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1.
It has been two decades since the last comprehensive standard model of ambient earth noise was published Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The PETERSON model was updated by analyzing the absolute quietest conditions for stations within the GSN (Berger et al. in J Geophys Res 109, 2005; Mcnamara and Buland in Bull Seism Soc Am 94:1517–1527, 2004; Ringler et al. in Seismol Res Lett 81(4) doi:10.1785/gssrl.81.4.605, 2010). Unfortunately, both the original model and the updated models did not include any deployed station in North Africa and Middle East, which reflects the noise levels within the desert environment of those regions. In this study, a survey was conducted to create a new seismic noise model from very broadband stations which recently deployed in North Africa. For this purpose, 1 year of continuous recording of seismic noise data of the Egyptian National Seismic Network (ENSN) was analyzed in order to create a new noise model. Seasonal and diurnal variations in noise spectra were recorded at each station. Moreover, we constructed a new noise model for each individual station. Finally, we obtained a new cumulative noise model for all the stations. We compared the new high-noise model (EHNM) and new low-noise model (ELNM) with both the high-noise model (NHNM) and low-noise model (NLNM) of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The obtained noise levels are considerably lower than low-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993) at ultra long period band (ULP band), but they are still below the high-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The results of this study could be considered as a first step to create permanent seismic noise models for North Africa and Middle East regions.  相似文献   

2.
On the selection of GMPEs for Vrancea subcrustal seismic source   总被引：2，自引：0，他引：2  
The Vrancea subcrustal seismic source is characterized by large magnitude ( $M_{W} \ge 7$ ) intermediate-depth earthquakes that occur two or three times during a century on average. In this study several procedures are used to grade four candidate ground motion prediction equations proposed for Vrancea source in the SHARE project. In the work of Delavaud et al. (J Seismol 16(3):451–473, 2012) four ground motion prediction models developed for subduction zones (Zhao et al. in Bull Seism Soc Am 96(3):898–913, 2006; Atkinson and Boore in Bull Seism Soc Am 93(4):1703–1729, 2003; Youngs et al. in Seism Res Lett 68(1):58–73, 1997; Lin and Lee in Bull Seism Soc Am 98(1):220–240, 2008) are suggested as suitable for Vrancea subcrustal seismic source. The paper presents the appropriateness analysis of the four suggested ground motion prediction equations done using a dataset of 109 triaxial accelerograms recorded during seven Vrancea seismic events with moment magnitude $M_{W}$ between 5.4 and 7.4, occurred in the past 35 years. The strong ground motions were recorded in Romania, as well as in Bulgaria, Republic of Moldova and Serbia. Based on the ground motion dataset several goodness-of-fit measures are used in order to quantify how well the selected models match with the recorded data. The compatibility of the four ground motion prediction models with respect to magnitude scaling and distance scaling implied by strong ground motion dataset is investigated as well. The analyses show that the Youngs et al. (Seism Res Lett 68(1):58–73, 1997) and Zhao et al. (Bull Seism Soc Am 96(3):898–913, 2006) ground motion prediction models have a better fit with the data and can be candidate models for Probabilistic Seismic Hazard Assessment.  相似文献   

3.
The North Pacific Subtropical Counter Current (STCC) is a weak zonal current comprising of a weak eastward flow near the surface (with speeds of less than 0.1 m/s and a thickness of approximately 50–100 m) and westward flow (the North Equatorial Current) beneath. Previous studies (e.g., Qiu J Phys Oceanogr 29: 2471–2486, 1999) have shown that the STCC is baroclinically unstable. Therefore, despite its weak mean speeds, nonlinear STCC eddies with diameters ~300 km or larger and rotational speeds exceeding the eddy propagation speeds develop (Samelson J Phys Oceanogr 27: 2645–2662, 1997; Chelton et al. Prog Oceanogr 91: 167–216, 2011). In this study, the authors present numerical experiments to describe and explain the instability and eddy-generation processes of the STCC and the seasonal variation. Emphasis is on finite-amplitude eddies which are analyzed based on the parameter of Okubo (Deep-Sea Res 17: 445–454, 1970) and Weiss (Physica D 48: 273–294, 1991). The temperature and salinity distribution in March and April offer the favorable condition for eddies to grow, while September and October are unfavorable seasons for the generation of eddies. STCC is maintained not only by subsurface front but also by the sea surface temperature (SST) front. The seasonal variation of the vertical shear is dominated by the seasonal surface STCC velocity. The SST front enhances the instability and lead to the faster growth of STCC eddies in winter and spring. The near-surface processes are therefore crucial for the STCC system.  相似文献   

