scholarly journals Broadband waveforms and site effects at a borehole seismometer in the Po alluvial basin (Italy)

2001 ◽  
Vol 44 (1) ◽  
Author(s):  
M. Cocco ◽  
F. Ardizzoni ◽  
R. M. Azzara ◽  
L. Dall'Olio ◽  
A. Delladio ◽  
...  
Keyword(s):  
Site Effects ◽  
Velocity Structure ◽  
Dynamic Range ◽  
Site Response ◽  
Seismic Exploration ◽  
Ratio Method ◽  
Data Set ◽  
Broadband Seismometer ◽  
Local Earthquakes ◽  
Teleseismic Events

Broadband seismograms recorded at a borehole three-component (high dynamic range) seismic station in the Po Valley (Northern Italy) were analyzed to study the velocity structure of the shallow sedimentary layers as well as the local site effects in soft sediments. The broadband borehole seismometer was installed at a depth of 135 m just below the quaternary basement, while a second digital broadband seismometer was installed in the same site at the Earth surface. The velocity structure in the shallower layers was determined both by means of cross-hole and up-hole measurements and by inverting seismic data recorded during a seismic exploration experiment.Velocity discontinuities are quite well related to the stratigraphy of the site. We are interested to record local earthquakes as well as regional and teleseismic events. The analyzed data set includes local, regional and teleseismic events, most of which were recorded during the seismic sequence that started on October 15, 1996, near Reggio Emilia 80 km away from the borehole site. The orientation of the borehole sensor is determined using the recordings of a teleseismic event and of some local earthquakes. The noise reduction for the borehole sensor is 2 decades in power spectral density at frequencies larger than 1.0 Hz. We studied the site amplification of the shallow alluvial layers by applying the spectral ratio method. We analyzed the spectral ratios of noise recorded by the surface and borehole seismometers as well as those from local earthquakes. We compared these observations with a theoretical model for the site response computed by the Haskell-Thomson method.

Methodology to identify the reference rock sites in regions of medium-to-high seismicity: an application in Central Italy

2020 ◽  
Vol 222 (3) ◽  
pp. 2053-2067 ◽  
Author(s):  
Giovanni Lanzano ◽  
Chiara Felicetta ◽  
Francesca Pacor ◽  
Daniele Spallarossa ◽  
Paola Traversa
Keyword(s):  
Ground Motion ◽  
Site Effects ◽  
Site Response ◽  
Central Italy ◽  
Large Data ◽  
Spectral Ratio ◽  
Data Set ◽  
Standard Spectral Ratio ◽  
Resonance Frequencies ◽  
The Impact

SUMMARY To evaluate the site response using both empirical approaches (e.g. standard spectral ratio, ground motion models (GMMs), generalized inversion techniques, etc.) and numerical 1-D/2-D analyses, the definition of the reference motion, that is the ground motion recorded at stations unaffected by site-effects due to topographic, stratigraphic or basin effects, is needed. The main objective of this work is to define a robust strategy to identify the seismic stations that can be considered as reference rock sites, using six proxies for the site response: three proxies are related to the analysis of geophysical and seismological data (the repeatable site term from the residual analysis, the resonance frequencies from horizontal-to-vertical spectral ratios on noise or earthquake signals, the average shear wave velocity in the first 30 m); the remaining ones concern geomorphological and installation features (outcropping rocks or stiff soils, flat topography and absence of interaction with structures). We introduce a weighting scheme to take into account the availability and the quality of the site information, as well as the fulfillment of the criterion associated to each proxy. We also introduce a hierarchical index, to take into account the relevance of the proposed proxies in the description of the site effects, and an acceptance threshold for reference rock sites identification. The procedure is applied on a very large data set, composed by accelerometric and velocimetric waveforms, recorded in Central Italy in the period 2008–2018. This data set is composed by more than 30 000 waveforms relative to 450 earthquakes in the magnitude range 3.2–6.5 and recorded by more than 450 stations. A total of 36 out of 133 candidate stations are identified as reference sites: the majority of them are installed on rock with flat topography, but this condition is not sufficient to guarantee the absence of amplifications, especially at high frequencies. Seismological analyses are necessary to exclude stations affected by resonances. We test the impact of using these sites by calibrating a GMMs. The results show that for reference rock sites the median predictions are reduced down to about 45 per cent at short periods in comparison to the generic rock motions.

