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At least 37 records · Page 2Linked to original sources
USGS1987

A model of geomagnetic secular variation for 1980-1983

We developed an updated model of the secular variation of the main geomagnetic field during 1980 through 1983 based on annual mean values for that interval from 148 worldwide magnetic observatories. The model consists of a series of 80 spherical harmonics, up to and including those of degree and order 8. We used it to form a proposal for the 1985 revision of the International Geomagnetic Reference Field (IGRF). Comparison of the new model, whose mean epoch is approximately 1982.0, with the Provisional Geomagnetic Reference Field for 1975-1980 (PGRF 1975), indicates that the moment of the centered-dipole part of the geomagnetic field is now decreasing faster than it was 5 years ago. The rate (in field units) indicated by PGRF 1975 was about -25 nT a-1, while for the new model it is -28 nT a-1. ?? 1987.

Physics of the Earth and Planetary Interiors↗
USGS1987

Evaluation of the 1985-1990 IGRF secular variation candidates

The IGRF secular variation model for 1985-1990 was adopted by the International Association of Geomagnetism and Aeronomy at its Prague meeting in August 1985 as an average of the three candidate models submitted to the committee. We compared the three models at epoch 1985.0 against each other and against a new model based on observatory data available as of July 1, 1985. These comparisons showed that one of the three candidate models disagreed more with the other two and our model, especially in the eastern Pacific. None of the candidate models was seen to respond to a change in the secular variation of the vertical component that appears to have taken place most strongly in the western Pacific area since 1982. The lack of satellite data was seen to be a significant handicap towards prediction of the field change over most of the Earth's surface, especially the southern oceans. Maximum errors of any model are estimated to be of the order of 80 nT a-1. ?? 1987.

Physics of the Earth and Planetary Interiors↗
USGS1989

Comparison of geoelectrical/tectonic models for suture zones in the western U.S.A. and eastern Europe: are black shales a possible source of high conductivities?

Large-scale geoelectrical anomalies have been mapped with geomagnetic depth sounding (GDS) and magnetotelluric (MT) surveys in the Carpathian Mountains region. These anomalies are associated with the zone of closure between stable Europe and a complex of microplates in front of the converging African plate. The zone of closure, or suture zone, is largely occupied by an extensive deformed flysch belt. The models derived to fit the observed geoelectrical data are useful in the study of other suture zones, and Carpathian structures have been compared with areas currently being studied in the western Cordillera of the U.S.A. Models derived for a smaller-scale suture zone mapped in western Washington State have features that are similar to the Carpathian models. The geoelectrical models for both the Carpathian and Washington anomalies require dipping conductive slabs of 1-5 ?? m material that extends to depths > 20 km. In both instances there is evidence that these materials may merge with lower crustal-mantle conductors along the down-dip margins of the slab. The main conductive units are interpreted to be sedimentary rocks that have been partially subducted due to collisional processes. Heat flow is low in both regions and it is difficult to explain fully the deep conduction mechanisms; however, evidence suggests that the conduction at depth may include electronic conduction in sulfide mineral or carbon films as well as ionic conduction in fluids or partial melt. ?? 1989.

Physics of the Earth and Planetary Interiors↗
USGS1990

What do we mean by accuracy in geomagnetic measurements?

High accuracy is what distinguishes measurements made at the world's magnetic observatories from other types of geomagnetic measurements. High accuracy in determining the absolute values of the components of the Earth's magnetic field is essential to studying geomagnetic secular variation and processes at the core mantle boundary, as well as some magnetospheric processes. In some applications of geomagnetic data, precision (or resolution) of measurements may also be important. In addition to accuracy and resolution in the amplitude domain, it is necessary to consider these same quantities in the frequency and space domains. New developments in geomagnetic instruments and communications make real-time, high accuracy, global geomagnetic observatory data sets a real possibility. There is a growing realization in the scientific community of the unique relevance of geomagnetic observatory data to the principal contemporary problems in solid Earth and space physics. Together, these factors provide the promise of a 'renaissance' of the world's geomagnetic observatory system. ?? 1990.

