Andrew Nicol

The shapes, major axis orientations and displacement patterns of fault surfaces

Abstract Displacement contour diagrams constructed using seismic reflection data and coal-mine plans are analysed to establish the factors determining the dimensions, shapes and displacement patterns of normal faults. For blind isolated normal faults in layered sequences the average aspect ratio is 2.15, with sub-horizontal major axes. Earthquake slip-surface aspect ratios range from 0.5 to 3.5 and are independent of slip orientation. The principal control on the shape of blind isolated faults is mechanical anisotropy associated with rock layering, resulting in layer-parallel elongation of fault surface ellipses. Faults that intersect the free surface and/or interact with nearby faults have aspect ratios ranging from 0.5 to 8.4, and are referred to as restricted. Restriction of fault growth has various effects including: (i) reduced curvature of the tip-line and of displacement contours; and (ii) increased displacement gradients in the restricted region. Many faults are restricted at more than one place on their tip-line loop and so have highly irregular shapes and displacement patterns. Subsequent linkage of interacting faults produces combined faults with aspect ratios within the normal range for unrestricted faults. Lateral interaction between faults does not necessarily lead to a change in the power-law exponent of the fault population.

An alternative model for the growth of faults

Conventional growth models suggest that faults become larger due to systematic increases in both maximum displacement and length. We propose an alternative growth model where fault lengths are near-constant from an early stage and growth is achieved mainly by increase in cumulative displacement. The model reconciles the scaling properties of faults and earthquakes and predicts a progressive increase in fault displacement to length ratios as a fault system matures. This growth scheme is directly applicable to reactivated fault systems in which fault lengths were inherited from underlying structure and established rapidly; the model may also apply to some non-reactivated fault systems. Near-constant fault lengths during subsequent growth are attributed to retardation of lateral propagation by interaction between fault tips. The model is validated using kinematic constraints from growth strata, which are displaced by a system of reactivated normal faults in the Timor Sea, NW Australia.

Growth of vertically segmented normal faults

The geometry and evolution of vertically segmented normal faults, with dip separations of < ca 11.5 m have been studied in a coastal outcrop of finely bedded Cretaceous chalk at Flamborough Head, U.K. Fault trace segments are separated by both contractional and extensional offsets which have step, overlap or bend geometries. The location of fault trace offsets is strongly controlled by lithology occurring at either thin (ca 1 mm-8 cm) and mechanically weak marl layers or partings between chalk units. Fault segmentation occurred during either fault nucleation within, or propagation through, the strongly anisotropic lithological sequence. An inverse relationship between fault displacement and number of offsets per length of fault trace reflects the progressive destruction of offsets during fault growth. The preservation of fault offsets is therefore dependent on offset width and fault displacement. Fault rock, comprising gouge and chalk breccia, may vary in thickness by 1.5–2.0 orders of magnitude on individual fault traces. Strongly heterogeneous fault rock distributions are most common on small faults (< 10 cm displacement) and are produced mainly by destruction of fault offsets. Shearing of fault rock with increasing displacement gives rise to a more homogeneous fault rock distribution on large faults at the outcrop scale.

Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand

An earthquake with a dozen faults The 2016 moment magnitude (Mw) 7.8 Kaikōura earthquake was one of the largest ever to hit New Zealand. Hamling et al. show with a new slip model that it was an incredibly complex event. Unlike most earthquakes, multiple faults ruptured to generate the ground shaking. A remarkable 12 faults ruptured overall, with the rupture jumping between faults located up to 15 km away from each other. The earthquake should motivate rethinking of certain seismic hazard models, which do not presently allow for this unusual complex rupture pattern. Science, this issue p. eaam7194 At least 12 faults spaced up to 15 kilometers apart ruptured during the magnitude 7.8 Kaikōura earthquake. INTRODUCTION On 14 November 2016 (local time), northeastern South Island of New Zealand was struck by a major moment magnitude (Mw) 7.8 earthquake. The Kaikōura earthquake was the most powerful experienced in the region in more than 150 years. The whole of New Zealand reported shaking, with widespread damage across much of northern South Island and in the capital city, Wellington. The earthquake straddled two distinct seismotectonic domains, breaking multiple faults in the contractional North Canterbury fault zone and the dominantly strike-slip Marlborough fault system. RATIONALE Earthquakes are conceptually thought to occur along a single fault. Although this is often the case, the need to account for multiple segment ruptures challenges seismic hazard assessments and potential maximum earthquake magnitudes. Field observations from many past earthquakes and numerical models suggest that a rupture will halt if it has to step over a distance as small as 5 km to continue on a different fault. The Kaikōura earthquake’s complexity defies many conventional assumptions about the degree to which earthquake ruptures are controlled by fault segmentation and provides additional motivation to rethink these issues in seismic hazard models. RESULTS Field observations, in conjunction with interferometric synthetic aperture radar (InSAR), Global Positioning System (GPS), and seismology data, reveal the Kaikōura earthquake to be one of the most complex earthquakes ever recorded with modern instrumental techniques. The rupture propagated northward for more than 170 km along both mapped and unmapped faults before continuing offshore at the island’s northeastern extent. A tsunami of up to 3 m in height was detected at Kaikōura and at three other tide gauges along the east coast of both the North and South Islands. Geodetic and geological field observations reveal surface ruptures along at least 12 major crustal faults and extensive uplift along much of the coastline. Surface displacements measured by GPS and satellite radar data show horizontal offsets of ~6 m. In addition, a fault-bounded block (the Papatea block) was uplifted by up to 8 m and translated south by 4 to 5 m. Modeling suggests that some of the faults slipped by more than 20 m, at depths of 10 to 15 km, with surface slip of ~10 m consistent with field observations of offset roads and fences. Although we can explain most of the deformation by crustal faulting alone, global moment tensors show a larger thrust component, indicating that the earthquake also involved some slip along the southern end of the Hikurangi subduction interface, which lies ~20 km beneath Kaikōura. Including this as a fault source in the inversion suggests that up to 4 m of predominantly reverse slip may have occurred on the subduction zone beneath the crustal faults, contributing ~10 to 30% of the total moment. CONCLUSION Although the unusual multifault rupture observed in the Kaikōura earthquake may be partly related to the geometrically complex nature of the faults in this region, this event emphasizes the importance of reevaluating how rupture scenarios are defined for seismic hazard models in plate boundary zones worldwide. Observed ground deformation from the 2016 Kaikōura, New Zealand, earthquake. (A and B) Photos showing the coastal uplift of 2 to 3 m associated with the Papatea block [labeled in (C)]. The inset in (A) shows an aerial view of New Zealand. Red lines denote the location of known active faults. The black box indicates the Marlborough fault system

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

Late Cenozoic evolution and earthquake potential of an active listric thrust complex above the Hikurangi subduction zone, New Zealand

