HoGBEN. — Nelson Earthquake of 12th Fehvuanj, 1893. 353 



places only the deep-seated shocks were felt. At Wanganui 

 Mr. Field noted vibrations for 4 or 5 minutes with several 

 distinct shocks (but possibly several maxima of the same 

 shock). At Timaru the first shock, and a later one (about 1^ 

 or 2 minutes later), were observed. Of all these positions 

 found for the epicentrum, Eg best corresponds with the Nelson 

 observations of direction ; but it is possible that, if these ob- 

 servations were those of the transverse vibrations, K, or a 

 place a little to the north of it, would agree equally with 

 them. 



It is, of course, most likely that the epicentrum would be 

 an area large enough to include all the places, K, F, Eg, E3 

 (epicentric area on map, PL XLL). The amount of damage 

 done at Nelson was greater — far greater — than that reported 

 from any other place. It is probable, therefore, that the angle 

 of emergence there was nearly that of the maximum intensity 

 — i.e., between 56° and 45'^. This would agree with either K 

 or E2, with a depth of 5 miles for the origin. 



The origin might be guessed at with a tolerable degree of 

 probability by the use of isoseismals. Looking at the last 

 column in the table given above, we see that the isoseismal of 

 intensity, vii. on the Eossi-Forel scale, would be drawn out- 

 side Picton, Takaka, Collingwood, "WelUngton, Blenheim ; but 

 would have all the other places outside it. An ellipse might 

 be so drawn with a focus not far from the epicentric area 

 (K, F, E3, E,). 



3. Intensity. — The maximum intensity of this earthquake 

 was as far above the average of our ordinary mild New Zealand 

 shocks as its velocity of propagation was. The intensity 

 at Nelson was evidently viii. (Eossi-Forel scale), or a little 

 above it. 



If a = amplitude of the largest vibration in the motion of 

 any earth-particle, and T = the period of the largest wave, then 



-7j^ — intensity of shock defined mechanically = destructive 



effect = maximum acceleration due to the impulse. 



Now, Dr. Holden, Director of the Lick Observatory, has 

 given equivalents of the degrees of earthquake-shocks on the 

 Eossi-Forel scale in terms of the acceleration due to the 

 velocity of the shock itself (American Journ. Sci., 1888, 

 No. 210). 



Thus a shock of intensity viii. corresponds to 500mm. 

 per second. We should not probably be far wrong if we gave 

 600mm. -800mm. per second as the measure of the intensity 

 of our present earthquake — or, in other words, from jJ^r to j-V 

 of the acceleration due to gravity. 



Summary. — The earthquake of the 12th February, 1893, 

 originated below an area within 5 or 6 miles of Nelson, 

 23 