Modeling of Empirical Transfer Functions with 3D Velocity Structure

2021 ◽  
Author(s):  
Zhifeng Hu ◽  
Daniel Roten ◽  
Kim B. Olsen ◽  
Steven M. Day
Keyword(s):  
Material Properties ◽  
Site Effects ◽  
Velocity Structure ◽  
Transfer Functions ◽  
Site Response ◽  
Hazard Analysis ◽  
Earthquake Hazard ◽  
Site Effect ◽  
3D Models ◽  
Site Amplification

ABSTRACT Empirical transfer functions (ETFs) between seismic records observed at the surface and depth represent a powerful tool to estimate site effects for earthquake hazard analysis. However, conventional modeling of site amplification, with assumptions of horizontally polarized shear waves propagating vertically through 1D layered homogeneous media, often poorly predicts the ETFs, particularly, in which large lateral variations of velocity are present. Here, we test whether more accurate site effects can be obtained from theoretical transfer functions (TTFs) extracted from physics-based simulations that naturally incorporate the complex material properties. We select two well-documented downhole sites (the KiK-net site TKCH05 in Japan and the Garner Valley site, Garner Valley Downhole Array, in southern California) for our study. The 3D subsurface geometry at the two sites is estimated by means of the surface topography near the sites and information from the shear-wave profiles obtained from borehole logs. By comparing the TTFs to ETFs at the selected sites, we show how simulations using the calibrated 3D models can significantly improve site amplification estimates as compared to 1D model predictions. The primary reason for this improvement in 3D models is redirection of scattering from vertically propagating to more realistic obliquely propagating waves, which alleviates artificial amplification at nodes in the vertical-incidence response of corresponding 1D approximations, resulting in improvement of site effect estimation. The results demonstrate the importance of reliable calibration of subsurface structure and material properties in site response studies.

Seismic site response in the greater Vancouver, British Columbia, area: spectral ratios from moderate earthquakes

1999 ◽  
Vol 36 (2) ◽  
pp. 195-209 ◽  
Author(s):  
John F Cassidy ◽  
Garry C Rogers
Keyword(s):  
Site Response ◽  
Strong Motion ◽  
River Delta ◽  
Data Sets ◽  
Fraser River ◽  
Spectral Ratios ◽  
Data Set ◽  
Seismic Site Response ◽  
Fraser River Delta ◽  
S Period

Three-component, digital recordings of two recent moderate earthquakes provide valuable new insight into the response to seismic shaking in the greater Vancouver area, particularly on the Fraser River delta. The 1996 M = 5.1 Duvall, Washington, earthquake (180 km southeast of Vancouver) triggered strong-motion seismographs at seven sites and the 1997 M = 4.3 Georgia Strait earthquake (37 km west of Vancouver) triggered instruments at 13 sites in the greater Vancouver area. The latter data set is especially important because it contains the first three-component recordings made on bedrock in greater Vancouver. Both data sets represent weak ground motion, with peak horizontal accelerations of 0.5-1.5% gravity (g) for the Duvall earthquake, and 0.2-2.4% g for the Georgia Strait earthquake. Using the method of spectral ratios, we estimate the site response for each of the strong-motion instrument soil sites. On the Fraser River delta amplification is observed over a relatively narrow frequency range of 1.5-4 Hz (0.25-0.67 s period), with peak amplification of 4-10 (relative to competent bedrock) for the thick soil delta centre sites, and about 7-11 for the delta edge sites. Relative to firm soil, the peak amplification ranges from 2 to 5 for the thick soil delta centre sites, and 2 to 6 for the delta edge sites. At higher frequencies, little or no amplification, and in many cases slight attenuation, is observed.Key words: seismic site response, Fraser delta, earthquakes.

Structure beneath Queen Charlotte Sound from seismic-refraction and gravity interpretations

1992 ◽  
Vol 29 (7) ◽  
pp. 1509-1529 ◽  
Author(s):  
Tianson Yuan ◽  
G. D. Spence ◽  
R. D. Hyndman
Keyword(s):  
Crustal Structure ◽  
Velocity Structure ◽  
Sedimentary Basin ◽  
Single Channel ◽  
Velocity Variation ◽  
Moho Depth ◽  
High Porosity ◽  
Upper Crust ◽  
Low Velocity ◽  
Data Set

A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.

