EARTHQUAKES OF MAY 1960 IN CHILE — SAINT-AIMAND 359 



movement. The very long period waves are probably not ordinary 

 surface waves, as pointed out by Prof. F. Press. 



The waves produced by this earthquake were gigantic and of ex- 

 tremely long periods. The whole earth was set in vibration in its 

 fundamental mode and in many of the harmonics thereof. These 

 oscillations continued for a period of some 2 weeks, exhibiting a Q 

 factor (or dissipation function) of 170 for 35-minute waves and 380 

 for 6-minute waves [19]. Both spheroidal and toroidal oscillations 

 were observed. The spheroidal oscillations showed a "Zeeman type" 

 splitting caused by the rotation of the earth. 



It is not unlikely that the surface waves, combined with the free 

 oscillations of the earth, produced movements of the earth below the 

 water and along the coast. The land rose and fell — like the chest of 

 a person breathing deeply while on his back in a bathtub — in such a 

 way that the land rose from the sea, the water running off, and then 

 slowly fell, letting the water surge over the subsiding land. Support 

 for this impression comes from the almost simultaneous appearance of 

 the wave at points all along the coast, including, for example, Achao 

 (fig. 3), a small town on the landward side of Isla Chiloe where the 

 tsunami began about 10 minutes after the earthquake [1]. If the 

 waves were produced solely in the Pacific, the disturbance would not 

 have reached Achao for possibly an hour. Isla Chiloe and the con- 

 tiguous sea bottom must have participated in the production of the 

 tsunami. 



SEISMIC HISTORY OF SOUTHERN CHILE 



The region affected by the recent earthquakes has been repeatedly 

 subjected to similar events. Greve [23] has summarized his labors 

 and observations, those of Montessus de Balore, and those of the staff 

 of the Instituto de Sismologia at Santiago, from 1520 to 1946. In 

 these 426 years there have been 47 notable earthquakes between San- 

 tiago and Castro. No notice was taken of the millions of tremors of 

 nondestructive size. Of these 47 earthquakes, 7 have been roughly 

 comparable in magnitude to the main shock of May 1960, and 8 have 

 been associated with tsunamis or similar disturbances of the sea. Of 

 special interest is the seism of 1575, which appears to have had a 

 similar zone of influence, even to having produced a landslide in Rio 

 San Pedro. 



SUMMARY OF EFFECTS 



As of June 1960 large destructive aftershocks were still occurring 

 at intervals of a few days. Many of these were strong enough to 

 have caused a disaster in a more populous region. Shocks in excess 

 of magnitude 6 were sufficiently common that people no longer took 

 much notice of them. These aftershocks w^ill continue for a period of 

 years. 



C72-174— 63 20 