4.
The 23 October 2011 Van (Mw 7.1) earthquake that occurred in Eastern Turkey resulted in heavy damage particularly in the city of Van and town of Ercis. This paper presents ground motion simulations of Van earthquake by using stochastic finite fault method (EXSIM, Motazedian and Atkinson in Bull Seismol Soc Am 95:995–1010, 2005; Boore in Bull Seismol Soc Am 99:3202–3216, 2009) that provides a simple and effective tool to generate high frequency strong motion. The input parameters related to source, path, and site effects are calibrated on the basis of minimizing the error functions between simulations and observations both in time and frequency domain. Validated model parameters are used to produce synthetics in regional extent with the aim of understanding the level and distribution of the ground shaking particularly in the near fault region where no recordings are available within the 40 km of the epicenter. This paper evaluates the effect of two different slip models on ground motion intensity measures over the area of interest and addresses the variability in the near fault region associated with the source effect. The synthetics are compared with the corresponding estimations of ground motion prediction equations by Boore and Atkinson (Earthq Spectra 24:99–138, 2008), Akkar and Bommer (Seismol Res Lett 81:195–206, 2010) and Akkar and Cagnan (Bull Seismol Soc Am 100:2978–2995, 2010). Our results indicate that despite the limitation of the method for incorporating the directivity effect and inadequate representation of the soil conditions at the individual stations, a satisfactory match between synthetics and observations are obtained both in time and frequency domain. Spatial distributions of the synthetics in regional level also show reasonable correlation with ground motion prediction equations and damage observations.  相似文献   

5.
We estimate the corner frequencies of 20 crustal seismic events from mainshock–aftershock sequences in different tectonic environments (mainshocks 5.7 < M W < 7.6) using the well-established seismic coda ratio technique (Mayeda et al. in Geophys Res Lett 34:L11303, 2007; Mayeda and Malagnini in Geophys Res Lett, 2010), which provides optimal stability and does not require path or site corrections. For each sequence, we assumed the Brune source model and estimated all the events’ corner frequencies and associated apparent stresses following the MDAC spectral formulation of Walter and Taylor (A revised magnitude and distance amplitude correction (MDAC2) procedure for regional seismic discriminants, 2001), which allows for the possibility of non-self-similar source scaling. Within each sequence, we observe a systematic deviation from the self-similar \( M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - 3} \) line, all data being rather compatible with \( M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - (3 + \varepsilon )} \) , where ε > 0 (Kanamori and Rivera in Bull Seismol Soc Am 94:314–319, 2004). The deviation from a strict self-similar behavior within each earthquake sequence of our collection is indicated by a systematic increase in the estimated average static stress drop and apparent stress with increasing seismic moment (moment magnitude). Our favored physical interpretation for the increased apparent stress with earthquake size is a progressive frictional weakening for increasing seismic slip, in agreement with recent results obtained in laboratory experiments performed on state-of-the-art apparatuses at slip rates of the order of 1 m/s or larger. At smaller magnitudes (M W < 5.5), the overall data set is characterized by a variability in apparent stress of almost three orders of magnitude, mostly from the scatter observed in strike-slip sequences. Larger events (M W > 5.5) show much less variability: about one order of magnitude. It appears that the apparent stress (and static stress drop) does not grow indefinitely at larger magnitudes: for example, in the case of the Chi–Chi sequence (the best sampled sequence between M W 5 and 6.5), some roughly constant stress parameters characterize earthquakes larger than M W ~ 5.5. A representative fault slip for M W 5.5 is a few tens of centimeters (e.g., Ide and Takeo in J Geophys Res 102:27379–27391, 1997), which corresponds to the slip amount at which effective lubrication is observed, according to recent laboratory friction experiments performed at seismic slip velocities (V ~ 1 m/s) and normal stresses representative of crustal depths (Di Toro et al. in Nature in press, 2011, and references therein). If the observed deviation from self-similar scaling is explained in terms of an asymptotic increase in apparent stress (Malagnini et al. in Pure Appl Geophys, 2014, this volume), which is directly related to dynamic stress drop on the fault, one interpretation is that for a seismic slip of a few tens of centimeters (M W ~ 5.5) or larger, a fully lubricated frictional state may be asymptotically approached.  相似文献   

