Jamie Howarth

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

Lake sediments record cycles of sediment flux driven by large earthquakes on the Alpine fault, New Zealand

Large earthquakes in mountain regions commonly trigger extensive landsliding and are important drivers of erosion, but the contribution of this landsliding to long-term erosion rates and seismic hazard remains poorly understood. Here we show that lake sediments record postseismic landscape response as a sequence of turbidites that can be used to quantify erosion related to large (moment magnitude, M w > 7.6) earthquakes on the Alpine fault, New Zealand. Alpine fault earthquakes caused a threefold increase in sediment flux over the ∼50 yr duration of each postseismic landscape response; this represents considerable delayed hazard following earthquake-induced strong ground motion. Earthquakes were responsible for 27% of the sediment flux from the lake catchment over the past 1100 yr, leading us to conclude that Alpine fault earthquakes are one of the most important drivers of erosion in the range front of the Southern Alps.

Extreme hydrothermal conditions at an active plate-bounding fault

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.

A 2000 yr rupture history for the Alpine fault derived from Lake Ellery, South Island, New Zealand

Determining the earthquake segmentation of plate-boundary transform faults remains a scientific challenge because paleoseismic data sets rarely resolve the end points of past ruptures. In this study, we test whether lacustrine paleoseismology can be used to assess rupture end points and the earthquake segmentation of the Alpine fault, one of the longest and fastest-slipping plate-boundary transform faults on Earth. Sediment cores from Lake Ellery record eight episodes of high-intensity shaking (modified Mercalli intensity [MM] IX) from Alpine fault earthquakes as event sequences of a turbidite produced by coseismic subaqueous mass wasting, overlain by deposits representing sediment flux from co- and postseismic landsliding in the fluvial catchment. Age-depth modeling constrains the timing of shaking events at a decadal resolution, facilitating correlation with two previously published lake records to reconstruct the spatial distribution of MM IX shaking along ~150 km of the Alpine fault. When resolved with existing on- and near-fault paleoseismic records, the lake data set demonstrates that independent ruptures of the South Westland and Central segments occurred in A.D. 845–775 and A.D. 739–646, and A.D. 646–592 and A.D. 416–370, respectively. Lakes adjacent to the Alpine fault provide paleoseismic records with sufficient spatial and temporal resolution to define along-strike differences in the pattern of rupture capable of distinguishing rupture termination at a geometric segment boundary. This multilake study suggests that locating the end points of ruptures using lacustrine paleoseismology will be most applicable in midlatitude convergent plate-boundary settings where along-strike topography and Quaternary glaciation have resulted in the widespread distribution of suitable lakes.

Tsunami runup and tide-gauge observations from the 14 November 2016 M7.8 Kaikōura earthquake, New Zealand

The 2016 Mw 7.8 Kaikōura earthquake was one of the largest earthquakes in New Zealand’s historical record, and it generated the most significant local source tsunami to affect New Zealand since 1947. There are many unusual features of this earthquake from a tsunami perspective: the epicentre was well inland of the coast, multiple faults were involved in the rupture, and the greatest tsunami damage to residential property was far from the source. In this paper, we summarise the tectonic setting and the historical and geological evidence for past tsunamis on this coast, then present tsunami tide gauge and runup field observations of the tsunami that followed the Kaikōura earthquake. For the size of the tsunami, as inferred from the measured heights, the impact of this event was relatively modest, and we discuss the reasons for this which include: the state of the tide at the time of the earthquake, the degree of co-seismic uplift, and the nature of the coastal environment in the tsunami source region.

Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the faults principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

Lidar reveals uniform Alpine fault offsets and bimodal plate boundary rupture behavior, New Zealand: COMMENT

De Pascale et al. (2014) present a new “bimodal” fault rupture behavior model for the dextral-reverse Alpine fault using evidence from several new, small (<30 m) dextral displacements identified on lidar along a 20 km stretch of the densely forested, central section of the Alpine fault near Whataroa. The proposed model asserts that partial fault ruptures (related to moderate to large earthquakes) occur in addition to full ruptures (related to major or great earthquakes). However, De Pascale et al. do not provide evidence for bimodal rupture behavior. The paper fails to: (1) identify a bimodal displacement pattern in the geologic record, (2) provide geological age control for any measured displacements, (3) adequately address the role of the ca. A.D. 1600 earthquake event, and (4) accommodate the lack of moderate to large earthquakes in the historical record. We briefly address these points here. De Pascale et al. demonstrate from lidar and field data that similar amounts of dextral offset (~7.1 ± 2.1 m dextral slip per event) occurred in each of the last three movements recorded on what appears to be the main dextral trace of the Alpine fault, south of Gaunt Creek. They concede that no smaller-sized displacements (<3 m) were identified, and if they are present they would likely be below the resolution of field studies (see their figure 4) or the 3 m lidar model published by Langridge et al. (2014). In addition, the authors neglect to mention the presence and importance of range-front thrust and oblique-slip fault traces and the likelihood that at least some partitioned dextral slip occurs on them, as is known from Gaunt Creek (Cooper and Norris, 1994; Barth et al., 2012). Preliminary paleoseismic results from Gaunt Creek emphasize the lack of well-dated late Holocene fault displacements along this part of the Alpine fault (De Pascale and Langridge, 2012). In place of independent age constraints on their measured offsets, De Pascale et al. (2014) explore the timing of individual earthquakes by dividing their singleevent displacement estimates by the average Late Quaternary slip rate. Although entirely hypothetical, the calculated ages appear broadly consistent with independent dating of past Alpine Fault earthquakes. However, the authors have neglected to properly propagate displacement and slip-rate uncertainties through their calculations. Correct propagation of uncertainty renders the resulting “ages” meaningless—the uncertainties are so great that each inferred displacement increment correlates with more than one dated Alpine fault event. In fact, we have been unable to replicate the values in their table 1 using the data and methods presented. We do not discount the possibility of “partial” ruptures or of events of Mw <7.0, but argue that a bimodal rupture behavior is supported by neither paleoseismic data nor the historical earthquake record. De Pascale et al. fail to evaluate the published paleoseismic data adequately to support their statement that moderate to large earthquakes (Mw ≥6.5) occur on the Alpine fault. They suggest that the ca. A.D. 1600 Alpine fault earthquake was such a moderate-sized earthquake. However, the available data documented from six on-fault trench sites and off-fault landscape impact sites suggest that this event likely ruptured the northern (with overlap onto the central) section of the fault in a Mw ~7.6 earthquake with slip over a length of 200–300 km (using the Alpine fault Mw scaling relationship of Stirling et al. [2012]; Sutherland et al., 2007; Yetton and Wells, 2010; Howarth et al., 2012). If this published scenario for the ca. A.D. 1600 event is accepted, then there is no paleoseismic evidence to support the De Pascale et al. hypothesis that partial ruptures occur in between full ruptures of the fault, nor is there a mismatch between displacements, timing, and slip rate for the central Alpine fault. Critically, no Mw 6.5–7.0 earthquakes (that a bimodal model would predict) have occurred on the Alpine fault during the New Zealand historical period (since ca. A.D. 1840). Furthermore, recently published recurrence times for earthquakes on the central (260 ± 70 yr) and southern (329 ± 68 yr) sections of the Alpine fault derived from long paleoearthquake records support relatively regular recurrence of similarsized (major) earthquakes at a point (Howarth et al., 2012; Berryman et al., 2012). In summary, De Pascale et al. have not identified any smaller offsets (as subsets of the ~7.1 m increments) caused by moderate to large earthquakes, or reliably dated the strike-slip displacements at their sites caused by major Alpine fault ruptures. Therefore, they present insufficient data to argue for bimodal fault behavior or new earthquake recurrence models that would impact on the current understanding of Alpine fault hazards. Their paper does, however, highlight the need for continued efforts to better locate and precisely date on-fault records of earthquake timing and slip along the fault.