Proceedings, Slope Stability 2011: International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Vancouver, Canada (September 18-21, 2011)

Back Analysis of Landslides Triggered by Earthquakes Some Implications for Future Practice

W. Murphy School of Earth & Environment, University of Leeds, United Kingdom

R.N. Parker Department of Geography, University of Durham, United Kingdom

G. Hancox GNS Science, 1 Faiway Drive, Avalon 5010, PO Box 30-368, Lower Hutt 5040, New Zealand

Abstract Earthquake induced landslides are an important secondary effect of strong earthquakes. While the driving forces for such instabilities (e.g. topographic amplification) have received considerable attention, the actual mechanics of how such slope failures occur has not. Current thinking suggests that the weakest sector of the slope will fail. Field evidence does not always support this assertion. Analysis of three landslides triggered by the 1929 Ms=7.8 Murchison Earthquake indicates that the geometry of the landslides are inconsistent with the critical slip surface for ambient stress conditions. The landslides at Little Wanganui Head and Whitecliffs show critical slip surfaces that do not match the locations or geometry of the subsequent failures. Analysis of the Goat Landslide also indicates critical slide surfaces that are inconsistent with the actual slope failure, but the existence of a disturbed zone, either from weathering or unloading, can be used to explain geometric differences.

It is suggested that zones within rock slopes that are characterised by low shear velocities are a more significant control on the stability of rock slopes during earthquakes than the ambient stress conditions. The implications of this on-going research may impact substantially on the design of mine slopes in seismically active regions.

1 Introduction The current thinking that underpins the analysis of slopes during earthquakes is relatively simple. The model presumes that for almost any earthquake induced landslide (EIL), the slope behaviour is governed by the weakest slope configuration. To put this is slope stability terminology, it is the critical slip surface for ambient effective stress states that will slide during an earthquake. The model is further developed to outline how, at some threshold level, the slope may deform when subjected to seismic shaking. It is this model that underpins the work of Newmark (1965) and which has effectively been developed by other workers who have considered how to analyse the stability of slopes dynamic conditions. The principle outlined here applies to both soil or rock slopes and has been employed at site specific and regional scales (see inter alia Sarma, 1981; Ambraseys and Menu, 1988; Ambraseys, & Srbulov, 1995; Jibson, 1993; Miles and Ho, 1998)

The other major issue associated with EIL is that the forcing function that is driving slope failure i.e. the earthquake ground motion is in many cases poorly described. For analytical purposes this can be described as a seismic coefficient a peak horizontal ground acceleration (PGA) or an Arias Intensity (Arias, 1970). True accelerograms can also be used employing a variety of methods. From an analytical perspective these tend to be conservative, however, due to the fact that many of these estimates have been derived from free field earthquake records few incorporate topographic amplification (Paolucci& Rimoldi, 2002; Sepulveda et al. 2005). The absence of topographic amplification effects may significantly underestimate ground motions acting on slopes. The work by Ashford et al. (1997) and Ashford and Sitar (1997) provided a valuable insight into the processes involved. Further challenges to effective assessment of slope stability during earthquakes has been outlined by Murphy et al (2002).

While there are specific problems with a variety of implementations that have been outlined by Murphy et al (2002), the basic presumption that the weakest sliding block is the one that will fail during an earthquake has

never been really questioned. In this paper the stability conditions relating to slopes which subsequently suffered from earthquake induced landslides have been evaluated. A comparison is made with the geometry of the critical slip surface derived from numerical stability analysis for ambient stress states and the geometry of the landslide mass that actually slipped during the 1929 Murchison (Buller) Earthquake in South Island, New Zealand. This comparison is considered in terms of the implications for the analysis of slopes during earthquakes, and whether the current approach is justified in the majority of cases.

2 Data sets used and methodology At each site a ground model was constructed based on field observations and limited a priori information. Rock mass data were collected from using subjective assessments and rock mass classification. These data were then validated using a test site near Karamea (see Figure 1 for locations) where stability conditions were assessed from field evidence. Additionally the methodology outlined by Banks (2005) to evaluate rock mass classification from each site using geomorphological data was used to provide additional site specific data. In addition to the Karamea sub division test site a more complex FEM model of the current geometry of the Little Wanganui Slump (marked as 1 on Figure 1) to match predicted stability conditions to observed stability conditions as a further control. Table 1 shows observed material and rock mass classification data in comparison with those required to reach limit state. GSI observations were made on an unweathered section to allow a better representation of rock mass state at the depth of the landslides which may have been as much as 300m. Input values were derived from Geological Strength Index (Hoek and Brown, 1997).

