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1.
Waves with a large incidence angle in deep water can drive a morphodynamic instability on a sandy coast whereby shoreline sand waves, cuspate forelands, and spits can emerge. This instability is related to bathymetric perturbations extending offshore in the shoaling zone. Here, we explore a different mechanism where the large incidence angle is supposed to occur at breaking and the bathymetric perturbations occur only in the surf zone. For wave incidence angles at breaking above ≈?45°, the one-line approximation of coastal dynamics predicts an unstable shoreline. This instability (EHAWI) is scale-free and the growth rate increases without bound for decreasing wavelength. Here we use a 2DH morphodynamic model resolving surf zone instabilities to investigate whether EHAWI could approximate a real instability in nature with a characteristic length scale. Assuming very idealized conditions on the bathymetric profile and sediment transport, we find a 2DH instability mode consisting of shore-oblique up-current bars coupled to a meandering of the longshore current. This mode grows for high-angle waves, above about 30° (offshore) and the maximum growth rate occurs for the angle maximizing the angle at breaking, about 70° (offshore). The dominant wavelength is of the order of the surf zone width. Interestingly, for long sand waves, the growth rate never becomes negative and it matches very well the anti-diffusive behavior of EHAWI. This distinguishes the present instability mode from other modes found in previous studies for other bathymetric and sediment transport conditions. Thus, we conclude that EHAWI approximates a real morphodynamic instability only for quite particular conditions. In such case, a characteristic length scale of the instability emerges thanks to surf zone processes that damp short wavelengths.  相似文献   

2.
Kilometric-scale shoreline sand waves (KSSW) have been observed in the north-east flank of the Dungeness Cuspate Foreland (southeastern coast of the UK). They consist of two bumps separated by embayments with a 350–450-m spacing. We have analysed 36 shoreline surveys of 2-km length using the Discrete Fourier Transformation (DFT), from 2005 to 2016, and seven topographic surveys encompassing the intertidal zone, from 2010 to 2016. The data set shows two clear formation events. In order to test the role of high-angle waves on the KSSW formation, the 10-year wave series is propagated from the wave buoy located at 43 m depth up to a location in front of the undulations at 4 m depth using the SWAN wave model. The dominating SW waves arrive with a very high incidence angle (～ 80°) while the NE waves arrive almost shore normal. The ratio R, which measures the degree of dominance of high-angle waves with respect to low-angle waves, correlates well with the shoreline DFT magnitude values of the observed wavelength undulations. In particular, the highest R values coincide with the formation events. Finally, a linear stability model based on the one-line approximation is applied to the Dungeness profile and the 10-year propagated wave series. It predicts accurately the formation moments, with positive growth rates in the correct order of magnitude for wavelengths similar to the observed ones. All these results confirm that the shoreline undulations in Dungeness are self-organized and that the underlying formation mechanism is the high-angle wave instability. The two detected formation events provide a unique opportunity to validate the existing morphodynamic models that include such instability.  相似文献   

3.
Analyses of shoreline and bathymetry change near Calais, northern coast of France, showed that shoreline evolution during the 20th century was strongly related with shoreface and nearshore bathymetry variations. Coastal erosion generally corresponds to areas of nearshore seabed lowering while shoreline progradation is essentially associated with areas of seafloor aggradation, notably east of Calais where an extensive sand flat experienced seaward shoreline displacement up to more than 300 m between 1949 and 2000. Mapping of bathymetry changes since 1911 revealed that significant variation in nearshore morphology was caused by the onshore and alongshore migration of a prominent tidal sand bank that eventually welded to the shore. Comparison of bathymetry data showed that the volume of the bank increased by about 10×107 m3 during the 20th century, indicating that the bank was acting as a sediment sink for some of the sand transiting alongshore in the coastal zone. Several lines of evidence show that the bank also represented a major sediment source for the prograding tidal flat, supplying significant amounts of sand to the accreting upper beach. Simulation of wave propagation using the SWAN wave model (Booij et al., 1999) suggests that the onshore movement of the sand bank resulted in a decrease of wave energy in the nearshore zone, leading to more dissipative conditions. Such conditions would have increased nearshore sediment supply, favoring aeolian dune development on the upper beach and shoreline progradation. Our results suggest that the onshore migration of nearshore sand banks may represent one of the most important, and possibly the primary mechanism responsible for supplying marine sand to beaches and coastal dunes in this macrotidal coastal environment.  相似文献   