In the center of the frontal wedge of the Hikurangi subduction zone, New Zealand, Mahia Peninsula and its submarine continuation, Lachlan Ridge, are being uplifted and folded above an active landward- dipping thrust-fault complex that is 80 km long. High-quality marine seismic reflection profiles reveal complex deformation of a Cretaceous to Holocene sedimentary section and enable a detailed analysis of the stratigraphy, structural evolution, deformation rates, and future earthquake potential. The structural analysis is facilitated by uplifted marine terraces on Mahia Peninsula and by 14 submarine unconformities in the hanging-wall sequence, five of which are correlated across the eroded crest of Lachlan Ridge and into the footwall basin. The ages of the unconformities are determined by seismic ties to an offshore exploration well, onshore outcrops on the peninsula, and seabed samples dated by pollen, coccolith nannoflora, and foraminifera biostratigraphy. Nine regional Quaternary unconformities, which developed in response to eustatic fluctuations in sea level and are not older than ca. 1 Ma, are correlated with oxygen isotope stages in equatorial Pacific Ocean Drilling Project core 677. The ages of fault-growth strata and progressive restorations of deformed stratigraphy indicate that Lachlan Ridge developed during three phases of deformation since subduction of the Pacific plate commenced beneath the Australian plate in the early Miocene. These include an initial phase of thrust faulting, a subsequent phase dominated by extensional faulting, and the current, mainly Pleistocene to Holocene phase of structural inversion, reactivated listric thrust faulting, and folding. Early to middle Miocene thrusts in the deeper core of the complex developed out of sequence, by sequentially stepping up into the hanging-wall section, creating an imbricate fan with emerging thrust tips now buried within the forelimb basin. In the middle Miocene–early Pliocene, listric extensional faults developed in the active thrust wedge—possibly as a result of substantial relief on the subducted Pacific plate—and controlled the development of Lachlan Basin to the west of the ridge. The principal active thrust is the Lachlan fault, a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right-stepping segments. Depth-converted seismic profiles indicate that the fault dips westward at 15°–20°, 6–8 km beneath the western flank of Lachlan Ridge, and steepens to 55°–70° or even steeper in the upper 1–2 km of section beneath the eastern flank. Syninversion Pleistocene fault-growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold-limb rotation resulting from the listric-fault geometry. A geometric analysis of the fault-growth strata and deformed terraces was used to derive a maximum dip-slip displacement rate of as fast as 3.0–6.5 mm/yr. The implied shortening rate of 2.6–6.3 mm/yr represents ∼8%–20% of the total 31 mm/yr of orthogonal plate convergence across this part of the upper plate of the Hikurangi subduction zone. The top of the pre–fault-growth Paleogene section reveals as much as 5.8 ± 1.5 km of vertical separation in the north, decreasing to ∼30%–50% of this value in the south. Temporal (10 3 –10 6 yr) and spatial (10 3 –10 4 m of strike length) variations in vertical-deformation rate have occurred during the past 1 m.y.; maximum rates occurred in the Holocene and middle Quaternary. A long-term increase in vertical- separation rate on all segments during the Pleistocene largely reflects a change in thrust kinematics associated with structural inversion. The relatively greater increase in uplift rate on the northern part of the fault during the past 1 m.y. could be related to the possibility that a subducted seamount lies >10 km beneath the peninsula. Estimates of earthquake source parameters, incorporating paleoseismic uplift data from Mahia Peninsula, indicate a potential moment magnitude of up to M w 7.6–8.0 for an earthquake that ruptures all three segments of the Lachlan fault. The average recurrence interval for such events is estimated to be 615–2333 yr, which is consistent with a mean recurrence interval of 1062 yr for four late Holocene earthquakes. Thus, the uplift and folding of Mahia Peninsula and Lachlan Ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large-magnitude earthquakes.

River response to an active fold-and-thrust belt in a convergent margin setting, North Island, New Zealand

High-resolution digital elevation data (TOPSAR 10-m grid) are used to reconstruct Late Quaternary growth histories of subtle folding in the Wairarapa fold-and-thrust belt, North Island, New Zealand. Outcrop data of deformed latest Miocene and younger strata are combined with observations of warped and faulted late Quaternary terrace surfaces to unravel the geomorphic and structural history of the Huangarua River valley. Optically stimulated luminescence (OSL) dating of loess that accumulated on these strath terraces and paleosol stratigraphy allow temporal correlation among the terraces and with glacial climate cycles since the Last Interglaciation. These data indicate that five intervals of strath cutting occurred, at ∼ 125, 60-70, ∼ 30, ∼ 15, and <10 ka. Strath beveling is largely independent of local folding or regional base-level change. We hypothesize that straths are cut when an increase in sediment supply, during cool climatic periods, brings river sediment load and river transport capacity into balance. In the Wairarapa, strath-cutting events appear to occur near the end of cool climatic cycles. This study shows that the identification of subtle departures from regional topographic trends becomes practical when a high-resolution DEM is available. After subtraction of an average valley gradient from the digital topography, the residual topography on the terrace treads reveals cross-valley and longitudinal tilting. Although rates of folding are slow and the magnitude of deformation is commonly limited to less than a few tens of meters, these topographic anomalies define fold axes that coincide with subsurface structures. When combined with time control, these anomalies serve to define the patterns and rates of fold growth over the past 125 kyears.

