Systematic relocation of early instrumental seismicity:: Earthquakes in the international seismological summary for 1960-1963

We have relocated 2556 events that were reported in the bulletins of the International Seismological Summary (ISS) during the period 1960-1963 using the phase arrival time data listed in the bulletins. Parts of the data were already available in digital form, and the rest we obtained by scanning the printed bulletins and applying an optical character recognition procedure. Earthquakes were then relocated with a teleseismic location method that uses an improved ID global travel-time Earth model, depth phases, and corrections for ellipticity, bounce-point bathymetry/topography, and station elevation. The aspherical velocity structure below stations was partially taken into account through the use of teleseismic station (patch) corrections. The quality of the newly obtained locations and the improvements in data fit are similar to relocations of earthquakes in the early bulletins of the International Seismological Centre (ISC). The most significant improvement of the new locations is in focal depth, and a significant number of blatant mislocations (such as earthquakes deeper than 100-km below midocean ridges) have also been corrected. The relocations presented here extend by 10% the time period for which homogeneous and reliable hypocenters are available on a global basis. This is the first of a series of studies addressed towards relocating, using the same methodology, all the instrumentally recorded earthquakes listed in the bulletins of the ISS (1918-1963).