Spatiotemporal analysis of repeating earthquakes near Porangahau, Hikurangi Margin, New Zealand

10.26686/wgtn.14760234.v1 ◽

2021 ◽

Author(s):

Olivia Pita Sllim

Keyword(s):

New Zealand ◽

Cross Correlation ◽

Slip Rate ◽

Pacific Plate ◽

Focal Mechanisms ◽

Slow Slip ◽

Slow Slip Events ◽

Repeating Earthquakes ◽

The Pacific ◽

Temporal And Spatial

The Hikurangi subduction zone beneath the eastern North Island, New Zealand exhibits a variety of fault-slip related phenomena including tsunami earthquakes, non-volcanic tremor, low-frequency earthquakes, episodic slow slip, and repeating earthquakes. The northern Hikurangi margin hosts shallow slow-slip and is weakly coupled to shallow depths. In contrast, the southern Hikurangi margin is strongly coupled, and only deep slow-slip has been observed. The transition in coupling occurs beneath the township of Porangahau, and is an exemplary focus region for studying how this change in locking is accommodated. To examine slip processes beneath Porangahau, we have constructed and analysed a catalogue of repeating earthquakes that occurred between 2004 and 2018. Repeating earthquakes are thought to re-rupture the same fault patch at different times, and thus have nearly identical waveforms, locations and magnitudes. Because repeating earthquakes represent cyclic loading, they can be used to detect temporal and spatial changes of slip-rate at depth and hence monitor how stress is transferred to seismogenic zones. To build a catalogue of repeating earthquakes we first clustered the GeoNet earthquake catalogue by distance and correlation to identify potential repeating events. We then used a stronger cross-correlation threshold of at least 0.95 normalised cross-correlation value at three or more stations to identify repeating earthquakes from the initial clusters. This threshold was determined by our group's previous work on the northern Hikurangi margin. We identified 225 families of repeating earthquakes, with each family having two or more earthquakes in the 14-year study period from 2004 to 2018. We carried out manual phase picking and polarity identification for the most recent event in each family and computed absolute locations, local magnitudes calibrated with moment magnitude, and high-quality focal mechanisms. For the rest of the events in each family, we conducted cross-correlation re-picking to obtain precise relative locations and relative magnitudes. With precise locations and well-constrained focal mechanisms, we determined whether the repeating earthquake families originated within the Pacific Plate, Australian Plate or on the subduction interface. Most of the families are located within the Pacific Plate, and the majority of families that originate on the subduction interface are located near the township of Porangahau. At least 220 of the 532 identified repeating earthquakes locate at the transition from strong- to weak-coupling of the subduction interface near the township of Porangahau. A variety of slow slip events have been detected near Porangahau in the last two decades. Even though some repeating earthquakes correlate spatially and temporally with slow slip events, temporal and spatial correlations between slow slip events and repeating earthquakes are scarce and sparse. The majority of repeating earthquakes are located up-dip or down-dip of modelled slow slip patches, with very few families having spatial correlation with slow slip events. We obtained a moment-recurrence interval relationship for the catalogue of repeating earthquakes near Porangahau and compared it to the relationship obtained by Nadeau and Johnson (1998) at Parkfield, California. Finally, we computed slip-rates using the families located on the subduction interface and obtained an average slip-rate of 13 mm/yr. The insights gained from this study lay the groundwork for future work constraining processes of strain accumulation at the creeping-to-locked transition zone near Porangahau.

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Using low-frequency earthquakes to monitor slow tectonic deformation in the central Southern Alps, New Zealand

10.26686/wgtn.17134781.v1 ◽

2021 ◽

Author(s):

◽

Laura-May Baratin Wachten

Keyword(s):

