In search of the source of the 1934 Pahiatua earthquake

Technical Abstract

The March 5, 1934 Ms 7.6 Pahiatua earthquake (epicentre 40.51 S, 176.29E) caused severe damage (MM8 and MM9) in a large part of the southern North Island. Although investigations immediately after the earthquake failed to identify a surface fault rupture, no direct observations were made of the faults in the area. In this study we present the results of field work and paleoseismicity studies on several of the major faults within the 1934 MM9 isoseismal to assess the likelihood that there was surface rupture on these, or other, faults in 1934, and to describe the geometry, kinematics, and rupture parameters of the recent historic and/or prehistoric surface rupture earthquakes in this region. We discuss the implications of our results for sesmic hazard assessment in the area.

We conducted an airphoto survey of the 1934 MM9 isoseismal region and compiled existing published and unpublished data to identify known and likely active faults. Field work examined several of the more prominent scarps in the region, with geomorphic and structural mapping designed to quantify the length, type of faulting, and offset characteristics, as well as assess the relative recency of faulting by an examination of scarp morphology and continuity. Of the faults studied (Alfredton, Saunders Road, Waitawhiti, Oporae Road, Waipukaka faults), the Alfredton and Waipukaka faults were identified as having extremely fresh scarps and these were chosen for more detailed analysis via mapping, trenching and 14C dating.

Mapping of the Alfredton fault, the northern continuation of the Wairarapa fault, revealed fresh scarps over a strike length of 12-16km from just north of Mauriceville to ~5km north of Alfredton. The fault is a dextral fault; mapping in this study and compilation of previous work suggest that the single-event displacement on this fault is at least 4m and possibly as high as 7m. Data from trenches dug in 1991 (unpublished data from IGNS) and in this study reveal evidence for the last two surface rupture earthquakes. The 1991 trenches reveal two surface-rupturing earthquakes occurred since 642±91 yBP (years before present =1950), with the most recent event occurring after 330±65 yBP, and possibly after AD1750. The trenches dug in this study also contain evidence for the two most recent surface ruptures, one at ~450yBP and the other at <240±70 yBP. Photographs taken of the fault scarp in December, 1935 by Ongley of the NZGS show a scarp that appears fresh, but does not look like a surface rupture that is only ~1.5 years old. Preliminary evidence thus suggests the Alfredton fault may have ruptured the surface in the 1855 earthquake. If our interruption is correct, the surface rupture for the 1855 earthquake ruptured across a major geometric barrier, a large right step in the fault trace.

The second fault investigated in detail, and mapped for the first time, is named here the Waipukaka fault zone. The fault is a west-dipping dextral-reverse fault comprising ~8km of extremely fresh scarps near the northern end of the 1934 MM9 isoseismal and within the error estimate of the earthquake epicentre of Downes et al. (1998). The few offset features identified suggest a dextral to reverse ratio of ~5:1 to 2:1, with single event slip components on the order of 3-6m dextral, 1-3m reverse (4-7m oblique slip). Two trenches along the fault reveal evidence for multiple surface-rupturing earthquakes. The youngest event in both trenches is interpreted to post-date a charcoal horizon that contains European historical artifacts (such as shoes and pottery) and thus is considered to be an historical event. Analysis of historical seismicity and evidence on settlement of the area suggests the most likely time of the earthquake is 1934. Radiocarbon results establish that at least four earthquakes have occurred since 7300±70 yBP and three since 2762±57 yBP.

Evidence from this study suggests that historical earthquakes occurred on both the Alfredton and the Waipukaka faults. Preliminary analysis of the historical and geological evidence suggests that the Alfredton fault may have ruptured in 1855 and the Waipukaka fault most likely ruptured in 1934. We also present an analysis of the implications of the rupture parameters, including the geometry, length, displacement, and recurrence interval, for seismic hazard estimation on the east coast of the North Island.