The Texas earthquake of August 16, 1931*

1934 ◽  
Vol 24 (2) ◽  
pp. 81-99
Author(s):  
Perry Byerly
Keyword(s):  
Travel Time ◽  
Travel Times ◽  
Time Curve ◽  
P Waves ◽  
First Order ◽  
First Motion ◽  
Order Discontinuity

Summary The travel-time curve of P for the Texas earthquake of August 16, 1931, shows that there is a definite break in the travel-time curve near Δ = 16°. This is interpreted as indicating a first-order discontinuity at a depth of about 300 kilometers. Another break in the travel-time curve at Δ = 25° is strongly suggested. Beyond Δ = 75° the curve has two branches, the lower following most existing curves, the upper following the Montana curve which latter seems to be a usual one for American earthquakes. This part of the curve is interpreted as indicating that the discontinuity at depth about 2,400 kilometers is a first-order one at which the speed of P waves drops discontinuously. From the direction of first motion on the records it is concluded that a sufficient source would have been motion on a fault of strike about N 35° W, the movement being up on the easterly side and down on the westerly side. The travel times of all waves read on the records are plotted on graphs. The scattering of all waves after P is marked.

scholarly journals P- waves reflected from the "20" discontinuity" beneath the Mediterranean region

2010 ◽  
Vol 28 (1) ◽  
Author(s):  
A BOTTARI ◽  
B. FEDERICO
Keyword(s):  
Mediterranean Area ◽  
P Wave ◽  
Travel Times ◽  
P Waves ◽  
First Order ◽  
P Wave Velocity ◽  
Different Depths ◽  
The Mediterranean ◽  
Regional Character ◽  
Order Discontinuity

The observed travel-times of the P-waves for twenty shallow, intermediate, and deep earthquakes, with epicenters in the Mediterranean area, are used in order to analyze some characteristics of the upper mantle. A first- order discontinuity, identifiable as the "20° discontinuity", is found at a depth of 505 ± 16 km in the area underneath the Mediterranean basin. The velocity contrast is equal to 12% (above T'= 8.9 km/sec; below V= 9.97 km/sec). Assuming that this discontinuity gives rise to reflected P-waves (PdP), the travel times of these waves are calculated for various hypocentral depths. The observation of impulses identified as PdP on the seismograms of Messina supports this hypothesis. This result and its implications are discussed in the contest of the conclusions of various authors who locate a P-wave velocity-discontinuity at different depths between 400 and 580 km. Finally, particular emphasis is given to the regional character of the analyzed structures in question.

The Hebgen Lake, Montana, earthquake of August 18, 1959: P waves

1962 ◽  
Vol 52 (2) ◽  
pp. 235-271
Author(s):  
Alan Ryall
Keyword(s):  
Travel Time ◽  
Sierra Nevada ◽  
Fault Plane ◽  
Time Curve ◽  
P Waves ◽  
Arrival Times ◽  
The Pacific ◽  
First Motion ◽  
Plane Solution ◽  
The Pacific Ocean

ABSTRACT The instrumental epicenter of the Hebgen Lake earthquake is found to lie within the region of surface faulting. The depth of focus had a maximum value of 25 kilometers. Times of P are studied in detail for epicentral distances less than 13 degrees. The apparent scatter of arrival times from 700 to 1400 kilometers can be explained by variations of the velocity of Pn between the physiographic provinces of the western United States. A comparison of observations for the Hebgen Lake earthquake with published times for blasts in Nevada and Utah indicates that the velocity of Pn in the central and eastern Basin and Range is about 7.5 km/sec, and that the crust in that region thickens toward the east and thins toward the south. A comparison of apparent velocities in northern California, in directions parallel and transverse to the structure, indicates that the crust thins by about 19 kilometers, from the edge of the Sierra Nevada to the Pacific Ocean. A discontinuity is observed in the travel-time curve at a distance of 24–25 degrees. Arrivals of P waves in the range 65–128 degrees fall on two parallel travel-time branches; this multiplicity in the travel-time curve may be related to repeated motion at the source. Travel-times of PKIKP appear to deviate from published curves. The fault-plane solution for the Hebgen Lake earthquake, together with a consideration of the first motion at Bozeman, Montana, indicates a focal mechanism of the dipole, or fault, type. The strike and dip of the instrumental fault plane agree well with observed ruptures at the surface.

scholarly journals On supposed discontinuities in the mantle of the Earth

1931 ◽  
Vol 21 (3) ◽  
pp. 216-223 ◽  
Author(s):  
B. Gutenberg ◽  
C. F. Richter
Keyword(s):  
Travel Time ◽  
Glassy State ◽  
Time Curve ◽  
P Waves ◽  
S Waves ◽  
The Earth ◽  
Straight Line

Summary Investigations of the Mexican shocks of January 2, 15, and 17, 1931, as recorded at stations in California have shown that the travel-time curve of the P-waves at distances between 9° and 15° is nearly a straight line. At these distances the amplitudes of the P-waves are very small, as is to be expected from theory. At greater distances dt/dΔ decreases, and the amplitudes are larger. The data are not sufficient to decide whether the changes are abrupt or not. No S-waves could be found between 9° and 15°. The calculated velocities of the P-waves are near 8.2 kilometers per second at depths between 40 and 100 kilometers, increasing slightly with greater depths. It is possible that the velocity decreases very slightly at some depths between 40 and 80 kilometers, but there is no sign of any discontinuity at depths between 40 and more than 500 kilometers. The S-waves seem to be affected a little more at depths between 40 and 100 kilometers than the P-waves. It is not impossible that at some depth between 40 and 80 kilometers there is a transition from the crystalline to the glassy state.

