L. M. Wallace

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

Simultaneous long‐term and short‐term slow slip events at the Hikurangi subduction margin, New Zealand: Implications for processes that control slow slip event occurrence, duration, and migration

[1] We document a sequence of simultaneous short-term and long-term slow slip events (SSEs) at the Hikurangi subduction zone during the 2010/2011 period. The sequence of short-term events (each 2–3 weeks in duration) ruptured much of the shallow plate interface (<15 km) at central and northern Hikurangi over a 6-month period, was accompanied by microseismicity and involved patchy, irregular migration of SSE slip. We suggest that the patchy migration of the short-term SSE is due to large-scale ( 100–3500 km) heterogeneities on the plate interface related to seamount subduction and sediment subduction and/or underplating. This is in contrast to a 2010/2011 long-term SSE at the central Hikurangi margin, which evolved steadily over 1.5 years and ruptured much of the plate interface between 20 and 70 km depth. We suggest that the occurrence of long-term versus short-term SSEs at Hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface. The long-term SSE sequence began 1 year before the short-term sequence. Coulomb stress change models suggest that the long-term SSE may have triggered initiation of the subsequent short-term SSE sequence. Initiation of the short-term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior. Alternatively, it could be characteristic of a region undergoing partial interseismic coupling. This is in contrast to SSEs observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior.

Slow slip near the trench at the Hikurangi subduction zone, New Zealand

Applying pressure to plate tectonics The full range of deformation behavior of subduction zone faults that are responsible for great earthquakes and tsunamis is now clearer. Wallace et al. observed the heave of the ocean floor near the Hikurangi trench, off the east coast of New Zealand, with a network of absolute pressure gauges (see the Perspective by Tréhu). The gauges sit on the ocean floor and detect changes in pressure generated from slow-slip deformation events. Detailed geodetic observation of deformation events will finally clarify the role that such aseismic events play at major plate boundaries. Science, this issue p. 701; see also p. 654 Absolute pressure gauges detect a slow-slip event near the Hikurangi trench. The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world’s largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist.

Time‐dependent modeling of slow slip events and associated seismicity and tremor at the Hikurangi subduction zone, New Zealand

We present a time-dependent slip model of 12 slow slip events (SSEs) occurring in the Hikurangi margin of New Zealand during 2010 and 2011. This model is obtained by inverting daily GPS solutions from GeoNets continuous GPS network on the North Island and northern South Island. We compare the properties of these SSEs to observations in Japan, Cascadia, and Mexico and find that Hikurangi SSEs have comparatively large amounts of slip (up to 27 cm), high slip rates (up to 1.4 cm/d), and a large range of depths (10–40 km), durations (7–270 days), and sizes (Mw 5.9–6.9). We further investigate the relationship between the Cape Turnagain SSE and an associated seismic swarm and find that observations are consistent with stress triggering outside the slowly slipping region; however, other explanations cannot be ruled out. We also compare slip during the long-term Manawatu SSE with the tremor epicenters found by Ide (2012) and note that tremor locations are offset in the downdip direction relative to the slipping region, similar to observations in the Bungo Channel of Japan and Guerrero, Mexico.

Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust

Eight slow-slip events over 6 years accommodated up to 50% of the fault slip on the Nankai megathrust. Silently taking up the slack Megathrust earthquakes occur when locked subduction zone faults suddenly slip, unleashing shaking and causing tsunamis. However, seismically silent slow earthquakes also relieve slip on these dangerous faults. Araki et al. present data from ocean boreholes with which they analyze eight slow-slip events near the Nankai trench off the coast of Japan. These events accommodated up to half of the plate convergence over 6 years. The events appear to occur regularly, which has a long-term impact on hazard assessment for the region. Science, this issue p. 1157 The discovery of slow earthquakes has revolutionized the field of earthquake seismology. Defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults. We report on a family of recurring slow-slip events (SSEs) on the plate interface immediately seaward of repeated historical moment magnitude (Mw) 8 earthquake rupture areas offshore of Japan. The SSEs continue for days to several weeks, include both spontaneous and triggered slip, recur every 8 to 15 months, and are accompanied by swarms of low-frequency tremors. We can explain the SSEs with 1 to 4 centimeters of slip along the megathrust, centered 25 to 35 kilometers (km) from the trench (4 to 10 km depth). The SSEs accommodate 30 to 55% of the plate motion, indicating frequent release of accumulated strain near the trench.

