Following the 22 February 2011, MW 6.2 earthquake located on a fault beneath the Port Hills of Christchurch, fissuring of up to several hundred metres in length was observed in the loess and loess-colluvium of foot-slope positions in north-facing valleys of the Port Hills. The fissuring was observed in all major valleys, occurred at similar low altitudes, showing a contour-parallel orientation and often accompanied by both lateral compression/extension features and spring formation in the valley floor below. Fissuring locations studied in depth included Bowenvale Valley, Hillsborough Valley, Huntlywood Terrace–Lucas Lane, Bridle Path Road, and Maffeys Road–La Costa Lane. Investigations into loess soil, its properties and mannerisms, as well as international examples of its failure were undertaken, including study of the Loess Plateau of China, the Teton Dam, and palaeo-fissuring on Banks Peninsula. These investigations lead to the conclusion that loess has the propensity to fail, often due to the infiltration of water, the presence of which can lead to its instantaneous disaggregation. Literature study and laboratory analysis of Port Hills loess concluded that is has the ability to be stable in steep, sub-vertical escarpments, and often has a sub-vertically jointed internal structure and has a peak shear strength when dry. Values for cohesion, c (kPa) and the internal friction angle, ϕ (degrees) of Port Hills loess were established. The c values for the 40 Rapaki Road, 3 Glenview Terrace loess samples were 13.4 kPa and 19.7 kPa, respectively. The corresponding ϕ values were thought unusually high, at 42.0° and 43.4°.The analysed loess behaved very plastically, with little or no peak strength visible in the plots as the test went almost directly to residual strength. A geophysics resistivity survey showed an area of low resistivity which likely corresponds to a zone of saturated clayey loess/loess colluvium, indicating a high water table in the area. This is consistent with the appearances of local springs which are located towards the northern end of each distinct section of fissure trace and chemical analysis shows that they are sourced from the Port Hills volcanics. Port Hills fissuring may be sub-divided into three categories, Category A, Category B, and Category C, each characterised by distinctive features of the fissures. Category A includes fissures which display evidence of, spring formation, tunnel-gullying, and lateral spreading-like behaviour or quasi-toppling. These fissures are several metres down-slope of the loess-bedrock interface, and are in valleys containing a loess-colluvium fill. Category B fissures are in wider valleys than those in Category A, and the valleys contain estuarine silty sediments which liquefied during the earthquake. Category C fissures occurred at higher elevations than the fissures in the preceding categories, being almost coincident with bedrock outcropping. It is believed that the mechanism responsible for causing the fissuring is a complex combination of three mechanisms: the trampoline effect, bedrock fracturing, and lateral spreading. These three mechanisms can be applied in varying degrees to each of the fissuring sites in categories A, B, and C, in order to provide explanation for the observations made at each. Toppling failure can describe the soil movement as a consequence of the a three causative mechanisms, and provides insight into the movement of the loess. Intra-loess water coursing and tunnel gullying is thought to have encouraged and exacerbated the fissuring, while not being the driving force per se. Incipient landsliding is considered to be the least likely of the possible fissuring interpretations.
Geomorphic, structural and chronological data are used to establish the late Quaternary paleoseismicity of the active dextral-oblique Northern Esk Fault in North Canterbury, New Zealand. Detailed field mapping of the preserved c. 35 km of surface traces between the Hurunui River and Ashley Head reveals variations in strike ranging from 005° to 057°. Along with kinematic data collected from fault plane striae and offset geomorphic markers along the length of the fault these variations are used to distinguish six structural subsections of the main trace, four dextral-reverse and two dextral-normal. Displacements of geomorphic markers such as minor streams and ridges are measured using differential GPS and rangefinder equipment to reveal lateral offsets ranging from 3.4 to 23.7 m and vertical offsets ranging from < 1 to 13.5 m. Characteristic single event displacements of c. 5 m and c. 2 m have been calculated for strike-slip and reverse sections respectively. The use of fault scaling relationships reveals an anomalously high displacement to surface rupture length ratio when compared to global data sets. Fault scaling relationships based on width limited ruptures and magnitude probabilities from point measurements of displacement imply earthquake magnitudes of Mw 7.0 to 7.5. Optically Stimulated Luminescence (OSL) ages from displaced Holocene alluvial terraces at the northern extent of the active trace along with OSL and radiocarbon samples of the central sections constrain the timing of the last two surface rupturing events (11.15 ±1.65 and 3.5 ± 2.8 ka) and suggest a recurrence interval of c. 5612 ± 445 years and late Quaternary reverse and dextral slip rates of c. 0.31 mm/yr and 0.82 mm/yr respectively. The results of this study show that the Northern Esk Fault accommodates an important component of the c. 0.7 – 2 mm/yr of unresolved strain across the plate boundary within the North Canterbury region and affirm the Esk Fault as a source of potentially damaging ground shaking in the Canterbury region.