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 Alaskan earthquake of July 22, 1937*

1940 ◽  
Vol 30 (4) ◽  
pp. 353-376
Author(s):  
John N. Adkins
Keyword(s):  
Travel Time ◽  
Epicentral Distance ◽  
Time Interval ◽  
Time Curve ◽  
Arrival Times ◽  
The Earth ◽  
Geographical Latitude ◽  
The World ◽  
First Motion ◽  
First Arrival

Summary The study of the Alaskan earthquake of July 22, 1937, is based on the examination of original seismograms and photographic copies from seismological observatories throughout the world. The arrival times of P at 71 stations were used in locating the epicenter. By Geiger's method and the use of Jeffreys' travel times, the position of the epicenter was found to be: geographical latitude, 64.67±.04° N, longitude, 146.58±.12° W, and the time of occurrence to be 17h 9m 30.0±.25s, U.T. The epicenter lies in the Yukon-Tanana upland in central Alaska, which is not a region of frequent major earthquakes. The disagreement caused by the apparently early arrivals at College and Sitka was reduced by replacing the standard travel-time curve of P by a linear travel-time curve in the interval of epicentral distance 0° to 16° and by interpreting the first arrival at College as P. It was possible to determine the direction of the first motion of P for 51 stations. The observed distribution of first motion and the geological trends in the region of the epicenter are consistent with the earthquake's having been caused by movement along a fault with strike between N 30° E and N 37° E, and dip between 64° and 71° to the southeast, in which the southeast side of the fault was displaced relatively northeastward with the line of movement pitching between 12° and 16° northeast. A wave designated F (for “false S”) was found to precede S on the records by 20 to 55 seconds, depending on the epicentral distance. The wave is longitudinal in type and the arrival times define a linear travel-time curve. It is suggested that this wave may be a longitudinal surface wave, of the type proposed by Nakano, produced at the surface of the earth by the arrival of a transverse wave which has been reflected at a surface of discontinuity within the earth. The records show two impulses near the time when S is expected. The average time interval between the two impulses is 11.3 sec. The first, called S1, has a plane of vibration intermediate in direction between the plane of propagation and the normal thereto. The second impulse, called S2, is nearly pure SH movement. The writer wishes to express his indebtedness to Professor Perry Byerly for invaluable suggestions and criticism during the course of the investigation.

On the question of dispersion in the first preliminary seismic waves1

1931 ◽  
Vol 21 (2) ◽  
pp. 87-158 ◽  
Author(s):  
H. Henrietta Sommer
Keyword(s):  
Continuous Function ◽  
Least Squares ◽  
Travel Time ◽  
Epicentral Distance ◽  
Anomalous Dispersion ◽  
Method Of Least Squares ◽  
Time Curve ◽  
Least Squares Adjustment ◽  
First Motion

Abstract Summary By use of the Byerly-Jeffreys travel-time curve for P, and Geiger's method of least-squares adjustment, the epicenter of the Alaskan earthquake of October 24, 1927, was placed at 5 7 ° 26 ' ± 5 0 ' N . 13 7 ° 03 ' ± 1 9 ' W . and the time of occurrence was placed at 15h 59m 55s ± 2s, G.M.C.T. A second solution was obtained using Mohorovičić's multiple travel-time curves for P. The co-ordinates of the epicenter were the same as those given above, but the time of occurrence was found to be 16h00m, G.M.C.T. It has been held by some seismologists that anomalous dispersion can be observed in the first preliminary waves; i.e., that shorter periods travel faster than long ones. Investigations of periods were made with a view to testing this hypothesis, with the following results: The general conclusion is that observation of periods gives no evidence for dispersion in waves of longitudinal type. It is shown that, if dispersion did exist, the travel time of the beginning would be a continuous function of epicentral distance, and, therefore, Mohorovičić's curves are not evidence for dispersion. The observations of the epicentral distances at which P2, P1, and Pn are most frequently recorded first are contrary to dispersion. In the Alaskan earthquake the distribution of first motion (condensation or rarefaction) is very complicated. Dispersion offers no explanation for this fact, and it is believed that complex movements at the source are responsible for the observed distribution.

