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° 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.

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