scholarly journals Accommodating ~9 m of dextral slip on the Kekerengu Fault through ground deformation during the Mw 7.8 Kaikōura Earthquake, November 2016

2021 ◽  
Author(s):  
◽  
Philippa Morris
Keyword(s):  
Strike Slip ◽  
Recurrence Interval ◽  
Rupture Zone ◽  
Interval Estimate ◽  
Large Magnitude ◽  
Near Surface ◽  
Before And After ◽  
Age Model ◽  
Kaikoura Earthquake ◽  
Dextral Strike Slip

<p>The Mw 7.8 Kaikōura earthquake of November 14th 2016 provided unprecedented opportunities to understand how the ground deforms during large magnitude strike-slip earthquakes. The re-excavation and extension of both halves of a displaced paleoseismic trench following this earthquake provided an opportunity to test, refine, and extend back in time the known late Holocene chronology of surface rupturing earthquakes on the Kekerengu Fault. As part of this thesis, 28 organic-bearing samples were collected from a suite of new paleoseismic trenches. Six of these samples were added to the preferred age model from Little et al. (2018); this updated age model is now based on 16 total samples. Including the 2016 earthquake, six surface rupturing earthquakes since ~2000 cal. B.P. are now identified and dated on the Kekerengu Fault. Based on the latest five events (E0 to E4), this analysis yields an updated mean recurrence interval estimate for the Kekerengu Fault of 375 ± 32 yrs (1σ) since ~1650 cal. B.P. The older, sixth event (E5) is not included in the preferred model, as it may not have directly preceded E4; however, if this additional event is incorporated into an alternative age model that embraces all six identified events, the mean recurrence interval estimate (considered a maximum) calculated is 433 ± 22 yrs (1σ) since ~2000 cal. B.P.   Comparison of structures on an identical trench wall logged both before and after the 2016 earthquake, and analysis of pre- and post-earthquake high resolution imagery and Digital Surface Models (DSMs), has allowed the quantification of where and how ~9 m of dextral-oblique slip was accommodated at this site during the earthquake. In addition to this, I analyse the coseismic structure of the adjoining segment of the 2016 ground rupture using detailed post-earthquake aerial orthophotography, to further investigate how geological surface structures (bulged-up moletrack structures) accommodated slip in the rupture zone. These combined analyses allowed me to identify two primary deformation mechanisms that accommodated the large coseismic slip of this earthquake, and the incremental effect of that slip on the structural geology of the rupture zone. These processes include: a) discrete slip along strike-slip faults that bound a narrow, highly deformed inner rupture zone; and b), distributed deformation within this inner rupture zone. The latter includes coseismic clockwise rotation of cohesive rafts of turf, soil and near-surface clay-rich sediment. During this process, these “turf rafts” detach from the underlying soil at a mean depth of ~0.7 m, shorten by ~2.5 m (in addition to shortening introduced by any local contractional heave), bulge upwards by < 1 m, and rotate clockwise by ~19° - while also separating from one another along fissures bounded by former (now rotated) synthetic Riedel faults. This rotational deformation accommodated ~3 m of dextral strike-slip (of a total of ~9 m), after which this rotation apparently ceased, regardless of the total slip or the local kinematics (degree of transpression) at any site. The remaining slip was transferred onto later forming, throughgoing faults as discrete displacement. Analysis of the morphology and amplitude of these moletracks suggests that an increase in the degree of transpression (value of contractional heave) at a site increases the magnitude of shortening and the finite longitudinal strain absorbed by the rotated turf rafts, but does not necessarily contribute to an increase in height (generally 0.33-0.53 m on all parts of the fault). Rather, the comparison of these moletracks with those described by other authors suggests that a more controlling factor on their height is the clay content and cohesion of material deformed into the moletracks.  Finally, comparison of the before and after cross-sections of the displaced paleoseismic trench has provided, for the first time, insight into how large magnitude strike-slip ruptures are expressed in the fault-orthogonal view typical of paleoseismic trenches. Although this rupture involved ~9 m of dextral strike-slip, the cross-sectional view of the re-excavated trenches was dominated by the much lesser component of fault-perpendicular contractional heave (~1.3 m) that occurred in 2016, which did not occur in previous paleoearthquakes at the same site (these were, by contrast, transtensional). This heave was expressed as up to ~2 m of fault-transverse shortening in the inner rupture zone of the trenches, while the ~9 m of strike-slip only created cm-scale offsets across faults. Previous earthquakes at the site were expressed as cm-dm scale, mostly normal dip-separations of sub-horizontal stratigraphic units across faults, suggesting that a change in local kinematics (of ~8°) must have occurred in 2016. Such a small kinematic change may drastically impact the overall ground expression of strike-slip earthquakes - producing also complicated structures including overprinting fault strands in the rupture zone (to a few metres depth). This information poses challenges for structural geologists and paleoseismologists when interpreting (the significance of) structures in future trench walls.</p>

