TY - JOUR A1 - Tolorza, Violeta A1 - Mohr, Christian Heinrich A1 - Carretier, Sebastien A1 - Serey, Amador A1 - Sepulveda, Sergio A. A1 - Tapia, Joseline A1 - Pinto, Luisa T1 - Suspended sediments in chilean rivers reveal low postseismic erosion after the maule earthquake (Mw 8.8) during a severe drought JF - Journal of geophysical research : Earth surface N2 - We address the question of whether all large-magnitude earthquakes produce an erosion peak in the subaerial components of fluvial catchments. We evaluate the sediment flux response to the Maule earthquake in the Chilean Andes (Mw 8.8) using daily suspended sediment records from 31 river gauges. The catchments cover drainage areas of 350 to around 10,000 km(2), including a wide range of topographic slopes and vegetation cover of the Andean western flank. We compare the 3- to 8-year postseismic record of sediment flux to each of the following preseismic periods: (1) all preseismic data, (2) a 3-year period prior to the seismic event, and (3) the driest preseismic periods, as drought conditions prevailed in the postseismic period. Following the earthquake, no increases in suspended sediment flux were observed for moderate to high percentiles of the streamflow distribution (mean, median, and >= 75th percentile). However, more than half of the examined stations showed increased sediment flux during baseflow. By using a Random Forest approach, we evaluate the contributions of seismic intensities, peak ground accelerations, co-seismic landslides, hydroclimatic conditions, topography, lithology, and land cover to explain the observed changes in suspended sediment concentration and fluxes. We find that the best predictors are hillslope gradient, low-vegetation cover, and changes in streamflow discharge. This finding suggests a combined first-order control of topography, land cover, and hydrology on the catchment-wide erosion response. We infer a reduced sediment connectivity due to the postseismic drought, which increased the residence time of sediment detached and remobilized following the Maule earthquake. KW - earthquake KW - suspended sediment KW - Maule megathrust KW - Chile KW - catchment Y1 - 2019 U6 - https://doi.org/10.1029/2018JF004766 SN - 2169-9003 SN - 2169-9011 VL - 124 IS - 6 SP - 1378 EP - 1397 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Manga, Michael A1 - Wald, David T1 - Stronger peak ground motion, beyond the threshold to initiate a response, does not lead to larger stream discharge responses to earthquakes JF - Geophysical research letters N2 - The impressive number of stream gauges in Chile, combined with a suite of past and recent large earthquakes, makes Chile a unique natural laboratory to study several streams that recorded responses to multiple seismic events. We document changes in discharge in eight streams in Chile following two or more large earthquakes. In all cases, discharge increases. Changes in discharge occur for peak ground velocities greater than about 7-11cm/s. Above that threshold, the magnitude of both the increase in discharge and the total excess water do not increase with increasing peak ground velocities. While these observations are consistent with previous work in California, they conflict with lab experiments that show that the magnitude of permeability changes increases with increasing amplitude of ground motion. Instead, our study suggests that streamflow responses are binary. Plain Language Summary Earthquakes deform and shake the surface and the ground below. These changes may affect groundwater flows by increasing the permeability along newly formed cracks and/or clearing clogged pores. As a result, groundwater flow may substantially increase after earthquakes and remain elevated for several months. Here we document streamflow anomalies following multiple high magnitude earthquakes in multiple streams in one of the most earthquake prone regions worldwide, Chile. We take advantage of the dense monitoring network in Chile that recorded streamflow since the 1940s. We show that once a critical ground motion is exceeded, streamflow responses to earthquakes can be expected. KW - earthquake KW - streamflow KW - shaking KW - Chile KW - modeling Y1 - 2018 U6 - https://doi.org/10.1029/2018GL078621 SN - 0094-8276 SN - 1944-8007 VL - 45 IS - 13 SP - 6523 EP - 6531 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Montgomery, David R. A1 - Huber, Anton A1 - Bronstert, Axel A1 - Iroume, Andres T1 - Streamflow response in small upland catchments in the Chilean coastal range to the M-W 8.8 Maule earthquake on 27 February 2010 JF - Journal of geophysical research : Earth surface N2 - Hydrological response to earthquakes has long been observed, yet the mechanisms responsible still remain unclear and likely vary in space and time. This study explores the base flow response in small upland catchments of the Coastal Range of south-central Chile after the M-W 8.8 Maule earthquake of 27 February 2010. An initial decline in streamflow followed by an increase of up to 400% of the discharge measured immediately before the earthquake occurred, and diurnal