Hikurangi Subduction Zone Research Articles

The Ocean's Impact on Slow Slip Events

AbstractWe test the hypothesis that ocean seafloor pressures impart stresses that alter the initiation or termination of transient slow slip events (SSEs) on shallow submarine and near‐coastal faults, using simulated seafloor pressures and a new catalog of SSEs in the Hikurangi subduction zone. We show that seafloor pressures may be represented by an average time history over the ~100‐km dimensions of the study area. We account for SSE uncertainties and the multiplicity of processes that affect SSEs statistically by estimating the probabilities of rejecting the null hypothesis that SSE initiation or termination pressures are those to be expected by chance sampling of known (modeled) seafloor pressures, with low probabilities indicating some causal connection. No impact of ocean pressure changes on SSE initiation is detectable, but a correlation with their terminations is suggested. SSE slip that weakens the fault and makes it more sensitive to small stress changes may explain results.

Sea Surface Gravity Waves Excited by Dynamic Ground Motions from Large Regional Earthquakes

Abstract Infragravity waves on the sea surface near coastlines are occasionally excited by static displacement caused by large local earthquakes and recorded as tsunamis. However, tsunamis induced by ground motions from seismic waves are rarely observed, especially far from earthquake focal areas. We investigated seafloor pressure variations in the infragravity band at the Hikurangi subduction zone following the M 7.8 Kaikōura and M 7.1 Te Araroa earthquakes. Anomalous infragravity waves were observed at 0.2–20 mHz at sites overlying a low-velocity accretionary wedge offshore of the east coast of New Zealand’s North Island accompanying the Rayleigh-wave arrivals. The maximum amplitude of these ultra-low-frequency waves was similar to the tsunami that propagated from the earthquake focal area hours later. The amplitude of the pressure signal from these waves observed offshore varied inversely with water depth, suggesting that sea surface gravity waves were excited by Rayleigh or Love waves amplified within the accretionary wedge.

Slow Slip Events in New Zealand

Continuously operating global positioning system sites in the North Island of New Zealand have revealed a diverse range of slow motion earthquakes on the Hikurangi subduction zone. These slow slip events (SSEs) exhibit diverse characteristics, from shallow (<15 km), short (<1 month), frequent (every 1–2 years) events in the northern part of the subduction zone to deep (>30 km), long (>1 year), less frequent (approximately every 5 years) SSEs in the southern part of the subduction zone. Hikurangi SSEs show intriguing relationships to interseismic coupling, seismicity, and tectonic tremor, and they exhibit a diversity of interactions with large, regional earthquakes. Due to the marked along-strike variations in Hikurangi SSE characteristics, which coincide with changes in physical characteristics of the subduction margin, the Hikurangi subduction zone presents a globally unique natural laboratory to resolve outstanding questions regarding the origin of episodic, slow fault slip behavior. ▪ New Zealand's Hikurangi subduction zone hosts slow slip events with a diverse range of depth, size, duration, and recurrence characteristics. ▪ Hikurangi slow slip events show intriguing relationships with seismicity ranging from small earthquakes and tremor to larger earthquakes. ▪ Slow slip events play a major role in the accommodation of plate motion at the Hikurangi subduction zone. ▪ Many aspects of the Hikurangi subduction zone make it an ideal natural laboratory to resolve the physical processes controlling slow slip.

Estimation of stress state and detailed structure at shallow plate boundary based on 3D seismic data

The Earth's crust is a dynamic place, consisting of the large tectonic plates that sit atop the mantle, the extremely hot, high pressure zone of material between the crust and the core. The first several tens kilometres of the Earth's shell, the crust and uppermost mantle, is moving and shifting, causing the oceanic and continental plates to bang and crash with one another, resulting in the earthquakes and tsunamis that dramatically impact the countries at the boundaries of the plates. While the world takes notice when one of these disasters happen, we don't realise that there is constant activity in these zones causing measurable seismic waves to regularly ripple through the crust. Using artificially-generated seismic waves can help researchers to understand more about how the crust and upper mantle behave. Depending on the positioning of the plates and the nature of the crust, i.e., thickness, direction of movement and composition, the behaviour will differ. By using 'controlled sources' in marine seismic experiments, such as air-gun shooting, scientists are able to gather as much data as possible regarding the factors influencing the behaviours of the plates to help provide a better understanding of the processes leading to earthquakes.One organisation that builds and operates via international collaboration is the Japan Agency For Marine-Earth Science and Technology (JAMSTEC). Dr Ryuta Arai joined the Structural Seismology Group of JAMSTEC in 2014 and is now a scientist at the Research Institute for Marine Geodynamics of JAMSTEC. Arai is part of a team of researchers from Japan, New Zealand, the United States and the United Kingdom who are investigating the physical properties of the seismogenic area in the Hikurangi subduction zone.

