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Seismic reflection images of active faults on New Zealand's South Island

A.G. Green1, F.M. Campbell

1, A.E. Kaiser

1, C. Dorn

1, S. Carpentier

1, J.A. Doetsch

1,

H. Horstmeyer1, D. Nobes

2, J. Campbell

2, M. Finnemore

2, R. Jongens

3, F. Ghisetti

4, A.R.

Gorman4, R.M. Langridge

5, A.F. McClymont

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1. Institute of Geophysics, ETH, Zürich, CH-8092, Switzerland

2. Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand

3. Dunedin Research Centre, GNS Science, Private Bag 1930, Dunedin, 9054, New Zealand

4. Department of Geology, University of Otago, Dunedin, 9054, New Zealand

5.GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand

6. Department of Geoscience, University of Calgary, Calgary, T2N1N4, Canada

Abstract

New Zealand is located along the boundary between the Australian and Pacific plates.

Although there are numerous faults associated with this plate boundary setting, few have ruptured

during the nearly 200 years of European settlement. Yet, paleoseismology provides clear

evidence of relatively recent activity on many of them. Knowledge of the shallow structure and

other characteristics of these faults is important for understanding the related seismic hazard and

risk. Key properties of faults that produce infrequent large earthquakes are usually determined or

inferred from paleoseismological investigations of surface outcrops, geomorphology, trenches,

and boreholes. In an attempt to improve our knowledge and understanding of active faults

beyond the reach of conventional paleoseismological methods (i.e., deeper than a few meters), we

have acquired high-resolution seismic reflection and ground-penetrating radar (GPR) data across

the following three fault systems on New Zealand's South Island: (i) a northern section of the

transpressive Alpine Fault zone, (ii) numerous reverse faults hidden beneath the very young

sediments that cover the northwest Canterbury Plains, and (iii) a critical portion of the reverse

Ostler Fault zone in the south-central part of the Island. After subjecting our data to diverse

processing procedures, the resultant seismic and GPR sections provide vivid images of the target

structures. On the 2D and 3D high-resolution seismic and GPR images of the Alpine Fault zone,

we see the principal fault dipping steeply through Quaternary sediments and offsetting the

basement. A distinct ~25 m vertical offset of basement provides a maximum ~1.4 mm/yr dip-slip

displacement rate. The more important strike-slip component of displacement has yet to be

estimated at this location. Our high-resolution seismic and GPR sections across parts of the

northwest Canterbury Plains display a complex pattern of faults and folds beneath a variably

thick veneer of flat-lying sediments. Structural restorations of the seismic images suggest

10 - 23% compressive strain, which would correspond to an average strain rate of

20 - 50 x 10-9

/yr if the onset of compression coincided with the accelerated uplift of the Southern

Alps approximately 5 Ma. Finally, multiple 2D high-resolution seismic images of the Ostler Fault

zone reveal a 45° - 55° west-dipping principal fault and two subsidiary 25 - 30° west-dipping

faults, one in the hanging wall and one in the footwall of the principal fault. Again, we are able to

structurally restore models based on the seismic images. These restorations are compatible with

440 - 800 m of vertical offset and 870 - 1080 m of horizontal shortening across the Ostler Fault

zone, which translate to a relatively constant deformation rate of 0.3 - 1.1 mm/yr since the Late

Pliocene - Pleistocene.

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Keywords: 2D and 3D high-resolution seismic reflection and GPR images, Australian - Pacific

plate boundary, Alpine Fault zone, faults underlying the northwest Canterbury Plains, Ostler

Fault zone.

Introduction

New Zealand's South Island is a dynamic landmass distinguished by two active subduction

zones and an intervening transform plate boundary. The west-directed Hikurangi subduction zone

in the north is connected to the east-directed Puysegur subduction zone in the south via the

Alpine Fault zone and a broad expanse of oblique compression (Figure 1). Paleoseismological

studies suggest the Alpine Fault is accommodating two-thirds to three-quarters of the

35 - 38 mm/y of relative motion between the Australian and Pacific plates in the central and

southern parts of the South Island, with the remainder being distributed over a 150 - 200-km-

wide region that encompasses the Southern Alps and adjacent regions to the east (Figure 1;

Norris et al., 1990; Pettinga et al., 2001; Sutherland et al., 2006; Cox and Sutherland, 2007;

Norris and Cooper, 2007).

