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1.
The Sumatra–Andaman Tsunami left distinctive sedimentological and geomorphological signatures in the area of Khao Lak. Fine-grained sediments, predominantly layers of cohesive, carbonate-rich, fine-sandy silt with thicknesses of 1–10 cm, erosionally overlying pre-tsunami sandy soils and sediments, represent the most common tsunami deposits in the study area. Petrographically, they differ significantly from other coastal sediments and affiliated soils. Due to their grain size and corresponding clay mineral content, muddy shelf sediments (sub-wave base) are indicated as a main source. The present results suggest that indications of shelf influence, although varying regionally, might contribute to the identification of fine-grained tsunami sediments and their differentiation from storm sediments. However, the observed differences of tsunami sediments to soils and other coastal sediments, especially with respect to carbonate mineralogy, might disappear in short geological time under conditions of intensive weathering and bioturbation. At Cape Pakarang, hundreds of boulders with up to 24 tons were deposited on the foreshore and upper shoreface. Applying Nott’s (Earth Planet Sci Lett 210:269–276, 2003) formulas, minimum flow velocities of 3.9 m/s are required to transport the largest boulders. The devastating tsunami effect of both, onshore flow and backflow, is documented by damaged human constructions. Geomorphological effects include intensive widening of estuary mouths and the development of erosional channels. Now, estuary mouths are reduced, and erosional channels cut off from the sea due to the formation of a post-tsunami beach ridge.  相似文献   

2.
To explore the local tsunami hazard from the Cascadia subduction zone we (1) evaluate geologically reasonable variability of the earthquake rupture process, (2) specify 25 deterministic earthquake sources, and (3) use resulting vertical coseismic deformations for simulation of tsunami inundation at Cannon Beach, Oregon. Maximum runup was 9–30 m (NAVD88) from earthquakes with slip of ~8–38 m and M w ~8.3–9.4. Minimum subduction zone slip consistent with three tsunami deposits was 14–15 m. By assigning variable weights to the source scenarios using a logic tree, we derived percentile inundation lines that express the confidence level (percentage) that a Cascadia tsunami will not exceed the line. Ninety-nine percent of Cascadia tsunami variation is covered by runup ≤30 m and 90% ≤16 m with a “preferred” (highest weight) value of ~10 m. A hypothetical maximum-considered distant tsunami had runup of ~11 m, while the historical maximum was ~6.5 m.  相似文献   

3.
A Late Holocene cliff-top deposit of large boulders well above the limits of modern storm waves is described from the southern coast of the Atacama Desert (northern Chile). The largest moved boulder weighs >40 t and field data point to a flood height >18·5 m above high tide level and an inland penetration greater than 284 m from the cliff edge. The minimum flow velocity needed for particle entrainment was estimated as 10·1 ms−1 and the most likely processes of sediment deposition for different boulders were deduced. The boulder distribution, sorting and orientation of imbricated debris, together with the significant wave height of extreme storms reported and the occurrence of interplate earthquakes in the study area indicate that the deposit records a single event, interpreted here as a tsunami wave train rather than exceptional storm waves. The boulder field was dated to between the 13th and the 16th Centuries ce and possibly correlates with the 1420 Oei orphan tsunami, that affected the eastern coast of Japan. A magnitude of 8·8 to 9·4 has been estimated for the earthquake, which may be one of the larger events of a super-cycle of earthquakes in the southern Atacama Desert. These cycle-ending earthquakes involve large rupture areas (lengths in excess of 600 km) and highly destructive ocean-wide tsunamigenic events.  相似文献   

