The 1934 Pahiatua earthquake sequence
Analysis of observational and instrumental data
Descriptive accounts and analysis of local seismograms establish that the epicentre of the 1934 March 5 Ms7.6 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 led early seismologists to name the earthquake after the town, but it has now been found that the highest intensities (MM9) occurred about 40 km to the east and southeast of Pahiatua, between Pongaroa and Bideford. Uncertainties in the location of the epicentre that have existed for sixty years are now resolved with the epicentre determined in this study lying midway between those calculated in the 1930's 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 descriptions and isoseismal maps. A stable solution for the epicentre of the mainshock has been obtained by analysing phase arrivals read from surviving seismograms of the rather small and poorly equipped 1934 New Zealand network of twelve stations (two privately owned). The addition of some teleseismic P arrivals to this solution shifts the location of the epicentre by less than 10 km. It lies within, and to the northern end of, the MM9 isoseismal zone. Using local instrumental data larger aftershocks and other moderate magnitude earthquakes that occurred within 10 days and 50 km of the mainshock have also been located. Approximate locations of other associated moderate magnitude earthquakes until October 1934 have been identified by their maximum intensity and S-P intervals read from the Wellington Wood-Anderson seismograph records. The distribution of S-P intervals of aftershocks (magnitudes M > 3.5) within 24 hours of the mainshock is used to delineate the probable mainshock rupture zone.
Neither contemporary sources nor recent inquiries directed to old residents yield historical evidence of a surface fault rupture. Nevertheless, the strike-slip mechanism at 20 km depth determined by preliminary teleseismic body wave modelling of Doser and Webb suggests that rupture could have extended to the surface. Recent investigation of two of the freshest-looking, active faults that lie within the MM9 isoseismal by Schermer and others indicates that one of them could have ruptured in the 1934 Pahiatua earthquake.
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Copyright (c) 1999 Gaye Downes, David Dowrick, Euan Smith, Kelvin Berryman
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