6.
We estimate seismological fracture energies from two subsets of events selected from the seismic sequences of L’Aquila (2009), and Northridge (1994): 57 and 16 selected events, respectively, including the main shocks. Following Abercrombie and Rice (Geophys J Int 162: 406–424, 2005), we postulate that fracture energy (G) represents the post-failure integral of the dynamic weakening curve, which is described by the evolution of shear traction as a function of slip. Following a direct-wave approach, we compute mainshock-/aftershock-source spectral ratios, and analyze them using the approach proposed by Malagnini et al. (Pure Appl. Geophys., this issue, 2014) to infer corner frequencies and seismic moment. Our estimates of source parameters (including fracture energies) are based on best-fit grid-searches performed over empirical source spectral ratios. We quantify the source scaling of spectra from small and large earthquakes by using the MDAC formulation of Walter and Taylor (A revised Magnitude and Distance Amplitude Correction (MDAC2) procedure for regional seismic discriminants, 2001). The source parameters presented in this paper must be considered as point-source estimates representing averages calculated over specific ruptured portions of the fault area. In order to constrain the scaling of fracture energy with coseismic slip, we investigate two different slip-weakening functions to model the shear traction as a function of slip: (i) a power law, as suggested by Abercrombie and Rice (Geophys J Int 162: 406–424, 2005), and (ii) an exponential decay. Our results show that the exponential decay of stress on the fault allows a good fit between measured and predicted fracture energies, both for the main events and for their aftershocks, regardless of the significant differences in the energy budgets between the large (main) and small earthquakes (aftershocks). Using the power-law slip-weakening function would lead us to a very different situation: in our two investigated sequences, if the aftershock scaling is extrapolated to events with large slips, a power law (a la Abercrombie and Rice) would predict unrealistically large stress drops for large, main earthquakes. We conclude that the exponential stress evolution law has the advantage of avoiding unrealistic stress drops and unbounded fracture energies at large slip values, while still describing the abrupt shear-stress degradation observed in high-velocity laboratory experiments (e.g., Di Toro et al., Fault lubrication during earthquakes, Nature 2011).  相似文献   

7.
Earthquake early warning systems (EEWS) are considered to be an effective, pragmatic, and viable tool for seismic risk reduction in cities. While standard EEWS approaches focus on the real-time estimation of an earthquake’s location and magnitude, innovative developments in EEWS include the capacity for the rapid assessment of damage. Clearly, for all public authorities that are engaged in coordinating emergency activities during and soon after earthquakes, real-time information about the potential damage distribution within a city is invaluable. In this work, we present a first attempt to design an early warning and rapid response procedure for real-time risk assessment. In particular, the procedure uses typical real-time information (i.e., P-wave arrival times and early waveforms) derived from a regional seismic network for locating and evaluating the size of an earthquake, information which in turn is exploited for extracting a risk map representing the potential distribution of damage from a dataset of predicted scenarios compiled for the target city. A feasibility study of the procedure is presented for the city of Bishkek, the capital of Kyrgyzstan, which is surrounded by the Kyrgyz seismic network by mimicking the ground motion associated with two historical events that occurred close to Bishkek, namely the 1911 Kemin (M?=?8.2; ±0.2) and the 1885 Belovodsk (M?=?6.9; ±0.5) earthquakes. Various methodologies from previous studies were considered when planning the implementation of the early warning and rapid response procedure for real-time risk assessment: the Satriano et al. (Bull Seismol Soc Am 98(3):1482–1494, 2008) approach to real-time earthquake location; the Caprio et al. (Geophys Res Lett 38:L02301, 2011) approach for estimating moment magnitude in real time; the EXSIM method for ground motion simulation (Motazedian and Atkinson, Bull Seismol Soc Am 95:995–1010, 2005); the Sokolov (Earthquake Spectra 161: 679–694, 2002) approach for estimating intensity from Fourier amplitude spectra; and the Tyagunov et al. (Nat Hazard Earth Syst Sci 6:573–586, 2006) approach for risk computation. Innovatively, all these methods are jointly applied to assess in real time the seismic risk of a particular target site, namely the city of Bishkek. Finally, the site amplification and vulnerability datasets considered in the proposed methodology are taken from previous studies, i.e., Parolai et al. (Bull Seismol Soc Am, 2010) and Bindi et al. (Soil Dyn Earthq Eng, 2011), respectively.  相似文献   