An automatic phase picker for local and teleseismic events

1987 ◽  
Vol 77 (4) ◽  
pp. 1437-1445
Author(s):  
M. Baer ◽  
U. Kradolfer
Keyword(s):  
Arrival Time ◽  
Detection Algorithm ◽  
Dynamic Threshold ◽  
Data Set ◽  
Automatic Phase ◽  
Nonlinear Amplifier ◽  
First Motion ◽  
Transient Events ◽  
Teleseismic Events ◽  
Automatic Detection Algorithm

Abstract An automatic detection algorithm has been developed which is capable of time P-phase arrivals of both local and teleseismic earthquakes, but rejects noise bursts and transient events. For each signal trace, the envelope function is calculated and passed through a nonlinear amplifier. The resulting signal is then subjected to a statistical analysis to yield arrival time, first motion, and a measure of reliability to be placed on the P-arrival pick. An incorporated dynamic threshold lets the algorithm become very sensitive; thus, even weak signals are timed precisely. During an extended performance evaluation on a data set comprising 789 P phases of local events and 1857 P phases of teleseismic events picked by an analyst, the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases. The accuracy of the automatic picks was “ideal” (i.e., could not be improved by the analyst) for 60 per cent of the local events and 63 per cent of the teleseismic events.

scholarly journals Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography

1996 ◽  
Vol 39 (6) ◽  
Author(s):  
C. Chiarabba ◽  
A. Amato
Keyword(s):  
Velocity Structure ◽  
P Wave ◽  
Depth Interval ◽  
Low Velocity ◽  
Data Set ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
Minimum Number ◽  
Velocity Images ◽  
Lower Crustal

In this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.

scholarly journals Sensitivity Of Differential Item Functioning Detection Methods On National Mathematics Examination In North Sumatera Province, Indonesia

2019 ◽  
Vol 8 (5C) ◽  
pp. 1538-1549
Keyword(s):  
Differential Item Functioning ◽  
Likelihood Ratio ◽  
Classical Test Theory ◽  
Test Theory ◽  
Detection Methods ◽  
Ratio Method ◽  
Suitable Model ◽  
Data Set ◽  
Item Functioning ◽  
Mathematics Examination

The purpose of this study was to examine the differences in sensitivity of three methods: IRT-Likelihood Ratio (IRT-LR), Mantel-Haenszel (MH) and Logistics Regression (LR), in detecting gender differential item functioning (DIF) on National Mathematics Examination (Ujian Nasional: UN) for 2014/2015 academic year in North Sumatera Province of Indonesia. DIF item shows the unfairness. It advantages the test takers of certain groups and disadvantages other group test takers, in the case they have the same ability. The presence of DIF was reviewed in grouping by gender: men as reference groups (R) and women as focus groups (F). This study used the experimental method, 3x1 design, with one factor (i.e. method) with three treatments, in the form of 3 different DIF detection methods. There are 5 types of UN Mathematics Year 2015 packages (codes: 1107, 2207, 3307, 4407 and 5507). The 2207 package code was taken as the sample data, consisting of 5000 participants (3067 women, 1933 men; for 40 UN items). Item selection was carried out based on the classical test theory (CTT) on 40 UN items, producing 32 items that fulfilled, and item response theory selection (IRT) produced 18 items that fulfilled. With program R 3.333 and IRTLRDIF 2.0, it was found 5 items were detected as DIF by the IRT-Likelihood Ratio-method (IRTLR), 4 items were detected as DIF by the Logistic Regression method (LR), and 3 items were detected as DIF by the MantelHaenszel method (MH). To test the sensitivity of the three methods, it is not enough with just one time DIF detection, but formed six groups of data analysis: (4400,40),(4400,32), (4400,18), (3000,40), (3000,32), (3000,18), and generate 40 random data sets (without repetitions) in each group, and conduct detecting DIF on the items in each data set. Although the data lacks model fit, the 3 parameter logistic model (3PL) is chosen as the most suitable model. With the Tukey's HSD post hoc test, the IRT-LR method is known to be more sensitive than the MH and LR methods in the group (4400,40) and (3000,40). The IRT-LR method is not longer more sensitive than LR in the group (4400,32) and (3000,32), but still more sensitive than MH. In the groups (4400,18) and (3000,18) the IRT-LR method is more sensitive than LR, but not significantly more sensitive than MH. The LR method is consistently tested to be more sensitive than the MH method in the entire analysis groups.

Regional propagation characteristics and source parameters of earthquakes in northeastern North America

1994 ◽  
Vol 84 (1) ◽  
pp. 1-15 ◽  
Author(s):  
John Boatwright
Keyword(s):  
Site Response ◽  
Source Parameters ◽  
Low Frequency ◽  
Response Spectra ◽  
Spectral Shape ◽  
Propagation Characteristics ◽  
S Wave ◽  
Data Set ◽  
Wave Trains ◽  
Source Site