Table 1. Observed and calculated rock mass properties. Intact strength properties were estimated from field indices (material could be broken easily by hammer) and the Mohr-Coulomb parameters are calculated in RocLab. *The calculated GSI is derived from the method outlined by Banks (2005). The modelled parameters are those that are required for limiting stability at the Little Wanganui Slump site at a range of depths from 20 to 100m selected on the basis of the likely depths of the slip surface. UCS observations are constrained based on Read & Millar (1990).

Parameter Observed Calculated Modelled

Intact strength (MPa) 1-5 5

GSI 80 82* 85

Cohesion (kPa) 181 432

Friction ( o) 28 32

Anisotropy cohesion (kPa) 0 0

Anisotropy Friction ( o) 15 15

Mass unit weight (kN/m3) 26

Figure 1. Landslides triggered by the 1929 MS=7.8 Murchison Earthquake (from after Hancox et al. 2002).

2.1 Little Wanganui and Whitecliffs landslides The Little Wanganui Slump (LW) is a large rock slump (Cruden and Varnes, 1996) located approximately 41 km NNW of the epicentre of the 1929 Earthquake (Table 2). However it is c. 15-20 km to the west of the likely projection of the White Creek Fault. The freefield acceleration was therefore likely to have been higher than that suggested by attenuation models that use the epicentre as a point source.

The geological model for this site was of an almost uniform succession of light grey, fine to medium grained, weak, siltstones and marls that weather to a yellow, fine grained, extremely weak surface material. Field index tests of these materials at the back of the Little Wanganui slump indicates Unconfined Compression Strengths of the order of 0.5-5MPa (crumbles under firm hammer blow and can be peeled with a pocket knife). There is little evidence on this landslide of ongoing deep seated landsliding. Using the method outlined by Banks, (2005) the slope angles of the scarp indicate RMR values of the order of 65-75. Field observations of GSI (Hoek and Brown, 1997) tend to suggest value of the order of 75-80. The major, persistent discontinuity present in the mass was bedding plane fractures. These dipped at c. 10-15o to the NE. The significance of these bedding planes in the formation of the Glasseye Landslide is a dip slope failure at the back of the little Wanganui Slump and is noted by the clear bedding plane control seen on aerial photography. That bedding is better exposed in the Whitecliff landslide to the south.

Goat Slide

Whitecliffs SlideLittle Wanganui Slump

Table 2. Landslides used in this study from Hancox et al 2002. (*classification according to Cruden and Varnes, 1996).

Both landslides outlined in this section are coastal failures where there is evidence of active coastal erosion and ongoing instability.

The geological model at this site had to allow four identifiable conditions (see Figure 2):

1. The presence of single and multiple rock block falls, and shallow slides on the coastal slopes;

2. The presence of shallow debris slides from weathered material forming on the main scarp of the landslide and evidence of instability on occasional exposed blocks showing small displacements;

3. The presence of water ponding at the foot of the main scarp indicating a high water table at this point;

4. The presence of a thick and undisturbed vegetation blanket at the foot of the main scarp and two secondary scarps within the landslide system indicating the lack of movement at these points.

Using Geological Strength Indices as a starting point for estimating apparent values of cohesion and friction a series of back analysis were run to establish a set of strength conditions that allowed for the modelled conditions shown in Table 1 to replicate the stability conditions outlined above using the finite element code Phase2 v7 from Rocscience to carry out the analysis. The initial estimate of GSI as observed in the field resulted in stability conditions that met conditions 1 and 2 above, but not condition 4 as movement was indicated on the main slip surface. Input parameters were systematically increased within the range of uncertainty of the field observations until all stability criteria were satisfied. Estimates of mi were derived from using the ratios of Unconfined Compressive Strength to Tensile strengths outlined in Read and Miller (1990) as this has been found to yield a good approximation (Read pers comm.) and a value of 4 was used. Figure 3 shows the outputs of the finite element model for the current stability conditions representing limit state conditions. Uncertainty associated with elastic properties (=0.25, E=10GPa) and the associated flow rule means that the magnitude of the predicted displacements must be treated with some caution.

