault structures (the South Boyalica and Iznik faults), belonging to the MNAF system. Sediment cores sampled on both sides of the Iznik Fault document an event deposit and a sedimentary unit vertically offset of ∼ 40 cm interpreted as the last rupture during the 1065 CE destructive earthquake. Older events are supposed on this fault more than a thousand years ago. Further studies will help to estimate the horizontal coseismic offset of this oblique-slip fault and the calendar of older ruptures. The current seismic gap of thousand years on this segment greatly increases the seismic hazard in this region and must be considered in the seismic risk assessment of the NAF system.

How to cite: Gastineau, R., De Sigoyer, J., Sabatier, P., Fabbri, S. C., Anselmetti, F. S., Develle, A.-L., Şahin, M., Gündüz, S., Niessen, F., and Gebhardt, A. C.: Active subaquatic fault segments in Lake Iznik along the middle strand of the North Anatolian Fault, NW Turkey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-784, https://doi.org/10.5194/egusphere-egu21-784, 2021.

Earthquake-induced soft sediment deformation structures (SSDS) can be used to resolve earthquake recurrence rates, but also to provide more quantitative information on past earthquake shaking intensities. Thorough understanding of the interplay between i) different ground motion characteristics, ii) sediment properties, iii) slope morphology and iv) seismic site effects is paramount for full exploitation of the paleoseismological potential of subaqueous SSDS records. However, we lack comparative studies investigating different SSDS records related to well-documented earthquakes, varying sediment types and site morphologies.

We investigated 17 slope and three basin sediment cores from two South-Central Chilean lakes: lakes Riñihue and Calafquén. Using X-ray computed tomography (CT) data, six different types of SSDS were observed: i) disturbed lamination, ii) folds, iii) intraclast breccia, iv) faults, v) load structures and vi) injection structures. We directly linked SSDS to five well-documented megathrust earthquakes using stratigraphic correlation of sediment sequences to well-dated basinal seismo-turbidite records.

Sediment of both lakes consists of varve couplets of diatomaceous ooze and organic-rich terrestrial material intercalated with coarse-grained tephras and fine-grained lahar deposits. From the 49 SSDS intervals, 61% are assigned to one of the five megathrust earthquakes. Of the SSDS intervals not assigned to megathrust earthquakes, 68% are located directly above a tephra or lahar deposit. We suggest that dewatering of volcanic deposits could have weakened overlying sediment and facilitated deformation during later earthquakes.

Slope gradient at coring sites range from 0.2-9.5° and 0.2-14.2° in lakes Riñihue and Calafquén, respectively. Deformation occurs from 0.2° and total deformation increases with slope angle in both lakes. Total deformation in lake Calafquén increases less with slope angle than in lake Riñihue. Our observations suggest seismically-induced shear stress alone can suffice to deform sediment, but even minor increases of gravitational downslope stress will ease deformation. Smaller increase of total deformation with slope angle for lake Calafquén could be explained by higher diatom content. Diatoms enhance shear strength through high particle interlocking and surface roughness. Therefore, we suggest that enhanced diatom content reduces sediment susceptibility to shear-induced deformation.

We evaluate the effect of ground motion characteristics by correlating SSDS to peak ground acceleration (PGA) and bracketed duration (BD) of the causative strong megathrust earthquakes as derived from ground motion prediction equations. As first suggested for SSDS in the Dead Sea area, disturbed lamination develops to folds and finally intraclast breccia and is driven by earthquake-induced shear causing Kelvin-Helmholtz Instability (KHI). In lake Riñihue, SSDS type and count correlates best with PGA suggesting amplitude of ground acceleration as the main control of KHI-driven deformation.

Future comparative analysis of lacustrine SSDS records in different geodynamic settings will put our findings in a broader perspective. Sediment types will be quantified by measuring characteristics like grain size, diatom and organic content, density, viscosity and Atterberg limits. Our study is the first to allow direct comparison of three different factors—sediment type, ground motion characteristics and slope morphology—with related earthquake-triggered SSDS, thereby advancing lacustrine paleoseismology towards a more quantitative interpretation of SSDS records.

