EARTH FAULTS

Hokitika Guardian, 4 July 1929, Page 2

EARTH FAULTS

THEIR RELATION TO EARTHQUAKES. PHENOMENA EXPLAINED. CHRISTCHURCH, ‘ July 3. “The'Nature of Earth Faults and their Relation to Earthquakes” was tlie title of a lecture given by Mr G. ’Jobberns before the AV.E.A. last night. The lecturer warned his audience at the outset that he did not intend to speculate about the cause of the recent earthquake, because its cause would be fully investigated, and anythin'' he might say might come home to roost when this was done. He did not attempt to explain the basic causes of earthquakes, which are in the realm of speculation, except to establish that they were the result of movements of earth blocks along certain lines of fault. He also emphasised that it was not this sudden movement which was the earthquake, but the resulting tremor or quiver.

Explaining the nature of faults, Mr fobberns said that when a level land surface was subjected to compression or squeezing, it might become arched or bent into folds. It might do this without breaking, but if the warped block did (break, the break represented d fault. It might be quite small, with a displacement of only a few •inches, or it might result in a differential movement of one part of the broken block, amounting to one or two miles.

AVbere, in fairly recent times, faulting on a big scale had occurred, it could usually bo recognised in the fault scarps left behind, They could be easily discerned as obvious breaks i-i the c ntin ’ity of the surface, e.g., steep descents from high land to ad- ! a cent, low land.

Faulting of such magnitude might produce block mountains, which were commonly simple tilted blocks of the earth’s crust. Such tilted blocks generally had a very even sky lino, with lung oentle sloue on one side, while the other, representing the fault scarp or hfoak, was \ cry steep. Surf) simple block mountains were typically developed in Central Otago. Again; it might happen that a rising block of land might be traversed by many faults running in a more or less parallel direction. In some cases reversed faults or thrust planes developed on a large scale, these being characterised by a movement of one part of the broken earth block over an adjacent part.. •• Besides the displacement of blocks due to forces of compression, there Were fault phenomena arising from forces of tension which dent the e'arth’s crust ■ to produce troughs or rifts." The best-known example of this was the rift extending throughout East Africa from Ihe mouth of the to the head of the Jordan river in Palestine.

Dealing with the relation between faults and earthquakes, Mr Jobberns said it might best be expressed by saying that earthquakes indicated that some faults .were still alive. Should the forces of compression, to which the faults were due, be still operative, it was unlikely that these forces would be relieved by a continuous slow sweep of an earth block subjected to them. It was much niore likely that a state of stress would b* set up and that this would be relieved by a sudden spasmodic movement of a block. .

In a land like New Zealand, mack up of an assemblage of faulted rocks, it was possible that one of those blocks might move considerably, quite independent of the rest of the adjacent country. Such sudden spasmodic movement of a block, which . might represent a mountain range, would give the rest of the land a jolt such as to make it tremble or shiver or quake. It was necessary to distinguish between the sudden movemeir of the block concerned and the re suiting tremor or shiver, which wa> the earthquake. After the mail movement which would cause a big disturbance, many minor shocks woulc naturally be expected until the ad jacent crust had been adjusted to the new conditions. These after shocks probably indicated nothing nijre tha> this phase of settling down again. “I do not want you to think thal there is necessarily any connexion be tween earthquakes and volcanoes, continued Mr Jobberns. It was ceitainly found that places subject; tc earthquakes were also scenes of volcanic activity. Earthquakes occurred chiefly when the crust was likely to give way suddenly under stress, and were common in places where there were large scale faults. It was probable that earthquakes and volcanoes were due to much the same fundamental cause, but many earthquakes occurred in regions .where there had been no volcanic activity in lecent times. Violent volcanic eruption might give rise to earthquakes, but since the point of origin was near the surface, the effect was only local. It was the deep-seated block displacement movements which caused the big shocks that were felt over large areas. In reply to a question, Mr Jobberm said that so far as was known, no two big earthquakes had had th«r origin in the same spot in New Zealand.