8.
This article presents the latest generation of ground-motion models for the prediction of elastic response (pseudo-) spectral accelerations, as well as peak ground acceleration and velocity, derived using pan-European databases. The models present a number of novelties with respect to previous generations of models (Ambraseys et al. in Earthq Eng Struct Dyn 25:371–400, 1996, Bull Earthq Eng 3:1–53, 2005; Bommer et al. in Bull Earthq Eng 1:171–203, 2003; Akkar and Bommer in Seismol Res Lett 81:195–206, 2010), namely: inclusion of a nonlinear site amplification function that is a function of $\text{ V }_\mathrm{S30}$ and reference peak ground acceleration on rock; extension of the magnitude range of applicability of the model down to $\text{ M }_\mathrm{w}$ 4; extension of the distance range of applicability out to 200 km; extension to shorter and longer periods (down to 0.01 s and up to 4 s); and consistent models for both point-source (epicentral, $\text{ R }_\mathrm{epi}$ , and hypocentral distance, $\text{ R }_\mathrm{hyp}$ ) and finite-fault (distance to the surface projection of the rupture, $\text{ R }_\mathrm{JB}$ ) distance metrics. In addition, data from more than 1.5 times as many earthquakes, compared to previous pan-European models, have been used, leading to regressions based on approximately twice as many records in total. The metadata of these records have been carefully compiled and reappraised in recent European projects. These improvements lead to more robust ground-motion prediction equations than have previously been published for shallow (focal depths less than 30 km) crustal earthquakes in Europe and the Middle East. We conclude with suggestions for the application of the equations to seismic hazard assessments in Europe and the Middle East within a logic-tree framework to capture epistemic uncertainty.  相似文献   

9.
In a companion article Akkar et al. (Bull Earthq Eng, doi:10.1007/s10518-013-9461-4, 2013a; Bull Earthq Eng, doi:10.1007/s10518-013-9508-6, 2013b) present a new ground-motion prediction equation (GMPE) for estimating 5 %-damped horizontal pseudo-acceleration spectral (PSA) ordinates for shallow active crustal regions in Europe and the Middle East. This study provides a supplementary viscous damping model to modify 5 %-damped horizontal spectral ordinates of Akkar et al. (Bull Earthq Eng, doi:10.1007/s10518-013-9461-4 2013a; Bull Earthq Eng, doi:10.1007/s10518-013-9508-6, 2013b) for damping ratios ranging from 1 to 50 %. The paper also presents another damping model for scaling 5 %-damped vertical spectral ordinates that can be estimated from the vertical-to-horizontal (V/H) spectral ratio GMPE that is also developed within the context of this study. For consistency in engineering applications, the horizontal and vertical damping models cover the same damping ratios as noted above. The article concludes by introducing period-dependent correlation coefficients to compute horizontal and vertical conditional mean spectra (Baker in J Struct Eng 137:322–331, 2011). The applicability range of the presented models is the same as of the horizontal GMPE proposed by Akkar et al. (Bull Earthq Eng, doi:10.1007/s10518-013-9461-4 2013a; Bull Earthq Eng, doi:10.1007/s10518-013-9508-6, 2013b): as for spectral periods $0.01 \hbox { s}\le \,\hbox {T}\le \,4\hbox { s}$ as well as PGA and PGV for V/H model; and in terms of seismological estimator parameters $4\le \hbox {M}_\mathrm{w} \le 8, \hbox { R} \le 200 \hbox { km}, 150\hbox { m/s}\le \hbox { V}_\mathrm{S30}\le $ 1,200 m/s, for reverse, normal and strike-slip faults. The source-to-site distance measures that can be used in the computations are epicentral $(\hbox {R}_\mathrm{epi})$ , hypocentral $(\hbox {R}_\mathrm{hyp})$ and Joyner–Boore $(\hbox {R}_\mathrm{JB})$ distances. The implementation of the proposed GMPEs will facilitate site-specific adjustments of the spectral amplitudes predicted from probabilistic seismic hazard assessment in Europe and the Middle East region. They can also help expressing the site-specific design ground motion in several formats. The consistency of the proposed models together with the Akkar et al. (Bull Earthq Eng, doi:10.1007/s10518-013-9461-4 2013a; Bull Earthq Eng, doi:10.1007/s10518-013-9508-6, 2013b) GMPE may be advantageous for future modifications in the ground-motion definition in Eurocode 8 (CEN in Eurocode 8, Design of structures for earthquake resistance—part 1: general rules, seismic actions and rules for buildings. European Standard NF EN 1998-1, Brussels, 2004).  相似文献   

