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B.E. Smith

Publications and source records attributed to B.E. Smith.

9 recordsLinked to original sources
USGS2017

Using surface velocities to calculate ice thickness and bed topography: A case study at Columbia Glacier, Alaska, USA

Information about glacier volume and ice thickness distribution is essential for many glaciological applications, but direct measurements of ice thickness can be difficult and costly. We present a new method that calculates ice thickness via an estimate of ice flux. We solve the familiar continuity equation between adjacent flowlines, which decreases the computational time required compared to a solution on the whole grid. We test the method on Columbia Glacier, a large tidewater glacier in Alaska, USA, and compare calculated and measured ice thicknesses, with favorable results. This shows the potential of this method for estimating ice thickness distribution of glaciers for which only surface data are available. We find that both the mean thickness and volume of Columbia Glacier were approximately halved over the period 1957–2007, from 281m to 143 m, and from 294 km 3 to 134 km 3 , respectively. Using bedrock slope and considering how waves of thickness change propagate through the glacier, we conduct a brief analysis of the instability of Columbia Glacier, which leads us to conclude that the rapid portion of the retreat may be nearing an end.

Alaska↗
USGS1980

Local magnetic field measurements and fault creep observations on the San Andreas fault

Simultaneous creep and magnetic field records have been obtained for more than 60 episodic creep events since early 1974, no clear magnetic transients or offsets, as suggested by Breiner and Kovach (1968), are observed at or up to several days before the occurrence times of these events. Although some patterns of creep onset times at adjacent stations over periods of weeks to months appear to correspond to some periods of longer term change in local magnetic field, these changes do not always occur and other groups of creep events have no corresponding changes in local magnetic field. Changes in stress related to the surface expression of episodic fault creep on the San Andreas fault can be estimated from dislocation models fit to observations of simultaneous strains and tilts at points near the fault. These stress values are generally less than 1 bar. For these stress levels and with the apparent limited extent of surface failure, tectonomagnetic models of creep events indicate that simultaneous observations of related magnetic field variations at detectable levels of a gamma or so are unlikely. Slip at greater depth may occur more smoothly and would load the near-surface material to failure. These data also argue against large-scale dilatant cracking occurring along the region of the fault presently monitored.

California↗
USGS1978

Local magnetic field measurements, fault creep observations and local earthquakes on the San Andreas fault

Simultaneous records of local variations in magnetic field, fault creep and the occurrence times of local earthquakes have been obtained for the period early 1974 through mid-1977 along the San Andreas fault between the most southern extent of the 1906 earthquake fault break and the most northern extent of the 1857 break. The data utilized are primarily from stations located near the two ends of this section of the fault where strains accumulation is expected. The magnetic data show that local magnetic field variations up to 1.8 gammas with durations of a few minutes to several months have occurred. The creep data indicate both episodic events and changes in creep rate of up to 10 mm/yr lasting for six months or more. No clear magnetic transients or offsets are evident either simultaneous with or preceding the occurrence times of the episodic creep events by up to a day or so. Although some patterns of creep onset times at adjacent stations appear to correspond to some periods of longer term change in local magnetic field, these changes do not always occur and other groups of creep events have no corresponding changes in local magnetic field. Earthquakes with magnitudes less than 4.0 do not appear to correspond in time to local changes in magnetic field greater than 0.8 gamma or variations in the creep rate. In order to explain the observations presented in this study, it appears necessary to allow for a substantial amount of deep aseismic slip without any obvious attendant changes in the time distribution or size of the local earthquakes. Changes in stress related to the surface expression of aseismic slip on the San Andreas fault can be estimated from dislocation models fit to these data and to observations of simultaneous strains and tilts at points near the fault. These stress values are on average less than one bar near the surface but are probably more than 10 bars in localized regions at depths of a kilometer or so.

California↗
USGS1978

Continuous tilt, strain, and magnetic field measurements near four earthquakes (ML = 3.6 to 3.8) on the San Andreas fault, California

Four moderate earthquakes (M = 3.6 to 3.8) have occurred on the San Andreas fault in central California since October, 1977. These earthquakes are the first since 1974 to occur at this magnitude level within the array of tilt, strain, and magnetic instruments between Chalome (35.726N, 12.249 W) and San Francisco (37.79N, 122.23W) shown in Figure 1. They offer, therefore, one of the few opportunities to search for indications of precursive ground deformation. Furthermore, several other geophysical parameters such as resistivity, creek, radon, seismicity, geodetic strain, etc, are also monitored and allow comparisons for some of these events. This note reports comparative data and some possible implications in continuous strain, tilt, magnetic field and other measurements obtained from instruments within 10 km of the epicenters.

California↗