Hydration of the crust and upper mantle of the Hikurangi Plateau as it subducts at the southern Hikurangi margin

Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (Vp) in the upper mantle under oceanic plateaus. Onshore-offshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (Vp≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (Vp≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle Vp≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered Vp in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in Vp is ≈10%, and a similar drop in Vp is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bending-induced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the “regular mantle” (Vp≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle Vp under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle Vp under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting.

Slow slip source characterized by lithological and geometric heterogeneity.

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

Observations of Laboratory and Natural Slow Slip Events: Hikurangi Subduction Zone, New Zealand

AbstractSlow slip events (SSEs) are recognized as an important component of plate boundary fault slip, and there is a need for laboratory friction data on natural samples to guide comparisons with natural SSEs. Here, we compile a comprehensive catalog of SSEs observed geodetically at the Hikurangi subduction zone offshore northern New Zealand, and compare it with results of laboratory friction experiments that produce laboratory SSEs under plate tectonic driving rates (5 cm/yr). We use samples from Ocean Drilling Program Site 1124 seaward of the Hikurangi subduction zone to represent the plate boundary that hosts shallow SSEs at Hikurangi. We find that laboratory SSEs exhibit a similar displacement record and range of stress drops as the natural SSEs. Results of velocity step tests, which can be used to evaluate frictional instability based on the critical stiffness criterion, indicate that the slow slip activity at Hikurangi is a form of stably‐accelerating slip. Our laboratory SSEs provide an alternative method of quantifying (in)stability by direct measurement of the unloading stiffness during the stress drop. The observed dependence of laboratory SSE parameters on effective normal stress is consistent with critical stiffness theory; however, depth‐increasing projections based on laboratory data do not match observations from natural SSEs. These differences are likely related to changing temperature and fault rock composition downdip but also complications related to scaling and/or limited sampling. Scientific drilling recently undertaken at the Hikurangi subduction zone should serve to improve and guide future studies of the role of frictional properties for the occurrence of SSEs.

Temporal and spatial variations in seismic anisotropy and VP/VS ratios in a region of slow slip

In September 2014, a five week long slow slip event (SSE) occurred near Gisborne at the northern Hikurangi subduction zone, New Zealand, and was recorded by offshore instruments deployed by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. Up to 25 cm of slip occurred directly below the HOBITSS array. We calculate shear wave splitting (SWS) and VP/VS ratios for event-station pairs on HOBITSS ocean bottom seismometers and onshore GeoNet seismic stations to determine the relationship in time and space between slow slip and these seismic properties. Spatial averaging of SWS fast azimuths yields trench-perpendicular fast azimuths in some areas, suggesting that compressive stress from plate convergence closes microcracks and controls anisotropy in the upper-plate. Variations from the trench perpendicular directions are observed near a subducting seamount, with directions closely resembling fracture and fault patterns created by subducting seamounts previously observed in both laboratory and field experiments. Temporal variations in fast azimuths are observed at three stations, two of which are located above the seamount, suggesting measurable variations in stress orientations. During the SSE, median VP/VS measurements across all offshore stations increase from 1.817 to 1.894 and SWS delay times decrease from 0.178 s to 0.139 s (both changes are significant within 95% confidence intervals). Temporal variations in VP/VS and delay time are consistent with fluid pressurization below a permeability barrier and movement of fluids during the rupture of a slow-slip patch.

Three‐Dimensional Modeling of Spontaneous and Triggered Slow‐Slip Events at the Hikurangi Subduction Zone, New Zealand

AbstractAt the southern Hikurangi margin, New Zealand, long‐term (duration of &gt;1 year), deep (&gt;25 km) slow‐slip events (SSEs) occur at intervals of approximately 5 years. In contrast, along the northern and central Hikurangi margin, short‐term and shallow (&lt;10 km) SSEs (2‐ to 3‐week duration) occur at 1‐ to 2‐year intervals. However, it is not clear what controls these differences in SSE behavior. In this study, we simulate SSEs along the entire Hikurangi subduction zone using a 3‐D model of the plate boundary incorporating a rate‐and‐state friction law with a cutoff velocity to the evolution effect. Overall, our models are able to replicate the along‐strike and depth‐dependent variations in SSE behavior at the Hikurangi margin. Our models also show that the slip velocities of the deep, long‐term SSEs increase when the slip velocities at the deeper extension of the seismogenic zone increase during the latter half of the interseismic period. By imposing static stress changes induced by the Kaikōura earthquake in the SSE regions, we also reproduce patterns of triggering of deep, long‐term SSEs that followed the earthquake. Our results further suggest that the propensity for SSE triggering via static stress transfer depends on the amplitude of perturbation and the timing with respect to the SSE cycle. Overall, both spontaneous and triggered SSEs require larger values of effective stress for deep, long‐term events (~5 MPa) compared to the shallow, short‐term events (~1 MPa). Such low effective stresses can be explained by the existence of high pore fluid pressure in these SSE source regions.

