William Ries

The New Zealand Active Faults Database

ABSTRACT The New Zealand Active Faults Database (NZAFD) is a national geospatial database of active faults – including their locations, names and degrees of activity – that have deformed the ground surface of New Zealand within the last 125,000 years. The NZAFD is used for geological research, hazard modelling and infrastructure planning and is an underlying dataset for other nationally significant hazard applications such as the National Seismic Hazard Model. Recent refinements to the data structure have improved the accuracy of active fault locations and characteristics. A subset of active fault information from the NZAFD, generalised for portrayal and use at a scale of 1:250,000 (and referred to as NZAFD250), is freely available online and can be downloaded in several different formats to suit the needs of a range of users including scientists, governmental authorities and the general public. To achieve a uniform spatial scale of 1:250,000 a simplification of detailed fault locational data was required in some areas, while in other areas new mapping was necessary to provide a consistent level of coverage. Future improvements to the NZAFD will include the incorporation of data on active folds and offshore active faults.

Associations between volcanic eruptions from Okataina volcanic center and surface rupture of nearby active faults, Taupo rift, New Zealand: Insights into the nature of volcano-tectonic interactions

From a data set of 50 published and new fault exposures, we establish a 26,000 year record of associations between the timing of fault rupture in two sectors of the Taupo rift, New Zealand, and deposition on the fault scarps of rhyolitic fall tephra from the adjacent Okataina volcanic center. We also investigate processes that could be responsible for the time associations. From 40 high-resolution couplets of fault rupture and volcanic eruption (located up to 30 km distant), we show that 30% of the fault ruptures occurred when the volcano was erupting, whereas in 70% of the cases volcanism and faulting were independent. Other geological and geophysical information indicates that faulting in the Taupo rift is essentially tectonic and, thus, most of the cases with time association between fault rupture and volcanic eruption found in the fault exposures in this study are interpreted to be a manifestation of stress transfer between faults and magmatic storage zones beneath the volcanic center. In a few cases close (

Determining Rockfall Risk in Christchurch Using Rockfalls Triggered by the 2010–2011 Canterbury Earthquake Sequence

The Canterbury earthquake sequence triggered thousands of rockfalls in the Port Hills of Christchurch, New Zealand, with over 6,000 falling on 22 February 2011. Several hundred families were evacuated after about 200 homes were hit. We characterized the rockfalls by boulder-size distribution, runout distance, source-area dimensions, and boulder-production rates over a range of triggering peak ground accelerations. Using these characteristics, a time-varying seismic hazard model for Canterbury, and estimates of residential occupancy rates and resident vulnerability, we estimated annual individual fatality risk from rockfall in the Port Hills. The results demonstrate the Port Hills rockfall risk is time-variable, decreasing as the seismic hazard decreases following the main earthquakes in February and June 2011. This presents a real challenge for formulating robust land-use and reconstruction policy in the Port Hills.

Map of the 2010 Greendale Fault surface rupture, Canterbury, New Zealand: application to land use planning

Abstract Rupture of the Greendale Fault during the 4 September 2010, M W7.1 Darfield (Canterbury) earthquake produced a zone of ground-surface rupture that severely damaged several houses, buildings and lifelines. Immediately after the earthquake, surface rupture features were mapped in the field and from digital terrain models developed from airborne Light Detection and Ranging (lidar) data. To enable rebuild decisions to be made and for future land use planning, a fault avoidance zone was defined for the Greendale Fault following the Ministry for the Environment guidelines on ‘Planning for the Development of Land on or Close to Active Faults’. We present here the most detailed map to date of the fault trace and describe how this was used to define and characterise the fault avoidance zone for land use planning purposes.

