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International audience; This paper presents preliminary results for three GPR profiles acquired across the Wellington active strike-slip fault within the Wellington urban area. In this sector, it is suggested that the subsurface geometry (8-10 m) of the fault comprises twomain deforming strands that bound narrow transpressive and transtensive sections. The location of fault planes interpreted from radargrams closely corresponds with the inferred location of the main fault at the ground surface. Despite noise due to the urban settings, GPR proved to be a technique capable of locating fault strands, thus potentially providing useful data in targeting areas for palaeoseismic studies, such as trenching.

Well-preserved mid-late Holocene coral reefs are exposed in low coastal cliffs in the vicinity of the Madang lagoon on the north coast of Papua New Guinea. Results from U/Th and 14 C dating of corals, surveying, and field mapping indicate several major changes in relative sea level over this period. Specifically, there is evidence for a relative sea level fall of ≥4.5 m about 3000 calendar years B.P., followed by relative sea level rises of ∼1.5 m about 2400 calendar years B.P. and ≥0.5 m about 1200 calendar years B.P. and a subsequent relative sea level fall of ≥3 m some time in the past 1000 years. Since regional eustatic sea levels are believed to have been dropping gradually over this time frame, these observed changes in relative sea level are interpreted as reflecting alternating tectonic uplift and subsidence. Furthermore, the detailed structure and age relationships of the coral deposits indicate that both uplift and subsidence occurred rapidly, most probably as coseismic events with vertical displacements of 0.5 to 4.5 m. These events may be related to rupture on NW-SE trending reverse faults which have been mapped in the nearby Adelbert Range and possibly on NE trending cross faults which have been inferred from seismicity. This interpretation implies a much greater degree of tectonic instability and potential seismic hazard in the region than previously recognized, although the inferred coseismic vertical displacements are shown to be consistent with present-day local seismicity. In a broader context, the study illustrates how detailed analysis of vertical changes in coral reef structure and assemblages may be used as a sensitive indicator of changing relative sea level, capable of resolving century timescale events and reversals.

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

[1] The 1957 Gobi Altay M8.3 earthquake in southern Mongolia was associated with the simultaneous rupture of several faults, including the Gurvan Bulag reverse fault, which is located about 25 km south of the main strike-slip Bogd fault. Our study of paleoseismic excavations across the Gurvan Bulag fault suggests that the penultimate surface rupture occurred after 6.0 ka, most likely between 2.6 and 4.4 ka, and a possible earlier rupture occurred after 7.3 ka. Our interpretation of the stratigraphic relations in one of the exposures suggests that at least five earthquakes have generated surface rupture of the Gurvan Bulag fault since the abandonment of an ancient alluvial fan surface. Luminescence dating of sediment associated with this surface indicates that it formed either 26.6 ± 2.1 ka or 16.1 ± 2.0 ka. These data imply that the recurrence intervals for surface faulting on the Gurvan Bulag and Bogd faults are similar, on the order of several thousands of years, but that the penultimate surface ruptures of the two faults did not occur during the same earthquake.

The National Science Foundation's (NSF) MARGINS research program examines the processes governing continental margin evolution through four science initiatives. By promoting research strategies that redirect traditional approaches, these initiatives will help to stimulate coordinated, interdisciplinary research in a few focus areas over the next decade. For the MARGINS Source-to-Sink science initiative, New Zealand and New Guinea have been selected as the two primary focus areas (http://www.ldeo.columbiaedu/margins/SedStrat.html).

Along the 500 km long eastern coastline of the North Island, New Zealand there are, at most, seven distinct Holocene marine terraces aged 7 ka B.P. or less. The highest of the terraces is 27 m above present-day mean sea level. The coastal region is subdivided into fourteen distinct subregions based on radiocarbon ages of marine deposits overlying wave-cut shore platforms and geographic distribution of similarly-aged terraces. Holocene marine terraces are the result of uplift associated with large earthquakes (co-seismic deformation). This conclusion is based on characteristic stepped terrace morphology, clustering of ages of terrace deposits within subrogions, and the occurrence of co-seismic uplift in historic time. Differential uplift across structures and distinct age variations at subregion boundaries are also characteristic. Recurrence intervals of uplift in any one terrace sequence vary from ∼ 0.4 ka to ∼ 2.0 ka, and individual amounts of uplift vary from ∼ 1.0 m to ∼ 4.0 m. In the past ∼ 2.5 ka ages from all terraces within subrogions indicate at least 21 paleoseismic events affecting coastal areas of eastern North Island in that period. These events cluster in time, and in separate parts of eastern North Island several earthquakes occurred ∼ 0.3, 0.6,1.0,1.5, 2.1 and 2.3 ka B.P. Our data strongly support the concept of segmentation of deformation along the subduction margin. The likely cause of coastal uplift is movement on steep reverse faults (local structures rather than the subduction thrust) that propagate from or near the subduction thrust some 20–25 km below the region. Earthquakes of moment magnitude 7.3–8.0 are estimated to be associated with these fault movements.

Histories of movement on several active faults have been determined and used to evaluate possible future seismic hazard in the Rangitaiki Plains. The eruption of Kaharoa Ash c. 800 years B. P. provides a datum to establish the history of faulting. The Matata, Edgecumbe, Onepu, and Rotoitipakau Faults, all located in the southern and western Rangitaiki Plains, have moved at least once during the last 800 years. In contrast, faults in the eastern part of the Rangitaiki Plains do not show evidence of post-Kaharoa movement. The past level of seismic hazard has, therefore, been lower in the eastern part of the plains compared with southern and western areas. We assume that rupture of the northeast-trending normal faults in the Rangitaiki Plains will produce isoseismal patterns similar to that observed during the 1987 March 2 Edgecumbe earthquake. By overlaying the MM IX isoseismal of the Edgecumbe earthquake over each fault in the Rangitaiki Plains, and taking into account geological data on the recur...

