ABSTRACT Time residuals from 75-km segments of 18 crustal seismic-refraction profiles in the Basin and Range province are used to investigate the validity of the linear-regression model and to make large sample estimates of the variance in the travel time distributions. A formula for unbiased estimates of velocity uncertainty is derived, assuming a linear trend with distance for the variances of the travel-time distributions. If the recording units are symmetric about the center of the recording interval, this formula is equivalent to the one derived assuming the variances are equal. At the 95-per cent confidence level the chi-squared test implied 84 per cent of the time-residual samples were inconsistent with the hypothesis that their parent populations had Gaussian distributions. If the number of recording locations expceeds 8, confidence limits computed without the Gaussian assumption suggest the departures from normality are not significant for velocity uncertainty estimates. The large sample estimates of the time-residual populations may be applicable to other areas. This evidence motivated the development of a method, requiring very little numerical calculation, for estimating uncertainties in velocities. The method requires, in addition to the large sample estimates of the travel time variances, information on the quality of the data, the location of the recording interval, and the number of recording units. The method is useful for the design of new experiments and independent estimates of uncertainty reported in the literature.
Dilatational earth strain, associated with the radiation fields for several hundred local, regional, and teleseismic earthquakes, has been recorded over an extended bandwidth and dynamic range at four borehole sites near the San Andreas fault, CA. The general theory of linear viscoelasticity is applied to account for anelasticity of the near-surface materials and to provide a mathematical basis for interpretation of seismic radiation fields as detected simultaneously by co-located volumetric strain meters and seismometers. The general theory is applied to describe volumetric strain and displacement for general (homogeneous or inhomogeneous) P and S waves in an anelastic whole space. Solutions to the free-surface reflection problems for incident general P and S-I waves are used to evaluate the effect of the free surface on observations from co-located sensors. Corresponding expressions are derived for a Rayleigh-type surface wave on a linear viscoelastic half-space. The theory predicts a number of anelastic wave field characteristics that can be inferred from observation of volumetric strains and displacement fields as detected by co-located sensors that cannot be inferred from either sensor alone. Volumetric strain meters respond to P waves but not S waves, with simultaneous observations permitting resolution of superimposed P and S wave fields into their respective components. The amplitude and phase for components of the displacement fields depend on angle of incidence, azimuth and inhomogeneity of the wave field. As volumetric strain shows no similar dependencies, simultaneous measurement permits inference of these characteristics as well as in situ material parameters. Conversion of S energy to dilatational strain energy by the free surface is largest at angles of incidence for which inhomogeneity of the reflected P wave is near its physical limit (that is, amplitudes vary rapidly along surfaces of constant phase). For such angles of incidence in a low-loss anelastic half-space, the particle motions for the reflected P waves are elliptical, amplitudes near the surface increase with depth and phase propagation is not parallel to the free surface. Volumetric strain for a Rayleigh-type surface wave shows an exponentially damped sinusoidal dependence on depth not evident for a Rayleigh wave on an elastic half-space.
abstract Measurements of ground motion generated by nuclear explosions in Nevada have been completed for 99 locations in the San Francisco Bay region, California. The recordings show marked amplitude variations in the frequency band 0.25 to 3.0 Hz that are consistently related to the local geological conditions of the recording site. The average spectral amplifications observed for vertical and horizontal ground motions are, respectively: (1, 1) for granite, (1.5, 1.6) for the Franciscan Formation, (3.0, 2.7) for the Santa Clara Formation, (3.3, 4.4) for alluvium, and (3.7, 11.3) for bay mud. Spectral amplification curves define predominant ground frequencies in the band 0.25 to 3.0 E for bay mud sites and for some alluvial sites. Amplitude spectra computed from recordings of seismic background noise at 50 sites do not generally define predominant ground frequencies. The intensities ascribed to various sites in the San Francisco Bay region for the California earthquake of April 18, 1906, are strongly dependent on distance from the zone of surface faulting and the geological character of the ground. Considering only those sites (approximately one square city block in size) for which there is good evidence for the degree of ascribed intensity, the intensities for 917 sites on Franciscan rocks generally decrease with the logarithm of distance as Intensity = 2 . 6 9 - 1 . 9 0 log ( Distance in kilometers ) . ( 1 ) For sites on other geological units, intensity increments, derived from this empirical relation, correlate strongly with the Average Horizontal Spectral Amplifications (AHSA) according to the empirical relation Intensity Increment = 0 . 2 7 + 2 . 7 0 log ( AHSA ) . ( 2 ) Average intensity increments predicted for the various geological units are −0.3 for granite, 0.2 for the Franciscan Formation, 0.6 for the Great Valley sequence, 0.8 for the Santa Clara Formation, 1.3 for alluvium, and 2.4 for bay mud. The maximum intensity map predicted on the basis of these data delineates areas in the San Francisco Bay region of potentially high intensity for large earthquakes on either the San Andreas fault or the Hayward fault. The map provides a crude form of seismic zonation for the region and may be useful for certain general types of land-use zonation.
