The times of origin and depths of focus of intermediate and deep focus earthquakes: Model calculations

1981 ◽  
Vol 71 (5) ◽  
pp. 1539-1552
Author(s):  
A. L. Hales ◽  
K. J. Muirhead ◽  
L. Maki-Lopez
Keyword(s):  
Average Velocity ◽  
Epicentral Distance ◽  
Depth Range ◽  
Depth Of Focus ◽  
P Waves ◽  
Model Calculations ◽  
The Earth ◽  
Deep Focus ◽  
The Times ◽  
Time Of Origin

abstract Two methods for determining the time of origin, depth of focus, and the average velocities from the focus to the surface are described. The first stage in the first method is to determine the time of origin using a modification of the Wadati method. As was pointed out in 1973 by Kisslinger and Engdahl, the relation between (ts − tp and tp is nonlinear and it is necessary to allow for this nonlinearity by including a term in tp2 in the analysis. Thereafter the depth of focus and the average velocity can be found by a modification of the procedure used to determine the depth to a reflector in seismic reflection prospecting. It is necessary to allow for the sphericity of the Earth in this analysis. In the second method, the depth of focus is determined first by analyzing (ts − tp)2 as a function of x2, x being the epicentral distance. The average velocity of separation of S and P waves is also determined at this stage. Thereafter the time of origin and the average P and S velocities are determined. The results of the analysis of the calculated travel times for three models show that systematic errors in the depth of focus using these procedures are less than 2 km over the depth range of 60 to 640 km. Preliminary results of the analysis of a limited set of Japanese earthquakes by these methods give estimates of depth smaller than those given by ISC for depths less than 300 km. For deeper earthquakes, these methods give foci deeper than the ISC, but in these cases the observations close to the epicenter are inadequate for reliable analysis.

A method of obtaining P and S travel-time curves within a “Stripped Earth”*

1952 ◽  
Vol 42 (4) ◽  
pp. 313-314
Author(s):  
V. C. Stechschulte
Keyword(s):  
Travel Time ◽  
Depth Distribution ◽  
Epicentral Distance ◽  
Travel Times ◽  
Depth Of Focus ◽  
Distance Curve ◽  
Simple Method ◽  
Time Distance ◽  
Deep Focus

Abstract A simple method is outlined for obtaining from a time-distance curve of a deep-focus earthquake a table of travel times within an earth “stripped” to the depth h, the depth of focus. The method depends on the fact that such a curve for a deep-focus earthquake has a point of inflection and therefore has the same slope at two different values of epicentral distance. The Herglotz-Wiechert method may then be applied to these travel times to obtain a velocity-depth distribution.

The Distribution of Deep-focus Earthquakes

1937 ◽  
Vol 74 (7) ◽  
pp. 316-324 ◽  
Author(s):  
Charles Davison
Keyword(s):  
Northern Hemisphere ◽  
Depth Of Focus ◽  
The Earth ◽  
Deep Focus

During the years 1918–1931, there were 270 earthquakes with unusually deep foci, 167 in the Northern Hemisphere, 101 in the Southern, and two with epicentres on the equator. The normal depth of focus is assumed to be about 50 km. or ·008 of the earth's radius. The focal depths of the above earthquakes range from ·005 to ·090 of the earth's radius below the normal depth, or from 50 to 380 miles beneath the surface. Throughout this paper, the depth, when given in terms of the earth's radius, is referred to the normal depth; when given in miles, to the surface of the earth.

Focal depth determination from the signal character of long-period P waves

1976 ◽  
Vol 66 (4) ◽  
pp. 1221-1232
Author(s):  
Robert B. Herrmann
Keyword(s):  
Epicentral Distance ◽  
Focal Depth ◽  
Time Function ◽  
P Wave ◽  
Source Time Function ◽  
P Waves ◽  
Long Period ◽  
The Earth ◽  
Resolution Capability ◽  
Earth Structures

abstract The shape of long-period teleseismic P-wave signals is a function of many factors, among which are focal depth, focal mechanism, the source time function, and the earth structures at both the source and receiver. The effect of focal depth is quite pronounced, so much so, that focal depths should be able to be determined to within 10 km on the basis of the long-period P-wave character. This resolution capability is demonstrated for events occurring in continental and oceanic crust as observed by seismographs in the 30° to 80° epicentral distance range.