Physics of the Earth and Planetary Interiors↗
USGS1990

An evaluation of the accuracy of geomagnetic data obtained from an unattended, automated, quasi-absolute station

A comparison is made of geomagnetic calibration data obtained from a high-sensitivity proton magnetometer enclosed within an orthogonal bias coil system, with data obtained from standard procedures at a mid-latitude U.S. Geological Survey magnetic observatory using a quartz horizontal magnetometer, a Ruska magnetometer, and a total field magnetometer. The orthogonal coil arrangement is used with the proton magnetometer to provide Deflected-Inclination-Deflected-Declination (DIDD) data from which quasi-absolute values of declination, horizontal intensity, and vertical intensity can be derived. Vector magnetometers provide the ordinate values to yield baseline calibrations for both the DIDD and standard observatory processes. Results obtained from a prototype system over a period of several months indicate that the DIDD unit can furnish adequate absolute field values for maintaining observatory calibration data, thus providing baseline control for unattended, remote stations. ?? 1990.

Physics of the Earth and Planetary Interiors↗
USGS1990

Determination of the accuracy and operating constants in a digitally biased ring core magnetometer

By using a very stable voltage reference and a high precision digital-to-analog converter to set bias in digital increments, the inherently high stability and accuracy of a ring core magnetometer can be significantly enhanced. In this case it becomes possible to measure not only variations about the bias level, but to measure the entire value of the field along each magnetometer sensing axis in a nearly absolute sense. To accomplish this, one must accurately determine the value of the digital bias increment for each axis, the zero field offset value for each axis, the scale values, and the transfer coefficients (or nonorthogonality angles) for pairs of axes. This determination can be carried out very simply, using only the Earth's field, a proton magnetometer, and a tripod-mounted fixture which is capable of rotations about two axes that are mutually perpendicular to the Earth's magnetic field vector. ?? 1990.

Physics of the Earth and Planetary Interiors↗
USGS1995

Frequency-moment distribution of deep earthquakes; Implications for the seismogenic zone at the bottom of slabs

We present a systematic investigation of the variation with depth of the frequency of earthquake occurrence vs. seismic moment based on 16 years of Harvard Centroid Moment Tensor (CMT) solutions. We analyze depth variations of earthquake size distribution in terms of variations in the absolute value of the slope of the regression of the logarithm of the population vs. seismic moment, a quantity known as the β parameter. The shallowest earthquakes (0–50 km depth) exhibit a well-defined and robust size distribution regime characterized by a discontinuous increase in β with increasing moment. Others have shown that this increase probably represents the effects of a physical limit in the dimensions of the area of seismogenic slip of shallow earthquake sources. The population of deep earthquakes in the depth interval 500–600 km shows two markedly different distributions. The deep earthquakes in the Tonga region feature an initially high β value (0.92) at small moments and a lower β value (0.41) at high moments. In contrast, the size distribution of non-Tonga deep events shows the reverse of those changes (β = 0.41 at low moment and β = 1.17 at higher moment). To help explain these observations, we propose a model of deep seismogenesis that assumes three-dimensional earthquake source regions that vary principally in their transverse dimensions. The two-β segment behavior in the Tonga region and other subduction zones is thought to represent, in part, constraints owing to the threshold of completeness of the CMT catalog and to its short time interval of sampling. We interpret the differences between Tonga and other deep Wadati-Benioff zones as being a consequence of Tonga's markedly higher subduction rate and, hence, its colder thermal structure and presumably thicker region of seismogenesis. We interpret the critical moments at which β values change in terms of variations in the transverse thickness of deep seismogenic zones and estimate that it is about 11 km for the Tonga region and about 3 km for other zones at depths of 500–600 km. These results are generally consistent with deep earthquakes being restricted to wedge-shaped regions of peridotite persisting metastably to as deep as 700 km in old, rapidly descending and hence cold slabs. Failure is thought to occur in metastable peridotite by transformational faulting. Great deep earthquakes present special challenges to any theory of deep earthquakes based on slab thermal structure. For example, a continuing question is how such large events can fit in a thermally controlled seismogenic zone that is diminishing in its transverse dimensions with increasing depth. The very concept of a scale-invariant earthquake size distribution may be inappropriate for these rare events and the unusual settings in which they are found.

