Kelvin Berryman

National Seismic Hazard Model for New Zealand: 2010 Update

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

A late Quaternary extension rate in the Taupo Volcanic Zone, New Zealand, derived from fault slip data

Abstract A northwest‐southeast oriented extension rate from faulting for a time‐averaged period of c. 50 000 yr (10 000–64 000 yr), across the Ngakuru‐Waikite depression (modern Taupo Fault Belt, central Taupo Volcanic Zone), has a best estimate of 1.9 mm/yr (in a range of 1.2–2.8 mm/ yr) in the near surface, but increases to a best estimate of 6.4 mm/yr (in a range of 3.6–10.2 mm/yr) at seismogenic depths of 6–10 km. We obtain this result by summing the vertical components of fault displacement across known‐age surfaces, or as the vertical component of displacement in stratigraphic units of known age, within the 14 km wide zone of active normal faulting. We convert the summed vertical slip rate of 7.2 ± 0.4 mm/yr to dip‐slip displacement rate and to northwest‐southeast extension by estimating a range of possible fault plane dips at the surface and at seismogenic depth. Fault displacement at seismogenic depth in large events is on average 1.6 times larger than at the surface, and for earthquake magnitudes of A/6.8 and smaller, about one‐third of the displacement occurring with the whole Gutenburg & Richter distribution of earthquakes in the modern Taupo Fault Belt will not rupture to the ground surface. Fault dip averages c. 75° in the near surface, but is poorly constrained at seismogenic depth in the Taupo Fault Belt. From a variety of local and literature considerations, we propose a dip of c. 60° at seismogenic depth in the Taupo Fault Belt. Our observations suggest only a minor component of extension at the surface (c. 5%) is contributed by small scale faulting below our observation threshold of 0.1–0.5 m of fault slip. The c. 4.5 mm/yr difference in extension rate between seismogenic depth and the ground surface may represent the surface extension rate caused by a combination of opening of extension fractures and penetrative grain‐scale extensional deformation.

A New Seismic Hazard Model for New Zealand

We present a new probabilistic seismic hazard analysis (PSHA) for New Zealand. An important feature of the analysis is the application of a new method for the treatment of historical (distributed) seismicity data in PSHA. The PSHA uses the seismicity recorded across and beneath the country to define a three-dimensional grid of a -values (i.e., parameter a of a Gutenberg-Richter distribution log N/yr = a - bM , in which N /yr is the number of earthquakes per year recorded inside each grid cell equal to or greater than magnitude M ); parameter b and the limiting maximum cutoff magnitude of the Gutenberg-Richter distribution are defined from the surrounding region (14 crustal and 23 subcrustal seismotectonic zones are defined for the country) and then smoothed across the boundaries of the zones. The methodology therefore combines the modern method of defining continuous distributions of seismicity parameters (Frankel, 1995; Frankel et al. , 1996) with the traditional method of defining large area sources and the associated seismicity parameters (e.g., Algermissen et al. , 1990). The methodology provides a means of including deep (subduction zone) seismicity in a PSHA, preserves the finer-scale spatial variations of seismicity rates across a region, avoids the undesirable edge effects produced in the traditional method when adjacent area sources enclose areas of significantly different seismicity rates, and also enables parameters most reliably defined at a regional scale (parameter b and maximum cutoff magnitude of a Gutenberg-Richter distribution, and slip type) to be incorporated into the PSHA. The PSHA combines the modeled seismicity data with geological data describing the location and earthquake recurrence behavior of 305 active faults and new attenuation relationships for peak ground acceleration and spectral acceleration developed specifically for New Zealand. Different attenuation expressions are used for crustal and subduction zone earthquakes. The resulting PSH maps for a 150-year return period show the highest hazard to occur in the center and southwest of the country, in the areas of highest historical crustal and deep subduction zone seismicity. In contrast, the longer return-period maps (475 and 1000 year return period) show the highest hazard to occur from the southwest to northeast ends of the country, along the faults that accommodate the majority of the motion between the Pacific and Australian plates. The maps are currently being used to revise New Zealands building code, which has previously been based on PSHAs that did not explicitly include individual faults as earthquake sources. Manuscript received 10 April 2001.