Surface Rupture of Multiple Crustal Faults in the 2016 Mw 7.8 Kaikōura, New Zealand, Earthquake

Multiple (>20 >20 ) crustal faults ruptured to the ground surface and seafloor in the 14 November 2016 M w Mw 7.8 Kaikōura earthquake, and many have been documented in detail, providing an opportunity to understand the factors controlling multifault ruptures, including the role of the subduction interface. We present a summary of the surface ruptures, as well as previous knowledge including paleoseismic data, and use these data and a 3D geological model to calculate cumulative geological moment magnitudes (M G w MwG ) and seismic moments for comparison with those from geophysical datasets. The earthquake ruptured faults with a wide range of orientations, sense of movement, slip rates, and recurrence intervals, and crossed a tectonic domain boundary, the Hope fault. The maximum net surface displacement was ∼12  m ∼12  m on the Kekerengu and the Papatea faults, and average displacements for the major faults were 0.7–1.5 m south of the Hope fault, and 5.5–6.4 m to the north. M G w MwG using two different methods are M G w MwG 7.7 +0.3 −0.2 7.7−0.2+0.3 and the seismic moment is 33%–67% of geophysical datasets. However, these are minimum values and a best estimate M G w MwG incorporating probable larger slip at depth, a 20 km seismogenic depth, and likely listric geometry is M G w MwG 7.8±0.2 7.8±0.2 , suggests ≤32% ≤32% of the moment may be attributed to slip on the subduction interface and/or a midcrustal detachment. Likely factors contributing to multifault rupture in the Kaikōura earthquake include (1) the presence of the subduction interface, (2) physical linkages between faults, (3) rupture of geologically immature faults in the south, and (4) inherited geological structure. The estimated recurrence interval for the Kaikōura earthquake is ≥5,000–10,000  yrs ≥5,000–10,000  yrs , and so it is a relatively rare event. Nevertheless, these findings support the need for continued advances in seismic hazard modeling to ensure that they incorporate multifault ruptures that cross tectonic domain boundaries.

GIS modelling in support of earthquake-induced rockfall and cliff collapse risk assessment in the Port Hills, Christchurch

Ground shaking associated with the 22 February 2011 Mw 6.2 Christchurch earthquake exceeded MM10 and dislodged boulders from cliffs on the upper slopes of the Port Hills, southeast of Christchurch City. Boulders rolled into the urban areas below. Cliffs on the lower slopes collapsed and debris avalanches inundated homes built at their bases and undercut homes built at their crests. Boulders and debris avalanches impacted over 200 buildings and killed five people, resulting in widespread evacuations. Large aftershocks caused further boulder rolls and cliff collapses. Before buildings could be reoccupied a life-safety risk assessment was required. This study included pilot investigations of the 19 worst affected areas and these were used to develop hazard and risk models. Once ground verified, the models were extended to the entire Port Hills area. GIS was the main tool used in the development of the models, but other tools and techniques were also utilised.

Slip rate estimates and slip gradient for the Alpine Fault at Calf Paddock, Maruia River, New Zealand

ABSTRACT The dextral–reverse Alpine Fault offsets alluvial terraces of the Maruia River at Calf Paddock. RTK‐GPS surveying of the faulted terraces yields detailed measurements of fault slip across terraces T1–T5. Pits were excavated to log the stratigraphy, date the deposits and to thereby constrain fault slip rates for this site. The mean of the four largest dextral offsets across terrace T2 is 12.0 ± 1.3 m. The corresponding mean vertical offset is 1.6 ± 0.6 m. A single charcoal date with an age of 1095–1275 cal yr BP is used to derive minimum dextral and reverse slip rates of 10 ± 2 and 1.3 ± 0.5 mm/yr, respectively. These rates are comparable with others observed nearby at Little and Lost Mary creeks and from previous studies at Calf Paddock. These new late Holocene slip rate estimates highlight the northeastward decrease in strike-slip and dip-slip rate along the northern section of the Alpine Fault as motion is partitioned onto faults of the Marlborough Fault System. The lack of aseismic creep observed across ‘Evison’s wall’ highlights that slip is achieved at Calf Paddock during metre-scale co-seismic displacements.