Physics of the Earth and Planetary Interiors↗
USGS1996

Rupture distribution of the 1977 western Argentina earthquake

Teleseismic P and SH body waves are used in a finite-fault, waveform inversion for the rupture history of the 23 November 1977 western Argentina earthquake. This double event consists of a smaller foreshock (M0 = 5.3 ?? 1026 dyn-cm) followed about 20 s later by a larger main shock (M0 = 1.5 ?? 1027 dyn-cm). Our analysis indicates that these two events occurred on different fault segments: with the foreshock having a strike, dip, and average rake of 345??, 45??E, and 50??, and the main shock 10??, 45??E, and 80??, respectively. The foreshock initiated at a depth of 17 km and propagated updip and to the north. The main shock initiated at the southern end of the foreshock zone at a depth of 25 to 30 km, and propagated updip and unilaterally to the south. The north-south separation of the centroids of the moment release for the foreshock and main shock is about 60 km. The apparent triggering of the main shock by the foreshock is similar to other earthquakes that have involved the failure of multiple fault segments, such as the 1992 Landers, California, earthquake. Such occurrences argue against the use of individual, mapped, surface fault or fault-segment lengths in the determination of the size and frequency of future earthquakes.

Physics of the Earth and Planetary Interiors↗
USGS1996

Calculation of earthquake rupture histories using a hybrid global search algorithm: Application to the 1992 Landers, California, earthquake

A method is presented for the simultaneous calculation of slip amplitudes and rupture times for a finite fault using a hybrid global search algorithm. The method we use combines simulated annealing with the downhill simplex method to produce a more efficient search algorithm then either of the two constituent parts. This formulation has advantages over traditional iterative or linearized approaches to the problem because it is able to escape local minima in its search through model space for the global optimum. We apply this global search method to the calculation of the rupture history for the Landers, California, earthquake. The rupture is modeled using three separate finite-fault planes to represent the three main fault segments that failed during this earthquake. Both the slip amplitude and the time of slip are calculated for a grid work of subfaults. The data used consist of digital, teleseismic P and SH body waves. Long-period, broadband, and short-period records are utilized to obtain a wideband characterization of the source. The results of the global search inversion are compared with a more traditional linear-least-squares inversion for only slip amplitudes. We use a multi-time-window linear analysis to relax the constraints on rupture time and rise time in the least-squares inversion. Both inversions produce similar slip distributions, although the linear-least-squares solution has a 10% larger moment (7.3 ?? 1026 dyne-cm compared with 6.6 ?? 1026 dyne-cm). Both inversions fit the data equally well and point out the importance of (1) using a parameterization with sufficient spatial and temporal flexibility to encompass likely complexities in the rupture process, (2) including suitable physically based constraints on the inversion to reduce instabilities in the solution, and (3) focusing on those robust rupture characteristics that rise above the details of the parameterization and data set.

Physics of the Earth and Planetary Interiors↗
USGS2004

Book review: Earth’s magnetism in the age of sail

For many centuries, the source, behavior, and even the essential nature of geomagnetism were enigmatic. Despite this, the effect of geomagnetism was familiar, by imparting a directional preference on the magnetized needle of the compass and providing a useful, if somewhat annoyingly complicated, reference for navigators. Although the compass seems to have first been invented in China, it was the Europeans who made the most systematic early studies of magnetism, who made the first elaborate and practical usage of the compass, and who developed most of the early theories as to the cause of the compass needle’s north-seeking tendency. From the centuries of the Middle Ages, through the late 16th century of the Renaissance, to the 17th century of philosophical enlightenment and the 18th century of discovery, the subject of magnetism and, more specifically, geomagnetism, evolved from a hodgepodge of mystical beliefs into something that we can today recognize as the object of modern scientific pursuit. Those same centuries witnessed the great transoceanic sailing voyages undertaken by European nations for reasons of exploration, territorial claim, religious mission, and mercantile trade. Naturally, the navigator’s compass, and therefore geomagnetism, played an important role in these developments. This romantic intersection of science and history is the subject of Earth’s Magnetism in the Age of Sail , a pleasantly written and scholarly book by A.R.T. Jonkers.

Physics of the Earth and Planetary Interiors↗
USGS2009

Localized double-array stacking analysis of PcP: D″ and ULVZ structure beneath the Cocos plate, Mexico, central Pacific, and north Pacific

