In this study, ground strains over a soft sediment-filled plain were estimated for nineteen moderate earthquakes in northeastern Taiwan. To this end, abundant accelerograms recorded by two independent seismic arrays, LLSST and SMART1, were analyzed. These two dense seismic arrays are both highly overlapping in terms of space and operation duration. Spatial analyses herein for the LLSST array suggest that good estimates of ground strains can be obtained using only three 3-component seismometers recording at stations configured in a simple triangle with maximum separation of about 100 m.
In the case of the Hualien earthquake (ML = 6.5) of 20 May 1986, a peak shear strain of 210μ occurs with direct S-waves whilst strain of 186μ occurs with surface waves dominated by a period of about 2.5 sec. However, the peak vertical gradient accompanied by shear waves is in the range 1500 ~ 2000μ, which is larger than that of horizontal differential motion by a factor of about 10.
For further applications in mapping ground deformation fields at a given region, the single-station method is examined by comparing its esti- mates with those of a displacement gradient algorithm. A moving cross- correlation method is proposed to obtain phase velocities for a sequence of waves, particularly when ground motion contains obvious surface-wave energy. Consequently, estimates of peak horizontal strain at the LLSST site are in the range 1 ~ 150μ for the nineteen moderate earthquakes and the logarithm of these appears to be proportional to earthquake magnitudes.