Detecting slow, long-duration slip of large earthquakes using very long-period orbital surface waves

Abstract

SUMMARY In this paper, we present a simple yet effective technique for measuring the source time-delay spectrum from very long-period world-circling surface waves without relying on assumed earth models or calculation of synthetic seismograms. By using autocorrelation and self-convolution functions derived from whole seismograms of large earthquakes recorded at a seismic station it is possible to isolate the source effects and precisely (5 5 s) determine the source delay spectrum at periods as long as 600 s. Measurements at such long periods are necessary for detecting slow precursory slip and slow creep after the main ‘fast’ seismic phase of large earthquakes. We apply this technique to five recent large earthquakes: the 1989 Macquarie Ridge (M, 8.2), the 1992 Nicaragua (M, 7.6), the 1993 Mariana Islands (M, 7.8), and the 1994 deep events in Fiji (M, 7.6) and Bolivia (M, 8.3). Our results are largely in agreement with the results obtained by other investigators. The Mariana Islands, Fiji, and Bolivia earthquakes had a normal rupture duration. The Nicaragua earthquake exhibits a source time-delay spectrum with the delay increasing from 31 s at the 100 s period to 50 s at the 600 s period, indicating an unusually slow rupture process. The Macquarie Ridge earthquake is anomalous by the discrepancy between the Love-wave and Rayleighwave estimates. Love waves, in the period band of 150 to 600 s, show a normal, flat source-delay spectrum with a delay of about 28 s. The Rayleigh-wave spectrum agrees with the Love-wave spectrum for periods smaller than 200 s and greater than 400 s; however, it shows a significantly shorter source delay (about 15 s) in the 250 to 350 s period range. The unusual nature of the Macquarie Ridge earthquake’s surface wave spectrum was pointed out by Ihmli, Harabaglia & Jordan (1993) who also provided an explanation in terms of a slow, precursory moment release. This explanation is not consistent with the flat Love-wave spectrum determined here.