Schmucker, 1970 (Scripps) - MTNet

ANOMALIES OF GEOMAGNETIC VARIATIONS
IN THE SOUTHWESTERN UNITED STATES
BY
ULRICH SCHMUCKER
ABSTRACT
Local differences of transient geomagnetic variations have been studied in
the southwestern United States with a network of temporary magnetic field
stations, equipped with Askania variographs. Large anomalous Z-variations,
accompanied by a slight reduction of the horizontal amplitude, have been
found along the California coast for fast and slow variations (bays, magnetic
storms, diurnal variations). These coastal Z -variations gradually disappear
inland, within about 200 km from the coast. The coastal anomaly of bays and
other fast disturbances are interpreted as an "edge effect" of oceanic induction
currents. This interpretation suggests relatively high internal conductivities
under the California coast when compared to Lahiri and Price's model
tid".
Less prominent anomalies of fast variations occur further inland in California
and Nevada which are explicable partially by superficial conductivity
contrasts. There remain, however, clear indications of a local uplift of
highly conductive mantle material along the eastern slope of the Sierra Nevada
and near Napa, north of San Francisco. Both areas are characterized
by young volcanic activity and high terrestrial heat flow, supporting the hypothetical
correlation between high internal temperatures and high conductivities.
A profile of ,stations from Tucson, Arizona, to SWeetwater, Texas, revealed
a general reduction of the Z-amplitude of fast and slow variations west
of the Rio Grande, accompanied by a local increase of the D-amplitude at
Las Cruces, New Mexico. These observations have been interpreted by a
general increase of mantle conductivity under the southern Arizona Rockies
and a zone of very high conductivity under the Rio Grande Rift Belt, which is
noted for its intensive vulcanism in recent times. The mantle under southern
Arizona and the California coast seems to be three times better conducting
than the mantle under West Texas at the same level between 50 and 300 km
pepth, which could be explained by a lateral temperature increase of 100° C
from east to west. These conclusions conform with changes of terrestrial
heat flow. Velocity and attenuation of seismic Pn -waves undergo likewise a
distinct change between West Texas and southern Arizona.
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