Technology and the public sector

Interfaces between technology and public need for improved systems and service

JPL spacecraft sterilization technology program - A status report

Facility description and procedures for heat and ethylene oxide sterilization of spacecraft instrumentation, components, and material

Sand Mining in Baja and Alta California

This Comment will examine some geologic, environmental, and legal aspects of the international sand trade. Looking at the state of sand mining in both countries will demonstrate that the United States and Mexico have parallel regulatory structures and similar environmental concerns and will show how municipal and state officials in Baja California are ale to piggyback their economic concerns onto environmental regulations. This Comment will also examine the sand trade issue for lessons applicable to cross border trade and suggest a certification mechanism that would allow continued sand exports while preserving environmental safeguards

State of stress in seismic gaps along the San Jacinto Fault

Data from the Southern California Seismic Network have been extensively used to map spatial and temporal variations of seismicity (for example, Hileman et al., 1973; Green, 1983; Webb and Kanamori, 1985; Doser and Kanamori 1986; Nicholson et al., 1986). A recent study by Sanders et al. (1986) clarified some of the important features of historical seismicity along the San Jacinto fault of southern California, one of the most prominent being the Anza seismic gap. Thatcher et al. (1975) investigated the spatial distribution of large earthquakes along the fault and indicated that a 40-km-long section from Anza to Coyote Mountain is deficient in seismic slip and can be considered a seismic gap (G1 in fig. 1). Sanders and Kanamori (1984) investigated the seismicity along an 18-km-long section (also often called the Anza seismic gap) centered near the town of Anza, and concluded that this section of the fault is locked and has the potential for a magnitude 6.5 event (G2 in fig. 1). In this paper, we review the most recent activity along the San Jacinto fault and assess the seismic potential of this fault zone in light of an empirical relation between fault length, seismic moment, and repeat time obtained from earthquakes along active fault zones around the world

Forward and inverse three-dimensional P wave velocity models of the southern California crust

We construct a three-dimensional P wave velocity model of the southern California crust by combining existing one-dimensional models, each describing a region defined by surface geology, and calibrate the model with travel times from three explosions. The model is expressed as blocks, each of a given slowness. The variance of the P wave travel time residuals of ≈1000 earthquakes relocated in and near the Los Angeles basin, where the model is most detailed, is half that of the catalog locations in the standard one-dimensional model for southern California. Starting from the forward model, we invert ≈21,000 P wave arrivals from earthquakes for hypocenters and block slownesses using the technique of Roecker (1981). The variance of these P wave travel time residuals decreases 47% during the inversion. Many of the blocks representing the upper crust and midcrust are well sampled and well resolved. The resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions. For example, the velocity structure of the Los Angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the Peninsular Ranges. Moho is between 26 and 32 km depth. In contrast, the Ventura basin has mostly slower sediments above a deeper, higher-velocity basement. Compared to catalog locations, relocations in the final three-dimensional model of 98 ML ≥4 earthquakes throughout southern California tend to deepen below sediment filled valleys and basins, shallow in regions without sedimentary cover, and have a 44% lower P wave travel time residual variance

Slip Along the Superstition Hills Fault Associated with the 24 November 1987 Superstition Hills, California, Earthquake

Surficial slip along the entire mapped length of the Superstition Hills fault in southern California occurred in association with the Superstition Hills earthquake (M_s 6.6)of 24 November 1987. We made repeated measurements of surface slip at 36 sites along the fault and occupied 64 sites at least once. At our sites, dextral slip was as high as 48.5 cm 1 day after the earthquake and 71 cm 2 months after. The measurements show that slip during the period from hr to several hundred hr following the event is described by a simple power law of time. Extrapolation to t = 1 min indicates that co-seismic slippage ranged from 5 to 23 cm at 10 of our best recorded sites, suggesting that finite co-seismic slippage occurred along the length of the fault. These extrapolations are supported by a measurement made at Imler Road 30 min after the shock. Measurements are complete through October 1988. At many sites, the form of slip-rate was decay changed from power law to a function of log time during the interval between 300 and 500 hr after the earthquake. Logarithmic slip-rate decay in time was observed for a period of several yr after the Parkfield, Borrego Mountain, and Imperial Valley earthquakes. Those measurements may have begun too late to resolve power-law behavior at early times. If current logarithmic behavior of the Superstition Hills fault persists, right-lateral slippage will approach 90 cm 10 yr after the rupture. Changes in the along-fault displacement profile correlate well with geometric features including a fault bend and a major fault step. Moreover, slip behavior appears to be correlated to the thickness of sedimentary cover along the fault. Also, the northern half of the fault is bounded by a large block of continental crystalline basement. The presence of this block may have contributed to the relatively uniform early slip behavior observed there

Rupture patterns and preshocks of large earthquakes in the southern San Jacinto fault zone

We relocated the large 1937, 1942, and 1954 earthquakes in the San Jacinto fault zone. The epicenters of the main shocks, aftershocks, and some preshocks were determined using empirical station corrections from recent small events in the study areas. The 1937 (ML 5.9) earthquake has an epicenter between the surface traces of the San Jacinto and Buck Ridge faults, and aftershocks suggest about 7 km of rupture predominantly to the northwest. A significant increase in small earthquake activity occurred about Formula yr before this event. The 1954 (M_L 6.2) earthquake is located at the southeast end of the mapped trace of the San Jacinto fault, and aftershocks suggest about 15 km of rupture further southeast into an area of folded young sediments with no surface fault trace. This event was preceded by a cluster of small earthquakes which occurred within an 8-hr period 10 weeks before the main event and in the eventual rupture zone. The 1942 (M_L 6.3) earthquake is located southwest of the southeast end of the Coyote Creek fault. Large aftershocks of this event are spread over a 15 by 18 km area southwest of the Coyote Creek fault and are not associated with any one fault. The relation of the 1942 event to the San Jacinto fault zone is not simple

Teleseismic source parameters and rupture characteristics of the 24 November 1987, Superstition Hills earthquake

Long-period body waves from the 24 November 1987, Superstition Hills earthquake are studied to determine the focal mechanism and spatial extent of the seismic source. The earthquake is a complex event consisting of two spatially distinct subevents with different focal mechanisms. Two consistent models of rupture are developed. For both models, the second subevent begins 8 sec after the initiation of the first subevent and the preferred centroid depth lies between 4 to 8 km. Model 1 consists of two point sources separated by 15 to 20 km along strike of the Superstition Hills fault. Model 2 consists of one point source and one line source with a rupture velocity of 2.5 km/sec with moment release distributed along strike of the focal plane at a distance of 10 to 22 km from the epicenter. These moment release patterns show that a significant amount of long-period energy is radiated from the southern segment of the fault. Total moment release for both models is approximately 8 × 10^(25) dyne-cm. Both models also suggest a change of dip from near vertical near the epicenter to steeply southwesterly dipping along the southern segment of the fault. The difference in rupture characteristics and fault dips seen teleseismically is also reflected in aftershock and afterslip data, and crustal structure underlying the two fault segments. The northern segment had more aftershocks and a smaller proportion of afterslip than the southern segment. The boundary between the two segments lies at a step in the basement that separates a deeper metasedimentary basement to the south from a shallower crystalline basement to the north