Subduction Interface Earthquakes Research Articles

11.04: Examination of the Canadian provisions for the seismic stability of steel structures

ABSTRACTThis article presents the seismic stability provisions of the CSA S16 design standard for steel structures in Canada. These provisions include notional lateral loads as well as lateral load amplification for P‐delta effects evaluated at maximum lateral displacements including inelastic response. The impact of these provisions is examined through nonlinear response history analyses performed on single‐degree‐of‐freedom models representing steel braced frames for buildings varying from 1 to 15 storeys in height. Buckling restrained braced frames and concentrically braced frames are investigated. The structures are located in eastern and western seismic regions of Canada to account for the difference in anticipated ground motion signature. For each location, firm ground and soft soil conditions are considered. The results show that current code provisions for seismic are sufficient to ensure stable seismic response of structures located in eastern Canada where ground motions are richer in high frequencies. Structural collapse was observed for one third of the structures located in western Canada. Most failures occurred for taller buckling restrained braced frames subjected to long duration subduction interface earthquakes. Collapse also took place for low‐rise concentrically braced frames.

Alternative Rupture‐Scaling Relationships for Subduction Interface and Other Offshore Environments

Abstract Alternative fault‐rupture‐scaling relationships are developed for M w 7.1–9.5 subduction interface earthquakes using a new database of consistently derived finite‐fault rupture models from teleseismic inversion. Scaling relationships are derived for rupture area, rupture length, rupture width, maximum slip, and average slip. These relationships apply width saturation for large‐magnitude interface earthquakes (approximately M w >8.6) for which the physical characteristics of subduction zones limit the depth extent of seismogenic rupture, and consequently, the down‐dip limit of strong ground motion generation. On average, the down‐dip rupture width for interface earthquakes saturates near 200 km (196 km on average). Accordingly, the reinterpretation of rupture‐area scaling for subduction interface earthquakes through the use of a bilinear scaling model suggests that rupture asperity area is less well correlated with magnitude for earthquakes M w >8.6. Consequently, the size of great‐magnitude earthquakes appears to be more strongly controlled by the average slip across asperities. The sensitivity of the interface scaling relationships is evaluated against geographic region (or subduction zone) and average dip along the rupture interface to assess the need for correction factors. Although regional perturbations in fault‐rupture scaling could be identified, statistical significance analyses suggest there is little rationale for implementing regional correction factors based on the limited number of interface rupture models available for each region. Fault‐rupture‐scaling relationships are also developed for intraslab (within the subducting slab), extensional outer‐rise and offshore strike‐slip environments. For these environments, the rupture width and area scaling properties yield smaller dimensions than interface ruptures for the corresponding magnitude. However, average and maximum slip metrics yield larger values than interface events. These observations reflect both the narrower fault widths and higher stress drops in these faulting environments. Although expressing significantly different rupture‐scaling properties from earthquakes in subduction environments, the characteristics of offshore strike‐slip earthquake ruptures compare similarly to commonly used rupture‐scaling relationships for onshore strike‐slip earthquakes. Electronic Supplement: Table of rupture parameters.

Ground‐Motion Prediction Equations for Subduction Interface Earthquakes in Japan Using Site Class and Simple Geometric Attenuation Functions

The frequency content of strong ground motions from subduction slab earthquakes differs significantly from that of ground motions produced by other categories (tectonic locations: shallow crustal, upper mantle, and subduction interface) of earthquakes in subduction zones. In the last two decades, a large number of records from subduction slab events have been obtained in Japan. We present a ground‐motion prediction equation (GMPE) for this category of earthquakes. We used a large dataset from reliably identified slab events up to the end of 2012. The GMPEs were based on a set of simple geometric attenuation functions, site classes were used as site terms, and nonlinear site amplification ratios were adopted. A bilinear magnitude‐scaling function was adopted for large earthquakes with moment magnitude M w≥7.1, with the scaling rates for large events being much smaller than for the smaller events. A magnitude‐squared term was used for events with M w<7.1 as well as the bilinear magnitude‐scaling function. We also modeled the effect of volcanic zones using an anelastic attenuation coefficient applied to a horizontal portion of the seismic‐wave travel distance within possible volcanic zones. We found that excluding the records from sites with inferred site classes improved the model goodness of fit. The within‐event residuals were approximately separated into within‐site and between‐site residuals, and the corresponding standard deviations were calculated using a random effects model. The separation of within‐event residuals into within‐site and between‐site components allows for the possibility of adopting different standard deviations for different site classes in a probabilistic seismic‐hazard analysis if desired. Online Material: Figures showing the distribution of between‐event residuals with respect to magnitude and fault‐top depth and the distribution of within‐event residuals with respect to magnitude and source distance.