A large, high quality P-wave data set comprising short-period and broadband signals sampling four separate regions in the lowermost mantle beneath the Cocos plate, Mexico, the central Pacific, and the north Pacific is analyzed using regional one-dimensional double-array stacking and modelling with reflectivity synthetics. A data-screening criterion retains only events with stable PcP energy in the final data stacks used for modelling and interpretation. This significantly improves the signal stacks relative to including unscreened observations, allows confident alignment on the PcP arrival and allows tight bounds to be placed on P-wave velocity structure above the core&ndash;mantle boundary (CMB). The PcP reflections under the Cocos plate are well modelled without any ultra-low velocity zone from 5 to 20&deg;N. At latitudes from 15 to 20&deg;N, we find evidence for two P-wave velocity discontinuities in the D &Prime; region. The first is &sim;182 km above the CMB with a &delta;ln V p of +1.5%, near the same depth as a weaker discontinuity (<+0.5%) observed from 5 to 15&deg;N in prior work. The other reflector is &sim;454 km above the CMB, with a &delta;ln V p of +0.4%; this appears to be a shallower continuation of the joint P- and S-wave discontinuity previously detected south of 15&deg; N, which is presumed to be the perovskite to post-perovskite phase transition. The data stacks for paths bottoming below Mexico have PcP images that are well matched with the simple IASP91 structure, contradicting previous inferences of ULVZ presence in this region. These particular data are not very sensitive to any D &Prime; discontinuities, and simply bound them to be <&sim;2%, if present. Data sampling the lowermost mantle beneath the central Pacific confirm the presence of a &sim;15-km thick ultra-low velocity zone (ULVZ) just above the CMB, with &delta;ln V p and &delta;ln V s of around &minus;3 to &minus;4% and &minus;4 to &minus;8%, respectively. The ULVZ models predict previous S-wave data stacks well. The data for this region indicate laterally varying V p discontinuities in D &Prime;, with one subregion having a &delta;ln V p of 0.5% 140 km above the CMB. Beneath the north Pacific, the PcP arrivals are compatible with only weak ULVZ (&delta;ln V p &sim; 0 to &minus;3%), and there is a weak D &Prime; reflector with &delta;ln V p = 0.5%, near 314 km above the CMB. These results indicate localized occurrence of detectable ULVZ structures rather than ubiquitous ULVZ structure and emphasize the distinctiveness between the large low shear velocity province under the central Pacific and circum-Pacific regions.

Physics of the Earth and Planetary Interiors↗
USGS2009

On the reported magnetic precursor of the 1989 Loma Prieta earthquake

Among the most frequently cited reports in the science of earthquake prediction is that by Fraser-Smith et al. (1990) and Bernardi et al. (1991). They found anomalous enhancement of magnetic-field noise levels prior to the 18 October 1989 Loma Prieta earthquake in the ultra-low-frequency range (0.0110-10.001 Hz) from a ground-based sensor at Corralitos, CA, just 7 km from the earthquake epicenter. In this analysis, we re-examine all of the available Corralitos data (21 months from January 1989 to October 1990) and the logbook kept during this extended operational period. We also examine 1.0-Hz (1-s) data collected from Japan, 0.0167-Hz (1-min) data collected from the Fresno, CA magnetic observatory, and the global Kp magnetic-activity index. The Japanese data are of particular importance since their acquisition rate is sufficient to allow direct comparison with the lower-frequency bands of the Corralitos data. We identify numerous problems in the Corralitos data, evident from both straightforward examination of the Corralitos data on their own and by comparison with the Japanese and Fresno data sets. The most notable problems are changes in the baseline noise levels occurring during both the reported precursory period and at other times long before and after the earthquake. We conclude that the reported anomalous magnetic noise identified by Fraser-Smith et al. and Bernardi et al. is not related to the Loma Prieta earthquake but is an artifact of sensor-system malfunction. ?? 2008 Elsevier B.V.

Physics of the Earth and Planetary Interiors↗
USGS2015

Double point source W-phase inversion: Real-time implementation and automated model selection

Rapid and accurate characterization of an earthquake source is an extremely important and ever evolving field of research. Within this field, source inversion of the W-phase has recently been shown to be an effective technique, which can be efficiently implemented in real-time. An extension to the W-phase source inversion is presented in which two point sources are derived to better characterize complex earthquakes. A single source inversion followed by a double point source inversion with centroid locations fixed at the single source solution location can be efficiently run as part of earthquake monitoring network operational procedures. In order to determine the most appropriate solution, i.e., whether an earthquake is most appropriately described by a single source or a double source, an Akaike information criterion (AIC) test is performed. Analyses of all earthquakes of magnitude 7.5 and greater occurring since January 2000 were performed with extended analyses of the September 29, 2009 magnitude 8.1 Samoa earthquake and the April 19, 2014 magnitude 7.5 Papua New Guinea earthquake. The AIC test is shown to be able to accurately select the most appropriate model and the selected W-phase inversion is shown to yield reliable solutions that match published analyses of the same events.

Physics of the Earth and Planetary Interiors↗