The 33 km-long Ragged Mountain fault (RMF) forms the northwestern corner of the Yakutat Terrane, which is colliding with the North American plate in south coastal Alaska at similar to 5.5 cm/yr. The fault zone contains three types of scarps in a zone up to 175 m wide: (1) antislope scarps on the lower range front, (2) a sinuous thrust scarp at the toe of the range front, and (3) a swarm of flexural-slip scarps on the footwall. Trenches across the first two scarp types reveal evidence for two Holocene surface ruptures, plus several late Pleistocene ruptures. In the antislope scarp trench, ruptures occurred at 0.5-3.9 ka; slightly younger than 8.3 ka; and at 18.1-21.8 ka (recurrence intervals 4.4-8 kyr and 9.8-13.3 kyr). Displacements per event ranged from 15 to 40 cm. In the thrust trench ruptures are dated at 2.8-5.9 ka; 5.9-17.2 ka, and 17.2-44.9 ka (mean recurrence intervals 7.2 kyr and 19.5 kyr). Displacements per event ranged from 26 to 77 cm. We interpret the thrust fault as the primary seismogenic structure, and its largest trench displacement (77 cm) equates to the average displacement expected for a 33 km-long reverse rupture. The flexural-slip scarp, in contrast, was rapidly formed ca. 4 ka but its sag pond sediments have continued to slowly fold up to present. The southern third of the fault is dominated by large gravitational failures of the range front (as large as 2.5 km wide, 0.6-0.7 km long, and 200-250 m thick), which head in a linear, 40 m-deep range-crest trough filled with lakes, a classic expression of deep-seated gravitational slope deformation. (C) 2019 Elsevier B.V. All rights reserved.

The production of a cartographic landslide inventory in the glaciated headwaters of the upper Gállego River, Axial Zone of the Spanish Pyrenees, revealed several paleolakes. These relict lake basins are related to the blockage of drainages by postglacial earthflows and slides, mainly developed on Paleozoic slates. The paleolakes are recorded either by non-dissected infilled basins or by hanging and incised lacustrine terraces that persisted for thousands of years. The development of lakes upstream of landslides, was favored, despite their erodibility, by high-displacement rates and the occurrence of paired landslides on both sides of the valley. Radiocarbon and single-grain optically stimulated luminescence (OSL) ages obtained on the lake sediments range between 41.5 ± 3.9 ka and 15.1 ± 0.3 ka. These chronologies indicate prolonged landslide activity long after the early deglaciation in the upper Gállego River Valley. The landslide-lake dating results support the interpretation that the regional maximum ice extent (MIE) in the Pyrenees occurred during the marine isotope stage 4 (MIS 4), long before the global Last Glacial Maximum (LGM).

This contribution presents a geomorphological map of the Rio Grio depression, located in the central sector of the Iberian Chain, NE Spain. In previous geological maps this depression, ca. 30 km long, has been interpreted as an erosional fluvial valley. Geomorphological mapping reveals that it corresponds to a neotectonic graben filled by alluvial fan deposits and subsequently dissected by the axial Grio River and its tributaries. The length of the normal faults indicates that some of the mapped neotectonic structures have the potential to generate earthquakes with moment magnitudes as high as 6.8. This work illustrates that there is limited knowledge on the distribution of recent faults in some sectors of Spain, and that a higher input on the geomorphology and Quaternary geology in the geological maps would help to overcome this weakness.