Source‐Scaling Relations of Interface Subduction Earthquakes for Strong Ground Motion and Tsunami Simulation

The recording on high‐resolution broadband seismic networks of several great interface subduction earthquakes during the last decade provide an excellent opportunity to extend source‐scaling relations to very large magnitudes and to place constraints on the potential range of source parameters for these events. At present, there is a wide range of uncertainty in the median rupture areas predicted for any given seismic moment by current relationships between magnitude and rupture area for subduction interface earthquakes. Our goal is to develop an updated set of earthquake source‐scaling relations that will reduce this current large degree of epistemic uncertainty and improve the accuracy of seismic‐hazard analysis and the prediction of the strong‐motion characteristics and tsunamis of future subduction earthquakes. To achieve this goal, we compiled a database including slip models of interface earthquakes that occurred worldwide with moment magnitudes ranging from M 6.75–9.1. We characterized the seismic sources based on well‐established criteria to estimate the asperity areas as well as the average slip on the faults, and we used these parameters to compute an updated set of magnitude scaling relations of the various characteristics of the fault. Additionally, we followed an alternative approach to quantifying slip models for use in developing characteristic slip models of future earthquakes. This involved analyzing the 2D Fourier transforms of the slip functions in the compiled database and deriving a wavenumber spectral model of the slip distribution.

Ground Motion Models Used in the 2014 U.S. National Seismic Hazard Maps

The National Seismic Hazard Maps (NSHMs) are an important component of seismic design regulations in the United States. This paper compares hazard using the new suite of ground motion models (GMMs) relative to hazard using the suite of GMMs applied in the previous version of the maps. The new source characterization models are used for both cases. A previous paper ( Rezaeian et al. 2014 ) discussed the five NGA-West2 GMMs used for shallow crustal earthquakes in the Western United States (WUS), which are also summarized here. Our focus in this paper is on GMMs for earthquakes in stable continental regions in the Central and Eastern United States (CEUS), as well as subduction interface and deep intraslab earthquakes. We consider building code hazard levels for peak ground acceleration (PGA), 0.2-s, and 1.0-s spectral accelerations (SAs) on uniform firm-rock site conditions. The GMM modifications in the updated version of the maps created changes in hazard within 5% to 20% in WUS; decreases within 5% to 20% in CEUS; changes within 5% to 15% for subduction interface earthquakes; and changes involving decreases of up to 50% and increases of up to 30% for deep intraslab earthquakes for most U.S. sites. These modifications were combined with changes resulting from modifications in the source characterization models to obtain the new hazard maps.

Tsunami inundation from heterogeneous earthquake slip distributions: Evaluation of synthetic source models

AbstractThis study investigates whether eight different synthetic finite fault models (SFFM) can simulate stochastic earthquake‐tsunami with similar statistical properties to “real” earthquake‐tsunami events, where the latter are represented using heterogeneous slip distributions from 66 Finite Fault Inversions (FFI) for oceanic subduction interface earthquakes. A new method is derived to estimate SFFM parameters from FFI, and predictive relations between the earthquake moment magnitude and the SFFM corner wave numbers are developed to support model applications. SFFM with more capacity to spatially localize slip are better able to simulate higher slip regions on the FFI, and this strongly influences their associated tsunami inundation, which is computed in two dimensions over idealized topography. The best performing SFFM generates tsunami inundation which envelopes the FFI inundation in 81% of cases using 10 synthetic events (close to the ideal value of 82%), while the other SFFM show greater tendencies to underestimate inundation. These differences are related to the capacity of each SFFM to produce spatially localized slip distributions. None of the SFFM showed a tendency to overpredict inundation. The results highlight that SFFM cannot be assumed to reliably quantify uncertainties in the tsunami inundation of real earthquakes, and the use of untested SFFM could create nonconservative bias in tsunami hazard assessments. However, the most successful model used here performs quite well, although it may still underestimate inundation more often than an optimal model.

