scholarly journals Automated detection and association of surface waves

1994 ◽  
Vol 37 (3) ◽  
Author(s):  
R. G. North ◽  
C. R. D. Woodgold
Keyword(s):  
Surface Waves ◽  
Group Velocity ◽  
Rayleigh Waves ◽  
Arrival Time ◽  
Narrow Band ◽  
Broad Band ◽  
Detection Algorithm ◽  
Automated Detection ◽  
Short Period ◽  
Group Velocities

An algorithm for the automatic detection and association of surface waves has been developed and tested over an 18 month interval on broad band data from the Yellowknife array (YKA). The detection algorithm uses a conventional STA/LTA scheme on data that have been narrow band filtered at 20 s periods and a test is then applied to identify dispersion. An average of 9 surface waves are detected daily using this technique. Beamforming is applied to determine the arrival azimuth; at a nonarray station this could be provided by poIarization analysis. The detected surface waves are associated daily with the events located by the short period array at Yellowknife, and later with the events listed in the USGS NEIC Monthly Summaries. Association requires matching both arrival time and azimuth of the Rayleigh waves. Regional calibration of group velocity and azimuth is required. . Large variations in both group velocity and azimuth corrections were found, as an example, signals from events in Fiji Tonga arrive with apparent group velocities of 2.9 3.5 krn/s and azimuths from 5 to + 40 degrees clockwise from true (great circle) azimuth, whereas signals from Kuriles Kamchatka have velocities of 2.4 2.9 km/s and azimuths off by 35 to 0 degrees. After applying the regional corrections, surface waves are considered associated if the arrival time matches to within 0.25 km/s in apparent group velocity and the azimuth is within 30 degrees of the median expected. Over the 18 month period studied, 32% of the automatically detected surface waves were associated with events located by the Yellowknife short period array, and 34% (1591) with NEIC events; there is about 70% overlap between the two sets of events. Had the automatic detections been reported to the USGS, YKA would have ranked second (after LZH) in terms of numbers of associated surface waves for the study period of April 1991 to September 1992.

An investigation of discriminants for events in Western USSR based on regional phases recorded at station Kabul

1981 ◽  
Vol 71 (1) ◽  
pp. 263-274
Author(s):  
Indra N. Gupta ◽  
J. A. Burnetti
Keyword(s):  
Group Velocity ◽  
Rayleigh Waves ◽  
Amplitude Ratio ◽  
Vertical Component ◽  
Maximum Amplitude ◽  
Amplitude Ratios ◽  
Nuclear Explosions ◽  
Short Period ◽  
Regional Phases ◽  
Group Velocities

abstract Short-period, vertical-component records at the WWSSN station, Kabul for 13 earthquakes and 8 nuclear explosions occurring within a region of efficient propagation of Lg are examined to explore the possibility of using ratios of amplitudes in different group velocity windows as a discriminant. Each seismogram is divided into 10 windows with boundaries representing Pn and group velocities of 6.0, 5.0, 4.5, 4.0, 3.8, 3.6, 3.4, 3.2, 3.0, and 2.8 km/sec. The first three windows include crustal phases Pn, Pg, and Sn, respectively, whereas the six windows from 4.0 to 2.8 km/sec encompass the expected group velocity of higher mode Rayleigh waves that include Lg. The ratio of the maximum amplitude before the arrival of Sn to the maximum amplitude thereafter is significantly larger for explosions than for earthquakes and provides the largest separation between earthquake and explosion populations. Considerable separation is also shown by amplitude ratios Pn/Lg and Pn/Pg. Amplitude ratios based on earlier- and later-arriving Lg phases and the amplitude ratio Sn/Lg show insignificant discrimination. The ratio (maximum before Sn)/(maximum after Sn) is expected to be a useful regional discriminant for events in the Western USSR.

