Characterization of global seismograms using an automatic-picking algorithm

1994 ◽  
Vol 84 (2) ◽  
pp. 366-376
Author(s):  
Paul S. Earle ◽  
Peter M. Shearer
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Low Frequency ◽  
Travel Times ◽  
Comparable Amount ◽  
Arrival Times ◽  
Long Period ◽  
Automatic Phase ◽  
Short Period ◽  
Sample Rate

Abstract An automatic phase picker is useful for quickly identifying and timing phase arrivals in large seismic data bases. We have developed an automatic phase picker that is sensitive to small changes in amplitude and applied it to over 7 yr of global data distributed by the National Earthquake Information Center (NEIC). Our phase-picking algorithm is based on a short-term-average to long-term-average ratio (STA/LTA) taken along an envelope function generated from the seismogram. The algorithm returns arrival times and corresponding pick qualities. The procedure requires few input parameters and is easily adapted to various types of data. We produce global travel-time plots from both high-frequency (20- or 40-Hz sample rate) and low-frequency (1-Hz sample rate) data. These plots clearly image the predominant high- and low-frequency phases in the NEIC data base. Picks made from the long-period seismograms are less precise, but they reveal far more phase arrivals than the short-period picks. A number of phases resulting from reflections and phase conversions at upper mantle discontinuities can be identified in the low-frequency picks; however, a search of the short-period picks for upper mantle discontinuity phases, between P and PP and prior to P′P′, has so far been unsuccessful. In the long-period S and SS picks, we observe a discrepancy in SV and SH travel times, a possible result of upper mantle anisotropy. To check the accuracy and consistency of our algorithm, we present comparisons between hand-picked times and automatic-picked times for identical seismograms. Travel-time residuals from the short-period automatic picks and data reported to the International Seismological Centre (ISC) picks exhibit a comparable amount of scatter. Histograms of the ISC residuals and automatic-pick residuals are similar in shape and width for P and PcP. These observations suggest that human picking errors are not a major contributor to the scatter observed in ISC travel times, although direct comparisons between ISC reported picks and automatic picks on particular seismograms occasionally identify operator mispicks.

scholarly journals Body-wave amplitude and travel-time correlations across North America

1983 ◽  
Vol 73 (4) ◽  
pp. 1063-1076
Author(s):  
Thorne Lay ◽  
Donald V. Helmberger
Keyword(s):  
North America ◽  
Travel Time ◽  
Body Wave ◽  
Rocky Mountain ◽  
S Wave ◽  
Arrival Times ◽  
S Waves ◽  
Long Period ◽  
Amplitude Data ◽  
Short Period

abstract Relationships between travel-time and amplitude station anomalies are examined for short- and long-period SH waves and short-period P waves recorded at North American WWSSN and Canadian Seismic Network stations. Data for two azimuths of approach to North America are analyzed. To facilitate intercomparison of the data, the S-wave travel times and amplitudes are measured from the same records, and the amplitude data processing is similar for both P and S waves. Short-period P- and S-wave amplitudes have similar regional variations, being relatively low in the western tectonic region and enhanced in the shield and mid-continental regions. The east coast has intermediate amplitude anomalies and systematic, large azimuthal travel-time variations. There is a general correlation between diminished short-period amplitudes and late S-wave arrival times, and enhanced amplitudes and early arrivals. However, this correlation is not obvious within the eastern and western provinces separately, and the data are consistent with a step-like shift in amplitude level across the Rocky Mountain front. Long-period S waves show no overall correlation between amplitude and travel-time anomalies.

Intensities and travel times of seismic rays in a sphere with a linear velocity function

1977 ◽  
Vol 67 (1) ◽  
pp. 33-42
Author(s):  
Mark E. Odegard ◽  
Gerard J. Fryer
Keyword(s):  
Velocity Profile ◽  
Travel Time ◽  
Upper Mantle ◽  
Intensity Ratio ◽  
Computation Time ◽  
Spherical Body ◽  
Travel Times ◽  
Velocity Function ◽  
Intensity Ratios ◽  
Distance Travel

Abstract Equations are presented which permit the calculation of distances, travel times and intensity ratios of seismic rays propagating through a spherical body with concentric layers having velocities which vary linearly with radius. In addition, a method is described which removes the infinite singularities in amplitude generated by second-order discontinuities in the velocity profile. Numerical calculations involving a reasonable upper mantle model show that the standard deviations of the errors for distance, travel time and intensity ratio are 0.0046°, 0.057 sec, and 0.04 dB, respectively. Computation time is short.

