University of Potsdam Cumulative Dissertation LEARNING MORE TO PREDICT LANDSLIDES Author: Supervisor: Ugur Öztürk Prof. Oliver Korup, Ph.D. Prof. Dr. Jürgen Kurths Publication-based dissertation submitted in fulfilment of the requirements for the degree of Doctor of Engineering (Doktor der Ingenieurwissenschaften, Dr.-Ing.) under the discipline of Geohazards Institute of Earth and Environmental Sciences Faculty of Science March 21, 2019 Published online at the Institutional Repository of the University of Potsdam: https://doi.org/10.25932/publishup-42643 https://nbn-resolving.org/urn:nbn:de:kobv:517-opus4-426439 iii Learning More to Predict Landslides by Ugur Öztürk Supervisor: Affiliation: Prof. Oliver Korup, Ph.D. University of Potsdam Prof. Dr. Dr. h.c. mult. Jürgen Kurths (Reviewer) Humboldt-Universität zu Berlin Potsdam Institute for Climate Impact Research (PIK) University of Potsdam Co-Supervisor: Dr. rer. nat. Norbert Marwan Potsdam Institute for Climate Impact Research (PIK) Assoc. Prof. Hitoshi Saito, Ph.D. Kanto Gakuin University Mentor: Prof. Dr.-Ing Jürgen Jensen University of Siegen Assessment Committee: Prof. Dr. Fabrice Cotton (Chair) German Research Centre for Geosciences (GFZ) University of Potsdam Prof. Dr.-Ing Axel Bronstert (Reviewer) University of Potsdam Prof. Eva Nora Paton, Ph.D. (Reviewer) Technische Universität Berlin Prof. Dr.-Ing Bruno Merz German Research Centre for Geosciences (GFZ) University of Potsdam Publication-based dissertation submitted in fulfilment of the requirements for the degree of Doc- tor of Engineering (Doktor der Ingenieurwissenschaften, Dr.-Ing.) within the DFG - Research Training Group "NatRiskChange" under the discipline of Geohazards at the Institute of Earth and Environmental Sciences, Faculty of Science, University of Potsdam. v Declaration of Authorship I, Ugur Öztürk, declare that this thesis titled, “Learning More to Predict Landslides” and the work presented in it are my own. I confirm that: • This work was done wholly or mainly while in candidature for a research degree at the University of Potsdam. • Where any part of this dissertation has previously been submitted for a degree or any other quali- fication at the University of Potsdam, or any other institution, this has been clearly stated. • Where I have consulted the published work of others, this is always clearly attributed. • Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. • I have acknowledged all main sources of help. • Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself. Signature: Date: vii “Raise your words, not voice. It is rain that grows flowers, not thunder.” Rumi ix Abstract Landslides are frequent natural hazards in rugged terrain, when the resisting frictional force of the surface of rupture yields to the gravitational force. These forces are functions of geological and morphological factors, such as angle of internal friction, local slope gradient or curvature, which remain static over hundreds of years; whereas more dynamic triggering events, such as rainfall and earthquakes, compro- mise the force balance by temporarily reducing resisting forces or adding transient loads. This thesis investigates landslide distribution and orientation due to landslide triggers (e.g. rainfall) at different scales (6–4·105 km2); and aims to link rainfall movement with the landslide distribution. It additionally explores the local impacts of the extreme rainstorms on landsliding and the role of precursory stability conditions that could be induced by an earlier trigger, such as an earthquake. Extreme rainfall is a common landslide trigger. Although several studies assessed rainfall intensity and duration to study the distribution of thus triggered landslides, only a few case studies quantified spatial rainfall patterns (i.e. orographic effect). Quantifying the regional trajectories of extreme rainfall could aid predicting landslide prone regions in Japan. To this end, I combined a non-linear correlation metric, namely event synchronization, and radial statistics to assess the general pattern of extreme rainfall tracks over distances of hundreds of kilometers using satellite based rainfall estimates. Results showed that, although the increase in rainfall intensity and duration positively correlates with landslide occurrence, the trajectories of typhoons and frontal storms were insufficient to explain landslide distribution in Japan. Extreme rainfall trajectories inclined northwestwards and were concentrated along some certain locations, such as coastlines of southern Japan, which was unnoticed in the landslide distribution of about 5000 rainfall-triggered landslides. These landslides seemed to respond to the