Gravity Studies of Earthquake-Related Structures Near Massena, New York

1979 ◽  
Vol 50 (1) ◽  
pp. 13-24
Author(s):  
Reinhard K. Frohlich ◽  
Robert L. Albert ◽  
Frank A. Revetta
Keyword(s):  
New York ◽  
Sea Level ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
River Valley ◽  
Density Contrast ◽  
Positive Anomaly ◽  
Eastern United States ◽  
St Lawrence River

Abstract The causes of the seismicity of the St. Lawrence River Valley are not well understood. As is the case for the entire east coast of North America, epicentral zones often occur in regions where no correlation exists between seismicity and mapped geologic structures. Several explanations have been proposed for such a phenomenon: a) earthquakes occur along unmapped surface faults; b) earthquakes occur along subsurface faults showing no surface expression; or c) the earthquakes are not related to existing faults. Conventional analytical techniques, such as upward and downward continuation, were applied to gravity data from the St. Lawrence River Valley in an attempt to delineate possible seismic–related structures. The analysis of the gravity data indicates that the anomalies trend in a north-northeast direction similar to the structural trends of the Precambrian rocks. The major feature of the Simple Bouguer anomaly map is an extensive positive gravity anomaly centered at Massena, New York. Profiles across the Bouguer gravity anomalies and the up-and downward continued gravity anomalies were reproduced with a two–dimensional modeling technique. Among the various non-unique anomaly-producing structures tested we prefer a model suggesting that the positive anomaly near Massena is derived from two bodies with different density contrasts. The first is a wedge (8 km deep by 35 km wide) located 6 km below sea level with a density contrast of +0.11 gm/cm3 and the second is a smaller body (2 km deep by 6 km wide) located 3.3 km below sea level with a density contrast of +0.2 gm/cm3. The large wedge may represent a sequence of interlayered metasediments and metavolcanics related to the Grenville sequence. The smaller body may represent a mafic intrusive. Several authors have suggested that high gradients of gravity (toward positive) produced by mafic intrusives are associated with earthquakes in the eastern United States. The possible existence of a mafic intrusive near Massena, New York, and its proximity to epicentral zones suggest a similar association for earthquakes in the study area.

Stable inversion of gravity anomalies of sedimentary basins with nonsmooth basement reliefs and arbitrary density contrast variations

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 754-764 ◽  
Author(s):  
Valéria C. F. Barbosa ◽  
João B. C. Silva ◽  
Walter E. Medeiros
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
A Priori ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Upper And Lower Bounds ◽  
Density Contrast ◽  
Good Precision ◽  
Rift Basins

We present a new, stable method for interpreting the basement relief of a sedimentary basin which delineates sharp discontinuities in the basement relief and incorporates any law known a priori for the spatial variation of the density contrast. The subsurface region containing the basin is discretized into a grid of juxtaposed elementary prisms whose density contrasts are the parameters to be estimated. Any vertical line must intersect the basement relief only once, and the mass deficiency must be concentrated near the earth’s surface, subject to the observed gravity anomaly being fitted within the experimental errors. In addition, upper and lower bounds on the density contrast of each prism are introduced a priori (one of the bounds being zero), and the method assigns to each elementary prism a density contrast which is close to either bound. The basement relief is therefore delineated by the contact between the prisms with null and nonnull estimated density contrasts, the latter occupying the upper part of the discretized region. The method is stabilized by introducing constraints favoring solutions having the attributes (shared by most sedimentary basins) of being an isolated compact source with lateral borders dipping either vertically or toward the basin center and having horizontal dimensions much greater than its largest vertical dimension. Arbitrary laws of spatial variations of the density contrast, if known a priori, may be incorporated into the problem by assigning suitable values to the nonnull bound of each prism. The proposed method differs from previous stable methods by using no smoothness constraint on the interface to be estimated. As a result, it may be applied not only to intracratonic sag basins where the basement relief is essentially smooth but also to rift basins whose basements present discontinuities caused by faults. The method’s utility in mapping such basements was demonstrated in tests using synthetic data produced by simulated rift basins. The method mapped with good precision a sequence of step faults which are close to each other and present small vertical slips, a feature particularly difficult to detect from gravity data only. The method was also able to map isolated discontinuities with large vertical throw. The method was applied to the gravity data from Reco⁁ncavo basin, Brazil. The results showed close agreement with known geological structures of the basin. It also demonstrated the method’s ability to map a sequence of alternating terraces and structural lows that could not be detected just by inspecting the gravity anomaly. To demostrate the method’s flexibility in incorporating any a priori knowledge about the density contrast variation, it was applied to the Bouguer anomaly over the San Jacinto Graben, California. Two different exponential laws for the decrease of density contrast with depth were used, leading to estimated maximum depths between 2.2 and 2.4 km.