The Acheron rock avalanche is located in the Red Hill valley almost 80 km west of Christchurch and is one of 42 greywacke-derived rock avalanches identified in the central Southern Alps. It overlies the Holocene active Porters Pass Fault; a component of the Porters Pass-Amberley Fault Zone which extends from the Rakaia River to beyond the Waimakariri River. The Porters Pass Fault is a dextral strike-slip fault system viewed as a series of discontinuous fault scarps. The location of the fault trace beneath the deposit suggests it may represent a possible source of seismic shaking resulting in the formation of the Acheron rock avalanche. The rock mass composition of the rock avalanche source scar is Torlesse Supergroup greywacke consisting of massive sandstone and thinly bedded mudstone sequences dipping steeply north into the centre of the source basin. A stability analysis identified potential instability along shallow north dipping planar defects, and steep south dipping toppling failure planes. The interaction of the defects with bedding is considered to have formed conditions for potential instability most likely triggered by a seismic event. The dTositional area of the rock avalanche covers 7.2 x 105 m2 with an estimated volume of 9 x 10 m3 The mobilised rock mass volume was calculated at 7.5 x 106 m3• Run out of the debris from the top of the source scar to the distal limit reached 3500m, descending over a vertical fall of almost 700m with an estimated Fahrboschung of 0.2. The run out of the rock avalanche displayed moderate to high mobility, travelling at an estimated maximum velocity of 140-160 km/hour. The rapid emplacement of the deposit is confirmed by highly fragmented internal composition and burial of forest vegetation New radiocarbon ages from buried wood retrieved from the base of Acheron rock avalanche deposit represents an emplacement age closely post-dating (Wk 12094) 1152 ± 51 years B.P. This differs significantly from a previous radiocarbon age of (NZ547) 500 ± 69 years B.P. and modal lichenometry and weathering-rind thickness ages of approximately 460 ± 10 yrs and 490 ± 50 years B.P. The new age shows no resemblance to an earthquake event around 700- 500 years B.P. on the Porters Pass-Amberley Fault Zone. The DAN run out simulation using a friction model rheology successfully replicated the long run out and velocity of the Acheron rock avalanche using a frictron angle of 27° and high earth pressure coefficients of 5.5, 5.2, and 5.9. The elevated earth pressure coefficients represent dispersive pressures derived from dynamic fragmentation of the debris within the mobile rock avalanche, supporting the hypothesis of Davies and McSaveney (2002). The DAN model has potential applications for areas prone to large-scale instability in the elevated slopes and steep waterways of the Southern Alps. A paleoseismic investigation of a newly identified scarp of the Porters Pass Fault partially buried by the rock avalanche was conducted to identify any evidence of a coseismic relationship to the Acheron rock avalanche. This identified three-four fault traces striking at 078°, and a sag pond displaying a sequence of overbank deposits containing two buried soils representing an earthquake event horizon. A 40cm vertical offset of the ponded sediment and lower buried soil horizqn was recorded, which was dated to (Wk 13112 charcoal in palosol) 653 ± 54 years B.P. and (Wk 13034 palosol) 661 ± 34 years B.P. The evidence indicates a fault rupture occurred along the Porters Pass Fault, west of Porters Pass most likely extending to the Red Lakes terraces, post-dating 700 years B.P., resulting in 40cm of vertical displacement and an unknown component of dextral strike slip movement. This event post dates the event one (1000 ± 100 years B.P) at Porters Pass previously considered to represent the most recent rupture along the fault line. This points to a probable source for resetting of the modal weathering-rind thicknesses and lichen size populations in the Red Hill valley and possibly the Red Lakes terraces. These results suggest careful consideration must be given to the geomorphic and paleoseismic history of a specific site when applying surface dating techniques and furthermore the origin of dates used in literature and their useful range