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...

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