3D Gravity modeling of the volcanic island of Surtsey, Iceland

2021 ◽  
Author(s):  
sara sayyadi ◽  
Magnús T. Gudmundsson ◽  
Thórdís Högnadóttir ◽  
James White ◽  
Joaquín M.C. Belart ◽  
...  
Keyword(s):  
Sea Level ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Gravity Modeling ◽  
Gravity Station ◽  
Effusive Activity ◽  
3D Gravity ◽  
Anomaly Map ◽  
Bouguer Anomaly Map ◽  
Drill Cores

<p>The formation of the oceanic island Surtsey in the shallow ocean off the south coast of Iceland in 1963-1967 remains one of the best-studied examples of basaltic emergent volcanism to date. The island was built by both explosive, phreatomagmatic phases and by effusive activity forming lava shields covering parts of the explosively formed tuff cones.  Constraints on the subsurface structure of Surtsey achieved mainly based on the documented evolution during eruption and from drill cores in 1979 and in the ICDP-supported SUSTAIN drilling expedition in 2017(an inclined hole, directed 35° from the vertical). The 2017 drilling confirmed the existence of a diatreme, cut into the sedimentary pre-eruption seafloor (Jackson et al., 2019). </p><p>We use 3D-gravity modeling, constrained by the stratigraphy from the drillholes to study the structure of the island and the underlying diatreme.  Detailed gravity data were obtained on Surtsey in July 2014 with a gravity station spacing of ~100 m. Density measurements for the seafloor sedimentary and tephra samples of the surface were carried out using the ASTM1 protocol. By comparing the results with specific gravity measurements of cores from drillhole in 2017, a density contrast of about 200 kg m<sup>-3</sup> was found between the lapilli tuffs of the diatreme and the seafloor sediments.  Our approach is to divide the island into four main units of distinct density: (1) tuffs above sea level, (2) tuffs below sea level, (3) lavas above sea level, and (4) a lava delta below sea level, composed of breccias over which the lava advanced during the effusive eruption.  The boundaries between the bodies are defined from the eruption history and mapping done during the eruption, aided by the drill cores. </p><p>A complete Bouguer anomaly map is obtained by calculating a total terrain correction by applying the Nagy formula to dense DEMs (5 m spacing out to 1.2 km from station, 200 m spacing between 1.2 km and 50 km) of both island topography and ocean bathymetry.  Through the application of both forward and inverse modeling, using the GM-SYS 3D software, the results provide a 3-D model of the island itself, as well as constraints on diatreme shape and depth.</p>

Gravity interpretation using Fourier transforms and simple geometrical models with exponential density contrast

Geophysics ◽  
1993 ◽  
Vol 58 (8) ◽  
pp. 1074-1083 ◽  
Author(s):  
D. Bhaskara Rao ◽  
M. J. Prakash ◽  
N. Ramesh Babu
Keyword(s):  
Los Angeles ◽  
Fourier Transforms ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Density Contrast ◽  
Model Parameters ◽  
Exponential Density ◽  
Exponential Density Contrast ◽  
Gravity Interpretation

The decrease of density contrast in sedimentary basins can often be approximated by an exponential function. Theoretical Fourier transforms are derived for symmetric trapezoidal, vertical fault, vertical prism, syncline, and anticline models. This is desirable because there are no equivalent closed form solutions in the space domain for these models combined with an exponential density contrast. These transforms exhibit characteristic minima, maxima, and zero values, and hence graphical methods have been developed for interpretation of model parameters. After applying end corrections to improve the discrete transforms of observed gravity data, the transforms are interpreted for model parameters. This method is first tested on two synthetic models, then applied to gravity anomalies over the San Jacinto graben and Los Angeles basin.