New Zealand ◽

Matched Filter ◽

Low Frequency ◽

Southern Alps ◽

Focal Mechanisms ◽

Slow Slip ◽

Slow Slip Events ◽

Event Time ◽

Alpine Fault ◽

Occurrence Patterns

This thesis involves the study of low-frequency earthquakes (LFEs) in the central Southern Alps. The Alpine Fault is the principal locus of deformation within the Australia–Pacific plate boundary in the South Island of New Zealand and it is late in its typical ∼300-year seismic cycle. Surveying the seismicity associated with slow deformation in the vicinity of the Alpine Fault may provide constraints on the stresses acting on a major transpressive margin prior to an anticipated great (≥M8) earthquake. Here, we use 8 years of data from the Southern Alps Microearthquake Borehole Array (SAMBA) (amongst those, 3 years of data were collected as part of this project) in order to: (1) generate an updated LFE catalogue using an improved matched-filter technique that incorporates phase-weighted stacking; (2) compute LFE focal mechanisms and invert them to infer the crustal stress field on the deep extent of the Alpine Fault; (3) expand the LFE catalogue to cover a wider range of spatial/temporal behaviours; (4) study LFE families’ characteristics to identify periods where slow slip might happen. We first use fourteen primary LFE templates in an iterative matched-filter and stacking routine, which allows the detection of similar signals and produces LFE families sharing common locations. We generate an 8-yr catalogue containing 10,000 LFEs that are combined for each of the 14 LFE families using phase-weighted stacking to produce signals with the highest possible signal-to-noise ratios. We find LFEs to occur almost continuously during the 8-yr study period and we highlight two types of LFE distributions: (1) discrete behaviour with an inter-event time exceeding 2 minutes; (2) burst-like behaviour with an inter-event time below 2 minutes. The discrete events are interpreted as small-scale frequent deformation on the deep extent of the Alpine Fault and the LFE bursts (corresponding in most cases to known episodes of tremor or large regional earthquakes) are interpreted as brief periods of increased slip activity indicative of slow slip. We compute improved non-linear earthquake locations using a 3D velocity model and find LFEs to occur below the seismogenic zone at depths of 17–42 km, on or near the hypothesised deep extent of the Alpine Fault. We then compute the first estimates of LFE focal mechanisms associated with continental faulting. Focal mechanisms, in conjunction with recurrence intervals, are consistent with quasi-continuous shear faulting on the deep extent of the Alpine Fault. We then generate a new catalogue that regroups hundreds of LFE families. This time 638 synthetic LFE waveforms are generated using a 3D grid and used as primary templates in a matched-filter routine. Of those, 529 templates yield enough detections during the first iteration of the matched-filter routine (≥ 500 detections over the 8-yr study period) and are kept for further analysis. We then use the best 25% of correlated events for each LFE family to generate linear stacks which create new LFE templates. From there, we run a second and final iteration of the matched-filter routine with the new LFE templates to obtain our final LFE catalogue. The remaining 529 templates detect between 150 and 1,671 events each totalling 300,996 detections over the 8-yr study period. Of those 529 LFEs, we manage to locate 378 families. Their depths range between 11 and 60 km and LFEs locate mainly in the southern part of the SAMBA network. We finally examine individual LFE family rates and occurrence patterns. They indicate that LFE sources seem to evolve from an episodic or ‘stepped’ to a continuous behaviour with depth. This transition may correspond to an evolution from a stick-slip to a stable-sliding slip regime. Hence, we propose that the distinctive features of LFE occurrence patterns reflect variations in the in-situ stress and frictional conditions at the individual LFE source locations on the Alpine Fault. Finally, we use this new extensive catalogue as a tool for in-depth analyses of the deep central Alpine Fault structure and its slip behaviour. We identify eight episodes of increased LFE activity between 2009 and 2017 and provide time windows for further investigations of tremor and slow slip. We also study the spatial and temporal behaviours of LFEs and find that LFEs with synchronous occurrence patterns tend to be clustered in space. We thus suggest that individual LFE sources form spatially coherent clusters that may represent localised asperities or elastic patches on the deep Alpine Fault interface. We infer that those clusters may have a similar rheological response to tectonic forcing or to potential slow slip events. Eventually, we discover slow (10km/day) and rapid (∼20-25km/h) migrations of LFEs along the Alpine Fault. The slow migration might be controlled by slow slip events themselves while the rapid velocities could be explained by the LFE sources’ intrinsic properties.

