Recent history of natural hazards in Chile : imprints of earthquakes and volcanic events in lacustrine and marine sediments

In the past decade, a series of major endogenic, natural catastrophic events caused hundreds of thousands of casualties, as well as a tremendous amount of structural and economic damage. Megathrust earthquakes at subduction zones (i.e. interplate earthquakes) resulted in damage and fatalities over areas of hundreds of kilometers, and, moreover, triggered ocean-crossing tsunamis that in turn caused devastation on several continents. Smaller, but shallow, earthquakes were locally even more destructive than most of the megathrust events. Also, volcanic eruptions disrupted air travel in several parts of the world. Along subduction zones, these three types of potentially catastrophic events occur in the same area and are genetically linked. The subduction process does not only directly produce the megathrust earthquakes, but can also influence the stress regime within the overriding (continental) plate, resulting in continental faults potentially causing local but shallow earthquakes. Moreover, the subduction process induces volcanism in the overriding plate. Assessing the past behavior of faults or volcanoes can lead us towards understanding their current behavior and hazard. Hence, there is a need to study the recurrence and magnitudes of past events. However, in many regions of the world, such as South-Central Chile (the study area of this thesis), historical records are not long enough and barely cover one or two cycles of some of these events. Therefore, prehistoric events should be studied. Sedimentary sequences in lakes and fjords can be used to reconstruct both past earthquakes and volcanic events. Earthquakes can trigger mass-transport deposits, in-situ deformation and several types of turbidites, which can, for example, be used to estimate local seismic intensities. Volcanic events can cause fallout ash layers, but also lahars and ashes that are deposited in the catchment can eventually leave traces in lakes or fjords. Moreover, continuous inter-event sedimentation supports preservation and allows achieving good, reliable age models. To be able to correctly interpret these event deposits, it is crucial to study the sedimentary impact of recent catastrophic events, and thereby “calibrate” the method. In this thesis, the event deposits of several recent, well-described natural events are studied: i) sedimentary event deposits related to volcanic activity of Villarrica Volcano in the last 100 years (Chapter 3); ii) the sedimentary imprint of the 1960 Mw 9.5 Valdivia and 2010 Mw 8.8 Maule Chilean megathrust earthquakes in 19 lakes (Chapter 4); and iii) the impact of the 2007 Mw 6.2 Aysén earthquake and related onshore landslides in Aysén fjord (chapters 5 and 6). Following a general introduction in Chapter 1, an overview of the methods that are used in this study is presented in Chapter 2. In Chapter 3, a multi-lake and multi-proxy analytical approach is used to obtain reliable and high-resolution records of past natural catastrophes from c. 600-year-old annually-laminated (varved) lake-sediment sequences extracted from three lakes, Villarrica, Calafquén and Riñihue, in the volcanically and seismically active Chilean Lake District. Using a combination of µXRF scanning, microfacies analysis, grain-size analysis, color analysis and magnetic susceptibility, four different types of event deposits (lacustrine turbidites, tephra-fall layers, runoff cryptotephras, lahar deposits) are detected and characterized, and the eruption record for the nearby volcanoes (i.e. Villarrica, El Mocho) is revised. This eruption record is unprecedented in its continuity and temporal resolution. Time series analysis shows 112 eruptions with a Volcanic Explosivity Index (VEI) ≥ 2 from Villarrica Volcano in the last c. 600 years. Mocho-Choshuenco Volcano has been significantly less active since 1900 compared to preceding centuries. Also deposits of eruptions from the more remote Carrán-Los Venados Volcanic Complex, Quetrupillán and Lanín or Huanquihue volcanoes are identified in the studied lake sediments. The last VEI ≥ 2 eruption of Villarrica Volcano occurred in 1991. The probability of the occurrence of future eruptions from Villarrica Volcano is estimated, and it is statistically demonstrated that the probability of a 21-year repose period (anno 2012) without eruptions is ≤ 1.9 %. This new perspective on the recurrence interval of eruptions and historical lahar activity will help improve volcanic hazard assessments for this rapidly expanding tourist region. In Chapter 4, short sediment cores, sometimes combined with high-resolution reflection seismics, are used to study the sedimentary impact of the 1960 Mw 9.5 Valdivia and the 2010 Mw 8.8 Maule earthquakes in 19 lakes in South-Central Chile (from north to south: Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Lanalhue, Butaco, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). By using image-analysis, magnetic susceptibility and grain-size analysis, five types of event deposits (seismites) are identified and are attributed to one of the abovementioned megathrust earthquakes: i) mass-transport deposits (MTDs), ii) in-situ deformation, iii) lacustrine turbidites with a similar composition as the background sediments (LT1s), iv) lacustrine turbidites with a different composition as the background sediments (more terrestrial; LT2s), and v) homogenites. These seismites are compared to local seismic intensities, eyewitness reports, post-earthquake observations, vegetation and geomorphology of the catchment. Homogenites consistently represent lake seiches and LT2s can be a result of local near-shore mass wasting, delta collapse or onshore landslides surging into the lake (direct imprint), but also of debris or mud flows entering the lake, or of fluvial reworking of landslide debris in