A very simple modification to the Dziewonski multiple filter analysis method for surface-wave data yields a 90 per cent reduction in computation time. The method involves a frequency shift such that the center frequency of the bandpass filter is moved to the origin. The effect is to greatly reduce the apparent bandwidth and allow a reduction of the inverse transform size down to only 256 points.
The new method was used to analyze Rayleigh waves for 32 paths to Guam in the southwest Pacific. Results support the existence of a very thick crustal region north of the Fiji Plateau. Data from Samoa exhibited the highest group velocities, peaking at 4.0 km/sec; other regions generally showed anomalously low-group velocities, reflecting the anomalous mantle structure known to exist in many parts of the region.