mean annual rainfall rates. Above mentioned findings suggest further investigation on a more local scale to better understand the mechanistic response of landscape to extreme rainfall in terms of landslides. On May 2016 intense rainfall struck southern Germany triggering high waters and landslides. The highest damage was reported at the Braunsbach, which is located on the tributary-mouth fan formed by the Orlacher Bach. Orlacher Bach is a ∼3 km long creek that drains a catchment of about ∼6 km2. I visited this catchment in June 2016 and mapped 48 landslides along the creek. Such high landslide activity was not reported in the nearby catchments within ∼3300 km2, despite similar rainfall intensity and duration based on weather radar estimates. My hypothesis was that several landslides were triggered by rainfall-triggered flash floods that undercut hillslope toes along the Orlacher Bach. I found that morphometric features such as slope and curvature play an important role in landslide distribution on this micro scale study site (<10 km2). In addition, the high number of landslides along the Orlacher Bach could also be boosted by accumulated damages on hillslopes due karst weathering over longer time scales. Precursory damages on hillslopes could also be induced by past triggering events that effect landscape evolution, but this interaction is hard to assess independently from the latest trigger. For example, an earthquake might influence the evolution of a landscape decades long, besides its direct impacts, such as landslides that follow the earthquake. Here I studied the consequences of the 2016 Kumamoto 2 Earthquake (MW 7.1) that triggered some 1500 landslides in an area of ∼4000 km in central Kyushu, Japan. Topography, i.e. local slope and curvature, both amplified and attenuated seismic waves, thus controlling the failure mechanism of those landslides (e.g. progressive). I found that topography fails in explaining the distribution and the preferred orientation of the landslides after the earthquake; instead the landslides were concentrated around the northeast of the rupture area and faced mostly normal to the rupture plane. This preferred location of the landslides was dominated mainly by the directivity effect of the strike-slip earthquake, which is the propagation of wave energy along the fault in the rupture direction; whereas amplitude variations of the seismic radiation altered the preferred orientation. I suspect that the earthquake directivity and the asymmetry of seismic radiation damaged hillslopes at those preferred locations increasing landslide susceptibility. Hence a future weak triggering event, e.g. scattered rainfall, could further trigger landslides at those damaged hillslopes. x xi Zusammenfassung Hangrutschungen treten häufig in steilem Gelände auf, wenn die Erdanziehungskraft die Scherkräfte an der Oberfläche übersteigt. Diese Kräfte beinhalten geologische und geomorphologische Faktoren wie den Reibungswinkel oder die Neigung und Krümmung von Hängen, die über Jahrhunderte statisch bleiben können. Dynamische Ereignisse wie Regenfälle und Erdbeben können hingegen das Kräftegleichgewicht beeinträchtigen, indem sie Widerstandskräfte vorübergehend reduzieren oder Lasten temporär hinzufü- gen. Diese Arbeit untersucht die Verteilung und Orientierung von Hangrutschungen in Abhängigkeit von Auslösern (z.B. Niederschlag) auf verschiedenen Skalenebenen (6–4·105 km2) und verknüpft die Bewe- gung des Niederschlagssystems mit der Hangrutschungsverteilung. Zudem werden lokale Auswirkungen von extremen Gewittern auf Hangrutschungen untersucht, sowie vorausgehende Stabilitätsbedingungen, die durch ein früheres Ereignis, beispielsweise ein Erdbeben, verändert werden können. Extremer Niederschlag ist ein weithin bekannter Auslöser für Hangrutschungen. Obwohl mehrere Stu- dien die Verteilung von Hangrutschungen mit der Niederschlagsintensität und -dauer verglichen haben, beachteten nur wenige Fallstudien das räumliche Bewegungsmuster des Niederschlags, i.e. den orographis- chen Effekt. Eine solche Quantifizierung könnte die Vorhersage von hangrutschungsgefährdete Regionen in Japan verbessern. Hierfür habe ich ein nicht-lineares Korrelationsmaß (event synchronization) auf re- gionale Sturmbahnen in Japan angewendet, um deren räumliche Verteilung durch satellitengestützen Re- genschätzungen nachzuvollziehen. Die durchgeführten Untersuchungen zeigten, dass sich die Verteilung von Hangrutschungen nur unzureichend mit den Zugbahnen von Taifunen und Sturmtiefen erklären lässt. Die Stabilität von Hängen scheint mehr durch mittlere Jahresniederschlagsmengen beeinflusst zu werden. Erzielte Ergebnisse zeigen, dass weitere Untersuchungen