This work introduces for the first time the concept of using sinkholes in fluvial valleys as recorders of past floods. The notion is illustrated through the investigation of a complex sinkhole located in a broad floodplain underlain by salt-bearing Cenozoic evaporites. This active sinkhole comprises a large subsidence depression affecting the floodplain and the edge of a terrace, and a nested collapse sinkhole that used to host a sinkhole pond. A borehole drilled in the buried sinkhole pond revealed an ~7.8-m-thick fill that records around 2700 yr of clayey lacustrine deposition interrupted by three types of detrital facies. Two thick pebble gravel beds have been attributed to major high-competence floods: a paleoflood that occurred in Visigothic times (1537–1311 cal yr BP) and the 1961 Great Ebro River Flood, which is the largest event of the instrumental record. A trench dug in the portion of the terrace affected by subsidence exposed a mid-Holocene slack-water paleoflood deposit. The disadvantages and advantages of sinkholes as archives of past flood histories are discussed.

On the basis of a multidisciplinary approach we have unraveled the palaeo-earthquake history of a trenched section across the Peel Boundary Fault. The area shows at present one of the largest contrasts in relative motion on both sides of the fault on the basis of repeated levelling. The geological record for the last 25 thousand years, recovered in the trench, shows evidence of two heavy earthquakes (moment magnitude between 6.0 and 6.6), that occurred in a relatively short timespan around 15 thousands years ago. A third less severe event occurred somewhere in the mid Holocene. The time interval between the two large events is in the order of 1500 years, an interval comparable to that between the last volcanic explosions in the nearby Eifel area. Both records together seem to suggest a relation between large-scale faulting and volcanic activity in the nearby Eifel area, but this interpretation is based on one trench only and should be tested by opening more trenches in the zone that is assumed to be affected by these large events.

Given that earthquakes do not occur only along high slip-rate faults, slow moving seismogenic faults should be characterized in order to minimize seismic hazard uncertainties. Although no historical earthquakes related to the El Camp fault have been documented, earlier regional geological analysis and the presence of a fault scarp provide evidence of its activity. A paleoseismological study on the southern part of the fault was performed in accordance with the following steps: 1) geological and géomorphologie analysis focussing on the detection of evidence for the seismogenic behavior of the fault, 2) near fault analysis to select the best trenching sites, and 3) trenching to establish and characterize the paleoseismic events. Different dating procedures were used in regional and trenching analyses (Thermoluminiscence, U/Th, Radiocarbon, Pollen analysis). The seismogenic nature of the fault was established by the presence of liquefaction features related to the fault, and by the presence of colluvial wedges composed of large angular blocks. We identified a segment boundary to the north of the Porquerola creek and we focussed our attention on the southern segment, which was active after 125000 yr. The slip rate in this southern segment is 0.02 mm/yr. A minimum of three seismic events were detected, from young to old: the last event Z took place some time prior to 1195 yr AD, the penultimate event Y between event X and the Holocene, and, finally, event X occurred after 125000 yr and prior to 60000 yr. The recurrence period is between 25000 and 35000 yr, the elapsed time is estimated to be no longer than 3000 yr; and the maximum estimated earthquake considering both the onshore and the offshore part of the fault is Mw 6.7 +/-0,5.

A first trench has been excavated for paleoseismological analysis in the German part of the Roer Valley graben, which has experienced several historical earthquakes with a maximum intensity up to VIII on the MSK-scale.The trench has exposed the Rurrand fault as a complex fault zone with at least five separate, SW-dipping, normal fault strands displacing an early Pleistocene terrace of the Rhine river by more than 7 m. The major part of the observed deformation was produced during or after deposition of an overlying unit of stratified loess of middle Weichselian to probably Saalian age. The faulting history is shown to be episodic, with different fault strands active at different times. Growth faulting that would be indicative of continuous, aseismic fault motion has not been observed. Our stratigraphic control is not sufficient to constrain the timing and to provide evidence of the coseismic nature for each observed fault displacement. However, two units of structureless, gravelly loess are interpreted as the result of extensive solifluction triggered by two large surface-rupturing events. This is suggested by the position of these units, which is controlled by the main faults, and by their remarkably young age (< 400 cal. BC), indicated by radiocarbon and OSL datings and by the presence of historic brick fragments. At least two faults show moderate activity that is even younger. Our interpretation is not in agreement with earlier hypotheses that ongoing vertical movements of circa 1 mm/a in the German part of the Lower Rhine graben are the result of aseismic fault creep, but is in line with the results of similar investigations on the southwestern border fault of the Roer Valley Graben in Belgium, which demonstrates the need for further paleoseismological research in this region. The Rurrand fault is presently experiencing aseismic slip on its superficial portion, induced by extensive groundwater lowering for mining purposes. This ongoing deformation seems to be expressed in the trench as diffuse bundles of anastomosing cracks extending up to, and in some cases even into the plough zone, rather than as sharp fault planes which are typical of older, tectonic fault movements.

Seedlings in tropical forests are ultimately the source of canopy trees, thus factors controlling their composition and growth potentially influence the composition of the forest. Seedlings are primarily limited by above-ground competition with trees, but below-ground competition is potentially also important. Over 4 y we experimentally reduced below-ground competition by trenching to 50 cm and reduced drought stress by irrigating in the dry seasons (6 cm every 2 d) in the understorey of a semi-evergreen rain forest in Panama. There were four irrigated plots and four unirrigated, in each plot there were eight subplots (four trenched, four untrenched); 32 seedlings (two per subplot) of each of four tree species were equally allocated to the four treaments; the four species were: Aspidosperma cruenta; Gustavia superba; Simarouba amara and Tachigali versicolor. Over all species together, trenching increased seedling height by 41% and leaf area by 140% over 4 y. The cause was likely to be increased nutrient supply, because the amounts of N, K and Ca were higher in trenched plants, though concentrations were not higher. Irrigation had no significant effect on growth. Irrigation, but not trenching, reduced seedling mortality. We conclude that below-ground competition was a major limitation for seedling growth for at least some common species (Gustavia and Tachigali in this experiment). More experiments are necessary to determine whether below-ground competition is also important in other tropical rain forests on fairly fertile soils.