the catchment (indirect imprint). LT1s are the most reliable seismites, since they can have almost no other triggers then earthquake shaking. Moreover, they most sensitively record different seismic intensities, which is reflected in a different thickness and distribution in a certain lake. For the other seismites, the lake sub-basins only have a minimum intensity threshold. In general, the minimum encountered seismic-intensity threshold for most seismites is VI, but it can differ between the lakes and/or sub-basins. Finally, coastal lakes on the border between the Valdivia and Concepción-Valparaíso rupture zones (i.e. lakes Lanalhue and Butaco), where seismic shaking is frequent, appear to record lake-level rise due to the coastal tilting rather than the seismic shaking itself. In Chapter 5, widespread basin-plain deformation that is caused by the 2007 Mw 6.2 earthquake in Aysén fjord is studied. This earthquake caused many onshore landslides which, in turn, triggered several destructive tsunamis in the fjord. Both superficial and buried deformed basin-plain deposits are mapped using multibeam bathymetry and seismic-reflection (sparker) profiling. The seismic signature of the sub-bottom is ground-truthed with short sediment cores on key locations. Deformed basin-plain deposits induced by the 2007 earthquake can be subdivided into frontally emergent and confined deformed basin-plain deposits, with a deep and shallow basal shear surface. All deformed basin-plain deposits with a deep basal shear surface are induced by the weight and impact of a slope-adjacent mass-flow wedge. The frontally emergent –most mobile– basin-plain deformation is triggered by mass flows originating from onshore landslides (i.e. debris flows, rock slides and avalanches) propagating into the fjord. This basin-plain deformation results in vertical seafloor offsets of up to 20 m and might be even more important for tsunami generation than the impact on the sea surface of the onshore landslides themselves. In the depressions created by the basin-plain deformation, megaturbidites occur, while more distally, sandy density-flow deposits cover the seafloor. The data also indicates that these density flows propagate slower than the basin-plain deformation. Based on correlations with the two main eruptions of the Hudson Volcano, it is hypothesized that during the Holocene three to four similar events have struck the fjord. Due to the constant structural characteristics of the Liquiñe-Ofqui Fault Zone in the northern Patagonian fjordland and historical seismic swarms in this area, it is concluded that similar hazards should be taken into account for most of the fjords in this region. Finally, there are indications that both the prehistoric events and the 2007 fluid-driven seismic swarm may have been accompanied by fluid flow from tectonic structures below the fjord to the seafloor. In Chapter 6, the density-flow deposits that are recognized in the previous chapter are studied in more detail. Twenty two short sediment cores taken in the inner Aysén fjord help to better understand the sedimentary-facies successions and intercalation of the density-flow deposits. By combining grain-size analysis with X-ray computed tomography (CT) scanning, it is possible to demonstrate that the encountered facies correspond to classical divisions of density-flow deposits and turbidites. The deposits consist of a succession of several sub-deposits with different paleoflow directions and can be interpreted as amalgamated density-flow deposits. Orientations of i) folds, ii) imbricated mud clasts, iii) back- and foresets of climbing ripples and iv) asymmetric convolute lamination, are used to determine relative paleoflow directions at the location of the cores. By assigning the basal flow of the amalgamated density-flow deposits to the closest principal mass-flow, the absolute flow directions of the sub-deposits is determined, which, in combination with multibeam basin-floor morphology, allows reconstruction of the 2007 density-flow successions in Aysén fjord. Furthermore, alternating flow directions observed in some of the cores provide evidence for a seiche induced by the density flows. Traction carpets, in which rip-up mud clasts correspond to the floating, coarsest grains, occur at the base of the density-flow deposits. The main conclusion is that X-ray CT scans provide crucial information for reconstructing paleoflows and can be a useful tool in marine and lacustrine sedimentology and paleoseismology. Multidirectional, amalgamated density-flow deposits and turbidites are an indication for simultaneous triggering of density flows and can therefore in most cases be attributed to earthquakes, ruling out other triggers, such as e.g. floods. In conclusion, this PhD dissertation shows that lake and fjord sediments are excellent recorders of natural events. The most frequently encountered type of deposits linked to catastrophic events are density-flow deposits (which include turbidites). It was possible to differentiate between mass-wasting generated turbidites triggered directly by earthquakes and turbidites triggered by onshore debris or mud flows, lahars (for the first time described in lakes), or floods reaching the lake shore. Amalgamated density-flow deposits were used to prove simultaneous triggering (and hence, earthquake triggering) of mass-wasting events. Another breakthrough is the recognition of deposits related to volcanic events that are not fallout, but have been transported from the catchment. Therefore, independent of wind directions during an eruption, even fine fallout ashes in the catchment can reach the lakes through runoff and leave a permanent sedimentary imprint. Finally, it can be concluded that the deposits in all the study areas match very well with reports, seismic intensities etc., and can therefore be used to reconstruct similar events in the past, in a semi-quantitative way

