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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.
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. <br /> 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.
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.
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.
Increased landslide activity on forested hillslopes following two recent volcanic eruptions in Chile
(2019)
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.
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.
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.
This special issue is the result of several fruitful conference sessions on disturbance hydrology, which started at the 2013 AGU Fall Meeting in San Francisco and have continued every year since. The stimulating presentations and discussions surrounding those sessions have focused on understanding both the disruption of hydrologic functioning following discrete disturbances, as well as the subsequent recovery or change within the affected watershed system. Whereas some hydrologic disturbances are directly linked to anthropogenic activities, such as resource extraction, the contributions to this special issue focus primarily on those with indirect or less pronounced human involvement, such as bark-beetle infestation, wildfire, and other natural hazards. However, human activities are enhancing the severity and frequency of these seemingly natural disturbances, thereby contributing to acute hydrologic problems and hazards. Major research challenges for our increasingly disturbed planet include the lack of continuous pre and postdisturbance monitoring, hydrologic impacts that vary spatially and temporally based on environmental and hydroclimatic conditions, and the preponderance of overlapping or compounding disturbance sequences. In addition, a conceptual framework for characterizing commonalities and differences among hydrologic disturbances is still in its infancy. In this introduction to the special issue, we advance the fusion of concepts and terminology from ecology and hydrology to begin filling this gap. We briefly explore some preliminary approaches for comparing different disturbances and their hydrologic impacts, which provides a starting point for further dialogue and research progress.
Spatial analysis of the impacts of the Chaiten volcano eruption (Chile) in three fluvial systems
(2016)
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.
Shaking water out of soil
(2015)
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.