4.
v--vWe present a preliminary study of T waves from Polynesian nuclear tests at Mururoa, recorded on digital stations of the Hawaii Volcano Observatory network, following their conversion to seismic waves at the southern shore of the Island of Hawaii, and subsequent propagation to the recording stations. We show that seismograms are composed of several packets, which can be interpreted as resulting from TMP and TMS conversions, and which feature distinct spectral characteristics. As the distance from the shoreline to the station increases, the relative importance of the several wave packets changes; a prominent shadow for TMP is found at 8-12 km from the shore. This pattern is affected by the local crustal structure; in a favorable case, propagation in deep, low-attenuation layers resulted in a clear record as far as 76 km from the shoreline. While these results are generally robust, they can be moderately affected by a change of location of the source inside Mururoa Atoll.  相似文献   

5.
—?T-phase propagation from ocean onto land is investigated by comparing data from hydrophones in the water column with data from the same events recorded on island and coastal seismometers. Several events located on Hawaii and the emerging seamount Loihi generated very large amplitude T phases that were recorded at both the preliminary IMS hydrophone station at Point Sur and land-based stations along the northern California coast. We use data from seismic stations operated by U. C. Berkeley along the coast of California, and from the PG&;E coastal California seismic network, to estimate the T-phase transfer functions. The transfer function and predicted signal from the Loihi events are modeled with a composite technique, using normal mode-based numerical propagation codes to calculate the hydroacoustic pressure field and an elastic finite difference code to calculate the seismic propagation to la nd-based stations. The modal code is used to calculate the acoustic pressure and particle velocity fields in the ocean off the California coast, which is used as input to the finite difference code TRES to model propagation onto land. We find both empirically and in the calculations that T phases observed near the conversion point consist primarily of surface waves, although the T phases propagate as P waves after the surface waves attenuate. Surface wave conversion occurs farther offshore and over a longer region than body wave conversion, which has the effect that surface waves may arrive at coastal stations before body waves. We also look at the nature of T phases after conversion from ocean to land by examining far inland T phases. We find that T phases propagate primarily as P waves once they are well inland from the coast, and can be observed in some cases hundreds of kilometers inland. T-phase conversion at tenuates higher frequencies, however we find that high frequency energy from underwater explosion sources can still be observed at T-phase stations.  相似文献   

6.
A deep-focus (H = 609 km) earthquake with M w = 8.3 occurred in the Sea of Okhotsk on May 24, 2013. This earthquake was felt in Moscow at a distance of about 6500 km from the epicenter but barely felt on the western coast of Kamchatka, which is located within 200 km of the source. In this paper, an attempt is made to discover the probable causes of this phenomenon in the instrumental records of the earthquake. It is most probable that the anomalously high amplitudes in the group of SSS phases, which are observed in the vertical component, appear as the result of their superimposition on the surface waves. Different mechanisms can be suggested to interpret the formation of the observed wave pattern.  相似文献   

7.
Results of the analysis of 15 unusual Pc1 pearl wave events with inverse dispersion in comparison with the dispersion of well-known electromagnetic ion-cyclotron (EMIC) waves in the form of classic pearl pulsations are presented. Pulsations with the dynamical spectrum consisting of both falling tones only (first type) and events with structures, which start with the falling tones and then develop into rising tones (second type), have been discovered. The first type corresponds to the frequency dispersion of magnetosonic waves (R-waves), and the second type corresponds to the mixed frequency dispersion of R-waves and EMIC waves (L-waves). All events were observed during quiet geomagnetic periods. The duration of the events is about 20–30 min. For the interpretation of these phenomena, the cyclotron instability driven by energetic proton beams with relative mean velocity v0 directed along the background magnetic field and corresponding to an energy ∼10–100 keV is considered. The interaction of such proton beams with waves having frequencies ω<ωi (ωi is the ion gyrofrequency) leads to the instability, which allows the fastest growth of electromagnetic oscillations with the dispersion of R-wave type. When the velocity of the proton beam decreases (v0≈0), R-waves attenuate and L-waves (for the proton temperature T⊥>T∥) will be amplified. This instability is the reason for the generation of classic Pc1 pearl pulsations with the usual dispersion and allows explaining the transition of the dispersion from R- to L-waves.  相似文献   

