TY - THES A1 - Illien, Luc T1 - Time-dependent properties of the shallow subsurface BT - groundwater and earthquake damage dynamics from seismic interferometry N2 - The shallow Earth’s layers are at the interplay of many physical processes: some being driven by atmospheric forcing (precipitation, temperature...) whereas others take their origins at depth, for instance ground shaking due to seismic activity. These forcings cause the subsurface to continuously change its mechanical properties, therefore modulating the strength of the surface geomaterials and hydrological fluxes. Because our societies settle and rely on the layers hosting these time-dependent properties, constraining the hydro-mechanical dynamics of the shallow subsurface is crucial for our future geographical development. One way to investigate the ever-changing physical changes occurring under our feet is through the inference of seismic velocity changes from ambient noise, a technique called seismic interferometry. In this dissertation, I use this method to monitor the evolution of groundwater storage and damage induced by earthquakes. Two research lines are investigated that comprise the key controls of groundwater recharge in steep landscapes and the predictability and duration of the transient physical properties due to earthquake ground shaking. These two types of dynamics modulate each other and influence the velocity changes in ways that are challenging to disentangle. A part of my doctoral research also addresses this interaction. Seismic data from a range of field settings spanning several climatic conditions (wet to arid climate) in various seismic-prone areas are considered. I constrain the obtained seismic velocity time-series using simple physical models, independent dataset, geophysical tools and nonlinear analysis. Additionally, a methodological development is proposed to improve the time-resolution of passive seismic monitoring. N2 - Die oberflächennahen Erdschichten sind ein Zusammenspiel vieler physikalischer Prozesse: Einige werden durch atmosphärische Einflüsse (Niederschlag, Temperatur...) angetrieben, während andere ihren Ursprung im Untergrund haben, beispielsweise Bodenerschütterungen aufgrund seismischer Aktivität. Diese Kräfte führen zu sich ständig ändernden mechanische Eigenschaften des Untergrunds, und damit auch Änderungen der Festigkeit der Geomaterialien der Oberfläche, sowie der Wasserflüsse. Als Gesellschaft leben wir auf der Erdoberfläche, und sind auf diese zeitabhängigen Eigenschaften angewiesen. Für unsere zukünftige, geographische Entwicklung ist es daher essentiell, die hydromechanischen Dynamiken des flachen Untergrunds zu verstehen. Eine Möglichkeit, die sich stetig ändernden physikalischen Eigenschaften unter unseren Füßen zu untersuchen, ist die Ableitung seismischer Geschwindigkeitsänderungen aus dem Umgebungslärm, mit Hilfe einer Technik namens seismische Interferometrie. In dieser Dissertation verwende ich diese Methode, um die Entwicklung der Grundwasserspeicherung, sowie durch Erdbeben verursachte Schäden zu untersuchen. Es werden zwei Forschungslinien untersucht, diese umfassen die Hauptkontrollen der Grundwasserneubildung in steilen Landschaften und die Vorhersagbarkeit und Dauer der transienten physikalischen Eigenschaften aufgrund von Erdbebenerschütterungen. Beide Dynamiken beeinflussen sich gegenseitig, sowie die Geschwindigkeitsänderungen im Untergrund in einer Weise, die schwer zu entschlüsseln ist. Ein Teil dieser Doktorarbeit behandelt daher auch diese Wechselwirkung. Wir verwenden seismische Daten, die unter verschiedenen klimatischen Bedingungen (feuchtes bis trockenes Klima) in mehreren, seismisch gefährdeten Gebieten erhoben wurden. Die gewonnenen seismischen Geschwindigkeitszeitreihen werden mit Hilfe einfacher, physikalischer Modelle, unabhängiger Datensätze, geophysikalischer Instrumente und nichtlinearer Analysen untersucht. Darüber hinaus wird eine methodische Entwicklung vorgeschlagen, um die zeitliche Auflösung des passiven, seismischen Monitorings zu verbessern. KW - seismology KW - seismic noise KW - mountain hydrology KW - earthquake damage KW - geomechanics KW - geophysics KW - Erdbebenschäden KW - Geomechanik KW - Geophysik KW - Gebirgshydrologie KW - seismisches Rauschen KW - Seismologie Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-599367 ER - TY - JOUR A1 - Illien, Luc A1 - Sens-Schönfelder, Christoph A1 - Andermann, Christoff A1 - Marc, Odin A1 - Cook, Kristen L. A1 - Adhikari, Lok Bijaya A1 - Hovius, Niels T1 - Seismic velocity recovery in the subsurface BT - transient damage and groundwater drainage following the 2015 Gorkha Earthquake, Nepal JF - Journal of geophysical research : Solid earth N2 - Shallow earthquakes frequently disturb the hydrological and mechanical state of the subsurface, with consequences for hazard and water management. Transient post-seismic hydrological behavior has been widely reported, suggesting that the recovery of material properties (relaxation) following ground shaking may impact groundwater fluctuations. However, the monitoring of seismic velocity variations associated with earthquake damage and hydrological variations are often done assuming that both effects are independent. In a field site prone to highly variable hydrological conditions, we disentangle the different forcing of the relative seismic velocity variations delta v retrieved from a small dense seismic array in Nepal in the aftermath of the 2015 M-w 7.8 Gorkha earthquake. We successfully model transient damage effects by introducing a universal relaxation function that contains a unique maximum relaxation timescale for the main shock and the aftershocks, independent of the ground shaking levels. Next, we remove the modeled velocity from the raw data and test whether the corresponding residuals agree with a background hydrological behavior we inferred from a previously calibrated groundwater model. The fitting of the delta v data with this model is improved when we introduce transient hydrological properties in the phase immediately following the main shock. This transient behavior, interpreted as an enhanced permeability in the shallow subsurface, lasts for similar to 6 months and is shorter than the damage relaxation (similar to 1 yr). Thus, we demonstrate the capability of seismic interferometry to deconvolve transient hydrological properties after earthquakes from non-linear mechanical recovery. KW - earthquake damage KW - earthquake hydrology KW - relaxation KW - Gorkha earthquake KW - seismic monitoring KW - ambient noise Y1 - 2022 U6 - https://doi.org/10.1029/2021JB023402 SN - 2169-9313 SN - 2169-9356 VL - 127 IS - 2 PB - American Geophysical Union CY - Washington ER - TY - GEN A1 - Dietze, Michael A1 - Krautblatter, Michael A1 - Illien, Luc A1 - Hovius, Niels T1 - Seismic constraints on rock damaging related to a failing mountain peak BT - The Hochvogel, Allgäu T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Large rock slope failures play a pivotal role in long-term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is due, to a large degree, to difficulties associated with instrumenting high mountain terrain and the local nature of classic monitoring methods, which does not allow integral observation of large rock volumes. Here, we analyse data from a small network of up to seven seismic sensors installed during July-October 2018 (with 43 days of data loss) at the summit of the Hochvogel, a 2592 m high Alpine peak. We develop proxy time series indicative of cyclic and progressive changes of the summit. Modal analysis, horizontal-to-vertical spectral ratio data and end-member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 260,000 m(3) large, instable rock volume, due to thermal forcing. Relative seismic wave velocity changes also indicate diurnal accumulation and release of stress within the rock mass. At longer time scales, there is a systematic superimposed pattern of stress increased over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on the development of large-scale slope instabilities towards catastrophic failure. (c) 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1360 KW - environmental seismology KW - fatigue KW - fundamental frequency KW - HVSR KW - mass KW - wasting KW - mountain geomorphology KW - natural hazard KW - noise cross KW - correlation KW - seismic monitoring KW - slope failure Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-568787 SN - 1866-8372 IS - 2 ER - TY - JOUR A1 - Dietze, Michael A1 - Krautblatter, Michael A1 - Illien, Luc A1 - Hovius, Niels T1 - Seismic constraints on rock damaging related to a failing mountain peak BT - The Hochvogel, Allgäu JF - Earth surface processes and landforms N2 - Large rock slope failures play a pivotal