Shape and depth solutions from gravity data using correlation factors between successive least‐squares residuals

Geophysics ◽  
1993 ◽  
Vol 58 (12) ◽  
pp. 1785-1791 ◽  
Author(s):  
El‐Sayed M. Abdelrahman ◽  
Hesham M. El‐Araby
Keyword(s):  
Least Squares ◽  
Shape Factor ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Vertical Cylinder ◽  
Gravity Anomalies ◽  
Horizontal Cylinder ◽  
Amplitude Coefficient ◽  
Regional Component ◽  
Correlation Factors

The gravity anomaly expression produced by most geologic structures can be represented by a continuous function in both shape (shape factor) and depth variables with an amplitude coefficient related to the mass. Correlation factors between successive least‐squares residual gravity anomalies from a buried vertical cylinder, horizontal cylinder, and sphere are used to determine the shape and depth of the buried geologic structure. For each shape factor value, the depth is determined automatically from the correlation value. The computed depths are plotted against the shape factor representing a continuous correlation curve. The solution for the shape and depth of the buried structure is read at the common intersection of correlation curves. This method can be applied to a Bouguer anomaly profile consisting of a residual component caused by local structure and a regional component. This is a powerful technique for automatically separating the Bouguer data into residual and regional polynomial components. This method is tested on theoretical examples and a field example. In both cases, the results obtained are in good agreement with drilling results.

Reproduction and Survival of Female Mallards in the St. Lawrence River Valley, New York

1995 ◽  
Vol 59 (1) ◽  
pp. 23 ◽  
Author(s):  
Michael P. Losito ◽  
Guy A. Baldassarre ◽  
Jeffrey H. Smith
Keyword(s):  
New York ◽  
River Valley ◽  
St Lawrence River

The Mid-Atlantic Ridge Near 45 °N. XX. The Gravity Field

1972 ◽  
Vol 9 (8) ◽  
pp. 942-959 ◽  
Author(s):  
J. M. Woodside
Keyword(s):  
Gravity Data ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
Spreading Rate ◽  
Survey Area ◽  
Free Air ◽  
Mid Atlantic Ridge ◽  
Sea Floor Spreading ◽  
Relationship Of ◽  
East West

Detailed maps of free-air, Bouguer, and residual gravity anomalies for a survey area 250 km wide across the Mid-Atlantic Ridge between 45° and 46 °N have been compiled. The Bouguer anomaly was terrain-corrected to a radius of 40 km. The residual anomaly was computed from the terrain-corrected Bouguer anomaly using an empirical linear relationship between the Bouguer anomaly and the bathymetry to predict a 'regional' Bouguer anomaly from the depth data. North–south and east–west trends in the gravity data are enhanced in the residual anomaly; and it is suggested that at least one short east–west transform fault may offset the ridge in a right-lateral sense. The offset is presumably a response to a change in sea-floor spreading direction from west–northwest/east–southeast to west/east about 10 m.y. ago. A change in spreading rate may have occurred at the same time. A difference in accretion rate on either side of the ridge axis is indicated by asymmetry in the gravity data and by differences in the topographic compensation across the axis. Variations in the relationship of terrain-corrected Bouguer anomaly to bathymetry within the survey area suggest that a density deficiency or buoyant forces in the upper mantle are responsible for the overall elevation of the crestal mountain region but that the topography of the high-fractured plateau may be partially compensated by undulations of the crust–mantle interface.

scholarly journals Seafloor Density Contrast Derived From Gravity and Shipborne Depth Observations: A Case Study in a Local Area of Atlantic Ocean

2021 ◽  
Vol 9 ◽  
Author(s):  
Xiaoyun Wan ◽  
Weipeng Han ◽  
Jiangjun Ran ◽  
Wenjie Ma ◽  
Richard Fiifi Annan ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Local Area ◽  
Density Variation ◽  
Mean Difference ◽  
Density Contrast ◽  
Data Sets ◽  
Marine Gravity ◽  
Band Pass

Marine gravity data from altimetry satellites are often used to derive bathymetry; however, the seafloor density contrast must be known. Therefore, if the ocean water depths are known, the density contrast can be derived. This study experimented the total least squares algorithm to derive seafloor density contrast using satellite derived gravity and shipborne depth observations. Numerical tests are conducted in a local area of the Atlantic Ocean, i.e., 34°∼32°W, 3.5°∼4.5°N, and the derived results are compared with CRUST1.0 values. The results show that large differences exist if the gravity and shipborne depth data are used directly, with mean difference exceeding 0.4 g/cm3. However, with a band-pass filtering applied to the gravity and shipborne depths to ensure a high correlation between the two data sets, the differences between the derived results and those of CRUST1.0 are reduced largely and the mean difference is smaller than 0.12 g/cm3. Since the spatial resolution of CRUST1.0 is not high and in many ocean areas the shipborne depths and gravity anomalies are much denser, the method of this study can be an alternative method for providing seafloor density variation information.