8.
The effect of climate change on extreme waves in front of the Dutch coast   总被引：1，自引：1，他引：0  
Coastal safety may be influenced by climate change, as changes in wave conditions (height, period, direction) may increase the vulnerability of dunes and other coastal defences. Dune erosion depends on mean water level, storm surge height and wave conditions. In this paper, we investigate the change in wave conditions in the North Sea in a changing climate. Until now, the effect of climate change on annual maximum wave conditions has been investigated, while events with higher return periods are actually most damaging for the coast (e.g. severe dune erosion). Here, we use the 17-member Ensemble SimulationS of Extreme weather under Non-linear Climate changeE (ESSENCE) change of climate change simulations, to analyse A1b-induced changes in the mean wave climate, the annual maxima and wave conditions with return periods of up to 1:10,000?years in front of the Dutch coast. The mean wave climate is not projected to differ between 1961–1990 and 2071–2100, with both wave height (H s) and wave period (T m) remaining unaltered. In the annual maximum conditions, a decrease is projected; especially, the annual T m maximum decreases significantly by 0.3 to 0.6?s over the whole study area. Furthermore, we find that the direction of the annual maximum wave conditions shifts from north and north-west to west and south-west for both H s and T m. This is induced by a similar shift in the direction of the extreme wind speeds. Despite the decrease in annual maximum conditions, the return H s and T m are not projected to change significantly as a result of climate change in front of the Dutch coast for the period 2071–2100 relative to 1961–1990.  相似文献   

9.
Wave data collected off Goa along the west coast of India during February 1996-May 1997 has been subjected to spectral analysis, and swell and wind sea parameters have been estimated by separation frequency method. Dominance of swells and wind seas on monthly and seasonal basis has been estimated, and the analysis shows that swells dominate Goa coastal region not only during southwest monsoon (93%), but also during the post-monsoon (67%) season. Wind seas are dominant during the pre-monsoon season (51%). The mean wave periods (Tm) during southwest monsoon are generally above 5 s, whereas Tm is below 5 s during other seasons. Co-existence of multiple peaks (from NW and NE) was observed in the locally generated part of the wave spectrum, especially during the post-monsoon season. NCEP reanalysis winds have been used to analyse active fetch available in the Indian Ocean, from where the predominant swells propagate to the west coast of India. A numerical model was set up to simulate waves in the Indian Ocean using flexible mesh bathymetry. The correlation coefficients between measured and modelled significant wave heights and mean wave periods are 0.96 and 0.85, respectively. Numerical simulations reproduced the swell characteristics in the Indian Ocean, and from the model results potential swell generation areas are identified. The characteristics of swells associated with tropical storms that prevail off Goa during 1996 have also been analysed.  相似文献   

10.
We analysed in detail three earthquakes recorded in a small-aperture accelerometric array in Mexico City, using the correlation of the records as a function of time along the accelerogram and frequency. Ground response is strongly conditioned by the fundamental period of the soft soils at the site of the array (T0). Energy at periods longer than 2T0 is guided by the crustal structure (with a thickness of 45 km). The wave field at periods between T0 and 2T0 also consists of surface waves but guided by the upper 2–3 km of volcanic sediments in central Mexico. For periods smaller than T0, ground motion is uncorrelated among the stations. Our results indicate that seismic response of Mexico City, including its very long duration, results from deeply guided surface waves (between 2 and 45 km depth) interacting with the very local response of the soft surficial clay layer.  相似文献   

