CHAPTER X. 



DETEEMINATION OF EAETHQUAKE ORIGINS. 



Approximate determination of an Origin — Earthquake-hunting in 

 Japan — Determinations by direction of motion — Direction indicated 

 by destruction of buildings — Direction determined by rotation — 

 Cause of rotation — The use of time observations — Errors in such 

 observations — Origin determined by the method of straight lines — 

 The method of circles, the method of hyperbolas, the method of 

 co-ordinates — Haughton's method— Difference in time between 

 sound, earth, and water waves— Method of Seebach. 



One of the most practical problems which can be sug- 

 gested to the seismologist is the determination of the 

 district or districts in any given country from which 

 earthquake disturbances originate. With a map of a 

 country before us, shaded with tints of different intensity 

 to indicate the relative frequency of seismic disturbance 

 in various districts, we at once see the localities where we 

 might dwell with the least disturbance, and those we 

 shouM seek if we wish to make observational seismology 

 a study. Before erecting observatories for the systematic 

 investigation of earthquakes in a country, it would be 

 necessary for us, in some way or other, to examine the 

 proposed country to find out the most suitable district. 

 The special problem of determining approximately the 

 origin or origins of a set of earthquakes would be given 

 to us. Having made this preliminary investigation, the 

 next point is, by means of observatories so arranged that 