The Edgecumbe earthquake, 1987 March 2 (M L 6. 3), was associated with renewed tectonic rupture on the Edgecumbe Fault, renewed movement on the Onepu and Rotoitipakau Faults, and several new surface breaks—the Awaiti, Otakiri, Te Teko, and Omeheu Faults. These northeast trending tectonic ruptures are widely distributed across the Rangitaiki Plains, range in length from 0. 5 to 7 km, and are mostly downthrown to the northwest. They are associated with warping and prominent fissuring. Maximum displacement, 2. 5 m vertical and 1. 8 m extensional, occurred near the middle of the Edgecumbe Fault trace. Trenching investigations revealed that perhaps two faulting events have occurred in the past 1850 years in addition to the 1987 event; the earlier one is unsubstantiated, but may have occurred at about 1850 years B. P., and another occurred at about 800 years B. P. (the time of deposition of the Kaharoa Ash). The 800 year B. P. event was associated with warping, but little fissuring. An average slip vec...

Both the South Island of New Zealand and the Transverse Ranges of California have (1) a major right-slip fault marking the plate boundary, (2) a left-stepping bend in the fault resulting in ranges and basins bounded by active reverse faults that strike 45° counterclockwise to the regional strike of the main fault, (3) a set of right-slip faults on one side of the constraining bend, with most of the total displacement concentrated On the fault stepped farthest left from the continuation of the fault on the other side of the bend, and (4) intersections between the set of faults and the main fault within the bend not present or poorly defined. The faults with most of the displacement (Alpine-Wairau, San Andreas) may pre-date the constraining bend, The reverse fault province southeast of the Alpine fault is separated from that fault by the Southern Alps, which may be underlain by crust that is too hot and too weak to sustain major reverse faults. Average reverse fault recurrence intervals in New Zealand and California are 10³ to 2 × 104 years as compared to less than 10³ years for the right-slip master fault, Seismicity is high but patchy in the Nelson-Buller region of the northern South Island, low but patchy in the Transverse Ranges, and very low in Central Otago in the southern South Island, all reverse fault provinces. Seismicity patterns in both regions, poorly related to overall late Quaternary fault distribution, correspond largely to individual rupture events on portions of Quaternary faults, and the seismic inactivity of many of these faults may simply represent quiescence between seismic faulting with long recurrence intervals. The geology and Seismicity of the three reverse-fault provinces support the suggestion of C. R. Allen [1975] that late Quaternary fault history is a better long-term guide for assessment of large-magnitude earthquake hazard than instrumental and historical Seismicity for faults with long recurrence intervals.

Depositional environments for Holocene terrestrial, estuarine, and marine sediments on the Wairoa River floodplain at Wairoa and coastal plain east of Wairoa, were determined from an analysis of stratigraphy, lithologies, and diatom assemblages obtained from auger holes and foundation and water-well drillholes. The sediments were deposited during the postglacial sea-level rise which started sometime prior to c. 9200 years ago and ended at c. 4000 years ago, when the shoreline was at least 2 km landward of the present position, and a ria-like morphology had developed at the innermost edge of the present-day coastal plain. The postglacial marine sediments usually occur slightly below the present sea level (down to -4 m a.m.s.l.) indicating net tectonic subsidence during the Holocene. Other evidence suggests that this tectonic subsidence may extend back into the middle Pleistocene.

Tectonic processes have both very pronounced and very subtle effects on the earth’s crust, which may be measured by the deformation of datum planes at the earth’s surface. The position of sea level with respect to adjacent landmasses may be regarded as a datum plane that records the interplay between climatic, geodynamic and tectonic processes through time.

Pleistocene marine terraces along the Marlborough coast, South Island, New Zealand, have been re‐examined with detailed stratigraphic observations, accurate height data, and amino acid and thermo‐luminescence (TL) geochronology. Marine terraces range in age from c. 220 ka (oxygen isotope stage 7) to c. 60 ka (oxygen isotope stage 3), in the area from Cape Campbell to Conway River. At Kaikoura Peninsula, five marine terraces are preserved. The marine fauna, loess stratigraphy, and amino acid dating of Tawera spissa, from the Kaikoura I (highest) terrace and from the highest terrace at Haumuri Bluffs (Tarapuhi Terrace), indicate a correlation to oxygen isotope substage 5c, with an age of 100 ka. North of the Clarence River, marine terraces (including the Parikawa Formation) are correlated to oxygen isotope substage 5e of the last interglacial. TL dating of loess supports this interpretation. The Winterholme Formation terrace at Kekerengu is reinterpreted as a last glaciation fluvioglacial terrace g...

Trenching of two fault strands that form part of the 21 km long, and up to 2 km wide Whirinaki Fault, Taupo Rift, New Zealand, shows that surface fault rupture has occurred at least eight times during the past 26.5 kyr. Single‐event displacements on each strand vary from a few decimetres to perhaps >2.5 m. Fault trace mapping, geomorphic analysis, and paleoseismic studies indicate a maximum magnitude for the Whirinaki Fault of c. Mw 6.6, with a recurrence interval of between 1000 and 4000 yr. The variability of single‐event displacement and timing of rupture on the two strands is attributed to various unsegmented and segmented rupture modes. Airfall tephra revealed in the trenches span the past 26.5 kyr, and provide time horizons to enable interpretation of how slip has accumulated on each of the strands of the fault. Temporal variation in slip rate ranges from 0.3 to 1.5 mm/yr and is indicative of spatial and temporal changes in the locus of strain in the Taupo Rift and the interdependence of th...