Several grabens superimposed on the previous contractional structures have been developed in the central sector of the Iberian Chain during the post-orogenic stage. Teruel and Calatayud grabens were formed in a first rifting phase (Miocene-Lower Pliocene). The second extensional phase is recorded by Late Pliocene-Quaternary grabens superimposed and/or inset with respect to the previous basins (Rio Grio, Munebrega, Daroca and Jiloca depressions). This article reviews the existing knowledge on the main Quaternary faults (distribution, seismogenic potential) in the study area and discusses its limitations: (1) Two new grabens (Rio Grio and Munebrega) have been recently documented on the basis of geomorphological mapping, suggesting that our knowledge on the distribution of Quaternary faults in the Iberian Chain is still limited. (2) Available data on cumulative displacement and long-term slip rate is scarce for a number of faults mainly due to the lack of correlatable markers on both sides of the faults and/or lack of numerical dates. (3) Our capability to estimate earthquake recurrence and elapsed time is largely limited due to the scarcity of paleoseismological data. Only the Munebrega W Fault and the Concud-Teruel Fault have been studied. The paleoearthquake record inferred for the former is probably under-represented and the latter fault has received different interpretations. The paper also includes the main results of a trenching investigation carried out in a graben generated by interstratal dissolution of evaporites, which allowed us to propose some criteria that may help to differentiate between tectonic (seismogenic) and gravitational (nonseismogenic) faults.

Multiple sinkhole susceptibility models have been generated in three study areas of the Ebro Valley evaporite karst (NE Spain) applying different methods (nearest neighbour distance, sinkhole density, heuristic scoring system and probabilistic analysis) for each sinkhole type separately (cover collapse sinkholes, cover and bedrock collapse sinkholes and cover and bedrock sagging sinkholes). The quantitative and independent evaluation of the predictive capability of the models reveals that: (1) The most reliable susceptibility models are those derived from the nearest neighbour distance and sinkhole density. These models can be generated in a simple and rapid way from detailed geomorphological maps. (2) The reliability of the nearest neighbour distance and density models is conditioned by the degree of clustering of the sinkholes. Consequently, the karst areas in which sinkholes show a higher clustering are a priori more favourable for predicting new occurrences. (3) The predictive capability of the best models obtained in this research is significantly higher (12.5–82.5%) than that of the heuristic sinkhole susceptibility model incorporated into the General Urban Plan for the municipality of Zaragoza. Although the probabilistic approach provides lower quality results than the methods based on sinkhole proximity and density, it helps to identify the most significant factors and select the most effective mitigation strategies and may be applied to model susceptibility in different future scenarios.

A quantitative method for assessing sinkhole susceptibility and hazard has been developed and independently tested in an evaporite karst of NE Spain. Three genetic types of sinkholes have been mapped: 947 small cover-collapse sinkholes, 23 large collapse sinkholes and 24 large subsidence depressions. By analysing the statistical relationships between the sinkholes and the potential conditioning factors, susceptibility models have been developed. These were evaluated using several strategies that have allowed us to assess quantitatively the quality of the models. The best susceptibility model for the small collapse sinkholes was transformed into a hazard map by considering the frequency and average size of an independent population of new occurrences. The susceptibility and hazard models obtained indicate that it is possible to produce reasonably good predictions on the distribution and frequency of sinkholes in the study area.

An active sinkhole around 100 m long has been investigated in the city of Zaragoza (NE Spain). Subsidence activity on this depression, including the sudden occurrence of a collapse sinkhole 5 m across, led to the abandonment of a factory in the 1990s. At the present time, a building with 100 flats and shallow pad foundations partially built on the sinkhole, is affected by rapid differential settlement. The development of the sinkhole results from the karstification of the halite- and glauberite- bearing bedrock and the sagging and collapse of the overlying bedrock and alluvium, more than 30 m thick. GPR and electrical resistivity profiles have provided information on the distribution and geometry of the subsidence structure. The application of the trenching technique and geochronological methods (AMS and OSL dating) has allowed us to infer objective and practical data on the sinkhole including (1) Limits of the subsidence structure, (2) subsidence mechanisms, (3) cumulative subsidence (> 408 cm), (4) subsidence rates on specific failure planes (> 1.8 cm/year), (5) episodic displacement regime of some fault planes. The available information indicates that the progressive deformation recorded in the building will continue and might be punctuated by events of more rapid displacement. This work illustrates the practicality of the trenching technique for the study of sinkholes in mantled karst areas.

