Tecronophysics, 163 (1989) 185-195 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

185

Holocene paleoseismicity in the fold and thrust belt of the Hikurangi subduction zone, eastern North Island,

New Zealand

K.R. BERRYMAN ‘, Y. OTA 2 and A.G. HULL **

I New Zealand Geological Suruey, P.O. Box 30368, Lower Hutt (New Zealand)

’ Ge0graph.v department, Yokoh~a Nationat university, Hodoga~a-K~, Yokohama 240 (~apan~

(Received April 20,1988; accepted May 8.1988)

Abstract

Berryman, K.R., Ota, Y. and Hull, A.G., 1989. Holocene paleoseismicity in the fold and thrust belt of the Hikurangi subduction zone, eastern North Island, New Zealand. In: N.-A. Miimer and J. Adams (Editors), Paleoseismicity and Neotectonics. Tectonophysics, 163: 185-195.

Along the 500 km long eastern coastline of the North Island, New Zealand there are, at most, seven distinct

Holocene marine terraces aged 7 ka B.P. or less. The highest of the terraces is 27 m above present-day mean sea level. The coastal region is subdivided into fourteen distinct subregions based on radiocarbon ages of marine deposits overlying wave-cut shore platforms and geographic distribution of similarly-aged terraces.

Holocene marine terraces are the result of uplift associated with large earthquakes (co-seismic deformation). This conclusion is based on characteristic stepped terrace morphology, clustering of ages of terrace deposits within

subregions. and the occurrence of co-seismic uplift in historic time. Differential uplift across structures and distinct age variations at subregion boundaries are also characteristic.

Recurrence intervals of uplift in any one terrace sequence vary from - 0.4 ka to - 2.0 ka, and individual amounts of uplift vary from - 1.0 m to - 4.0 m. In the past - 2.5 ka ages from all terraces within subregions indicate at least 21 paleoseismic events affecting coastal areas of eastern North Island in that period. These events cluster in time, and

in separate parts of eastern North Island several earthquakes occurred - 0.3,0.6,1.0, 1.5, 2.1 and 2.3 ka B.P. Our data strongly support the concept of segmentation of deformation along the subduction margin.

The likely cause of coastal uplift is movement on steep reverse faults (local structures rather than the subduction thrust) that propagate from or near the subduction thrust some 20-2.5 km below the region. Earthquakes of moment magnitude 7.3-8.0 are estimated to be associated with these fault movements.

In~~uction

Part of the present boundary between the Australian and Pacific plates in the southwest Pacific is along the Hikurangi Trough, east of the 500 km long eastern coastline of the North Island, New Zealand. The magnitude of the relative plate

* Present address: Department of Geological Sciences, Uni- versity of California, Santa Barbara, CA 93106 (U.S.A.).

Q 1989 Elsevier Science Publishers B.V.

vector varies from 45 to 55 mm/yr, and is ori- ented approximately normal to the Hikurangi Trough in the north and increasingly oblique to the south.

Recent studies of the age and distribution of Holocene marine terraces along the east coast of the North Island (Fig. 1) have been described in terms of a paleoseismicity record in various parts of the region (Berryman, 1983; Ota et al., 1983; Hull, 1987; Ota et al., in press, a). This paper

186

I I I I 178’E 178-E 179-E 18t

IIIw -p Pakarae Rwe~

Cape KIdnappers

Fw 2

0 50 100 km

Paleogene

0 Neogene and Quaternary

175’E 176’E 177-E 178-E 179-E I I 1 I I

38-S

Fq 5

39-s

40’S

41’S

18C I

Fig. 1. Locality map showing plate tectonic setting and place names mentioned in text. After Walcott (1978a).

summaries these data, presents the case that each

terrace in each subregion represents co-seismic

uplift and assesses the spatial and temporal distri-

bution of the seismic events causing the uplift.

Tectonic setting

The present boundary between the Australian Outcropping rocks of the continental border-

and Pacific plates extends northeast through New land are composed largely of terrigenous, late

Zealand from the Puysegur Trench in the south- Cenozoic trench-fill sediment offscraped from the .

west to the Hikurangi Trough, 50-150 km east of

the shoreline of the North Island (Weissel et al.,

1976) (Fig. 1). Along the east coast of the North

Island the Pacific Plate is being subducted be-

neath the Australian Plate at the Hikurangi Trough

(Adams and Ware, 1977; Walcott 1978a, b; Re-

yners, 1980).