scholarly journals Accommodating ~9 m of dextral slip on the Kekerengu Fault through ground deformation during the Mw 7.8 Kaikōura Earthquake, November 2016

2021 ◽  
Author(s):  
◽  
Philippa Morris
Keyword(s):  
Strike Slip ◽  
Recurrence Interval ◽  
Rupture Zone ◽  
Interval Estimate ◽  
Large Magnitude ◽  
Near Surface ◽  
Before And After ◽  
Age Model ◽  
Kaikoura Earthquake ◽  
Dextral Strike Slip

<p>The Mw 7.8 Kaikōura earthquake of November 14th 2016 provided unprecedented opportunities to understand how the ground deforms during large magnitude strike-slip earthquakes. The re-excavation and extension of both halves of a displaced paleoseismic trench following this earthquake provided an opportunity to test, refine, and extend back in time the known late Holocene chronology of surface rupturing earthquakes on the Kekerengu Fault. As part of this thesis, 28 organic-bearing samples were collected from a suite of new paleoseismic trenches. Six of these samples were added to the preferred age model from Little et al. (2018); this updated age model is now based on 16 total samples. Including the 2016 earthquake, six surface rupturing earthquakes since ~2000 cal. B.P. are now identified and dated on the Kekerengu Fault. Based on the latest five events (E0 to E4), this analysis yields an updated mean recurrence interval estimate for the Kekerengu Fault of 375 ± 32 yrs (1σ) since ~1650 cal. B.P. The older, sixth event (E5) is not included in the preferred model, as it may not have directly preceded E4; however, if this additional event is incorporated into an alternative age model that embraces all six identified events, the mean recurrence interval estimate (considered a maximum) calculated is 433 ± 22 yrs (1σ) since ~2000 cal. B.P.   Comparison of structures on an identical trench wall logged both before and after the 2016 earthquake, and analysis of pre- and post-earthquake high resolution imagery and Digital Surface Models (DSMs), has allowed the quantification of where and how ~9 m of dextral-oblique slip was accommodated at this site during the earthquake. In addition to this, I analyse the coseismic structure of the adjoining segment of the 2016 ground rupture using detailed post-earthquake aerial orthophotography, to further investigate how geological surface structures (bulged-up moletrack structures) accommodated slip in the rupture zone. These combined analyses allowed me to identify two primary deformation mechanisms that accommodated the large coseismic slip of this earthquake, and the incremental effect of that slip on the structural geology of the rupture zone. These processes include: a) discrete slip along strike-slip faults that bound a narrow, highly deformed inner rupture zone; and b), distributed deformation within this inner rupture zone. The latter includes coseismic clockwise rotation of cohesive rafts of turf, soil and near-surface clay-rich sediment. During this process, these “turf rafts” detach from the underlying soil at a mean depth of ~0.7 m, shorten by ~2.5 m (in addition to shortening introduced by any local contractional heave), bulge upwards by < 1 m, and rotate clockwise by ~19° - while also separating from one another along fissures bounded by former (now rotated) synthetic Riedel faults. This rotational deformation accommodated ~3 m of dextral strike-slip (of a total of ~9 m), after which this rotation apparently ceased, regardless of the total slip or the local kinematics (degree of transpression) at any site. The remaining slip was transferred onto later forming, throughgoing faults as discrete displacement. Analysis of the morphology and amplitude of these moletracks suggests that an increase in the degree of transpression (value of contractional heave) at a site increases the magnitude of shortening and the finite longitudinal strain absorbed by the rotated turf rafts, but does not necessarily contribute to an increase in height (generally 0.33-0.53 m on all parts of the fault). Rather, the comparison of these moletracks with those described by other authors suggests that a more controlling factor on their height is the clay content and cohesion of material deformed into the moletracks.  Finally, comparison of the before and after cross-sections of the displaced paleoseismic trench has provided, for the first time, insight into how large magnitude strike-slip ruptures are expressed in the fault-orthogonal view typical of paleoseismic trenches. Although this rupture involved ~9 m of dextral strike-slip, the cross-sectional view of the re-excavated trenches was dominated by the much lesser component of fault-perpendicular contractional heave (~1.3 m) that occurred in 2016, which did not occur in previous paleoearthquakes at the same site (these were, by contrast, transtensional). This heave was expressed as up to ~2 m of fault-transverse shortening in the inner rupture zone of the trenches, while the ~9 m of strike-slip only created cm-scale offsets across faults. Previous earthquakes at the site were expressed as cm-dm scale, mostly normal dip-separations of sub-horizontal stratigraphic units across faults, suggesting that a change in local kinematics (of ~8°) must have occurred in 2016. Such a small kinematic change may drastically impact the overall ground expression of strike-slip earthquakes - producing also complicated structures including overprinting fault strands in the rupture zone (to a few metres depth). This information poses challenges for structural geologists and paleoseismologists when interpreting (the significance of) structures in future trench walls.</p>

