No abstract

No abstract

Frequency and amplitude characteristics were studoeed for microearthquakes recorded on a local network of vertical-component seismographs in the Fairview Peak area of central Nevada. Maximum trace amplitude o.f the events decreased with focal distance approximately as R -•'9. Site amplification, by a factor of 2, was observed for the only station not located on rock. Comparison of observed spectra for these events with theoretical spectra given by Brune indicates stress drops ranging from 0.04 to 0.60 bar. Calculated source dimensions and seismic moments for microearthquakes in this area are in agreement with results of earlier studies.When location of the events is considered, the variation in stress drop appears to be related to differences in the faulting mechanisms.In a recent study Brune [1970] used an earthquake dislocation model to derive nearand far-field theoretical spectra for seismic shear waves and suggested that comparisons of observed and theoretical spectra could be used to estimate the effective stress, stress drop, and source dimensions of earthquakes. In this paper we consider amplitudes and spectral characteristics of microearthquakes recorded in the Fairview Peak area of central Nevada and use the method outlined by Brune to estimate various source parameters of these events. Results of this study indicate that effective stresses vary within the Fairview fault zone, and this variation may be related to differences in focal mechanism for events in different parts of the zone. Previous investigations of microearthquakedistribution and focal mechanisms in the Fairview Peak area have been published by Stauder and Ryall [1967] and Ryall and Malone [1971], and average spectral characteristics of events in the same area were described by Douglas and Ryall [1970]. A discussion of previous theoretical work on elastic wave radiation for seismic dislocations, as well as the various assumptions Copyright •¸ 1972 by the American Geophysical Union. in his own derivation, are given by Brune and will not be repeated here except as they bear on the observations and analysis of this study. The data for this investigation were obtained in early 1970 near Fairview Peak. Eight earthquakes were included in the analysis; the locations of these events are shown in Figure 1 with the recording sites, recent fault breaks, and epicenters of small earthquakes recorded by stations of a regional network during late 1969 and 1970. Records used for amplitude and spectral analysis were obtained on portable tape-recording field seismographs designed by Hunt [1969]. This system uses an Electrotech EV-17 moving-coil seismometer with a free period of 1.0 sec and has the response characteristics shown in Figure 2. ANALYSIS AND RESULTS Focal coordinates for the eight earthquakes of this study were calculated by using times of P and S waves from four to six of the field stations in a computer location routine developed by Malone [1969]. In the process of focal determination, site corrections were obtained by a comparison of near-reg...

SUMMARYThis paper presents the results of a unique field test on a curved highway overpass. In the test, large horizontal loads were applied to the superstructure of the bridge and quickly released, causing the bridge to vibrate. The resulting largeamplitude vibrations were intended to be similar to the vibrations caused by earthquakes (horizontal accelerations of up to 25 per cent of gravity were measured on the bridge deck). Well-defined lateral, longitudinal, vertical and torsional vibration modes were identified from the test data.The vibration modes were used to verify an analytical model of the bridge's dynamic response. For this paper, the model was verified using only the fundamental vibration mode, which was primarily a horizontal vibration mode. Using a system identification procedure, the dynamic response model was adjusted until its frequency and mode shape matched the measured frequency and mode shape. Parameters in the verified model were compared with the same parameters calculated from information in the structural drawings. Because the fundamental mode represents a horizontal mode, the bridge parameters identified in this paper were those parameters which strongly influence the horizontal response of the bridge.

An experimental study to evaluate the fatigue strength of double-angle connections used to connect stringers to floor beams in steel railroad bridges is described. The tests examined connections that could be used for either new construction or replacement of old, damaged connections. Double-angle connections are always assumed to be subjected only to shear stresses. But, in reality, some bending moment also is developed by this type of connection, causing additional flexural stresses, which, in turn, can produce fatigue damage by exceeding the constant amplitude fatigue limit. The amount of the transferred moment was estimated in the tests. Two full-scale specimens were built, representing the stringer-to-floor beam connection of a one-span, open-deck, through-plate girder bridge located in Nevada, typical of construction in the early 1900s. The specimens were tested under a cyclic loading of ± 195.8 kN, representing the equivalent load, which produces a total maximum shear reaction at the connection equal to the one produced in the actual bridge. However, the load cycles were from 0 to 391.6 kN in real bridges. The specimens also were tested beyond the fatigue initiation to observe the crack-propagation patterns and their growth rate. The tested specimens were modeled by the finite element program ADINA, and a comparison between the analytical and experimental results was made.