A detailed sinkhole map has been produced in a stretch of the Ebro Valley (40.8 km(2)) including the western sector of Zaragoza city (NE Spain). During the last few decades, around 70% of the original sinkhole area has been filled with anthropogenic sediments causing the disappearance of 137 ha of wetlands. The interstratal karstification of salts (halite and glauberite) and a WNW-ESE-trending joint set have played a major control in the development of sinkholes. Three morphometric types of sinkholes have been differentiated, each attributed to a specific subsidence mechanism inferred from the paleosinkholes exposed in the surrounding of Zaragoza city; sagging of bedrock and cover, collapse of bedrock and cover, and collapse of cover material related to the downward migration of particles through dissolutional conduits. Each type of sinkhole is characterised by a distinctive behaviour in terms of controlling factors, spatio-temporal distribution and kinematics, and consequently the proposed differentiation may have a practical utility. The vast majority of the Subsidence damage identified in the area occurs within the boundaries of pre-existing sinkholes identifiable in old aerial photographs and topographical maps. This fact demonstrates that the application of preventive planning strategies based on detailed geomorphological maps would have allowed avoidance of most of the large financial losses caused by subsidence in the area, of the order of hundreds of thousands of euros per year. (C) 2009 Elsevier B.V. All rights reserved.

A method for quantitatively assessing sinkhole susceptibility (spatial probability) and hazard (spatio-temporal probability) has been developed and independently tested in a 50 km(2) sector of the Ebro Valley evaporite karst. Three genetic types of sinkholes have been mapped in the floodplain and a terrace surface: 947 small cover-collapse sinkholes (type 1, terrace), large collapse sinkholes (type 2, floodplain) and large subsidence depressions (type 3, floodplain). The type 1 sinkhole inventory includes two temporal populations: 447 sinkholes formed before 24 November 2005, and 500 between that date and 2 November 2006. Sinkhole susceptibility models have been elaborated analysing the statistical relationships between the sinkholes of the 2005 inventory and a set of potential conditioning factors. The independent evaluation (validation) of the susceptibility models by means of several strategies (random, sequentially excluded, and temporal) has allowed us to select the most significant variables for each sinkhole type and assess quantitatively the quality of models; which are reasonable for the three sinkhole types. Validation has also provided information on the contribution of specific variables and the effect of changing their accuracy to the prediction capability of models. Susceptibility models for type 3 sinkholes have been validated satisfactorily with the 2006 sinkhole inventory (temporal validation). The best susceptibility model has been transformed into a hazard map considering the frequency of sinkholes that occurred in each Susceptibility class between 2005 and 2006, as well as their average size. The susceptibility and hazard models obtained could be used as an objective basis for the application of mitigation measures, either of preventive or corrective nature. Copyright (C) 2008 John Wiley & Sons, Ltd.

A preliminary sinkhole susceptibility analysis has been carried out in a stretch 50 km(2) in area of the Ebro valley alluvial evaporite karst (NE Spain). A spatial database consisting of a sinkhole layer and 27 thematic layers related to causal factors was constructed and implemented in a GIS. Three types of sinkholes were differentiated on the basis of their markedly different morphometry and geomorphic distribution: large subsidence depressions (24), large collapse sinkholes (23), and small cover-collapse sinkholes (447). The susceptibility models were produced analysing the statistical relationships between the mapped sinkholes and a set of conditioning factors using the Favourability Functions approach. The statistical analyses indicate that the best models are obtained with 6 conditioning factors out of the 27 available ones and that different factors and processes are involved in the generation of each type of sinkhole. The validation of two models by means of a random-split strategy shows that reasonably good predictions on the spatial distribution of future dolines may be produced with this approach; around 75% of the sinkholes of the validation sample occur on the 10% of the pixels with the highest susceptibility and about 45% of the area can be considered as safe. (C) 2008 Elsevier B.V. All rights reserved.