On how X-ray (micro) computed tomography on turbidites can help us unravel paleoflow successions, directions and dynamics

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Reconstructing East African monsoon variability from grain-size distributions : end-member modeling and source attribution of diatom-rich sediments from Lake Chala

Grain-size analysis and end-member modeling of the clastic fraction of the 25-kyr sediment sequence from Lake Chala, a meromictic crater lake on the lower east slope of Mt. Kilimanjaro, reveal crucial aspects of climate-driven environmental change in equatorial East Africa since the Last Glacial Maximum. The finely laminated sediments of Lake Chala contain only up to 40% of clastic components, the rest are mainly diatom frustules and amorphous organic matter. Measured grain-size distributions were split into six statistically meaningful end members, of which four could be linked to a distinct source and transport mechanism of clastic mineral input: fine aeolian dust from distal sources (EM1), fine catchment runoff (EM2), coarser aeolian dust from proximal sources (EM5) and coarse erosive material from the upper crater slopes (EM6). The two other end members (EM3 and EM4) represented frustules of the two most common diatom taxa in Lake Chala, Afrocymbella barkeri and Nitzschia fabiennejansseniana, which had (partly) survived sample pre-treatment. Temporal variation in normalized abundance of the two dust-derived end members are valuable proxies for past changes in monsoon circulation over equatorial East Africa. During Northern Hemisphere cold periods (e.g., Last Glacial Maximum and Younger Dryas) the Inter-Tropical Convergence Zone shifted southward, enhancing northeasterly monsoon winds in the Lake Chala area and increasing advection of fine dust from the dry Horn of Africa region. Simultaneously, more modest continental heating reduced the prevalence of small-scale atmospheric turbulence, and thus the occurrence of dust devils, resulting in reduced influx of coarse dust from drylands nearby. Conversely, abrupt intensification of the southeasterly monsoon at the onset of the Holocene is recorded by an abrupt increase in the amount of coarse dust delivered to Lake Chala. Temporal variation in the end members representing catchment run-off (EM2) and erosion (EM6) mainly reflect changes in lacustrine sedimentation dynamics associated with major lake-level fluctuation, as evidenced by other paleoenvironmental proxies. Overall this study shows that subdivision of the clastic fraction of lacustrine sediments into statistically robust grain-size end members can provide multiple independent and quantitative proxies which help constrain reconstructions of a region’s multi-faceted climate history

Holocene Event Record of Aysen Fjord (Chilean Patagonia): An Interplay of Volcanic Eruptions and Crustal and Megathrust Earthquakes

In the first months of 2007, the Aysen region in southern Chile was affected by a crustal seismic swarm. Its largest earthquake (M-w 6.2) occurred in April and had its epicenter in Aysen Fjord. Seismic intensities became so high that hundreds of onshore mass movements were triggered, several of which entered into the fjord, resulting in mass transport deposits (MTDs) preserved at the fjord bottom. Here we present a Holocene record of paleo-earthquakes in the previously unstudied Patagonian fjordland based on MTD stratigraphy. High-resolution seismic data retrieved using two different seismic systems (sparker and TOPAS) reveal multiple older MTDs on different stratigraphic levels. Correlation of the seismic stratigraphy with sedimentological data obtained from a long Calypso core (MD07-3117) allows conclusion on the seismic origin of these deposits. Additionally, radiocarbon dating permits constructing an age model, validated by tephrochronology, providing an age for the different MTD levels. We thus present a highly detailed paleoseismological history of the Aysen region, including at least six major Holocene earthquakes, one of which is likely related to a known megathrust earthquake. Other earthquakes are related to activity of the Liquine-Ofqui Fault Zone (LOFZ), forming the main source of seismic hazard in the area. We can infer a general average recurrence time for LOFZ earthquakes of -2,100years in the vicinity of Aysen Fjord with clustered events during the early and late Holocene. Finally, we argue that cascading events (causal link between volcanic and seismic events) may be a frequent phenomenon along the LOFZ