Three-dimensional geometry and growth of conjugate normal faults

Abstract Good quality 2-D and 3-D seismic reflection data from the Timor Sea are used to determine the threedimensional geometry, displacement patterns and development of intersecting conjugate normal faults. These data are supplemented by data from previous physical modelling studies. Conjugate structures, which comprise two intersecting opposed-dipping normal faults or fault sets, form synchronously on a geological time scale and develop due to the incidental intersection of the faults, factors which affect both the formation and imaging of these structures include: the fault density, the spatial distribution of opposed-dipping faults, the seismic resolution and the vertical extent of the imaged fault data. Large conjugate structures grow from smaller ones; larger conjugates are associated with more numerous and larger faults than small structures. On the scale of the seismic data (fault throws range from ca 10–400 m). synchronous fault movements are accommodated by a reduction of displacements on discrete fault surfaces towards the fault intersection zone, and a corresponding increase in ductile strain of this region. High strains in the volume proximal to the fault intersection zone are expressed as thinning of stratigraphic units between the conjugate faults, and are believed to be accommodated by numerous small sub-seismic faults. Intersection of two opposed-dipping faults does not prevent their continued synchronous movement and does not result in mechanical locking of the system.

Oblique back arc rifting of Taupo Volcanic Zone, New Zealand

] Taupo Volcanic Zone (TVZ) is a back arc rift inNorth Island, New Zealand. Its geometry andkinematics are investigated using shaded reliefimages, field examination of faults and offset streamchannels. The results show that TVZ trends NNE, is 250 km long by Hikurangi margin in the North Island of New Zealand [20 km wide and consists of fivesegments. Extension is principally manifest as steeplydipping (60 –90 ) normal faults parallel to TVZ;these, in the last 300 kyr, have experienced acomponent of dextral shear. TVZ is therefore anoblique back arc rift. The dextral shear is 37% of thetotal displacement, which, for previously estimatedspreadingrates 7mm/yr,correspondsto 2.6mm/yr.Thisvalueissimilartopreviousestimatesofthedextralshear from the back arc to the forearc domains in theNorthIsland.DistributeddextralshearacrossTVZthussuggests that strain partitioning across the plateboundary at latitudes of TVZ is less significant thanpreviously thought.

Geometry and origin of a polygonal fault system

A fault array in South Australia, interpreted from a 3D onshore seismic survey, shows fault traces on the lowermost mapped horizon of a shale‐dominated sequence which outline polygonal cells averaging 1.4 km in diameter. The cell boundaries coincide approximately with the downward terminations and near convergence of conjugate pairs of normal faults. The pattern becomes less spatially ordered on higher horizons where faults still show a near‐isotropic strike distribution. Maximum throws, c. 80 m, occur c. 400 m above the downward terminations of the faults. The faults have a systematic geometric relationship with folds, with anticlines in the mutual hanging walls of fault pairs and broader footwall synclines that define the shallow dish forms of the polygons. Polygon boundaries coincide with anticlinal ridges on the interface between the faulted sequence and an underlying 35 m thick low velocity, low density, overpressured layer. Although the pattern of ridges defining the polygon boundaries is strikingly similar to experimental spoke and hub patterns formed at the boundaries between viscous materials with density inversion, the data do not exclude the possibility of lateral extension.