Russ Van Dissen

National Seismic Hazard Model for New Zealand: 2010 Update

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

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

Surface rupture earthquakes over the last ∼1000 years in the Wellington region, New Zealand, and implications for ground shaking hazard

The Wellington region is cut by five active right-lateral strike-slip faults : Wairarapa, Wellington, Ohariu, Shepherds Gully/Pukerua, and Wairau faults that have average recurrence intervals of meter-scale surface rupture that range from ∼500 years to 5000 years, and lateral slip rates that range from 1 to 10 mm/yr. Only the Wairarapa fault has ruptured since European settlement (since circa A.D. 1840). Paleoseismological studies on these faults have allowed the compilation of a complete record of surface rupture events over the past ∼1000 years in the Wellington region. Within this time period, there does not appear to be any temporal clustering of surface rupture events on adjacent faults. The M 8 A.D. 1855 Wairarapa earthquake did not trigger rupture on any other fault in the region. The most recent surface-faulting event on the Wellington fault (290-440 cal years B.P.) (cal years are calendar years before A.D. 1950) does not coincide with rupture of any other onland fault, and over 300 years separate the timing of the second most recent rupture on the Wellington fault (660-720 cal years B.P.) and the most recent rupture of the Ohariu fault (1060-1140 cal years B.P.). The most recent rupture of the Shepherds Gully/Pukerua fault is probably older than that of the Ohariu fault. The apparent nonclustering of surface rupture earthquakes in the Wellington region has been documented only for the on-land strike-slip faults. There are other possible seismogenic sources in the region, and thus important issues remain to be addressed regarding the history of large earthquakes in the Wellington region : (1) the seismogenic potential and earthquake recurrence interval of the subduction thrust beneath Wellington is not known ; (2) the timing of rupture events on the offshore portion of the Wairau fault is not known ; and (3) paleoseismic data are not available for the section of the Wellington fault north of the Wellington-Hutt Valley segment. Estimates of earthquake hazard in the Wellington region, for all return times greater than 50 years, that incorporate paleoseismicity data are between one and two Modified Mercalli (MM) intensity units higher than the hazard based solely on the historical seismicity catalog, and the hazard is spatially more variable. Using a deterministic attenuation model, the level of shaking hazard approaches near maximum values within a return time of ∼500 years, largely reflecting the recurrence interval (500-770 years) of surface rupture earthquakes on the Wellington fault. Inclusion of a plausible model for magnitude 8 subduction zone earthquakes does not affect the level of MM intensity in Wellington region at return times greater than 500 years but does make a small contribution to the hazard at return times between 50 and 500 years.

Ground Motion and Seismic Source Aspects of the Canterbury Earthquake Sequence

This paper provides an overview of the ground motion and seismic source aspects of the Canterbury earthquake sequence. Common reported attributes among the largest earthquakes in this sequence are complex ruptures, large displacements per unit fault length, and high stress drops. The Darfield earthquake produced an approximately 30 km surface rupture in the Canterbury Plains with dextral surface displacements of several meters, and a subordinate amount of vertical displacement, impacting residential structures, agricultural land, and river channels. The dense set of strong ground motions recorded in the near-source region of all the major events in the sequence provides significant insight into the spatial variability in ground motion characteristics, as well as the significance of directivity, basin-generated surface waves, and nonlinear local site effects. The ground motion amplitudes in the 22 February 2011 earthquake, in particular, produced horizontal ground motion amplitudes in the Central Business District (CBD) well above those specified for the design of conventional structures.

Tilting of active folds and faults in the Manawatu region, New Zealand: Evidence from surface drainage patterns

Abstract We examine the drainage system on four anticlinal ridges in Manawatu that affect a mid‐Quaternary (c. 300 000 yr old) marine horizon. The folds are all located above buried, west‐dipping, reverse faults in the basement that are c. 15–20 km long and capable of generating earthquakes of c. MW 6.5–7.0. The drainage systems allow us to distinguish a regional tectonic tilt from the normal plunge of an anticline axis towards its end. We estimate tilt rates of around 4 × 10‐8 rad/yr towards the south averaged over the last c. 300 000 yr. The regional tilting is related to the development and southward migration of the Pliocene‐Pleistocene depocentre in the offshore South Wanganui Basin.

