The Akatarawa Fault: a Newly Discovered Active Fault in the Wellington Region, and Implications for Increased Hazard on the Wellington Fault

Paper No. 4.06.01

The Akatarawa Fault: a newly discovered active fault in the Wellington region, and implications for increased hazard on the Wellington fault

R.J. Van Dissen, J.G. Begg & R. Robinson NZSEE 2001 Institute of Geological & Nuclear Sciences, PO Box 30-368, Lower Hutt, New Zealand Conference

ABSTRACT: The active Akatarawa fault extends northeastward for ca 18 km from its junction with the Wellington fault near the Whakatikei terraces area of Upper Hutt to its junction with the Moonshine and Otaki Forks faults at Cloustonville in the Akatarawa valley. Geomorphic mapping and trenching of the fault trace in the Akatarawa valley indicate that the Akatarawa fault has a minimum dextral slip rate of 0.4 mm/yr, and a maximum average earthquake recurrence interval of ca 9000 yrs. However, given dating and measurement uncertainties, the actual slip rate may be considerably higher, and the recurrence interval may be considerably less. Coulomb failure stress modelling of the Wellington and Akatarawa faults suggests that rupture of the Wellington fault enhances the likelihood of rupture of the Akatarawa fault. An important implication, given the Akatarawa fault’s sense of slip and geometry relative to the Wellington fault, is that the hazard posed by the Wellington fault south of its junction with the Akatarawa fault may currently be underestimated.

1 INTRODUCTION The Wellington region is located in the boundary zone between the obliquely convergent Pacific and Australian plates (Figure 1). Deformation within this zone is to a large extent accommodated by movement on the active faults within, including under, the region. The vast majority of slip on these faults, at least those located within the upper (Australian) plate, is believed to result from metre-scale displacements that generate large earthquakes. The location and rates of slip on these faults thus provide a general estimation of the long-term pattern of stress release, and can therefore be used as good indicators of location and frequency of future large, damaging earthquakes (e.g. Stirling et al. 2000). Within the upper plate (Australian plate) the major active faults within the Wellington region include the Wairau, Shepherd’s Gully, Ohariu, Wellington, and Wairarapa right-lateral strike- slip faults (Figure 1). The location of these faults, and to a lesser extent their hazard potential, have been known for some time (e.g. Ota et al. 1981). They have slip rates that range between 1-10 mm/yr, average surface-rupture earthquake recurrence intervals ranging from several hundred to several thousand years, and collectively they accommodate more than half of the margin-parallel motion between the two plates (e.g. Van Dissen & Berryman 1996). More recently, as a result of continued geological and geophysical research, about half-a-dozen “new” active faults have been discovered (or rediscovered) in the region (e.g. Van Dissen et al. 1998). The Akatarawa fault, the focus of this paper, is one of these newly discovered active faults. The purpose of this paper, a condensed version of Begg & Van Dissen (2000), is three-fold: 1) document evidence of recent activity on the Akatarawa fault, 2) discuss possible stress interactions between the Akatarawa fault and surrounding faults, and 3) comment on the important implication that, given the location and sense of slip of the Akatarawa fault, relative to the Wellington fault, the hazard of the Wellington fault, south of its intersection with the Akatarawa fault, may currently be underestimated.

Paper No. 4.06.01

Palmerston North

North Australian Island Plate

Wellington ?

40 mm/yr FORKS FAULT LT UL AU FA Pacific U

OTAKI South IU RI Plate Levin HA Island Levin OH N ER TH OR NO

WAIRAU FAULT Otaki N (location approximate only) OTAKI FORKS FAULT

0 10 20 kilometres Waikanae GF Paraparaumu Paraparaumu FAULTFAULT Trench site Masterton (Fig. 2) Cloustonville Cloustonville TON MaF AKATARAWA FAULT TARARUA RANGE Tararua section PF WELLINGTONWELLING CF

MOONSHINE FAULT FAULT

Upper Hutt MOONSHINE FAULT

FAULT WV WAIRARAPA FAULT SGF WELLINGTON Lower Lower RIMUTAKA RANGE Martinborough MF Martinborough OHARIU Hutt TF OHARIU HF