The Alpine Fault became a significant plate boundary 22 - 23 Ma (Cooper et al., 1987; King,

2000; Cox and Sutherland, 2007), about the same time the Hikurangi subduction zone started to

consume the Pacific Plate (Stern et al., 2006). Since then, the zone of deformation has gradually

broadened, with accelerated levels of faulting, folding, tilting, and uplift in the Southern Alps

commencing ~5.0 Ma (Sutherland, 1995; Coates, 2002). The zone of deformation extended

further east into the region of the Ostler Fault zone prior to the Late Pliocene and into the

northwest Canterbury Plains either prior to or during the Quaternary (Forsyth et al., 2008). There

are areas of active and Quaternary deformation close to the South Island's two largest cities,

Christchurch and Dunedin (Figure 1) with populations of ~370,000 and ~124,000, respectively.

The identification and characterization of active faults are critical to studies of regional

seismic hazard (McCalpin, 1996; Yeats et al., 1996). Probabilistic seismic hazard analyses

require seismic source models, which are usually constructed by assigning earthquakes to active

faults based on seismological, other geophysical, and geological information. Once all seismic

sources are defined, their characteristic earthquake magnitudes and recurrence frequencies are

determined (Thenhaus and Campbell, 2003). Historical and instrumental records of seismicity do

not completely characterize the earthquake cycle of many active fault zones, because the records

are generally much shorter than the repeat times of the largest earthquakes. This is a particular

problem in New Zealand, where relatively complete records of historical and instrumental

seismicity exist for only the past ~200 and ~50 years, respectively, whereas large active faults on

the islands are estimated to have recurrence intervals of many hundreds of years (Stirling et al,

1998). Consequently, paleoseismic studies are important for characterizing the numerous active

faults that extend along the length of the country (e.g., Berryman, 1980; Van Dissen and

Berryman, 1996; Benson et al., 2001; Villamor and Berryman, 2001; Sutherland et al., 2007).

Conventional paleoseismological techniques based on surface outcrops, geomorphology,

trenches, and boreholes (McCalpin, 1996; Yeats et al., 1996) usually supply details on major

faults that have disrupted the upper few meters of the ground. To provide structural and other

information at greater depths, we have acquired high-resolution seismic reflection and ground-

penetrating radar (GPR) data across two active fault zones and an active fault network on the

South Island. Our first target (1 in Figure 1) is the Alpine Fault zone at a rare accessible site

where a ~2-m-high linear fault scarp cuts across a sequence of abandoned river terraces. Apart

from a nearby campground, several houses, and a small village, this location in the north of the

South Island is relatively isolated. A network of faults hidden beneath variably thick Quaternary

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sediments that characterize the remarkably flat northwest Canterbury Plains is our second target

(2 in Figure 1). A major buried or blind fault beneath the southernmost part of this target area is

less than 40 km from Christchurch. The third target (3 in Figure 1) is a region of the Ostler Fault

zone where thick sediments of the Mackenzie Basin are offset. This location in the south-central

part of the Island is quite close to eight hydro-electric power dams that supply a large proportion

of the South Island's electricity.

After briefly describing the geology at the study sites, we show examples of the high-

resolution seismic reflection images at all three locations.

Figure 1. Plate tectonic setting of New Zealand's South Island showing the west-directed Hikurangi subduction zone in the north, the east-directed Puysegur subduction zone in the south, and the Alpine Fault. Also shown are our study sites: 1 - northern section of the Alpine Fault zone, 2 - northwest Canterbury Plains, 3 - Ostler Fault zone.

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Geology in the vicinity of the three study sites

Alpine Fault zone

The Alpine Fault is the principal element of the onshore boundary between the Pacific and

Australian plates (Figure 1). Offset of basement terrains by ~470 km along the fault since the late

Oligocene to early Miocene is evidence for the importance of this structure in accommodating

relative plate motion. At our Alpine Fault study site, a sequence of faulted abandoned river

terraces is derived from glaciofluvial valley sediments dating from periods of aggradation that

followed phases of advance and retreat of a late Pleistocene glacier. Basement southeast of the

fault is schist derived from Triassic-age sediments, whereas that to the northwest is Paleozoic-age

marble. Paleoseismic trenches on the youngest river terrace intercept a narrow zone of variably

dipping (40o - 90

o) faults within the upper 1 - 2 m of gravels (Yetton, 2002). Such gravels are

either exposed at the surface or covered by a thin layer of undisturbed topsoil.