Time‐Dependent Behavior of a Near‐Trench Slow‐Slip Event at the Hikurangi Subduction Zone

AbstractStudying offshore slow‐slip events (SSEs) along subduction zone interfaces is important for constraining the overall slip budget and potential for seismic slip and the relationship with large megathrust earthquakes. Models using only onshore data increasingly lack model resolution the further from the shore the SSE occurs. In this study, we combine data from the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip seafloor absolute pressure gauge (APG) network with daily position time series from New Zealand's GeoNet to create time‐dependent models of slip during the 2014 Gisborne, New Zealand SSE using the Network Inversion Filter. We compare models assuming heterogeneous versus homogenous elastic properties to explore their influence on our models. The time‐dependent results show that slip uncertainties under the APGs drop by about 23%. We also find that the peak value of slip increases with heterogeneous elastic properties as compared to homogenous models. The inclusion of the offshore APG data in our models places more slip near the trench and detects a more defined migration of slip, especially in the heterogeneous model. These differences are important for interpreting the relationship between the SSE and associated tremor, which occurs after the peak SSE slip rate. Additionally, we use a static “potency bounding” technique in order to gauge the range of models that can fit the data. This analysis demonstrates that the inclusion of offshore data helps to substantially narrow the range of plausible slip models.

Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

AbstractThe northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid pressures and conditionally stable frictional conditions. Using 2‐D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2‐D P wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the prestack depth‐migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of slow‐slip events to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intrawedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest that they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from International Ocean Discovery Program drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3‐D using a recently acquired, longer‐offset, seismic data set.

Episodic stress and fluid pressure cycling in subducting oceanic crust during slow slip

Slow slip events are part of a spectrum of aseismic processes that relieve tectonic stress on faults. Their spatial distribution in subduction zones has been linked to perturbations in fluid pressure within the megathrust shear zone and subducting oceanic crust. However, physical observations of temporal fluid pressure fluctuations through slow slip cycles remain elusive. Here, we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to show that crustal stresses and fluid pressures within subducting oceanic crust evolve before and during slow slip events. Specifically, we observe that the retrieved stress ratio, which describes the relative magnitudes of the principal compressive stresses, systematically decreases before slow slip events in New Zealand’s northern Hikurangi subduction zone, and subsequently increases during the evolution of each slow slip event. We propose that these changes represent the accumulation and release of fluid pressure within overpressured subducting oceanic crust, the episodicity of which may influence the timing of slow slip event occurrence on subduction megathrusts. This work contributes an improved understanding of the physical driving forces underlying slow subduction earthquakes, and a potential means by which to monitor stress and fluid pressure accumulation in such regions. Stress cycling in subducting crust before and during slow slip events is due to accumulation and release of fluid pressure, according to analysis of small earthquakes in the Hikurangi subduction zone.