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Repeating Earthquake Detection and Characterisation in New Zealand: A Catalogue for the Raukumara Peninsula, Northern Hikurangi Subduction Margin

10.26686/wgtn.17148299 ◽

2021 ◽

Author(s):

◽

Laura Hughes

Keyword(s):

New Zealand ◽

Cross Correlation ◽

Active Faults ◽

P Wave ◽

Slow Slip ◽

Normal Faulting ◽

Repeating Earthquakes ◽

Long Duration ◽

Recurrence Intervals ◽

Repeating Earthquake

Repeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. In this thesis, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand and present New Zealand’s first long-duration repeating earthquake catalogue. This thesis addresses three primary objectives: (1) develop a method and composite criterion for identifying repeating earthquakes; (2) build a long-duration catalogue of repeating earthquakes for the Raukumara Peninsula; and (3) apply the method and composite criterion in different tectonic settings to investigate whether it can be applied more broadly elsewhere in New Zealand. The systematic identification of repeating earthquakes in New Zealand provides the first step in being able to monitor the state of stresses of New Zealand’s active faults in situ throughout the earthquake cycle. Studies elsewhere, particularly in Japan and California, have developed case-specific criteria for identifying repeating earthquakes. Building on these studies, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand, focusing on seismicity around the Raukumara Peninsula. Our composite criterion states that for events to be identified as repeating earthquakes, two or more events must have a normalised cross-correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. Sensitivity to correlation window length, filtering frequency-band and correlation threshold were tested during the development of the composite criterion. These tests indicated that small perturbations to the parameter thresholds did not affect our ability to detect repeating earthquakes using the composite criterion. By applying our composite criterion to seismicity around the Raukumara Peninsula, we identified 62 repeating earthquake families occurring between 2003 and 2018, consisting of 160 individual earthquakes. These families have a magnitude range of MW 1.5–4.5, and have recurrence intervals and family durations of < 1–12 years. High-precision absolute and relative locations were calculated using manual phase picks and cross-correlation re-picking. Focal mechanisms for 56 of the families were also determined, using P-wave first motions, revealing predominantly strike-slip and normal faulting at shallow depths, low-angle reverse faulting along the subduction interface, and normal faulting in the subducting plate. We compared the timing of the repeating earthquakes to slow-slip events previously identified using geodetic measurements around the Raukumara Peninsula and observed that repeating earthquakes occurred during 26 of the 31 identified periods of slow-slip. We also compared the seismic moment– recurrence interval relationship of the Raukumara Peninsula repeating earthquakes to that of earthquakes near Parkfield, California, identified by Nadeau and Johnson (1998), and observed a similar functional relationship. Slip-rates of the Raukumara Peninsula repeating earthquake families were also calculated using a slip-rate–moment relationship and were found to vary from < 10mm/yr to 80mm/yr. We applied the method and composite criterion developed for the Raukumara Peninsula to two other locations to ensure it could be applied successfully in other New Zealand regions with different seismotectonic characteristics. Using our workflow, we successfully identified four families in Marlborough, and three families around Fiordland. These families differ from those identified around the Raukumara Peninsula in that they had relatively short recurrence intervals and family durations, of 2 minutes– 15 months. The ability of the composite criterion to identify these families confirms its suitability for further studies of repeating earthquakes throughout New Zealand.

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Repeating Earthquake Detection and Characterisation in New Zealand: A Catalogue for the Raukumara Peninsula, Northern Hikurangi Subduction Margin

10.26686/wgtn.17148299.v1 ◽

2021 ◽

Author(s):

◽

Laura Hughes

Keyword(s):