Evidence for Past Subduction Earthquakes at a Plate Boundary with Widespread Upper Plate Faulting: Southern Hikurangi Margin, New Zealand

Abstract At the southern Hikurangi margin, New Zealand, we use salt marsh stratigraphy, sedimentology, micropaleontology, and radiocarbon dating to document evidence of two earthquakes producing coseismic subsidence and (in one case) a tsunami over the past 1000 yrs. The earthquake at 520–470 yrs before present (B.P.) produced 0.25±0.1 m of subsidence at Big Lagoon. The earthquake at 880–800 yrs B.P. produced 0.45±0.1 m of subsidence at Big Lagoon and was accompanied by a tsunami that inundated ≥360 m inland with a probable height of ≥3.3 m. Distinguishing the effects of upper plate faulting from plate interface earthquakes is a significant challenge at this margin. We use correlation with regional upper plate paleoearthquake chronologies and elastic dislocation modeling to determine that the most likely cause of the subsidence and tsunami events is subduction interface rupture, although the older event may have been a synchronous subduction interface and upper plate fault rupture. The southern Hikurangi margin has had no significant ( M >6.5) documented subduction interface earthquakes in historic times, and previous assumptions that this margin segment is prone to rupture in large to great earthquakes were based on seismic and geodetic evidence of strong contemporary plate coupling. This is the first geologic evidence to confirm that the southern Hikurangi margin ruptures in large earthquakes. The relatively short‐time interval between the two subduction earthquakes (∼350 yrs) is shorter than in current seismic‐hazard models. Online Material: Historical accounts, description of vertical deformation, core names, foraminifera census and abundance, diatom census, modern analog samples, map of cores collected, stratigraphic correlation diagram for all cores, and detailed core logs.

Foraminiferal record of Holocene paleo-earthquakes on the subsiding south-western Poverty Bay coastline, New Zealand

Foraminiferal faunas in 29 short cores (maximum depth 7 m) of estuarine and coastal wetland sediment were used to reconstruct the middle–late Holocene (last 7 ka) elevational history on the southern shores of Poverty Bay, North Island, New Zealand. This coast is on the southwest side of a rapidly subsiding area beneath western Poverty Bay. Modern Analogue Technique paleo-elevation estimates based on fossil foraminiferal faunas indicate that the four study areas have gradual late Holocene (<3.5 ka) subsidence rates that increase from the southwest (mean c. 0.5 m ka–1) to northeast (mean c. 1.0 m ka–1). Only two rapid, possibly co-seismic, vertical displacement events are recognised: (1) c. 1.2 m of subsidence at 5.7 ± 0.4 ka (cal yr BP), which may have been generated by a subduction interface earthquake centred offshore and recorded in other published studies in northern Hawkes Bay, c. 35 km to the south; and (2) c. 1 m of uplift (relative sea-level fall) at c. 4.5 ± 0.3 ka, which might have been generated by rupture on an offshore upper plate fault that also uplifted coastal terraces at Pakarae and Mahia, 40 km to the north and south of the study area, or by rupture on the subduction interface penetrating beneath Poverty Bay. No sudden displacement events are recognised during the last 4 ka although subsidence, possibly aseismic, has continued.

Effect of seismic sequences in reinforced concrete frame buildings located in soft-soil sites

On September 19 and 20, 1985, two strong subduction interface earthquakes struck Mexico City leading to a large stock of damaged, or even collapsed, reinforced concrete (RC) building structures located in soft soil sites of the former lake-bed zone. The aim of this study is to gain further understanding on the effects of soft-soil seismic sequences on the seismic response of RC framed-buildings. This investigation employed artificial sequences since only two real sequences were gathered during the 1985 earthquakes. The nonlinear response, in terms of peak and residual lateral inter-story drift demands of four RC buildings having 4, 8, 12, and 16 stories, was evaluated. Results show that the relationship of the damaged period of the building (i.e. period of the building after the mainshock) to the predominant period of the aftershock, which is a measure of its frequency content, has a significant impact in the building response.

