123 THE DUNEDIN EARTHQUAKE, 9 APRIL, 1974 PART 2: LOCAL EFFECTS D. G. Bishop* ABSTRACT The earthquake of 9 April, 1974 was the strongest experienced in the Dunedin area in historic times. It was centred at sea about 10 km south of the city and had a magnitude of 5.0. The felt intensity reached MMVII in the St. Clair area, where a ground acceleration of 0.27 g was recorded. Variations in felt intensity were determined from a survey of grocery stores. The intensity decreased rapidly away from a maximum on the alluvial ground of the southern suburbs and correlated strongly with the underlying rock type. The number of claims received by the Earthquake and War Damage Commission was extraordinarily large for an earthquake of this magnitude. Damage, generally of a rather minor nature, was reported from all parts of the city, but was greatly concentrated in the South Dunedin - St. Clair area. About half of the 3000 claims received included chimney damage. The effects of the earthquake highlight the need to assess the safety of public buildings in Dunedin, particularly those sited on areas of thick alluvium. INTRODUCTION central city area and an intensity of MMVII at St. Clair. A sharp earthquake was felt throughout the Dunedin City area at 7.50 p.m. on GEOLOGY Tuesday, 9 April, 1974. The earthquake had a shallow focus (probably about 20 km) Dunedin City is built on the southern about 10 km south of the city? its magnitude flank of the Dunedin Volcanic Complex was 5.0 (see Part 1 for details). The main (late Miocene), a series of basaltic lava shock was felt as a double event, with an flows and associated rocks which overlie initial tremor followed a second or two a succession of lower Tertiary marine later by the main shock. The two events sedimentary rocks. Only the uppermost may correspond to the arrival of the P and sedimentary unit, the Caversham Sandstone S waves respectively. After-shocks were (Oligocene), a thick (c. 230 m), creamy felt at 8.20 p.m. and 9.50 p.m. The main white, calcareous sandstone, outcrops shock was accompanied (many reports state within the urban area. The older sedi• preceded) by a loud roaring noise, and mentary rocks outcrop a few kilometres to caused considerable public alarm with the west, where they unconformably overlie telephone circuits being jammed and widespread the Otago Schist. damage occurring to masonry, particularly chimneys, in the St. Clair, St. Kilda and Younger alluvial deposits occur Caversham areas. Power supplies to mainly around the head of the harbour, Corstorphine were interrupted for 4 5 especially in the south, where a tombolo minutes when high tension switches in the has developed to link the volcanic hills foreshore sub-station were tripped, and of the Peninsula with the mainland. all available fire appliances were called out to false alarms caused by the trigger• GEOLOGICAL EFFECTS ing of automatic warning systems. Following the earthquake some 3 000 claims totalling The only fault known to have been about $250,000 were received by the active in Recent geological time in the Earthquake and Warm Damage Commission, making Dunedin region is the Akatore Fault, which this earthquake second only to the 19 68 has well-preserved Recent fault traces at Inangahua earthquake as the most damaging Taieri Mouth, Big Creek, and Nobles in the Commission's records. Stream, some 30 km south-west of Dunedin. As early indications indicated the Almost all observers reported a strong epicentre might be associated with a vertical motion. Swaying of hanging submarine north-eastern extension of this objects was not noticed except in a few fault, the fault traces were inspected peripheral areas, such as near Milton. but no signs of surface rupture was seen. The earthquake had a felt intensity of No instances of landsliding attributable MMVI on the Modified Mercalli Scale in the to the earthquake were detected. Found• ation subsidence due to compaction did occur * N.Z. Geological Survey, Department of beneath a house at St. Clair, however, Scientific and Industrial Research, about two days after the earthquake. Box 5342, Dunedin. BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, VOL.7, NO.3. SEPTEMBER 1974. 