New Zealand ◽

Cross Correlation ◽

Active Faults ◽

P Wave ◽

Slow Slip ◽

Normal Faulting ◽

Repeating Earthquakes ◽

Long Duration ◽

Recurrence Intervals ◽

Repeating Earthquake

Repeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. In this thesis, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand and present New Zealand’s first long-duration repeating earthquake catalogue. This thesis addresses three primary objectives: (1) develop a method and composite criterion for identifying repeating earthquakes; (2) build a long-duration catalogue of repeating earthquakes for the Raukumara Peninsula; and (3) apply the method and composite criterion in different tectonic settings to investigate whether it can be applied more broadly elsewhere in New Zealand. The systematic identification of repeating earthquakes in New Zealand provides the first step in being able to monitor the state of stresses of New Zealand’s active faults in situ throughout the earthquake cycle. Studies elsewhere, particularly in Japan and California, have developed case-specific criteria for identifying repeating earthquakes. Building on these studies, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand, focusing on seismicity around the Raukumara Peninsula. Our composite criterion states that for events to be identified as repeating earthquakes, two or more events must have a normalised cross-correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. Sensitivity to correlation window length, filtering frequency-band and correlation threshold were tested during the development of the composite criterion. These tests indicated that small perturbations to the parameter thresholds did not affect our ability to detect repeating earthquakes using the composite criterion. By applying our composite criterion to seismicity around the Raukumara Peninsula, we identified 62 repeating earthquake families occurring between 2003 and 2018, consisting of 160 individual earthquakes. These families have a magnitude range of MW 1.5–4.5, and have recurrence intervals and family durations of < 1–12 years. High-precision absolute and relative locations were calculated using manual phase picks and cross-correlation re-picking. Focal mechanisms for 56 of the families were also determined, using P-wave first motions, revealing predominantly strike-slip and normal faulting at shallow depths, low-angle reverse faulting along the subduction interface, and normal faulting in the subducting plate. We compared the timing of the repeating earthquakes to slow-slip events previously identified using geodetic measurements around the Raukumara Peninsula and observed that repeating earthquakes occurred during 26 of the 31 identified periods of slow-slip. We also compared the seismic moment– recurrence interval relationship of the Raukumara Peninsula repeating earthquakes to that of earthquakes near Parkfield, California, identified by Nadeau and Johnson (1998), and observed a similar functional relationship. Slip-rates of the Raukumara Peninsula repeating earthquake families were also calculated using a slip-rate–moment relationship and were found to vary from < 10mm/yr to 80mm/yr. We applied the method and composite criterion developed for the Raukumara Peninsula to two other locations to ensure it could be applied successfully in other New Zealand regions with different seismotectonic characteristics. Using our workflow, we successfully identified four families in Marlborough, and three families around Fiordland. These families differ from those identified around the Raukumara Peninsula in that they had relatively short recurrence intervals and family durations, of 2 minutes– 15 months. The ability of the composite criterion to identify these families confirms its suitability for further studies of repeating earthquakes throughout New Zealand.

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Using low-frequency earthquakes to monitor slow tectonic deformation in the central Southern Alps, New Zealand

10.26686/wgtn.17134781 ◽

2021 ◽

Author(s):

◽

Laura-May Baratin Wachten

Keyword(s):