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

In order to trace the cycling of fluid-mobile elements (FMEs) through subduction zone forearcs, we collected water samples from two warm and 16 cold springs along the subaerially exposed forearc of the Hikurangi subduction zone in New Zealand. Water samples were analyzed for their cation and anion concentrations, as well as their B, Li, Cl, and O stable isotope compositions. Fluids discharging through the prism have high concentrations of Cl (2400–16,000 mg/L), Br (6–70 mg/L), I (0.4–72 mg/L), Sr (0.1–200 mg/L), B (3–130 mg/L), Li (0.1–13 mg/L), and Na (33–6600 mg/L), consistent with data from previous studies. Most of these elements decrease overall in concentration from north to south, have a concentration peak in the central part of the margin, and have had limited concentration variability during the last three decades. Because Li, Cl, and B are all fluid-mobile elements, their incompatibility potentially limits modification by fluid-rock interaction, making them reliable tracers of fluid source. δ37Cl, δ11B, and δ7Li values range from −1.3‰ to +0.4‰ (n = 36), +11.8‰ to +41.9‰ (n = 25), and −3.1‰ to +29.0‰ (n = 29), respectively. Despite the change in concentrations along the margin, there is no corresponding trend in isotopic composition. Chlorine and boron isotope compositions are consistent with fluids dominated by seawater (δ37Cl = 0‰; δ11B = 40‰) and sedimentary pore fluids (δ37Cl ≈ −8‰ to 0‰; δ11B > ∼17‰). Br/Cl (0.0025–0.005) and I/Cl (0.00005–0.007) weight ratios also support a dominant seawater and pore-fluid source. Lithium isotope data also suggest fluids sourced from seawater (δ7Li = +31‰) as well as dehydrating sediments and/or modified by interaction with local sediment. The fluid geochemical data cannot be explained by a change in the fluid source along the margin, but rather by a change in the upper-plate structural permeability. In the north, extension likely results in a highly permeable forearc, whereas transpression in the south traps fluids within the upper plate and dilutes saline fluids with groundwater. Such changes in upper-plate structural permeability may influence fluid pressure conditions within the forearc, which in turn may influence the observed change in slip behavior on the interface from north Hikurangi (aseismic creep) to south Hikurangi (deep locking). This work highlights the role the upper plate may play in the geochemical modification and transport of slab-derived fluids, and supports the long-standing but poorly documented assumption that seawater and pore fluids are expelled at shallow levels in the subduction zone (

Subducted seamount diverts shallow slow slip to the forearc of the northern Hikurangi subduction zone, New Zealand

Fluid migration and pore fluid pressure have been implicated in generating the transitional fault zone properties thought to be important for slow slip events (SSEs) in subduction zones. At the northern Hikurangi margin, New Zealand, overpressurized sediments spatially correlated with shallow SSEs are imaged downdip of subducted seamounts, providing an excellent environment to study the relationship between subducted oceanic relief, fluids, shallow SSEs, and related microseismicity. Using data from ocean-bottom seismometers, we detect “burst-type” repeating earthquakes coincident with tremor on an upper-plate fracture network above a subducted seamount. This activity occurred at the edge of a large, shallow SSE recorded offshore of Gisborne, New Zealand, in September–October 2014, but began days to weeks after the SSE and continued for nearly two months. We propose that during the large plate-boundary SSE, fluids migrated from the downdip overpressurized sediments into the fracture network, diverting aseismic slip to multiple faults in the upper plate. Thus, seamount subduction appears to play a key role in controlling the mechanics of shallow slow slip and microseismicity at the northern Hikurangi margin.

Temperature-dependent frictional properties of heterogeneous Hikurangi Subduction Zone input sediments, ODP Site 1124

The Hikurangi Subduction Zone (HSZ), New Zealand, accommodates westward subduction of the Pacific Plate. Where imaged seismically, the shallow HSZ décollement (<10–15km depth) occurs within or along the upper contact of Late Cretaceous-Paleogene (70–32 million-year-old) sediments. The frictional properties of Paleogene sediments recovered from Ocean Drilling Program Leg 181, Site 1124 were measured at 60MPa effective normal stress and varying sliding velocities (V=0.3–30μm/s) and temperatures (T=25–225°C). Velocity-stepping experiments were conducted at temperatures of 25°C, 75°C, 150°C, and 225°C to determine the friction rate parameter (a–b). Paleocene and Oligocene clay-bearing nannofossil chalks (μ=0.45–0.61) and a middle Eocene clayey nannofossil chalk (μ=0.35–0.51) are frictionally stronger than smectite-bearing Eocene clays (μ=0.16–0.31). With increasing temperature, chalks show rate-strengthening to rate-weakening frictional stability trends; clays show rate-weakening and rate-neutral to rate-strengthening frictional stability trends. The results obtained from Site 1124 sediments indicate that: (1) fault-zone weakness may not require pore-fluid overpressures; (2) clays and chalks can host frictional instabilities; and (3) heterogeneous frictional properties can promote variable slip behaviour.

Geophysical Constraints on the Relationship Between Seamount Subduction, Slow Slip, and Tremor at the North Hikurangi Subduction Zone, New Zealand

AbstractWe use a prestack depth migration reflection image and magnetic anomaly data across the northern Hikurangi subduction zone, New Zealand, to constrain plate boundary structure and geometry of a subducting seamount in a region of shallow slow slip and recent International Ocean Discovery Program drilling. Our 3‐D model reveals the subducting seamount as a SW‐NE striking, lozenge‐shaped ridge approximately 40 km long and 15 km wide, with relief up to 2.5 km. This seamount broadly correlates with a 20‐km‐wide gap separating two patches of large (&gt;10 cm) slow slip and the locus of tectonic tremor associated with the September–October 2014 Gisborne slow slip event. Largest slow slip magnitudes occurred where the décollement is underlain by a 3.0‐km‐thick zone of highly reflective subducting sediments. Wave speeds within this zone are 7% lower than adjacent and overlying strata, supporting the view that high fluid pressures within subducting sediments may facilitate shallow slow slip along the north Hikurangi margin.