Wellington WF DRF

OF Cook StraitStrait Palliser Bay

Turakirae Head

Figure 1 Location of the Akatarawa fault in relation to other active faults in the greater Wellington region. Active tectonic setting of New Zealand is summarised in the inset with the azimuth and rate of Pacific plate motion shown relative to the Australian plate at the latitude of Wellington. Fault name abbreviations are as follows: CF, Carterton fault; DRF, Dry River fault; GF, Gibbs fault; HF, Huangarua fault; MF, Martinborough fault; MaF, Masterton fault; OF, Otaraia fault; PF, Pukerua fault; SGF, Shepherds Gully fault; TF, Terawhiti fault; WF, Wharekauhau fault; WV, Whitemans Valley fault.

2 TF SGF PF Hutt Upper OF DRF MF HF

Paper No. 4.06.01

2 THE AKATARAWA FAULT

2.1 Location, expression of active surface trace, and slip rate The Akatarawa fault zone consists of a number of faults that splay from the Wellington fault in the Whakatikei terraces area of Upper Hutt, striking NE through the Akatarawa hills to join the Moonshine and Otaki Forks faults at Cloustonville in the Akatarawa valley (Figure 1). For much of its ca 18 km length, the Akatarawa fault extends through bedrock hills, making assessment of its recent activity difficult. However, about 2 km south of Cloustonville (grid ref. R26 879186), the fault extends obliquely across the Akatarawa valley and displaces geologically young alluvial fans and terraces, though the present flood plane is not displaced. Here, each displaced alluvial surface is downthrown on the SE side of the fault; the youngest by ca 1.5 m, and the oldest by 1.5-2 m. At two places, we measured metre-scale right-lateral displacements. A low terrace is dextrally displaced 2.5-3.5 m, and a higher, older terrace is displaced ca 5-10 m. These displaced alluvial surfaces range in elevation above the present riverbed from ca 5-20 m. They are located below, and are thus younger than, the main aggradation terrace surface in the valley which we assume is late last glacial in age (ca 14,000 yrs, see also Section 2.2). Taken together, these data provide evidence for more than one surface rupture on the Akatarawa fault within the last ca 14,000 yrs because the lateral displacement of an older terrace is about two to three times greater than that of a younger terrace. A minimum dextral slip-rate of 0.4-0.7 mm/yr is estimated for the Akatarawa fault based on a 5-10 m horizontal displacement of a post-glacial terrace with an assumed maximum age of 14,000 yrs. The actual slip rate may be higher than 0.4-0.7 mm/yr if the age of the displaced terrace is significantly younger than 14,000 yrs.

2.2 Trench exposure across the fault trace To further constrain the timing, size and style of past ruptures on the Akatarawa fault, a trench was excavated across the fault scarp on the low-level terrace that is dextrally displaced by 2.5- 3.5 m (Figure 2, Table 1). In the middle of the trench, the fault was exposed. It displaces sediments to within 0.3 m of the ground surface, and comprises a zone of steeply dipping (75° NW) faults that have a component of reverse displacement, NW side up. Near the surface, some shear planes decrease in dip to as little as 15°. Three cycles of alluvial sedimentation are recognised in the trench based on unconformable relationships and minor lithological variation (e.g. clast-supported vs. matrix-supported gravel). Gravel clasts within all units are derived from Torlesse greywacke sandstone and are essentially unweathered, suggesting no significant age difference between alluvial Units 1, 2, 3, 6, 7 & 8 (Figure 2). All alluvial units in the trench are truncated by the fault, and faults also truncate younger colluvial coverbed Units 4, 9 & C. Faults extend upward to above the base of Units 4, 9 & C, but do not apparently penetrate the full thickness of these units (see also Section 2.3). No carbonaceous material suitable for dating was found in the trench, and age estimates are based only on indirect evaluation using coverbed stratigraphy, relative elevation of alluvial surfaces, and weathering of alluvial gravel clasts. The youngest alluvial units in the trench (Units 3 & 8) are overlain by silty gravel interpreted as overbank silt and colluvium (Units 4, 9, 10, 11, A & C) having moderate soil development. In the Wellington region, surfaces older than ca 14,000 yrs commonly have a capping of loess. The absence of a loess-cap on the trenched terrace is consistent with the surface’s age being <14,000 yrs, as is the limited development of the soil B-horizon and the freshness of the gravel clasts. Also, the trenched terrace is ca 15 m below the inferred late last glacial aggradation terrace and only ca 5 m above the modern flood plane. Though equivocal, we consider the trenched terrace to be Holocene in age.