Name Size (x 106m3)

Epicentral distance (km)

Geology Type* Slope angle (Dip/Dip Direction)

Little Wanganui Slump

210 41 km NNW Tertiary calcareous mudstones/siltstones with limestone interbeds. The materials are generally weak and weather to a buff yellow, medium grained extremely weak soil,

Rock slump 20 o/315o

Whitecliffs

Slump 120 39 km NNW Rock slump 50 o/315o

Goat

3.8 26 km N Rock slide /

avalanche 30o/135o

Figure 2. a) top left - Looking down the LWS; b) top right - the body of the LWS the break in slope in the

centre of the image is the location of a secondary slip surface; and c) bottom - The main scarp of the LWS showing water at an elevation of c. 200m above sea level.

Figure 3. Phase 2 v7.0 Finite Element Model of the Little Wanganui Slump. Sliding can be seen on the

seaward sections of the slope (c. 150-200m horizontal distance) and small displacements can be seen on the main scarp. No movement is indicated at the daylighting slip planes at c. 400m, 820m and 1090m horizontal distance. This corresponds with field observations.

2b

Second slideblock

2c

2a

main scarp

main slide block

second slideblock

landslide toe

ponding indicatingbacktilted block

third slideblock

scarpscarp scarp

100

200

300

400

00

200 400 600 800 1000 1200 1400 1600 1800

Horizontal Distance (m)

Elev

atio

n (m

)

Little Wanganui Slump - current conditionsMaximum displacement 1.3mat eroded toe on the coast Main scarp (see fig 2c)

Groundwater observedat land surface

Secondary scarp (see fig 2b)

Secondary scarp (see fig 2b)

Toe (fig 2a)

?

??? postulated slip

surfaces

While Figure 3 relates to testing the model parameters against the stability conditions for the current slope surface, to look at the evolution of the slumps it is necessary to reconstruct the slope profile prior to failure. In the absence of pre-1929 topographic maps of this area, the pre-earthquake long profile was constructed by two complimentary methods. The first was by simply extending the current contours across the landslide system, connecting the 20m contour intervals at each side of the landslide system, partly informed by the slope geometry in other sections of the coastal land system. The second is by rotating the block back up the slip surface. This process was carried out for both the Little Wanganui and Whitecliffs landslides. The latter approach was not possible at the Goat landslide as the whole slide mass had effectively vacated the scar. In this case the contours were extended around the slide mass. Sensitivity analysis was used to investigate the impact of errors in long slope profile associated with these approaches, and it was found that the variation in modelled slip surface were significantly less than the errors in rock mass properties and groundwater. Groundwater was considered to be below the slip plane in the Goat Landslide.

Figure 4. The Whitecliffs Slump looking south along the coast (4a) with the rotated block to the right hand

side of the photograph. Erosion of the main scarp can be seen as shallow earthflows on 4b. The bedding can be seen tilting backward into the main scarp at the top 4b.

2.2 The Goat Landslide The Goat Landslide (see Figure 1) is a predominantly planar failure surface and should in theory at least be the easiest landslide to model. Figures 5a-f show the landslide. Available evidence suggests that the rock mass is somewhat blockier than that seen at LW and the WC landslides. However, bedding plane fracture traces seen in the rock face suggest bedding that dips into the slope. Initial assessment indicated that dips would be of the order of 25o and greater, however it is recognized in the absence of good 3-D control that such estimates of orientation could be considerably steeper. The bedrock geology was a marly limestone similar to that seen elsewhere in the area. Sensitivity analyses were carried out to look at the impact of using different shear strength parameters for this landslide and these did not improve the fit between the actual and predicted landslide geometry.

There was however clear visible differences between the rock mass observed in the flanks of the landslide and the basal shear plane evident by the vacated scar and there is some indication that bedding at the top of the slope is much more steeply inclined than that seen towards the base of hill. There are two additional observations that are worth making about the rock mass at this site. Firstly, it is evident that the top 20m or so of the rock mass is disturbed. Although disturbance here is natural in origin, a D value of c. 0.5 (Hoek et al 2002) could easily be ascribed to the rock mass (Figures 5c, 5e) based on visual inspection and a comparison D values for engineered rock masses. This degree of disturbance can be seen in the flanks of the vacated landslide, while the slip surface exposed in the scar shows significantly less disturbance and the rock mass is more compact (Figure 5d). The implications of this observation will be discussed later.