Records of strong ground shaking obtained from damaging earthquakes provide the basis for improving earthquake-resistant design and for understanding the nature of seismogenic failure in the Earth's crust.Strong-motion data acquired from the national network operated by the U.S. Geological Survey form a national resource for scientific research, engineering design, siting of critical structures, and safety evaluations for large-scale structures such as dams and bridges.This report series, first initiated by the late R. B. Matthiesen after the damaging San Fernando earthquake of February 9, 1971, is intended to provide a catalog of strong-motion records recovered from the U.S. national network as well as summaries of related topics of special interest to the international earthquake-engineering research community.The collation of information presented in table l of this report series provides a complete list of all strong-motion records obtained from the national network, including those of relatively small amplitude (less than 0.05 .9. peak acceleration) that generally would not be digitized or otherwise processed unless warranted by a special studies request.Publication deadlines for this catalog are necessarily determined by instrumentation maintenance intervals (6 months to 2 years), which in turn determine the recovery period for much of the small amplitude data.Recent developments in relational data base technology and improvements in mini-and micro-computer technology have provided opportunities to develop and implement improvements in data-dissemination procedures.Some of these improvements have been incorporated into standard dissemination procedures and others are in the process of being incorporated.Standard procedures currently implemented include: l) Publication of data reports presenting processed recordings of large and damaging earthquakes of special interest, initially in the Open-file series and subsequently in the Bulletin series of the U.S. Geological Survey;2) Commercial distribution of digital copies of published data by the National Geophysical Data Center, Boulder, Colorado;3) Automatic dissemination via computer terminal of information in the Strong-Motion Information Retrieval System (SMIRS) for access by any interested user; and 4) A continuation of the catalog of strong-motion data recovered from the National Network, published annually in this series.Procedures that are presently being developed to further facilitate dissemination of strong-motion data are based on recently developed relational data base technology.These efforts are directed toward development of a user friendly environment that will extend the capabilities of SMIRS by providing computer access to the digital time series, catalogs such as those published in this series, and the capability to quickly retrieve data subsets on the basis of a wide variety of user specified parameters.These efforts, in support of the National Strong-Motion Program operated by the U.S. Geological Survey, which has responsibility for the acquisition, processing, and dissemination of all strong-motion data collected on the national network, are intended to facilitate the use of a scarce national resource, by the earthquake engineering and scientific research communities.
Microprocessor technology has permitted the development of a General Earthquake-Observation System (GEOS) useful for most seismic applications. Central-processing-unit control via robust software of system functions that are isolated on hardware modules permits field adaptability of the system to a wide variety of active and passive seismic experiments and straightforward modification for incorporation of improvements in technology. Various laboratory tests and numerous deployments of a set of the systems in the field have confirmed design goals, including: wide linear dynamic range (16 bit/96 dB); broad bandwidth (36 hr to 600 Hz; >36 hr available); selectable sensor-type (accelerometer, seismometer, dilatometer); selectable channels (1 to 6); selectable record mode (continuous, preset, trigger); large data capacity (1.4 to 60 Mbytes); selectable time standard (WWVB, master, manual); automatic self-calibration; simple field operation; full capability to adapt system in the field to a wide variety of experiments; low power; portability; and modest costs. System design goals for a microcomputer-controlled system with modular software and hardware components as implemented on the GEOS are presented. The systems have been deployed for 15 experiments, including: studies of near-source strong motion; high-frequency microearthquakes; crustal structure; down-hole wave propagation; teleseismicity; and earth-tidal strains. These studies have yielded recordings of near-source radiation fields in the frequency band of 1 to 300 Hz with signal resolution greater than 84 dB, documented seismic signals of 80 Hz at distances of 190 km with implications for nuclear detection, provided complete onscale high-resolution recording of several aftershock sequences with signal amplitudes ranging over 180 dB, and records of Earth dilational strain over the period band 0.1 sec to 28 hr, with superimosed radiation fields for nuclear explosions at regional distances and near-source earthquakes. Data sets recorded on the GEOS illustrate the importance of broad bandwidth, high resolution, and wide linear dynamic range for future earthquake studies. Field deployments of a minicomputer system compatible with the GEOS have emphasized the usefulness of portable field computers for experiments using microcomputer-controlled data-acquisition systems.