streamflow oscillations intensified after the earthquake. Neither response time, nor time to maximum streamflow discharge showed any relationship with catchment topography or size, suggesting non-uniform release of water across the catchments. The fast response, unaffected stream water temperatures and a simple diffusion model point to the sandy saprolite as the source of the excess water. Base flow recession analysis reveals no evidence for substantial enhancement of lateral hydraulic conductivity in the saprolite after the earthquake. Seismic energy density reached similar to 170 J/m(3) for the main shock and similar to 0.9 J/m(3) for the aftershock, exceeding the threshold for liquefaction by undrained consolidation only during the main shock. Although increased hydraulic gradient due to ground acceleration-triggered, undrained consolidation is consistent with empirical magnitude-distance relationships for liquefaction, the lack of independent evidence for liquefaction means that enhanced vertical permeability (probably in combination with co-seismic near-surface dilatancy) cannot be excluded as a potential mechanism. Undrained consolidation may have released additional water from the saturated saprolite into the overlying soil, temporarily reducing water transfer to the creeks but enlarging the cross-section of the saturated zone, which in turn enhanced streamflow after establishment of a new hydraulic equilibrium. The enlarged saturated zone facilitated water uptake by roots and intensified evapotranspiration. Y1 - 2012 U6 - https://doi.org/10.1029/2011JF002138 SN - 0148-0227 VL - 117 IS - 23 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Ulloa, H. A1 - Iroume, A. A1 - Picco, L. A1 - Mohr, Christian Heinrich A1 - Mazzorana, B. A1 - Lenzi, Mario Aristide A1 - Mao, L. T1 - Spatial analysis of the impacts of the Chaiten volcano eruption (Chile) in three fluvial systems JF - Journal of South American earth sciences N2 - The eruption of the Chaiten volcano in May 2008 generated morphological and ecological disturbances in adjacent river basins, and the magnitude of these disturbances depended on the type of dominant volcanic process affecting each of them. The aim of this study is to analyse the morphological changes in different periods in river segments of the Blanco, El Amarillo and Rayas river basins located near the Chaiten volcano. These basins suffered disturbances of different intensity and spatial distribution caused by tephra fall, dome collapses and pyroclastic density currents that damaged hillslope forests, widened channels and destroyed island and floodplain vegetation. Changes continued to occur in the fluvial systems in the years following the eruption, as a consequence of the geomorphic processes indirectly induced by the eruption. Channel changes were analyzed by comparing remote images of pre and post eruption conditions. Two periods were considered: the first from 2008 to 2009-2010 associated with the explosive and effusive phases of the eruption and the second that correspond to the post-eruption stage from 2009-2010 to 2013. Following the first phases channel segments widened 91% (38 m/yr), 6% (7 m/yr) and 7% (22 m/yr) for Blanco, Rayas and El Amarillo Rivers, respectively, compared to pre-eruption condition. In the second period, channel segments additionally widened 42% (8 m/yr), 2% (2 m/yr) and 5% (4 m/yr) for Blanco, Rayas and El Amarillo Rivers, respectively. In the Blanco River 62 and 82% of the islands disappeared in the first and second period, respectively, which is 6-8 times higher than in the El Amarillo approximately twice the Rayas. Sinuosity increased after the eruption only in the Blanco River but the three study channels showed a high braiding intensity mainly during the first post-eruption period. The major disturbances occurred during the eruptive and effusive phases of Chaiten volcano, and the intensity of these disturbances reflects the magnitude of the dominant volcanic processes affecting each basin. Inputs of sediment from dome collapses and pyroclastic density currents and not ash fall seem to explain morphologic channel change magnitudes in the study segments. The resulting knowledge can facilitate land use planning and design of river restoration projects in areas affected by volcanic eruptions disturbances. (C) 2016 Elsevier Ltd. All rights reserved. KW - Volcanic disturbances KW - Changes in fluvial systems KW - Island KW - Riparian vegetation KW - Chaiten volcano KW - Chile Y1 - 2016 U6 - https://doi.org/10.1016/j.jsames.2016.04.008 SN - 0895-9811 VL - 69 SP - 213 EP - 225 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Manga, Michael A1 - Wang, Chi-yuen A1 - Kirchner, James W. A1 - Bronstert, Axel T1 - Shaking water out of soil JF - Geology N2 - Moderate to large earthquakes can increase the amount of water flowing in streams. Previous interpretations and models assume that the extra water originates in the saturated zone. Here we show that earthquakes may also release water from the unsaturated zone when the seismic energy is sufficient to overcome the threshold of soil