4b4a

Backtilted block

Main backtilted block

Main scarp

Partial obscuredsecondary slipplane

Regionalbedding dip

~15o

~30o

Backtilted bedding dip

Main scarp

3 Back analysis of earthquake induced landslides

3.1 Analyses of the landslides The aim of the analysis of each of these landslides was to identify whether two common methods of slope stability analysis yielded a reasonable prediction of the sections of the slopes that failed during this earthquake.

5a 5b

5c 5d

Slip surface

Slip surface

~25o

~25o

Regionalbedding dip

Dis

turb

ed ro

ck

mas

s D

~ 0.

5

Undisturbed rockmass below slip plane

10m

10m10m

5e 5f

Figure 5. The Goat Landslide c. 26km from the epicentre of the 1929 Murchison Earthquake. The overview of the landslide can be seen in 5a and the debris that remains on the largely vacated slide scar can be seen in 5b-5f. The left flank of the landslide showing the disturbed rock mass can be seen in 5e while the less disturbed rock mass forming the slip plane is evident in 5f.

These methods are Janbus (1954) Method and Bishops (1955) Method. The analyses were carried out using the Slide program from Rocscience. Field observations, limited strength data and the technical literature were used to obtain input parameters for the analysis. Two broad groups of analyses were used: the first used a Mohr-Coulomb strength model with c and derived from rock mass classification; the second used an anisotropic strength model with a c and being the same as that used in the Mohr-Coulomb model, but with a strength anisotropy dipping parallel with the bedding plane fractures. These were assigned strength values of c=0 and =15o. This value was selected on the basis of angles of repose bedding plane controlled landslides in the area.

3.2 Little Wanganui and Whitecliffs Slumps The results of these analyses in the absence of seismic accelerations can be conveniently described together. Figures 6a and 6b shows the results of the stability analyses for the Little Wanganui and Whitecliffs Slumps. Both of these analyses use a reconstructed slope angle to consider the most critical slip surface for sliding analyses were run using both non-circular and circular slip surface options to investigate the goodness of fit. In both cases an anisotropic strength model was used allowing for a set of weaknesses orientated dipping into the slope. This anisotropy is consistent with field evidence and explains the stability conditions seen at the Glasseye Landslide some 500m to the East of the Little Wanganui Slump.

In the case of the Little Wanganui Slump, the critical slip surface yields a value of 1.04 using Bishops method of slices. This slip surface would produce a main scarp at about 550m along the cross section. This marginal Factor of Safety (F) is significant, since it indicates that even relatively low ground accelerations would result in failure of this lower block and subsequent unloading of the upper sections of the slope. However, whilst the geomorphology supports progressive failure of this slope during the earthquake, the location of the scarps is not consistent with the critical slide surface indicated by the analysis. In fact the slip surfaces closest to those observed from field data all have significantly higher values for F ranging from 1.67 to 1.93. Modification of the slope long profile and re-analysing the stability conditions allowing for the removal or partial removal of the block bounded by the critical slip surface shown in green in 6a, yields values of F ranging from 1.1-1.4 depending on the degree of removal of the sliding block. The most seaward section of this slump has not completely vacated the scar and so modelling such conditions is not supported by field evidence. A non-circular slip model yields a slightly better estimate of current slip surface geometries than a circular one for the Little Wanganui Slump. This is not the case with the Whitecliffs landslide where a circular slip model yields better results.

The conditions at Whitecliffs are somewhat simpler due to having only one or possibly two slip surfaces. A consideration of the observations highlighted in 4a demonstrates that there is a large backtilted block but there may be a small secondary slide, indicated by a variation in bedding plane dips (Figure 4b) as a result of unloading by the main slump. The analysis shown in Figure 6b again shows a critical slip surface at the over-steepened sea cliff. The factor of safety (1.22) indicates stability for ambient stress conditions. Analysis of the closest approximation to the actual slip surface assuming one large and a smaller secondary block indicates values of F of the order of 2.39 and 2.49. The larger slip surface encompassing the whole slide mass does not fit well with the available field evidence.