Abstract The vertical components of the S wave trains recorded on the Eastern Canadian Telemetered Network (ECTN) from 1980 through 1990 have been spectrally analyzed for source, site, and propagation characteristics. The data set comprises some 1033 recordings of 97 earthquakes whose magnitudes range from M ≈ 3 to 6. The epicentral distances range from 15 to 1000 km, with most of the data set recorded at distances from 200 to 800 km. The recorded S wave trains contain the phases S, SmS, Sn, and Lg and are sampled using windows that increase with distance; the acceleration spectra were analyzed from 1.0 to 10 Hz. To separate the source, site, and propagation characteristics, an inversion for the earthquake corner frequencies, low-frequency levels, and average attenuation parameters is alternated with a regression of residuals onto the set of stations and a grid of 14 distances ranging from 25 to 1000 km. The iteration between these two parts of the inversion converges in about 60 steps. The average attenuation parameters obtained from the inversion were Q = 1997 ± 10 and γ = 0.998 ± 0.003. The most pronounced variation from this average attenuation is a marked deamplification of more than a factor of 2 at 63 km and 2 Hz, which shallows with increasing frequency and increasing distance out to 200 km. The site-response spectra obtained for the ECTN stations are generally flat. The source spectral shape assumed in this inversion provides an adequate spectral model for the smaller events (Mo < 3 × 1021 dyne-cm) in the data set, whose Brune stress drops range from 5 to 150 bars. For the five events in the data set with Mo ≧ 1023 dyne-cm, however, the source spectra obtained by regressing the residuals suggest that an ω2 spectrum is an inadequate model for the spectral shape. In particular, the corner frequencies for most of these large events appear to be split, so that the spectra exhibit an intermediate behavior (where |ü(ω)| is roughly proportional to ω).

Urban DAS recording of a vibroseismic campaign with a 21km-long dark fibre in Potsdam, Germany

2021 ◽  
Author(s):  
CharLotte Krawczyk ◽  
Christopher Wollin ◽  
Stefan Lüth ◽  
Martin Lipus ◽  
Christian Cunow ◽  
...  
Keyword(s):  
Urban Areas ◽  
District Heating ◽  
Telecommunication Networks ◽  
Seismic Exploration ◽  
City Centre ◽  
Data Set ◽  
Geothermal Potential ◽  
First Results ◽  
Local Geology ◽  
The City

<p>The de-carbonization strategy of the city of Potsdam, Germany, incorporates the utilization of its geothermal potential.  As a first step of developing a deep geothermal project for district heating, an urban seismic exploration campaign of the Stadtwerke Potsdam took place in December 2020 in the city centre of Potsdam.  Since urban measurements are often difficult to setup and a low-footprint alternative is sought for, we supplemented the contractor-performed Vibroseis survey along three profiles by distributed acoustic sensing (DAS).  In close cooperation with the municipal utilities, we interrogated a 21 km-long dark telecommunication fibre whose trajectory followed the seismic lines as close as possible.  This was accompanied by a network of 15 three-component geophones for further control and research.</p><p>In this contribution we present the data set, the approach for geo-referencing the fibre, and first results regarding DAS recording capabilities of vibroseismic signals in an urban environment.  Following the paradigm that the high density of telecommunication networks in urban areas may facilitate the exploration of the often insufficiently known local geology, we strive to further shed light on the possibilities of their employment for urban exploration.  In this respect we aim at tackling the question of the accuracy of fibre localization, recording sensitivity and range of active stimulation.</p>

The response of the time-term method to simulated crustal structures

1979 ◽  
Vol 69 (5) ◽  
pp. 1455-1473
Author(s):  
D. N. Whitcombe ◽  
P. K. H. Maguire
Keyword(s):  
Velocity Structure ◽  
Seismic Refraction ◽  
Critical Distance ◽  
Structural Variations ◽  
Data Set ◽  
Term Model ◽  
Time Term ◽  
Seismic Refraction Data ◽  
Relationship Of ◽  
The Relationship

abstract The time-term method of interpreting seismic refraction data is analyzed to examine inadequacies in the chosen time-term model by relating observational errors to the solution variance. The results obtained from data that has been simulated for various structures are investigated. This is done quantitatively for simple structures and semi-quantitatively for more complex cases. Velocity and topographic variations of the refractor are considered as signals having dominant wavelengths. It is found that the response of the time-term method to these structural variations depends on the relationship of the structural wavelength to the dimensions of the experiment and the critical distance. For all but the simplest structures, the standard error estimates that can be obtained from a time-term solution are likely to be completely inadequate as estimates of the true error. It is demonstrated that if the refractor is anything other than uniform, the effects of a complicated velocity structure may be absorbed into the time terms. Similarly it is argued that in situations in which the refractor is not horizontal, erroneous values for complex velocity coefficients (e.g., gradient, anisotropy, etc.) can be obtained if these coefficients are included in the chosen time-term model. Finally, it is indicated that reduced travel times can be used in a way that removes the “stirring pot” aspect of time-term analysis, and to determine if a data set is suitable for examination by the time-term method.

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