should be verified. An event at 700-500 years B.P did not trigger the Acheron rock avalanche as previously assumed supporting Howard's conclusions. The lack of similar aged rupture evidence in either of the Porters Pass and Coleridge trenches supports Howard's hypothesis of segmentation of the Porters Pass Fault; where rupture occurs along one fault segment but not along another. The new rock avalanche age closely post-dating 1200-1100 years B.P. resembles the poorly constrained event one rupture age of 1700-800 years B.P for the Porters Pass Fault and the tighter constrained Round Top event of 1010 ± 50 years B.P. on the Alpine Fault. Eight other rock avalanche deposits spread across the central Southern Alps also resemble the new ages however are unable to be assigned specific earthquake events due to the large associated error bars of± 270 years. This clustering of ages does represent compelling lines of evidence for large magnitude earthquake events occurring over the central Southern Alps. The presence of a rock avalanche deposit does not signify an earthquake based on the historical evidence in the Southern Alps however clustering of ages does suggest that large Mw >7 earthquakes occurred across the Southern Alps between 1200-900 years BP.
A number of reverse and strike-slip faults are distributed throughout mid-Canterbury, South Island, New Zealand, due to oblique continental collision. There is limited knowledge on fault interaction in the region, despite historical multi-fault earthquakes involving both reverse and strike-slip faults. The surface expression and paleoseismicity of these faults can provide insights into fault interaction and seismic hazards in the region. In this thesis, I studied the Lake Heron and Torlesse faults to better understand the key differences between these two adjacent faults located within different ‘tectonic domains’. Recent activity and surface expression of the Lake Heron fault was mapped and analysed using drone survey, Structure-from-Motion (SfM) derived Digital Surface Model (DSM), aerial image, 5 m-Digital Elevation Model (DEM), luminescence dating technique, and fold modelling. The results show a direct relationship between deformation zone width and the thickness of the gravel deposits in the area. Fold modelling using fault dip, net slip and gravel thickness produces a deformation zone comparable to the field, indicating that the fault geometry is sound and corroborating the results. This result Is consistent with global studies that demonstrate deposit (or soil thickness) correlates to fault deformation zone width, and therefore is important to consider for fault displacement hazard. A geomorphological study on the Torlesse fault was conducted using SfM-DSM, DEM and aerial images Ground Penetrating Radar (GPR) survey, trenching, and radiocarbon and luminescence dating. The results indicate that the Torlesse fault is primarily strike-slip with some dip slip component. In many places, the bedding-parallel Torlesse fault offsets post-glacial deposits, with some evidence of flexural slip faulting due to folding. Absolute dating of offset terraces using radiocarbon dating and slip on fault determined from lateral displacement calculating tool demonstrates the fault has a slip rate of around 0.5 mm/year to 1.0 mm/year. The likelihood of multi-fault rupture in the Torlesse Range has been characterised using paleoseismic trenching, a new structural model, and evaluation of existing paleoseismic data on the Porters Pass fault. Identification of overlapping of paleoseismic events in main Torlesse fault, flexural-slip faults and the Porters Pass fault in the Torlesse Range shows the possibility of distinct or multi-fault rupture on the Torlesse fault. The structural connectivity of the faults in the Torlesse zone forming a ‘flower structure’ supports the potential of multi-fault rupture. Multi-fault rupture modelling carried out in the area shows a high probability of rupture in the Porters Pass fault and Esk fault which also supports the co-rupture probability of faults in the region. This study offers a new understanding of the chronology, slip distribution, rupture characteristics and possible structural and kinematic relationship of Lake Heron fault and Torlesse fault in the South Island, New Zealand.
