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.

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.

scholarly journals Global earthquakes in the 2020 second half according to the GS RAS

2021 ◽  
Vol 3 (1) ◽  
pp. 7-26
Author(s):  
Yuri Vinogradov ◽  
Mariya Ryzhikova ◽  
Natalia Petrov ◽  
Svetlana Poygina ◽  
Marina Kolomiets
Keyword(s):  
Seismic Energy ◽  
Aegean Sea ◽  
Focal Mechanisms ◽  
Intraplate Earthquake ◽  
Strong Earthquakes ◽  
Tatar Strait ◽  
The Earth ◽  
Deformation Processes ◽  
The Russian Federation ◽  
Deep Focus

The data on the seismicity of the Earth in the second half of 2020 at the level of strong earthquakes with magnitudes mb≥6.0 are given according to the data of the Alert Service of the Geophysical Survey RAS. The review also includes information on 54 tangible earthquakes in Russia and five earthquakes in adjacent territories that were felt in the settlements of the Russian Federation. Two of 67 strong earthquakes of the Earth with mb≥6.0 for the period under consideration were registered in the territory of Russia. For 15 strong earthquakes, the Alert Service published Information Messages within one or two days after their occurrence, for 14 earthquakes the information on focal mechanisms is provided. The strongest earthquake of the Earth with MS=7.9 occurred on July, 22 in the region of the Alaska Peninsula. The maximum human casualties and material damage during the study period were the result of the catastrophic intraplate earthquake with MS=6.8, which occurred on October, 30 in the Aegean Sea, near the Samos Island. As a result of the earthquake, 117 people died, 1054 were injured. The strongest earthquake on the territory of Russia was the deep-focus one with mb=6.4, which took place on November, 30 in the Tatar Strait, separating Sakhalin Island from continental Eurasia. The crustal Bystrinsk earthquake on September, 21 with MS=5.2, which occurred in the area of Lake Baikal, was felt with a maximum intensity I=6–7 on the territory of Russia. Comparative analysis of the rate of seismic energy released in the Globe in 2010-2020 showed that its value in the second half of 2020, as well as for 2019-2020 on average, is one of the lowest for the eleven-year period and indicates a seismic calm, which should be replaced by a period of intensification of global seismic and deformation processes

A new analytical method for finding the upper mantle velocity structure from P and S wave travel times of deep earthquakes

1969 ◽  
Vol 59 (2) ◽  
pp. 755-769
Author(s):  
K. L. Kaila
Keyword(s):  
Velocity Structure ◽  
Focal Depth ◽  
Least Square ◽  
Travel Times ◽  
S Wave ◽  
The Earth ◽  
Straight Line ◽  
Wave Travel ◽  
Deep Focus ◽  
Mantle Velocity

abstract A new analytical method for the determination of velocity at the hypocenter of a deep earthquake has been developed making use of P- and S-wave travel times. Unlike Gutenberg's method which is graphical in nature, the present method makes use of the least square technique and as such it yields more quantitative estimates of the velocities at depth. The essential features of this method are the determination from the travel times of a deep-focus earthquake the lower and upper limits Δ1 and Δ2 respectively of the epicentral distance between which p = (dT/dΔ) in the neighborhood of inflection point can be considered stationary so that the travel-time curve there can be approximated to a straight line T = pΔ + a. From p = (1/v*) determined from the straight line least-square fit made on the travel-time observation points between Δ1 and Δ2 for various focal depths, upper-mantle velocity structure can be obtained by making use of the well known relation v = v*(r0 − h)/r0, h being the focal depth of the earthquake, r0 the radius of the Earth, v* the apparent velocity at the point of inflection and v the true velocity at that depth. This method not only gives an accurate estimate of p, at the same time it also yields quite accurate value of a which is a function of focal depth. Calibration curves can be drawn between a and the focal depth h for various regions of the Earth where deep focus earthquakes occur, and these calibration curves can then be used with advantage to determine the focal depths of deep earthquakes in those areas.

Understanding earthquake from the granular physics point of view — Causes of earthquake, earthquake precursors and predictions

2018 ◽  
Vol 32 (07) ◽  
pp. 1850081 ◽  
Author(s):  
Kunquan Lu ◽  
Meiying Hou ◽  
Zehui Jiang ◽  
Qiang Wang ◽  
Gang Sun ◽  
...  
Keyword(s):  
Large Scale ◽  
Continuum Theory ◽  
Earthquake Precursors ◽  
Point Of View ◽  
The Earth ◽  
Discrete Nature ◽  
Granular Physics ◽  
Deep Focus ◽  
Tectonic Forces ◽  
Precursory Information

We treat the earth crust and mantle as large scale discrete matters based on the principles of granular physics and existing experimental observations. Main outcomes are: A granular model of the structure and movement of the earth crust and mantle is established. The formation mechanism of the tectonic forces, which causes the earthquake, and a model of propagation for precursory information are proposed. Properties of the seismic precursory information and its relevance with the earthquake occurrence are illustrated, and principle of ways to detect the effective seismic precursor is elaborated. The mechanism of deep-focus earthquake is also explained by the jamming–unjamming transition of the granular flow. Some earthquake phenomena which were previously difficult to understand are explained, and the predictability of the earthquake is discussed. Due to the discrete nature of the earth crust and mantle, the continuum theory no longer applies during the quasi-static seismological process. In this paper, based on the principles of granular physics, we study the causes of earthquakes, earthquake precursors and predictions, and a new understanding, different from the traditional seismological viewpoint, is obtained.

scholarly journals On the diurnal variation of the magnetic declination of St. Helena

1851 ◽  
Vol 5 ◽  
pp. 664-664
Keyword(s):  
Diurnal Variation ◽  
Northern Hemisphere ◽  
Opposite Direction ◽  
Diurnal Variations ◽  
The Sun ◽  
The Earth ◽  
Magnetic Declination ◽  
St Helena ◽  
Magnetic Needle

It has long been known that the diurnal variation of the magnetic needle is in an opposite direction in the southern, to what it is in the northern hemisphere; and it was therefore proposed as a pro­blem by Arago, Humboldt and others, to determine whether there exists any intermediate line of stations on the earth where those diurnal variations disappear. The results recorded in the present paper are founded on observations made at St. Helena during the five consecutive years, from 1841 to 1845 inclusive; and also on similar observations made at Singapore, in the years 1841 and 1842; and show that at these stations, which are intermediate between the northern and southern magnetic hemispheres, the diurnal variations still take place; but those peculiar to each hemisphere prevail at opposite seasons of the year, apparently in accordance with the position of the sun with relation to the earth’s equator.

Global Synoptic Maps *

1952 ◽  
Vol 33 (10) ◽  
pp. 435-437 ◽  
Author(s):  
Leo Alpert
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
North Pole ◽  
Weather Bureau ◽  
The North ◽  
The Earth ◽  
Antarctic Continent ◽  
Map Analysis ◽  
The Antarctic

Synoptic map analysis of the Earth from the North Pole to the shores of the Antarctic Continent is now attained by combining the Southern Hemisphere map analysis of the U. S. Weather Bureau-M.I.T. Southern Hemisphere Map Analysis Project, and the Northern Hemisphere map analysis of the published Daily Historical Weather Maps. Sample synoptic maps of the Earth for 19 and 20 March 1949 are presented.

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