Submarine paleoseismology of the northern Hikurangi subduction margin of New Zealand as deduced from Turbidite record since 16 ka

Paleoseismic studies seek to characterise the signature of pre-historical earthquakes by deriving quantitative information from the geological record such as the source, magnitude and recurrence of moderate to large earthquakes. In this study, we provide a ∼16,000 yr-long paleo-earthquake record of the 200 km-long northern Hikurangi Margin, New Zealand, using cm-thick deep-sea turbidites identified in sediment cores. Cores were collected in strategic locations across the margin within three distinct morphological re-entrants – the Poverty, Ruatoria and Matakaoa re-entrants. The turbidite facies vary from muddy to sandy with evidence for rare hyperpycnites interbedded with hemipelagites and tephra. We use the Oxal probabilistic software to model the age of each turbidite, using the sedimentation rate of hemipelagite deduced from well-dated tephra layers and radiocarbon ages measurements on planktonic foraminifera.Turbidites are correlated from one core to the other using similarity in sedimentary facies, petrophysical properties and ages. Results show that 46 turbidites are synchronous along the entire margin. Amongst them 41 are interpreted as originating from the upper continental slope in response to earthquake-triggered slope failures between 390 ± 170 to 16,450 ± 310 yr BP. Using well-established empirical relationships that combine peak ground acceleration, magnitude and location of earthquakes, we calculate that synchronous slope failures were triggered by the rupture of 3 of the 26 known active faults in the region, each capable of generating Mw 7.3 to 8.4 earthquakes – two are crustal reverse faults and one is the subduction interface. The 41 Mw ≥ 7.3 earthquakes occurred at an average recurrence interval of ∼400 yr over the last ∼16,000 yr. Among them, twenty are interpreted as subduction interface earthquakes that occurred at an average recurrence interval of ∼800 yr, with alternating periods of high activity and low return times (305–610 yr) and quiescence periods with high return times (1480–2650 yr). Based on turbidite paleoseismology, we propose that subduction interface earthquakes were of lower magnitude during active periods (Mw > 7.5) than during quiescence periods (Mw ≥ 8.2).

Probabilistic Seismic Hazard Assessment for Bridgetown-Barbados, Employing Subduction Interface Characteristic Earthquakes

A new probabilistic seismic hazard analysis was performed for the city of Bridgetown, Barbados, West Indies. Hazard computations have been performed using the standard Cornell-McGuire approach based on the definition of appropriate seismogenic sources and expected maximum magnitudes, the authors take into consideration the possibility of large subduction interface earthquakes of magnitude 8.0-9.0 beneath the Barbados accretionary prism via application of a characteristic model and slip rates. The analysis has been conducted using a standard logic-tree approach. Uniform hazard spectra have been calculated for the 5% of critical damping and the horizontal component of ground motion for rock site conditions setting 5 return periods (95, 475, 975, 2,475 and 4,975 years) and spectral accelerations for 34 structural periods ranging from 0 to 3 s. The disaggregation results suggest that the magnitude-distance pair that dominates the hazard yields M 7.4 and 8.6 and a distance of 42.5 km in the Interface Subduction Zone beneath Barbados for the 475 and 975 years RP (return period), respectively. An event with an M 8.0 at a distance of 107.5 km in the Intraplate Subduction Zone is the second scenario that dominates the hazard for both 475 and 975 years RP.