Errors Introduced in Estimation of Surface Wave Phase and Group Velocities Due to Wrong Assumptions: An Assessment Using a Simple Model For Love Wave

2021 ◽  
Author(s):  
Akash Kharita ◽  
Sagarika Mukhopadhyay
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Love Wave ◽  
Arrival Time ◽  
Epicentral Distance ◽  
Horizontal Distance ◽  
Wave Analysis ◽  
Time Period ◽  
Phase And Group Velocities ◽  
Group Velocities

<p>The surface wave phase and group velocities are estimated by dividing the epicentral distance by phase and group travel times respectively in all the available methods, this is based on the assumptions that (1) surface waves originate at the epicentre and (2) the travel time of the particular group or phase of the surface wave is equal to its arrival time to the station minus the origin time of the causative earthquake; However, both assumptions are wrong since surface waves generate at some horizontal distance away from the epicentre. We calculated the actual horizontal distance from the focus at which they generate and assessed the errors caused in the estimation of group and phase velocities by the aforementioned assumptions in a simple isotropic single layered homogeneous half space crustal model using the example of the fundamental mode Love wave. We took the receiver locations in the epicentral distance range of 100-1000 km, as used in the regional surface wave analysis, varied the source depth from 0 to 35 Km with a step size of 5 km and did the forward modelling to calculate the arrival time of Love wave phases at each receiver location. The phase and group velocities are then estimated using the above assumptions and are compared with the actual values of the velocities given by Love wave dispersion equation. We observed that the velocities are underestimated and the errors are found to be; decreasing linearly with focal depth, decreasing inversely with the epicentral distance and increasing parabolically with the time period. We also derived empirical formulas using MATLAB curve fitting toolbox that will give percentage errors for any realistic combination of epicentral distance, time period and depths of earthquake and thickness of layer in this model. The errors are found to be more than 5% for all epicentral distances lesser than 500 km, for all focal depths and time periods indicating that it is not safe to do regional surface wave analysis for epicentral distances lesser than 500 km without incurring significant errors. To the best of our knowledge, the study is first of its kind in assessing such errors.</p>

Short-period seismic noise

1967 ◽  
Vol 57 (1) ◽  
pp. 55-81
Author(s):  
E. J. Douze
Keyword(s):  
Surface Waves ◽  
Rayleigh Waves ◽  
Seismic Noise ◽  
Body Waves ◽  
Deep Hole ◽  
Period Range ◽  
Short Period ◽  
The Subject ◽  
Hole Arrays

abstract This report consists of a summary of the studies conducted on the subject of short-period (6.0-0.3 sec period) noise over a period of approximately three years. Information from deep-hole and surface arrays was used in an attempt to determine the types of waves of which the noise is composed. The theoretical behavior of higher-mode Rayleigh waves and of body waves as measured by surface and deep-hole arrays is described. Both surface and body waves are shown to exist in the noise. Surface waves generally predominate at the longer periods (of the period range discussed) while body waves appear at the shorter periods at quiet sites. Not all the data could be interpreted to define the wave types present.

The dispersion of surface waves on multilayered media*

1953 ◽  
Vol 43 (1) ◽  
pp. 17-34 ◽  
Author(s):  
N. A. Haskell
Keyword(s):  
Surface Waves ◽  
Rayleigh Waves ◽  
Matrix Formalism ◽  
Multilayered Media ◽  
Solid Media ◽  
Phase Velocity Dispersion ◽  
Phase And Group Velocities ◽  
Horizontal Heterogeneity ◽  
The Difference ◽  
Group Velocities

abstract A matrix formalism developed by W. T. Thomson is used to obtain the phase velocity dispersion equations for elastic surface waves of Rayleigh and Love type on multilayered solid media. The method is used to compute phase and group velocities of Rayleigh waves for two assumed three-layer models and one two-layer model of the earth's crust in the continents. The computed group velocity curves are compared with published values of the group velocities at various frequencies of Rayleigh waves over continental paths. The scatter of the observed values is larger than the difference between the three computed curves. It is believed that not all of this scatter is due to observational errors, but probably represents a real horizontal heterogeneity of the continental crusts.

