Borehole Stations Research Articles

The new accelerograph network for Santa Fe De Bogota, Colombia and implications for microzonation

The new accelerographic network of Santa Fe de Bogotá is composed of 29 three-component stations with sensors at the surface and three additional six-component borehole stations with three sensors at the surface and three at depth (115, 126 and 184 m). In total, 32 stations have been operative in the metropolitan area of Bogotá since 1999. During this period of time, a significant number of weak motion are recorded and used for a preliminary analysis of local site effects. Using the SH-wave response spectra we verify the behavior of the different seismic zones proposed by the previous microzonation study of the city. A comparison between normalized SH-wave response spectra and the normalized design spectra for each zone clearly depicts that parts of the design spectra should be revised, as well as the boundaries between different zones may require some changes. The spectral amplification levels reach up to a factor of 5. The predominant periods obtained by the amplification spectra in different stations in the city, show variability from 0.3 to 3.0 s. A comparison is also made between the predominant periods obtained using H/ V spectral ratios of microtremors and those using weak motion. In general, microtremors tend to predict slightly lower values of dominant periods than those calculated by the weak motion spectra. However, there is a general correlation between the two data sets. Using the data recorded by one of the borehole station, an equivalent linear seismic response analysis was conducted. The modeled and recorded response spectra show similarities in period peaks, however, the modeled soil amplification is underestimated for periods less than 0.8 s. Since the available record is weak motion which represents mostly the linear response of the soils, further analysis is required.

Differential analysis of codaQusing similar microearthquakes in seismic gaps. Part 2: Application to seismograms recorded by the Parkfield High Resolution Seismic Network

AbstractNearly two-thirds of the microseismicity occurring at Parkfield, California, consists of repeating clusters of closely located events exhibiting waveform cross-correlation values exceeding 0.95. This observation, coupled with a high probability for an M ≈ 6 earthquake within the next decade (Working Group on California Earthquake Probabilities, 1988), make this segment of the San Andreas fault system a prime location to search for temporal variations in coda Q associated with crustal changes that may occur during the earthquake cycle. The coda comparison analysis method described in Part 1 of this study (Aster et al., 1996) is applied to data from four borehole stations of the Parkfield High Resolution Seismic Network (HRSN) for the period January 1987 through February 1994. We examine 21 of the nearly 300 repeating clusters observed to date. The restriction of the data set to only the most highly similar sources is observed to be critical; small differences in the source process, even for event pairs with centroid locations within approximately 20 m, significantly increase the spurious noise level in the parameter estimates. Absolute 68% confidence bound estimates obtained on the repeatability of coda Q for all microearthquake pairs are ±10% for frequencies below 20 Hz. Tighter constraints are obtainable by reconstructing the temporal history of coda Q from the first difference measurements between microearthquake pairs. These functional reconstructions show that the mean value of coda Q has not varied by more than approximately 5% from 3 to 30 Hz at the 1σ level during the study period. However, other observables, such as an increased level of microseismicity, suggest that a preparatory stage for the next Parkfield earthquake has begun, indicating that coda Q is so far showing no sign of sensitivity to preparatory processes at Parkfield.

Natural selection of PAH-degrading bacterial guilds at coal-tar disposal sites.

Microbial activity patterns at buried coal-tar disposal sites have been under investigation for several years to determine the response of naturally occurring microflora to polycyclic aromatic hydrocarbons (PAHs) at the sites. At one site in upstate New York, data have shown enrichment of PAH-degrading bacteria in subsurface contaminated zones but not in uncontaminated zones. Similar work at a midwestern site showed that the same trends existed in a heterogeneous disposal site except that a borehole outside the plume showed some PAH-mineralization activity. Polymerase chain reaction amplification of DNA extracted from sediment samples from the New York site indicated the presence of naphthalene metabolism genes nahAc and nahR, similar to those found on the NAH7 plasmid of Pseudomonas putida G7. Significant sequence polymorphism was observed in amplified nahAc products, indicating that divergent homologs of nahAc were present in the native community. Protozoan numbers were elevated in sediment samples displaying relatively high PAH-degrading activity, suggesting that a food chain was established based on PAH-degrading bacteria. Removal of the coal-tar source at the site occurred in 1991. In 1992, sampling of three key borehole stations revealed that mixing and backfilling operations had introduced soil microorganisms into the source area and introduced 14C-PAH-mineralization activity into the previously inactive pristine area. Thus removal of the source of the contaminants and restoration at the site have altered the microbial activity patterns outside the contaminant plume as well as in the source area.