We read with great interest the news report in Eos (June 16, 1989) on the very large Macquarie earthquake of May 23, 1989, which left U.S. scientists divided as to whether this earthquake was of strike slip or thrust type or a mixture of both.
Indeed, we think we have the solution to this dilemma, since thanks to teletransmitted data from 11 Geoscope stations recording both very long-period and broad band data, we were able to obtain a reliable preliminary mechanism less than 48 hours after this earthquake occurred. Figure 1 shows this mechanism, which is a practically pure right-lateral strike slip, consistent with the general trend in the Macquarie ridge area as indicated from the CMT catalog for the past 10 years [Dziewonski et al., 1989], and in particular, very similar to the mechanisms of the two largest events which occurred in this area in this time period (May 25, 1981 and July 7, 1982). By moment tensor inversion of very long-period Rayleigh waves, we obtained a moment of 2.2×1028 dyne-cm and a centroid time of 28 s, which yields a rating for this earthquake as “average-to-fast” with respect to the moment/duration relation. So far, we haven't found any conclusive evidence for strong horizontal directivity, while it seems that the rupture may have extended down to (or up from) at least 40 km. These features of the solution offer a good explanation as to why no tsunami was observed for this earthquake. The confidence in our “very-long period” solution comes from its very good compatibility with broad band data from the three teletransmitted Geoscope stations that lie within the epicentral distance range 30°− 90° from the epicenter of the Macquarie event, both in terms of mechanism as well as source duration. For shallow earthquakes the earthquake depth and the vertical dip-slip components of faulting are poorly constrained by the very-long period data. The addition of only a few broad band seismograms in the analysis can remove this uncertainty. Figure 2 shows two examples of fits for SH waves obtained in a combined analysis of long-period surface waves and broad band body waves [Ekstrom, 1989]. The teleseismic SH arrival at Papeete had an amplitude of 0.9 mm. The Macquarie region lies so far south in the southern hemisphere that very few records from existing broad band digital stations appropriate for mantle P and S wave analysis will ever be available. For Geoscope (which presently counts 20 operational broad band digital stations), besides PPT (Tahiti), KIP (Hawaii) and RER (Reunion Island), which were teletransmitted (CAN, Australia,is too close), we still expect data in this distance range from stations NOU (New Caledonia), DRV (Dumont d'Urville), PAF (Kerguelen) and CRZF (Crozet Island), the last three of which will take several months to arrive because they will travel part of the way by ship. Table 1 lists the coordinates and position with respect to the epicenter of the 11 stations whose data were teletransmitted.