Frontal Rtdge Forearc Basin Htghest Accretlonary Ridge

maln ranges coastal hills

sea

I

0 50km I 4

Fig. 2. Diagrammatic cross-section of eastern North Island just south of Hawke Bay. Black represents pre-late Cretaceous greywacke

basement. Cross-hatching represent late Cretaceous. Coarse stipple represents early Tertiary. Fine stipple represents late Tertiary and

white represents Quatemary. The coastal study area is on the coastal margin of the highest accretionary ridge. Based on sections by

Kingma (1962) and Van der Lingen and Pettinga (1980).

187

Fig.

rive]

3. Aerial view westwards of Holocene marine terraces at the mouth of the Pakarae River. The prominent, high terrace above the

. is TI of Fig. 5. An active fault disrupts the terrace sequence on the far (southwest) side of the river. Photo A14791a. D.L.

Homer, N.Z. Geological Survey.

subducting plate and subsequently uplifted (Lewis,

1980). An imbricate thrust system underlying

asymmetric fold structures has developed roughly

parallel to the continental slope and the Hikurangi

Trough (Lewis, 1971; Lewis and Bennett, 1985;

Davey et al., 1986).

The western portion of the imbricate thrust

system is exposed near the present coastline, where

a zone of coastal hills records rapid uplift (Pet-

tinga, 1982) (Fig. 2). To the west, late Cenozoic

sedimentary basins have been uplifted and dis-

rupted by normal faulting, and further west, near

the axial ranges of the North Island, the region is

disrupted by both contractional and translational

structures (Fig. 2).

Uplifted marine terraces of both Holocene and

last interglacial age occur extensively in coastal

areas (Figs. 3 and 4). Terraces are often landward

tilted (Yoshikawa et al., 1980; Berryman, 1983;

Ota et al., 1983) and suggest deformation in re-

sponse to convergence on west-dipping faults of

the imbricate thrust system.

Age estimates of Holocene marine terraces

Ages of individual terraces have been obtained

from radiocarbon dating of shell and wood

material incorporated in beach deposits that over-

lie marine-cut shore platforms. Throughout much

of the study area, shore platforms are cut in soft

late Cenozoic sedimentary rocks, but in the south

they are cut in harder Cretaceous and older, weakly

metamorphosed, flysch sediments (Fig. 1). To the

northeast of Hawke Bay (Fig. 1) airfall volcanic

ash layers are also found within marine and ter-

restrial deposits underlying the terraces, providing

additional age control.

Uncertainties in the amount and timing of up-

lift of the elevated marine terrace sequences arise

from sample height with respect to present mean

188

# 9 : 3 Terraces

3200.5400~.5600 Y6P

,y.y , 176’ (+3). 6700 yBP 178’E I

Fig. 4. Fourteen tectonic subregions on east coast of North

Island, on the basis of radiocarbon ages of Holocene marine

terraces. Faults associated with coastal uplift are tentatively

located in subregions 3, 5, 13, and 14.

sea level, sample height with regard to former

mean sea level, the radiocarbon dating method

and relation of dated sample to time of uplift. In

the following paragraphs we discuss each of these

considerations.

Sample elevation with respect to present mean sea

level

All samples were from beach deposits overlying

marine-cut shore platforms. The elevation of the

shore platform at the sample collection site (or

ideally at the shoreline-angle) was measured by

Abney or automatic level as an elevation above

local sea level. These heights are regarded as accu-

rate to kO.5 m.

Elevations were then corrected to mean sea

level by interpolation to published tide tables for

several locations along the east coast (Ministry of

Transport, 1984). Maximum tidal range is 1.9 m

for outer coastal areas with a mean of about 1.6

m. Errors introduced by this factor are estimated

to be less than +0.3 m.

Sample height with regard to former mean sea level

Gibb (1986) has shown that the sea level

fluctuated by only about kO.5 m since the

culmination of the postglacial transgression about

6.5 ka B.P.

All shell samples dated in the present study are

intertidal in habitat and most cannot tolerate more

than 3 m water depth. Two genera that were

commonly dated, Nerita and Melagraphia, re-

quire emergence during each tidal cycle for

survival.

