AN ANALYSIS OF THE TRAVEL TIMES OF S WAVES TO NORTH AMERICAN STATIONS, IN THE DISTANCE RANGE 28 DEGREES TO 82 DEGREES

1967 ◽  
Author(s):  
H. A. Doyle ◽  
A. L. Hales
Keyword(s):  
North American ◽  
Travel Times ◽  
S Waves ◽  
Distance Range

An analysis of the travel times of S waves to North American stations in the distance range 28° to 82°

1967 ◽  
Vol 57 (4) ◽  
pp. 761-771 ◽  
Author(s):  
H. A. Doyle ◽  
A. L. Hales
Keyword(s):  
United States ◽  
Upper Mantle ◽  
Univariate Analysis ◽  
Western United States ◽  
Travel Times ◽  
Eastern United States ◽  
S Waves ◽  
Distance Range ◽  
Time Term ◽  
Mantle Velocity

abstract The travel times of S waves from 20 earthquakes to stations in North America in the distance range 28° to 82° have been studied. The deviations from J-B times were analyzed into station, source and distance components using the least-squares time-term approach of Cleary and Hales. Station anomalies had a range of about eight seconds, as compared to three seconds for the P anomalies, and are believed to be caused largely by variations in the upper mantle velocity distribution. S residuals, like the P residuals, were generally positive in the western United States, and negative in the central and eastern United States. P and S residuals at the same station correlated with a coefficient of 0.75, the slope of the regression of S anomaly on P anomaly being 3.72. Corrections to J-B times for S were of the order of the standard errors of the determinations. Within the distance range of 28° to 82° large changes of the S travel times, such as were required by the lower mantle velocities proposed by MacDonald and Ness (1961), are not permitted by the present data. The analysis was checked by carrying out a univariate analysis of variance of the same data.

The reliability of North American seismological stations

1957 ◽  
Vol 47 (1) ◽  
pp. 1-5
Author(s):  
Eugene Herrin
Keyword(s):  
Standard Deviation ◽  
North American ◽  
Systematic Difference ◽  
Travel Times ◽  
Seismic Stations

Abstract Estimates are given for the reliabilities of fifty-eight North American seismic stations, based on I.S.S. P-time residuals of Mexican earthquakes for 1941, 1942, 1943, and 1944. The ten most reliable stations were found to be Tinemaha, Riverside, Pasadena, Mount Wilson, Tucson, Lick, Weston, Palomar, Haiwee, and Seven Falls. The standard deviation of a single P arrival for the more reliable stations is ± 2.0 seconds or greater. A systematic difference in travel times between the California stations and eastern stations, and particularly between Ottawa and the Pasadena group, is strongly suggested. P arrivals at Ottawa are about one second faster than for the Pasadena group.

scholarly journals Seismic Attenuation Anisotropy in the Southernmost Part of the Taupo Volcanic Zone, North Island, New Zealand

2022 ◽  
Author(s):  
◽  
Syuhada, Syuhada
Keyword(s):  
Frequency Dependence ◽  
Seismic Attenuation ◽  
Taupo Volcanic Zone ◽  
S Wave ◽  
Volcanic Zone ◽  
S Waves ◽  
Distance Range ◽  
Uncertainty Estimates ◽  
Q Model ◽  
Attenuation Anisotropy