Integrated geological and geophysical studies for delineation of chromite deposits: A case study from Tangarparha, Orissa, India

Geophysics ◽  
2011 ◽  
Vol 76 (5) ◽  
pp. B173-B185 ◽  
Author(s):  
William K. Mohanty ◽  
Animesh Mandal ◽  
S. P. Sharma ◽  
Saibal Gupta ◽  
Surajit Misra
Keyword(s):  
Bouguer Anomaly ◽  
Geophysical Methods ◽  
Gravity Anomalies ◽  
High Density ◽  
Geological Mapping ◽  
Magnetic Data ◽  
Ultramafic Rocks ◽  
Dc Resistivity ◽  
Positive Anomaly ◽  
Very High

In Orissa, India, chromite deposits occur in a NE-SW trending belt as discontinuous pods associated with tectonically deformed and metamorphosed ultramafic rocks. Geological mapping and detailed geophysical survey (including gravity, magnetic, electrical, and electromagnetic methods) for exploring chromite were conducted in a [Formula: see text] area at Tangarparha, located within the belt. Lithologies include sheared granite, quartzofeldspathic gneiss, and mafic/ultramafic rocks. The calculated Bouguer anomaly map shows a distinct positive anomaly (up to 16 mGal) in the northern part of the area, indicating the existence of a very high density rock in the subsurface. The trend-surface analysis technique was applied to the gravity and magnetic data for regional-residual separation. The 2D and 2.5D forward modelings of the residual gravity anomaly suggest the presence of lithologies with densities higher than mafic/ultramafic rocks in the subsurface. Chromite fragments recovered from pits within the soil cover around the location indicate that the very high density material is likely to be chromite. Correlation of magnetic and gravity anomalies further emphasizes this possibility. The results of very low frequency (VLF) and DC-resistivity surveys reveal that the suspected chromite deposit is about 250–300 m long in a south-north direction, and 300–350 m wide in the east-west direction. The estimated depth of the deposit varies from 35–100 m. VLF and DC-resistivity methods suggest that chromite occurs in the form of a small disseminated body within a mafic/ultramafic rock matrix. The ambiguity of interpretation is reduced by systematic integration of complementary geophysical methods, compared to that from any single geophysical technique.

New gravity data and 3-D density model constraints on the Ivrea Geophysical Body (Western Alps)

2020 ◽  
Vol 222 (3) ◽  
pp. 1977-1991 ◽  
Author(s):  
M Scarponi ◽  
G Hetényi ◽  
T Berthet ◽  
L Baron ◽  
P Manzotti ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Sea Level ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Western Alps ◽  
Density Model ◽  
Density Contrast ◽  
Surface Rock ◽  
Spatial Coverage ◽  
Density Map

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

Moho depth and sediment thickness estimation beneath the Red Sea derived from satellite and terrestrial gravity data

Geophysics ◽  
2013 ◽  
Vol 78 (5) ◽  
pp. G89-G101 ◽  
Author(s):  
Ahmed Salem ◽  
Chris Green ◽  
Simon Campbell ◽  
J. Derek Fairhead ◽  
Lorenzo Cascone ◽  
...  
Keyword(s):  
Red Sea ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Density Contrast ◽  
Moho Depth ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Terrestrial Gravity ◽  
Residual Gravity ◽  
Northern Red Sea

We sought to map the depth and density contrast of the Mohorovičić discontinuity (Moho) across the Red Sea area and to model sedimentary thickness from gravity data. The gravity data that are used are a combination of satellite and terrestrial gravity data processed into a Bouguer anomaly grid. A 200-km low-pass filter was used to separate this grid into regional and residual gravity grids. We inverted the regional gravity grid to a Moho depth map based on a density contrast map that is constrained by published seismic results. The interpreted Moho is shallowest ([Formula: see text]) along the axis of the central Red Sea, [Formula: see text] along the axis to the south, and [Formula: see text] in the northern Red Sea. The depth increased to [Formula: see text] at the coast and 35–40 km in the adjacent continents. The residual gravity data provided insights into the tectonic fabric along the whole rift and provided a good correlation with magnetic lineaments where these are available. We used the complete Bouguer anomaly to model sedimentary thicknesses constrained by wells and the interpreted Moho. The modeling results are consistent with the presence of large-scale, ridge parallel tilted fault blocks forming subbasins with a maximum depth of about 6–7 km. Our models suggest that the northern Red Sea has an asymmetric basement surface with the western side deeper than the eastern side. The results indicate the presence of oceanic crust in the central and southern parts of the Red Sea, but not in the north. The very thin crust and interpreted oceanic crustal density in the central Red Sea suggest that this area is fully oceanic—although possibly quite young.

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