2.3 Paleo-earthquake rupture history, magnitude, and recurrence interval Evaluation of both the faulted terraces and trench exposure in the Akatarawa valley suggests that the Akatarawa fault has ruptured the ground surface repeatedly within the last ca 14,000 yrs. Below, we briefly outline evidence for individual rupture events, from oldest to youngest: Event 1: Lateral displacement of the trenched low-level terrace (2.5-3.5 m) is about one-half to one-third that of a higher, older, post-glacial terrace. This suggests that there is at least one

Paper No. 4.06.01

2 1 ftec algrid wall trench of lines vertical of Location SW Trench

P oain(lbla 212) (GlobalNav location GPS ZG28956018607 2687935 NZMG 8

6 SE WWall SW rnhN wall NE Trench

7 wall

5 10 6 lnView Plan 4 0123456789

5 .3m 2.43 6c 11

3 8a 1

metre

.. ..

2 .. ..

......

ttec floor trench at faults of Location ......

......

0 ......

......

.. ..

......

.. 3 ..

0 1

metre

......

...... 8c

6b ..

......

1 .. ..

......

N ......

......

2 ......

......

...... 7 6c 2 irriaeo cullog actual of image mirror Wall NE SE ......

......

...... E ......

......

9 ......

......

...... 6

......

..... KTRW AL TRENCH FAULT AKATARAWA

......

6a ......

......

7+8 ......

......

...

......

...... ? ...... ?

C ......

......

. .

. . .

......

. . .

. 3 ......

. . .

......

. . .

. .

? ?

. . .

. .

. . .

. . . . .

. . 12 . .

. . .

. ..

......

. .

......

. . . .

......

. .

. . .

. .

......

. . . .

. .

__ . .

. . . .

. .

. 6d .

. .

......

. .

. . .

. .

. . .

. .

......

. . .

. .

? ?

......

. .

......

E .

. . .

. .

. . .

......

. 8c

___ . .

. .

7+8 . . . .

. .

___

......

. .

. . .

. .

. . .

. .

. . .

.... .

......

. . .

D .

. .

. . .

......

. .

......

. .

. . .

......

. .

. 12 .

. ...

......

. . .

. .

......

. . .

. .

. . ___

. .

. ___ . .

. ? .

......

. .

. 8b

......

. . .

......

. . . . .

. . .

... . .

......

F . .

. .

. . .

. .

. . .

. .

F . . .

6e . . . .

. .

__ .

. . . ___ . .

. .

. . .

. . 3 . .

. . . .

......

. . . .

......

6 . . .

......

...... al zone fault .

......

. . . 4 ......

......

...... F ...... ? ...... 2 ......

. . . 3 . . . . . al zone fault 2 .. 2 2 .. 5 3 2 1 B NW A 6 2 1 0 1 5 al ntebsso osatthickness constant of basis the the to on trench fault of end NW 3 the Unit from of top the of Extrapolation 2 1.0 HORIZONTAL ETCL= VERTICAL SCALE metre usVnDse,Jh Begg John Dissen, Van Russ 4 3 ..

......

...... 4Fbur 2000 February 24

...... 0

......

1.0 0

......

metre A

......

...... NW

Figure 2 Log of the Akatarawa fault trench, showing full length of SW wall and only central faulted section of NE wall. Horizontal and vertical scales are the same, but scale of plan view is one-half that of the log. Stratigraphic units on SW wall are numbered, and those on NE wall are lettered. See Table 1 for unit descriptions, and cross-trench and cross-fault correlations.

