Allan R. Sanford

New measurements of crystal doming over the Socorro magma body, New Mexico

Recent releveling measurements conducted in the Socorro area of the Rio Grande rift specifically to study possible crustal deformation indicate uplift of the central part of the rift (Socorro) relative to bench marks to the west (Magdalena). Total relative uplift measured along the newly releveled line between 1934 and 1978.7 reached 10 ± 1 cm. Combining this new information with previously reported releveling data suggests a roughly elliptical uplift affecting at least 7,000 km 2 . Maximum observed uplift near the center of this area reaches about 20 cm relative to the periphery. The uplift occurs directly above an unusual crustal discontinuity at a depth of about 20 km below this part of the rift. This discontinuity, discovered on the basis of seismic evidence, has been inferred to be the upper boundary of an extensive magma body. The spatial coincidence of the zone of uplift and the presumed magma body, the persistent microearth-quake activity, and the modeling results strongly suggest that the observed movements result from expansion of the Socorro magma body. Although available data are not sufficient to determine uniquely the temporal behavior of the doming, the observations are consistent with more or less continuous uplift averaging about 5 mm/yr during the time the movements were monitored (1909 to 1979). The average inflation rate within the magma body required to produce the observed uplift is on the order of 1 to 2 × 10 −2 km 3 /yr and is consistent with measurements in volcanic regions. Although less clearly defined, the leveling data also suggest relative uplift within the Albuquerque-Belen Basin well beyond the presumed northern boundary of the Socorro magma body. If this uplift is associated with magmatic activity, the Socorro magma body extends considerably farther north than previously expected, or another magma body lies beneath the Albuquerque-Belen Basin.

Improving Regional Earthquake Locations Using a Modified G Matrix and Fuzzy Logic

A simple procedure is presented to resolve locations of regional earthquakes with poor quality of recorded phases and/or a very large gap in seismograph recording geometry. In solving earthquake locations, we use a modified G matrix containing S - P time intervals and P - P and S - S time differences between stations and a forward method. Unlike the regular G matrix, which consists of three spatial parameters ( x, y, z ) and one timing parameter ( t ), the modified G matrix contains only two spatial parameters ( x, y ) and a fixed depth ( z ). The origin time parameter is eliminated by using only relative time intervals. In the new G matrix, two base equations instead of one are used. The S - P time intervals constrain epicentral distance, and P - P and S - S time differences constrain the distribution of azimuth. In searching for epicenters, we first divide the modified G matrix based on individual time intervals then map deviations between theoretical and observed time intervals into logic space using the fuzzy logic technique. Resolutions of earthquake locations are enhanced in the logic space by applying logic operations among individual G matrices. Final locations are derived by searching for a center of gravity in the output matrix. Test results indicate that this method effectively avoids the local minimum problem encountered by generalized inverse methods when data are recorded by small aperture arrays from earthquakes outside the arrays or when large errors occur in phase readings.