Three types of sinkhole have been mapped in a 50 km2 stretch of the Ebro River valley downstream of Zaragoza: large collapse sinkholes, large shallow subsidence depressions and small cover‐collapse sinkholes. The sinkholes relate to the karstification of evaporitic bedrock that wedges out abruptly downstream, giving way to a shale substratum. Twenty‐three collapse sinkholes, up to 50 m in diameter by 6 m deep, and commonly hosting saline ponds, have been identified in the floodplain. They have been attributed to the upward stoping of dissolutional cavities formed within the evaporitic bedrock by rising groundwater flows. Twenty‐four large shallow subsidence depressions were mapped in the floodplain. These may reach 850 m in length and were formed by structurally controlled interstratal karstification of soluble beds (halite or glauberite? and gypsum) by rising groundwater flow and the progressive settlement of the overlying bedrock and overburden sediments. A total of 447 small cover‐collapse, or dropout, sinkholes have been recognized in a perched alluvial level along the southern margin of the valley. These sinkholes result from the upward propagation of voids through the alluvial mantle caused by the downward migration of detrital sediments into dissolutional voids. The majority of these sinkholes, commonly 1·5–2 m in diameter, are induced by human activities. Over the karstic bedrock, there is a significant increase in sinkhole density downstream. This is interpreted as being a result of the evaporitic bedrock wedging out and the convergence of the groundwater flow lines in the karstic aquifer. The collapse sinkholes in this area, locally with a probability of occurrence higher than 140 sinkholes/km2/year, cause substantial damage to the linear infrastructures, buildings and agriculture, and they might eventually cause the loss of human lives. Copyright © 2006 John Wiley & Sons, Ltd.

The halite-bearing Barbastro Formation crops out in the core of the Barbastro Anticline (Ebro Tertiary Basin). This anticline is traversed perpendicularly by some of the most important Pyrenean drainages such as the Cinca and Noguera-Ribagorzana Rivers. The terrace sequences of these fluvial systems have been used as markers to identify and assess dissolution-induced subsidence and salt tectonics. In the limbs of the anticline, terrace deposits underlain by detrital bedrock do not show any evidence of deformation and have a consistent thickness of less than 10 m. The deposits of certain terrace levels of the Noguera-Ribagorzana River and its tributary, the Lo Reguer Creek, are locally thickened filling basins generated by dissolution-induced synsedimentary subsidence up to several kilometers long and more than 100 m deep. Conversely, terraces of the Cinca River do not show anomalously high thicknesses, but local uplifts related to differential upward flow of the halite-bearing bedrock. Locally, a minimum uplift rate of 0.3 mm/year has been estimated from a 64-ka terrace tilted away from the valley. The subsidence hazards occur chiefly in areas where the ground receives artificial water recharge. Serviceability of some canals has been notoriously affected by evaporite karstification. The problem has been mitigated to acceptable levels by grouting. Numerous buildings of Ivars de Noguera are severely damaged by dissolution subsidence, and possibly, by hydrocompaction of gypsiferous silts. The pipe network has been replaced to ameliorate the subsidence risk. In the Cinca River valley, cavities with a total volume of about 180,500 m 3 have been created by solution mining at depths greater than 500 m. No investigation methods are applied in the brine field to monitor the distribution and evolution of artificial voids. Substantial increase in salinity of the Cinca River is another evidence of subjacent evaporite dissolution.