3.3 The Goat Landslide The Goat Landslide provides a somewhat different state of conditions. The slopes are considerably higher and there is no marine erosion to complicate stability conditions. Additionally, given the nature of the slope it is likely that groundwater is likely to be well below the landslide shear plane as seen in Figure 5. Observations made in the exposed rock faces indicate bedding dipping into the slope, there does appear to be differences in the discontinuity pattern at the bottom and the top of the slope. Therefore these were modelled as an anisotropic mass, with different anisotropy orientations (Figure 6c) therefore were treated as two different materials. The overview of the landslide seen in Figure 5a indicates a slip surface c. 20m deep. The variations in fracture pattern seen in Figure 5e and 5f are evident. Additionally there is a significant degree of rock mass disturbance

evident in 5e. In this example two rock masses units were considered reflecting different bedding fracture orientations within the material.

Analysis indicates that the critical slip surface (F=1.25) is broadly consistent in shape (i.e. significantly more planar) with the landslide scar, but is far too deep (c. 65m). Shallower slip surfaces show high Factors of Safety, with slip surfaces of consistent depth to that observed to have been triggered by the 1929 earthquake to show F>2.

Figure 6. Stability analyses of the Little Wanganui Slump (6a); The Whitecliffs Slump (6b) and the Goat

Landslide (6c). Stability analyses show high factors of safety with the exception of a potential slide block on the Little Wanganui Slump. The somewhat blocky nature of the slip surfaces highlighted in black in 6a relate to mudstone forming the anisotropy within the rock mass.

4 Discussion What is apparent from the analyses of these three landslide systems is that standard limit-equilibrium methods do not replicate the seismic slope stability conditions well. There are a number of possible reasons for this that is worth consideration.

1.98

1.73

1.25

550

620

690

760

830

900

970

10 80 150 220 290 360 430 500 570 640 710 780

Ele

vatio

n (m

)

Horizontal distance (m)

1.00

Fact

or o

f Saf

ety 1.20

1.40

1.60

1.80

2.00+

1.71

Ele

vatio

n (m

)

Horizontal distance (m)

1.22

2.392.402.49

80

160

240

320

400

70 210 350 490 630 770 910

Fact

or o

f Saf

ety

1.00

2.00

3.00

4.005.00

6.00+

closest slip circle

approximation

Ele

vatio

n (m

)

Horizontal distance (m)

1.674

1.929

1.76

1.93

1.04

150

0

300

450

Fact

or o

f Saf

ety

1.00

1.20

1.40

1.60

1.80

2.0+

0 150 300 450 600 750 900 1050

1.67

6a

6c

6b

Firstly, there is the very reasonable consideration that the assumptions that have had to be employed in these analyses are wrong. In order to consider the impacts of these, sensitivity analyses was carried out on the output of each model. The main and most obvious potential error is that groundwater has been incorrectly located. Groundwater level was selected to mirror the land surface that was reconstructed from topographic data and knowledge of the geometry of sliding blocks (in two of the three cases). In none of these landslides was there evidence of surface seepage emerging from the slopes in the near vicinity, and this does tend to suggest that water levels are normally low. Sensitivity analysis indicates that an error in the groundwater level would result in a change in Factor of Safety for the critical and actual slip surfaces, but did not significantly change the geometry or location of the critical slip surface. There is no evidence to suggest that there was an ongoing stability problem prior to the 1929 Murchison Earthquake in these slopes. It is however recognised that the area was sparsely populated.

Secondly, there is the likelihood that errors in rock mass classification and parameter selection has resulted in significant miscalculation. This is in some respects more likely. Material strength was estimated from field index testing and technical literature. As such they are prone to wide uncertainties. Additionally, the rock masses evident in Figures 2, 4 and 5 cannot be said to be homogenous and isotropic. In fact a significant anisotropy was introduced onto the rock mass in the form of weak layers within the stratigraphy that was evident at the Little Wanganui and Whitecliffs Slumps. In an attempt to overcome this problem, the selected parameters to be used in the analysis of the failures were used to evaluate the current stability conditions at Little Wanganui Head. In order to replicate the stability conditions seen i.e. only localised instability on the steeper slopes, it was actually necessary to increase strength parameters, so the initial rock mass classification and parameter selection was possibly tending towards the conservative. This probably stems from the weathered nature of the materials, and the rock mass quality is likely to be better at depth. Sensitivity analysis demonstrated that large errors were required to significantly change the critical slip surface geometry (large in this case being 500-600kPa in cohesion and 4-5o in terms of friction angle). However while these changed slip surface geometry they did not get the location of the slip surface correct. In the Goat Landslide, the changes in strength actually produced a slip surface less like that observed in the field.