10.
A previous study (Lyman et al., Nature 465:334–337, 2010) showed a robust warming signal of the global upper ocean (0–700 m). They examined several sources of uncertainty that contribute to differences among heat content estimations. However, their focus was limited to globally averaged estimation. This study presents the spatial pattern of the global heat content change based on observed gridded datasets (Levitus et al., Geophys Res Lett 36:L07608, 2009). The western Pacific, Atlantic, and Indian Oceans showed significant warming trends, whereas eastern Pacific and some areas of the Gulf Stream experienced negative trends during 1993–2009. Steady warming trend was obtained from the first EOF mode when El Nino and Southern Oscillation (ENSO)-related signals were removed. This result implies that the rapid increase in heat content of the upper ocean around 2000–2005 is not related to a sampling transition from XBT to Argo observations but is associated with a natural variability dominated by strong ENSO-related signals.  相似文献   

11.
Historically, surface subsidence as a result of subsurface groundwater fluctuations have produced important and, at times, catastrophic effects, whether natural or anthropogenic. Over the past 30?years, numerical and analytical techniques for the modeling of this surface deformation, based upon elastic and poroelastic theory, have been remarkably successful in predicting the magnitude of that deformation (Le Mouélic and Adragna in Geophys Res Lett 29:1853, 2002). In this work we have extended the formula for a circular-shaped aquifer (Geertsma in J Petroleum Tech 25:734–744, 1973) to a more realistic elliptical shape. We have improved the accuracy of the approximation by making use of the cross terms of the expansion for the elliptic coordinates in terms of the eccentricity, e, and the mean anomaly angle, M, widely used in astronomy. Results of a number of simulations, in terms of e and M developed from the transcendental Kepler equation, are encouraging, giving realistic values for the elliptical approximation of the vertical deformation due to groundwater change. Finally, we have applied the algorithm to modeling of groundwater in southern California.  相似文献   

12.
One of the major challenges related with the current practice in seismic hazard studies is the adjustment of empirical ground motion prediction equations (GMPEs) to different seismological environments. We believe that the key to accommodating differences in regional seismological attributes of a ground motion model lies in the Fourier spectrum. In the present study, we attempt to explore a new approach for the development of response spectral GMPEs, which is fully consistent with linear system theory when it comes to adjustment issues. This approach consists of developing empirical prediction equations for Fourier spectra and for a particular duration estimate of ground motion which is tuned to optimize the fit between response spectra obtained through the random vibration theory framework and the classical way. The presented analysis for the development of GMPEs is performed on the recently compiled reference database for seismic ground motion in Europe (RESORCE-2012). Although, the main motivation for the presented approach is the adjustability and the use of the corresponding model to generate data driven host-to-target conversions, even as a standalone response spectral model it compares reasonably well with the GMPEs of Ambraseys et al. (Bull Earthq Eng 3:1–53, 2005), Akkar and Bommer (Seismol Res Lett 81(2):195–206, 2010) and Akkar and Cagnan (Bull Seismol Soc Am 100(6):2978–2995, 2010).  相似文献   