scholarly journals Coseismic deformation of the ground during large-slip strike-slip ruptures: Finite evolution of “mole tracks”

Geosphere ◽  
2021 ◽  
Author(s):  
T.A. Little ◽  
P. Morris ◽  
M.P. Hill ◽  
J. Kearse ◽  
R.J. Van Dissen ◽  
...  
Keyword(s):  
Ground Deformation ◽  
Kinematic Model ◽  
Vertical Axis ◽  
Strike Slip ◽  
Rupture Zone ◽  
Coseismic Deformation ◽  
Near Surface ◽  
Main Fault ◽  
Fault Trace ◽  
Kaikoura Earthquake

To evaluate ground deformation resulting from large (~10 m) coseismic strike-slip displacements, we focus on deformation of the Kekerengu fault during the November 2016 Mw 7.8 Kaikōura earthquake in New Zealand. Combining post-earthquake field observations with analysis of high-resolution aerial photography and topographic models, we describe the structural geology and geomorphology of the rupture zone. During the earthquake, fissured pressure bulges (“mole tracks”) initiated at stepovers between synthetic Riedel (R) faults. As slip accumulated, near-surface “rafts” of cohesive clay-rich sediment, bounded by R faults and capped by grassy turf, rotated about a vertical axis and were internally shortened, thus amplifying the bulges. The bulges are flanked by low-angle contractional faults that emplace the shortened mass of detached sediment outward over less-deformed ground. As slip accrued, turf rafts fragmented into blocks bounded by short secondary fractures striking at a high angle to the main fault trace that we interpret to have originated as antithetic Riedel (R¢) faults. Eventually these blocks were dispersed into strongly sheared earth and variably rotated. Along the fault, clockwise rotation of these turf rafts within the rupture zone averaged ~20°–30°, accommodat­ing a finite shear strain of 1.0–1.5 and a distributed strike slip of ~3–4 m. On strike-slip parts of the fault, internal shortening of the rafts averaged 1–2 m parallel to the R faults and ~1 m perpendicular to the main fault trace. Driven by distortional rotation, this contraction of the rafts exceeds the magnitude of fault heave. Turf rafts on slightly transtensional segments of the fault were also bulged and shortened—relationships that can be explained by a kinematic model involving “deformable slats.” In a paleoseismic trench cut perpendicular the fault, one would observe fissures, low-angle thrusts, and steeply dipping strike-slip faults—some cross-cutting one another—yet all may have formed during a single earthquake featuring a large strike-slip displacement.