New Zealand ◽

Matched Filter ◽

Low Frequency ◽

Southern Alps ◽

Focal Mechanisms ◽

Slow Slip ◽

Slow Slip Events ◽

Event Time ◽

Alpine Fault ◽

Occurrence Patterns

This thesis involves the study of low-frequency earthquakes (LFEs) in the central Southern Alps. The Alpine Fault is the principal locus of deformation within the Australia–Pacific plate boundary in the South Island of New Zealand and it is late in its typical ∼300-year seismic cycle. Surveying the seismicity associated with slow deformation in the vicinity of the Alpine Fault may provide constraints on the stresses acting on a major transpressive margin prior to an anticipated great (≥M8) earthquake. Here, we use 8 years of data from the Southern Alps Microearthquake Borehole Array (SAMBA) (amongst those, 3 years of data were collected as part of this project) in order to: (1) generate an updated LFE catalogue using an improved matched-filter technique that incorporates phase-weighted stacking; (2) compute LFE focal mechanisms and invert them to infer the crustal stress field on the deep extent of the Alpine Fault; (3) expand the LFE catalogue to cover a wider range of spatial/temporal behaviours; (4) study LFE families’ characteristics to identify periods where slow slip might happen. We first use fourteen primary LFE templates in an iterative matched-filter and stacking routine, which allows the detection of similar signals and produces LFE families sharing common locations. We generate an 8-yr catalogue containing 10,000 LFEs that are combined for each of the 14 LFE families using phase-weighted stacking to produce signals with the highest possible signal-to-noise ratios. We find LFEs to occur almost continuously during the 8-yr study period and we highlight two types of LFE distributions: (1) discrete behaviour with an inter-event time exceeding 2 minutes; (2) burst-like behaviour with an inter-event time below 2 minutes. The discrete events are interpreted as small-scale frequent deformation on the deep extent of the Alpine Fault and the LFE bursts (corresponding in most cases to known episodes of tremor or large regional earthquakes) are interpreted as brief periods of increased slip activity indicative of slow slip. We compute improved non-linear earthquake locations using a 3D velocity model and find LFEs to occur below the seismogenic zone at depths of 17–42 km, on or near the hypothesised deep extent of the Alpine Fault. We then compute the first estimates of LFE focal mechanisms associated with continental faulting. Focal mechanisms, in conjunction with recurrence intervals, are consistent with quasi-continuous shear faulting on the deep extent of the Alpine Fault. We then generate a new catalogue that regroups hundreds of LFE families. This time 638 synthetic LFE waveforms are generated using a 3D grid and used as primary templates in a matched-filter routine. Of those, 529 templates yield enough detections during the first iteration of the matched-filter routine (≥ 500 detections over the 8-yr study period) and are kept for further analysis. We then use the best 25% of correlated events for each LFE family to generate linear stacks which create new LFE templates. From there, we run a second and final iteration of the matched-filter routine with the new LFE templates to obtain our final LFE catalogue. The remaining 529 templates detect between 150 and 1,671 events each totalling 300,996 detections over the 8-yr study period. Of those 529 LFEs, we manage to locate 378 families. Their depths range between 11 and 60 km and LFEs locate mainly in the southern part of the SAMBA network. We finally examine individual LFE family rates and occurrence patterns. They indicate that LFE sources seem to evolve from an episodic or ‘stepped’ to a continuous behaviour with depth. This transition may correspond to an evolution from a stick-slip to a stable-sliding slip regime. Hence, we propose that the distinctive features of LFE occurrence patterns reflect variations in the in-situ stress and frictional conditions at the individual LFE source locations on the Alpine Fault. Finally, we use this new extensive catalogue as a tool for in-depth analyses of the deep central Alpine Fault structure and its slip behaviour. We identify eight episodes of increased LFE activity between 2009 and 2017 and provide time windows for further investigations of tremor and slow slip. We also study the spatial and temporal behaviours of LFEs and find that LFEs with synchronous occurrence patterns tend to be clustered in space. We thus suggest that individual LFE sources form spatially coherent clusters that may represent localised asperities or elastic patches on the deep Alpine Fault interface. We infer that those clusters may have a similar rheological response to tectonic forcing or to potential slow slip events. Eventually, we discover slow (10km/day) and rapid (∼20-25km/h) migrations of LFEs along the Alpine Fault. The slow migration might be controlled by slow slip events themselves while the rapid velocities could be explained by the LFE sources’ intrinsic properties.

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Estimation of frictional properties and slip evolution on a long-term slow slip event fault with the ensemble Kalman filter: numerical experiments

Geophysical Journal International ◽

10.1093/gji/ggz415 ◽

2019 ◽

Vol 219 (3) ◽

pp. 2074-2096 ◽

Cited By ~ 2

Author(s):

Kazuro Hirahara ◽

Kento Nishikiori

Keyword(s):

Surface Displacement ◽

Slip Rate ◽

Slow Slip ◽

Slow Slip Events ◽

Actual Distribution ◽

Frictional Properties ◽

Data Analyses ◽

Bungo Channel ◽

Megathrust Faults ◽

Gnss Data

Summary A variety of slow slip events at subduction zones have been observed. They can be stress meters for monitoring the stress state of megathrust faults during their earthquake cycles. In this study, we focus on long-term slow slip events (LSSEs) recurring at downdip portions of megathrust faults among such slow earthquakes. Data analyses and simulation studies of LSSEs have so far been executed independently. In atmosphere and ocean sciences, data assimilations that optimally combine data analyses and simulation studies have been developed. We develop a method for estimating frictional properties and monitoring slip evolution on an LSSE fault, with a sequential data assimilation method, the ensemble Kalman filter (EnKF). We executed numerical twin experiments for the Bungo Channel LSSE fault in southwest Japan to validate the method. First, based on a rate- and state-dependent friction law, we set a rate-weakening circular LSSE patch on the rate-strengthening flat plate interface, whose critical nucleation size is larger than that of the patch, and reproduced the observed Bungo Channel LSSEs with recurrence times of approximately 7 yr and slip durations of 1 yr. Then, we synthesized the observed data of surface displacement rates at uniformly distributed stations with noises from the simulated slip model. Using our EnKF method, we successfully estimated the frictional parameters and the slip rate evolution after a few cycles. Secondly, we considered the effect of the megathrust fault existing in the updip portion of the LSSE fault, as revealed by kinematic inversion studies of Global Navigation Satellite System (GNSS) data and added this locked region with a slip deficit rate in the model. We estimated the slip rate on the locked region only kinematically, but the quasi-dynamic equation of motion in each LSSE fault cell includes the stress term arising from the locked region. Based on this model, we synthesized the observed surface displacement rate data for the actual distribution of GNSS stations and executed EnKF estimations including the slip rate on the locked region. The slip rate on the locked region could be quickly retrieved. Even for the actual distribution of GNSS stations, we could successfully estimate frictional parameters and slip evolution on the LSSE fault. Thus, our twin numerical experiments showed the validity of our EnKF method, although we need further studies for actual GNSS data analyses.

