Research Papers
The 1942 June 24 M7.2, August 1 M7.0 and December 2 M6.0, Wairarapa, New Zealand Earthquakes: Analysis of Observational and Inst
GL Downes, DJ Dowrick, RJ Van Dissen, JJ Taber, GT Hancox, EGC Smith - IGNS
In 1942, two large earthquakes, on June 24 (Ms 7.2) and August 1 (Ms7.0) strongly shook the Wairarapa, Manawatu and Wellington districts, causing moderate to severe damage. A third earthquake, of magnitude Ms 6.0, occurred in the same area on December 2. The June and August main shocks were each preceded within a few hours by a moderate magnitude "foreshock" (ML 5.2 June; Ms 5.3 August), previously assumed to originate from the same location. The seismological aspects of the earthquakes were not studied in detail at the time, although their locations were identified instrumentally, and by the distribution of intensity (Rossi-Forel scale), and by the discovery of a scarp about 15km east of Masterton that was thought to be the surface rupture of a fault. Although two Wellington-based engineers produced reports on aspects of damage in Wellington, no engineering report on the damage in the Wairarapa, closest to the epicentres, was published.
The locations and effects of these earthquakes have now been studied in detail, by re-reading and analysing seismograms from 1942 using present knowledge and modern techniques and by the collection and analysis of contemporary descriptive accounts and technical information from many archival sources. New isoseismal maps and new damage (casualties, building, lifelines, landslides and ground damage) summaries for the three main earthquakes are given, as well as new locations for the main shocks and other major shocks in the sequence.
Data on the damage caused by the larger and shallower June earthquake (depth 12km) provide valuable insight into the performance of buildings, domestic and non-domestic, and lifelines in urban or small town environments at high intensities (³MM8). The good performance of reinforced concrete and retrofitted brick buildings in the Wairarapa is of particular interest. Although the August 1 main shock was far less damaging overall because of its greater depth (40km), the effect of strong shaking on previously damaged, as yet unrepaired, and newly repaired structures was readily apparent.
The distribution of damage in the central business district of Wellington from both earthquakes has been previously interpreted as a microzone effect. Among new information found during this study, is a map (of unknown authorship), which identifies and classifies by age damaged non-Government buildings. The map shows the greater percentage of damage to pre-1914 buildings. The distribution of damage to these buildings shows broad agreement with previously published shaking zones, with some discrepancies, but also suggests a correlation of the damage with particular reclamations.
The area in which earthquake-induced landsliding, ground cracking, subsidence and liquefaction effects occurred (June ~ 11,500km2; August ~ 5,600km2) was extensive and much greater than previously recognised. Evidence suggests that the June 24 earthquake did not rupture to the surface and that the scarp-like features described in 1943 as surface fault rupture are probably landslide-related, and not tectonic in origin.
This study has a number of important implications for seismic hazard assessment of this part of the Hikurangi Margin, particularly the failure of a large magnitude shallow earthquake to produce a surface fault rupture and the occurrence of several large earthquakes close in space and time.