The Cardona Diapir in NE Spain, with a salt outcrop about 0.9 km2 in area, has a well-developed endokarstic system that used to discharge into the Cardener River. Underground mining for potassium salt carried out from 1930 to 1990 caused significant changes in the topography and hydrology of the diapir. The accumulation of two halite slag heaps, totalling around 10 million tons, locally dammed the surface drainage, creating closed depressions and preferential zones of water recharge. The waters that infiltrated in one of these depressions, largely derived from uncontrolled sewage disposal, led to the generation of a 335-m-long human-induced cave excavated in one of the slag heaps. Moreover, the inflow of freshwater from the surrounding sandstone aquifer, caused by the excavation of a ventilation gallery, resulted in the generation of a 280-m-long cave. In March 1998, the interception of a phreatic conduit by a halite mine gallery 50 m deep caused dramatic changes in the hydrology and geomorphology of the diapir, including: (a) a sudden decline in the piezometric level of the karstic aquifer; (b) the inflow of freshwater and debris from the Cardener River into the endokarstic system and the mine galleries. A tunnel had to be constructed to divert the river flow from the salt outcrop; (c) massive dissolution of salt, creating new cavities and enlarging the pre-existing ones, including both mine galleries or cave passages. The 4,300-m-long Salt Meanders Cave was largely generated by the inrush of water from the Cardener River into the mine galleries; and (d) the generation of a large number of sinkholes in the vicinity of the Cardener River. An inventory of 178 sinkholes has allowed us to estimate minimum probability values of 4.7 and 8 sinkholes/km2·year for time intervals previous and subsequent to the 1998 mine flood event, respectively.

The paper analyses uphill-facing scarps and associated troughs developed in the oversteepened slopes of two neighbouring glacial valleys in the central Spanish Pyrenees. Previous studies of sackung landforms in the Pyrenees have argued for deglacial unloading as the genetic mechanism, but this causal and temporal relationship has not been proved due to the lack of chronological data. The antislope scarps in the two studied locations, Vallibierna and Estós, are developed in Palaeozoic metasedimentary rocks (parallel to the contour lines and the structural grain), occur in the intermediate sector of the hillslope, and are up to 0.5 km long and several meters high. A trench was excavated in a sackung trough fill in each of the valleys in order to gain information about their chronology and genesis. Charcoal from the lowermost unit in Vallibierna provided an age of 5.9 cal. ka for the sackung and extrapolation of the three dates obtained in Estós indicates that the trough formed ca. 7.6–7.8 cal. ka. Deglaciation of the studied sectors of the valleys occurred between 16 and 13 ka. The time lag of >5 ka suggests that glacial erosion and the subsequent debutressing of the oversteepened valley walls created slopes predisposed to sackung development, but did not initiate the movement. Seismic shaking is proposed as a probable triggering factor. This hypothesis, although supported by the sudden deformation event recorded by a failure plane exposed in Vallibierna trench, and by the seismic and neotectonic activity of the area, cannot be proved due to the lack of chronological information about paleoearthquakes.

The present work suggests that preventive planning based on the avoidance of hazardous areas recognizable through detailed geomorphological studies may contribute significantly to reduce the risk in urban areas. Subsidence, floods and slope movements cause significant damage in numerous buildings of Cadrete village, located in the Huerva River valley (NE Spain). This multihazard village is built on a terrace surface placed between the flood-plain and a scarp made up of horizontally lying gypsum and argillaceous Tertiary sediments. The low recurrence floods that affect the floodplain of the Huerva River have caused severe damage in recently built houses and industrial states. Regardless of the application of the projected structural correction measures, the mitigation of the flood risk requires a preventive planning based on the avoidance of the flood-prone areas. A detailed geomorphological map shows that the scarp is affected by a large density of slope movements (translational and rotational slides, rock-falls, topples, lateral spreading). The majority of the movements are restricted to the lower litostratigraphic unit made up of shales and gypsum. The damage in buildings derived from slope movements is always related to the reactivation of old rotational landslides caused by anthropogenic alterations in the slopes (excavation, loading, additional water supply). This fact indicates the inadequateness of the old landslides for the construction of structures in spite of their apparent favourable topography. On the other hand, a large number of buildings are affected by cracking and tilting due to ground subsidence. The most severely damaged areas include the foot of the scarp. The subsidence damage is attributed to the karstification of gypsum and possibly halite in the bedrock and to the hydrocompaction of gypsiferous silts. A reversible hydraulic connection controlled by the precipitation regime between the Tertiary karstic aquifer and the Quaternary alluvium is proposed.