The first preliminary waves of the Nevada earthquake of December 20, 1932*

1935 ◽  
Vol 25 (1) ◽  
pp. 62-80 ◽  
Author(s):  
Perry Byerly
Keyword(s):  
Sudden Change ◽  
Simple Explanation ◽  
Depth Of Focus ◽  
Time Curve ◽  
P Waves ◽  
Long Period ◽  
First Motion ◽  
Straight Lines ◽  
Suggested Explanation ◽  
Period Motion

Summary The P travel-time curve of the Nevada earthquake is presented. It is drawn as a series of straight lines. It is near Δ = 28° that the data outline most clearly the sudden change in slope of the curve, but definite evidence of overlapping of the branches is lacking. At Δ = 67° (approximately) the curve branches into two parts as did the curve of the Texas earthquake. Between 4° and 12° three parallel P curves are drawn. The suggested explanation of them is that they are due to transformations of an original P or S at boundaries near the focus. This would indicate a depth of focus of about fifteen kilometers. The nature of the first motion at the various stations shows a complex distribution which does not lead to a simple explanation of the forces acting at the focus. A long-period component of P waves is observed, and it is concluded that it is present from very near the beginning of the record, although often masked by shorter-period motion. It begins in opposite phase to the shorter-period motion accompanying it.

Travel times and amplitudes of the Salmon nuclear explosion

1969 ◽  
Vol 59 (2) ◽  
pp. 959-966
Author(s):  
I. Lehmann
Keyword(s):  
Nuclear Explosion ◽  
The State ◽  
Mountainous Region ◽  
Travel Times ◽  
The West ◽  
Time Curve ◽  
P Waves ◽  
Continuous Bending

Abstract Travel times and amplitudes of the Pn and P waves generated by the nuclear explosion Salmon fired in the State of Mississippi are studied. To the west the Pn waves are found to be cut off at a distance of about 1800 km, where they are about to enter the mountainous region. An increase of P amplitude with distance goes to confirm that the P velocity increases with depth and to indicate that the time-curve is a continuous, bending curve.

scholarly journals Teleseismic P waves at the AlpArray seismic network: wave fronts, absolute travel times and travel-time residuals

Solid Earth ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 1635-1660
Author(s):  
Marcel Paffrath ◽  
Wolfgang Friederich ◽  
Keyword(s):  
Travel Time ◽  
Cross Correlation ◽  
Field Experiments ◽  
Epicentral Distance ◽  
Seismic Network ◽  
Corner Frequency ◽  
Travel Times ◽  
Wave Fronts ◽  
P Waves ◽  
Waveform Cross Correlation

Abstract. We present an extensive dataset of highly accurate absolute travel times and travel-time residuals of teleseismic P waves recorded by the AlpArray Seismic Network and complementary field experiments in the years from 2015 to 2019. The dataset is intended to serve as the basis for teleseismic travel-time tomography of the upper mantle below the greater Alpine region. In addition, the data may be used as constraints in full-waveform inversion of AlpArray recordings. The dataset comprises about 170 000 onsets derived from records filtered to an upper-corner frequency of 0.5 Hz and 214 000 onsets from records filtered to an upper-corner frequency of 0.1 Hz. The high accuracy of absolute and residual travel times was obtained by applying a specially designed combination of automatic picking, waveform cross-correlation and beamforming. Taking travel-time data for individual events, we are able to visualise in detail the wave fronts of teleseismic P waves as they propagate across AlpArray. Variations of distances between isochrons indicate structural perturbations in the mantle below. Travel-time residuals for individual events exhibit spatially coherent patterns that prove to be stable if events of similar epicentral distance and azimuth are considered. When residuals for all available events are stacked, conspicuous areas of negative residuals emerge that indicate the lateral location of subducting slabs beneath the Apennines and the western, central and eastern Alps. Stacking residuals for events from 90∘ wide azimuthal sectors results in lateral distributions of negative and positive residuals that are generally consistent but differ in detail due to the differing direction of illumination of mantle structures by the incident P waves. Uncertainties of travel-time residuals are estimated from the peak width of the cross-correlation function and its maximum value. The median uncertainty is 0.15 s at 0.5 Hz and 0.18 s at 0.1 Hz, which is more than 10 times lower than the typical travel-time residuals of up to ±2 s. Uncertainties display a regional dependence caused by quality differences between temporary and permanent stations as well as site-specific noise conditions.

scholarly journals Longitudinal waves through the Earth's Core*

1952 ◽  
Vol 42 (2) ◽  
pp. 119-134
Author(s):  
M. E. Denson
Keyword(s):  
Velocity Distribution ◽  
Travel Time ◽  
Focal Point ◽  
Epicentral Distance ◽  
Outer Core ◽  
Travel Times ◽  
Time Curve ◽  
Longitudinal Waves ◽  
The Core ◽  
The Waves

Abstract Amplitudes, periods, and travel times of the longitudinal P′ or PKP core waves have been investigated. Results indicate that the epicentral distance of the main focal point and the travel time of P′ phases vary with the periods of the waves. This variation would seem reasonably explained in terms of dispersion. The point of reversal in the travel-time curve of the waves through the outer core is believed to lie near 157°. Data suggest a discontinuity between 120° and 125° rather than 110°. Anomalies existing in energy, period, and travel-time relationships of the P′ phases indicate that current concepts of velocity distribution and of propagation paths within the core are in need of modification.

Sign in / Sign up

Export Citation Format

Share Document