Characterisation of a Pleistocene debris-avalanche deposit in the Tenteniguada Basin,Gran Canaria Island,Spain

We studied a large debris-avalanche deposit of Pleistocene age in the Tenteniguada Basin, Gran Canaria Island, Spain. This deposit, which is well preserved because it is mostly covered by basanite lava flows, has distinctive matrix and block facies, hummocky topography and internal structures typical of debris avalanches. However, neither syneruptive lavas nor some characteristic features of volcanic debris-avalanche deposits, such as a stratovolcano edifice or a horseshoe-shaped crater, are present. The occurrence of internal features characteristic of volcanic avalanche deposits could be attributed to the volcanic materials involved in the movement rather than to the triggering of the avalanche during a volcanic eruption. The conditioning factors are shown to be associated with specific structural and hydrological conditions, such as the presence of old volcanic domes, strength reduction of the rocks, effective stress decrease, active gully erosion and water table rise during Pleistocene humid episodes. We finally suggest that the possible triggering factor of the avalanche was a neighbouring volcanic or tectonic earthquake.     

Plant macrofossil assemblages from surface sediment represent contemporary species and growth forms of aquatic vegetation in a shallow Mediterranean lake

Macrofossils are known as a useful tool in reconstructing their original plant communities. However, most studies have been focused on comparing the composition and distribution of living plant communities and their remains in temperate lakes. Mediterranean shallow lakes have been historically far less studied and little is known about the relationships between Mediterranean macrophyte communities and their remains. The aim of our study is to assess how contemporary aquatic macrophyte communities are represented by their sedimentary remains in terms of composition, distribution and concordance between the contemporary and the subfossil assemblages in a procrustean superimposition space, and to determine which surface sediment cores, collected along a depth gradient, may represent best the whole-lake macrofossil assemblage. These analyses were carried out for both species and macrophyte growth forms (submerged hydrophytes, floating-leaved hydrophytes, helophytes and charophytes) in order to check which of the two (species and growth forms) were represented best by their macro-remains. The most abundant present-day species (Myriophyllum alterniflorum DC. and Potamogeton trichoides L.) were under-represented while Characeae and some floating-leaved hydrophytes (Polygonum amphibium L. and Ranunculus peltatus Schrank) were over-represented in sedimentary samples. Additionally, macro-remains of submerged hydrophytes and helophytes were generally found in the central areas and in close proximity to contemporary vegetation, whereas floating-leaved hydrophytes distributed close to the near-shore. Notwithstanding some disparities between contemporary vegetation and their macrofossil assemblages, we found a good agreement between present-day and sedimentary datasets for both species and macrophyte growth forms. Furthermore, our study suggests that sediment cores from deep areas are more likely to represent best the whole-lake macrofossil assemblage because of their high diversity, equitability and heterogeneity. We conclude that aquatic macrophyte subfossils from the central areas of the basin can be a very useful tool in tracking the species composition and structure of the original macrophyte communities in shallow Mediterranean lakes. Additionally, when considering the use of macro-remains to reconstruct the composition and structure of macrophyte growth forms, we recommend a multicore approach that uses transects running from the shore to the lake center.     

Refining the palaeoecology of lacustrine testate amoebae: insights from a plant macrofossil record from a eutrophic Scottish lake

Lakes are common natural systems for which sedimentation is considered to be relatively simple, generally dominated by fluvial processes along the margin and prevailing low-energy settling in the central, deeper parts. However, for many lakes, higher-energy wind-driven processes dominate. As such, a new category of lakes is proposed, herein referred to as wind-driven waterbodies (WWB). WWB display a sedimentation largely dominated by wave related processes and wind-induced lake-scale water circulation evidenced by the construction of beach ridges, spits or cuspate spits along their shorelines, and by sediment drifts, sedimentary shelf progradation and erosional surfaces in their deeper, offshore domains. WWB are observed worldwide, they share a common physiography that favours wind-forced hydrodynamics and related sedimentation patterns. This physiography is expressed by the I_WWB index, a ratio of the maximum representative fetch relative to mean basin depth. It is proposed that an index value greater than three favours the evolution of a lake as a WWB. The WWB concept represents a new end-member in an alternative, sedimentologically-based lake classification that is proposed in this paper.     

Assessment of flood inundation mapping of Surat city by coupled 1D/2D hydrodynamic modeling: a case application of the new HEC-RAS 5

Surat city of India, situated 100 km downstream of Ukai Dam and 19.4 km upstream from the mouth of River Tapi, has experienced the largest flood in 2006. The peak discharge of about 25,770 m³ s^?1 released from the Ukai Dam was responsible for a disaster. To assess the flood and find inundation in low-lying areas, simulation work is carried out under the 1D/2D couple hydrodynamic modeling. Two hundred ninety-nine cross sections, two hydraulic structures and five major bridges across the river are considered for 1D modeling, whereas a topographic map at 0.5 m contour interval was used to produce a 5 m grid, and SRTM (30 and 90 m) grid has been considered for Surat and the Lower Tapi Basin. The tidal level at the river mouth and the release from the Ukai Dam during 2006 flood are considered as the downstream and upstream boundaries, respectively. The model is simulated under the unsteady flow condition and validated for the year 2006. The simulated result shows that 9th August was the worst day in terms of flooding for Surat city and a maximum 75–77% area was under inundation. Out of seven zones, the west zone had the deepest flood and inundated under 4–5 m. Furthermore, inundation is simulated under the bank protection work (i.e., levees, retaining wall) constructed after the 2006 flood. The simulated results show that the major zones are safe against the inundation under 14,430 m³ s^?1 water releases from Ukai Dam except for the west zone. The study shows the 2D capability of new HEC-RAS 5 for flood inundation mapping and management studies.     

