314 C. Davison — British Earthquakes of 1889. 



we may take this point as indicating very nearly the position of the 

 epiceutrum, which is therefore close to the intersection of the Irwell 

 fault with the surface, and on the downthrow side of it. 



The centre of the sound-area, as drawn upon the map, lies, how- 

 ever, about 3|- miles S.S.W. of the epicentrum, and therefore to the 

 south-west of the line of the Irwell fault. But since, as before 

 remarked, there may be a slight error in this determination, and since 

 the minor faults of the district are excluded, by their position with, 

 respect to the epicentrum, from any relation with the earthquake, I 

 think we may with some probability connect the origin of the earth- 

 quake with the Irwell fault. 



There is, however, other evidence bearing on the point, (1) It 

 has been shown that, as a general rule, in the neighbourhood of 

 the continuation of the Irwell fault near Bolton, the number of 

 vibrations was greater than elsewhere, and the sound- phenomena 

 were also more lasting ; and this is what we should expect if the 

 seismic focus were a plane whose strike is N.W. and S.E. (2) The 

 maximum intensity was recorded at Tottington, about two miles 

 east of the epicentrum, and this, it will be seen, is not far from the 

 point where the perpendicular to the fault-plane through the centre 

 of the seismic focus meets the surface of the earth. Other conditions 

 being the same, it is evident that the intensity should be greatest 

 in the neighbourhood of this point. 



The inclination of the Irwell fault near Bolton is 28° from the 

 vertical, according to the horizontal section (sheet 67) of the 

 Geological Survey, which is drawn across the fault. If this in- 

 clination be approximately constant to within a few miles from the 

 surface, it follows that the centre of the seismic focus must be at a 

 depth of about 3| miles. 



Again, as in the case of the Edinburgh earthquake, the movement 

 at four places (Bury, Heywood, Prestwich, and Eamsbottom) on 

 the downthrow side of the fault, was upwards first and then down- 

 wards. There are no records of a contrary movement on the 

 upthrow side of the fault ; but the evidence, so far as it goes, in- 

 dicates that the earthquake was probably caused by a slip which 

 slightly increased the throw of the fault. 



Lastly, that the distance in a horizontal direction over which the 

 slip extended was very short, may be inferred : (1) from the 

 approximate circularity of the isoseismal lines, and (2) from the 

 short duration of the earthquake- shock and the accompanying sound- 

 phenomena. Since the duration is rarely mentioned as being more 

 than a few seconds, it is possible that this distance was not much 

 greater than a mile. 



The whole evidence being taken together, the following conclu- 

 sions seem to me probable : (1) the Lancashire earthquake was 

 caused by a slip of the Irwell fault, a few miles from its north- 

 western extremity as traced upon the map : (2) the horizontal 

 length of the slip-area was short, possibly less than a mile : (3) the 

 slip resulted in an increase of the throw of the fault : and (4) the 

 slip must have extended upwards to within a short distance from, if 