Curved slickenlines preserve direction of rupture propagation

Geology ◽  
2019 ◽  
Vol 47 (9) ◽  
pp. 838-842
Author(s):  
Jesse Kearse ◽  
Yoshihiro Kaneko ◽  
Tim Little ◽  
Russ Van Dissen
Keyword(s):  
Cohesive Zone ◽  
Strike Slip ◽  
Rupture Propagation ◽  
Dynamic Rupture ◽  
Slip Direction ◽  
Rupture Dynamics ◽  
Earthquake Rupture Dynamics ◽  
Kaikoura Earthquake ◽  
Rupture Direction ◽  
Dextral Strike Slip

Abstract Slip-parallel grooves (striations) on fault surfaces are considered a robust indicator of fault slip direction, yet their potential for recording aspects of earthquake rupture dynamics has received little attention. During the 2016 Kaikōura earthquake (South Island, New Zealand), >10 m of dextral strike-slip on the steeply dipping Kekerengu fault exhumed >200 m2 of fresh fault exposure (free faces) where it crossed bedrock canyons. Inscribed upon these surfaces, we observed individual striae up to 6 m long, all of which had formed during the earthquake. These were typically curved. Using simulations of spontaneous dynamic rupture on a vertical strike-slip fault, we reproduce the curved morphology of striae on the Kekerengu fault. Assuming strike-slip pre-stress, our models demonstrate that vertical tractions induced by slip in the so-called cohesive zone result in transient changes in slip direction. We show that slip-path convexity is sensitive to the direction of rupture propagation. To match the convexity of striae formed in 2016 requires the rupture to have propagated in a northeast direction, a prediction that matches the known rupture direction of the Kaikōura earthquake. Our study highlights the potential for fault striae to record aspects of rupture dynamics, including the rupture direction of paleo strike-slip earthquakes.

scholarly journals Surface Fault Rupture and Slip Distribution of the Jordan-Kekerengu-Needles Fault Network during the 2016 Mw 7.8 Kaikōura Earthquake, New Zealand

2021 ◽  
Author(s):  
◽  
Jesse Kearse
Keyword(s):  
Ground Surface ◽  
Strike Slip ◽  
Airborne Lidar ◽  
Rupture Zone ◽  
Fault Rupture ◽  
Slip Direction ◽  
Surface Rupture ◽  
Linear Features ◽  
Surface Rupture Zone ◽  
Kaikoura Earthquake

<p>During the 2016, Mw 7.8 Kaikōura earthquake the Kekerengu fault ruptured the ground surface producing a maximum of ~12 m of net displacement (dextral-slip with minor reverse- slip), one of the largest five co-seismic surface rupture displacements so far observed globally. This thesis presents the first combined onshore to offshore dataset of co-seismic ground-surface and vertical seabed displacements along a near-continuous ~83 km long strike-slip dominated earthquake surface rupture of large slip magnitude. Onshore on the Kekerengu, Jordan Thrust, Upper Kowhai, and Manakau faults, we measured the displacement of 117 cultural and natural markers in the field and using airborne LiDAR data. Offshore on the dextral-reverse Needles fault, multibeam bathymetric and high-resolution seismic reflection data image a throw of the seabed of up to 3.5±0.2 m. Mean net slip on the total ~83 km rupture was 5.5±1 m, this is an unusually large mean slip for the rupture length compared to global strike-slip surface ruptures. Surveyed linear features that extend across the entire surface rupture zone show that it varies in width from 13 to 122 m. These cultural features also reveal the across-strike distribution of lateral displacement, 80% of which is, on average, concentrated within the central 43% of the rupture zone. Combining the near-field measurements of fault offset with published, far-field InSAR, continuous GPS, and coastal deformation data, suggests partitioning of oblique plate convergence, with a significant portion of co-seismic contractional deformation (and uplift) being accommodated off-fault in the hanging-wall crust to the northwest of the main rupturing faults.  This thesis also documents in detail the onshore extent of surface fault rupture on the Kekerengu, Jordan Thrust, Upper Kowhai and Manakau faults. I present large-scale maps (up to 1:3,000) and documentary field photographs of this 53 km-long onshore surface rupture zone utilizing field data, post-earthquake LiDAR-derived Digital Elevation Models (DEMs), and post-earthquake ortho-rectified aerial photography. Ground deformation data is most detailed near the Marlborough coast where the 2016 rupture trace is well-exposed on agricultural grassland on the Kekerengu fault. In the southwest, where surface fault rupture traversed the alpine slopes of the Seaward Kaikoura ranges, fault mapping relied heavily on the LiDAR-derived DEMs.   At 24 sites along the Kekerengu fault, I document co-seismic wear striae that were formed during the earthquake and were preserved on free face fault exposures. Nearly all of these striae were distinctly curved along their length, demonstrating that the direction of near-surface fault slip changed with time during rupture of the Kekerengu fault. Co-seismic displacement on the Kekerengu fault initiated as oblique-dextral (mainly dextral-reverse), and subsequently rotated to become nearly-pure dextral slip. These slip trajectories agree with directions of net displacements derived from offset linear features at nearby sites. Temporal rotation of the slip direction may suggest a state of low shear stress on the Kekerengu fault before the earthquake, and a near-complete reduction in stress during the earthquake, as has been inferred for other historic earthquakes that show evidence for changing slip direction with time.</p>