The formation of marine-cut shore platforms in

relation to mean sea level also introduces uncer-

tainties in calculating uplift. In microtidal en-

vironments, such as along the east coast of the

North Island, wave-cut shore platforms are almost

always formed within the tidal range (Kirk, 1977;

Trenhaile and Layzell, 1981; Hull, 1987). Half the

mean tidal range (kO.8 m) is considered a rea-

sonable overall uncertainty due to these factors.

The overall error due to height and sea level

uncertainties is estimated as the square root of the

sum of the squares, i.e.:

/(0.5)2 + (0.3)2 + (0.5)2 + (0.8)2 = 1.1 m

Radiocarbon dating

All ages quoted are calculated on the Libby

half-life of 5568 years. Errors are derived from

laboratory counting statistics. At the two standard

deviation level they generally represent 5-10% of

the age of the sample. Where multiple dates are

available from a single terrace they are generally

indistinguishable at two standard deviations.

Relationship of dated sample to time of uplift

In most places along the outer, exposed, parts

of the eastern North Island coastline, the present

shore platform has only a sporadic and ephemeral

cover of beach deposits. Therefore the ages of

sediments preserved on uplifted shore platforms

do not vary appreciably and are considered to be

close to the time of emergence above marine con-

ditions.

A feature of tectonic deformation on coasts in

many circum-Pacific areas such as Alaska and

Japan is subsidence between major co-seismic up-

lift events. In Alaska, subsidence for up to 1300

years occurred prior to the uplift associated with

the 1964 Prince William Sound Earthquake

(Plafker, 1969) and in Japan rapid subsidence of

10 mm/y occurred for 20-30 yrs prior to the 1923

Kanto Earthquakes (Matsuda et al., 1978). Hull

(1987) noted that on an uplifted shore platform at

Cape Kidnappers radiocarbon ages of samples

tend to decrease as one approached the former

shoreline angle, consistent with subsidence. In

general, the date of the youngest sample will closely

approximate the time of uplift. In this study we

accept the youngest age obtained from each ter-

race (provided it falls within 95% confidence limits

of ages of other samples from that terrace) as the

time of uplift.

Summary of age estimates: spatial and temporal

relations

About 150 samples from beach and estuarine

deposits of Holocene age have been dated from

marine terrace sequences along the east coast of

the North Island (Singh, 1971; Berryman, 1983;

Ota et al., 1983; Hull 1987; Ota et al., in press, a)

(Fig. 4). The samples have been obtained from

terrace sequences that range in elevation from

only about 1 m above mean sea level (a.m.s.1.) up

to 27 m a.m.s.1. at Pakarae River (Fig. 1) and

have, at most, seven distinct levels at Pakarae

River and south Wairarapa. At Mahia Peninsula

(Fig. 1) six distinct levels have been mapped, but

in other areas the Holocene plain consists of less

than six terraces in areas further from the

Hikurangi Trough, such as in Hawke’s Bay and

Poverty Bay (Fig. 1). These areas are to the west

of the zone of rapid uplift in the coastal hills and

suggests there is either vertical tectonic stability or

subsidence in the forearc basin (Fig. 2).

The oldest terrace in many of the sequences

corresponds to the culmination of transgressive

estuarine deposits that accumulated during the

189

rise in sea level from the last glacial lowstand. The

age of the culmination terrace ranges from about 6

ka B.P. to 7 ka B.P. being older in more rapidly

uplifted areas and younger in lower uplift areas

(Ota et al., in press, b). The mean age of the

culmination is in agreement with Gibb’s (1986)

generalised New Zealand Holocene sea level curve

that did not utilise data from the study area.

Marine terrace data in the Pakarae River area

(Figs. 3, 5 and Table 1) serves as a representative

example of the geomorphology and terrace age

data obtained. A total of seven terraces of marine

origin are distributed on either side of the Pakarae

River. All terraces have been dated and range in

age from c. 0.6 ka B.P. to c. 6.7 ka B.P. The

morphology is characterised by subhorizontal ter-

TABLE 1

Radiocarbon dates from Holocene marine terrace deposits at

Pakarae River, subregion 3

Site Material Code No. 14C date Height Terrace

(N.Z.) a (yrs B.P.) b a.m.s.1. ’

(m)