Rapid Evolution of Subduction-Related Continental Intraarc Rifts: The Taupo Rift, New Zealand

The evolution of the continental intra-arc Taupo Rift in the North Island, New Zealand is rapid, significantly faster than comparative intra-continental rifts such as the African Rifts. Based on our faulting data and published geological, geophysical and borehole data, we show that activity in the ~2 Ma Taupo Rift has rapidly and asymmetrically narrowed via inward and eastward migration of faulting (at rates of ca. 30 km My-1 and 15 km My-1, respectively) and has propagated southwards along its axis ~70 km in 350 kyr. The loci of voluminous volcanic eruptions and active faulting are correlated in time and space, suggesting that a controlling factor in the rapid rift narrowing is the presence of large shallow heterogeneities in the crust, such as large rhyolitic magma bodies generated by subduction processes, which weaken the crust and localize deformation. Eastward migration of faulting also follows the eastward migration of the volcanic arc which may be related to rollback of the Pacific crust slab at the Hikurangi subduction zone. Southward propagation of the rift is linked with southward migration of the Hikurangi plateau/Chatham Rise subduction point and occurs episodically aided by stress changes associated with voluminous local volcanism. The large magma supply during early continental intra-arc rift stages explains faster evolution (from tectonic to magmatic) than intracontinental rifts. However, the fast changes in magma supply from the subduction zone can also lead to evolution reversals (more evolved magmatic stages reverting to less evolved tectonic stages), rift cessation, and thus failed continental break-up.

Coseismic Rupture and Preliminary Slip Estimates for the Papatea Fault and Its Role in the 2016 Mw 7.8 Kaikōura, New Zealand, EarthquakeCoseismic Rupture and Preliminary Slip Estimates for the Papatea Fault

Coseismic rupture of the 19‐km‐long north‐striking and west‐dipping sinistral reverse Papatea fault and nearby structures and uplift/translation of the Papatea block are two of the exceptional components of the 14 November 2016 Mw 7.8 Kaikōura earthquake. The dual‐stranded Papatea fault, comprising main (sinistral reverse) and western (dip‐slip) strands, ruptured onshore and offshore from south of Waipapa Bay to George Stream in the north, bounding the eastern side of the Papatea block. Fault rupture mapping was aided by the acquisition of multibeam bathymetry, light detection and ranging (lidar) topography and other imagery, as well as differential lidar (D‐lidar) from along the coast and Clarence River valley. On land, vertical throw and sinistral offset on the Papatea fault was assessed across an aperture of ±100  m using uncorrected D‐lidar and field data to develop preliminary slip distributions. The maximum up‐to‐the‐west throw on the main strand is ∼9.5±0.5  m⁠, and the mean throw across the Papatea fault is ∼4.5±0.3  m⁠. The maximum sinistral offset, measured near the coast on the main strand, is ∼6.1±0.5  m⁠. From these data, and considering fault dip, we calculate a maximum net slip of 11.5±2  m and an average net slip of 6.4±0.2  m for the Papatea fault surface rupture in 2016. Large sinistral reverse displacement on the Papatea fault is consistent with uplift and southward escape of the Papatea block as observed from Interferometric Synthetic Aperture Radar (InSAR) and optical image correlation datasets. The throw and net slip are exceedingly high for the length of the Papatea fault; such large movements likely only occur during multifault Kaikōura‐type earthquakes that conceivably have recurrence times of ≥5000–12,000  yrs⁠. The role of the Papatea fault in the Kaikōura earthquake has significant implications for characterizing complex fault sources in seismic hazard models.

The Kerepehi Fault, Hauraki Rift, North Island, New Zealand: active fault characterisation and hazard

ABSTRACT The Kerepehi Fault is an active normal fault with a total onshore length of up to 80 km comprising six geometric/rupture segments, with four more offshore segments to the north. For the last 20 ± 2.5 ka the slip rate has been 0.08–0.4 mm a–1. Average fault rupture recurrence intervals are 5 ka or less on the central segments and 10 ka or more on low slip rate segments to the north and south. Characteristic earthquakes for a single segment rupture range from Mw 5.5 to 7.0, and up to Mw 7.2 or 7.4 in the unlikely event of rupture of all the onshore fault segments. Fault rupture would result in damage to unreinforced masonry buildings, chimneys and parapets in Auckland (45 km nearest distant). Very severe damage to buildings in towns within the Hauraki Plains without specific seismic design (those built before 1960) may pose a significant risk to life and livelihood.