11.
— On the evening of 17 July 1998, on th e Aitape Coast of Papua New Guinea, a strongly felt earthquake was followed some 10–25 minutes later by a destructive tsunami. The tsunami comprised three waves, each estimated to be about 4 m high. The second of the three waves rose to a height of 10–15 m above sea level after it had crossed the shoreline and caused most damage. Maximum wave heights and the greatest damage were recorded along a 14-km sector of coast centered on Sissano Lagoon. In this sector the wavefronts moved from east to west along the coast; all structures were destroyed, and in the two main villages 20–40 percent of the population was killed. Partial destruction extended 23 km to the southeast and 8 km to the northwest, and effects of the tsunami were felt as far as 250 km to the west–northwest, beyond the international border. More than 1600 people are known to have died, with some estimates as high as 2200; 1000 were seriously injured, and 10,000 survivors were displaced. This paper presents information from interviews with eye-witnesses and from mapping of damage and inundation, and includes new information on the height, shape and timing of the waves; on the possible escape of petroleum and other gases from beneath the seafloor before and during the tsunami; on unusual sound effects that preceded the waves, and lighting effects that followed; on possible deep circulation (to 250 m) of sea water in the waves; on subsidence of the order of 50–70 cm at the coastal sand barrier; and on the resilience and potential protective capacity of certain species of trees. Eye-witness accounts indicate that the tsunami reached the shore at between 09:00 and 09:08 UT, which is earlier than is proposed in published models of the timing and location of the source of the tsunami.  相似文献   

12.
Crustal receiver functions have been calculated from 128 events for two three-component broadband seismomenters located on the south coast (FOMA) and in the central High Plateaux (ABPO) of Madagascar. For each station, crustal thickness and V p /V s ratio were estimated from H- κ plots. Self-consistent receiver functions from a smaller back-azimuthal range were then selected, stacked and inverted to determine shear wave velocity structure as a function of depth. These results were corroborated by guided forward modeling and by Monte Carlo error analysis. The crust is found to be thinner (39 ± 0.7 km) beneath the highland center of Madagascar compared to the coast (44 ± 1.6 km), which is the opposite of what would be expected for crustal isostasy, suggesting that present-day long wavelength topography is maintained, at least in part, dynamically. This inference of dynamic support is corroborated by shear wave splitting analyses at the same stations, which produce an overwhelming majority of null results (>96 %), as expected for vertical mantle flow or asthenospheric upwelling beneath the island. These findings suggest a sub-plate origin for dynamic support.  相似文献   

13.
The January 18, 2010, shallow earthquake in the Corinth Gulf, Greece (M w  5.3) generated unusually strong long-period waves (periods 4–8 s) between the P and S wave arrival. These periods, being significantly longer than the source duration, indicated a structural effect. The waves were observed in epicentral distances 40–250 km and were significant on radial and vertical component. None of existing velocity models of the studied region provided explanation of the waves. By inverting complete waveforms, we obtained an 1-D crustal model explaining the observation. The most significant feature of the best-fitting model (as well as the whole suite of models almost equally well fitting the waveforms) is a strong velocity step at depth about 4 km. In the obtained velocity model, the fast long-period wave was modeled by modal summation and identified as a superposition of several leaking modes. In this sense, the wave is qualitatively similar to P long or Pnl waves, which however are usually reported in larger epicentral distances. The main innovation of this paper is emphasis to smaller epicentral distances. We studied properties of the wave using synthetic seismograms. The wave has a normal dispersion. Azimuthal and distance dependence of the wave partially explains its presence at 46 stations of 70 examined. Depth dependence shows that the studied earthquake was very efficient in the excitation of these waves just due to its shallow centroid depth (4.5 km).  相似文献   