4.
The tsunami of 2004 in the Indian Ocean transported thousands of meters-long boulders shoreward at Pakarang Cape, Thailand. We investigated size, position and long axis orientation of 467 boulders at the cape. Most of boulders found at the cape are well rounded, ellipsoid in shape, without sharp broken edges. They were fragments of reef rocks and their sizes were estimated to be < 14m3 (22.7t). The distribution pattern and orientation of long axis of boulders reflect the inundation pattern and behavior of the tsunami waves. It was found that there is no clear evidence indicating monotonous fine/coarse shoreward trends of these boulders along each transect line. On the other hand, the large boulders were deposited repeatedly along the three arcuate lines at the intertidal zone with a spacing of approximately 136m interval. This distribution pattern may suggest that long-lasting oscillatory flows might have repositioned the boulders and separated the big ones from small. No boulders were found on land, indicating that the hydraulic force of the tsunami wave rapidly dissipated on reaching the land due to the higher bottom friction and the presence of a steep slope. We further conducted numerical calculation of tsunami inundation at Pakarang Cape. According to the calculation, the sea receded and the major part of the tidal bench (area with boulders at present) was exposed above the sea surface before the arrival of the first tsunami wave. The first tsunami wave arrived at the cape from west to east at approximately 130min after the tsunami generation, and then inundated inlands. Our calculation shows that tsunami wave was focused around the offshore by a small cove at the reef edge and spread afterwards in a fan-like shape on the tidal bench. The critical wave velocities necessary to move the largest and average-size boulders by sliding can be estimated to be approximately 3.2 and 2.0m/s, respectively. The numerical result indicates that the maximum current velocity of the first tsunami wave was estimated to be from 8 to 15m/s between the reef edge and approximately 500m further offshore. This range is large enough for moving even the largest boulder shoreward. These suggest that the tsunami waves that were directed eastward, struck the reef rocks and coral colonies, originally located on the shallow sea bottom near the reef edge, and detached and transported the boulders shoreward.  相似文献   

5.
The Yucatán Peninsula, Mexico, has typically been considered a tectonically stable region with little significant seismic activity. The region though, is one that is regularly affected by hurricanes. A detailed survey of ca 100 km of the eastern Yucatán and Cozumel coast identified the presence of ridges containing individual boulders measuring >1 m in length. The boulder ridges reach 5 m in height and their origin is associated with extreme wave event activity. Previously modelled tsunami waves from known seismically active zones in the region (Muertos Thrust Belt and South Caribbean Deformed Belt) are not of sufficient scale in the area of the Yucatán Peninsula to have produced the boulder ridges recorded in this study. The occurrence of hurricanes in this region is more common, but two of the most destructive (Hurricane Gilbert 1988 and Hurricane Wilma 2005) produced coastal waves too small to have created the ridges recorded here. In this paper, a new tsunami model with a source area located on the Motagua/Swan Island Fault System has been generated that indicates a tsunami event may have caused the extreme wave events that resulted in the deposition of the boulder ridges.  相似文献   

6.
Natural disasters can neither be predicted nor prevented. Urban areas with a high population density coupled with the construction of man-made structures are subjected to greater levels of risk to life and property in the event of natural hazards. One of the major and densely populated urban areas in the east coast of India is the city of Chennai (Madras), which was severely affected by the 2004 Tsunami, and mitigation efforts were severely dampened due to the non-availability of data on the vulnerability on the Chennai coast to tsunami hazard. Chennai is prone to coastal hazards and hence has hazard maps on its earth-quake prone areas, cyclone prone areas and flood prone areas but no information on areas vulnerable to tsunamis. Hence, mapping has to be done of the areas where the tsunami of December 2004 had directly hit and flooded the coastal areas in Chennai in order to develop tsunami vulnerability map for coastal Chennai. The objective of this study is to develop a GIS-based tsunami vulnerability map for Chennai by using a numerical model of tsunami propagation together with documented observations and field measurements of the evidence left behind by the tsunami in December 2004. World-renowned and the second-longest tourist beach in the world “Marina” present in this region witnessed maximum death toll due to its flat topography, resulting in an inundation of about 300 m landward with high flow velocity of the order of 2 m/s.  相似文献   

7.
Based on the general physical nature of tsunami generation, it is established that it is an attribute of seismically hazardous areas and regions adjacent to large water reservoirs and is threatening to the population and infrastructure of the coastal zones. The main preconditions and possibilities for the occurrence of tsunami on Lake Baikal are considered: the information on earthquakes in the Baikal hollow during the instrumental-historical period (1724–2011) is generalized in the map of epicenters of shocks of magnitude M ⩾ 5 and histograms of the distribution of numbers of shocks with respect to magnitude. It is shown that the tsunami waves start forming on Baikal if the earthquake magnitude M is ≈5, but since a system of tsunami monitoring on Baikal is absent, it can be observed only during the strongest earthquakes of M > 7. The catastrophic Tsagan earthquake (1861, M ≈ 7.5) is given as an example. It happened near the eastern coast of Lake Baikal and caused a tsunami with people’s deaths.  相似文献   