13.
This paper describes a new method for the evaluation of the static eccentricity $e_{s}$ and the ratio $\Omega _{\uptheta } $ of uncoupled torsional to lateral frequencies in real multi-storey buildings. The above-mentioned parameters greatly affect the lateral-to-torsional coupling of the response of asymmetric systems and thus are of paramount importance in the assessment of the in-plan irregularity of buildings. The proposed method, which is a generalization of that suggested by Calderoni et al. (Earthq Spectra 18(2):219–231, 2002), allows the calculation of the static eccentricity $e_{s}$ and the ratio $\Omega _{\uptheta } $ from the structural response to arbitrary distributions of forces and torsional couples. The effectiveness of the method is validated on some regularly and non-regularly asymmetric buildings characterised by different in-plan irregularity. The analyses demonstrate that the results of the method are rigorous in the case of regularly asymmetric systems and only slightly depend upon the heightwise distribution of the forces in the case of non-regularly asymmetric systems. Finally, the values of the static eccentricity $e_{s}$ and the ratio $\Omega _{\uptheta } $ resulting from the proposed method are compared to those obtained by means of the procedure suggested by Makarios and Anastassiadis in (Struct Des Tall Spec Build 7(1):33–55, 1998a; Struct Des Tall Spec Build 7(1):57–71, 1998b) .  相似文献   

14.
The effect of location errors in the performance of seismicity-based forecasting methods was studied here using one particular binary forecast technique, the Pattern Informatics (PI) technique (Rundle et al., Proc Nat Acad Sci USA 99, 2514–2521, 2002; Tiampo et al., Pure Appl Geophys 159, 2429–2467, 2002). The Southern Californian dataset was used to generate a series of perturbed catalogs by adding different levels of noise to epicenter locations. The PI technique was applied to these perturbed datasets to perform retrospective forecasts that were evaluated by means of skill scores, commonly used in atmospheric sciences. These results were then compared to the effectiveness obtained from the original dataset. Isolated instances of decline of the PI performance were observed due to the nature of the skill scores themselves, but no clear trend of degradation was identified. Dependence on the total number of events in a catalog also was studied, with no systematic degradation in the performance of the PI for catalogs with events in the cases studied. These results suggest that the stability of the PI method is due to the invariance of the clustering patterns identified by the TM metric (Thirumalai and Mountain, Phys Rev A 39, 3563–3573, 1989) when applied to seismicity.  相似文献   

15.
Unloaded natural rock masses are known to generate seismic signals (Green et al., 2006; Hainzl et al., 2006; Husen et al., 2007; Kraft et al., 2006). Following a 1,000 m3 mass failure into the Mediterranean Sea, centimeter-wide tensile cracks were observed to have developed on top of an unstable segment of the coastal cliff. Nanoseismic monitoring techniques (Wust-Bloch and Joswig, 2006; Joswig, 2008), which function as a seismic microscope for extremely weak seismic events, were applied to verify whether brittle failure is still generated within this unconsolidated sandstone mass and to determine whether it can be detected. Sixteen days after the initial mass failure, three small-aperture sparse arrays (Seismic Navigation Systems-SNS) were deployed on top of this 40-m high shoreline cliff. This paper analyzes dozens of spiky nanoseismic (?2.2 ≥ M L ≥ ?3.4) signals recorded over one night in continuous mode (at 200 Hz) at very short slant distances (3–67 m). Waveform characterization by sonogram analysis (Joswig, 2008) shows that these spiky signals are all short in duration (>0.5 s). Most of their signal energy is concentrated in the 10–75 Hz frequency range and the waveforms display high signal similarity. The detection threshold of the data set reaches M L ?3.4 at 15 m and M L ?2.7 at 67 m. The spatial distribution of source signals shows 3-D clustering within 10 m from the cliff edge. The time distribution of M L magnitude does not display any decay pattern of M L over time. This corroborates an unusual event decay over time (modified Omori’s law), whereby an initial quiet period is followed by regained activity, which then fades again. The polarization of maximal waveform amplitude was used to estimate spatial stress distribution. The orientation of ellipses displaying maximal signal energy is consistent with that of tensile cracks observed in the field and agrees with rock mechanics predictions. The M L– surface rupture length relationship displayed by our data fits a constant-slope extrapolation of empirical data collected by Wells and Coppersmith (1994) for normal fault features at much larger scale. Signal characterization and location as well as the absence of direct anthropogenic noise sources near the monitoring site, all indicate that these nanoseismic signals are generated by brittle failure within the top section of the cliff. The atypical event decay over time that was observed suggests that the cliff material is undergoing post-collapse bulk strain accommodation. This feasibility study demonstrates the potential of nanoseismic monitoring in rapidly detecting, locating and analyzing brittle failure generated within unconsolidated material before total collapse occurs.  相似文献   

