WIDE‐BAND EXTRACTION OF MANTLE P WAVES FROM AMBIENT NOISE

Geophysics ◽  
1964 ◽  
Vol 29 (5) ◽  
pp. 672-692 ◽  
Author(s):  
Milo Backus ◽  
John Burg ◽  
Dick Baldwin ◽  
Ed Bryan
Keyword(s):  
Ambient Noise ◽  
Wide Band ◽  
P Wave ◽  
Cumberland Plateau ◽  
P Waves ◽  
Single Output ◽  
Short Period ◽  
Multichannel Filtering ◽  
On Line ◽  
Element Array

The spatial correlation characteristics of ambient short‐period (0.5 to 5 cps) noise at Ft. Sill, Oklahoma, and on the Cumberland Plateau in Tennessee were investigated on “permanent” arrays with 3–4 kilometer diameter. Dominant ambient noise at the two locations is spatially organized, and to first order may be treated as a combination of seismic propagating wave trains. At the Tennessee location noise energy above one cps is dominantly propagating with velocities from 3.5 to 4.5 km/sec, and must be carried in deeply trapped, high‐order modes. Generalized multichannel filtering (Burg) can be used to preserve a large class of mantle P‐wave signals, wide‐band, in a single output trace, while at the same time specifically rejecting ambient noise on the basis of its organization. Results of generalized multichannel filtering applied on‐line at the nineteen‐element array in Tennessee and applied off‐line are discussed.

SEISMIC EXPERIMENTS WITH VERTICAL ARRAYS

Geophysics ◽  
1968 ◽  
Vol 33 (2) ◽  
pp. 270-284 ◽  
Author(s):  
Robert B. Roden
Keyword(s):  
Ambient Noise ◽  
Vertical Component ◽  
P Wave ◽  
Signal Interference ◽  
Vertical Array ◽  
Reflected Waves ◽  
Deep Well ◽  
Vertical Arrays ◽  
Multichannel Filtering ◽  
Selection Of

Experiments with vertical arrays of seismometers were conducted from 1963 to 1965. Data of exceptionally high quality were obtained through the use of special 1‐cps, deep‐well seismometers and direct digital recording techniques. Arrays studied experimentally contained up to 7 vertical‐component seismometers and extended to depths as great as 3.1 km. P‐wave signals observed at depth are generally distorted because of interference between incident and surface‐reflected waves. It is shown that the outputs of two or more deep‐well instruments can be combined to reconstruct signal waveforms. Small additional improvements were obtained through the application of optimum multichannel filtering to vertical array outputs, but the best signal‐to‐noise ratios obtained were never more than 6 db above those available from single deep‐well seismometers located at points of constructive signal interference. Results obtained from analysis of experimental data support the hypothesis that severe attenuation of seismic noise with depth is a characteristic only of sites where ambient noise is very intense at the surface. It is concluded that improvements in record quality which can be obtained through the application of vertical array processing at noisy sites are probably not greater than the improvements which would result from careful selection of a quiet site.

scholarly journals The effects of tectonic release on short-period P waves from NTS explosions

1984 ◽  
Vol 74 (3) ◽  
pp. 819-842
Author(s):  
Thorne Lay ◽  
Terry C. Wallace ◽  
Don V. Helmberger
Keyword(s):  
Frequency Component ◽  
Temporal Variations ◽  
P Wave ◽  
High Frequency Component ◽  
Stress Regime ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Long Period ◽  
Short Period ◽  
Amplitude Pattern

Abstract The first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particulary for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.

Teleseismic p-wave transmission through slabs

1973 ◽  
Vol 63 (4) ◽  
pp. 1349-1373
Author(s):  
Norman H. Sleep
Keyword(s):  
P Wave ◽  
P Waves ◽  
Nuclear Explosions ◽  
Velocity Models ◽  
Shallow Earthquakes ◽  
Short Period ◽  
Systematic Effects ◽  
First Motion ◽  
Low Amplitude ◽  
Good Agreement

abstract Theoretical ray paths through velocity models constructed from numerically calculated thermal models of slabs were computed. The results were in good agreement with observed travel times. First motion amplitudes of P waves at teleseismic distances were measured from long- and short-period WWSSN records of intermediate focus earthquakes in the Tonga, Kermadec, and Kurile regions and of nuclear explosions and shallow earthquakes in the Aleutian region. These amplitudes were corrected for source mechanism. The Aleutian data were sufficient to show that intermediate focus earthquakes in that region occur in the colder regions of the slab. At short periods, for regions other than the Aleutians, shadowing effects which could be associated with the slab were not very marked, less than a factor of 2 reduction for epicentral distances between 30° and 50°. No systematic effects due to plates were found in the long-period data. Some stations in the predicted shadow zone of a Tonga earthquake recorded low amplitude precursors which probably were greatly defocused waves which ran the full length of the slab. Simple diffraction is incapable of explaining the short-period results.