6 6

F 7+8

7+8

C 2

6c 2

7 8b 1

2 8a 8c

12 2

11 3

3 11

5 4

Paper No. 4.06.01

Table 1 Akatarawa fault trench unit descriptions, and cross-trench and cross-fault correlations.

Unit SW wall unit descriptions 1 Medium grey, loose, coarsely cross-stratified, poorly sorted, matrix-supported sandy gravel. Clasts sub-angular to sub-rounded, up to 20 cm. 2 Orange-brown stained, medium grey, loose, coarsely cross-stratified, moderately to well sorted, clast-supported “open” gravel. Clasts sub-angular to sub-rounded, up to 18 cm (commonly 3-4 cm). 3 Medium grey (basal) to orange-brown stained (upper), loose, coarsely cross-stratified, poorly sorted, matrix-supported sandy gravel. Clasts sub-angular to sub-rounded, up to 10 cm, commonly 2-3 cm. Unit grossly upward fining. Upper part is part of soil B- horizon. 4 Yellow-brown, loose, massive, poorly sorted matrix-supported silty gravel. Clasts sub- angular to sub-rounded, up to 5 cm. Unit is within soil B-horizon. 5 Dark grey-brown, loose, A-horizon; massive, poorly sorted gravely silt. Maximum clast size c. 5 cm (rare). This unit is predominantly the topsoil. 6a Medium grey, with some orange-red staining, loose, massive, poorly sorted sandy gravel. Clasts sub-angular to sub-rounded, up to c. 15 cm. 6b Grey, loose, crudely bedded, moderately sorted (basal) to well sorted (top half) medium to very coarse sand. Pebbly at base, with angular to sub-angular clasts up to c. 1 cm. 6c Medium grey, orange-brown stained, loose, crudely bedded, moderately sorted, matrix- supported sandy gravel. Clasts sub-angular to sub-rounded, rarely up to 8 cm, mostly 1-3 cm. 6d Medium grey, loose, crudely bedded, moderately sorted fine gravely sand. Clasts sub- angular to sub-rounded and up to 2 cm. 6e Medium grey with some dark-stained clasts, massive, poorly sorted, matrix-supported coarse gravel. Clasts sub-angular to sub-rounded and up to c. 20 cm. 7 Orange-brown stained, medium grey, loose, coarsely cross-bedded, moderately to moderately well sorted clast-supported “open” gravel. Clasts sub-angular to sub- rounded, rarely up to 12 cm diameter, commonly 1-5 cm. 8a Medium grey-brown, loose, massive to crudely cross-bedded, poorly sorted, matrix supported sandy gravel. Clasts sub-angular to sub-rounded, up to 15 cm, commonly 3- 8 cm. 8b Medium to dark grey, loose, crudely bedded, moderately sorted, medium to coarse sand with some granules. Granules sub-rounded to sub-angular. 8c Orange-brown stained, medium grey, loose, massive, moderately to well sorted gravel. Clasts sub-angular to sub-rounded, up to c. 1 cm, commonly < 1 cm. 9 Probable colluvial wedge - fault generated. Brownish yellow, firm to poorly consolidated massive, poorly sorted silt with some sand and fine gravel. Gravel clasts rarely up to 8 cm, commonly < 2 cm. 10 Brownish yellow firm to poorly consolidated, massive poorly sorted fine gravely silt. Soil B horizon developed in the top of this unit. 11 Dark brown, firm to poorly consolidated, massive, poorly sorted silt with fine gravel. Clasts sub-angular to sub-rounded, up to 2 cm, commonly < 1 cm. 12 Brownish grey, loose, tectonically mixed, poorly sorted gravel. Clasts sub-rounded to sub- angular, up to 5 cm, commonly 1-3 cm. Colluvial wedge.