GPR surveys at the site reveal a narrow fault zone dipping steeply to the southeast at ~80o to

a depth of ~15 m (McClymont et al., 2008a, 2010). Notable changes in GPR reflection geometry

are observed across the main strand of the fault. Additional changes in reflection characteristic

~30 m on either side of the main strand are interpreted as secondary left-stepping en echelon fault

strands and associated deformation. This pattern of left-stepping traces is a consistent feature of

the surface geomorphology within 1 km to the the SW of the site. The relationship between the

main fault and secondary faults below the sediment cover is unknown.

Northwest Canterbury Plains

Lithologies and structures beneath the surficial sediments of the 8000 km2 Canterbury Plains

are only poorly known. The landscape is flat and even in most areas, with little surface evidence

of underlying geological complexity. Limited lithological information is provided by four

moderately deep hydrocarbon exploration boreholes, a large number of shallow water-well holes,

and outcrops in the hills and along the river banks (McLennan, 1981; Cowan, 1992; Evans, 2000;

Estrada, 2003; May, 2004; Finnemore, 2004; Forsyth et al., 2008). Useful structural details are

supplied by these same outcrops, sparse hydrocarbon exploration seismic lines (recorded with the

intention of imaging much deeper features; Jongens et al., 1999), and a number of short seismic

lines recorded by the University of Canterbury geophysics group (Estrada, 2003; Finnemore,

2004). Collectively, these sources of information suggest that the subsurface beneath the

Canterbury Plains comprises Permian - Triassic basement rocks overlain successively by Late

Cretaceous - Tertiary interbedded sedimentary and volcanic layers and Quaternary sediments.

However, the details of most buried structures are either unknown or based on speculative

extrapolations.

Ostler Fault zone

The Ostler Fault zone is one of a number of distinct structures east of the Alpine Fault that

accounts for a significant component of strain between the two plates. It accommodates

1.1 - 1.7 mm/yr of east - west compression (Amos et al. 2007) and 0.8 - 1.0 mm/yr of vertical

deformation that is mostly taken up by buckling in the fault hangingwall (Blick et al., 1989). In

the area of the Mackenzie Basin transected by the Ostler Fault zone, a substantial late

Pliocene - Pleistocene fluviolacustrine sequence is overlain by a series of Holocene glacial and

fluvioglacial gravels and associated terraces (Ghisetti et al. 2007). The north - south striking

Ostler Fault zone, which can be traced for a distance exceeding 50 km, comprises a highly

segmented series of predominantly west-dipping surface-rupturing pure reverse faults. Fault

segments are nearly orthogonal to inferred maximum horizontal-stress directions. They occur

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over an area up to 3 km wide, with complex arrays of multiple fault segments in the central parts

of the fault zone, where basin fill is thicker.

The surface dip of the principal Ostler Fault plane appears to vary along its length. Davis et

al. (2005) measured an average westerly dip of 50 ± 9°. A GPR survey in the northern part of the

Clearburn area revealed a complex pattern of linked faulting, with multiple strands dipping to the

west at 20 - 30° (McClymont et al. 2008b). Another 3D GPR survey in the northern section of the

Ostler Fault zone found the principal fault plane to be westerly dipping at 51 ± 10° (Amos et al.,

2007; Wallace et al., 2010).

Seismic reflection data acquisition and processing

For each study site we designed a target-specific data acquisition strategy and a data-driven

processing scheme. General information on the data acquisition strategies, all of which involved a

240-channel recording system and single 30 Hz geophones, are provided in the sections below,

but we refer the readers to the relevant papers for details on the individual processing schemes

(Kaiser et al., 2009, 2010; Dorn et al., 2010a, 2010b; Campbell et al., 2010a, 2010b, 2010c).

