ABSOLUTE FAR-FIELD DISPLACEMENTS FROM THE 28 JUNE 1992 LANDERS EARTHQUAKE SEQUENCE

ON 28 June 1992, the largest earthquake in California in 40 years (surface-wave magnitude M(s) = 7.5) occurred near the small town of Landers, in southeastern California, and was followed three hours later by the nearby M(s) 6.5 Big Bear earthquake1. Fortuitously, the Landers earthquake sequence coincided with the first week of the official three-month test period of the International Global Positioning System and Geodynamics Service2 (IGS), giving us an unprecendented opportunity to detect absolute pre-, co- and post-seismic displacements at a distance of 50-200 km from the main rupture with millimetre-level precision. Mutual and independent confirmation of some of our geodetic results are demonstrated by Bock et al. in this issue3. For the Landers earthquake, the observed displacements indicate that the depth of the bottom of the rupture is shallower towards the northern end, displacements were dominantly symmetric, and the rupture extended further south on the Johnson Valley fault than has been mapped on the basis of surface ground offsets. The combined geodetic moment for the Landers and Big Bear earthquakes (1.1 x 10(20) N m-1) agrees well with teleseismic estimates.