Shear velocity from differential travel times of short-period ScS-P in New Hebrides, Fiji-Tonga, and Banda Sea regions

1975 ◽  
Vol 65 (6) ◽  
pp. 1787-1796
Author(s):  
Mansur A. Choudhury ◽  
Georges Poupinet ◽  
Guy Perrier
Keyword(s):  
Upper Mantle ◽  
Focal Depth ◽  
Shear Velocity ◽  
Mean Value ◽  
Travel Times ◽  
Velocity Models ◽  
Depth Dependence ◽  
Short Period ◽  
New Hebrides ◽  
The Antarctic

abstract Behavior of P, S and ScS residuals as well as those of differential travel times of ScS-P from the Jeffreys-Bullen tables are analyzed. The phases have been read from short-period records of the Antarctic station, Dumont d'Urville (DRV); the earthquakes originating in New Hebrides, Fiji-Tonga, and Banda Sea regions. P residuals from all regions show a mean value of about −1 sec. On the contrary, S and ScS residuals, well correlated among themselves, show important regional as well as focal-depth dependence. ScS-P residuals from shallow and intermediate shocks are largely positive for New Hebrides and largely negative for Banda Sea; those from intermediate shocks are moderately positive for Fiji-Tonga. The anomalies disappear at depths greater than about 200 km. Upper mantle shear velocity models are presented for the three regions. The models are discussed in relation to a sinking lithosphere.

scholarly journals Relative amplitudes of P and S waves as a mantle reconnaissance tool

1969 ◽  
Vol 59 (3) ◽  
pp. 1189-1200
Author(s):  
John R. McGinley ◽  
Don L. Anderson
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Poisson’S Ratio ◽  
Basin And Range ◽  
The Other ◽  
Poisson's Ratio ◽  
P Waves ◽  
S Waves ◽  
High Attenuation ◽  
Short Period

abstract The unified magnitude, the ratio of the amiplitudes of S to P waves, and travel-time residuals were compiled from published data for the five Seismological Observatories, TFO, UBO, BMO, WMO and CBO. Using one of the stations as a reference, a relative measure of the above quantities was calculated for each of the other stations for each of a number of earthquakes. The stations in the Basin and Range Province are consistent with a markedly higher attentuation of P waves and a high attenuation of S relative to P when compared to the other stations. This latter observation indicates a high Poisson's ratio in the mantle under the Basin and Range. The delay times to these stations are also consistent with the high Poisson's ratio and with a low-velocity upper mantle. The ratio of the amplitudes of long-period S waves to short-period P waves varies by a factor of 4 among these stations. BMO, in eastern Oregon, has a high S/P amplitude ratio compared to other stations and a travel-time residual that is comparable to the observatories in the mid-continent. This may be another example of a seismic “window” into the upper mantle that is generated by underthrusting of the oceanic lithosphere.

Teleseismic P-wave travel time tomography of the Alpine upper mantle using AlpArray seismic network data

2020 ◽  
Author(s):  
Marcel Paffrath ◽  
Wolfgang Friederich ◽  
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
High Velocity ◽  
Eastern Alps ◽  
Alpine Region ◽  
Seismic Network ◽  
P Wave ◽  
Travel Times ◽  
Travel Time Tomography ◽  
Wave Travel

