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A sedimentary sequence that was highly sensitive to fault rupture–driven changes in water level and sediment supply has been used to extract a continuous record of 22 large earthquakes on the Alpine fault, the fastest-slipping fault in New Zealand. At Hokuri Creek, in South Westland, an 18 m thickness of Holocene sediments accumulated against the Alpine fault scarp from ca. A.D. 800 to 6000 B.C. We used geomorphological mapping, sedimentology, and paleoenvironmental reconstruction to investigate the relationship between these sediments and Alpine fault rupture. We found that repeated fault rupture is the most convincing mechanism for explaining all the features of the alternating peat and silt sedimentary sequence. Climate has contributed to sedimentation but is unlikely to be the driver of these cyclical changes in sediment type and paleoenvironment. Other nontectonic causes for the sedimentary alternations do not produce the incremental increase in basin accommodation space necessary to maintain the shallow-water environment for 6800 yr. Our detailed documentation of this near-fault sedimentary basin sequence highlights the advantages of extracting paleoearthquake records from such sites—the continuity of sedimentation, abundance of dateable material, and pristine preservation of older events.

From a data set of 50 published and new fault exposures, we establish a 26,000 year record of associations between the timing of fault rupture in two sectors of the Taupo rift, New Zealand, and deposition on the fault scarps of rhyolitic fall tephra from the adjacent Okataina volcanic center. We also investigate processes that could be responsible for the time associations. From 40 high-resolution couplets of fault rupture and volcanic eruption (located up to 30 km distant), we show that 30% of the fault ruptures occurred when the volcano was erupting, whereas in 70% of the cases volcanism and faulting were independent. Other geological and geophysical information indicates that faulting in the Taupo rift is essentially tectonic and, thus, most of the cases with time association between fault rupture and volcanic eruption found in the fault exposures in this study are interpreted to be a manifestation of stress transfer between faults and magmatic storage zones beneath the volcanic center. In a few cases close (

Paleoecological investigations of three Holocene marginal-marine sedimentary sequences provide information on vertical tectonic deformation in a transect across the forearc basin adjacent to the Hikurangi subduction zone, New Zealand. The elevation of maximum postglacial sea level indicators at Te Paeroa Lagoon and Opoho is between 4 and 6 m below present mean sea level, indicating net subsidence since 7200 yr B.P. Opoutama is closer to the Hikurangi Trench and appears to lie near the edge of the zone of subsidence, as evidence for vertical movement there is equivocal. Some of the subsidence at Te Paeroa Lagoon and Opoho is likely to be a result of compaction. However, a component of subsidence probably happened coseismically in two events at ca. 7100 and 5550 yr B.P. Event signatures consist of tsunami deposits overlain by chaotically mixed, reworked sediment that appears to have filled rapidly created accommodation space at marine inlet sites 10 km apart. Large offshore earthquakes are suggested by the coincidence of tsunami inundation with sudden subsidence. Forward elastic-dislocation models indicate that the observed subsidence could be achieved in ∼M w 7.9 earthquakes on either the subduction interface or the Lachlan Fault, which would involve synchronous uplift of Mahia Peninsula. Combined rupture of the interface and the Lachlan Fault, either simultaneously in a ∼M w 8.1 earthquake, or consecutively, could explain larger amounts (>1.5 m) of coastal subsidence.

Glacial landforms at 12 localities in 9 river valleys are offset by the southern end of the onshore Alpine fault. Offsets cluster at ∼435, 1240, and 1850 m, consistent with evidence for glacial retreat at 18, 58, and 79 calendar ka. The peak of an offset fluvial aggradation surface is correlated with the Last Glacial Maximum at 22 ka. Displacement rates derived from features aged 18, 22, 58, and 79 cal. ka are 24.2 ± 2.2, 23.2 ± 4.9, 21.4 ± 2.6, and 23.5 ± 2.7 mm/yr, respectively, with uncertainties at the 95% confidence level. The joint probability, weighted mean, and arithmetic mean of all observations pooled by rank are 23.1 ± 1.5, 23.2 ± 1.4, and 23.1 ± 1.7 mm/yr, respectively. We conclude that the mean surface displacement rate for this section of the Alpine fault is 23.1 mm/yr, with standard error in the range of 0.7–0.9 mm/yr. The reduction in estimated long-term slip rate from 26 ± 6 mm/yr to 23 ± 2 mm/yr results in an increase in estimated hazard associated with faulting distributed across the rest of the plate boundary. Model-dependent probabilities of Alpine fault rupture within the next 50 yr are in the range 14%–29%. The 36 ± 3 mm/yr of total plate motion (NUVEL-1A) is partitioned into 23 ± 2 mm/yr of Alpine fault dextral strike slip, 12 ± 4 mm/yr of horizontal motion by clockwise block rotations and oblique dextral-reverse faulting up to 80 km southeast of the Alpine fault, and 5 ± 3 mm/yr of heave on reverse faults at the peripheries of the plate boundary.

The Poukawa fault zone, on the North Island of New Zealand within the forearc of the Hikurangi subduction zone, consists of a series of en echelon reverse faults and companion hanging-wall anticlines. The geomorphically expressed length of the fault zone is 34 km. However, on the basis of coseismic deformation associated with an M s 7.8 earthquake in 1931 and the presence of blind faults north of the geomorphically expressed fault zone, it appears that the seismogenic length of the fault zone may be as much as 130 km. On the basis of chronostratigraphic horizons identified in each of three trenches evenly distributed along the exposed fault zone, from which a paleoseismological record for the past ∼25 k.y. can be determined, there is not a characteristic rupture length for earthquakes. Some slip events are confined to the ∼10–20-km-long southern part of the fault zone, whereas other slip events may have ruptured the entire 34 km length of the geomorphically expressed fault zone. At least two slip events that occurred in the northern part of the fault zone did not occur in the southern part of the zone. The largest earthquake recorded in the trenches had a maximum reverse slip in excess of 10 m. We infer that this prehistoric earthquake, similar to the 1931 earthquake, entailed slip on faults along the geomorphically expressed fault zone and on blind faults to the north. This prehistoric earthquake may have had a rupture length (surface plus subsurface) in excess of 100 km. Average earthquake repeat times on the fault zone range from 3–7.5 k.y. for the southern and middle part of the zone to 7–12 k.y. for the northern part of the fault zone. Average single-event slip ranges from 3 m to as much as 6 m. Slip was initially accommodated at the surface primarily by folding. With successive slip events, however, coseismic displacements propagated to the surface and surface deformation became increasingly dominated by reverse slip on fault planes. The Poukawa fault zone is part of a foreland-propagating fold and thrust belt in the forearc of the Hikurangi subduction zone. Older, actively eroding hanging-wall anticlines are present to the west of the fault zone toward the volcanic arc, whereas younger folds are developing above blind reverse faults east of the main fault trace. In addition to propagating to the east, the fault zone is propagating northward beneath the Heretaunga Plains. This active propagation testifies to ongoing and evolving contractional forearc deformation in response to oblique plate convergence.

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 (

Paleoearthquakes at Earth's surface often generate faults with variable displacement rates over short time intervals (e.g.