Isostatic Gravity Anomalies Research Articles

Seismicity and isostasy

Both a theoretical treatment and observed data show that the isostatic gravity field and the seismicity are closely related. The equation for harmonic loading shows that large deviations from isostatic equilibrium lead to large shear stresses in the crust. High seismic activity correlates well with large isostatic gravity anomalies and with large horizontal gradients of isostatic gravity anomalies. Studies of seismicity should consider the isostatic gravity field.

Earthquake mechanisms and island arc tectonics in the Indonesian-Philippine region

abstract Forty-four new focal mechanisms from shallow-focus earthquakes in the Indonesian-Philippine region have been determined from P and S waves recorded on long-period instruments. The dominant modes of deformation are thrust and normal faulting rather than strike-slip faulting. Along the Sunda and Philippine arcs the most active zone of shallow focus activity occurs between the ocean trench and the line of active volcanoes. Mechanism solutions from earthquakes in this zone are all of the thrust type with the sense of motion on the shallower dipping of the two nodal planes consistent with underthrusting beneath the island arc. Seismic slip vectors strike in a northeasterly direction along the convex side of the western Sunda arc and strike in a westerly direction along the Philippine arc and its extension along the eastern margin of the Celebes Sea. Normal faulting mechanisms from one earthquake under the Java Trench and another under the Philippine Trench may result from extension in the upper surface of the lithosphere as it bends beneath the island arc. The curvature of the Sunda arc may have caused the inclined seismic zone beneath the arc to shoal toward the northwest. There is a correlation between negative isostatic gravity anomalies, maximum water depth in the trench and the length and shape of the seismic zone beneath the Sunda arc. Shoaling of the inclined seismic zone beneath the nearly linear Phillippine arc can be explained by a decrease in the slip rates from south to north along the arc. Complex regions such as western New Guinea, the Sulu Spur and lesser Philippine islands may include a number of small plates of lithosphere.

Gravity, seismicity, and tectonics of the North Island, New Zealand

Abstract The distribution of earthquakes and gravity anomalies in New Zealand are compared. Shocks classified as 12 km or less are more widespread than those classified as 33 km, but all crustal shocks are symmetrically disposed about the axis of the negative isostatic gravity anomalies. A minimum of seismicity occurs between the base of the crust and about 80 km depth which conflicts with the author's previous suggestion that low densities between these depths are associated with seismicity. The Benioff zone dips at 63° and high densities associated with this zone would be consistent with the positive gravity anomaly. In plan, the main belt of shallow earthquakes follows the negative gravity anomaly axis in changing direction from SW to SSE through Cook Strait. A relatively aseismic region in the upper crust following a parallel strike from south Taranaki through Tasman Bay to Kaikoura separates the main zone from a further active belt running from New Plymouth through north-west Nelson to Canterbury. All these directions cross the main geological structures almost at right angles in the Cook Strait-Marlborough area. There is some evidence that the deeper earthquakes tend to change their strike in a similar manner

VARIATIONS OF VERTICAL GRAVITY GRADIENT IN NEW YORK CITY AND ALPINE, NEW JERSEY

Careful gravity measurements with La Coste‐Romberg geodetic gravimeters were carried out in tall buildings on a floor‐to‐floor basis in New York City and on the Armstrong Tower, Alpine, New Jersey. Corrections for the instrumental drift and tidal gravity variation and for the Bouguer effect, topography, mass of the buildings, and subway and basement excavations have been applied to the gravity data, which are tied to the absolute gravity value of the National Gravity Base Station of Washington, D. C. The observed gravity versus elevation curves are nonlinear, particularly near the surface of the ground; the slope of the observed gravity anomaly versus elevation curves reverses sign at an elevation of about 170 ft for the campus buildings and about 350 ft for the downtown buildings, and is nearly linear without a reversal for the Armstrong Tower. The vertical gradients vary substantially even within short distances. Comparisons of the corrected observed gradients with the theoretical gradients of gravity are made. The anomalous gradient anomalies are positive and are correlated with the positive isostatic surface gravity anomalies. Calibration of gravimeters against the observed vertical gradient of gravity to an accuracy of ±2 μgal is definitely feasible provided the gradient is predetermined to a comparable accuracy by a standard instrument.