Near earthquakes in Central California*

1939 ◽  
Vol 29 (3) ◽  
pp. 427-462 ◽  
Author(s):  
Perry Byerly
Keyword(s):  
Least Squares ◽  
Travel Time ◽  
Sierra Nevada ◽  
Accurate Determination ◽  
Depth Of Focus ◽  
Surface Layers ◽  
P Waves ◽  
Central California ◽  
The Waves

Summary Least-squares adjustments of observations of waves of the P groups at central and southern California stations are used to obtain the speeds of various waves. Only observations made to tenths of a second are used. It is assumed that the waves have a common velocity for all earthquakes. But the time intercepts of the travel-time curves are allowed to be different for different shocks. The speed of P̄ is found to be 5.61 km/sec.±0.05. The speed for S̄ (founded on fewer data) is 3.26 km/sec. ± 0.09. There are slight differences in the epicenters located by the use of P̄ and S̄ which may or may not be significant. It is suggested that P̄ and S̄ may be released from different foci. The speed of Pn, the wave in the top of the mantle, is 8.02 km/sec. ± 0.05. Intermediate P waves of speeds 6.72 km/sec. ± 0.02 and 7.24 km/sec. ± 0.04 are observed. Only the former has a time intercept which allows a consistent computation of structure when considered a layer wave. For the Berkeley earthquake of March 8, 1937, the accurate determination of depth of focus was possible. This enabled a determination of layering of the earth's crust. The result was about 9 km. of granite over 23 km. of a medium of speed 6.72 km/sec. Underneath these two layers is the mantle of speed 8.02 km/sec. The data from other shocks centering south of Berkeley would not fit this structure, but an assumption of the thickening of the granite southerly brought all into agreement. The earthquakes discussed show a lag of Pn as it passes under the Sierra Nevada. This has been observed before. A reconsideration of the Pn data of the Nevada earthquake of December 20, 1932, together with the data mentioned above, leads to the conclusion that the root of the mountain mass projects into the mantle beneath the surface layers by an amount between 6 and 41 km.

The Hindu Kush earthquake of March 4, 1949 as recorded in Europe

1964 ◽  
Vol 54 (6A) ◽  
pp. 1915-1925 ◽  
Author(s):  
I. Lehmann
Keyword(s):  
Amplitude Ratio ◽  
Epicentral Distance ◽  
Focal Region ◽  
Travel Times ◽  
Period Range ◽  
Characteristic Appearance ◽  
Hindu Kush ◽  
The Difference ◽  
Deep Focus ◽  
The Many

abstract The European records from distances 36°-50° of the deep Hindu Kush earthquake of March 4, 1949 were studied. The many clearly recorded deep-focus reflections lend to the records a characteristic appearance which is repeated in many other shocks from the same focal region. The ratios of the amplitudes of these phases vary somewhat from one shock to another. In the shock here considered sP and sPP are exceptionally large at most stations; in the Italian stations they are not so large, while pP is a clear phase. pP is not very well defined at most other stations. Most of the 1949 records were from the old type long-period instruments having their highest magnification for periods from about 5 sec to 12 sec. Present day instruments of quite short or of very long proper period while admirable for many purposes do not record waves in this period range very well and therefore do not produce a satisfactory picture of the forerunners of earthquakes. The difference between the records obtained on different instruments is illustrated. It is shown in examples that the amplitude ratio PP:P may differ strongly at the same epicentral distance and also that pP may vary greatly with azimuth. The deficiency of station readings is noted. Travel times and their residuals are tabulated and travel times plotted versus epicentral distances.