A shell 7088 6920 + 240 23.4 Tl

A shell 5572 6740 f 300 22.0 Tl

A wood 10460 + 9160+360 3.6 Tl

A shell 10459 * 9960 + 500 3.6 Tl

B she11 7113 549Ok 120 12.7 T2

B shell 7126 8120+200 9.1 Tl

C shell 7047 2700* 90 7.0 T4

D shell 10464 * 7710 f 300 3.4 Tl

E shell 7101 168Ok 120 4.1 T5

F shell 7078 1570* 130 1.9 T5

G shell 7131 682Ok 180 c.10 Tl

G shell 7124 6880 f 200 c.11 Tl

H shell 7130 3910 f 140 12.8 T3

I she11 7104 2450+100 11.0 T4

J shell 6489 1030+ 80 4.2 T6

J shell 6490 1000+ 70 3.2 T6

K shell 6460 1625+ 70 7.5 T5

Code number refers to dates from the Institute of Nuclear

Sciences (I.N.S.), D.S.I.R. Radiocarbon Laboratory, New

Zealand. * indicates samples dated at Gakushuin University

Laboratory, Japan.

Radiocarbon ages are in relation to Libby f life with 2

standard deviation confidence limits. Shell ages from INS

calculated with respect to N.Z. Shell std; Ai4C of -41%.

All INS ages are corrected for isotopic fractionation by

normalizing to A13C of - 25% relative to PDB.

Heights are considered accurate to + 1.1 m relative to former

sea level position at time of beach deposit accumulation.

190

(a)

Tl A _~n

m volcanic ash

m drift pumice

B s&

m sand

m gravel

m basement mudstone

- wood

Q +Q shell

BR beach ridge

0 sand dune

T4 T3 J,

10

0 0

(bf

Fig. 5. G~rno~hoio~c~ map (a) and cross-sections (b) of the coastal Pakarae River area. The sequence of marine terraces is

labelted TI-T7. Letters refer to location where the samples have been obtained for radiocarbon dating. Cross-section locations are

shown. All radiocarbon dates have 2 standard deviation uncertainties. P.F.-Pakarae Fault. Fine stippling-dunes; dashes-beach

ridges, Wk-Wbakatane Ash (- 4800 yrs B.P.).

191

race treads that represent marine abrasion during

tectonic and distinct terrace risers that represent

each uplift event. The terrace sequence is also

disrupted by the active Pakarae Fault.

In coastal Wairarapa (Figs. 1 and 4) four subre-

gions have been distinguished, each of which has

different terrace age sequences adjacent to one

another. For example a subregion to the southwest

of Flat Point (Figs. 1 and 4) has a terrace se-

quence comprising seven levels, the lower three of

which have ages of c. 1, 2, and 3.5 ka B.P. To the

northeast of Flat Point the youngest two terraces

of the sequence have ages of c. 0.6 and 1.5 ka B.P.

Dates are not available from many of the higher

and older terraces in the sequences.

Mechanism of uplift of Holocene marine terraces

The Holocene marine terrace sequences dis-

cussed above have identical morphology along the

whole east coast of the North Island and we

propose that co-seismic uplift is the mechanism of

formation of the terraces. Our arguments (out-

lined below) for a co-seismic uplift mechanism are

similar to those of Hull (1987).

That the uplift is co-seismic is supported by

three main lines of evidence: (1) characteristic

stepped terrace morphology, (2) clustering of ages

of terrace deposits with statistically significant

gaps between successive terraces, and (3) historic

occurrence of co-seismic coastal uplift in New

Zealand in 1855 (Ongley, 1943; Wellman, 1967)

and 1931 (Henderson, 1933; Marshall, 1933; Hull,

1986). Differential uplift across structures and dis-

tinct age differences for the timing of uplift of

adjacent structures rule out episodes of increased

storminess on aseismically uplifting coasts as the

mechanism of terrace formation (cf. Lajoie, 1986,

p. 102).

The assigned age for each terrace is therefore

considered to be the time of a past, moderate-large

magnitude earthquake uplifting the coastal area.