Quake clamps down on slow slip

Using continuous GPS (cGPS) data from the Hikurangi subduction zone in New Zealand, we show for the first time that stress changes induced by a local earthquake can arrest an ongoing slow slip event (SSE). The cGPS data show that the slip rate in the northern portion of the 2013/2014 Kapiti SSE decreased abruptly following a nearby intraslab earthquake. We suggest that deceleration of the Kapiti SSE in early 2014 occurred due to a tenfold increase in the normal stress relative to shear stress in the SSE source, induced by the nearby Mw 6.3 earthquake, consistent with expectations of rate and state friction. Our observation of an abrupt halting/slowing of the SSE in response to stress changes imposed by a local earthquake has implications for the strength of fault zones hosting SSEs and supports the premise that static stress changes are an important ingredient in triggering (or delaying) fault slip.

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.

Crustal deformation and stress transfer during a propagating earthquake sequence: The 2013 Cook Strait sequence, central New Zealand

The 2013 Cook Strait earthquake sequence began on 18 July 2013 with two foreshocks of Mw 5.7 and Mw 5.8 and culminated in the Mw 6.6 Cook Strait and Lake Grassmere events on 21 July and 16 August, respectively. Located ∼50 km south of New Zealands capital, Wellington, the earthquakes generated the most significant ground shaking in the Wellington and Marlborough regions in recent decades. During the first event, located under Cook Strait, continuously recording GPS instruments across central New Zealand recorded up to 5 cm of horizontal displacement. Modeling suggests that the rupture was 25 km long with up to 90 cm of dextral strike slip. The second event, located 20 km to the southwest, caused displacements of up to 25 cm at GPS sites located around the Clifford bay area. In addition, two interferograms from RADARSAT-2 and TerraSAR-X showed up to 30 cm of line-of-sight displacement in the vicinity of Lake Grassmere. Modeling indicates predominantly dextral strike slip of up to 2.1 m. Coulomb Stress changes induced by the earlier foreshocks suggest that the Cook Strait event was triggered by the preceding events and that the Lake Grassmere event was subsequently triggered by the Cook Strait earthquake.

Near‐field observations of an offshore Mw 6.0 earthquake from an integrated seafloor and subseafloor monitoring network at the Nankai Trough, southwest Japan

An Mw 6.0 earthquake struck ~50 km offshore the Kii Peninsula of southwest Honshu, Japan on 1 April 2016. This earthquake occurred directly beneath a cabled offshore monitoring network at the Nankai Trough subduction zone and within 25–35 km of two borehole observatories installed as part of the International Ocean Discovery Programs NanTroSEIZE project. The earthquakes location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate-sized offshore earthquake. We use the offshore seismic network to locate the main shock and aftershocks, seafloor pressure sensors, and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation, and seafloor pressure observations to model the resulting tsunami. Contractional strain estimated from formation pore pressure records in the borehole observatories (equivalent to 0.37 to 0.15 μstrain) provides a key to narrowing the possible range of fault plane solutions. Together, these data show that the rupture occurred on a landward dipping thrust fault at 9–10 km below the seafloor, most likely on the plate interface. Pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock. The earthquake and its aftershocks are located within the coseismic slip region of the 1944 Tonankai earthquake (Mw ~8.0), and immediately downdip of swarms of very low frequency earthquakes in this region, illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone.

Effects of material property variations on slip estimates for subduction interface slow-slip events

We investigate the influence of elastic heterogeneity on geodetic inversions of slow-slip events by inverting for slip distributions of four events along the Hikurangi Margin, New Zealand. We generate Greens functions using a finite element code in conjunction with a New Zealand-wide seismic velocity model to assign elastic properties. We find that these heterogeneous models typically require ~20% less slip than homogeneous models in cases where the slip is deep or there is reasonable geodetic coverage above the slipping region. In cases where the slip is shallow (and mostly offshore) and there is little geodetic coverage directly above the slipping region, the heterogeneous models can predict significantly larger amounts of slip (42% in our study). These changes in the predicted amounts of slip have important implications for quantifying slip budgets accommodated by slow slip at subduction zones worldwide.