water retention. Soil water may then be released into aquifers, increasing streamflow. After the M8.8 Maule, Chile, earthquake, the discharge in some headwater catchments of the Chilean coastal range increased, and the amount of extra water in the discharge was similar to the total amount of water available for release from the unsaturated zone. Assuming rapid recharge of this water to the water table, a groundwater flow model that accounts for evapotranspiration and water released from soils can reproduce the increase in discharge as well as the enhanced diurnal discharge variations observed after the earthquake. Thus the unsaturated zone may play a previously unappreciated, and potentially significant, role in shallow hydrological responses to earthquakes. Y1 - 2015 U6 - https://doi.org/10.1130/G36261.1 SN - 0091-7613 SN - 1943-2682 VL - 43 IS - 3 SP - 207 EP - 210 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Zimmermann, Andreas A1 - Korup, Oliver A1 - Iroume, A. A1 - Francke, Till A1 - Bronstert, Axel T1 - Seasonal logging, process response, and geomorphic work JF - Earth surface dynamics N2 - Deforestation is a prominent anthropogenic cause of erosive overland flow and slope instability, boosting rates of soil erosion and concomitant sediment flux. Conventional methods of gauging or estimating post-logging sediment flux often focus on annual timescales but overlook potentially important process response on shorter intervals immediately following timber harvest. We resolve such dynamics with non-parametric quantile regression forests (QRF) based on high-frequency (3 min) discharge measurements and sediment concentration data sampled every 30-60 min in similar-sized (similar to 0.1 km(2)) forested Chilean catchments that were logged during either the rainy or the dry season. The method of QRF builds on the random forest algorithm, and combines quantile regression with repeated random sub-sampling of both cases and predictors. The algorithm belongs to the family of decision-tree classifiers, which allow quantifying relevant predictors in high-dimensional parameter space. We find that, where no logging occurred, similar to 80% of the total sediment load was transported during extremely variable runoff events during only 5% of the monitoring period. In particular, dry-season logging dampened the relative role of these rare, extreme sediment-transport events by increasing load efficiency during more efficient moderate events. We show that QRFs outperform traditional sediment rating curves (SRCs) in terms of accurately simulating short-term dynamics of sediment flux, and conclude that QRF may reliably support forest management recommendations by providing robust simulations of post-logging response of water and sediment fluxes at high temporal resolution. Y1 - 2014 U6 - https://doi.org/10.5194/esurf-2-117-2014 SN - 2196-6311 SN - 2196-632X VL - 2 IS - 1 SP - 117 EP - 125 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Coppus, Ruben A1 - Iroume, Andres A1 - Huber, Anton A1 - Bronstert, Axel T1 - Runoff generation and soil erosion processes after clear cutting JF - Journal of geophysical research : Earth surface N2 - Timber harvesting by clear cutting is known to impose environmental impacts, including severe disturbance of the soil hydraulic properties which intensify the frequency and magnitude of surface runoff and soil erosion. However, it remains unanswered if harvest areas act as sources or sinks for runoff and soil erosion and whether such behavior operates in a steady state or evolves through time. For this purpose, 92 small-scale rainfall simulations of different intensities were carried out under pine plantation conditions and on two clear-cut harvest areas of different age. Nonparametrical Random Forest statistical models were set up to quantify the impact of environmental variables on the hydrological and erosion response. Regardless of the applied rainfall intensity, runoff always initiated first and yielded most under plantation cover. Counter to expectations, infiltration rates increased after logging activities. Once a threshold rainfall intensity of 20mm/h was exceeded, the younger harvest area started to act as a source for both runoff and erosion after connectivity was established, whereas it remained a sink under lower applied rainfall intensities. The results suggest that the impact of microtopography on surface runoff connectivity and water-repellent properties of the topsoil act as first-order controls for the hydrological and erosion processes in such environments. Fast rainfall-runoff response, sediment-discharge-hystereses, and enhanced postlogging groundwater recharge at catchment scale support our interpretation. At the end, we show the need to account for nonstationary hydrological and erosional behavior of harvest areas, a fact previously unappreciated in predictive models. KW - infiltration KW - runoff KW - erosion KW - connectivity KW - rainfall simulation KW - catchment Y1 - 2013 U6 - https://doi.org/10.1002/jgrf.20047 SN - 2169-9003 VL - 118 IS - 2 SP - 814 EP - 831 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Manga, Michael