A Discussion on the measurement and interpretation of changes of strain in the Earth - New Zealand Earth strain measurements

Abstract

Three invar wire lever type Earth strain meters are being operated in geophysically interesting areas in New Zealand. T he oldest has been operational since early 1970. The main object of the work has been to detect possible strain changes occurring before an earthquake. None have been detected, the most likely earthquake to have produced an effect before hand having M s = 5.3 and lying at A = 1° from the nearest strain meter. Strain steps accompanying earthquakes appear to be rather smaller than expected. A great deal of information has been accumulated on strains induced by wind and rain at the most exposed site which is but 10 m below the top of a very exposed 300 m hill. Strain changes produced by several days’ heavy rain can, at this site, be as m uch as 3 x 10-6, and this indicates th at great care m ust be taken in interpreting apparent changes in the length of a baseline m easured by high precision surveying.