8.
Aerial photographs taken in the 1963 and 2001 and bathymetric charts, in conjunction with coastal processes are analyzed to assess changes in rate of shoreline position, seabed level, and seabed grain sizes along the Tabarka–Berkoukech beach at the north-western Tunisian coastline. The littoral cell of this beach, 12-km-long, is bounded by pronounced embayments and rocky headlands separated by sandy stretches. Although not yet very much undeveloped, this littoral is still experienced degradation and modification, especially along its shoreline, with significant coastal erosion at some places. Results obtained from analysis of shoreline position indicate that El Morjene Beach is experienced a landward retreat of more than −62 m, at a maximum rate of −1.64 m/year, whereas the El corniche beach is advanced about 16–144 m, at an average rate of 0.42 m–3.78 m/year. This beach accretion has been formed on the updrift side of the Tabarka port constructed between 1966 and 1970. Comparison of bottom contours deduced from bathymetric charts surveyed in 1881 and 1996 off the coastline between Tabarka Port and El Morjene Beach identifies erosional areas (sediment source) and accretionary zones (sediment sink). Erosion (0.87–4.35 cm/year) occurs between El kebir River Mouth and El Morjene beach, whereas accretion exists in the zone down wind of the port ranges between 0.87 and 5.21 cm/year. Morphological analyses of the shoreline and the seabed of the study nearshore area indicate that shoreline retreat corresponds to areas of seabed scour (sediment source) while shoreline accretion is associated with areas of seabed deposition (sediment sink). Furthermore, simulation of wave propagation using STWAVE model combined with grain size distributions of the seabed shows that fine sands are much dominated in depositional areas with low wave energy, whereas coarser sands in erosive zones with high wave energy. The results obtained suggest that the change of seabed morphology, wave height pattern and grain size sediment have a great influence on the modification of shoreline morphology and dynamics.  相似文献   

9.
The tsunami of 26th December 2004 severely affected Yemen’s Socotra Island with a death at a distance of 4,600 km from the epicenter of the Magnitude 9.0 earthquake. Yemen allowed a detailed assessment of the far-field impact of a tsunami in the main propagation direction. The UNESCO mission surveyed 12 impacted towns on the north and south shores covering from the east to the west tip of Socotra. The international team members were on the ground in Yemen from 11 to 19 October 2006. The team measured tsunami run-up heights and inundation distances based on the location of watermarks on buildings and eyewitness accounts. Maximum run-up heights were typically on the order of 2–6 m. Each measurement was located by means of global positioning systems (GPS) and photographed. Numerous eyewitness interviews were recorded on video. The tsunami impact on Socotra is compared with other locations along the shores of the Indian Ocean.  相似文献   

10.
Coastal boulder deposits and chevrons are two features whose origin have triggered controversial discussions. Boulders are often used as indicators of past tsunamis and storms, with the former interpretation in many cases preferred due to the clast size. Chevrons, defined as large parabolic sand bodies, were previously attributed to (mega-)tsunami, potentially caused by oceanic impacts, because of their dimensions, height above sea level and alignment of the central axis. This study documents that chevrons along the Quobba coast in Western Australia are parabolic dunes and not related to tsunami inundation; their age is consistent with an arid period at about 3·9 to 2·3 ka when the sea level was 1 to 2 m higher than today. The internal age distribution proves an inland migration. Weakly developed soil horizons represent phases of intermittent dune stabilization and later reactivation. The calculated velocities required for wind transport and the prevailing wind directions are consistent with on-site meteorological parameters. The boulders at Quobba are most likely to be remnants of in situ platform denudation that produces shell hash, coral clasts and boulders. An unknown portion of the boulders was certainly moved by tropical cyclones. A previously proposed tsunami origin is unsustainable because the observed features can be explained by processes other than tsunamis. Boulders were tilted during gravitative platform collapse, standing water caused dissolution of the boulder bottoms, creating ‘pseudo-rockpools’, consequently not applicable as upside-down criteria, and ages of attached encrusting organisms document their colonization at higher sea level and (sub)recent frequent inundation by wave splash during rough seas.  相似文献   