After assigning coordinates and editing traces, we tested a full range of processing procedures on

all data sets: (i) mutes of various types, including airwave attenuation, (ii) amplitude

enhancement and trace balance, (iii) spectral balance, (iv) surface-consistent predictive

deconvolution, (v) elevation statics, (vi) refraction statics with complementary P-wave velocity

tomograms derived from first arrivals, (vii) surface-consistent residual statics, (viii) various types

of velocity analysis, (ix) normal-moveout corrections, (x) a variety of dip-moveout (DM0)

analyses and corrections, (xi) frequency filtering, (xii) frequency-wavenumber filtering, (xiii)

frequency-space filtering, (xiv) various types of stacking, (xv) time migration followed by time-

to-depth conversion, and (xvi) depth migration. The processes were not necessarily applied in the

order listed and not all processes were successful in improving image quality. For example,

application of refraction static corrections was essential for imaging the Alpine Fault, but resulted

in markedly inferior images of the northwest Canterbury Plains and Ostler Fault regions. In

contrast, DMO processing improved the images for the latter two regions, but yielded variable

quality images across the Alpine Fault. Several of the processes were applied both pre- and post-

stack and some processes were applied in an iterative fashion (e.g., surface-consistent residual

statics and velocity analyses).

Results of seismic surveying the Alpine Fault zone

Across the Alpine Fault zone, we recorded an ultra-high-resolution 2D seismic reflection data

set along a 360-m-long line and a pseudo-3D seismic reflection data set that spanned a

500 x 200 m area (Kaiser et al., 2009, 2010). Energy generated by 6 blows of a hammer at 1 m

intervals recorded on receivers located every 0.5 m provided 60-fold ultra-high-resolution data.

For the pseudo-3D survey, energy from small 30 g explosive charges detonated every 8 m was

recorded simultaneously by receivers located every 4 m along two parallel lines separated by

8 m; one of the two receiver lines was also the source line. This strategy provided >20-fold data

on a uniform 2 x 4 m CMP grid. Together, these two data sets yielded very high-resolution

details along a single profile and reliable information over a large area in a cost effective manner.

Figures 2 and 3 show a depth-converted time-migrated 2D ultra-high-resolution image and a

depth-migrated 3D high-resolution image of structures in the vicinity of the principal Alpine

Fault (AF in the figures) strand. Seismic units A1, A2, and A3 of strong semicontinuous

reflections on either side of the fault from near the surface to as deep as ~60 m are interpreted to

be late Pleistocene gravels; trenches and pits at the site reveal alluvial gravel in the uppermost

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2 m. Undifferentiated gravel was intersected from the near surface to 26 m depth in the borehole

southeast of the surface fault trace (Figure 2b; Garrick and Hatherton, 1974); note the good

correlation between the thickness of gravels observed in the seismic images and the borehole

information. Along the southeastern half of the two data sets, reflections from seismic unit A3

appear to grade into gently dipping continuous reflections B0 - B2 at greater depth. We interpret

seismic units B0 - B2 as originating from layered glaciolacustrine sediments.

The sediment-basement interface produces strong reflections C1 and C2 that are offset across

the Alpine Fault AF and at other locations along the profile. From mapped surface outcrops, we

infer that marble basement C1 to the northwest is juxtaposed against schist basement C2 to the

southeast. In Figures 2 and 3, much of the reverse slip along the Alpine Fault at Calf Paddock

appears to be confined to a single major fault strand. More minor faults in Figure 2 are identified

by dashed lines, with the most significant labeled I and II. Minor fault II projects to one of the

subsidiary faults(SF in Figure 2) mapped in GPR data (McClymont et al., 2010).

Figure 2. (a) Depth-converted migrated section derived from the ultra-high-resolution seismic data set recorded across the Alpine Fault (AF). (b) Interpretation of depth-converted migrated section. The various units are described in the text. Information from a nearby ~80 m deep borehole is projected on to the line at about 143 m distance.

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Based on reflection truncations, the principal strand of the Alpine Fault has an apparent

southeasterly dip of 75°–80° from its surface scarp through the Quaternary sediments to the offset

basement at ~60 m depth. The dip of the Alpine Fault within the shallow basement is not so well

constrained. As a consequence, we show a range of plausible depth trajectories for the Alpine

Fault in Figure 2, with possible dip estimates in the basement varying between 50° and 80°.

Because reflections B0 - B2 from the former flat-lying glaciolacustrine units are only gently

tilted, our preferred interpretation is that the Alpine Fault continues to dip relatively steeply

within the shallow basement (Figure 3). The average ~25 m apparent vertical offset of basement

across the principal Alpine Fault strand together with the estimated maximum age of the eroded

basement surface and the measured fault dip within the Quaternary sediments yield a maximum

dip-slip strain rate of ~1.4 mm/yr (estimated range of 0.9 – 2.2 mm/yr). The much larger strike-

slip displacement rate has yet to be determined for this region of the Alpine Fault zone.