A statistical survey of earthquakes in the main seismic region of New Zealand

Abstract Maps are presented showing contours of frequency of earthquake occurrence in the main seismic region of New Zealand from 1942 to 1961. Shallow, intermediate, and deep earthquakes are considered separately; the deeper earthquakes have a more regular and concentrated distribution, and their highest concentration is to the north-west of that of the shallower shocks. Close relationships between the patterns of seismicity and isostatic gravity anomalies are not apparent.

A Proposed Correlation between Elastic Strain Rebound increments and Isostatic Gravity Anomalies

ELASTIC strain rebound increments have been extensively considered by Benioff and others1–6. The results of their investigations indicate that strain rebound increments vary from region to region. Their characteristics are found to be linear and non-linear, tending to asymptotic and irregular. Such elastic strain rebound increments are simply a plot of the cumulative strain factor, J½, versus time, where J is the seismic wave energy and can be calculated from Gutenberg–Richter's relation7 as used by Benioff: where M is the magnitude of the earthquake.

Isostasy and its Meaning

The theory of isostasy supposes that, in regions which have not been disturbed recently, each vertical column of the earth's crust with a certain minimum radius and extending to a depth of about 100 km has approximately the same mass. To find the deviation from this approximation in a given region, the density must be assumed as a function of depth. Such assumptions used at present for calculations are discussed critically. The resulting errors are greater than it is normally beleaved; errors in the calculated isostatic gravity anomalies exceeding ten milligals must be expected in certain regions. Systematic errors result from the usual assumption in routine calculations that the mean density in the earth's crustal layers under the bottom of the Pacific and in the continental areas is the same, and that in both the difference between the density of the layers above about 30 km and the layers below this depth is 0.6. The processes producing and maintaining isostatic equilibrium are discussed.

Isostasy and its Meaning

The theory of isostasy supposes that, in regions which have not been disturbed recently, each vertical column of the earth's crust with a certain minimum radius and extending to a depth of about 100 km has approximately the same mass. To find the deviation from this approximation in a given region, the density must be assumed as a function of depth. Such assumptions used at present for calculations are discussed critically. The resulting errors are greater than it is normally beleaved; errors in the calculated isostatic gravity anomalies exceeding ten milligals must be expected in certain regions. Systematic errors result from the usual assumption in routine calculations that the mean density in the earth's crustal layers under the bottom of the Pacific and in the continental areas is the same, and that in both the difference between the density of the layers above about 30 km and the layers below this depth is 0.6. The processes producing and maintaining isostatic equilibrium are discussed.

The density of the island of Hawaii, and density distribution in the earth's crust

Goranson examines isostatic gravity anomalies from the conterminous United States and Hawaii as background for an attempt to determine the density of Hawaii. He discusses the computation of the theoretical value of gravity and of isostatic compensation. Based on gravity measurements by the U.S. Coast and Geodetic Survey, the upper part of Mauna Kea has a density of 2.12, and Mauna Loa 2.52. He disputes Preston9s (1894) calculated density for Hawaii (3.26) as being larger than the rock density (2.96) calculated by Washington (1923e). Goranson correctly concludes that the Hawaiian gravity anomalies suggest an Airy-type compensation model but that the island of Hawaii is isostatically uncompensated, constituting a load upon the suboceanic crust. Hawaii, therefore, must either be supported by the strength of the crust, or be slowly sinking. He calculates limits on the sinking, noting that if the crust had zero strength, the island should sink 3,100 m, but as the crust must have some rigidity, the actual sinking would be less. He considers this hypothesis also to be a simpler explanation for drowned coral reefs.