A1 - Wang, Chi-Yuen A1 - Korup, Oliver T1 - Regional changes in streamflow after a megathrust earthquake JF - Earth & planetary science letters N2 - Moderate to large earthquakes can increase the amount of water feeding stream flows, mobilizing excess water from deep groundwater, shallow groundwater, or the vadose zone. Here we examine the regional pattern of streamflow response to the Maule M8.8 earthquake across Chile's diverse topographic and hydro-climatic gradients. We combine streamflow analyses with groundwater flow modeling and a random forest classifier, and find that, after the earthquake, at least 85 streams had a change in flow. Discharge mostly increased () shortly after the earthquake, liberating an excess water volume of >1.1 km3, which is the largest ever reported following an earthquake. Several catchments had increased discharge of >50 mm, locally exceeding seasonal streamflow discharge under undisturbed conditions. Our modeling results favor enhanced vertical permeability induced by dynamic strain as the most probable process explaining the observed changes at the regional scale. Supporting this interpretation, our random forest classification identifies peak ground velocity and elevation extremes as most important for predicting streamflow response. Given the mean recurrence interval of ∼25 yr for >M8.0 earthquakes along the Peru–Chile Trench, our observations highlight the role of earthquakes in the regional water cycle, especially in arid environments. KW - Maule earthquake KW - streamflow response KW - permeability KW - groundwater flow modeling KW - earthquake hydrology Y1 - 2016 U6 - https://doi.org/10.1016/j.epsl.2016.11.013 SN - 0012-821X SN - 1385-013X VL - 458 SP - 418 EP - 428 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Mohr, Christian Heinrich A1 - Korup, Oliver A1 - Ulloa, Hector A1 - Iroume, Andres T1 - Pyroclastic Eruption Boosts Organic Carbon Fluxes Into Patagonian Fjords JF - Global biogeochemical cycles N2 - Fjords and old-growth forests store large amounts of organic carbon. Yet the role of episodic disturbances, particularly volcanic eruptions, in mobilizing organic carbon in fjord landscapes covered by temperate rainforests remains poorly quantified. To this end, we estimated how much wood and soils were flushed to nearby fjords following the 2008 eruption of Chaiten volcano in south-central Chile, where pyroclastic sediments covered >12km(2) of pristine temperate rainforest. Field-based surveys of forest biomass, soil organic content, and dead wood transport reveal that the reworking of pyroclastic sediments delivered similar to 66,500+14,600/-14,500tC of large wood to two rivers entering the nearby Patagonian fjords in less than a decade. A similar volume of wood remains in dead tree stands and buried beneath pyroclastic deposits (similar to 79,900+21,100/-16,900tC) or stored in active river channels (5,900-10,600tC). We estimate that bank erosion mobilized similar to 132,300(+21,700)/(-30,600)tC of floodplain forest soil. Eroded and reworked forest soils have been accreting on coastal river deltas at >5mmyr(-1) since the eruption. While much of the large wood is transported out of the fjord by long-shore drift, the finer fraction from eroded forest soils is likely to be buried in the fjords. We conclude that the organic carbon fluxes boosted by rivers adjusting to high pyroclastic sediment loads may remain elevated for up to a decade and that Patagonian temperate rainforests disturbed by excessive loads of pyroclastic debris can be episodic short-lived carbon sources. Plain Language Summary Fjords and old-growth forests are important sinks of organic carbon. However, the role of volcanic eruptions in flushing organic carbon in fjord landscapes remains unexplored. Here we estimated how much forest vegetation and soils were lost to fjords following the 2008 eruption ofunknownChaiten volcano in south-central Chile. Pyroclastic sediments obliterated near-pristine temperateunknownrainforest, and the subsequent reworking of these sediments delivered in less than a decade similar to 66,000 tC of large wood to the mountain rivers, draining into the nearby Patagonian fjords. A similar volume of wood remains in dead tree stands and buried beneath pyroclastic deposits or stored in active riverunknownchannels. We estimate that similar to 130,000 tC of organic carbon-rich soil was lost to erosion, thus adding to the carbon loads. While much of the wood enters the long-shore drift in the fjord heads, the finerunknownfraction from eroded forest soils is likely to be buried in the fjords at rates that exceed regional estimates by an order of magnitude. We anticipate that these eruption-driven fluxes will remain elevated forunknownthe coming years and that Patagonian temperate rainforests episodically switch from carbon sinks to hitherto undocumented carbon sources if disturbed by explosive volcanic eruptions. KW - Chile KW - Patagonia KW - rainforest KW - volcanic eruption KW - organic carbon KW - biomass Y1 - 2017 U6 - https://doi.org/10.1002/2017GB005647 SN - 0886-6236 