scholarly journals Crustal structure of central Myanmar (Burma) By surface wave dispersion

MAUSAM ◽  
2022 ◽  
Vol 44 (4) ◽  
pp. 347-352
Author(s):  
S. N. BHATTACHARYA
Keyword(s):  
Surface Waves ◽  
Fundamental Mode ◽  
Rayleigh Waves ◽  
Seismic Waves ◽  
Digital Data ◽  
Low Velocity ◽  
Surface Wave Dispersion ◽  
Gangetic Plain ◽  
Indo Gangetic Plain ◽  
Group Velocities

Digital records of seismic waves observed at Seismic Research Observatory, Cheng Mai. Thailand have been analysed for two earthquakes in western Nepal. Digital data are processed by the floating filter and phase equalization methods to obtain surface waves free from noise. Group velocities of Love and Rayleigh waves are obtained by frequency time analysis of these noise free surface waves. The period of group velocities ranges from 17 to 62 sec for fundamental mode Rayleigh waves and from 17 to 66 sec for fundamental mode Love waves. The wave paths cross both central Myanmar (Burma) and the Indo-Gangetic plain. The group velocity data of surface waves across central Myanmar (Burma) have been obtained after correction of the data for the path across the Indo-Gangetic plain. Inversion of data gives the average crustal and subcrustal structure of central Myanmar (Burma). The modelled structure shows two separate sedimentary layers each of  8 km thick, The lower sedimentary layer forms the low velocity zone of the crust. The total thickness of central Myanmar (Burma) crust is found to be 55 km

scholarly journals Worldwide distribution of group velocity of mantle Rayleigh waves as determined by spherical harmonic inversion

1982 ◽  
Vol 72 (4) ◽  
pp. 1185-1194 ◽  
Author(s):  
Ichiro Nakanishi ◽  
Don L. Anderson
Keyword(s):  
Group Velocity ◽  
Rayleigh Waves ◽  
Least Squares Method ◽  
Active Regions ◽  
Northwestern Pacific ◽  
Regional Studies ◽  
Worldwide Distribution ◽  
Tectonically Active ◽  
Harmonic Inversion ◽  
Group Velocities

abstract We have determined the worldwide distribution of group velocity of mantle Rayleigh waves for periods between 100 and 300 sec without assuming any regionalization. Group slowness 1/u(θ, φ) is expressed by spherical harmonics, and the coefficients, up to angular order 7, have been determined from travel times of Rayleigh waves by a least-squares method. From these, u(θ, φ) has been synthesized. Since we cannot obtain information about the odd terms of the expansion from one circuit measurements around the world, we have used group velocities of mainly R2 and R3. The overall pattern of u(θ, φ) for periods between 100 and 200 sec is consistent with results of previous pure-path and regional studies. Group velocities for tectonically active regions are low, and those of the shields and the northwestern Pacific are high.

scholarly journals Imaging Seismic Wave-Fields with AlpArray and Neighboring European Networks

2020 ◽  
Author(s):  
Marcel Tesch ◽  
Johannes Stampa ◽  
Thomas Meier ◽  
Edi Kissling ◽  
György Hetényi ◽  
...  
Keyword(s):  
Surface Waves ◽  
Wave Field ◽  
Seismic Wave ◽  
Time Window ◽  
Deep Structure ◽  
Body Wave ◽  
Broad Band ◽  
Wave Fields ◽  
Short Period ◽  
Spatio Temporal

