Paul Denys

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

The Mw 6.2 Christchurch earthquake of February 2011: preliminary report

Abstract A moment magnitude (Mw) 6.2 earthquake struck beneath the outer suburbs of Christchurch, New Zealands second largest city, on 22 February 2011 local time. The Christchurch earthquake was the deadliest in New Zealand since the 1931 Mw 7.8 Hawkes Bay earthquake and the most expensive in New Zealands recorded history. The effects of the earthquake on the regions population and infrastructure were severe including 181 fatalities, widespread building damage, liquefaction and landslides. The Christchurch earthquake was an aftershock of the Mw 7.1 Darfield Earthquake of September 2010, occurring towards the eastern edge of the aftershock zone. This was a low recurrence earthquake for New Zealand and occurred on a fault unrecognised prior to the Darfield event. Geodetic and seismological source models show that oblique-reverse slip occurred along a northeast–southwest-striking fault dipping southeast at c. 69°, with maximum slip at 3–4 km depth. Ground motions during the earthquake were unusually large at near-source distances for an earthquake of its size, registering up to 2.2 g (vertical) and 1.7 g (horizontal) near the epicentre and up to 0.8 g (vertical) and 0.7 g (horizontal) in the city centre. Acceleration response spectra exceeded 2500 yr building design codes and estimates based on standard New Zealand models. The earthquake was associated with high apparent stress indicative of a strong fault. Furthermore, rupture in an updip direction towards Christchurch likely led to strong directivity effects in the city. Site effects including long period amplification and near-surface effects also contributed to the severity of ground motions.

Geodetic estimate of seismic hazard in the Gulf of Korinthos

The recent 15 June 1995, M0 = 6.0 × 1018 N m, Aigion earthquake in the western Gulf of Korinthos has focussed attention on the seismic hazard of the region. Although there have been few large earthquakes in the region during this century, the historical record suggests that there may have been many large earthquakes there in the interval 1750–1900. We present geodetic data that give estimates of the rate of extension of the Gulf of Korinthos during this century and which suggest that less than half of the elastic strain in the central and western Gulf of Korinthos has been released by earthquakes during this century. In contrast, the seismic and geodetic strains in the eastern Gulf of Korinthos are in agreement with each other. If the discrepancy between seismic and geodetic strains in the western Gulf of Korinthos that has accumulated during this century is removed in earthquakes, the moment release will be equivalent to several Ms > 6.5 earthquakes.

Kinematic Constraints From GPS on Oblique Convergence of the Pacific and Australian Plates, Central South Island, New Zealand

GPS observations since 1992 provide estimates of horizontal motion (surface velocity) at the 1 mm/yr level, or better, over much of New Zealands South Island and between the interiors of the neighboring Australian and Pacific plates. In a few cases vertical velocities have been estimated with ∼1 mm/yr precision. Australia-Pacific Euler vectors from GPS are mildly inconsistent with those from longer-term (> 1 Myr) estimates, and predict ∼3 mm/yr faster deformation integrated across the plate boundary. We review the kinematic models that have been proposed to fit surface velocity data in central South Island. We extend the rotating, interacting, elastic block model of Wallace et al. [2006] to show that the central Alpine Fault is not presently slipping significantly between the surface and ∼12 km depth, consistent with the observed distribution of moderate seismicity and with occasional very large earthquakes on the Alpine Fault itself. We show that the block boundary faults in the central Southern Alps can be chosen to correlate spatially with high-conductivity zones inferred from magnetotelluric studies. We review numerical modeling studies which show that observed surface velocities can be used to constrain the width of present-day deformation in the lower crust to ∼30 km, and in the upper mantle to ∼100 km. The GPS data are consistent with the doubly-vergent nature of deformation in many of these numerical models. We briefly review models that combine the long-term evolution of collision-related deformation and elastic seismic-cycle behavior; these provide insight into the relative contributions of elastic and inelastic deformation during the seismic cycle.

New Zealand GPS velocity field: 1995–2013

ABSTRACT We collate nearly two decades of campaign GPS data gathered at over 900 sites throughout New Zealand to release a New Zealand nationwide GPS velocity field. The data span the entire North and South islands of New Zealand with a typical spacing of 10–20 km and a denser network (c. 2–8 km spacing) in the Wellington region, central Taupo Volcanic Zone and parts of the Arthurs Pass area. The dataset provides the most comprehensive-to-date view of crustal deformation within the Australia–Pacific plate boundary zone in the New Zealand region. We discuss the data acquisition, processing and derivation of the velocities and uncertainties. We also undertake corrections for earthquake displacements to obtain a velocity field that is largely representative of interseismic deformation between 1995 and 2013.