How to cite: Molenaar, A., Van Daele, M., Vandorpe, T., Degenhart, G., De Batist, M., Urrutia, R., Pino, M., Strasser, M., and Moernaut, J.: Which Factors Modulate Earthquake-Triggered Soft Sediment Deformation? Moving Towards Quantitative Lacustrine Paleoseismology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3412, https://doi.org/10.5194/egusphere-egu21-3412, 2021.

Earthquake doublets form a particular challenge for seismic hazard assessment and can provide insights into potentially characteristic fault behaviour. However, knowledge on this type of earthquake sequences is limited to information provided by historical archives as their identification in paleoseismic records is ambiguous. The continuous sedimentation records provided by lacustrine settings might be able to resolve closely-timed earthquakes, but confident identification of earthquake doublets has, up to now, not been made. To reveal the potential of these high-resolution records, we perform a detailed analysis of a multi-pulsed turbidite that has been identified in the sedimentary infill of Lake Singkarak and that was generated by the March 2007 West Sumatra earthquake doublet (i.e. two M_w>6 shocks on adjacent fault segments at 2 hours apart). In order to distinguish non-synchronously generated pulses in this turbidite (different earthquake, same turbidite source area) from those that are potentially synchronously-generated (same earthquake, different turbidite source areas), we develop a new methodology that allows analysing paleoflow directions by using grain-size analysis, natural remanent magnetization measurements and high-resolution X-ray computed tomography. Combining these techniques allows us to reveal the absolute geographical orientation of elongated grains, which are considered to be deposited aligned to the dominant paleoflow direction. Application to the 2007 turbidite in Lake Singkarak allows identifying the presence of non-synchronously generated pulses, thus confirming that each earthquake in the 2007 West Sumatra doublet triggered separate turbidity currents in the lake. Our study thus underscores the invaluable sensitivity of lacustrine paleoseismic records and outlines a promising methodology to analyse previously-described multi-pulsed lacustrine turbidites to reveal the occurrence of, up to now, unknown earthquake doublets.

How to cite: Wils, K., Deprez, M., Kissel, C., Vervoort, M., Van Daele, M., Daryono, M. R., Cnudde, V., Natawidjaja, D. H., and De Batist, M.: Multiple pulses in lacustrine turbidites reveal earthquake doublets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-924, https://doi.org/10.5194/egusphere-egu21-924, 2021.

In formerly glaciated intraplate settings with moderate seismicities, such as the Eastern Alps, recurrence intervals of strong earthquakes (M_w >6) typically exceed the short time span of instrumental (~100 years) and historical (~1000 years) data. To assess the seismic hazards and draw conclusions about the role of postglacial isostatic rebound on earthquake recurrence in these regions, lakes have been increasingly used as natural seismographs over the last two decades.

We present paleoseismic records from three glacigenic lakes (Wörthersee, Millstätter See, and Klopeiner See) situated at the south-eastern rim of the Alps, Austria. This region, although located in an intraplate setting, has experienced several devastating historically and instrumentally recorded earthquakes with intensities ranging from V to IX (EMS-98) in our study area, e.g., in AD1348 (M_w ~7; possibly the strongest historical earthquake in the Alps), AD1511 (Mw 6.9), AD1690 (Mw 6.5), AD1857 (Mw 5) and AD1976 (Mw 6.4).

The lakes were investigated with multibeam bathymetry and a very dense grid of reflection seismic profiles (~1.3, 3.5, and 8 kHz; 640 km in total). Numerous short (~1.5 m; ~80 cores) and long (~up to 14 m; 22 cores) sediment cores were retrieved from all lakes and their respective subbasins and were independently dated (varve counting in the last ~1000 years, radiocarbon, and ²¹⁰Pb/¹³⁷Cs dating). This spatially and temporally high-resolution approach allows to construct a complete picture of the sedimentary imprint of strong earthquakes in these lakes.