Tertiary extension in the central British Columbia Intermontane Belt: magnetic and paleomagnetic evidence from the Endako region

2001 ◽  
Vol 38 (4) ◽  
pp. 657-678 ◽  
Author(s):  
Carmel Lowe ◽  
Randolph J Enkin ◽  
Lambertus C Struik
Keyword(s):  
British Columbia ◽  
Strike Slip ◽  
Paleomagnetic Data ◽  
Magnetic Data ◽  
Euler Deconvolution ◽  
The West ◽  
Near Surface ◽  
Dextral Strike Slip Fault ◽  
Slip Displacement ◽  
Dextral Strike Slip

New magnetic and paleomagnetic data for central British Columbia support and quantify the hypothesis that the area underwent significant Tertiary-age transtensional deformation. Paleomagnetically determined tilts in Eocene rocks indicate that four fault-bounded pits, which constitute the Endako molybdenum mine, were displaced on a series of normal (probably listric) faults that have separations of less than a kilometre. The interpretation also suggests there can be little vertical offset on the Denak West Fault, which separates the Denak East and Denak West pits. Regional paleomagnetic data indicate a predominance of easterly directed tilts to the east of the Casey Fault, but to the west a large variation in the orientation and magnitude of tilts is observed. Results at one site proximal to the Casey Fault indicate a component of dip-slip displacement on this dominantly dextral strike-slip fault. Mapped northeast- and northwest-trending faults commonly correspond to linear zones of steep magnetic gradient and near-surface magnetic sources. Several additional northwest- and northeast-trending lineaments are imaged in the magnetic data where no faults are mapped (particularly over massive and lithologically homogeneous phases of the Endako batholith). Euler deconvolution solutions confirm most such lineaments are also associated with shallow magnetic sources. In profile, they have either a fault or dyke character and are interpreted to be unmapped faults, some locally intruded by mafic dykes, which cut the region into a series of fault-bounded blocks.

scholarly journals Effect of the Kaikōura Earthquake on Velocity Changes In and Around the Ruptured Region: A Noise Cross-Correlation Approach

2021 ◽  
Author(s):  
◽  
Megan Kortink
Keyword(s):  
Ambient Noise ◽  
Seismic Velocity ◽  
Earthquake Sequence ◽  
Large Magnitude ◽  
Velocity Increase ◽  
Before And After ◽  
Seismic Velocity Changes ◽  
Velocity Changes ◽  
Kaikoura Earthquake ◽  
Velocity Decrease