Spectral and magnitude characteristics of anomalous Eurasian earthquakes

1977 ◽  
Vol 67 (2) ◽  
pp. 463-478
Author(s):  
So Gu Kim ◽  
Otto W. Nuttli
Keyword(s):  
Focal Mechanism ◽  
Rayleigh Waves ◽  
Main Shock ◽  
Focal Depth ◽  
P Wave ◽  
Wave Spectra ◽  
P Waves ◽  
Aftershock Sequences ◽  
Short Period ◽  
Amplitude Spectra

Abstract A number of main shock-aftershock sequences in the Eurasian interior contain some aftershocks whose mb:MS values are close to those of underground explosions. This paper is concerned with a study of the amplitude spectra of the P waves and Rayleigh waves for earthquakes of those main shock-aftershock sequences. It is found that for any given sequence studied, there is little if any variation in focal depth or focal mechanism. This rules out variations in these quantities as being the cause of anomalous mb:MS values. A study of the P-wave spectra establishes that one or both of the corner periods of anomalous earthquakes are smaller than those of non-anomalous earthquakes of the same moment. Thus the cause of anomalous mb:MS values of the earthquakes studied is a relative enrichment of the short-period portion of the spectrum of the anomalous events, which cannot be attributed to focal depth or focal mechanism.

DETECTION OF P‐WAVES FROM WEAK SOURCES AT GREAT DISTANCES

Geophysics ◽  
1964 ◽  
Vol 29 (2) ◽  
pp. 197-211 ◽  
Author(s):  
Jon F. Claerbout
Keyword(s):  
Time Series ◽  
Prediction Error ◽  
P Wave ◽  
Pass Band ◽  
Multiple Time ◽  
Multiple Time Series ◽  
P Waves ◽  
Short Period ◽  
Band Pass ◽  
Detection Of Signals

Optimum (Wiener sense) filters for suppression of noise in multiple time series are computed by a new method due to E. A. Robinson. Filters for prediction error and interpolation error have been used to detect P‐wave signals from three teleseismic events. These filters facilitate detection of signals in noise with low signal‐to‐noise ratios. The instrumentation consists of short‐period Benioff seismometers, both three‐component stations and surface arrays of verticals. It was found that microseismic noise in the pass band of these instruments is more accurately termed “Brownian motion of a surface” than “random waveforms with characteristic direction(s) of propagation.” Thus, single time‐series filters work almost as well as multiple time‐series matrix filters. Prediction‐error filters gave results substantially more satisfactory than simple band‐pass filters.

Azimuthal anomalies of short-period P-wave arrivals from Nahanni aftershocks, Northwest Territories, Canada, and effects of surface topography

1990 ◽  
Vol 80 (5) ◽  
pp. 1272-1283
Author(s):  
Goetz G. R. Buchbinder ◽  
R. A. W. Haddon
Keyword(s):  
Surface Topography ◽  
Northwest Territories ◽  
P Wave ◽  
Digital Data ◽  
Aftershock Activity ◽  
P Waves ◽  
Short Period ◽  
Order Of Magnitude ◽  
Large Earthquakes ◽  
Theoretical Results

Abstract Following two large earthquakes in the Northwest Territories of Canada in 1985 and one in 1988, four field surveys were undertaken to study the extensive aftershock activity. Some of the seismographs employed recorded three-component digital data, which allowed the directions of approach of short-period P-wave arrivals to be analyzed. At three stations, observed azimuthal deviations reached as much as 40° from theoretically expected azimuths as computed from the inferred hypocenters for the events. Theoretical results are presented that support the hypothesis that the observed azimuthal deviations are caused principally by local topographic slopes in the vicinity of the recording instruments. Similar theoretical results indicate that effects of local surface topography on arrival azimuths of SH and SV waves are nearly an order of magnitude less than for P waves, so that such effects will generally be unimportant in shear wave splitting studies.

SPECTRAL STUDY OF SHORT-PERIOD P-WAVES

1967 ◽  
Vol 4 (6) ◽  
pp. 1049-1063 ◽  
Author(s):  
Gabriel Leblanc ◽  
Benjamin F. Howell Jr.
Keyword(s):  
P Wave ◽  
Inverse Filtering ◽  
Frequency Range ◽  
P Waves ◽  
Quantitative Measurements ◽  
Short Period ◽  
Oscillatory Character ◽  
Quantitative Results ◽  
Almost All ◽  
Amplitude Level

The variability of 37 short-period P-wave spectra was studied. Amplitude level is affected by distance and crustal effects. Similarity of the over-all shape of the spectra is attributed to the source. An oscillatory character, present on almost all spectra, is related to the crustal effect. Acting as a mask, the crustal effect prevents quantitative measurements of the attenuation and of the radiation pattern. By an averaging of spectra, the crustal masking can be partly overcome, and an approximation of the spectrum of the pulse incident at the mantle-crust boundary obtained. Selected average spectra suggest that over the frequency range 0.4 to 0.9 cps, very little, if any, differential attenuation takes place. The use of crustal inverse filtering, difficult though it may be, seems required for any quantitative results.