Evaluation of seismic displacement demands from the September 19, 2017 Puebla‐Morelos (Mw = 7.1) earthquake in Mexico City

This short communication presents the assessment of seismic inelastic and elastic displacement demands computed from earthquake ground motions (EQGMs) recorded in Mexico City during the intermediate‐depth intraslab Puebla‐Morelos earthquake on 19 September 2017 (Mw = 7.1). Evaluation is conducted by means of peak elastic and inelastic displacement demand spectra, inelastic displacement ratio, C_R, spectra, and generalized interstory drift spectra computed for selected recording stations located in different soil sites of Mexico City, including those located in areas of reported collapsed buildings. Results of this study confirm previous observations made from interplate (subduction) EQGMs that peak inelastic displacement demands are greater than corresponding elastic counterparts for short‐to‐medium period structures, while the opposite is true for medium‐to‐long period structures. Possible basin site effects were identified from generalized interstory drift spectra. It is also shown that an equation introduced in the literature to obtain estimates of C_R developed from interplate EQGMs provides also a good estimate for mean C_R computed from the intermediate‐depth intraslab EQGMs.     

A simple two-step method for spatio-temporal design-based balanced sampling

We introduce a two-step method to perform spatio-temporal balanced sampling in a design-based approach. For populations with spatio-temporal trends and with anisotropic effects in the variable of interest, the prediction can be further improved by selecting samples that are well spread over the entire population in space and time. We control the spread of the sample over the population by using the volume of the corresponding three-dimensional Voronoi tessellation. Indeed, spatio-temporal design-based balanced sampling is even more efficient under the presence of a trend and anisotropic effects. We present an intensive simulation study comparing our method to other available methods for spatio-temporal sampling. Finally, we analyze real data by sampling from a population of temperature stations over six European countries.     

Stretch-free generalized normal moveout correction

The effective application of normal moveout correction processes mainly depends on four factors: the chosen traveltime approximation, the stretching associated with the given traveltime, crossing events and phase changes, the last two being inherent to the seismic data. In this context, we conduct a quantitative analysis on stretching considering a general traveltime expression depending on half-offset and midpoint coordinates. Through this analysis, we propose a mathematically proven procedure to eliminate stretching, which can be applied to any traveltime approximation. The proposed method is applied to synthetic and real data sets, considering different traveltime approximations and achieved complete elimination of stretching.     

Statistical analysis of the 2012–2013 Torreperogil–Sabiote seismic series,Spain

The study of earthquake swarms and their characteristics can improve our understanding of the transient processes that provoke seismic crises. The spatio-temporal process of the energy release is often linked with changes of statistical properties, and thus, seismicity parameters can help to reveal the underlying mechanism in time and space domains. Here, we study the Torreperogil–Sabiote 2012–2013 seismic series (southern Spain), which was relatively long lasting, and it was composed by more than 2000 events. The largest event was a magnitude 3.9 event which occurred on February 5, 2013. It caused slight damages, but it cannot explain the occurrence of the whole seismic crises which was not a typical mainshock–aftershock sequence. To shed some light on this swarm occurrence, we analyze the change of statistical properties during the evolution of the sequence, in particular, related to the magnitude and interevent time distributions. Furthermore, we fit a modified version of the epidemic type aftershock sequence (ETAS) model in order to investigate changes of the background rates and the trigger potential. Our results indicate that the sequence was driven by an aseismic transient stressing rate and that the system passes after the swarm occurrence to a new forcing regime with more typical tectonic characteristics.     

Comparative assessment of nonlinear static and dynamic methods for analysing building response under sequential earthquake and tsunami

This paper presents a comprehensive comparison of different dynamic and static approaches for assessing building performance under sequential earthquakes and tsunami. A 10-storey reinforced concrete seismically designed Japanese vertical evacuation structure is adopted as a case study for the investigation. The case study building is first assessed under sequential earthquake and tsunami nonlinear response history analyses: the first time this is done in the literature. The resulting engineering demand parameters are then compared with those obtained when the analysis procedure is systematically simplified by substituting different static approaches for the nonlinear response history analyses in both the earthquake and tsunami loading phases. Different unloading approaches are also tested for the cases when an earthquake pushover is adopted. The results show that an earthquake nonlinear response history analysis, followed by a transient free vibration and a tsunami variable depth pushover, provides the best alternative to full dynamic analyses in terms of accuracy and computational efficiency. This structural analysis combination is recommended and has the advantage that it does not require the tsunami inundation time history to be known in advance. The proposed double pushover approach is instead deemed only suitable for the collapse assessment of regular low to mid-rise buildings and for the development of collapse fragility functions. An important observation made is that sustained earthquake damage seems not to affect the tsunami resistance of the case study building when the fully dynamic analysis is carried out for the sequential loading. This observation will be the subject of future work.     

Hydroclimatology of the Upper Madeira River basin: spatio-temporal variability and trends

Rising in the Andes, the Madeira River drains the southwestern part of the Amazon basin, which is characterized by high geographical, biological and climatic diversity. This study uses daily records to assess the spatio-temporal runoff variability in the Madeira sub-basins. Results show that inter-annual variability of both discharge and rainfall differs between Andean and lowland tributaries. High-flow discharge variability in the Andean tributaries and the Guaporé River is mostly related to sea surface temperature (SST) in the equatorial Pacific in austral summer, while tropical North Atlantic (TNA) SST modulates rainfall and discharge variability in the lowlands. There also is a downward trend in the low-flow discharge of the lowland tributaries which is not observed in the Andes. Because low-flow discharge values at most lowland stations are negatively related to the SST in the tropical North Atlantic, these trends could be explained by the warming of this ocean since the 1970s.

EDITOR A. Castellarin

ASSOCIATE EDITOR A. Viglione