Seismic hazard from the Hispaniola subduction zone: Correction to “Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region”

[1] In the paper “Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region” by U. S. ten Brink et al. (Journal of Geophysical Research, 116, B12318, doi:10.1029/2011JB008497, 2011), we incorrectly inferred an expected average slip of 17 m on the subduction interface rather than 1.7 m (p. 11 of 15, paragraph 33). We used this incorrect seismic slip estimate and a slip accumulation rate of 14 mm/yr to infer an incorrect recurrence interval of 1200 years for this segment of the subduction zone. Eight M ≥ 7 earthquakes took place along this segment in the twentieth century, with two events dominating the seismic moment, the M7.7–7.9 1943 and the M7.8–8.1 4 August 1946 earthquakes [ten Brink et al., 2011, Table 3]. No other comparable great historical earthquakes were felt, even though the historical record of subduction interface earthquakes in Hispaniola is likely complete for the past 500 years [ten Brink et al., 2011]. Ten Brink et al. [2011] suggested that a 1200 year recurrence interval was consistent with these observations, but a 120 year recurrence interval is considerably shorter than the historical record. We hereafter consider the questions: Why have there been no other great earthquakes, like those in 1943 and 1946, on the subduction interface north of Hispaniola since Europeans settled in Hispaniola in 1494? What are the implications to seismic hazard from this segment of the subduction zone? [2] Reanalyzing the 1943–1953 earthquake sequence in the region, Dolan and Wald [1998] noted the progression of seismic activity from east to west throughout this period. They determined the rupture area of the 1943 and 1946 earthquakes on the basis of aftershocks as 100 × 65 km and 195 × 95 km, respectively. It is thus possible that these two events, with additional smaller events during the twentieth century, have ruptured the entire 415 km long segment, defined by these earthquakes (Figure 1). P wave tomographic images show that this segment is the only segment with a continuous slab extending from the surface to a depth of 500 km in the northern Caribbean [ten Brink and Villasenor, 2012, Figures 1c–1e]. These tomographic images were derived using the P wave global tomographic technique of Bijwaard et al. [1998], improved with additional arrival times from well-located earthquakes at both teleseismic and regional distances [Villasenor et al., 2003]. A horizontal high-velocity layer at depths <150 km underlies Haiti (Figure 2b), whereas north of Puerto Rico, the slab “is missing” at depths of 50–150 km (Figure 2f). Therefore, this 415 km long region likely defines a subduction segment. [3] Why have there been no other great earthquakes like the 1946 earthquake in the preceding historically documented 450 years? The expected average slip during an earthquake, d, is given by

Magnitude-Scaling Rate in Ground-Motion Prediction Equations for Response Spectra from Large Subduction Interface Earthquakes in Japan

Magnitude‐scaling rates (MSRs; the rates of increase in predicted response spectra with increasing moment magnitude) are evaluated for three ground‐motion prediction equations for response spectra from subduction interface earthquakes, including two empirical models developed for data from Japan and a model based on synthetic records generated by using a stochastic finite‐fault model. MSRs vary significantly among the three models, and the difference between the two empirical models is unacceptably large. A set of 2100 strong‐motion records from subduction interface events with a magnitude of 6.5 or larger from Japan, including the 11 March 2011 magnitude 9 earthquake, were compiled. The earthquakes were grouped according to magnitude, so that the magnitude spread in each group is less than 0.2 magnitude units. Each earthquake group was treated as a single event with magnitude equal to the average magnitude for the group. A simple attenuation model was fitted to the normalized and grouped data. The model has a constant term for each earthquake group to represent the effect of magnitude. Three separate functions of magnitude (a linear model for events with a magnitude greater than 7, a bilinear model, and a curved model) were then fitted to the constants, and MSRs were derived from these functions. At short periods, the derived MSRs are only a fraction of those from two of the three attenuation models. At spectral periods greater than 0.5 s, the derived MSRs are between about 1/3 and 1/2 of those of the two empirical models but are closer to those based on a set of synthetic records.

The kinematics of a transition from subduction to strike‐slip: An example from the central New Zealand plate boundary