11.
A detailed assessment of the impact of a far-field tsunami on the Australian coastline was carried out in the Steep Point region of Western Australia following the July 17 2006 Java tsunami. Tsunami inundation and run-up were mapped on the basis of eyewitness accounts, debris lines, vegetation damage and the occurrence of recently deposited fish, starfish, corals and sea urchins well above high-tide mark. A topographic survey using kinematic GPS with accuracies of 0.02 m in the horizontal and 0.04 m in the vertical recorded flow depths of between 1 and 2 m, inundation of up to 200 m inland, and a maximum recorded run-up of 7.9 m AHD (Australian Height Datum). The tsunami impacted the sparsely populated Steep Point coastline close to low tide. It caused widespread erosion in the littoral zone, extensive vegetation damage and destroyed several campsites. Eyewitnesses reported three waves in the tsunami wave train, the second being the largest. A sand sheet, up to 14 cm thick and tapering landwards over 200 m, was deposited over coastal dunes. The deposits are predominantly composed of moderately well-sorted, medium-grained carbonate sand with some gravel and organic debris. A basal unconformity defines the boundary between tsunami sediments and underlying aeolian dune sand. Evidence for up to three individual waves is preserved as normally graded sequences mantled by layers of dark grey, organic-rich fine silty sand. Given the strong wind regimes in the area and the similarity of the underlying dune deposits to the tsunami sediments, it is likely that seasonal erosion will remove all traces of these sediment sheets within years to decades.  相似文献   

12.
Among trawling grounds for Pacific cod (Gadus macrocephalus) off western Canada, Amphitrite Bank is small (∼220 km2) but important both because it is a major spawning area and because it sustains a relatively high yield At the south and east central part of the 40- to 60-m deep bank, sediments consist mainly of densely packed, poorly sorted, very coarse, immobile gravels Boulders which are commonly 3–5 m wide and 1–2 m high offer the most severe hazards on the bank to trawling gear. On the west central part of the bank, areas containing generally finer, but still poorly sorted gravels alternate with areas having coarse rippling (wavelength=4–5m, height=0.4–0.5m as measured from sonograms) and size-segregated gravels. Along the outer bank, areas having coarse and fine rippling alternate with relatively smooth patches where large boulders are generally absent. Local wave regime is adequate to mobilize the coarsest (mean size=−4ϕ) relatively well-sorted (σ=≤1.9ϕ) sediment and generate the largest observed oscillatory ripples in materials of this size. Incubating eggs were not observed directly on the bank surface, but Canadian trawl captains have reported catching actively spawning cod exclusively over areas of the bank which we have identified as heavily rippled. This substrate may be suitable for Pacific cod, whose eggs are demersal and temporarily adhesive, possibly because it is moved sufficiently to maintain gravel particle surfaces free of deposits which may inhibit adhesion, but not frequently enough during the spawning and incubation period to damage eggs  相似文献   

13.
Tsunami deposits present an important archive for understanding tsunami histories and dynamics. Most research in this field has focused on onshore preserved remains, while the offshore deposits have received less attention. In 2009, during a coring campaign with the Italian Navy Magnaghi, four 1 m long gravity cores (MG cores) were sampled from the northern part of Augusta Bay, along a transect in 60 to 110 m water depth. These cores were taken in the same area where a core (MS06) was collected in 2007 about 2·3 km offshore Augusta at a water depth of 72 m below sea level. Core MS06 consisted of a 6·7 m long sequence that included 12 anomalous intervals interpreted as the primary effect of tsunami backwash waves in the last 4500 years. In this study, tsunami deposits were identified, based on sedimentology and displaced benthic foraminifera (as for core MS06) reinforced by X-ray fluorescence data. Two erosional surfaces (L1 and L2) were recognized coupled with grain-size increase, abundant Posidonia oceanica seagrass remains and a significant amount of Nubecularia lucifuga, an epiphytic sessile benthic foraminifera considered to be transported from the inner shelf. The occurrence of Ti/Ca and Ti/Sr increments, coinciding with peaks in organic matter (Mo incoherent/coherent) suggests terrestrial run-off coupled with an input of organic matter. The L1 and L2 horizons were attributed to two distinct historical tsunamis (ad 1542 and ad 1693) by indirect age-estimation methods using 210Pb profiles and the comparison of Volume Magnetic Susceptibility data between MG cores and MS06 cores. One most recent bioturbated horizon (Bh), despite not matching the above listed interpretative features, recorded an important palaeoenvironmental change that may correspond to the ad 1908 tsunami. These findings reinforce the value of offshore sediment records as an underutilized resource for the identification of past tsunamis.  相似文献   