Major Earthquakes Occur Regularly on an Isolated Plate Boundary Fault

The Sedimentary Life of Earthquakes Estimating the hazards associated with possible large earthquakes depends largely on evidence of prior seismic activity. The relatively new global seismic networks installed to monitor earthquakes, however, have only captured the very recent history of fault zones that can remain active for thousands of years. To understand the recurrence of large earthquakes along the Alpine Fault in New Zealand, Berryman et al. (p. 1690) looked to the sediments near an old creek for evidence of surface ruptures and vertical offset. Along this fault segment, 24 large earthquakes seem to have occurred over the last 6000 years, resulting in a recurrence interval of ∼329 years. The activity is more regular than other similar strike-slip faults, such as the San Andreas Fault in California. Evidence of past earthquakes from sediments along New Zealand’s Alpine Fault improves seismic hazard estimates. The scarcity of long geological records of major earthquakes, on different types of faults, makes testing hypotheses of regular versus random or clustered earthquake recurrence behavior difficult. We provide a fault-proximal major earthquake record spanning 8000 years on the strike-slip Alpine Fault in New Zealand. Cyclic stratigraphy at Hokuri Creek suggests that the fault ruptured to the surface 24 times, and event ages yield a 0.33 coefficient of variation in recurrence interval. We associate this near-regular earthquake recurrence with a geometrically simple strike-slip fault, with high slip rate, accommodating a high proportion of plate boundary motion that works in isolation from other faults. We propose that it is valid to apply time-dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide.

Comparison of Earthquake Scaling Relations Derived from Data of the Instrumental and Preinstrumental Era

Estimates of surface rupture displacement and magnitude for crustal earthquakes from the preinstrumental era (pre-1900) tend to be greater than the corresponding estimates derived from modern scaling relations. We investigate this tendency using an expanded and updated version of the earthquake dataset of Wells and Coppersmith (1994) to fit regression relations of moment magnitude on surface rupture length and rupture area and average surface displacement on surface rupture length. Separate relations are fitted to preinstrumental and instrumental data and the results compared to the equivalent relations of Wells and Coppersmith. We find that our relations for instrumental data remove some, but not all, of the differences between the preinstrumental data and the relations of Wells and Coppersmith. We attribute the remaining differences largely to natural censoring of surface displacements less than about 1 m and surface rupture lengths less than about 5 km from the dataset for the preinstrumental era because regressions constructed from similarly censored instrumental data are indistinguishable from the preinstrumental regressions. Since the regressions for our censored instrumental data (i.e., restricted to moderate to large earthquakes) are different from regressions for our complete dataset of instrumental earthquakes and from the regressions of Wells and Coppersmith (both with a larger proportion of small-to-moderate earthquakes), the results may indicate that large earthquakes have different scaling relationships from those of smaller earthquakes.

Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand

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.

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

Surface rupture earthquakes over the last ∼1000 years in the Wellington region, New Zealand, and implications for ground shaking hazard

