Correlation of strong ground motion for the 1994 Northridge earthquake

The correlation of strong ground motions recorded during the 1994 Northridge earthquake is examined for stations located within and near the Van Norman Complex. This station cluster is located about 10 km north of the epicentre and contains eight free-field or nearly free-field instruments set at arbitrary orientations and irregular locations in a circular area of about 6 km in diameter. Correlation studies are carried out by comparing waveforms of the time histories and patterns for the orbital motions. The similarity in waveforms is most striking for the radial and transverse displacements. The correlation of the vertical component is much less pronounced than for the horizontal. The similarity in waveforms, visible throughout the complex for the radial and transverse time histories, decreases markedly as the high-frequency content is increased when the velocities and accelerations are examined. Patterns for the horizontal orbits also show an analogous similarity as a function of station separation. Maps constructed by plotting the orbits as a function of station location exhibit pattern similarities between adjacent stations such that specific nodes and segments can be tracked throughout the station ensemble to a great extent for the displacements and to lesser extents for the velocities and accelerations. Pattern similarity is examined for different station spacings, ranging from 0.4 to 3 km. By annotating the orbital traces with respect to time it is possible to estimate the duration of specific segments of the motion from station to station throughout the complex. A review of the traces indicates that the Rinaldi Receiving Station triggered late and began recording approximately 0.8 s after the start of the records at the remaining seven stations.