Epicenter and Origin Time of the Main Shock on July 21 and Travel Times of Major Phases

The epicenter of the main earthquake on July 21, 1952, was determined a) from the arrival times of P at near-by stations; b) from comparison of these times with those found previously for shocks in the same region in which, contrasting with the present shock, the motion was not so large that the light spot left the paper shortly after the beginning and in which the onset of the transverse waves (S) could be found on records of several stations (mainly shocks no. 13-16, Gutenberg 1943, with origin times revised in 1951); c) from similar comparison with times in records of aftershocks originating near the main shock, but for which seismograms from portable or temporary stations at short distances furnished additional data. For details of the method see Gutenberg (1943, p. 502). If arrival times of Pn (longitudinal wave leaving the source downward and refracted twice at the Mohorovicic discontinuity; see fig. 1) at the stations near the Sierra Nevada (Haiwee, Tinemaha, Reno) are used, the effects of the difference in crustal structure at the station must be considered. For Pn, which has to go deeper down than usual as a consequence of the Sierra Nevada root, this may result in a delay of as much as 4 seconds.