292 RECONNAISSANCE REPORT ON THE ORMOND EARTHQUAKE 10 AUGUST 1993 S.A.L. Read 1 , 2 and S. Sritharan1 ,2 ABSTRACT On the 10th of August 1993, an earthquake with a magnitude of 6.3 occurred near Gisborne. This report presents observations made by a reconnaissance team which visited the Gisborne area in the days following the earthquake. Structural damage in Gisborne City and damage to services and roading was limited. Stronger ground-motions were apparent between Ormond and Te Karaka, 20km northwest of Gisborne, where soil liquefaction occurred on the Waipaoa River flats, and some natural slopes failed. INTRODUCTION An earthquake shook the Gisborne area on Tuesday 10 August 1993, at 21:47 hours New Zealand Standard Time (NZST). The earthquake epicentre location is at 38.52°S N 177.93°E between Ormond and Waimata 20km north of Gisborne (Figures 1 and 2). The Richter local magnitude (ML) is 6.3, and the focal depth 48km (Seismological Observatory, Institute of Geological and Nuclear Sciences Ltd). Initial reports indicated damage over a reasonably wide area, including Wairoa, as well as in the immediate Gisborne region. The earthquake was felt noticeably in Napier, and as Figure 2 l far away as Wellington and East Cape. The Gisborne area was visited by a field team from the Talaga Bay Institute of Geological and Nuclear Sciences Ltd (GNS) between 11 August and 13 August with the following objectives: 0 • Deployment of portable seismographs in the Tuai epicentral area to record the aftershocks. • Servicing of strong motion accelerographs to obtain -Wairoa the records from the main earthquake shock. • Make an initial reconnaissance of structural damage caused by the earthquake, on behalf of the New r Zealand National Society for Earthquake Engineering 0 (NZNSEE). Hastings • Carry out an initial reconnaissance evaluation of ground damage caused by the earthquake, including soil liquefaction and landslides. 0 50 100 The GNS team was joined on 12 August by a team from the km University of Canterbury (UoC) to study soil liquefaction. Strong-motion Liaison was established with Gisborne Civil Defence, 0 accelerogaph site Gisborne District Council, Works Consultancy Services (Gisborne and Wairoa), Wairoa Civil Defence, and Wairoa District Council, and discussions were held with local Figure 1 Location map for the Gisbome area. residents. 1 Institute of Geological and Nuclear Sciences Ltd, Lower Hutt 2 Member BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, Vol. 26, No. 3, September 1993 293 N / • Whatatutu I • I Kanakanaia ) / ,,- - . -:.. ',, :____~ '\ ) ,,..,- ,,,- ,. fv Te Karaka * -• \', ' * \.• I I \._ I _I * ' /==' '- \ \ ) ~, Waimata .... -> l * • 11 • I \ Kaitaratahi / Epicentre \ *10 August 1993 I \ ,, ✓ I / Gisborne / City ( Poverty Bay --- --- / Epicentre * 5 March 1966 Waingake 0 LEGEND Approximate area of reported damage - - ==MMVI 0 5 10 -• - == MMVII Area with liquefaction effects kilometres ~ Sand boils ., Lateral spreading ~ Active fault trace Area of most intense -<])- Stale highway slope damage Poverty Bay flats * Landslide Figure 2 Prdiminary map ofdamage feature3 fort~ 10 August 1993 Ormond earthquake. 294 During the reconnaissance it became apparent that damage mm/year. The interface between the plates lies at a depth caused by the earthquake was more concentrated in the rural of about 15 to 20 km beneath the Gisborne area. The most area between Ormond and Te Karaka, 20km northwest of significant recent earthquake to affect the Gisborne area was Gisbome (Figure 2). This included damage to housing, the ML6.2 event of 5 March 1966 [Hamilton et al. 1969], bridge approaches, soil liquefaction effects, and slope failures, which had an epicentre located 15km to the southeast of including damage to ridge crests. In Gisbome City, damage Gisbome City (Figure 2). to structures and services was minor, as was damage to roading throughout the wider Gisborne area. ENGINEERING ASPECTS AND SHAKING DAMAGE Systematic field inspections of ground damage by staff from GNS and UoC continued into the week following the Civil Defence in Gisbome went into 'Alert Phase' for several earthquake. During this period teams from Works hours following the earthquake [S Clare, pers comm]. Consultancy Services, Wellington and Gracefield, also carried Incidents, mostly minor, were successfully dealt with by local out inspections of bridges and areas of soil liquefaction Police and Fire Service staff, and since there were no reports respectively. Systematic inspections of buildings in Gisbome of fires or serious injuries to residents, a state of emergency were carried out by the Gisborne District Council. was not declared. Power supply in central Gisbome was lost for _a few hours, and was not restored in some outlying areas