The terrace data indicate that paleoseismic

events have uplifted only discrete parts of the

coastline and that the boundaries between regions

are reasonably fixed in time and space. For exam-

ple, terrace age data from Mahia Peninsula and

Pakarae River (Fig. 1) illustrate similar ages for

some terraces (paleoseismic events) but significant

differences for others. At Mahia Peninsula, the

width of the zone of uplift associated with the

Lachlan Anticline (Fig. 1) is about 10 km. The

Lachlan Anticline extends to the northeast and

lies about 20 km offshore of the Pakarae River

locality. Parallel to the Lachlan Anticline, but

only 2-3 km offshore of Pakarae River, bathymet-

ric data indicate a possible fault which may be

responsible for coastal uplift there.

From these studies, we believe that the identifi-

cation of co-seismic terraces younger than about

2.5 ka B.P. is reasonably complete. The age distri-

bution of terraces provides a criterion for subdi-

viding the study area into subregions with unique

terrace age sequences (Fig. 4). A total of fourteen

subregions are recognised. Some subregions are

characterised by rapid uplift, some by a lack of

Holocene uplift, and some (for instance in the

inner Hawke Bay area associated with co-seismic

uplift in 1931) by subsidence interrupted by infre-

quent uplift at perhaps 10 ka intervals (Hull,

1986).

Discussion

Analyses of Holocene marine terraces indicate

there is a distinct spatial and temporal distribu-

tion of paleoseismic events in the study area (Fig.

6). Episodes of activity occurred 0.3, 0.5-0.6,

0.9-1.0, 1.5-1.6, 2.0-2.1 and 2.3-2.4 ka B.P. Two

subregions experienced uplift about 0.3 ka B.P.,

three subregions about 0.5-0.6 ka B.P., three sub-

regions 0.9-1.0 ka B.P. and five subregions about

1.5 ka B.P. The data show that there have been

periods of high seismicity separated by periods of

relative quiescence. Dating resolution is not good

enough to show the spatial sequence of earth-

quakes within any one episode of high activity.

Recurrence intervals of periods of increased

seismicity are not constant and range from 200 to

500 years. The last active period, with the excep-

tion of the 1931 Hawke’s Bay Earthquake, was

about 0.3 ka B.P.

The 1931 event may signal the beginning of a

new phase of activity, although its occurrence in a

subregion experiencing uplift with only very long

recurrence intervals makes this unlikely.

192

CAPE PALLISER 1931 Earthquake

EAST CAPE

(a)

SUBREGIONS i 1716lErl [Ij

1 14 1131121 11 1 lalsl I 3 I 2 I k I

0 100 200

1 1 4

300 400 500 km

(b)

Fig. 6. a. Coast-parallel plot showing the clustering in time of paleoseismic events in coastal areas of eastern North Island into five

episodes (shading) for the period back to 2.5 ka B.P. Symbol height encompasses 2 standard deviation dating uncertainty (dashed line

in upper zone is 1 standard deviation) and symbol length represents the length of the region that may be affected by individual

paleoseismic events. b. Schematic plan view of the spatial distribution of subregions. Subregions closer to the Hikurangi Trough are

toward the bottom of the figure. Length of symbols in (a) correspond to subregion positions in (b).

For a few of the subregions, we identify faults

that are likely to cause coastal uplift (Fig. 4).

These are reverse faults at the trailing edge of the

imbricate thrust system associated with deforma-

tion of the continental borderland. The reverse

faults may propagate upward from a decollement

surface a little above the subducting oceanic crust

(Davey et al., 1986). The depth of the decollement

beneath the study area is 20-25 km (Reyners,

1980; Davey et al., 1986).

The dimensions of coastal uplift in some of the

subregions can be used to calculate the seismic

moment (Hanks and Kanamori, 1979) (Table 2)

and thus estimate the possible moment magnitude

range of earthquakes associated with coastal up-

lift. For five subregions where a reliable estimate

of their length and the likely location of causative

faults are known the derived moment magnitude

is in the range 7.3-8.0 (Table 2). Some of the

parameters we use for moment magnitude calcula-

tions are derived from dislocation modelling of

the 1931 Hawkes Bay earthquake by Haines and

Darby (1987). Dislocation modelling of that event,

using both geologic and vertical and horizontal

geodetic data, has shown that coastal uplift was

associated with movement on a west-dipping re-

verse fault that ruptured from near the subduction

thrust. There are close similarities between the

1931 event and prehistoric coastal uplift events in

terms of the style of deformation, length of re-

gions affected by a single uplift and the magnitude

of uplift. Haines and Darby (1987) have calcu-

lated that there was an average of about 8 m of

dip-slip movement on the fault causative of about

2.5 m of coastal uplift near Napier during the

1931 earthquake. Thus the 1931 event is a very

useful analogy and the moment magnitudes

calculated in Table 2 are entirely reasonable for

the region.