124 FELT INTENSITY items fell was also recorded. As items were restrained in one direction in almost A survey was made of fallen items in all instances, the direction is plotted 105 small grocery stores throughout the as a non-polar azimuth in Fig. 3. A city and as far south at Taieri Mouth pronounced concentration is found in the and north to Warrington. Such stores have north-east - south-west quadrants in similar stock, stacked in similar fashion harmony with the accelerograph records. and although variations might be expected to arise from differences in building DAMAGE CLAIMS construction, foundation conditions, and types of shelving, it became clear during The number of claims received (3000) the course of the survey that variations is surprisingly high for an earthquake of from such causes were significant only in this magnitude. The majority of claims exceptional and obvious cases, and that were from houses and at least half included the shops were in fact rather satisfactory chimney damage (see below). Successively sites for making comparative observations. less important categories of damage were:- The results were initially plotted at a cracking of interior plaster, cracking of scale of 1: 12,500 and contoured at intervals external masonry, damage to plumbing, of 0 - 5, 6 - 15, 16 - 100, and more than breaking of household contents, and damage 100 fallen items. A reduced version of to tile roofs. this map is included as Fig. 1. The claim localities were plotted on The siting of the two Dunedin a street map, initially at a scale of accelerographs allows an approximate 1:12 500, and contoured (Fig. 2). Claims calibration of the isoseismals. The were received from all parts of Dunedin and 2 Dunedin Central Post Office lies just the lowest contour, 20 claims per km , outside the isoseismal marking the reflects the limits of the built-up area consistent appearance of a small number of except in the northern suburbs and displaced articles. Here, where the Waverley. The maximum contour (320 claims intensity was MMVI, a ground acceleration per sq km) is strongly affected locally of 0.12 g towards the south-west was by such factors as variations in the recorded and hence this value appears to number of houses per unit area and by be about the threshold necessary for dis• regions of older houses. The north-eastwards placement of cans from stacks or shelves. extension of the area of maximum claim The St. Clair Telephone Exchange on the density (relative to the intensity maximum other hand lies within the maximum felt of Fig. 1) is due to a combination of intensity contour and here an acceleration these factors, as the South Dunedin - St. of 0.27 g towards the north-east was Kilda area is a high density region of recorded. older housing. Such effects could be reduced by expressing the data as the It is estimated that, in a stronger ratio of the number of damage claims to earthquake, another one or two contours the number of buildings present. This could be drawn on a similar basis provided has not been attempted, and the map should that interference from structural damage therefore be interpreted with some caution. did not occur. Nevertheless it vividly displays the con• centration of damage on the alluvial Notable points of this map are:- ground of the southern suburbs, in addition to complementing the felt intensity map. 1. The very localised nature of the maximum. CHIMNEY DAMAGE 2. The rapid falloff in intensity. 3. The strong correlation between Two categories of chimney damage intensity and lithology. were differentiated during an assessment 4. The slight but distinct high along by A. R. Mutch, N.Z. Geological Survey, the topographic ridge from Lookout Dunedin. A zone of light damage (chimney Point, through Mornington, Roslyn pot displaced, outer plaster dislodged) and up Taieri Road to Wakari. extended throughout the city and as far afield as Brighton, Port Chalmers and The strongest effects were felt on the Mosgiel. In a more intense inner zone unconsolidated alluvium of the southern (Fig. 2) bricks were dislodged and in a suburbs, with the intensity falling off few instances a substantial part of the very steeply on the surrounding consolidated chimney collapsed. In many cases a poor sedimentary and volcanic rocks. The Lookout bond between brick and mortar was indicated Point - Wakari high is apparently a topo• by the complete disintegration of the graphic effect, which might have been fallen section into its component bricks significant in the case of a major earth• (Fig. 4). quake. Comparison of this map with the map of damage claim localities (Fig. 2) suggests RELATIONSHIP OF INTENSITY TO GEOLOGY that similar but less well defined areas of increased intensity occur on the ridge The effect of the geology is indicated crests in Brockville, Pinehill and Waverley. by the departure of the isoseismals from a Unfortunately no shops were available for series of concentric arcs. The most comparison on the reclaimed land at the marked effect is the concentrated intensity eastern end of the harbour, where an even maximum on the alluvial ground of the more pronounced response than that on southern suburbs, where the western and alluvial ground might be expected. northern edges of the maximum correspond closely to the edge of the alluvium. The DIRECTION OF MOVEMENT contrast between damage on this area of alluvium and that on the alluvial ground Where known, the direction in which of the northern part of the city is striking.