The August 10 earthquake is currently being referred to as until the following morning. the Ormond earthquake (Seismological Observatory, GNS). This report, which is based on the observations made during No major damage had been reported when the authors from the reconnaissance, provides a preliminary overview of the GNS arrived in Gisbome on 11 August. Over the next few earthquake and its effects. The records from the portable days it became apparent that the most severe effects of the seismographs installed following the earthquake have not earthquake were concentrated in rural areas outside of been analysed, and felt reports to determine Modified Gisbome City, in particular between Ormond and Te Karaka Mercalli (MM) intensities have not yet been systematically 20km northwest of Gisbome (Figure 2). More distant urban evaluated. Chapman (1993) and Christensen (1993) give centres (e.g., Wairoa, Tolaga Bay) did not appear to have specific details on bridging and soil liquefaction respectively.· suffered significant damage. The Earthquake Commission (EQC) has received 1700 claims for damage totalling $3 million. About 1000 of the claims are for damage to GEOLOGICAL AND SEISMOLOGICAL SETIING chimneys, and few of the individual claims are for more than $50,000. Bedrock in the Gisborne area comprises mainly sedimentary mudstones, siltstones, and sandstones of Tertiary (mainly There was no disturbance of the ground surface at the active Miocene or Pliocene) age [Moore et al. 1989]. The rocks fault trace west of Ormond to indicate that surface fault are folded along a southwest-northeast axis resulting in a rupture was associated with the earthquake. This is reasonably simple geological structure, with bedding having consistent with the size· and preliminary depth estimates for dips of up to 30°. The rocks are typical of many New the earthquake, well below the tectonic plate interface. Zealand sedimentary 'soft' rocks, which have unconfined compressive strengths between 1 and 10 MPa. Jointing is Felt Effects widely spaced, and weathering is generally to shallow depths ( < 10m). There were reports of moderate to strong shaking over more than 1000km2 (""MMVI isoseismal boundary on Figure 2) in The Waipaoa River drains a catchment area of 2000km2 to the Gisbome area. The occurrence of the earthquake close the north and west of Gisbome. In its lower reaches there to 10 o'clock in the evening resulted in many people being are well developed river terraces and flats that have been sharply awakened from sleep by the shaking, and also objects formed along a meandering river course. The river flats being thrown off shelves in houses. The loss of electricity, extend up to !Om above present river level, and south of particularly in rural areas, and the noise associated with the Kaitaratahi are known as the Poverty Bay flats (Figure 2). earthquake heightened the level of alarm experienced by These flats µave ari area of 20km2, and are up to 8km wide many people. near the Waipaoa River mouth. Extensive deposits of silts and sands have accumulated on the flats adjacent to and Further afield in Tolaga Bay, Wairoa and Tuai the effects above river level over the past 3,300 years (Pullar & Penhale were less dramatic. A low frequency motion was felt in 1970), and the total thickness of sediments overlying Tertiary Wairoa, and there were two reports of goods being thrown basement has a maximum of about 200m (L Brown, pers off shelves. Many of the earliest reports indicated that the comm). The hills surrounding the river and its flood plain Wairoa area had been affected, as much as, or more than the rise to altitudes of 500m, and are generally characterised by Gisbome area, but this did not subsequently prove to be the steep ( > 200) slopes. case. Sharp jolts were felt by some people living on hills in Napier and Hastings and a rolling motion was reported by The only active fault mapped in the Gisborne area is located some of those living on soft ground, but there were no about 7km west of Ormond where there is a short (1km) reports of damage. northeast trending trace (Figure 2). The age of last movement is not known. Strong Motion Records The east coast of the North Island, which includes the Twenty eight strong-motion accelerographs from the New Gisborne area, lies in the main seismic region of New Zealand-wide network maintained by GNS are located within Zealand. Offshore the Pacific Plate is being subducted down an epicentral distance of 150km of the earthquake. Those in to the west under the Indo-Australian Plate at a rate of 50 the Gisbome and Hawkes Bay regions were inspected during 295 Table 1 Peak horizontal accelerations for records obtained at twelve sites in the Ormond Earthquake of 10 August 1993 Accelerograph Type