<p>We investigate the mechanisms of seismic anisotropy and attenuation (1/Q) beneath the southernmost part of the Taupo Volcanic Zone (TVZ) by computing variations in S-wave attenuation factors with the direction of wave polarization. We rotate pairs of horizontal components in steps of 22.5◦ from 0◦ to 67.5◦ and into the radial and transverse directions to search for the optimal separation of the attenuation curves and thereby determine an anisotropy symmetry system. The frequency dependence of Q for the rotated S-waves is estimated by means of the non-parametric generalized inversion technique (GIT) of Castro et al. (1990) using shallow earthquakes (< 40 km depth) recorded by GeoNet within 100 km of Mt. Ruapehu. To analyze the effects on computed attenuation properties of source locations, we divide our dataset into two groups: a “TVZ” group containing earthquakes within the TVZ in a distance range of 5–55 km and a “non-TVZ” group containing earthquakes outside the TVZ in a distance range of 5–50 km. To measure Q, we compute the spectral amplitude decay with distance in terms of empirical functions at 20 separate frequencies in the frequency bands 2–10 Hz and 2– 12 Hz for the TVZ and non-TVZ datasets respectively. We construct homogeneous and two-layer Q models for the TVZ dataset based on characteristic features of the attenuation function, while for outside TVZ we only analyse a homogeneous Q model. The homogeneous Q models obtained for the two datasets indicate that S-waves are more attenuated within the TVZ than outside. The homogeneous Q model for the TVZ dataset reveals that the S-wave is anisotropic at high frequencies ( f > 6 Hz) along N–S/E– W directions with the relation QSE ( f ) = (6.15±1.22) f (1.73±0.12) and QSN ( f ) = (4.14± 1.26) f (2.06±0.14), while the non-TVZ dataset shows a weak frequency dependence of attenuation anisotropy at low frequencies in NE–SW/SE–NW directions giving the power law function QSNE ( f ) = (50.93±1.18) f (0.20±0.10) and QSSE ( f ) = (22.60±1.10) f (0.53±0.06). Here, the uncertainty estimates are 95% confidence intervals. To investigate the variation of attenuation anisotropy with depth within the TVZ, we first calculate Q along propagation paths (< 25 km, which corresponds to a maximum turning point depth of 9 km ) and then using paths of 25–55 km length. Small attenuation anisotropy with low attenuation in the N–S direction for the upper crust of TVZ may be related to heterogenous structure as reported by previous studies. Attenuation anisotropy in the northwest direction yielding lower attenuation inferred for the deeper crust suggests the presence of connected melt aligned with the extension direction of TVZ .</p>

Stability of P an S velocities from Central California quarry blasts

1974 ◽  
Vol 64 (2) ◽  
pp. 343-353 ◽  
Author(s):  
T. V. McEvilly ◽  
L. R. Johnson
Keyword(s):  
Strike Slip ◽  
Travel Times ◽  
Mean Values ◽  
Central California ◽  
S Waves ◽  
Source Regions ◽  
Reading Errors ◽  
The Mean ◽  
Velocity Variations

abstract Travel times of crustal P and S waves from 70 quarry blasts in Central California between July 1961 and June 1973 have been measured for seven paths 46 to 168 km long passing in several cases within 10 km of hypocenters of moderate (ML 4.5 to 5.4) earthquakes. All P and S times and their ratios TS/TP lie within 2.3 per cent of the mean values and over 97 per cent of the TS/TP ratios lie within 1 per cent of the mean values. Variations can be explained by reading errors and uncertainties in source times and locations. There are no indications of velocity variations related to earthquake occurrences. Mean VP/VS ratios range from 1.73 to 1.84 for the various paths. The phenomenon of premonitory dilatancy accompanied by a 10 to 20 per cent reduction in VP or VP/VS cannot be reconciled with these observations unless the affected source regions have lateral dimensions less than about 5 km for these moderate strike-slip earthquakes.

Origin of precursors to teleseismic S waves

1991 ◽  
Vol 81 (4) ◽  
pp. 1216-1230
Author(s):  
Lev. P. Vinnik ◽  
Barbara A. Romanowicz
Keyword(s):  
Free Surface ◽  
Continental Crust ◽  
Apparent Velocity ◽  
Wave Fields ◽  
S Waves ◽  
Distance Range ◽  
Long Period ◽  
Converted Phases

Abstract In the literature, there are descriptions of precursors to teleseismic S waves that are polarized as teleseimic P. They are commonly regarded as phases converted from SV to P underneath the seismographic stations. We present observations of precursors at the broadband digital GEOSCOPE stations in the distance range from 45° to 95° and at periods around 10 sec. These precursors are polarized as the teleseismic P but cannot be interpreted in terms of conversion underneath the station. We propose that they are formed by conversion and scattering from S to P at the free surface and scattering of the resulting P in the lithosphere of the region between the source and the receiver. The apparent velocity of S in the region of scattering is usually around 7 km/sec, which implies a long wavepath of the converted P in the continental crust. Apparently, S-to-P and P-to-P scattering play an important role in forming teleseismic wave fields even in the relatively long-period band around 10 sec. A possibility of observing true S-P converted phases originating in the lithosphere underneath the station is certainly not ruled out. However, our analysis is a warning that scattered waves sometimes can be erroneously taken for converted phases.

Sign in / Sign up

Export Citation Format

Share Document