SN - 1944-9224 VL - 31 SP - 1626 EP - 1638 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Parra Hormazábal, Eric A1 - Mohr, Christian Heinrich A1 - Korup, Oliver T1 - Predicting Patagonian landslides BT - roles of forest cover and wind speed JF - Geophysical research letters : GRL / American Geophysical Union N2 - Dense tree stands and high wind speeds characterize the temperate rainforests of southern Chilean Patagonia, where landslides frequently strip hillslopes of soils, rock, and biomass. Assuming that wind loads on trees promote slope instability, we explore the role of forest cover and wind speed in predicting landslides with a hierarchical Bayesian logistic regression. We find that higher crown openness and wind speeds credibly predict higher probabilities of detecting landslides regardless of topographic location, though much better in low-order channels and on midslope locations than on open slopes. Wind speed has less predictive power in areas that were impacted by tephra fall from recent volcanic eruptions, while the influence of forest cover in terms of crown openness remains.
Plain Language Summary Chilean Patagonia hosts some of Earth's largest swaths of temperate rainforests, where frequent landslides erode soil, rock, and vegetation. We explore the role of forest cover and wind disturbances in promoting such landslides with a model that predicts from crown openness and wind speed the probability of detecting landslide terrain. We find that both forest cover and wind speed play important, yet previously underappreciated, roles in this context, especially when grouped by landform types and previous volcanic disturbance, which may override the comparable modest control of wind on landslides. Our study is the first of its kind in one of the windiest spots on Earth and encourages a more discerning approach to landslide prediction. Y1 - 2021 U6 - https://doi.org/10.1029/2021GL095224 SN - 0094-8276 SN - 1944-8007 VL - 48 IS - 23 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Korup, Oliver A1 - Seidemann, Jan A1 - Mohr, Christian Heinrich T1 - Increased landslide activity on forested hillslopes following two recent volcanic eruptions in Chile JF - Nature geoscience N2 - Large explosive eruptions can bury landscapes beneath thick layers of tephra. Rivers subsequently overloaded with excess pyroclastic sediments have some of the highest reported specific sediment yields. Much less is known about how hillslopes respond to tephra loads. Here, we report a pulsed and distinctly delayed increase in landslide activity following the eruptions of the Chaiten (2008) and Puyehue-Cordon Caulle (2011) volcanoes in southern Chile. Remote-sensing data reveal that land-slides clustered in densely forested hillslopes mostly two to six years after being covered by tephra. This lagged instability is consistent with a gradual loss of shear strength of decaying tree roots in areas of high tephra loads. Surrounding areas with comparable topography, forest cover, rainfall and lithology maintained landslide rates roughly ten times lower. The landslides eroded the landscape by up to 4.8 mm on average within 30 km of both volcanoes, mobilizing up to 1.6 MtC at rates of about 265 tC km(-2) yr(-1). We suggest that these yields may reinforce the elevated river loads of sediment and organic carbon in the decade after the eruptions. We recommend that studies of post-eruptive mass fluxes and hazards include lagged landslide responses of tephra-covered forested hillslopes, to avoid substantial underestimates. Y1 - 2019 U6 - https://doi.org/10.1038/s41561-019-0315-9 SN - 1752-0894 SN - 1752-0908 VL - 12 IS - 4 SP - 284 EP - 289 PB - Nature Publ. Group CY - New York ER - TY - JOUR A1 - Korup, Oliver A1 - Mohr, Christian Heinrich A1 - Manga, Michael M. T1 - Bayesian detection of streamflow response to earthquakes JF - Water resources research : an AGU journal N2 - Detecting whether and how river discharge responds to strong earthquake shaking can be time-consuming and prone to operator bias when checking hydrographs from hundreds of gauging stations. We use Bayesian piecewise regression models to show that up to a fifth of all gauging stations across Chile had their largest change in daily streamflow trend on the day of the M-w 8.8 Maule earthquake in 2010. These stations cluster distinctly in the near field though the number of detected streamflow changes varies with model complexity and length of time window considered. Credible seismic streamflow changes at several stations were the highest detectable in eight months, with an increased variance of discharge surpassing the variance of discharge following rainstorms. We conclude that Bayesian piecewise regression sheds new and unbiased insights on the duration, trend, and variance of streamflow response to strong earthquakes, and on how this response compares to that following rainstorms. KW - Bayesian analysis KW - Chile KW - discharge KW - earthquake KW - streamflow changes Y1 - 2021 U6 - https://doi.org/10.1029/2020WR028874 SN - 0043-1397 SN - 1944-7973 VL - 57 IS - 7 PB - Wiley CY - Hoboken, NJ ER -