14.
The availability of accelerometric data for the Montenegro earthquake of 15th April 1979 makes it possible to investigate seismic Q of the lithosphere in that region, in particular, its dependence on frequency, on the depth reached by seismic waves, and on the length of time windows in which signals are processed. Two different spectral methods, S phase energy ratio and coda envelope decay, are applied, respectively, to direct and scattered shear waves. Similar results are obtained using different portions of the recordings, i.e., coda waves for the envelope decay fit and the S wave train, with a significant duration of ～ 10 s, for the energy ratios. The same apparent Q (Q ～ 40 f, where f is the frequency expressed in Hz) that is found for other neighbouring central Mediterranean regions (e.g., Ancona, on the central Italian Adriatic coast; Valnerina, in the central Apennines; Irpinia, in the southern Apennines) is also found for the southern Yugoslavian coast, in the band 1–25 Hz up to a maximum range of ～ 120 km from the focus. This strong frequency dependence is probably connected with the type of small-scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas.In order to compare the apparent Q of the whole S wave train, ～ 10 s long, with the (intrinsic) apparent Q of the single direct S wave (usually 1 s or less), the maximum entropy method is applied in the energy spectrum computation for shorter wave trains. The use of shorter time windows does not reveal any significant variation in the tendency of Q to increase linearly with frequency as the length of the time window containing the sample of the S waves decreases. This seems to indicate that scattering-dependent Q is generally inseparable from intrinsic Q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used. While the type of linear growth with frequency does not seem to undergo any variations (it remains of the Q = qf type), the data show there are a considerable decrease in the coefficient of proportionality Q with decreasing duration of the window of S waves analysed, probably as a result of variations in seismic attenuation with depth.  相似文献   

15.
The transient planetary waves in the atmosphere and ionosphere seem to occur in the form of bursts of a couple of waves with limited persistence. To study persistence of planetary wave events in the lower ionosphere, data from two radio paths from Central Europe are used, Luxembourg – Panská Ves (f = 6.09 MHz, f eq = 2.1-2.2 MHz) and Deutschlandfunk – Panská Ves (f = 1539 kHz, f eq = 650-700 kHz). The absorption along the former radio paths is formed very predominantly at altitudes of about 90-100 km, whereas the latter absorption is formed mostly at altitudes of about 85-90 km. The persistence of planetary wave type oscillations is studied in three period bands centred at 5, 10 and 16 days. Waves with period T near 5 days reveal a typical persistence of wave events around 5 cycles. Waves with T = 10 days are less persistent with a typical persistence of 3-4 cycles. The typical persistence of waves T = 16 days is no more than 3 cycles. In terms of number of cycles, the persistence of oscillations evidently decreases with increasing period. On the other hand, in terms of number of days, the persistence seems rather to increase with increasing period.  相似文献   

16.
Intertidal bars are common in mesotidal/macrotidal low-to-moderate energy coastal environments and an understanding of their morphodynamics is important from the perspective of both coastal scientists and managers. However, previous studies have typically been limited by considering bar systems two-dimensionally, or with very limited alongshore resolution. This article presents the first multi-annual study of intertidal alongshore bars and troughs in a macrotidal environment using airborne LiDAR (light detection and ranging) data to extract three-dimensional (3D) bar morphology at high resolution. Bar and trough positions are mapped along a 17.5 km stretch of coastline in the northwest of England on the eastern Irish Sea, using eight complete, and one partial, LiDAR surveys spanning 17 years. Typically, 3–4 bars are present, with significant obliquity identified in their orientation. This orientation mirrors the alignment of waves from the dominant south-westerly direction of wave approach, undergoing refraction as they approach the shoreline. Bars also become narrower and steeper as they migrate onshore, in a pattern reminiscent of wave shoaling. This suggests that the configuration of the bars is being influenced by overlying wave activity. Net onshore migration is present for the entire coastline, though rates vary alongshore, and periods of offshore migration may occur locally, with greatest variability between northern and southern regions of the coastline. This work highlights the need to consider intertidal bar systems as 3D, particularly on coastlines with complex configurations and bathymetry, as localized studies of bar migration can overlook 3D behaviour. Furthermore, the wider potential of LiDAR data in enabling high-resolution morphodynamic studies is clear, both within the coastal domain and beyond. © 2019 John Wiley & Sons, Ltd.  相似文献   

17.
The current study presents the effects of seasonal variations in the depth-of-closure(Dc)on shoreline evolution using a numerical,one-line shoreline model.Beach erosion of the southern beach of the Nha Trang Coast,which is located in south central of Vietnam,is selected as the study area.This study area is immensely influenced by the tropical monsoon climate that has a clear pattern of large waves in the northeast monsoon season and calm waves in the non-monsoon season.The analysis of the long-term measured shoreline variations from a video-camera system has found a strong correlation of these variations to the monsoon-dominated wave characteristics in the Nha Trang Bay.Therefore,a new approach for determining the depth-of-closure with consideration of the seasonal wave climate changes is purposed in the current study.By implementing this new approach into a numerical,one-line shoreline model,it is found that the seasonal variations of Dc appear to better describe the periodical shoreline evolution due to the monsoon-dominated wave characteristics for the Nha Trang Coast.Such important findings are considered to commonly apply for monsoon-dominated coastal regions in general.These findings are useful information not only for scientific readers but also for the coastal authorities and managers in order to make better countermeasure plans against this kind of erosion mechanism in the future.  相似文献   