16.
“Repeating earthquakes” identified by waveform cross-correlation, with inter-event separation of no more than 1 km, can be used for assessment of location precision. Assuming that the network-measured apparent inter-epicenter distance X of the “repeating doublets” indicates the location precision, we estimated the regionalized location quality of the China National Seismograph Network by comparing the “repeating events” in and around China by Schaff and Richards (Science 303: 1176–1178, 2004; J Geophys Res 116: B03309, 2011) and the monthly catalogue of the China Earthquake Networks Center. The comparison shows that the average X value of the China National Seismograph Network is approximately 10 km. The mis-location is larger for the Tibetan Plateau, west and north of Xinjiang, and east of Inner Mongolia, as indicated by larger X values. Mis-location is correlated with the completeness magnitude of the earthquake catalogue. Using the data from the Beijing Capital Circle Region, the dependence of the mis-location on the distribution of seismic stations can be further confirmed.  相似文献   

17.
Three-dimensional frequency dependent S-wave quality factor (Qβ(f)) value for the central Honshu region of Japan has been determined in this paper using an algorithm based on inversion of strong motion data. The method of inversion for determination of three-dimensional attenuation coefficients is proposed by Hashida and Shimazaki (J Phys Earth. 32, 299–316, 1984) and has been used and modified by Joshi (Curr Sci. 90, 581–585, 2006; Nat Hazards. 43, 129–146, 2007) and Joshi et al. (J. Seismol. 14, 247–272, 2010). Twenty-one earthquakes digitally recorded on strong motion stations of Kik-net network have been used in this work. The magnitude of these earthquake ranges from 3.1 to 4.2 and depth ranging from 5 to 20 km, respectively. The borehole data having high signal to noise ratio and minimum site effect is used in the present work. The attenuation structure is determined by dividing the entire area into twenty-five three-dimensional blocks of uniform thickness having different frequency-dependent shear wave quality factor. Shear wave quality factor values have been determined at frequencies of 2.5, 7.0 and 10 Hz from record in a rectangular grid defined by 35.4°N to 36.4°N and 137.2°E to 138.2°E. The obtained attenuation structure is compared with the available geological features in the region and comparison shows that the obtained structure is capable of resolving important tectonic features present in the area. The proposed attenuation structure is compared with the probabilistic seismic hazard map of the region and shows that it bears some remarkable similarity in the patterns seen in seismic hazard map.  相似文献   

18.
We performed three-dimensional (3-D) finite difference simulations of long-period ground motions (2–10 s) in the Kanto basin using the Japan Seismic Hazard Information Station (J-SHIS 2009), Yamada and Yamanaka (Exploration Geophysics 65(3):139–150, 2012) (YY), and Head Quarter for Earthquake Research Promotion (HERP 2012) velocity models for two intermediate depth (68–80 km) moderate earthquakes (Mw 5.8–5.9), which occurred beneath the Kanto basin. The models primarily differ in the basic data set used in the construction of the velocity models. The J-SHIS and HERP models are the results of integration of mainly geological, geophysical, and earthquake data. On the other hand, the YY model is oriented towards the microtremor-array-observation data. We obtained a goodness of fit between the observed and synthetic data based on three parameters, peak ground velocities (PGVs), smoothed Fourier spectra (FFT), and cross-correlations, using an algorithm proposed by Olsen and Mayhew (Seism Res Lett 81:715–723, 2010). We found that the three models reproduced the PGVs and FFT satisfactorily at most sites. However, the models performed poorly in terms of cross-correlations especially at the basin edges. We found that the synthetics using the YY model overestimate the observed waveforms at several sites located in the areas having V s 0.3 km/s in the top layer; on the other hand, the J-SHIS and HERP models explain the waveforms better at the sites and perform similarly at most sites. We also found that the J-SHIS and HERP models consist of thick sediments beneath some sites, where the YY model is preferable. Thus, we have concluded that the models require revisions for the reliable prediction of long-period ground motions from future large earthquakes.  相似文献   

