William H. Prescott

Strain accumulation across the Eastern California Shear Zone at latitude 36°30′N

The motion of a linear array of monuments extending across the Eastern California Shear Zone (ECSZ) has been measured from 1994 to 1999 with the Global Positioning System. The linear array is oriented N54°E, perpendicular to the tangent to the local small circle drawn about the Pacific-North America pole of rotation, and the observed motion across the ECSZ is approximated by differential rotation about that pole. The observations suggest uniform deformation within the ECSZ (strike N23°W) (26 nstrain yr−1 extension normal to the zone and 39 nstrain yr−1 simple right-lateral shear across it) with no significant deformation in the two blocks (the Sierra Nevada mountains and southern Nevada) on either side. The deformation may be imposed by right-lateral slip at depth on the individual major fault systems within the zone if the slip rates are: Death Valley-Furnace Creek fault 3.2±0.9 mm yr−1, Hunter Mountain-Panamint Valley fault 3.3±1.6 mm yr−1, and Owens Valley fault 6.9±1.6 mm yr−1. However, this estimate of the slip rate on the Owens Valley fault is 3 times greater than the geologic estimate.

Deformation across the Pacific-North America plate boundary near San Francisco, California

We have detected a narrow zone of compression between the Coast Ranges and the Great Valley, and we have estimated slip rates for the San Andreas, Rodgers Creek, and Green Valley faults just north of San Francisco. These results are based on an analysis of campaign and continuous Global Positioning System (GPS) data collected between 1992 and 2000 in central California. The zone of compression between the Coast Ranges and the Great Valley is 25 km wide. The observations clearly show 3.8±1.5 mm yr−1 of shortening over this narrow zone. The strike slip components are best fit by a model with 20.8±1.9 mm yr−1 slip on the San Andreas fault, 10.3±2.6 mm yr−1 on the Rodgers Creek fault, and 8.1±2.1 mm yr−1 on the Green Valley fault. The Pacific-Sierra Nevada-Great Valley motion totals 39.2±3.8 mm yr−1 across a zone that is 120 km wide (at the latitude of San Francisco). Standard deviations are one σ. The geodetic results suggest a higher than geologic rate for the Green Valley fault. The geodetic results also suggest an inconsistency between geologic estimates of the San Andreas rate and seismologic estimates of the depth of locking on the San Andreas fault. The only convergence observed is in the narrow zone along the border between the Great Valley and the Coast Ranges.

The 1984 Morgan Hill, California, Earthquake

The Morgan Hill, California, earthquake (magnitude 6.1) of 24 April 1984 ruptured a 30-kilometer-long segment of the Calaveras fault zone to the east of San Jose. Although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected, no short-term precursors were noted and so the time of the 1984 earthquake was not predicted. Unilateral rupture propagation toward the south-southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion, including an exceptionally large horizontal acceleration of 1.29g at a site on a dam abutment near the southeast end of the rupture zone.

Crustal deformation rates in Central and Eastern U.S. inferred from GPS

Analysis of continuous GPS observations between 1996 and 2000 at 62 stations distributed throughout the central and eastern United States suggests that the area is generally stable. Seven of the 62 stations show anomalous velocities, but there is reason to suspect their monument stability. Assuming the remaining 55 stations are stable with respect to interior North America, we have found the North America-ITRF97 Euler vector (-1.88° ± 1.04° N, 77.67° ± 0.39° W, 0.201° ± 0.004° Myr -1 ) that minimizes the RMS station velocity. Referred to fixed North America, all of these velocities are less than 3.2 mm yr -1 . Motion of several stations suggests the Mississippi embayment may be moving southward away from the rest of the continent at a rate of 1.7±0.9 mm yr -1 . The motion of the embayment produces a large gradient in velocity which, in turn, implies the highest seismic moment accumulation rate that we found. Although the highest rate is only marginally significant, the fact that it occurs near New Madrid, where earthquake risk is thought to be high, argues that the anomaly may be real. Nevertheless, the identification of the anomaly remains tentative.

Global Positioning System Measurements for Crustal Deformation: Precision and Accuracy

Analysis of 27 repeated observations of Global Positioning System (GPS) position-difference vectors, up to 11 kilometers in length, indicates that the standard deviation of the measurements is 4 millimeters for the north component, 6 millimeters for the east component, and 10 to 20 millimeters for the vertical component. The uncertainty grows slowly with increasing vector length. At 225 kilometers, the standard deviation of the measurement is 6, 11, and 40 millimeters for the north, east, and up components, respectively. Measurements with GPS and Geodolite, an electromagnetic distance-measuring system, over distances of 10 to 40 kilometers agree within 0.2 part per million. Measurements with GPS and very long baseline interferometry of the 225-kilometer vector agree within 0.05 part per million.

Strain in Southern California: Measured Uniaxial North-South Regional Contraction

The plate tectonics model of the Pacific moving northwest relative to North America implies that the regional strain in California should be simple shear across a vertical plane striking N45�W or equivalently equal parts of north-south contraction and east-west extension. Measurements of the strain accumulation at seven separate sites in southern California in the interval 1972 through 1978 indicate a remarkably consistent uniaxial north-south contraction of about 0.3 part per million per year; the expected east-west extension is absent. It is not clear whether the period from 1972 through 1978 is anomalous or whether the secular strain in southern California is indeed a uniaxial north-south contraction.

Will a continuous GPS array for L.A. help earthquake hazard assessment

The striking landscapes and hospitable climate of Southern California are home to more than 20 million people and vital elements of the nations economy. Unfortunately, the region is also laced with many active faults that can produce strong earthquakes. Scientists from several institutions are pursuing a new approach to studying earthquake hazards in a high-risk metropolitan area. The Southern California Integrated GPS Network (SCIGN) is currently an array of about 40 Global Positioning System (GPS) stations distributed throughout the greater Los Angeles metropolitan region. There have been informal discussions about expanding the array to 250 stations, and formal proposals have been submitted to begin this expansion. To achieve high precision, the sites will be carefully monumented, and all the GPS receivers will operate continuously. The goals of the array are to provide an accurate and detailed velocity field from which to identify the deformation from known faults, test current models of the geologic structure, and make better estimates of the seismic potential in the populous parts of southern California.

Yes: The L.A. array will radically improve seismic risk assessment

SCIGN will provide a key component in improving our understanding of the scientific and earthquake hazard issues. Because of the advantages, dense permanent GPS stations are being used in many earthquake research studies. The time is right for a project like SCIGN in the greater Los Angeles metropolitan region, scientifically, technically, economically, and socially. SCIGN will contribute key data to the current geologic debate between “thick-skinned” and “thin-skinned” models. With a global network of tracking stations and modern data processing, GPS techniques have matured adequately to handle the data volume and provide the precision required. Considering potential losses from earthquakes in the area, SCIGN is inexpensive relative to the scientific payoff. And the earthquake hazard in the region demands that we do everything we can to reduce the risks.

Will AGU survive in the 21st century

AGU is facing a difficult transition during the next few years. A major shift is underway in the communication of scientific information. Printed journals occupying meters of shelf space will be replaced by some form of electronic distribution. All scientific societies must rethink their role in communicating scientific research, if they want to continue to play a significant part. Otherwise, the 21st century will bring the same fate to scientific societies that the 20th century brought to railroads: obsolescence.