Seismo‐turbidites in Aysén Fjord (Southern Chile) reveal a complex pattern of rupture modes along the 1960 megathrust earthquake segment

Grainsize analysis and end‐member modeling of a long sediment core from Aysén Fjord (southern Chile) allows to identify over 25 seismo‐turbidites in the last 9,000 years. Considering the shaking intensities required to trigger these turbidites (V½‐VI½), the majority can be related to megathrust earthquakes. Multiple studies in south‐central Chile have aimed at finding traces of giant, tsunamigenic megathrust earthquakes leading to the current 5,500‐year‐long paleoseismological record of the Valdivia segment. However, none of these cover the southern third of the segment. Aysén Fjord allows to fill this data gap and presents the first, crucial paleoseismic data to demonstrate that the 1960 event was not unique for the Valdivia segment, yielding a recurrence rate of 321 ± 116 years in the last two millennia. Moreover, the oldest identified events in Aysén Fjord date back to 9,000 cal years BP and, thus, also extend the regional paleoseismological record in time. We infer a large temporal variability in rupture modes, with successions of full‐segment ruptures alternating with partial and cascading ruptures. The latter seems to significantly postpone the occurrence of another full rupture when consecutively occurring in different parts of the segment. Additionally, one outstanding period of seismic quiescence ¿during which no megathrust earthquake evidence has been found at any paleoseismic site¿occurred after a full rupture in AD ~745 that presents an unusual uplift/subsidence pattern. Such variability makes it highly speculative to anticipate the rupture mode of the next megathrust earthquake along the Valdivia segment

Synchronisation of sedimentary records using tephra : a postglacial tephrochronological model for the Chilean Lake District

Well-characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives. The use of tephra as a chronological tool is however still widely underutilised in southern Chile and Argentina. In this study we develop a postglacial tephrochronological model for the Chilean Lake District (ca. 38 to 42 degrees S) by integrating terrestrial and lacustrine records. Tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout. By combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date. We present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental, palaeoclimatological and palaeoseismological purposes. Most volcanoes in the studied segment of the Southern Volcanic Zone, between Llaima and Calbuco, have produced at least one regional marker deposit resulting from a large explosive eruption (magnitude >= 4), some of which now have a significantly improved age estimate (e.g., the 10.5 ka Llaima Pumice eruption from Llaima volcano). Others, including several units from Puyehue-Cordon Caulle, are newly described here. We also find tephra related to the Cha1 eruption from Chaiten volcano in lake sediments up to 400 km north from source. Several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives. Our chronological model suggests three distinct phases of eruptive activity impacting the area, with an early-to-mid-Holocene period of relative quiescence. Extending our tephrochronological framework further south into Patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial

Late Holocene current patterns in the northern Patagonian fjords recorded by sediment drifts in Aysén Fjord

Present-day circulation patterns in the southeastern Pacific Ocean are driven by the Antarctic Circumpolar Current, directing subantarctic surface water into the Patagonian fjords since at least the early Holocene. In this way, bottom current patterns in the area are regulated by the regional climate, although the complex bathymetry of the fjords has a significant impact as well. To understand the potential interplay of climate, seafloor topography and circulation patterns, we study the sedimentary infill of Aysén Fjord (~45°S) and reveal the first active sediment drifts in the region. These allow constraining the present-day circulation patterns in northern Patagonia and show an incoming (southward) as well as returning (northward) flow direction. While the general sedimentary evolution of the fjord (and thus also the sediment drifts) is climate-driven (i.e., it reflects variability in southern westerly wind strength), the onset of drift formation at ~3.7 ka does not seem to have originated from an abrupt change in regional climate. Instead, we propose that a megathrust earthquake described in paleoseismic records in the area could have resulted in subsidence of one (or more) of the many bathymetric highs in the Patagonian fjords, thus contributing to enhanced spilling of subantarctic water into the fjord. This study underscores the importance of multidisciplinary research to understand past and present bottom current circulation patterns and disentangle different possible feedback mechanisms