Late Pleistocene surface rupture history of the Paeroa Fault, Taupo Rift, New Zealand

Abstract The 30 km long Paeroa Fault is one of the largest and fastest slipping (c. 1.5 mm/yr vertical displacement rate) normal faults of the currently active Taupo Rift of North Island, New Zealand. Along its northern section, seven trenches excavated across 5 of 11 subparallel fault strands show that successive ruptures of individual strands probably occurred at the same time, but were individually and collectively highly variable in size and recurrence, and most fault strands have ruptured three or four times in the past 16 kyr. In the c. 16 kyrtimeframe, four surface‐rupturing earthquakes took place when Okataina volcano was erupting, and six occurred between eruptions. Large earthquakes on the Paeroa Fault comprise a significant component of the seismic hazard in the region between the Okataina and Taupo Volcanic Centres, and there are partial associations between these large earthquakes and volcanism.

Rates of active faulting during late Quaternary fluvial terrace formation at Saxton River, Awatere fault, New Zealand

A flight of faulted fluvial terraces at Saxton River on the Awatere fault, northeast South Island, New Zealand, preserves the incremental slip history and detailed paleoearthquake chronology of this major strike-slip fault. Here, six fluvial terraces have been progressively displaced across the inland Molesworth section of the fault, with horizontal displacements ranging from -6 m for an ephemeral channel on the youngest terrace to 81 m for the riser above the oldest terrace. New optically stimulated luminescence ages for abandonment of the two oldest terrace treads are 14.5 ± 1.5 and 6.7 ± 0.7 ka. When combined with new measurements of incremental horizontal displacements and previous age data, these new ages indicate that strike-slip on this part of the Awatere fault has been occurring at a near-constant rate of 5.6 ± 0.8 mm/yr since ca. 15 ka. This rate is similar to recent slip-rate estimates for an adjoining section of the same fault to the east, which suggests that | there is near-complete slip transfer across the junction between the two fault strands. Comparison of the magnitudes and ages of the terrace riser displacements to the timing of paleoearthquakes on the Molesworth section allows the mean per event horizontal displacement over the eight most recent surface-rupture events to be estimated at 4.4 ± 0.8 m. Between ca. 5 ka and ca. 2 ka, surface-rupturing earthquakes increased in frequency and decreased in their mean coseismic displacements to <2.6 m. During this time, the sense of local dip slip also shifted from north-side-up to south-side-up. Rapid incision of Saxton River in the mid-Holocene may have caused a perturbation in the near-surface stress directions acting on the fault plane beneath the Saxton River valley, forcing a change in the near-surface fault geometry that resulted in the shift in sense of dip slip.

Fault kinematics and surface deformation across a releasing bend during the 2010 MW 7.1 Darfield, New Zealand, earthquake revealed by differential LiDAR and cadastral surveying

Dextral slip at the western end of the east-west–striking Greendale fault during the 2010 M_W 7.1 Darfield earthquake transferred onto a northwest-trending segment, across an apparent transtensional zone, here named the Waterford releasing bend. We used detailed surface mapping, differential analysis of pre- and postearthquake light detection and ranging (LiDAR), and property boundary (cadastral) resurveying to produce high-resolution (centimeter-scale) estimates of coseismic ground-surface displacements across the Waterford releasing bend. Our results indicate that the change in orientation on the Greendale fault incorporates elements of a large-scale releasing bend (from the viewpoint of westward motion on the south side of the fault) as well as a smaller-scale restraining stepover (from the viewpoint of southeastward motion on the north side of the fault). These factors result in the Waterford releasing bend exhibiting a decrease in displacement to near zero at the change in strike, and the presence within the overall releasing bend of a nested, localized restraining stepover with contractional bulging. The exceptional detail of surface deformation and kinematics obtained from this contemporary surface-rupture event illustrates the value of multimethod investigations. Our data provide insights into strike-slip fault bend kinematics, and into the potentially subtle but important structures that may be present at bends on historic and prehistoric rupture traces.

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.

Late Quaternary movement on the Ohariu Fault, Tongue Point to MacKays Crossing, North Island, New Zealand

Abstract The Ohariu Fault is one of the major active dextral strike‐slip faults in the Wellington region. It extends northeastward from offshore at Tongue Point to Waikanae and beyond, but in the north the fault trace becomes intermittent. Extrapolation of the trend of the Ohariu Fault across Porirua Harbour suggests a 1.5 km right step. South of Porirua Harbour the fault is characterised by lateral displacement of drainage features of up to 450 m, a single‐event lateral displacement of 4–5 m, an average horizontal slip rate of 1–2 mm/yr, and a recurrence interval of 2000–5000 yr. The timing of the last event was between 150 and 1130 cal. BP. North of the harbour the fault is characterised by lateral displacement of drainage features of up to 250 m, a single‐event lateral displacement of 2.9 m, a horizontal slip rate of 0.6–1.9 mm/yr, and a recurrence interval of 1530–4830 yr. The timing of the last event was between 1070 and 2310 cal. BP. Although the possibility exists that Porirua Harbour is a pull‐apa...