<p>We perform a teleseismic P-wave travel time tomography to examine geometry and slab structure of the upper mantle beneath the Alpine orogen. Vertical component data of the extraordinary dense seismic network AlpArray are used which were recorded at over 600 temporary and permanent broadband stations deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Mantle phases of 347 teleseismic events between 2015 and 2019 of magnitude 5.5 and higher are evaluated automatically for direct and core diffracted P arrivals using a combination of higher-order statistics picking algorithms and signal cross correlation. The resulting database contains over 170.000 highly accurate absolute P picks that were manually revised for each event. The travel time residuals exhibit very consistent and reproducible spatial patterns, already pointing at high velocity slabs in the mantle.</p><p>For predicting P-wave travel times, we consider a large computational box encompassing the Alpine region up to a depth of 600 km within which we allow 3D-variations of P-wave velocity. Outside this box we assume a spherically symmetric earth and apply the Tau-P method to calculate travel times and ray paths. These are injected at the boundaries of the regional box and continued using the fast marching method. We invert differences between observed and predicted travel times for P-wave velocities inside the box. Velocity is discretized on a regular grid with an average spacing of about 25 km. The misfit reduction reaches values of up to 75% depending on damping and smoothing parameters.</p><p>The resulting model shows several steeply dipping high velocity anomalies following the Alpine arc. The most prominent structure stretches from the western Alps into the Apennines mountain range reaching depths of over 500 km. Two further anomalies extending down to a depth of 300 km are located below the central and eastern Alps, separated by a clear gap below the western part of the Tauern window. Further to the east the model indicates a possible high-velocity connection between the eastern Alps and the Dinarides. Regarding the lateral position of the central and eastern Alpine slabs, our results confirm previous studies. However, there are differences regarding depth extent, dip angles and dip directions. Both structures dip very steeply with a tendency towards northward dipping. We perform various general, as well as purpose-built resolution tests, to verify the capabilities of our setup to resolve slab gaps as well as different possible slab dipping directions.</p>

On the prediction of low‐frequency geomagnetic pulsations for geophysical prospecting

Geophysics ◽  
1989 ◽  
Vol 54 (5) ◽  
pp. 635-642 ◽  
Author(s):  
Hing‐Lan Lam
Keyword(s):  
Background Noise ◽  
Low Frequency ◽  
Field Work ◽  
Magnetic Activity ◽  
Time Of Day ◽  
Virtual Absence ◽  
Long Period ◽  
The Earth ◽  
Short Period ◽  
Geomagnetic Soundings

Pulsations of the earth’s magnetic field due to solar‐terrestrial interaction are used for geomagnetic soundings to study conductive structures in the earth. However, the pulsations are also the unwanted background noise for magnetic exploration surveys. It is, therefore, desirable to conduct the sounding or survey work during geomagnetic conditions optimum for that particular undertaking. This paper relates the pulsational activity to a parameter (the DRX index) which characterizes the magnetic activity for each individual day and which is routinely forecast in Canada. By using the relations established in this study, it is possible to infer the future levels of pulsations from the forecast DRX, and a period of desired pulsational activity can thus be selected for the field work. In the course of a day, the afternoon is appropriate for conducting magnetic surveys because of a generally lower level of pulsational activity and a virtual absence of intense short‐period pulsations, while morning is optimal for carrying out induction soundings because of more intense pulsational activity and a tendency of enhanced long‐period pulsations to recur during that time of day.

Teleseismic P-wave travel time tomography of the Alpine upper mantle using AlpArray seismic network data

2021 ◽  
Author(s):  
Marcel Paffrath ◽  
Wolfgang Friederich ◽  
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
High Velocity ◽  
Alpine Region ◽  
Seismic Network ◽  
P Wave ◽  
Travel Times ◽  
Massif Central ◽  
Travel Time Tomography ◽  
Wave Travel

<p>We perform a teleseismic P-wave travel time tomography to examine geometry and slab structure of the upper mantle beneath the Alpine orogen. Vertical component data of the extraordinary dense seismic network AlpArray are used which were recorded at over 600 temporary and permanent broadband stations deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Mantle phases of 370 teleseismic events between 2015 and 2019 of magnitude 5.5 and higher are evaluated automatically for direct and core diffracted P arrivals using a combination of higher-order statistics picking algorithms and signal cross correlation. The resulting database contains over 170.000 highly accurate absolute P picks that were manually revised for each event. The travel time residuals exhibit very consistent and reproducible spatial patterns, already pointing at high velocity slabs in the mantle.</p><p>For predicting P-wave travel times we consider a large computational box encompassing the Alpine region up to a depth of 600 km within which we allow 3D-variations of P-wave velocity. To account for influences of the strongly heterogeneous crust that cannot be resolved with teleseismic data, we integrate a complex three-dimensional crustal model directly into our model. Outside the box we assume a spherically symmetric earth and apply the Tau-P method to calculate travel times and ray paths. These are injected at the boundaries of the regional box and continued using the fast marching method (Rawlinson et al. 2005). We invert differences between observed and predicted traveltimes for P-wave velocities inside the box. Velocity is discretized on a regular grid with a spacing of about 25x25x15 km. The misfit reduction reaches values of over 80% depending on damping and smoothing parameters.</p><p>The resulting model shows several steeply dipping high velocity anomalies following the Alpine arc. The most prominent structure stretches from the western Alps into the Apennines mountain range reaching depths of over 500 km. Two further anomalies of high complexity extending down to a depth of 300 km are located below the central and eastern Alps, both being detached from the lithosphere and separated by a clear gap below the western part of the Tauern window. The central anomaly shows mainly southwards dipping, whereas the eastern anomaly is mainly dipping to the northeast. We compare our results to former studies, confirming lateral positions of the anomalies. However, the new results can benefit from the superior resolution capabilities of the dense AlpArray seismic network, providing more accurate insights into depth extent, dip angle and directions. We perform various general, as well as purpose-built resolution tests, to verify the capabilities of our setup to resolve slab gaps as well as different possible slab dipping directions.</p>