The lowest 17-km long reach of the Huerva River valley, down to its confluence with the Ebro River in Zaragoza city, flows across salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). Upstream, the horizontally lying Miocene evaporites are interfingered with non-soluble distal alluvial fan facies (shales and sandstones). The proportion of soluble facies in the Huerva River valley increases in a downstream direction towards the basin depocenter. On the basis of the type and magnitude of the paleosubsidence features, the valley has been divided into four reaches. Along reach I, undeformed terrace deposits less than 4 m thick rest on insoluble detrital bedrock. In reaches II and III, dissolution at the alluvium–bedrock boundary has generated local thickening, deformation and paleocollapse structures, which only affect the alluvial mantle. In reach IV, terrace deposits thicken to over 60 m resulting from a large-scale synsedimentary subsidence. In this sector, subsidence locally affects to both the alluvium and the underlying bedrock. This indicates that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The development of an intraevaporitic karst in reach IV is attributed to gypsum and salt dissolution. Irregular terrace gravel bodies (gravel pockets) embedded in a fine-grained matrix associated with paleocollapse structures have been interpreted as liquefaction–fluidization structures resulting from ground acceleration and suction induced by catastrophic collapses. Subsidence is currently active in the region affecting areas with a thin alluvial cover in reaches III and IV. The low subsidence activity in most of Zaragoza city is explained by the presence of thickened (around 50 m) and indurated alluvial deposits. In the surrounding area, numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Natural and human-induced subsidence favours the development of slope movements in the gypsum scarp overlooking Cadrete village. Several transport routes including the Imperial Canal (irrigation canal) and the recently completed Madrid–Barcelona high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring road under construction point to hazardous locations underlain by cavities and collapse structures where special protection measures should be applied. Rigid structures are recommended beneath the high-speed railway with sufficient strength to span the larger sinkholes with no deformation. Electronic monitoring devices linked to a warning system can detect subtle subsidence-induced deformations in carriageways or railways. This research demonstrates that the study of the paleokarst helps to understand the processes involved in the present-day subsidence phenomena and their general spatial distribution.

The last 17 km-long reach of the Huerva River valley, up to its confluence with the Ebro River in Zaragoza city, flows through salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). The alluvium overlying evaporite bedrock shows spectacular paleosinkholes and soft sediment deformations interpreted as liquefaction structures resulting from ground acceleration induced by catastrophic subsidence. In the lower most reach of the valley, where the substratum has a higher solubility, terrace deposits thicken over 60 m. Subsidence affects both the alluvium and the underlying bedrock indicating that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The intra-evaporitic karst seems to be due to salt dissolution. Numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Several linear infrastructures like the Imperial Channel and an almost completed high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring-road under construction point to hazardous locations underlain by cavities, where special prevention measures should be applied.

The Barbastro Anticline is an east-west trending salt structure located in the northern margin of the Tertiary Ebro Basin (NE Spain). This anticline is cored by Late Eocene-Early Oligocene evaporites, made up of intensively folded gypsum (in outcrop), halite and anhydrite (in the subsurface). The nine levels of Quaternary aggradation terraces deposited by the Cinca River where it traverses the anticline constitute a datum to infer recent deformations caused by evaporite dissolution subsidence or evaporite flow. Anomalous height of T-4 and T-6 Cinca River terraces in the evaporitic core of the anticline and local tilting of the T-4 terrace are attributed to diapirism.The terrace deposits overlying the soluble Barbastro Gypsum Formation also show abundant gravitational deformations generated by dissolution-induced subsidence during the sedimentation of the alluvium. Nowadays, the subsidence due to evaporite dissolution seems to be restricted to areas where the natural hydrological conditions have been modified by human activity. Evidence of active subsidence includes sinkholes and costly stability problems associated with irrigation canals like the Ariestolas Canal and the Aragon and Cataluna Canal.A salt mine located 500 m away from the Cinca River floodplain extracts around 20,000 tons of NaCl per year by solution mining. The generated voids may propagate toward the surface by stoping to eventually generate catastrophic sinkholes. In addition, NaCl-rich waters derived from the mine enhance evaporite karst processes as revealed by the swallow holes developed between the mining area and the Cinca River floodplain.