Seismic wave-field observations at a dense, small-aperture array located on a landslide in the Santa Cruz Mountains, California

1996 ◽  
Vol 86 (3) ◽  
pp. 655-669
Author(s):  
Zhengyu Xu ◽  
Susan Y. Schwartz ◽  
Thorne Lay
Keyword(s):  
Wave Field ◽  
P Wave ◽  
Santa Cruz ◽  
P Waves ◽  
Small Aperture ◽  
Near Surface ◽  
Loma Prieta Earthquake ◽  
Slowness Vector ◽  
Short Period ◽  
Loma Prieta

Abstract A rectangular (4 by 5) array of short-period three-component seismometers with 15-m spacing was deployed to record several U.S. Geological Survey calibration explosions detonated around the Santa Cruz Mountains. The array was located at a site where an earlier station had recorded frequency-dependent polarized site resonances for aftershocks of the 1989 Loma Prieta earthquake. The site is on a hillside believed to be a landslide structure, with the near surface consisting of poorly sorted sediments and weathered rocks with dipping subsurface layers. The primary objective was to explore the site effects in this complex three-dimensional soft-rock environment, characteristic of much of the Loma Prieta source region. The direct P waves from four nearby (15 to 20 km) explosions at easterly azimuths from the array show counterclockwise arrival azimuth anomalies of 30° to 50°. These deflections are attributed to the presence of more than one dipping velocity contrast beneath the array, with dips of from 10° to 50° and dip directions generally toward the south. One such boundary may correspond to the landslide slip surface, and the presence of dipping velocity contrasts underlying the site is probably responsible for some of the observed directional site resonance. A slowness vector analysis demonstrates that arrivals early in the P coda have similar azimuthal anomalies, while later scattered arrivals come from many azimuths. Particle motions indicate that the more coherent arrivals in the coda are comprised of scattered P waves and Rayleigh waves, probably associated with scattering from the rough topography in the region. The coda displays greater spatial coherency along the hill strike than down the slope, consistent with a wedge-shaped landslide. The overall wave-field spatial coherence, CCC(f, Δx), decreases with increasing frequency, f, and spatial offset, Δx, and on average can be well represented by CCC(f, Δx) = e−cfΔx, with c = 0.6 km−1 Hz−1 for the vertical P wave in the first 1-sec window. This behavior is comparable to that found for previously studied hard-rock locations.

Synthesis of teleseismic P waves from sources near sinking lithospheric slabs

1975 ◽  
Vol 65 (6) ◽  
pp. 1667-1680
Author(s):  
Ronald W. Ward ◽  
Keiiti Aki
Keyword(s):  
Wave Amplitude ◽  
Wave Theory ◽  
P Wave ◽  
Wave Form ◽  
Shadow Zone ◽  
P Waves ◽  
Wave Source ◽  
Long Period ◽  
Short Period ◽  
Dominant Period

abstract A wave theory method is used to determine the effect of a sinking lithospheric slab on short-period and long-period waves. We consider a simplified model of the lithospheric slab with a 10 per cent velocity contrast and compute both short-period and long-period theoretical seismograms from a P-wave source located in or near the slab. For this model, the ray-theoretical amplitude agrees quite well with the short-period amplitude. In the ray-theoretical shadow zone the long-period seismograms (15- to 25-sec dominant period) typically have amplitudes 50 per cent (or greater) of the direct P-wave amplitude and exhibit wave-form broadening. Similar wave-form broadening has been attributed to the dynamics of earthquake faulting. The effect of the lithosphere on long-period waves from nearby sources must be taken into account in studies which utilize the observed variation in wave-form broadening to infer earthquake source dynamics.

Magnitude calibration of the Central Minnesota Seismic Array

1981 ◽  
Vol 71 (4) ◽  
pp. 1089-1103
Author(s):  
S. A. Greenhalgh ◽  
C. C. Mosher ◽  
H. M. Mooney
Keyword(s):  
Epicentral Distance ◽  
Seismic Array ◽  
P Wave ◽  
Total Charge ◽  
P Waves ◽  
Attenuation Rate ◽  
Short Period ◽  
The Individual ◽  
Mine Blasts ◽  
Best Fit

abstract Magnitude calibration for the Central Minnesota Seismic Array presents special problems because the infrequuent local and near-regional earthquake are usually too small to be recorded at teleseismic distances. Two procedures have been applied. The first uses teleseisms recorded by the array for which independent magnitudes are available. The second uses large near-regional mine blasts to determine the attenuation rate for short-period P waves in the upper crust, together with Evernden's (1970) magnitude-charge size relationship. The teleseismic correlations establish that the individual stations of the array yield consistent magnitudes, that the P wave amplitudes behave predictably with respect to epicentral distance and azimuth, but that a magnitude-dependent bias must be removed. Measured ground amplitudes for mine blasts are found to be proportional to total charge size to the power 1.0. Using this value, we find amplitude attenuation proportional to D−B for distance D in kilometers, with best fit given by B = 2.57 for P waves. The final local magnitude scale for D up to 250 km takes the form m b = 2.57 log ⁡ D + log ⁡ A − 3.97.

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