18.
—?Modal summation technique is used to generate 5000, three-component theoretical seismograms of Love and Rayleigh waves, assuming modified PREM (PREM-C) and AK135F global earth models. The focal depth h and the geometrical fault parameters are randomly chosen so as to uniformly cover possible source mechanisms and obtain uniform distribution of log h in the interval 1?h?h?M s of the form:¶ΔM s (h)=0 forh< 20km, ΔM s (h)=0.314log(h)-0.409 for 20≠h< 60km, ΔM s (h)=1.351log(h)-2.253 for 60≠h< 100km, ΔM s (h)=0.400log(h)-0.350 for 100≠h< 600km .¶After applying the above correction, the relationship between the surface wave magnitude and the scalar seismic moment for the observational data set significantly improves, and becomes independent of the source depth. In relation to CTBT, no depth correction is needed for M S when the m b ???M S discriminant is computed, because the proposed correction is zero for earthquakes with foci above 20?km.  相似文献   

19.
Abstract

An analysis is presented of the propagation of barotropic non-divergent oscillations along the western side of an ocean basin along which the persistent circulation in the basin is strongly intensified and laterally sheared. Because the Rossby number of a western boundary current is near unity, the properties of these waves are strongly affected by the steady circulation pattern. It is shown that for relatively long wavelengths, these waves can travel along the shelf in both directions; however, for a small range of short wavelengths they can only propagate northward and are unstable. Along the southeastern coast of North America, the unstable waves have wavelengths of order 150 km and periods of order 10 days. However, these waves can become stable oscillations in the deeper water northeast of Cape Hatteras. These oscillations are a possible explanation of the initiation of Gulf Stream meanders along the continental rise.  相似文献   

20.
—?Plans for a hydroacoustic network intended to monitor compliance with the CTBT call for the inclusion of five T-phase stations situated at optimal locations for the detection of seismic phases converted from ocean-borne T phases. We examine factors affecting the sensitivity of land-based stations to the seismic T phase. The acoustic to seismic coupling phenomenon is described by upslope propagation of an acoustic ray impinging at a sloping elastic wedge. We examine acoustic to seismic coupling characteristics for two cases; the first in which the shear velocity of the bottom is greater than the compressional velocity of the fluid (i.e., v p > v s > v w ), the second is a weakly elastic solid in which v s << v w < v p . The former is representative of velocities in solid rock, which might be encountered at volcanic islands; the latter is representative of marine sediments. For the case where v s > v w , we show that acoustic energy couples primarily to shear wave energy, except at very high slope angles. We show that the weakly elastic solid (i.e., v s << v w ) behaves nearly like a fluid bottom, with acoustic energy coupling to both P and S waves even at low slope angles.¶We examine converted T-wave arrivals at northern California seismic stations for two event clusters; one a series of earthquakes near the Hawaiian Islands, the other a series of nuclear tests conducted near the Tuamoto archipelago. Each cluster yielded characteristic arrivals at each station which were consistent from event to event within a cluster, but differed between clusters. The seismic T-phases consisted of both P- and S-wave arrivals, consistent with the conversion of acoustic to seismic energy at a gently sloping sediment-covered seafloor. In general, the amplitudes of the seismic T phases were highest for stations nearest the continental slope, where seafloor slopes are greatest, however noise levels decrease rapidly with increasing distance from the coastline, so that T-wave arrivals were observable at distances reaching several hundred kilometers from the coast. Signal-to-noise levels at the seismic stations are lower over the entire frequency spectrum than at the Pt. Sur hydrophone nearby, and decrease more rapidly with increasing frequency, particularly for stations furthest from the continental slope.  相似文献   

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