The Wellington region is cut by five active right-lateral strike-slip faults : Wairarapa, Wellington, Ohariu, Shepherds Gully/Pukerua, and Wairau faults that have average recurrence intervals of meter-scale surface rupture that range from ∼500 years to 5000 years, and lateral slip rates that range from 1 to 10 mm/yr. Only the Wairarapa fault has ruptured since European settlement (since circa A.D. 1840). Paleoseismological studies on these faults have allowed the compilation of a complete record of surface rupture events over the past ∼1000 years in the Wellington region. Within this time period, there does not appear to be any temporal clustering of surface rupture events on adjacent faults. The M 8 A.D. 1855 Wairarapa earthquake did not trigger rupture on any other fault in the region. The most recent surface-faulting event on the Wellington fault (290-440 cal years B.P.) (cal years are calendar years before A.D. 1950) does not coincide with rupture of any other onland fault, and over 300 years separate the timing of the second most recent rupture on the Wellington fault (660-720 cal years B.P.) and the most recent rupture of the Ohariu fault (1060-1140 cal years B.P.). The most recent rupture of the Shepherds Gully/Pukerua fault is probably older than that of the Ohariu fault. The apparent nonclustering of surface rupture earthquakes in the Wellington region has been documented only for the on-land strike-slip faults. There are other possible seismogenic sources in the region, and thus important issues remain to be addressed regarding the history of large earthquakes in the Wellington region : (1) the seismogenic potential and earthquake recurrence interval of the subduction thrust beneath Wellington is not known ; (2) the timing of rupture events on the offshore portion of the Wairau fault is not known ; and (3) paleoseismic data are not available for the section of the Wellington fault north of the Wellington-Hutt Valley segment. Estimates of earthquake hazard in the Wellington region, for all return times greater than 50 years, that incorporate paleoseismicity data are between one and two Modified Mercalli (MM) intensity units higher than the hazard based solely on the historical seismicity catalog, and the hazard is spatially more variable. Using a deterministic attenuation model, the level of shaking hazard approaches near maximum values within a return time of ∼500 years, largely reflecting the recurrence interval (500-770 years) of surface rupture earthquakes on the Wellington fault. Inclusion of a plausible model for magnitude 8 subduction zone earthquakes does not affect the level of MM intensity in Wellington region at return times greater than 500 years but does make a small contribution to the hazard at return times between 50 and 500 years.

Strain Partitioning between structural domains in the forearc of the Hikurangi Subduction Zone, New Zealand

The Pacific plate obliquely converges with the Australian plate at latitude 39°50′S along the Hikurangi margin off the east coast of the North Island of New Zealand. An extensive and youthful subaerially exposed forearc on the east coast of the North Island in the Hawkes Bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting. Geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension, contraction, and strike-slip. The domains are elongate and trend parallel to the margin. Measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface. Deformation rates calculated for the past 1–2 m.y. for structures in all six forearc domains account for 50–70% of the margin-parallel motion required by Pacific-Australian plate convergence and about 6% of the plate motion perpendicular to the plate boundary. At the surface in the forearc, this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike-slip fault zone and an accompanying contractional fault-and-fold zone on the trenchward side. There are two such transcurrent faulting-and-contraction couplets, one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin. The small amount of shortening, relative to strike-slip, in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust.

Tectonic and paleoclimatic significance of Quaternary river terraces of the Waipaoa River, east coast, North Island, New Zealand

Abstract Remnants of four aggradational terraces in the lower 45 km of the main branch of the Waipaoa River have been correlated with cold/cool climate episodes of the Otiran glaciation. The youngest of the aggradation levels—the Waipaoa‐1 terrace—has the c. 14.7 kaRerewhakaaituTephra as the oldest part of the coverbed sequence, indicating cessation of aggradation about 16 ka BP. This terrace is broadly correlated with Ohakean‐aged terraces in other parts of the North Island. The second most recent episode of aggradation—the Waipaoa‐2 terrace—is slightly older than the c. 28 ka Mangaone Tephra, and is broadly correlated with the Rata terrace. The third most recent aggradation episode— the Waipaoa‐3 terrace—is slightly older than the c. 55–57 ka Rotoehu Tephra (age estimate based on stratigraphic relationships in this study), indicating cessation of aggradation at c. 65 ka BP, and correlative with the Porewa terrace. The fourth, and oldest, aggradation episode we identify in the present landscape—the Waipaoa‐4 terrace—has poor age constraints, but is probably related to the cool period of late oxygen isotope stage 5 at c. 90 ka BP or the glacial period of oxygen isotope stage 6 at c. 140 ka BP. Tectonic deformation in the middle reaches of the Waipaoa catchment is deduced from the elevation difference of pairs of aggradation terraces, and takes the form of broad regional uplift in the range of 0.5–1.1 mm/yr. Uplift is probably driven by subduction processes in the middle part of the catchment and by a combination of deep‐seated subduction processes and local deformation associated with active faults and folds in the lower valley area. Downcutting rates of up to 7 mm/yr occur in upper reaches of the river. In the middle reaches of the valley, where there are both uplift and downcutting data, we find that downcutting is about four times faster than tectonic uplift. Thus, climate fluctuations are interpreted to be the primary control on formation of fluvial terrace landscapes in the region.