The geophysical data image an archive of multiple simultaneous subaqueous landslides. In the sediment cores, which cover the last 7500 years in Millstätter See and reach back into the Late Glacial in Wörthersee and Klopeiner See, these landslides are represented as turbidite deposits (from mm to m-scale) interspersed in the partly finely laminated background sediments. By comparing the seismic intensities of the well-documented historical earthquakes to the spatial distribution of sedimentary imprint in the lakes, we revealed the earthquake recording thresholds (EQRT) of different depositional areas. Most of the sites record local intensities ≥VI. In shallow basins with low sedimentation rates, however, the EQRT is significantly higher, either solely recording the AD1348 event (VIII-IX) or showing no evidence of seismic shaking at all. Contrastingly, sites close to alluvial fans in Wörthersee also record the AD1857 and the AD1976 earthquakes (V½). Quantification of the earthquake-related deposits (e.g., cumulative turbidite thickness, percentage of depositional areas recording an event) shows a linear size-scaling relationship with the respective intensities. This provides us with a tool to constrain the local seismic intensity of prehistoric earthquakes.

Our data show that the AD1348 earthquake generated the strongest earthquake shaking in the study area for the entire Holocene. Generally, the seismicity peaked in the Late Glacial, around ~3.5 ka, and in the last ~1000 years, whereas the early- to mid-Holocene was a relatively calm period. Other paleo-earthquake studies from both the Fennoscandian Peninsula and the Swiss Alps show a similar seismicity pattern, suggesting that seismicity in the Alps is governed by postglacial rebound rather than tectonically induced stress.

How to cite: Daxer, C., Ortler, M., Huang, J.-J. S., Fabbri, S., Hilbe, M., De Batist, M., Hajdas, I., Piechl, T., Strasser, M., and Moernaut, J.: Calibrated long-term lacustrine paleoseismic records from Carinthia (Austria): implications for earthquake hazard in the south-eastern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3099, https://doi.org/10.5194/egusphere-egu21-3099, 2021.

Seismogenic turbidites are widely used for geohazard assessment. The use of turbidites as an earthquake indicator requires a clear demonstration that an earthquake, rather than non-seismic factors, is the most plausible trigger. The seismic origin is normally verified either by correlating the turbidites to historic earthquakes, or by demonstrating synchronous deposition over large areas of a basin. Correlating historic earthquakes could potentially constrain the seismic intensities necessary for triggering turbidites, however this method is not applicable to prehistoric events. In addition, the synchronous deposition of turbidites cannot be verified for a single core record.

Here, we propose a new approach to establish the seismic origin of prehistoric turbidites that involves analyzing in situ deformation that underlies each turbidite, as recorded in a 457 m-long core from the Dead Sea depocenter. These in situ deformations have been previously verified as seismites and could thus authenticate the trigger for each overlying turbidite. We also constrain the seismic intensities that triggered prehistoric turbidites by analyzing the degree of in situ deformation underlying each turbidite. Moreover, our high-resolution chemical and sedimentological data validate a long-lasting hypothesis that soft-sediment deformation in the Dead Sea formed at the sediment-water interface. In addition, we use our results to propose seven basic earthquake-related depositional scenarios preserved in depocenters located in tectonically active regions like the Dead Sea. These techniques and findings permit a more confident geohazard assessment in the region and act as a model for other similar tectonic settings, by improving the completeness of a paleoseismic archive.

How to cite: Hubert-Ferrari, A., Moernaut, J., Bookman, R., Waldmann, N., Wetzler, N., Agnon, A., Marco, S., Alsop, G. I., Strasser, M., and Lu, Y.: A new approach to constrain the seismic origin for prehistoric turbidites as applied to the Dead Sea Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11057, https://doi.org/10.5194/egusphere-egu21-11057, 2021.