Our study also sheds some light on the seismic

193

TABLE 2

Calculation of moment magnitudes

Subregion Fault plane dimensions MO c

M, d

3

5

12

13

14

length uplift fault slip a fault area b

(km) (m) (m) (m2 X 10”)

25 2-4 6-8 0.5-1.1

150 l-4 5-8 2.5-5.2

25 l-2 5-6 0.4-0.6

30 l-2 5-6 0.5-0.8

75 l-3 5-8 1.3-2.6

(Nm x 102’)

1.5- 3.2 7.4-7.6

7.7-15.5 7.8-8.0

1.2- 1.9 7.3-7.4

1.5- 2.3 7.4-7.5

3.8- 1.7 7.6-7.8

a Assessed by analogy with dislocation modelling of the 1931 Hawke’s Bay Earthquake (Haines and Darby, 1987).

’ Assumes rupture initiation on or close to the subduction thrust 20-25 km below the site and that the faults dip at - 60”

’ MO = seismic moment assuming a rigidity of 3 X 10” Nrn-’

d M, = moment magnitude calculated from the equation: M, = ~(log,,M, - 9.1). (Hanks and Kanamori, 1979).

potential of other reverse faults deforming the

borderland offshore towards the Hikurangi

Trough. There are as many as fourteen ridges with

associated reverse faults in various transects across

the offshore part of the continental borderland

(Lewis and Bennett, 1985). We have shown that

along the inner margin of this borderland a series

of major earthquakes affect parts of eastern North

Island for 100-200 year periods, followed by

quiescence for perhaps 200-500 years. While there

have been some historic earthquakes in the off-

shore area, there is a major deficiency if the rate

of activity recognised from the Holocene marine

terrace sequence is extrapolated to each of the

many structures recognised offshore. This de-

ficiency leads us to conclude that the steepest and

probably oldest faults of the accretionary border-

land, which now occur within the highest accre-

tionary ridge, have the highest large earthquake

potential. While it seems likely that the border-

land is extending outward by progressive deforma-

tion, at the leading edge of the imbricate thrust

system (Davey et al., 1986), the movement on the

equivalent reverse faults may be aseismic because

the sediment is too soft or pore-water pressures

are too high.

Conclusions

We conclude that the sudden coastal uplift

recorded by uplifted marine terraces is associated

with movement on the older individual faults of

the imbricate thrust system within the continental

borderland. Although a large coastal landslide,

with a submarine toe region resembling a reverse

fault, has been described from one part of the

study area (Pettinga, 1985) the phenomenon does

not seem to be widespread in coastal areas. Simi-

larly, although normal faults are described from

onshore parts of eastern North Island (Mazen-

garb, 1984) they appear to be largely westward of

the zone of reverse faults causing coastal uplift.

The style of coastal uplift with strong landward

tilting is inconsistent with normal faulting.

No rupture of the subduction thrust occurred

during the 1931 Hawke’s Bay earthquake (Haines

and Darby, 1987) and similarly we have found no

evidence that any of the 21 paleoseismic events

inferred to have occurred in the past - 2.5 ka B.P.

along the inner margin of the Hikurangi subduc-

tion system, have been associated with major co-

seismic movement of the subduction thrust. We

believe coastal deformation can be primarily at-

tributed to movement on west-dipping reverse

faults within the accretionary prism.

Acknowledgements

Support for this work came from Japanese

Ministry of Education, Science and Culture (grants

60041029 and 61043025) and New Zealand Geo-

logical Survey. We thank Nozomi Iso, Takahiro

Miyauchi, Katsuhiko Ishibashi, Len Brown, Deb-

bie Fellows, Brad Pillans, Dave Francis, Colin

194

Mazenbarb, Kenichiro Y amashina and Masumi

Miyoshi for assistance with various parts of the

fieldwork. Keith Lewis, Simon Lamb, Harvey

Kelsey and John Adams provided very helpful

reviews of the manuscript.

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