Unit NE wall unit descriptions A Brownish-yellow, firm, massive, poorly sorted, pebbly silt. Clasts rarely up to 2 cm. B Dark grey-brown, firm, massive, poorly sorted, pebbly silt. Clasts up to 7 cm, commonly 1-3 cm. Soil A horizon has developed in this unit. C Probable colluvial wedge. Brownish yellow, firm, massive, poorly sorted pebbly silt. Cobbles sub-rounded to sub-angular, and up to 6 cm. D Grey-brown, firm, massive, poorly sorted pebbly sandy silt. Pebbles sub-angular to sub- rounded and up to 1 cm. Uncertain whether this unit is colluvial or alluvial. E Grey-brown to brownish grey, firm, crudely bedded, poorly sorted, pebbly sandy silt. Pebbles sub-angular to sub-rounded and up to 3 cm. Uncertain whether this is a colluvial or alluvial unit.

Probable correlations across fault zone Probable correlations across trench Foot wall unit Hanging wall unit SW wall NE wall

Paper No. 4.06.01

Unit 6 Unit 1 Unit 9 Unit C Unit 7 Unit 2 Unit 5 Unit B Unit 8 Unit 3 Unit 12 Units E & D

additional post-glacial surface rupture event older than the faulting exposed in the trench. Event 2: In the trench, the oldest probable scarp-forming rupture is represented by deposition of a coarse colluvial wedge comprising Units 12, E & D. These units are collectively interpreted as a scarp-derived colluvial wedge because no primary stratification is recognisable, they only occur close to and on the downthrown side of the fault, and they thicken towards the fault. Also, on the upthrown side of the fault, alluvial Unit 3 thins towards the fault. This thinning is consistent with colluvial degradation (erosion) of a fault scarp formed of Unit 3, and provides a source for the material that comprises colluvial Units 12, E & D. Event 3: This event is represented by deposition of a second colluvial wedge, comprising Units 9 & C, and rupture of the older, Event 2, colluvial wedge. The poor sorting, fine texture and lack of stratification of Units 9 & C suggest a colluvial origin, as does the observation that both units thicken towards the fault and are restricted to the downthrown side of the fault. The thickness of the Event 3 colluvial wedge, and also that of the older Event 2 colluvial wedge, is about 0.5 m, and provides a minimum estimate for the amount of vertical separation during these two events at this site. Event 4: The most recent event is represented by vertical separation of the basal contact of the youngest colluvial wedge (Units 9 & C), which is nowhere greater than ca 0.1 m. Faulting, however, cannot be followed through the entire thickness of Units 9 & C, and does not displace the top of these units. Event 4 appears to be different in character to earlier events. It did not produce a scarp (no colluvial wedge was deposited), and apparent vertical displacement was much smaller (cm-scale vs. dm-scale). As no paleosol or depositional hiatus is evident within Units 9 & C, we suspect that Event 4 took place soon after Event 3. We suggest two alternative explanations for Event 4: it may represent “after slip” on the Akatarawa fault following Event 3 rupture, or it may represent triggered slip on the Akatarawa fault resulting from rupture of another nearby fault (see Section 3). As noted previously, the trench was excavated on a terrace that is right-laterally displaced by 2.5-3.5 m. Stratigraphic and structural relationships exposed in the trench reveal at least one, but probably two, large surface ruptures since abandonment of the trenched terrace. This suggests that single-event surface-ruptures at this site are in the range of 1.25-3.5 m. Average co-seismic single-event surface-rupture displacements of 1.25-3.5 m are indicative of earthquakes of magnitude Mw 7.1-7.5 (Wells & Coppersmith 1994). In summary, there have been at least three, probably at least four, surface ruptures of the Akatarawa fault within the last ca 14,000 yrs, suggesting a maximum recurrence interval of 3500-4700 yrs. Independently, a maximum average earthquake recurrence interval of 4000- 9000 yrs is calculated based on a single event displacement size of 1.25-3.5 m and a minimum slip rate of 0.4-0.7 mm/yr.

