The 1934 Pahiatua earthquake sequence: analysis of observational and instrumental data

Technical Abstract

Descriptive accounts and analysis of local seismograms establish that the epicentre of the 1934 March 5 Ms 7.6 earthquake, known as the Pahiatua earthquake, was nearer to Pongaroa than to Pahiatua. Conspicuous and severe damage (MM8) in the business centre of Pahiatua in the northern Wairarapa lead early seismologists to name the earthquake after the town, but it has now been found that the highest intensities (MM9) occurred about 40km to the east-southeast of Pahiatua, between Pongoroa and Bideford. Uncertainties in the location of the epicentre that have existed for 60 years are now resolved with the epicentre determined in this study lying midway between those calculated in the 1930s by Hayes and Bullen.

Damage and intensity summaries and a new isoseismal map derived from extensive newspaper reports and from 1934 Dominion Observatory "felt reports", replace previous isoseismal maps. Surviving seismograms from the rather small and poorly equipped 1934 New Zealand network of 12 stations (two privately owned) have been read and analysed. The addition of some teleseismic P arrivals results in a stable solution for the epicentre of the main shock within, and to one end of, the MM9 isoseismal. Larger aftershocks and other moderate magnitude earthquakes that occurred within 10 days and 50km of the main shock have also been located using local instrumental data. Approximate locations of other moderate magnitude earthquakes until October 1934 have been identified by their maximum intensity and S-P intervals on the Wellington Wood-Anderson seismograph. The distribution of S-P intervals of aftershocks (magnitudes M>3.5) within 24 hours of the main shock is used to delineate the probable main shock rupture zone.

Neither contemporary sources nor recent enquiries directed to old residents yield any evidence of a fault break. Nevertheless, the strike-slip mechanism at 20km depth determined by preliminary teleseismic body wave modelling of Doser and Webb suggests that it is a possibility that needs to be considered. Final body wave modelling results will determine whether further investigation of a recently identified fresh-looking, active fault that lies within the MM9 isoseismal is warranted.