We develop a kinematic model for the transition from subduction beneath the North Island, New Zealand, to strike‐slip in the South Island, constrained by GPS velocities and active fault slip data. To interpret these data, we use an approach that inverts the kinematic data for poles of rotation of tectonic blocks and the degree of interseismic coupling on faults in the region. Convergence related to the Hikurangi subduction margin becomes very low offshore of the northern South Island, indicating that in this region the majority of the relative plate motion has been transferred onto faults within the upper plate, as suggested by previous studies. This result has implications for understanding the likely extent of subduction interface earthquake rupture in central New Zealand. Easterly trending strike slip faults (such as the Boo Boo fault) are the key features that facilitate the transfer of strike‐slip motion from the northern South Island faults further north into the southern North Island and onto the Hikurangi subduction thrust. Our results also indicate that the transition from rapid forearc rotation adjacent to the Hikurangi subduction margin to a strike‐slip dominated plate boundary (with negligible vertical‐axis rotation) in the South Island occurs via a crustal‐scale hinge or kink in the upper plate, compatible with paleomagnetic and structural geological data. Despite the ongoing tectonic evolution of the central New Zealand region, our study highlights a remarkable consistency between data sets spanning decades (GPS), thousands of years (active faulting data), and millions of years (paleomagnetic data and bedrock structure).

Earthquake shaking probabilities for communities on Vancouver Island, British Columbia, Canada

Comprehensive risk assessments are fundamental to effective emergency management. These assessments need to identify the range of hazards (or perils) an entity is exposed to and quantify the specific threats associated with each of those hazards. While hazard identification is commonly, if not formally, conducted in most circumstances, specific threat analysis is often overlooked for a variety of reasons, one of which is poor communication with subject matter experts. This poor communication is often attributable to an adherence to scientific jargon and missed opportunities to simplify information. In Canada, for example, earthquake hazard calculations have been readily available to engineers and scientists for decades. This hazard information, however, is expressed in terms of peak ground accelerations (PGA) or spectral accelerations (SA) that are foreign concepts to most emergency managers, community decision-makers and the public-at-large. There is, therefore, a need to more clearly, simply and effectively express seismic hazard information to the non-scientific community. This paper provides crustal, sub-crustal and subduction interface earthquake shaking probabilities, expressed as simple percentages for each of 57 locations across Vancouver Island, British Columbia, Canada. Calculations present the likelihood of earthquake shaking on Vancouver Island as the probabilities of exceeding each of three shaking intensity thresholds (“widely felt”; onset of “non-structurally damaging” shaking; and onset of “structurally damaging” shaking) over four timeframes (10, 25, 50 and 100 years). Results are based on the latest Geological Survey of Canada hazard models used for the 2010 national building code and are presented in both tabular and graphic formats. This simplified earthquake hazard information is offered to aid local residents, organizations and governments in understanding and assessing their risk and to encourage and facilitate sound earthquake preparedness funding decisions.

Oblique slip on the Puysegur subduction interface in the 2009 July MW 7.8 Dusky Sound earthquake from GPS and InSAR observations: implications for the tectonics of southwestern New Zealand

SUMMARY The MW 7.8 Dusky Sound earthquake of 2009 July 15 was the largest earthquake in New Zealand in the past ∼80 yr and is the only major subduction interface earthquake in the New Zealand historical record. We have estimated the coseismic and early post-seismic slip distribution in the earthquake by inversion of GPS and differential interferometric synthetic aperture radar (DInSAR) observations. We show that slip during the earthquake was highly obliqueandessentiallyintheAustralia–Pacificrelativeplatemotiondirection.Failureoccurred onaca.80by50kmpatchofthesubductioninterfacewithmaximumslip5–6m.Inthisregion, the offshore Alpine Fault carries up to ∼30 mm yr –1 of slip before terminating at Resolution Ridge near the southern end of the 2009 slip patch. The southernmost part of the Alpine Fault trace lies near the up-dip end of the 2009 slip patch and the fault probably exists only within the crust of the over-riding plate in this region, terminating at shallow depth where it meets the subduction interface. There is no evidence for rupture of the Alpine Fault during the 2009 event. However, Coulomb stress changes from the subduction earthquake have brought this section of the Alpine Fault closer to failure. In contrast to many subduction zones undergoing oblique subduction, where the slip in major earthquakes is partitioned into more-or-less pure thrustearthquakesonthesubductioninterfaceandstrike-slipearthquakesinthebackarc,thisis a case where the majority of the subduction interface slip is not partitioned, with only a shallow convergence zone west of the strike-slip Alpine Fault undergoing contraction approximately normal to subduction zone strike. North of the 2009 earthquake region the Alpine Fault lies further east relative to the subduction interface and partitioning between strike-slip on the Alpine Fault and trench-normal thrusting on the subduction interface takes place in a more typical fashion. Our results provide the first clear demonstration of non-partitioned oblique slip between the Australian and Pacific plates offshore of southern New Zealand.