role in long-term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is due, to a large degree, to difficulties associated with instrumenting high mountain terrain and the local nature of classic monitoring methods, which does not allow integral observation of large rock volumes. Here, we analyse data from a small network of up to seven seismic sensors installed during July-October 2018 (with 43 days of data loss) at the summit of the Hochvogel, a 2592 m high Alpine peak. We develop proxy time series indicative of cyclic and progressive changes of the summit. Modal analysis, horizontal-to-vertical spectral ratio data and end-member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 260,000 m(3) large, instable rock volume, due to thermal forcing. Relative seismic wave velocity changes also indicate diurnal accumulation and release of stress within the rock mass. At longer time scales, there is a systematic superimposed pattern of stress increased over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on the development of large-scale slope instabilities towards catastrophic failure. (c) 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd KW - environmental seismology KW - fatigue KW - fundamental frequency KW - HVSR KW - mass KW - wasting KW - mountain geomorphology KW - natural hazard KW - noise cross KW - correlation KW - seismic monitoring KW - slope failure Y1 - 2021 U6 - https://doi.org/10.1002/esp.5034 SN - 0197-9337 SN - 1096-9837 VL - 46 IS - 2 SP - 417 EP - 429 PB - Wiley CY - Hoboken ER - TY - GEN A1 - Marc, Odin A1 - Behling, Robert A1 - Andermann, Christoff A1 - Turowski, Jens M. A1 - Illien, Luc A1 - Roessner, Sigrid A1 - Hovius, Niels T1 - Long-term erosion of the Nepal Himalayas by bedrock landsliding BT - the role of monsoons, earthquakes and giant landslides T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - In active mountain belts with steep terrain, bedrock landsliding is a major erosional agent. In the Himalayas, landsliding is driven by annual hydro-meteorological forcing due to the summer monsoon and by rarer, exceptional events, such as earthquakes. Independent methods yield erosion rate estimates that appear to increase with sampling time, suggesting that rare, high-magnitude erosion events dominate the erosional budget. Nevertheless, until now, neither the contribution of monsoon and earthquakes to landslide erosion nor the proportion of erosion due to rare, giant landslides have been quantified in the Himalayas. We address these challenges by combining and analysing earthquake- and monsoon-induced landslide inventories across different timescales. With time series of 5 m satellite images over four main valleys in central Nepal, we comprehensively mapped landslides caused by the monsoon from 2010 to 2018. We found no clear correlation between monsoon properties and landsliding and a similar mean landsliding rate for all valleys, except in 2015, where the valleys affected by the earthquake featured ∼ 5–8 times more landsliding than the pre-earthquake mean rate. The longterm size–frequency distribution of monsoon-induced landsliding (MIL) was derived from these inventories and from an inventory of landslides larger than ∼ 0.1 km 2 that occurred between 1972 and 2014. Using a published landslide inventory for the Gorkha 2015 earthquake, we derive the size–frequency distribution for earthquake-induced landsliding (EQIL). These two distributions are dominated by infrequent, large and giant landslides but under-predict an estimated Holocene frequency of giant landslides (> 1 km 3 ) which we derived from a literature compilation. This discrepancy can be resolved when modelling the effect of a full distribution of earthquakes of variable magnitude and when considering that a shallower earthquake may cause larger landslides. In this case, EQIL and MIL contribute about equally to a total long-term erosion of ∼ 2 ± 0.75 mm yr −1 in agreement with most thermo-chronological data. Independently of the specific total and relative erosion rates, the heavy-tailed size–frequency distribution from MIL and EQIL and the very large maximal landslide size in the Himalayas indicate that mean landslide erosion rates increase with sampling time, as has been observed for independent erosion estimates. Further, we find that the sampling timescale required to adequately capture the frequency of the largest landslides, which is necessary for deriving long-term mean erosion rates, is often much longer than the averaging time of cosmogenic 10 Be methods. This observation presents a strong caveat when interpreting spatial or temporal variability in erosion rates from this method. Thus, in areas where a very large, rare landslide contributes heavily to long-term erosion (as the Himalayas), we recommend 10 Be sample in catchments with source areas > 10 000 km 2 to reduce the method mean bias to below ∼ 20 % of the long-term erosion. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 646 KW - rainfall thresholds KW - global database KW - sediment flux KW - mountain belt KW - rates KW - river KW - size KW - exhumation KW - precipitation KW - inventories Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-425022 SN - 1866-8372 IS - 646 ER - TY - JOUR A1 - Marc, Odin A1 - Behling, Robert A1 - Andermann, Christoff A1 - Turowski, Jens M. A1 - Illien, Luc A1 - Roessner, Sigrid A1 - Hovius, Niels T1 - Long-term erosion of the Nepal Himalayas by bedrock landsliding BT - the role of monsoons, earthquakes and giant landslides JF - Earth surface dynamics N2 - In active mountain belts with steep terrain, bedrock landsliding is a major erosional agent. In the Himalayas, landsliding is driven by annual hydro-meteorological forcing due to the summer monsoon and by rarer, exceptional events, such as earthquakes. Independent methods yield erosion rate estimates that appear to increase with sampling time, suggesting that rare, high-magnitude erosion events dominate the erosional budget. Nevertheless, until now, neither the contribution of monsoon and earthquakes to landslide erosion nor the proportion of erosion due to rare, giant landslides have been quantified in the Himalayas. We address these challenges by combining and analysing earthquake- and monsoon-induced landslide inventories across different timescales. With time series of 5 m satellite images over four main valleys in central Nepal, we comprehensively mapped landslides caused by the monsoon from 2010 to 2018. We found no clear correlation between monsoon properties and landsliding and a similar mean landsliding rate for all valleys, except in 2015, where the valleys affected by the earthquake featured similar to 5-8 times more landsliding than the pre-earthquake mean rate. The longterm size-frequency distribution of monsoon-induced landsliding (MIL) was derived from these inventories and from an inventory of landslides larger than similar to 0.1 km(2) that occurred between 1972 and 2014. Using a published landslide inventory for the Gorkha 2015 earthquake, we derive the size-frequency distribution for earthquakeinduced landsliding (EQIL). These two distributions are dominated by infrequent, large and giant landslides but under-predict an estimated Holocene frequency of giant landslides (> 1 km(3)) which we derived from a literature compilation. This discrepancy can be resolved when modelling the effect of a full distribution of earthquakes of variable magnitude and when considering that a shallower earthquake may cause larger landslides. In this case, EQIL and MIL contribute about equally to a total long-term erosion of similar to 2 +/- 0.75 mm yr(-1) in agreement with most thermo-chronological data. Independently of the specific total and relative erosion rates, the heavy-tailed size-frequency distribution from MIL and EQIL and the very large maximal landslide size in the Himalayas indicate that mean landslide erosion rates increase with sampling time, as has been observed for independent erosion estimates. Further, we find that the sampling timescale required to adequately capture the frequency of the largest landslides, which is necessary for deriving long-term mean erosion rates, is often much longer than the averaging time of cosmogenic Be-10 methods. This observation presents a strong caveat when interpreting spatial or temporal variability in erosion rates from this method. Thus, in areas where a very large, rare landslide contributes heavily to long-term erosion (as the Himalayas), we recommend Be-10 sample in catchments with source areas > 10 000 km(2) to reduce the method mean bias to below similar to 20 % of the long-term erosion. Y1 - 2019 U6 - https://doi.org/10.5194/esurf-7-107-2019 SN - 2196-6311 SN - 2196-632X VL - 7 IS - 1 SP - 107 EP - 128 PB - Copernicus CY - Göttingen ER -