scholarly journals Imaging the Mediterranean upper mantle by p- wave travel time tomography

1997 ◽  
Vol 40 (4) ◽  
Author(s):  
C. Piromallo ◽  
A. Morelli
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Three Dimensional ◽  
Velocity Model ◽  
Structural Heterogeneity ◽  
Linear Interpolation ◽  
P Wave ◽  
Travel Times ◽  
P Waves ◽  
Tectonic Lineament

Travel times of P-waves in the Euro-Mediterranean region show strong and consistent lateral variations, which can be associated to structural heterogeneity in the underlying crust and mantle. We analyze regional and tele- seismic data from the International Seismological Centre data base to construct a three-dimensional velocity model of the upper mantle. We parameterize the model by a 3D grid of nodes -with approximately 50 km spacing -with a linear interpolation law, which constitutes a three-dimensional continuous representation of P-wave velocity. We construct summary travel time residuals between pairs of cells of the Earth's surface, both inside our study area and -with a broader spacing -on the whole globe. We account for lower mantle heterogeneity outside the modeled region by using empirical corrections to teleseismic travel times. The tomo- graphic images show generai agreement with other seismological studies of this area, with apparently higher detail attained in some locations. The signature of past and present lithospheric subduction, connected to Euro- African convergence, is a prominent feature. Active subduction under the Tyrrhenian and Hellenic arcs is clearly imaged as high-velocity bodies spanning the whole upper mantle. A clear variation of the lithospheric structure beneath the Northem and Southern Apennines is observed, with the boundary running in correspon- dence of the Ortona-Roccamonfina tectonic lineament. The western section of the Alps appears to have better developed roots than the eastern, possibly reflecting à difference in past subduction of the Tethyan lithosphere and subsequent continental collision.

The identification and interpretation of upper mantle travel-time branches from measurements of dT/dΔ made on data recorded at the Yellowknife seismic array

1978 ◽  
Vol 15 (2) ◽  
pp. 227-236 ◽  
Author(s):  
A. Ram ◽  
R. F. Mereu ◽  
D. H. Weichert
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Body Wave ◽  
Processing Technique ◽  
The Body ◽  
Seismic Array ◽  
P Wave ◽  
Time Curve ◽  
Transition Zones ◽  
Short Period

There is broad agreement among various seismological studies that the upper mantle has two regions where very high positive velocity gradients or transition zones exist. In most cases, the presence of these zones implies that two major triplications are likely to exist in the body-wave travel-time curve for distances less than 30°. Because of the difficulties in observing and identifying later arrivals belonging to the various travel-time branches, the inversion of the seismic data is often very difficult. In this paper an adaptive processing technique was employed to examine the variations in slowness that occur along the first 36 s of the short-period P-wave trains recorded at the Yellowknife medium aperture seismic array. Over 100 earthquakes from the Alaska Peninsula and California regions were selected. From the California results we were able to clearly observe the 12–13 s/deg slowness branch as a later arrival out to distances as great as 26°. Other later arrival branches as well as cusps associated with the 400 and 650 km discontinuities were not well defined even though the cross-over point as determined from slowness measurements on first arrivals were clearly located. An inversion of the data showed that the '650 km' transition zone occurred at a much shallower depth west of the array compared to the corresponding region to the south.

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