Figure 3. (a) Depth-migrated volume of the pseudo-3D data set recorded across the Alpine Fault. (b) As for (a), but highlighting the Alpine Fault strand AF and geological units A1-C2 discussed in the text. (c) 3D representation of the horizons picked in b). Dashed lines indicate where horizons onlap basement.

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Results of seismic surveying across the northwest Canterbury Plains

We recorded high-resolution seismic reflection data along 4 lines within the northwest

Canterbury Plains (S1- S4 in Figure 4a - 4e; Dorn et al., 2010a, 2010b; Campbell et al., 2010b)

using ~60 g explosive charges in 1-m-deep shotholes every 10 m. By recording the resultant

energy on receivers every 5 m, we acquired 60-fold data at an interval of 2.5 m. Extensive rock

exposures to the north and west of our study area and isolated outcrops to the east (Figure 4e)

provide important constraints on the interpretation of the various seismic reflection patterns. The

most important geological units for this study are: (i) unsaturated, largely unconsolidated young

Quaternary gravel and sand, (ii) saturated, consolidated older Quaternary gravel and sand, (iii)

interbedded Late Cretaceous - Tertiary sediments and volcanics, and (iv) Permian - Triassic

Torlesse basement rocks. Additional critical information is supplied by refraction velocity

tomograms derived from inversions of the first-arrival traveltimes recorded on the raw seismic

data. Velocities in these tomograms vary from less than 1200 m/s in the shallow subsurface

through to 3000 m/s and greater than 3500 m/s. By combining this information, we discover that

the uppermost quasi-continuous subhorizontal reflections with velocities less than 1200 m/s

originate from young Quaternary gravel and sand. The next deeper unit with reflections

distinguished by distinct offsets and folding and velocities up to 3000 m/s is likely to be the older

Quaternary gravel and sand, whereas the underlying unit with discontinuous to laterally

continuous reflectivity and velocities up to 3000 m/s is very probably interbedded Late

Cretaceous - Tertiary sediments and volcanics. Finally, reflection-free regions with velocities

greater than 3500 m/s represent tightly folded and steeply dipping basement rocks.

The interpreted seismic reflection images in Figure 4a - 4d are based on an integrated

analysis of all available information. Abrupt truncations of prominent layered reflections from the

Late Cretaceous - Tertiary and older Quaternary sequences and juxtaposition of reflections from

these units with contrasting dips delineate a number of important faults. Moderate southeasterly

dips and distinct curvature of the layered reflections define regions of tilting and folding. The

vast majority of interpreted faults are distinguished by an anticlinal fold in the hanging wall and a

synclinal fold in the footwall, both features being indicative of normal drag during reverse

faulting (Grasemann et al., 2005). Of the 8 identified primary faults, 5 are plausible or probable

extensions of exposed faults in the neighboring hills (i.e., FA1, FA2, FA2a, FA3a, and FA6). In

particular, FA6 at the southern end of seismic section S2 (Figure 4c) may be a major basin

bounding fault. A major fault-propagated fold (FO) recognized on the same seismic section can be

confidently traced to an exposed anticline. The seismic sections also provide images of numerous

secondary faults (thin dashed lines).

Restorations to the top of the basement on the four longest seismic lines provide evidence for

10 - 23% compressive strain (Campbell et al., 2010b). This corresponds to an average strain rate

of (i) 4.5 - 11.0 x 10-9

/yr if deformation commenced when movement along the Alpine Fault was

initiated 23 Ma or (ii) 20 - 50 x 10-9

/yr if compression began when uplift of the Southern Alps

accelerated ~5 Ma. The strain appears to be spatially concentrated in the northern area of our

study site.

The geometry of the Quaternary units suggests that deformation is ongoing in this region. A

small discrete offset of reflections from very recent sediments (not shown here, Campbell et al.,

2010b), weak curvature of reflections from the youngest Quaternary sediments, and minor

undulations in the surface topography (May, 2004, Forsyth et al., 2008) are evidence for blind or

buried active faults beneath some of the seismic lines.