Frictional Behavior of Input Sediments to the Hikurangi Trench, New Zealand

AbstractThe Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes.

Spatial-temporal evolution of slow slip movements triggered by the 2016 Mw 7.8 Kaikoura earthquake, New Zealand

The 2016 Mw 7.8 Kaikoura earthquake involved complex rupture across multiple faults and triggered large-scale slow slip movements on the Hikurangi subduction zone. The complex seismically triggered slow slip events (SSEs) contribute to the understanding of triggering mechanisms of SSEs in New Zealand. Here, we invert the continuous Global Positioning System (cGPS) observations to investigate the temporal evolution of the seismically triggered slow-slip behaviour on the Hikurangi subduction interface. Similar to previous studies, our optimal geodetic inversion model reproduces deep (30–50 km) Marlborough afterslip of over 60 cm, deep Kapiti slow slip of up to 31 cm and shallow (<15 km), moderate (>10 cm) east coast slow slip on the Hikurangi subduction interface. The slow accumulated seismic moment over the past eight months from the large-scale slow slip motion is ~3.12 × 1020 Nm, which is equivalent to a magnitude 7.6 earthquake. Both the Marlborough afterslip and Kapiti slow slip were considerably active during the first week after the mainshock and gradually slowed down in the following months. These deep ongoing fault motions may continue for at least several months and are expected to decay logarithmically with time. In agreement with previous studies, the east coast slow slip motion occurred rapidly within 2–3 weeks, which stretched from the central to northern Hikurangi subduction margin. The coseismic Coulomb failure stress change investigation suggests that large-scale stress increase (over 1–2 MPa) appears on the southern Hikurangi subduction interface, which encourages afterslip underlying the Marlborough fault system and slow slip beneath the Kapiti coast. Consistent with previous studies, the long-distance slow slip motion offshore the east coast was likely dynamically triggered by the Kaikoura earthquake, where the static Coulomb stress increase is insignificant (<5 kPa).

Numerical Modeling of Dynamically Triggered Shallow Slow Slip Events in New Zealand by the 2016 Mw 7.8 Kaikoura Earthquake

AbstractThe 2016 Mw 7.8 Kaikoura earthquake triggered widespread slow slip events (SSEs) in the northern Hikurangi subduction zone, providing a unique opportunity to study the mechanism of dynamic triggering of SSEs. Here we simulate SSEs near Gisborne, New Zealand, in the framework of rate‐and‐state friction. Low effective normal stress (~0.4 MPa) on the shallow subduction interface is needed to reproduce the observed repeating, spontaneous SSEs. Dynamic stress perturbations from the Kaikoura mainshock are adequate to trigger SSEs with characteristics similar to observation. SSE propensity to dynamic triggering mainly depends on the timing of perturbation with respect to the SSE cycle and the maximum Coulomb stress change. Once the perturbation amplitude exceeds an initial threshold, prolonged stress perturbations tend to decrease the triggering threshold hence promote dynamic triggering of SSEs. Therefore, shallow SSEs are more likely to be dynamically triggered than their deep counterparts because of enhanced stress perturbation (magnitude and duration) from the sedimentary wedge.

Triggered Slow Slip and Afterslip on the Southern Hikurangi Subduction Zone Following the Kaikōura Earthquake

AbstractThe 2016 MW7.8 Kaikōura earthquake ruptured a complex sequence of strike‐slip and reverse faults in New Zealand's northeastern South Island. In the months following the earthquake, time‐dependent inversions of Global Positioning System and interferometric synthetic aperture radar data reveal up to 0.5 m of afterslip on the subduction interface beneath the northern South Island underlying the crustal faults that ruptured in the earthquake. This is clear evidence that the far southern end of the Hikurangi subduction zone accommodates plate motion. The MW7.8 earthquake also triggered widespread slow slip over much of the subduction zone beneath the North Island. The triggered slow slip included immediate triggering of shallow (&lt;15 km), short (2–3 weeks) slow slip events along much of the east coast, and deep (&gt;30 km), long‐term (&gt;1 year) slow slip beneath the southern North Island. The southern Hikurangi slow slip was likely triggered by large (0.5–1.0 MPa) static Coulomb stress increases.