The Lake Coleridge Rock Avalanche Deposits (LCRADs) are located on Ryton Station in the middle Rakaia Valley, approximately 80 km west of Christchurch. Torlesse Supergroup greywacke is the basement material and has been significantly influenced by both active tectonics and glaciation. Both glacial and post-glacial processes have produced large volumes of material which blanket the bedrock on slopes and in the valley floors. The LCRADs were part of a regional study of rock avalanches by WHITEHOUSE (1981, 1983) and WHITEHOUSE and GRIFFITHS (1983), and a single rock avalanche event was recognised with a weathering rind age of 120 years B.P. that was later modified to 150 ± 40 years B.P. The present study has refined details of both the age and the sequence of events at the site, by identifying three separate rock avalanche deposits (termed the LCRA1, LCRA2 and LCRA3 deposits), which are all sourced from near the summit of Carriage Drive. The LCRA1 deposit is lobate in shape and had an estimated original deposit volume of 12.5 x 10⁶ m³, although erosion by the Ryton River has reduced the present day debris volume to 5.1 x 10⁶ m³. An optically stimulated luminescence date taken from sandy loess immediately beneath the LCRA1 deposit provided a maximum age for the rock avalanche event of 9,720 ± 750 years B.P., which is believed to be realistic given that this is shortly after the retreat of Acheron 3 ice from this part of the valley. Emplacement of rock avalanche material into an ancestral Ryton riverbed created a natural dam with a ~17 M m³ lake upstream. The river is thought to have created a natural spillway over the dam structure at ~557 m (a.s.l), and to have existed for a number of years before any significant downcutting occurred. Although a triggering mechanism for the LCRA1 deposit was poorly constrained, it is thought that stress rebound after glacial ice removal may have initiated failure. Due to the event occurring c.10,000 years ago, there was a lack of definition for a possible earthquake trigger, though the possibility is obvious. The LCRA₂ event had an original deposit volume of 0.66 x 10⁶ m³, and was constrained to the low-lying area adjacent to the Ryton River that had been created by river erosion of the LCRA1 deposit. Further erosion by the Ryton River has reduced the deposit volume to 0.4 x 10⁶ m³. A radiocarbon date from a piece of mānuka found within the LCRA2 deposit provided an age of 668 ± 36 years B.P., and this is thought to reliably date the event. The LCRA2 event also dammed the Ryton River, and the preservation of dam-break outwash terraces downstream from the deposit provides clear evidence of rapid dam erosion and flooding after overtopping, and breaching by the Ryton River. Based on the mean annual flow of the Ryton River, the LCRA2 lake would have taken approximately two weeks to fill assuming that there were no preferred breach paths and the material was relatively impermeable. The LCRA2 event is thought to have been coseismic with a fault rupture along the western segment of the PPAFZ, which has been dated at 600 ± 100 years B.P. by SMITH (2003). The small LCRA3 event was not able to be dated, but it is believed to have failed shortly after the LCRA2 event and it may in fact be a lag deposit of the second rock avalanche event possibly triggered by an aftershock. The deposit is only visible at one locality within the cliffs that line the Ryton River, and its lack of geomorphic expression is attributed to it occurring closely after the LCRA2 event, while the Ryton River was still dammed from the second rock avalanche event. A wedge-block of some 35,000 m³ of source material for a future rock avalanche was identified at the summit of Carriage Drive. The dilation of the rock mass, combined with unfavourably oriented sub-vertical bedding in the Torlesse Supergroup bedrock, has allowed toppling-style failure on both of the main ridge lines around the source area for the LCRADs. In the event of a future rock avalanche occurring within the Ryton riverbed an emergency response plan has been developed to provide a staged response, especially in relation to the camping ground located at the mouth of the Ryton River. A long-term management plan has also been developed for mitigation measures for the Ryton riverbed and adjacent floodplain areas downstream of a future rock avalanche at the LCRAD site.
