Mat 21, 1909] 



SCIENCE 



833 



SYMPOSIUM ON EAETHQUAKES 



Introduction, Classification, Discussion of Vol- 

 canic Earthquakes, Description, with Illustra- 

 tions, of the Charleston, S. C, and Kingston, 

 Jamaica, Disasters: Professor Edmund 0. 

 HovEY, New York. 



The Present Status and the Outlook of Seismic 

 Geology: Professor Wiixiam H. Hobbs, of Ann 

 Arbor, Mieh. 



Conditions Leading to Tectonic Earthquakes — In- 

 struments used in the Study of Earthquakes — 

 Suggestions for a National Seismological Bu- 

 reau.: Professor Habey F. Eeid, of Baltimore. 

 Professor Hovey's paper served as an introduc- 

 tion to the subject. The ideas in regard to the 

 cause of earthquakes were considered, especial 

 attention being given to a discussion of volcanic 

 earthquakes. Professor Hovey described the 

 Charleston and Kingston earthquakes and illus- 

 trated his description by lantern slides. At the 

 conclusion of his paper he exhibited a series of 

 views of the Messina earthquake of December, 

 1908. 



Professor W. H. Hobbs spoke of the manner in 

 which the subject of seismology had been retarded 

 by the publications of Robert Mallet, who advo- 

 cated the idea that the shock originates at some 

 point underground called the seismic focus or 

 centrum, and from which center elastic waves are 

 propagated in all directions. Professor Hobbs 

 pointed out the fact that the " centrum " as 

 determined by the method of Mallet was at best 

 a line and practically had no existence. He ex- 

 plained the production of earthquakes by the 

 shifting of segments of the crust along already 

 existing fissures due to geotectonic movements, 

 and insisted upon the tectonic origin of earth- 

 quakes. Volcanic activity and earthquake activity 

 may be associated, as volcanoes are located along 

 lines of fissure in the crust. Release of strain by 

 shifting of the crust blocks, accompanied by earth- 

 quakes, as has been going on in the earthquake 

 regions surrounding the Pacific Ocean in the last 

 few years, probably produces in such a region 

 an establishment of approximate equilibrium so 

 that earthquakes may not be so severe for some 

 time. The probability of earthquakes occurring 

 in a given region may be tentatively predicted by 

 a study of such crustal adjustment, but such 

 predictions must be considered as tentative only. 

 The upward or downward movement of the sur- 

 face during earthquakes may be considerable, pro- 

 ducing raised beaches, as has recently occurred in 

 the Alaskan Islands. Along some coast lines the 



evidence of such movement is to be seen in the 

 raising of sea caves, and also the undercut por- 

 tions of sea-cliflfs, above the water level. As these 

 movements involve many feet of elevation during 

 an earthquake, perhaps the ideas of Lyell on the 

 rate of changes of level of the land and sea may 

 have to be modified. 



Professor Harry Fielding Reid considered three 

 phases of the subject: (a) conditions leading to 

 tectonic earthquakes, (6) instruments used in the 

 study of earthquakes, (c) suggestions for a na- 

 tional seismological bureau. 



(a) The rocks have the properties of elastic 

 substances and can only break after they have 

 been deformed by the action of external forces. 

 When they are strained beyond their strength a 

 break occurs and the rocks return to an un- 

 strained position, but it is only in the general 

 neighborhood of the rupture that any distinct dis- 

 placement takes place at the time of the earth- 

 quake. 



(6) Earlier investigators attempted to obtain 

 a " steady point " ; that is, a point which would 

 remain at rest when the earth vibrated under the 

 earthquake disturbances. This, however, is im- 

 practicable. All earthquake instruments must 

 have a proper period of vibration and when the 

 disturbance affects them the resulting record is a 

 combination of the movement of the earth and the 

 movement of the instrument. When the period of 

 vibration approaches that of the instrument, the 

 latter has a very large swing and therefore the 

 records unduly magnify special vibrations. This 

 can be avoided by introducing strong damping. 

 The different forms of instruments were briefly 

 noticed. 



(c) The work of a national bureau would be 

 very varied. It would require the collection of 

 data regarding all felt earthquakes, and a geolog- 

 ical examination, in special cases, of the regions 

 where earthquakes occur. The instrumental rec- 

 ords from the whole country should also be col- 

 lected and studied to throw light on the nature of 

 the earth's interior, and to discover the centers 

 of earthquake disturbances in the surrounding 

 oceans. This would require the general coopera- 

 tion of many departments of the government and 

 could probably be best undertaken by the Smith- 

 sonian Institution. 



At the conclusion of the discussion following 

 the symposium Professor Hobbs presented the 

 following resolutions, which were unanimously 

 adopted: 



Wheeeas: Earthquakes have been the cause of 