3 STRESS INTERACTIONS WITH OTHER FAULTS The Akatarawa fault is active, and appears to link the active Wellington fault with the Moonshine and Otaki Forks faults (Figure 1). This suggests that the latter two faults should also be regarded as active. The distinct topographic expression of these two faults, especially that of the Otaki Forks fault, is consistent with this inference. Of the upper-plate faults in the Wellington region, the Wellington fault has the shortest surface-rupture earthquake recurrence interval, and the second highest slip rate (e.g. Van Dissen & Berryman 1996, Nicol & Van Dissen 1997). The Akatarawa fault splays from the Wellington fault in the vicinity of a major (ca 15°) bend in the trace of the Wellington fault. Irregularities along the rupture plane of an earthquake, including asperities, terminations, and bends, are known to influence the pattern of induced stresses resulting from earthquake rupture. These induced stresses may either enhance or retard the likelihood of rupture of other nearby faults. To investigate how rupture of the high-slip Wellington fault may impact on the Akatarawa fault, we

Paper No. 4.06.01

modelled the induced Coulomb failure stresses (CFS) on the Akatarawa fault resulting from various plausible rupture scenarios of the Wellington-Hutt Valley segment of the Wellington fault (see Robinson & McGinty (2000) for a detailed account of the theory and methods involved in the CFS modelling, and Begg & Van Dissen (2000) for a full description of the rupture scenarios considered). For brevity, we present results for only one of the rupture scenarios (Figure 3). However, in all considered scenarios, the Akatarawa fault lies in the middle of areas of positive induced CFS. Rupture of the Wellington fault loads the Akatarawa fault in such a manner as to bring it closer to failure, or, put another way, rupture of the Wellington fault enhances the likelihood of rupture of the Akatarawa fault. Repeated rupture of the Wellington fault would incrementally load the region now occupied by the Akatarawa fault, almost necessitating the latter’s existence, perhaps growing from a pre-existing weakness. Also, as discussed in Section 2.3, Event 4 appears to be different (smaller) in nature to the other rupture events. A possible explanation is that Event 4 represents more-or-less immediate triggered-slip resulting from rupture of the Wellington fault. Such triggered events are often smaller than more characteristic events. The positive induced CFS on the Akatarawa fault resulting from various Wellington fault rupture scenarios, though not conclusive, is at least consistent with the triggered-slip hypothesis. However, confirmation lies in the precise dating of Event 4 and recent Wellington fault ruptures.

4 INCREASED HAZARD ON THE WELLINGTON FAULT An important implication, given the location and orientation of the Akatarawa fault relative to the right-lateral Wellington fault, and the fact that the Akatarawa fault is active and has a predominantly right-lateral sense of displacement, is that the hazard posed by the Wellington fault south of its junction with the Akatarawa fault may be currently underestimated. This is because current slip-rate and single event displacement estimates for the Wellington fault are based on data almost exclusively from NE of its junction with the Akatarawa fault (Berryman 1990). The slip rate of the Wellington fault south of the junction of the two faults should be, to a first approximation, the sum of slip rates on both the Wellington fault north of the junction and the Akatarawa fault. This implied higher slip rate on the Wellington fault south of the junction may be accommodated either by additional surface rupture earthquakes (i.e. shorter recurrence interval for large earthquakes) and/or as larger single event displacements relative to those north of the junction.

A) o 1. All faults vertical, dextral o B)B) strike-slip 2. 5 m slip on Wellington F. (sth) 3. 4 m slip on Wellington F. (nth) x 0102030 _ + OTAKI FORKS FAULT N40 E Scale (km) OTAKI FORKS FAULT N40 E N o o

N31 E AKATARAAKATARAWAWA FAULTFAULT N31 E plus southernsouthern OTAKI FORKSFORKS FFAULTAULT x

o o

xo x o WELLINGTONWELLINGTON F.F. (nth)(nth) Ohariu Fault N67N67 E E + intersection 44 m m slip/event slip/event WELLINGTON F.F. (sth)(sth) oo MOONSHINE FAULT FAULTN51N51 N51 N51 E E E E

55 m m slip/event slip/event xWellington-Hutt Valley Segment + of WELLINGTON FAULT _

marks mid-points of x modelled fault planes _ +

Figure 3 Modelled induced Coulomb failure stress on the Akatarawa fault resulting from rupture of the Wellington-Hutt Valley segment of the Wellington fault. In this, and other scenarios, rupture of the

Paper No. 4.06.01

Wellington-Hutt Valley segment enhances the likelihood of rupture of the Akatarawa fault. Model is shown in A) and results in B). All faults are modelled as vertical, right-lateral strike-slip faults that extend from the ground surface to a depth of 20 km. In this scenario, rupture of the Wellington-Hutt Valley segment of the Wellington fault is composed of two sections – Wellington fault south with 5 m of dextral slip, and Wellington fault north with 4 m of dextral slip. Induced stresses are resolved onto planes having the same attitude and sense of slip as that of the Akatarawa fault.