ELECTRIC EARTH TRANSIENTS IN GEOPHYSICAL PROSPECTING

Geophysics ◽  
1936 ◽  
Vol 1 (2) ◽  
pp. 271-277 ◽  
Author(s):  
Louis Statham
Keyword(s):  
Electric Current ◽  
Salt Dome ◽  
Geophysical Prospecting ◽  
The Earth ◽  
Electrical Methods ◽  
The Times

A suddenly applied electric current is passed through the earth by means of spaced electrodes. The form of the potential transient as it appears outside the current electrodes is studied. The potential transient is extremely rapid and refined methods of recording are necessary. Means for measuring the relative times of the transient potentials received from different points are discussed. A survey taken over a known deep salt dome is shown; anomalous times of the transients are found to exist over the dome. No correlation is seen between the times of the transients and the resistivity as found by ordinary electrical methods.

The Role of Chlorine in Stratospheric Chemistry

1996 ◽  
Vol 68 (9) ◽  
pp. 1749-1756 ◽  
Author(s):  
M. J. Molina
Keyword(s):  
Laboratory Experiments ◽  
Stratospheric Ozone ◽  
The Sun ◽  
Plastic Foam ◽  
Model Calculations ◽  
Stratospheric Chemistry ◽  
Blowing Agents ◽  
Stratospheric Ozone Layer ◽  
The Earth

The chlorofluorocarbons (CFCs) are industrialchemicals used as solvents, refrigerants, plastic foam blowing agents,etc. These compounds are eventually released to the environment; theyslowly drift into the stratosphere, where they decompose, initiatinga catalytic process involving chlorine free radicals and leading toozone destruction. The stratospheric ozone layer is important for theearth's energy budget, and it shields the surface of the earth fromultraviolet radiation from the sun. Very significant depletion of theozone layer has been observed in the spring months over Antarctica duringthe last 10-15 years. Laboratory experiments, model calculations andfield measurements, which include several aircraft expeditions, haveyielded a wealth of information which clearly points to the CFCs asthe main cause of this depletion.

scholarly journals The depth range of the Earth'snatural pulse electromagneticfield (or ENPEMF)

2019 ◽  
Vol 27 (3) ◽  
pp. 466-477 ◽  
Author(s):  
E. D. Kuzmenko ◽  
S. M. Bahrii ◽  
U. O. Dzioba
Keyword(s):  
Electromagnetic Field ◽  
Pulse Electromagnetic Field ◽  
Depth Range ◽  
Advantages And Disadvantages ◽  
The Earth ◽  
Mechanical Deformations ◽  
Specific Electric Resistance ◽  
Conducting Research ◽  
The University ◽  
The Impact

On the basis of the analysis of the literature sources, we determined the possible range of using the method of the Earth`s natural pulse electromagnetic field. As a result of detailed analysis of domestic and foreign research, we demonstrated the relevance of conducting research focused on development of the Earth'snatural pulse electromagneticfield (or ENPEMF). Using the results of theoretical studies, the advantages and disadvantages of the ENPEMF method were determined. A complex of physical processes which preceded the development of the pulse electromagnetic field of the Earth was characterized, and the impact of mechanical deformations of rocks on the change in the condition of the electromagnetic field was experimentally proven. The main fundamentals on the determination of depth range of the ENPEMF method were examined and a new approach to interpretation of the data was suggested. We conducted an analysis of methods developed earlier of calculating geometric parameters of the sources which generate electromagnetic impulses. Their practicability at a certain stage of solving the data of geological tasks was experimentally tested. We determined the factors which affect the depth range of the ENPEMF method. A mathematical solution of the effectiveness of the ENPEMF method was suggested and determined the relations between the depth parameter of the study and the frequency of measuring and effective value of specific electric resistance. On the example of different objects, the effectiveness and correctness of the suggested method of determining the depth range parameter was proven. In particular, the theoretical results of the study were tested and confirmed on objects of different geological-morphological and engineering-technical aspects, i.e. Novo-Holyn mine in the Kalush-Holynske potash deposit and the multi-storey educational building of the University in Ivano-Frankivsk. The practicability of using the ENPEMF method in combination with other methods of electrometry for solving practical geological tasks was experimentally proven.

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