14.
During two distinct earthquakes occurred on March 7, 1867 and October 6, 1944, tsunami waves were also observed at some localities around the Gulf of Edremit, NE Aegean Sea. The first event (M w = 6.8) mostly affected the city of Mitilini of Lesvos Island while the Gulf of Edremit-Ayvacık earthquake (M S = 6.8) largely affected the northern and eastern coastal areas of the Gulf of Edremit. In 1944 earthquake, numerous surface cracks and water gushes were reported. The coastal neighborhoods of the town of Ayvalık in the east were flooded by tsunami waves. At the WSW extend of the main fault observed on land, which is parallel to the present-day slip vectors, some normal-oblique faults were observed close and subparallel to the northern coast. On the basis of historical documents, reports, interviews, geological setting, field observations and marine seismic reflection data, the 1944 earthquake was not triggered by one of the main fault segments but by a secondary fault or fault group which was described in this study. Depending on the distribution of tensional and compressional forces in the region, which rotates clockwise under the control of the middle strand of the North Anatolian fault, secondary fault groups become important. The moment tensor parameters of such small-size events have been determined and have obtained consistent results with the faults proposed in this study.  相似文献   

15.
Geological Indicators of Large Tsunami in Australia   总被引：1，自引：0，他引：1  
Bryant  E. A.  Nott  J. 《Natural Hazards》2001,24(3):231-249
Tsunami waves can produce four general categories of depositional and erosional signatures that differentiate them from storm waves. Combinations of items from these categories uniquely define the impact of palaeo-tsunami on the coastal landscape. The largest palaeo-tsunami waves in Australia swept sediment across the continental shelf and obtained flow depths of 15–20 m at the coastline with velocities in excess of 10 m -1. In New South Wales, along the cliffs of Jervis Bay, waves reachedelevations of more than 80 m above sea-level with evidence of flow depths in excess of 10 m. These waves swept 10 km inland over the Shoalhaven delta. In northern Queensland, boulders more than 6 m in diameter and weighing 286 tonnes were tossed alongshore above cyclone storm wave limits inside the Great Barrier Reef. In Western Australia waves overrode and breached 60 m high hills up to 5 km inland. Shell debris and cobbles can be found within deposits mapped as dunes, 30 km inland. The array of signatures provide directional information about the origin of the tsunami and, when combined with radiocarbon dating, indicate thatat least one and maybe two catastrophic events have occurred during the last 1000 years along these three coasts. Only the West Australian coast hashistorically been affected by notable tsunami with maximum run-up elevations of 4–6 m. Palaeo-tsunami have been an order of magnitude greater than this. These palaeo-tsunami are produced most likely by large submarine slides on the continental slope or the impactof meteorites with the adjacent ocean.  相似文献   

