- © 2008 by the Seismological Society of America
Modern broadband seismometers generally have well-known and stable instrument parameters. Typically, the manufacturer's specifications indicate that the gain of each component of a three-component seismometer is known to within 1% and that the orthogonality of the components is true to within a fraction of a degree. Such precision makes possible many types of quantitative seismological analyses that were difficult with earlier instruments. In particular, different components of earlier three-component seismometers did not necessarily have the same response functions (e.g., free period of the seismometer), making any analysis based on the rotation of ground motion into the transverse and longitudinal directions difficult. Such technical problems have now largely been overcome in the most common broadband instrumentation, and it is routine to perform rotational transformations of the horizontal components of motion in modern seismological analyses, such as earthquake source investigations, S and SKS splitting studies, receiver-function determinations, and body- and surface-wave-polarization studies.
An essential station variable for the rotational transformation of horizontal components of motion is the geographical orientation of the original components in the horizontal plane. Horizontal seismometers are typically installed with output sensitivity aligned to the north-south and east-west directions, and the standard names of seismometer channels (e.g., BHN, BHE) reflect this convention. Nontraditional orientations are common for borehole and ocean-bottom seismometers, for which it is cumbersome or impossible to install the seismometer with a specified orientation, and the orientation is instead determined after deployment. In general, instruments with nontraditional orientations have channel names that reflect this (e.g., BH1, BH2). Regardless of how the seismometer is oriented at installation, the azimuths of sensitivity of the horizontal components are subsequently distributed as auxiliary data. In the SEED convention, as well as in other data distribution formats, the precision of this parameter is given to at least 0.1°.