ALBERT

Description: Most instruments used in seismological practice to record ground motion are pendulum seismographs, velocigraphs, or accelerographs. In most cases it is assumed that seismic instruments are only sensitive to the translational motion of the instrument"s base. In this study the full equation of pendulum motion, including the inputs of rotations and tilts, is considered. It is shown that tilting the accelerograph's base can severely impact its response to the ground motion. The method of tilt evaluation using uncorrected strong-motion accelerograms was first suggested by Graizer (1989), and later tested in several laboratory experiments with different strong-motion instruments. The method is based on the difference in the tilt sensitivity of the horizontal and vertical pendulums. The method was applied to many of the strongest records of the M (sub w) 6.7 Northridge earthquake of 1994. Examples are shown when relatively large tilts of up to a few degrees occurred during strong earthquake ground motion. Residual tilt extracted from the strong-motion record at the Pacoima Dam-Upper Left Abutment reached 3.1 degrees in N45 degrees E direction, and was a result of local earthquake-induced tilting due to high-amplitude shaking. This value is in agreement with the residual tilt measured by using electronic level a few days after the earthquake. The method was applied to the building records from the Northridge earthquake. According to the estimates, residual tilt reached 2.6 degrees on the ground floor of the 12-story Hotel in Ventura. Processing of most of the strongest records of the Northridge earthquake shows that tilts, if happened, were within the error of the method, or less than about 0.5 degrees .

Description: Receiver function is defined as the spectral ratio of the radial component and the vertical component of the ground motion. It is used to characterize converted waves. We extend the use of the receiver function to downhole data using waves recorded in a borehole, excited by an earthquake of magnitude 4.0 near San Francisco, California, on 26 June 1994. The focal depth of the event was 6.6 km and the epicenter was located at a distance of 12.6 km from the borehole array. Six three component sensors were located at different depths in a borehole. To extract a coherent response of the near-surface from the incoherent earthquake waves, we deconvolve the waves recorded by the sensors at different depths with the waves recorded by the sensor on the surface. Deconvolution applied to the waves in the S-time window recorded by the radial component result in an upgoing and a downgoing wave propagating with S-wave velocity. For the waves in the P-time window recorded by the radial component, deconvolution also gives an upgoing and a downgoing wave propagating with S-wave velocity. This interesting result suggests a P- to-S conversion at a depth below the deepest sensor. To diagnose this we compute the receiver function for the borehole recording of the earthquake waves. The receiver function shows an upgoing wave with an arrival close to time t = 0 for the deepest sensor. The agreement of the upgoing wave in the receiver function with the travel-time curve for the P-to-S converted wave, calculated using the P- and the S-wave velocity profile, supports the hypothesis of a pronounced P-to-S conversion. We present a synthetic example to illustrate that the first arrival of the receiver function applied to borehole data gives the upward-propagating P-to-S converted wave. To corroborate the observation of the mode conversion, we apply receiver function to a different earthquake data recorded by the same borehole array in 1998. The focal depth of the event was 6.9 km and the epicenter was located at a distance of 13 km from the borehole array. The receiver function for these data also show an upgoing wave with a pulse close to time t = 0 at the deepest sensor. The moveout of the upgoing wave agrees with the travel-time curve for the P-to-S converted wave, hence supporting our observation of the mode conversion.

Description: This article considers a classical approach of using a combination of pendulums to measure rotations. The idea of using two identical pendulums installed on different sides of the same axis of rotation for separate measurement of rotational and translational motion was apparently first suggested by Golitzin (1912). It was implemented by Kharin and Simonov (1969) in an instrument designed to record strong ground motion (VBPP--a seismograph of large translational motions and rotations). Unfortunately, difficulty in building identical mechanical systems resulted in unreliable measurements of the rotational component. We modified Golitzin's idea by using the same configuration of pendulums (a two-pendulum system) without the requirement that the pendulums be identical (Graizer et al., 1989). Instead of building two identical pendulums, one needs to calibrate the instrument to obtain the natural parameters of each pendulum and apply postprocessing to separate the rotational and translational motions. The two-pendulum system for separate measurements of large amplitude rotations was implemented at the end of the 1980s at the Institute of the Physics of the Earth in Moscow, Russia, using commercially available pendulum instruments. The system was tested using a basic shake table and later successfully applied to measurements in the near field of two large underground nuclear explosions. In this article I updated and generalized the approach to measuring translational and large amplitude rotational motion formulated in previous publications (Graizer, 1989; Graizer et al., 1989). Numerical testing demonstrated that using a combination of pendulums for measuring rotations may be limited for recording relatively large amplitudes of rotations of the order of 10 (super -4) and higher for the two-pendulum system of about 100 cm size.

Description: Most seismological instruments recording ground motion use three sensors oriented north, east, and upward. In this cardinal configuration horizontal and vertical sensors differ in their construction because of the gravitational acceleration affecting the vertical sensor. An alternative sensor arrangement was first introduced by Galperin (1955) for petroleum exploration. In this arrangement three identical sensors are also positioned orthogonally to each other but are tilted at the same angle of 54.7 degrees to the vertical axis (an orthogonal triaxial system of coordinates balanced on its corner). Records obtained using this sensor configuration must be rotated into an Earth referenced cardinal X, Y, Z coordinate system for most analyses. A number of recent seismological instruments (e.g., STS-2 and Trillium seismometers) use Galperin sensor configuration. In most seismological studies it is assumed that the rotational components of earthquake ground motion are small enough to be neglected. However, examples of significant rotational components have been noted (e.g., Bouchon and Aki, 1982; Graizer, 1991; Takeo, 1998; Huang, 2003; Zahradnik and Plesinger, 2005; Cochard et al., 2006; Graizer, 2006a; Schreiber et al., 2006; Spudich and Fletcher, 2008). The response of pendulums when installed in a cardinal configuration to input motions that include rotations has been studied in a number of publications (Golitzin, 1912; Rodgers, 1968; Wong and Trifunac, 1977; Graizer, 1991; Todorovska, 1998; Trifunac and Todorovska, 2001; Graizer, 2005, 2006b; Graizer and Kalkan, 2008). This article considers the response to input motions of pendulums in a Galperin sensor configuration as well as the resulting cardinal orientation system response. Given the benefits of identical designs for all three sensors in a Galperin configuration, this geometry may be useful for strong-motion measurements as well. The disadvantage of this sensor configuration is that if any of the sensors is not working properly or there are misalignments of sensor axes, then all three cardinal components are degraded.