Integration of on-land and offshore geomorphological and structural investigations coupled to extensive radiometric dating of co-seismically uplifted Holocene beaches allows characterization of the geometry, kinematics and seismotectonics of the Scilla Fault, which borders the eastern side of the Messina Strait in Calabria, Southern Italy. This region has been struck by destructive historical earthquakes, but knowledge of geologically-based source parameters for active faults is relatively poor, particularly for those running mostly offshore, as the Scilla Fault does. The 30 km-long normal fault may be divided into three segments of 10 km individual length, with the central and southern segments split in at least two strands. The central and northern segments are submerged, and in this area marine geophysical data indicate a youthful morphology and locally evidence for active faulting. The on-land strand of the western segment displaces marine terraces of the last interglacial (124 to 83 ka), but seismic reflection profiles suggest a full Quaternary activity. Structural data collected on bedrock faults exposed along the on-land segment provide evidence for normal slip and NW-SE extension, which is consistent with focal mechanisms of large earthquakes and GPS velocity fields in the region. Detailed mapping of raised Holocene marine deposits exposed at the coastline straddling of the northern and central segments supplies evidence for two co-seismic displacements at 1.9 and 3.5 ka, and a possible previous event at 5 ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour for the fault. The 1.5–2.0 m average co-seismic slips during these events document Me 6.9–7.0 earthquakes with 1.6–1.7 ka recurrence time. Because hanging-wall subsidence cannot be included into slip magnitude computation, these slips reflect footwall uplift, and represent minimum average estimates. The palaeoseismological record based on the palaeo-shorelines suggests that the last rupture on the Scilla Fault during the February 6, 1783 Mw = 5.9–6.3 earthquake was at the expected time but it may have not entirely released the loaded stress since the last great event at 1.9 ka. Comparison of the estimated co-seismic extension rate based on the Holocene shoreline record with available GPS velocities indicates that the Scilla Fault accounts for at least 15–20% of the contemporary geodetic extension across the Messina Strait. 相似文献
We analyzed thin sections from two palaeoseismic trenches across the low-slip-rate Geleen Fault in the Belgian Maas River valley to help identifying the most recent large palaeoearthquake on this fault segment. In the first trench we sampled silty sediment below and above a prehistoric stone pavement that was supposedly at or near the surface at the time of the event, and subsequently thrown down. The samples below show a well-developed in situ argillic Bt soil horizon in parent sediment containing remnants of stratification, whereas the sediment above is a structureless colluvium reworked at least partly from Bt-horizon material. Below the stone pavement, we also found evidence of contorted stratification, which is in agreement with macroscopic observations of both the sediment and the stone pavement itself, and which is attributed to co-seismic soft-sediment deformation. In the second trench, we sampled a sequence of vaguely discernible soil horizons in the hanging-wall, interpreted as a buried soil profile (Bt, E, and possibly A horizons), overlain by a featureless deposit. Thin-section analysis supports the colluvial nature of the latter, and also provides evidence that both the base of this layer and the top of the poorly developed A horizon below have occupied a shallow position in a soil profile. A sample from the same depth in the footwall is composed of very different material. Instead of colluvium, we find patches of Bt soil, most likely representing the same pedogenic level as the in situ Bt horizon at larger depth in the hanging-wall, but displaced and subsequently degraded. Furthermore, thin sections confirm that vertical structures cutting this Bt horizon are sand dykes. These dykes could be traced macroscopically upward to the base of the colluvium. In both trenches, we have thus identified a stratigraphic boundary in the hanging-wall, close to the surface, separating an in situ soil below from colluvium above. We interpret this limit and the overlying colluvium as the event horizon and the colluvial wedge, respectively, of a surface-rupturing palaeoearthquake. In addition, in both cases we found evidence of soft-sediment deformation (related to liquefaction) contemporaneous with the event within the stratigraphic resolution. 相似文献
A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to Mw 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent Mw 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region. 相似文献
We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map.
We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning.
The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4.
ZS9 is made out of 36 zones where earthquakes with Mw > = 5 are expected. It also assumes that earthquakes with Mw up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates.
Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates. 相似文献
Borehole data reveals that during Late Quaternary, the Ganga river was non-existent in its present location near Varanasi.
Instead, it was flowing further south towards peripheral craton. Himalayan derived grey micaceous sands were being carried
by southward flowing rivers beyond the present day water divide of Ganga and mixed with pink arkosic sand brought by northward
flowing peninsular rivers. Subsequently, the Ganga shifted to its present position and got incised. Near Varanasi, the Ganga
river is flowing along a NW-SE tectonic lineament. The migration of Ganga river is believed to have been in response to basin
expansion caused due to Himalayan tectonics during Middle Pleistocene times.
Multi-storied sand bodies generated as a result of channel migration provide excellent aquifers confined by a thick zone of
muddy sediments near the surface. Good quality potable water is available at various levels below about 70 m depth in sandy
aquifers. Craton derived gravelly coarse-to-medium grained sand forms the main aquifer zones of tens of meter thickness with
enormous yield. In contrast, the shallow aquifers made up of recycled interfluve silt and sandy silt occur under unconfined
conditions and show water-level fluctuation of a few meters during pre-and post-monsoon periods. 相似文献
The backward particle tracking method, an effective and powerful tool that can be used to delineate groundwater protection
zones, is presented. The theoretical background and insights on the applicability of this method are provided. Moreover, the
present work enriches the backward particle tracking method with an uncertainty analysis concerning the porosity values, applying
a Monte Carlo (MC) approach, coupled with the use of geographical information systems (GIS). As an application example, a
wellfield in the Komotini area, Greece, is investigated. The present study may serve as a potential guideline for wellfield
delineation, particularly in areas like Greece where lack of data related to the hydrogeological system is often a problem. 相似文献
The Hill Country of Central Texas, USA, is undergoing rapid socioeconomic development, but environmental management of this
region is hampered by misconceptions about local bedrock, soils, terrain, and hydrologic processes. The Hill Country is underlain
mostly by Glen Rose Limestone (Lower Cretaceous) and exhibits a stepped terrain, which has been incorrectly attributed to
alternating hard and soft bedrock strata. Other characteristics mistakenly attributed to this landscape include thin soils
with scant water-retention capabilities, and rapid runoff as the dominant hydrologic process. This report presents new findings:
unweathered bedrock is well indurated, but interbeds exhibit variable weathering rates. Recessive slopes (“risers”) on this
stepped terrain result from rapid deterioration of strata having generally heterogeneous depositional fabrics (bioturbation
and irregular clay partings) in contrast to ledge-forming strata having homogeneous fabrics. A stony regolith is thus formed
beneath risers, providing porous and permeable ground that retards runoff and promotes the formation of moderately deep to
deep (two-tiered) regolith/soil zones. These surficial materials on local steep slopes compose important natural environmental
buffers; they support diverse biota and enhanced geochemical cycling of nutrients; they also exhibit significant water retention
and enhanced erosion abatement. Proper land management demands recognition of these attributes in the siting, design, and
construction of facilities. 相似文献
This modeling study evaluated the capability of alternative funnel-and-gate structures with three gates for capturing contaminated
groundwater in a hypothetical unconfined aquifer. Simulated interceptor structures were linear and 45 m wide, consisting of
three gates and two funnels (walls). One gate occupied the center and two gates occupied the ends of the interceptor structures.
The structures, positioned perpendicular to regional groundwater flow, traversed the entire thickness of the aquifer. A total
of four structures were evaluated (numbers designate widths of end, center, and end gates, respectively, in meters): 3-3-3,
2-5-2, 1-7-1, and 4-1-4. Particle tracking and zonal water budgets identified shapes of capture zones and discharge patterns
for each interceptor structure. A mass transport model, accounting for advection and hydrodynamic dispersion, tested the capability
of each structure for capturing a contaminant plume. Results suggest that: time-dependent capture zones underestimate the
amount of time to capture a contaminant plume, wide center gates facilitate plume capture, and wide end gates facilitate lateral
containment of contaminants. Of the structures simulated, the 2-5-2 configuration was relatively efficient at processing and
containing the simulated contaminant plume. 相似文献