E F F E C T S OF T H E M A R C H 2 1987 E D G E C U M B E E A R T H Q U A K E O N W H A L E I S L A N D

by R R . Moore New Zealand Geological Survey, P.O. Box 30368, Lower Hutt

(Present address: State Highway 25, RD1, Waihi)

S U M M A R Y Damage caused by the Edgecumbe earthquake on Whale Island was largely

confined to minor rockfalls and two larger failures (on old slip scars) on the steep coastal cliffs. The total amount of debris involved in slope failures is estimated to be in excess of 1500 m 3 . No surface ruptures along old faults, or changes in thermal activity, were recorded.

I N T R O D U C T I O N

Whale Island is situated 8 km off the Bay of Plenty coast, near Whakatane. During the mainshock (M6.3) of the Edgecumbe earthquake, which was cen­tred about 18 km to the WSW (Beanland 1987), a large dust cloud was seen rising above the island, and subsequent aerial surveys indicated that a large

Fig. 1. Map of Whale Island showing position of main slope failures resulting from the Edgecumbe earthquake.

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Fig. 2. Aerial view of the western end of Whale Island, and Pa Hill slip (foreground), in June 1982. West Bay to right. Photo: Lloyd Homer, New Zealand Geological Survey.

amount of rock had been dislodged from the high northern cliffs. However, a detailed ground survey was not carried out.

This paper is based on observations made during a 5 day visit to the island from 13 to 18 July 1987, organised by the Department of Conservation. The main aim was to determine the extent of damage caused by the earthquake, and nature and amount of debris involved in slope failures. A check was made for indications of recent movement on known or inferred NNE-trending faults transecting the island along Camp Gully and Sulphur Valley (Fig. 1), but no surface ruptures were seen. In addition, temperatures of hot springs, fumaroles, and warm ground were measured in case any changes in thermal activity had occurred1. However, no significant differences (taking into account climatic con­ditions and slight differences in measurement points) were recorded. The 2 huts on the island were not damaged.

Geology of the island is described by Ramsay and Hayward (1971).

'Temperatures were last measured in January 1986 (Moore, 1990).

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Fig. 3. Aerial view of Whale Island from the east in June 1982; head of the Trig slip arrowed. Photo: Lloyd Homer, New Zealand Geological Survey.

E X T E N T OF D A M A G E

Slope failures were largely confined to the northwestern cliffs and high cliff face forming the northern side of the island. The largest features are the 'Pa Hi l l slip' and 'Trig slip' (Figs. 1-5). Many small failures occurred in West Bay (informal name) and along the low southern cliffs. The majority of slips were technically rock falls (Selby 1970), although some could be classified as rock or debris slides.

Pa Hi l l slip (Fig.2) This failure is situated in massive, well-jointed, andesite of the Pa Hi l l In­

trusive (Ramsay & Hayward 1971). Aerial photos taken in 1982 (Fig. 2) indicate that it was initiated many years ago, but a considerable amount of new material was dislodged during the Edgecumbe earthquake. Relatively fresh rock has fallen away over much of the slip face, some large rocks were loosened near the top of the cliff, and cracks up to 6 m long (some showing downthrow to the west) appeared near the crest of the ridge. The quantity of debris involved is dif­ficult to estimate, but from an examination of oblique air photos about 400-500 m 3 of rock was probably displaced during the earthquake.

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Fig. 4. View down Trig slip, July 1987.

West Bay (Figs. 1, 2) There are numerous small slope failures on the cliffs around West Bay. This

area has been actively eroding for some time, but several slips are relatively fresh and almost certainly occurred during the earthquake.

A l l the failures are in sedimentary rocks of the Camp Bay Formation (Ramsay & Hayward 1971), and mostly in more weathered material at the top of the cliffs. A few originated in breccia (debris flow) deposits present within the sedimentary sequence, particularly around the southern part of the cliffs. The largest slips involved about 100 m 3 of rock, and the total amount of debris in West Bay is probably in the order of 300-400 m 3 .

Northern cliffs (Figs. 1, 3, 4, 5) As indicated from newspaper reports (Whakatane Beacon 5 March 1987)

and oblique aerial photos taken 2-3 days after the main shock, the largest failure (Trig slip) was on the 340 m - high northern cliff face, originating from near the highest point. Previous aerial photos (June 1982, Fig. 3) and old newspaper reports reveal that this slip has been active at various times over the past 80 years. The last recorded movement was during the July 1944 earthquake (Whakatane Beacon 7 July 1944), although vegetation covering the slip prior to the Edgecumbe earthquake was only 10-15 years old ( M . McGlynn, pers. comm.).

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Fig. 5. Headscarp of Trig slip, July 1987; view looking west.

The most recent failure involved a rockfall of relatively fresh, loose blocks of andesite lava, and minor intercalated agglomerate, from the upper part of the old slip scar (Fig. 5). Falling debris cleared a 20-30 m wide path down to the sea, removing all existing vegetation and forming a prominent debris fan at the base of the cliff (Fig. 4). Large fissures, sub-parallel to the cliff edge and up to 2.5 m deep, presently extend 15 m inland from the headscarp, and boulders as far as 30 m to the west were shifted slightly. Unfortunately, it was not possible to gain access around the toe of the slip, but from aerial photos and observations from a distance the latest failure probably involved 500 m 3 of rock.

There were several other small rockfalls on the northern cliffs, mainly near the west end, which produced a total of perhaps 200-300 m 3 of debris.

Other parts of island Very little damage was evident around the N E end of the island, although

a few large blocks of andesite lava had obviously fallen off the cliffs and others loosened. Numerous slips occurred on the low cliffs around the southern side of the main cone, but these involved only a small amount of material (mostly agglomerate).

A few large boulders situated on the upper slopes of the main cone shifted slightly, or rolled a short distance downslope.

I l l

C O N C L U S I O N S

On Whale Island, damage caused by the Edgecumbe earthquake was large­ly confined to steep coastal cliffs at the west end, along the northern side (main cone) and around the southern shore. The 2 major slope failures occurred on old slip scars (Pa H i l l slip and Trig slip) and further movement on these can be expected in future, particularly during heavy rainstorms and larger earth tremors. The total amount of debris brought down as a result of the earth­quake is estimated to be in excess of 1500 m 3 .

ACKNOWLEDGEMENTS

I am grateful to the Rotorua District Office, Department of Conservation, for the opportunity to visit Whale Island, and Wayne Price (DOC, Whakatane) and Mike McGlynn (DOC, Gisborne) for their assistance. My thanks also to Anton van der Wouden (Curator, Whakatane Museum) for his hospitality, and providing information. Aerial photos were supplied by Lloyd Homer (NZGS, Lower Hutt).

REFERENCES

Beanland, S. 1987: The Edgecumbe earthquake, March 2, 1987 earth deformation studies. Geological Society of New Zealand Newsletter 76: 15-17.

Moore, P.R. 1990. Observations on the thermal activity at Whale Island (Motuhora). Tane 32: 101-105.

Ramsay, W.R.H.; Hayward, B.W. 1971: Geology of Whale Island (Motuhora). Tane 17: 9-32. Selby, M.J. 1970: Slopes and slope processes. Publication No. 1 of Waikato Branch of the New Zealand Geographical Society.

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