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Figure 4. High-resolution seismic reflection data recorded across the northwest Canterbury Plains. Sketch of interpreted geology superimposed on time-migrated seismic sections for (a) S4, (b) S1, (c) S2 and (d) S3. Solid and dashed lines indicate the level of confidence in the interpreted features. The very approximate depth scale on the right is based on a single velocity of 2300 m/s. (e) Location of the S1 - S4 seismic lines in the northwest Canterbury Plains. Regions of rock outcrop are shaded.

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Results of seismic surveying across the Ostler Fault zone

Our high-resolution seismic reflection data in the region of the Ostler Fault zone spanned a

transfer zone from one principal fault strand to another. Six pairs of subparallel 1.2-km-long

seismic reflection lines were recorded orthogonal to the fault traces and a single 1.6-km-long

crossline was collected in the hanging wall parallel to fault strike (see top right diagram of Figure

5). Since time and financial constraints prevented the acquisition of a sufficiently high-resolution

3D data set across the 1.2 x 1.6 km survey area, the seismic survey was designed to supply high-

resolution cross-sections through the predominantly 2D structures, while providing constraints on

the along-strike character of these structures at length scales of both ~50 m (between lines in each

pair) and 200 - 300 m (between adjacent pairs of lines). The pairs of lines gave us confidence that

the vast majority of reflected energy seen on the seismic images originated from within the

vertical planes defined by the respective recording lines. The acquisition parameters were very

similar to those described for the northwest Canterbury Plains data, except the source, receiver

and fold were 6, 3, and 1.5 m, respectively, and the shotholes were generally shallower at ~0.6 m.

The upper two diagrams in the left of Figure 5 show depth-converted migrated sections from

inline 3 and the crossline (Campbell et al., 2010a, 2010c). Nearby rock outcrops allow us to

identify the most prominent reflections. The shallowest subhorizontal events P - P' and R - R'

originate from fluvioglacial gravel beds, whereas the high-amplitude dipping reflections Q - Q'

and deeper subhorizontal reflections U - U' originate from layered Late Pliocene - Pleistocene

fluviolacustrine units (Ghisetti et al., 2007). We interpret T - T' as a rare package of reflections

from a fault. Correlation of reflections, reflection packages, and reflection truncations on all 13

seismic sections suitably constrained by the outcrop data leads to the interpretation presented in

the lower two diagrams of Figure 5. Major unknowns in our interpretation are the types of rock

underlying the deepest imaged reflections U - U'. Nevertheless, it is possible to interpret the

seismic images in terms of a principal 45 - 55° west-dipping Ostler Fault plane running the entire

length of the study site and two subsidiary 25 - 30° west-dipping faults, one in the hanging wall

and one in the footwall of the principal fault. The principal fault and its shallow splays offset the

Late Pliocene to Quaternary units. Deformation style varies along the length of the site, with

folding playing an increasingly important role towards the north.

Structural restorations of inline sections (Campbell et al., 2010a) demonstrate that 440 - 800

m of vertical strain and 870 - 1080 m of shortening has occurred since deposition of the earliest

Late Pliocene - Pleistocene fluviolacustrine unit (L1 in Figure 5). Furthermore, they suggest that

there has been 70 - 180 m of shortening since the start of Quaternary fluvioglacial deposition.

These estimates together with poorly constrained timing information translate to a relatively

constant deformation rate of ~0.3 - 1.1 mm/yr from the Late Pliocene - Pleistocene to the present

day.

Conclusions

Ultra-high-resolution and high-resolution seismic reflection sections and 3D GPR data (not

presented here) have revealed important details on the structure of two major active fault zones

and a network of potentially active faults on New Zealand's South Island. We have imaged the

transpressive plate-boundary Alpine Fault in 2D and 3D and reverse faults of the Ostler Fault

zone and the region underlying the northwest Canterbury Plains.

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Figure 5 High-resolution seismic reflection data recorded across the Ostler Fault. Top right diagram shows the field layout. Six pairs of parallel lines were recorded perpendicular to the fault strike (indicated by the lineations in the topography) and one was recorded perpendicular (i.e., the crossline). Depth-converted migrated section 3 and the crossline (both marked by heavier lines in the top right diagram) are shown in the top left and center diagrams. Interpreted versions of these sections are shown below.

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Acknowledgments

We thank all members of the field crews who worked so hard on our surveys. This project

was funded by the Swiss National Science Foundation and ETH Zurich.

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