Abstract. The modern-day coverage and availability of broad-band stations in the greater Alpine area offered by AlpArray, Swath-D and the European seismological networks allows for imaging seismic wave-fields at yet unprecedented resolution. In the AlpArray area and in Italy, the distance of any point to the nearest station is less than 30 km, resulting in an average inter-station distance of about 45 km. With a much denser deployment in a smaller region of the Alps (320 km in length and 140 km wide), the Swath-D network possesses an average inter-station distance of about 15 km. We provide single event seismogram sections, time slices of teleseismic and regional wave-fields, and wave-field animations to reveal both the resolution capabilities of this dense station distribution as well as the enormous spatio-temporal complexity of seismic wave propagation. The time slices and wave-field animations demonstrate the need for dense regional arrays of broad-band stations, such as provided by AlpArray and neighboring networks, to resolve properties of teleseismic wave-fields. Here we present the images of coherent arrivals of direct body and surface waves, multiple body wave reflections, and multi-orbit phases for teleseismic and regional events with moment magnitudes larger than 6 over a time window of at least 2:45 hours. Spatial observations of the wave-fields illustrate e.g. the decrease in horizontal wavelength from P to S to surface waves and the way in which they considerably deviate from plane waves, due to heterogeneous earth structures along the path from the source to the array and beneath the regional array itself. Tomographic imaging techniques for the deep structure beneath the regional array have to take this spatio-temporal variability into account and correct for it. The lateral resolution of the regional broad-band array is however dependent on station density, in this case limited to about 100 km. Only even denser station distributions like those provided by Swath-D suffice to recover wave-fields of short period body and surface waves.

The effect of oceanic sediments on surface-wave propagation

1975 ◽  
Vol 65 (6) ◽  
pp. 1531-1552
Author(s):  
Donald J. Weidner
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Rayleigh Waves ◽  
Propagation Path ◽  
Surface Wave Propagation ◽  
Oceanic Surface ◽  
Oceanic Sediments ◽  
Wave Phase Velocity ◽  
Short Period ◽  
Mid Atlantic Ridge

abstract Several characteristics of oceanic surface waves can be altered by the presence of low rigidity sediments along the propagation path. Love and Rayleigh waves from mid-Atlantic ridge earthquakes bear many effects of oceanic sediments. The general absence of these surface waves for periods shorter than about 15 sec can be attributed to either attenuation or scattering in thin sediments. Thin sediments also disperse short-period Love waves. Sediments whose thickness exceeds about 2 km are responsible for removing surface-wave energy with periods up to 40 sec. These sediments also alter the particle motion of Rayleigh waves and are responsible for a complicated dispersion relation. These thick sediments substantially reduce the surface-wave phase velocity at periods in excess of 100 sec.

scholarly journals Group Velocity Measurements of Earthquake Rayleigh Wave by S Transform and Comparison with MFT

2020 ◽  
Vol 24 (1) ◽  
pp. 91-95
Author(s):  
Chanjun Jiang ◽  
Youxue Wang ◽  
Gaofu Zeng
Keyword(s):  
Rayleigh Wave ◽  
Group Velocity ◽  
Rayleigh Waves ◽  
Accurate Method ◽  
Gaussian Filter ◽  
Velocity Measurements ◽  
Filter Parameter ◽  
S Transform ◽  
Real Earthquake ◽  
Group Velocities

Based upon the synthetic Rayleigh wave at different epicentral distances and real earthquake Rayleigh wave, S transform is used to measure their group velocities, compared with the Multiple Filter Technique (MFT) which is the most commonly used method for group-velocity measurements. When the period is greater than 15 s, especially than 40 s, S transform has higher accuracy than MFT at all epicenter distances. When the period is less than or equal to 15 s, the accuracy of S transform is lower than that of MFT at epicentral distances of 1000 km and 8000 km (especially 8000 km), and the accuracy of such two methods is similar at the other epicentral distances. On the whole, S transform is more accurate than MFT. Furthermore, MFT is dominantly dependent on the value of the Gaussian filter parameter α, but S transform is self-adaptive. Therefore, S transform is a more stable and accurate method than MFT for group velocity measurement of earthquake Rayleigh waves.

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