16.
Geodynamic status, seismo-tectonic environment, and geophysical signatures of the Bay of Bengal do not support the occurrence of seismogenic tsunami. Since thrust fault and its intensity and magnitude of rupture are the key tectonic elements of tsunamigenic seismic sources, the study reveals that such characteristics of fault-rupture and seismic sources do not occur in most of the Bay of Bengal except a small segment in the Andaman–Nicobar subduction zone. The inferred segment of the Andaman–Nicobar subduction zone is considered for generating a model of the deformation field arising from fluid-driven source. The model suggests local tsunami with insignificant inundation potential along the coast of northern Bay of Bengal. The bathymetric profile and the sea floor configuration of the northern Bay of Bengal play an important role in flattening the waveform through defocusing process. The direction of motion of the Indian plate makes an angle of about 30° with the direction of the opening of Andaman Sea. The opening of Andaman Sea and the direction of plate motion of the Indian plate results in the formation of Andaman trench where the subducting plate dives more obliquely than that in the Sunda trench in the south. The oblique subduction reduces significantly the possibilities of dominant thrust faulting in the Andaman subduction zone. Further, north of Andaman subduction in the Bengal–Arakan coast, there is no active subduction. On the otherhand, much greater volume of sediments (in excess of 20 km) in the Bengal–Arakan segment reduces the possibilities of mega rupture of the ocean floor. The water depth (≈1,000 m) along most of the northern Bay of Bengal plate margin is not optimum for any significant tsunami generation. Hence, very weak possibility of any significant tsunami is suggested that based on the interpretation of geodynamic status, seismo-tectonic environment, and geophysical signatures of the Andaman subduction zone and the Bengal–Arakan coast.  相似文献   

17.
In this article, we assess the human and economic hazard posed by tsunami waves generated from impacts of sub-2 km diameter asteroids. Annually, on average, 182(+197/−123) people will be affected by impact-induced waves with a corresponding infrastructure loss of $18(+20/−12)M/y. Half of the tsunami hazard stems from impactors with diameters less than 300 m. One near Earth asteroid will survive atmospheric transit and strike somewhere into Earth’s oceans every 5880 years, on average. In the mean generic scenario, the tsunami from the impact affects 1.1 million people and destroys $110B of infrastructure.  相似文献   

18.
Rockfall hazard and risk analysis for Monte da Lua,Sintra, Portugal   总被引：1，自引：0，他引：1  
The prediction of rockfall trajectories below a rock cliff is essential in susceptibility, hazard and risk maps, particularity close to populated areas. The Monte da Lua hill area in Portugal, a tourist destination close to the historic city of Sintra (UNESCO World Heritage), is a typical granite boulder chaos landscape where from time to time rockfalls occur, the last such event having occurred on 29 January 2002. This area is therefore suitable to develop a rockfall study in order to provide hazard and risk maps a basis for mitigation measures. A preliminary investigation of the area leads to the identification of 188 potentially dangerous boulders. Detailed locations and geotechnical characteristics in terms of geometry, strength and context were sampled for each boulder. Digital elevations at 1 × 1 m resolution, known rockfall trajectory and building locations are provided in a GIS project for the study together with the spatial database of boulder characteristics. The modelling approach was conducted in two steps: (1) discrimination of the boulders in terms of static and dynamic mobility behaviour with multivariate analysis; (2) stochastic simulation of rockfall trajectories. The rockfall trajectory algorithm proposed is straightforward and is only dependent on elevation data, initial location of boulders and a friction angle. Due to the slope of the area, it assumes that rockfall is always of the rolling or sliding type. The friction angle was calibrated on the basis of the rockfall travel distance recorded on 29 January 2002 and generates simulated “realistic” trajectories. A smaller friction angle increases all simulated trajectories, leading to more “pessimistic” scenarios. The combined analysis of trajectories and potential damage to buildings and discrimination in terms of static and dynamic behaviour provides a final table in which all 188 sampled boulders are classified in one of the five risk grades.  相似文献   

19.
The shear wave velocity (VS) profile based on the dispersive characteristics of fundamental mode of Rayleigh type surface waves indicate underground stiffness change with depth as well as near surface stiffness. The most important utility of shear wave velocity (VS) is to estimate the liquefaction hazard potential of an area particularly in seismically active region. Rayleigh type surface waves were utilized to estimate the velocity (VS) of shallow subsurface covering a depth range of 30–50 m employing multichannel analysis of surface waves. The liquefaction hazard map predicts an approximate percentage of an area that will have surface manifestation of liquefaction during an earth quake. The surface wave data acquired in an earth quake prone region of Jabalpur (Seismic zone III), India, yields a velocity (VS) range of 200–750 m/s corresponding to the subsurface depth of 30–35 m. The results were analyzed for possible liquefaction hazard in the study area and presented here besides the N values.  相似文献   

20.
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