Influence of the 3D Distribution of Q and Crustal Structure on Ground Motions from the 2003 Mw 7.2 Fiordland, New Zealand, Earthquake

The M w 7.2 earthquake of 21 August 2003 provides an unprecedented opportunity to evaluate the applicability of the New Zealand acceleration response spectrum attenuation model (McVerry et al. , 2006) in the Fiordland region and to consider effects from 3D Q heterogeneity. This was the largest shallow earthquake in New Zealand in the past 40 yrs. It was well recorded by the GeoNet seismograph and strong-motion network. We compare the observed 5% damped response spectral accelerations (SAs) to the New Zealand model values, for both crustal reverse and subduction interface earthquakes. On average, both models underpredict the observed SAs, by approximately 0.4 in lnSA, for 0–400 km distance; however, the data show great variability. Using the South Island 3D Q model, we compute the path-averaged attenuation rate, CQ for synthetic Fiordland hypocenters to distributed sites. Compared to the standard model, the relatively high Q in South Island produces lower attenuation rates, although the low- Q crustal root of the Southern Alps produces a 10 times greater attenuation rate for certain paths. The average attenuation rate estimated from the SA data is similar to CQ for frequencies <5 Hz and constant above 5 Hz. For an acceleration model for Fiordland earthquakes, 0–60 km depth, an applicable anelastic attenuation rate is the mean CQ from the synthetic hypocenters, which is 0.006 km-1 at 2.5 Hz. For the observed SAs, the most significant regional effect is high relative SAs at some sites that may be due to postcritical Moho reflections from the Southern Alps crustal root.

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 (&gt;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 thrust's seismogenic zone.

The 2003, Mw 7.2 Fiordland earthquake, and its near-source aftershock strong motion data

The 2003 Fiordland earthquake was not only the best ever recorded subduction interface earthquake to occur in New Zealand, it also provided the opportunity to collect near-source strong-motion data produced by its aftershocks covering a wide magnitude range. Near source strong-motion data had been lacking in the New Zealand data set, on which current attenuation models are based. Here the author presents some preliminary results relating recorded peak ground accelerations in the near-source field to current attenuation models. The near-source data from the 2003 Fiordland earthquake sequence has shown that the observed data has a greater magnitude-dependence than that predicted by the current attenuation models. This new data will help to improve current models and will lead to a better understanding of the attenuation process associated with New Zealand subduction interface earthquakes.

The magnitude 8.3 June 23 2001 southern Peru earthquake and tsunami

We present a precis of our reconnaissance trip to the area of the magnitude 8.3 June 23 2001 southern Peru earthquake and tsunami. The trip was undertaken because of the relevance of the event to hazard assessment in New Zealand. It is the best example in nearly 40 years of the maximum-size earthquake that might occur on the Hikurangi subduction zone, an event that is absent from the historical record of New Zealand (since 1840) and therefore of unknown potential in terms of hazard. Despite the great magnitude of this subduction interface earthquake, it produced only "moderately strong" levels of earthquake shaking (peak ground acceleration of 0.3g on alluvium from the one strong motion accelerograph in the earthquake area, and Modified Mercalli Intensity 8 in the epicentral area), and relatively minor ground damage (liquefaction and landslides). It did however produce a large and devastating tsunami. Our comparison of the one accelerograph record and attenuation curves for subduction interface earthquakes shows that the strength of shaking was typical for subduction interface earthquakes. If we apply our observations to New Zealand, they imply that a Hikurangi subduction interface earthquake may be less damaging to built-up areas in the southeastern part of the North Island (e.g. Wellington and Napier/Hastings) than earthquakes on major active faults in the shallow crust. However, the lateral extent of the strongest shaking in a subduction earthquake (300 km for the southern Peru event) and the associated tsunami generation will make the earthquake very significant in the national context.