EARTHQUAKES ON THE PACIFIC COAST 17 



Santa Clara, Napa, San Joaquin, Salinas, Amador, Clear Lake, 

 Pitt Eiver, etc., rather than to attempt wider ranges. It may thus 

 be possible to fix the origin of the local shocks, and finally to be 

 reasonably certain of its permanency. It also appears to me that 

 the data seem to indicate that the greater number of California 

 earthquakes have been the result of faulting in the underlying 

 strata rather than due to volcanic causes directly. 



Earthquake Shocks felt at Sea off Cape Mendocino, etc. 



The list of recorded earthquakes contains notices of several 

 shocks reported in this general neighborhood, as follows: 1868, 

 May 18; 1870, December 4; 1873, November 22; 1876, August 16; 

 1877, October 26; 1884, June 12; 1884, November 4; 1895, March 

 1, October 24. 



A relief map of the ocean bed near Cape Mendocino, made by 

 Professor George Davidson and Mr. Winston, shows the coast to be 

 very " steep-to "; and it further shows two submarine mountains 

 in the neighborhood. The slipping of the earth at the junction of 

 the steep submarine cliff with the (comparatively) flat ocean floor, 

 may very well be the cause of some of these disturbances. It is 

 also possible, on the other hand, that they are connected with the 

 two submarine elevations mentioned. More observations are 

 needed to decide this question. It is a little remarkable that we 

 have reports of shocks felt at sea in this vicinity and none, or few, 

 at other points along the coast. (See Plate V, page 31.) 



Self-Eegisteeing Seismometers. 



The Lick Observatory possesses a set of earthquake recorders 

 made from the designs of Professor J. A. Ewing of Cambridge. The 

 following description of them is extracted from Professor Ewing's 

 note in Nature of August 12, 1886. A similar set is installed in 

 the Students' Observatory of the University of California at 

 Berkeley. (See Plates III and IV, pages 18 and 20.) 



(1) A Horizontal Seismograph, with clock and driving plate. 

 The clock is started by an electric contact at the beginning of the 

 earthquake, and the two rectangular components of the horizontal 

 motion (N and S, and E and W) are registered side by side on a 

 rotating plate. 



(2) A Vertical Motion Seismograph, to register the vertical 

 movement of the surface of the earth on the same plate. 



2 