The third possible explanation is that even with the ground perfectly characterised, the methods outline would not provide an adequate description of stability conditions during a strong earthquake. The rationale behind the Newmark sliding block model is that the weakest component of the slope is the one that will fail and that the earthquake load merely contributes to the driving force. This is a simplification of earthquake ground motions since during an earthquake effective normal and shearing stresses will all change markedly during shaking. However, the analysis of the Goat landslide throws up an interesting observation: that is, there is a marked velocity contrast between the disturbed rock mass evident in Figure 5e and the undisturbed rock mass in Figure 5f. Calculations of shear wave velocity (Vs) based on rock mass classification indicate a significant difference in Vs. For the undisturbed rocks below the slip plane, shear waves will be transmitted at approximately 2400ms-1. However in the disturbed rock mass evident in the flanks of the landslide wave propagation speeds were likely to have been of the order of 1500-1600ms-1. In practical terms the energy moving into the low velocity layer will be unable to exit at the same speed so the particle amplitude will increase. That is, there will be higher ground accelerations in the low velocity layer.

The practical implications of this are potentially significant. The first is that the method outlined by Newmark (1965) while being valid for homogenous and isotropic materials does not translate well to natural heterogeneous and anisotropic slopes. It is therefore, potentially inappropriate to use such methods without careful consideration of the ground conditions. If either a Newmark Sliding block or a pseudostatic method of analysis is being applied to a slope there are some reasonable questions that may guide further investigation:

1. Does a low velocity layer exist in the stratigraphy? Such low velocity layers may include, but not be limited to weathered zones; disturbance from engineering, mining or quarrying; unloading or stratigraphic variations.

2. If such a zone is a discrete layer in the geological succession, is it orientated in a manner which will impact on the stability of the slope? Should it be treated in the same way that discontinuities are treated?

3. If such a low velocity layer outcrops at the surface of the slope, will the weakest materials merely be shaken off the top, and the depth of the landslide be limited by the availability of low velocity rock mass that can be displaced, rather than the shear strength of the material within the slope? I.e. does the landslide deepen progressively during the earthquake, not form as a discrete slip plane that the overburden slides on?

Figure 7 shows two rockfalls triggered by the 1999 Chi Chi Earthquake in Central Taiwan. Both failures occurred in a weathered rock mass where loose material, that is material with low shear wave velocities, was merely shaken off the rock mass. While neither of these landslides constitutes a shear failure, there is no rational reason why poorly consolidated materials at lower slope angles should not show deformation when subjected to forced vibration.

Figure 7. Rockfalls triggered by the M=7.9 Chi Chi earthquake in 1999. Are the low velocity layers

associated with weathering and unloading responsible for the large number of shallow earthquake induced landslides?

The observations presented here must be regarded as preliminary. There are large uncertainties in almost every stage of the process and more rigour is required. However, the investigation does tend to suggest that the presumption that the critical slip circle i.e. the weakest component of a given slope is the block that will slide does not appear to be supported by field evidence. This is the subject of research in progress.

5 Summary and implications This work currently outlines the some observations that have arisen from research in progress. The analysis of three landslides, albeit it with observations from the authors of many others, does not prove a hypothesis and these comments have to be regarded as preliminary. Having made that statement, it is felt that these are valid observations and not merely coincidental. These observations can be summarised as follows:

1. Slope stability analysis does not accurately predict the geometry of landslides triggered by earthquakes. Where the geometry of such landslides are known back analysis can be used to derive some information about forcing functions active during the earthquake or the potential seismic origin of a landslide

2. Is ambient stress state a valid predictor of seismic slope stability? Should there be some other parameter be used to guide further analysis of the stability of slopes during earthquakes (such as shear wave velocity)?

3. Is the simplest way to mitigate earthquake-induced landslides in rock masses to try to maintain an almost uniform velocity profile throughout the slope? Does the uniform transmission of seismic waves through the ground only present a problem when there are differences in the velocity profile? In the absence of a low velocity layer such as a systematic discontinuity zone; a weak layer within the stratigraphy or even a landslide shear zone to trap/scatter seismic waves (see Rial, 1996) wave propagation may occur through the rock mass without significant difficulties.

6 Acknowledgements GNS Science (New Zealand) is gratefully acknowledged for logistical and financial support during fieldwork. Rob Parker is funded by the Willis Re Research Network and their support is gratefully acknowledged.

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