19.
Waves, circulation and vertical dependence   总被引：1，自引：1，他引：0  
Longuet-Higgins and Stewart (J Fluid Mech 13:481–504, 1962; Deep-Sea Res 11:529–562, 1964) and later Phillips (1977) introduced the problem of waves incident on a beach, from deep to shallow water. From the wave energy equation and the vertically integrated continuity equation, they inferred velocities to be Stokes drift plus a return current so that the vertical integral of the combined velocities was nil. As a consequence, it can be shown that velocities of the order of Stokes drift rendered the advective term in the momentum equation negligible resulting in a simple balance between the horizontal gradients of the vertically integrated elevation and wave radiation stress terms; the latter was first derived by Longuet-Higgins and Stewart. Mellor (J Phys Oceanogr 33:1978–1989, 2003a), noting that vertically integrated continuity and momentum equations were not able to deal with three-dimensional numerical or analytical ocean models, derived a vertically dependent theory of wave–circulation interaction. It has since been partially revised and the revisions are reviewed here. The theory is comprised of the conventional, three-dimensional, continuity and momentum equations plus a vertically distributed, wave radiation stress term. When applied to the problem of waves incident on a beach with essentially zero turbulence momentum mixing, velocities are very large and the simple balance between elevation and radiation stress gradients no longer prevails. However, when turbulence mixing is reinstated, the vertically dependent radiation stresses produce vertical velocity gradients which then produce turbulent mixing; as a consequence, velocities are reduced, but are still larger by an order of magnitude compared to Stokes drift. Nevertheless, the velocity reduction is sufficient so that elevation set-down obtained from a balance between elevation gradient and radiation stress gradients is nearly coincident with that obtained by the aforementioned papers. This paper includes four appendices. The first appendix demonstrates the numerical process by which Stokes drift is excluded from the turbulence stress parameterization in the momentum equation. A second appendix determines a bottom slope criterion for the application of linear wave relations to the derivation of the wave radiation stress. The third appendix explores the possibility of generalizing results by non-dimensionalization. The final appendix applies the basic theory to a problem introduced by Bennis and Ardhuin (J Phys Oceanogr 41:2008–2012, 2011).  相似文献   

20.
This study investigates how Mattituck Sill influences circulation patterns and physical dynamics in eastern Long Island Sound, a major estuary on the U.S. east coast. Observations show there is pronounced across-estuary transport in the area and suggest there may be subtidal anticyclonic flow around the sill. Model runs, with and without sill bathymetry, exhibit this across-estuary transport and anticyclonic circulation. Comparison between these runs indicates that the sill intensifies the anticyclonic circulation. This study finds the sill does not exert internal hydraulic control during neap, mean, or spring tidal conditions. Nevertheless, along-estuary exchange is reduced over the sill and across-estuary fluxes are increased. The Connecticut River plume enters close to the estuary mouth. The sill deflects more of the plume waters towards the mouth, causing less freshwater to take the long looping route through the estuary. The subtidal circulation balance around the sill indicates a barotropic balance between the tidal advection of tidal vorticity and friction. The subtidal vorticity balance indicates the net effect of tidal advection of relative vorticity is balanced with frictional curl associated with lateral speed gradients and vorticity dissipation. Previously developed scalings based on the circulation balance (Nature 290:549–555, 1981), frictional vorticity generation mechanisms (Deep-Sea Res 28:195–212, 1981), and tidal diffusion of potential vorticity (J Phys Oceanogr 29:821–827, 1999) are applicable to Mattituck Sill and predict circulation with a similar magnitudes to model results.  相似文献   

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