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Slip-rate-dependent friction as a universal mechanism for slow slip events

Nature Geoscience ◽

10.1038/s41561-020-0627-9 ◽

2020 ◽

Vol 13 (10) ◽

pp. 705-710

Author(s):

Kyungjae Im ◽

Demian Saffer ◽

Chris Marone ◽

Jean-Philippe Avouac

Keyword(s):

Slip Rate ◽

Slow Slip ◽

Slow Slip Events ◽

Rate Dependent ◽

Universal Mechanism

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Simple Physical Model for the Probability of a Subduction- Zone Earthquake Following Slow Slip Events and Earthquakes: Application to the Hikurangi Megathrust, New Zealand

Geophysical Research Letters ◽

10.1029/2018gl077641 ◽

2018 ◽

Vol 45 (9) ◽

pp. 3932-3941 ◽

Cited By ~ 7

Author(s):

Yoshihiro Kaneko ◽

Laura M. Wallace ◽

Ian J. Hamling ◽

Matthew C. Gerstenberger

Keyword(s):

New Zealand ◽

Physical Model ◽

Subduction Zone ◽

Slow Slip ◽

Slow Slip Events ◽

Simple Physical Model ◽

Subduction Zone Earthquake

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Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: The North Mernoo Fault Zone, offshore New Zealand

Tectonics ◽

10.1029/94tc00798 ◽

1994 ◽

Vol 13 (4) ◽

pp. 735-754 ◽

Cited By ~ 17

Author(s):

Philip M. Barnes

Keyword(s):

New Zealand ◽

Subduction Zone ◽

Fault Zone ◽

Pacific Plate ◽

Continental Extension ◽

The North ◽

The Pacific

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A preliminary catalogue of Hikurangi, New Zealand, Slow Slip Earthquakes, from January 2000 to February 2014

10.26686/wgtn.17012783.v1 ◽

2021 ◽

Author(s):

◽

B. Peter Baxter

Keyword(s):

New Zealand ◽

Temporal Evolution ◽

Processing Route ◽

Slow Slip ◽

Slow Slip Events ◽

Slip Rates ◽

The North ◽

General Terms ◽

Significant Step

This thesis documents processing carried out on cGPS data from 115 sites in the North Island and the top of the South Island of New Zealand in order to produce a catalogue of slow slip events (SSEs) for the Hikurangi Margin covering the period Jan 2000 to Feb 2014. It covers the background to the concept of SSEs and the reporting to date on their occurrence along the Margin, the methods used in the processing and analysis, the results of each significant step, and discussion of the results. It has been shown that the processing route adopted in this work has reduced the average noise levels in the cGPS data by up to 67%, and has eliminated virtually all correlated (“pink”) noise, thus enabling the detection of small-amplitude events (~ 2mm in cGPS signals). One hundred and fifty events are catalogued in total, of which 137 are considered likely to be SSEs or similar. The catalogue includes estimates of the uncertainty in each parameter and is thus considered the most comprehensive to date. Sixteen of the inversion results were able to be directly compared with published information and showed satisfactory agreement on location and equivalent moment magnitudes. The important aspects of the project that have been developed further than has been documented to date in the literature include: partitioning of the secular velocity field over the margin to allow the underlying tectonic signal to be better understood; detailed characterization of the temporal evolution of the SSEs; the identification of approximately 40 events that show slips in the opposite direction to that expected; and some preliminary conclusions concerning event scaling. One of the objectives of the project was to identify whether there were fundamental differences in the characteristics of SSEs in the northeast and southwest of the margin. On the basis of the analyses to date, it appears that the events form a continuum, at least in terms of depth, temporal evolution, source slip rates and scaling, but in general terms the events in the southwest have been confirmed to be of longer duration than those in the northeast. The project has identified further work that needs to be carried out or is ongoing in order to maximize the value of these new results.

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