Geodetic constraints on the kinematics of the Alpine Fault in the southern South Island of New Zealand, using results from the Hawea‐Haast GPS Transect

Repeated GPS measurements at 21 sites along a profile extending 100 km southeast of the Alpine Fault demonstrate that active deformation occurs across much of the South Island of New Zealand and that slip on the Alpine Fault cannot accommodate all of the Pacific-Australian plate motion. Projecting the velocity field as normal and parallel components to the Alpine Fault, shows that the fault-parallel component of motion falls on a characteristic dislocation curve that suggests elastic strain accumulation in the upper crust. This type of deformation is associated with deep aseismic slip in the ductile regions below the brittle-ductile transition. The derived slip rate, of greater than 20 mm/yr, is in reasonable agreement with geological observations. The best fitting locking depth is about 10 km. There is also evidence that approximately 25% of the relative plate motion is accommodated either by distributed deformation or by slip on a second structure located within the Southern Alps, approximately 80 km east of the Alpine Fault.

Transponder altimetry: Precise height measurements over land

A novel experiment, coordinated by the University of Newcastle and Rutherford Appleton Laboratory, was conducted to demonstrate the feasibility of measuring relative height differences using transponder altimetry. This technique used the radar altimeter onboard the first European Remote-sensing Satellite, ERS-1, in conjunction with active ground-based microwave transponders. Two transponders were placed 127 km apart, one in the foothills close to Venice and the second, 1000 m higher in the Austrian Alps. The sites were chosen to coincide with the ground track of ERS-1 and also to be within the altimeter calibration zone which provided reliable ERS-1 orbits. Coincident observations at both transponder sites were made for a total of five overpasses. These measurements were subsequently compared with a height difference determined using the Global Positioning System (GPS). The experiment has estimated the precision of the transponder derived height difference by differencing the observed and computed transponder ranges at both transponder sites. The precision of one transponder-derived height difference, based on four of the recorded passes, is estimated as ±3.5 cm and the standard deviation of the mean is ±1.7 cm. This implies that the standard deviation of one transponder range at a single site is ±2.5 cm. The precision of the height difference derived from the GPS network was ±3.1 cm and thus the ground-based transponders have performed, as a height difference measurement system, to a comparable accuracy to that of GPS. Ground-based transponders do, however, have the significant practical advantage of being able to be operated in a simpler, automatic, and possibly more economical mode of operation. Data collection takes place onboard the space vehicle and power requirements can be made such that the equipment could be left unattended for many months. This makes the equipment especially suitable for hostile environments, for example, measuring the vertical precursory motion of volcanoes and for situations where long-period observations are needed such as the monitoring of subsidence or sea level changes.

A Geodetic Study of the Otago Fault System of the South Island of New Zealand

The Otago Fault System in the southern South Island of New Zealand is an area dominated by actively growing asymmetric anticlines above buried reverse faults. Geological studies indicate that the average total shortening rate across the system is 2–3 mm/year. This region was the subject of several early geodetic studies based on terrestrial techniques, which reported high (0.5 ppm/year) strain rates that are difficult to reconcile with geological estimates of fault slip rates.

A geodetic study of Otago: results of the central Otago deformation network 2004–2014

ABSTRACT We have analysed 11 years of geodetic data from 30 stations distributed over the Otago Fault System in the South Island of New Zealand. Velocities were estimated from time series corrected for coseismic displacements from the 2004 Macquarie Island and 2007 George Sound earthquakes and the coseismic and the short term postseismic deformation associated with the 2009 Dusky Sound earthquake. By dividing the corrected time series in half we were able to demonstrate the existence of a systematic difference between the pre- and post-earthquake velocity fields, associated with a longer term viscoelastic transient related to the 2009 Dusky Sound earthquake. In the northern part of our study area, the geodetic strain rate data are consistent with elastic strain accumulation on the Alpine Fault while in the south and east, the strain rate tensors are consistent with the Otago Fault System. There is a significant change in orientation in the axis of contraction from east to west across the network that correlates with a transition between the Otago and Waihemo Fault Systems. We also demonstrate significant spatial variation in the rates of strain accumulation that may correlate with active and quiescent parts of the Otago Fault System. However these strain rates represent the average values for the 11 years that the COD network has been observed and may also be influenced by the longer term viscoelastic transient related to the Dusky Sound earthquake.

Implementing Localised Deformation Models into a Semi-Dynamic Datum

The New Zealand Geodetic Datum 2000 (NZGD2000) has an associated deformation model that allows horizontal coordinates to be reduced to a reference epoch of 2000.0 and is therefore a semi-dynamic datum. Currently, the deformation model does not account for either discrete or continuous horizontal deformation that can be associated with localised events such as earthquakes and landslides. Such events will distort the geodetic network by as little as a few millimetres a year, or as much as several metres in a matter of seconds.