The Porter's Pass-Amberley Fault Zone (PPAFZ) is a complex zone of anastomosing faults and folds bounding the south-eastern edge of the transition from subducting Pacific Plate to continental collision on the Australia Plate boundary. This study combines mapping of a 2000 km2 zone from the Southern Alps northeast to the coast near Amberley, 40 km north of metropolitan Christchurch, with an analysis of seismicity and a revision of regional seismic hazard. Three structural styles: 1) a western strike-slip, and 2) a more easterly thrust and reverse domain, pass into 3) a northwest verging fold belt on the northern Canterbury Plains, reflecting the structural levels exposed and the evolving west to east propagation. Basal remnants of a Late Cretaceous-Cenozoic, largely marine sedimentary cover sequence are preserved as outliers that unconformably overlie Mesozoic basement (greywacke and argillite of the Torlesse terrain) in the mountains of the PPAFZ and are underlain by a deeply leached zone which is widely preserved. Structure contouring of the unconformity surface indicates maximum, differential uplift of c.2600 m in the southwest, decreasing to c.1200 m in the coastal fold belt to the northeast. Much lower rates (or reversal) of uplift are evident a few kilometres southeast of the PPAFZ range-front escarpment. The youngest elements of the cover sequence are basement-derived conglomerates of Plio-Pleistocene age preserved on the SE margin. The source is more distant than the intervening mountains of the PPAFZ, probably from the Southern Alps, to the west and northwest. The absence of another regional unconformity on Mesozoic basement, older than Pleistocene, indicates that this uplift is post-Pliocene. Late Pleistocene(<100 kyr) differential uplift rates of c.0.5-2.7 m/kyr from uplifted marine terraces at the east coast, and rates of 2.5-3.3 m/kyr for tectonically-induced river-down cutting further west, suggest that uplift commenced locally during the last 1 Ma, and possibly within the last 0.5 Ma, if average rates are assumed to be uniform over time. Analysis of seismicity, recorded during a 10 week regional survey of micro earthquakes in 1990, identified two seismic zones beneath North Canterbury: 1) a sub-horizontal zone of activity restricted to the upper crust (≤12 km); and 2) a seismic zone in the lower crust (below a ceiling of ≤17 km), that broadens vertically to the north and northwest to a depth of c.40 km, with a bottom edge which dips 10°N and 15°NW, respectively. No events were recorded at depths between 12 km and 17 km, which is interpreted as a relatively aseismic, mid-crustal ductile layer. Marked differences (up to 60°) in the trend of strain axes for events above and below the inferred ductile layer are observed only north of the PPAFZ. A fundamental, north-to-south increase in the Wave-length of major geological structures occurs across the PPAFZ, and is interpreted as evidence that the upper crust beneath the Canterbury Plains is coupled to the lower crust, whereas the upper crust further north is not. Most of the recorded micro earthquakes <12 km deep beneath the PPAFZ have strike-slip mechanisms. It is probable that faults splay upward into the thrusts and folds at the surface as an evolving transpression zone in response to deep shear in basement. There have been no historic surface ruptures of the PPAFZ, but the zone has been characterised historically by frequent small earthquakes. Paleoseismic data (dated landslides and surface ruptures) compiled in this study, indicate a return period of 1500-1900 years between the last two M>7-7.5 earthquakes, and 500-700 years have elapsed since the last. The magnitudes of these events are estimated at c.M7.5, which represents a probable maximum magnitude for the PPAFZ. There are insufficient data to determine whether or not the frequency of large earthquakes conforms to a recognised model of behaviour, but comparison of the paleoseismic data with the historic record of smaller earthquakes, suggests that the magnitudes of the largest earthquakes in this zone are not exponentially distributed. A seismicity model for the PPAFZ (Elder et al., 1991) is reviewed, and a b-value of 1.0 is found to be consistent with the newly acquired paleoseismic data. This b-value reduces the predicted frequency of large earthquakes (M≥7.0) in this zone by a factor of 3.5, while retaining a conservative margin that allows for temporal variations in the frequency of large events and the possibility that the geological database is incomplete, suggesting grounds for revising the hazard model for Christchurch.