5 CONCLUSIONS There is no doubt that the Akatarawa fault is active and capable of generating metre-scale surface rupture earthquakes with a maximum recurrence interval of ca 3,500-9,000 yrs. The recurrence interval could be considerably less if the faulted low-level alluvial surfaces in the Akatarawa valley are significantly younger than 14,000 yrs. We consider that the Moonshine and Otaki Forks faults should also be regarded as active, primarily because the active Akatarawa fault links the former two faults with the active Wellington fault. Additionally, given the location, orientation and sense of slip of the Akatarawa fault, in relation to the Wellington fault, we suspect that the hazard posed by the Wellington fault, south of its junction with the Akatarawa fault, is currently underestimated, and that this increased hazard on the Wellington fault may be expressed as more frequent earthquakes and/or larger single event displacements.

ACKNOWLEDGMENTS This study would not have been possible without financial support from both the EQC Research Foundation and FRST, and the willing co-operation of the landowners (families Cosgrove, Redington, Spicer, Wood, and Woollett). Various versions of the manuscript were improved by the reviews of K. Berryman, K. Gledhill, S. Nathan, J. Van Dissen, and J. Zachariasen. Institute of Geological & Nuclear Sciences contribution 2105.

REFERENCES Begg, J.G. & Van Dissen, R.J. 2000. Documentation of multiple post-glacial ruptures on the Akatarawa Fault, Wellington region, New Zealand. Institute of Geological & Nuclear Sciences Client Report 2000/81 (a research report prepared for EQC). Berryman, K.R. 1990. Late Quaternary movement on the Wellington fault in the Upper Hutt area, New Zealand. New Zealand journal of geology & geophysics 33: 257-270. Nicol, N. & Van Dissen, R.J. 1997. Late Miocene to recent fault and fold deformation east of Martinborough, Wairarapa. Geological Society of New Zealand miscellaneous publication 95B, Geological Society of New Zealand Annual Conference field trip guide FT 3, November 1997, 17 p. Ota, Y., Williams, D.N. & Berryman, K.R. 1981. Late Quaternary tectonic map of New Zealand, part sheets Q27, R27 and R28, Wellington, scale 1:50,000, with notes. DSIR, Lower Hutt. Robinson, R. & McGinty, P.J. 2000. The enigma of the Arthur’s Pass, New Zealand, earthquake 2: The aftershock distribution and its relation to regional and induced stress fields. Journal of geophysical research 105: 16,119-16,137. Stirling, M., McVerry, G., Berryman, K., McGinty, P., Van Dissen, R., Dowrick, D., Cousins, J. & Sutherland, R. 2000. Probabilistic seismic hazard assessment of New Zealand: new active fault data, seismicity data, attenuation relationships and methods. Institute of Geological & Nuclear Sciences Client Report 2000/53 (a research report prepared for EQC). Van Dissen, R.J. & Berryman, K.R. 1996. Surface rupture earthquakes over the last c. 1000 years in the Wellington region, New Zealand, and implications for ground shaking hazard. Journal of geophysical research 101 (B3): 5999-6019. Van Dissen, R.J., Begg, J.G., Palmer, A., Nicol, A., Darby, D. & Reyners, M. 1998. Newly discovered active faults in the Wellington region, New Zealand. In Proceedings, New Zealand National Society for Earthquake Engineering Technical Conference, Taupo, New Zealand: 1-7. Wells, D.L. & Coppersmith, K.J. 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the Seismological Society of America 84: 974-1002.

Paper No. 4.06.01

6 RETURN TO INDEX