<p>Seismic velocity changes before and after large magnitude earthquakes carry information about damage present within the faults in the surrounding region. In this thesis, temporal velocity changes are measured before and after the 2016 Kaikōura earthquake using ambient noise interferometry between 2012 - 2018. This period contains the Mw 7.8 2016 Kaikoura earthquake as well as the 2013 Cook Strait earthquake sequence and a few deep large magnitude earthquakes in 2015 - 2016. Three primary objectives are identified: (1) investigate seismic velocity changes in the Kaikōura region and their connection to the 2016 Kaikōura earthquake to try and determine if there was a change before/after the earthquake, (2) determine how this change varied across the region, and (3) consider if ambient noise can lead to improved detection and understanding of geological hazard.   The primary approach used to measure velocity changes in the Kaikōura region involved cross correlating noise recorded by seismic stations across the region. Velocity changes are sought by averaging the best result from multiple onshore station pairs. A secondary approach was also used, in which specific station pairs were averaged to determine if there were more localised velocity changes over more specific regions. This was to determine if the velocity changes observed following the 2016 Kaikōura earthquake occurred over the entire ruptured region.   Following the 2016 Kaikōura earthquake a velocity decrease of 0.24±0.02% was observed on the average of the vertical-vertical components for eight stations. The remaining eight cross-component pairs showed a smaller seismic decrease with an average value of 0.22±0.05%. After the decrease following the Kaikōura earthquake, there is a steady velocity increase of 0.13±0.02% over a one-and-a-half-year period. This indicates that prior to the earthquake, seismic velocity was at a steady state until it was perturbed by the Kaikōura earthquake, and seismic velocities rapidly decreased over all stations. Across the region, stations with a longer interstation distance and further away from ruptured faults had a smaller decrease in velocity than station pairs with a smaller interstation distance that were closer to ruptured faults. We interpret the velocity decrease following the Kaikōura earthquake as a result of cracks opening during the earthquake. The velocity increase following the earthquake is indicative of the cracks slowly healing.   The Cook Strait earthquake sequence that occurred in 2013 did not cause any velocity changes at the stations used in this thesis. This has been interpreted to be because the changes were too small compared to the background noise or the stations were not recording during the time of the earthquake sequence. Two other decreases were also observed in the region following two deep earthquakes in April 2015 (Mw 6.2, depth = 52km) and February 2016 (Mw 5.7, depth = 48km). Both of these events resulted in a small seismic decrease of 0.1±0.02%. Although these earthquakes were close to seismic stations when they occurred, they were much deeper and had a smaller magnitude than the Kaikōura earthquake so did not cause a large velocity decrease. By understanding what causes velocity changes it is possible to have an improved understanding of the geological hazard in the region.</p>

scholarly journals Surface Fault Rupture and Slip Distribution of the Jordan-Kekerengu-Needles Fault Network during the 2016 Mw 7.8 Kaikōura Earthquake, New Zealand

2021 ◽  
Author(s):  
◽  
Jesse Kearse
Keyword(s):  
Ground Surface ◽  
Strike Slip ◽  
Airborne Lidar ◽  
Rupture Zone ◽  
Fault Rupture ◽  
Slip Direction ◽  
Surface Rupture ◽  
Linear Features ◽  
Surface Rupture Zone ◽  
Kaikoura Earthquake

<p>During the 2016, Mw 7.8 Kaikōura earthquake the Kekerengu fault ruptured the ground surface producing a maximum of ~12 m of net displacement (dextral-slip with minor reverse- slip), one of the largest five co-seismic surface rupture displacements so far observed globally. This thesis presents the first combined onshore to offshore dataset of co-seismic ground-surface and vertical seabed displacements along a near-continuous ~83 km long strike-slip dominated earthquake surface rupture of large slip magnitude. Onshore on the Kekerengu, Jordan Thrust, Upper Kowhai, and Manakau faults, we measured the displacement of 117 cultural and natural markers in the field and using airborne LiDAR data. Offshore on the dextral-reverse Needles fault, multibeam bathymetric and high-resolution seismic reflection data image a throw of the seabed of up to 3.5±0.2 m. Mean net slip on the total ~83 km rupture was 5.5±1 m, this is an unusually large mean slip for the rupture length compared to global strike-slip surface ruptures. Surveyed linear features that extend across the entire surface rupture zone show that it varies in width from 13 to 122 m. These cultural features also reveal the across-strike distribution of lateral displacement, 80% of which is, on average, concentrated within the central 43% of the rupture zone. Combining the near-field measurements of fault offset with published, far-field InSAR, continuous GPS, and coastal deformation data, suggests partitioning of oblique plate convergence, with a significant portion of co-seismic contractional deformation (and uplift) being accommodated off-fault in the hanging-wall crust to the northwest of the main rupturing faults.  This thesis also documents in detail the onshore extent of surface fault rupture on the Kekerengu, Jordan Thrust, Upper Kowhai and Manakau faults. I present large-scale maps (up to 1:3,000) and documentary field photographs of this 53 km-long onshore surface rupture zone utilizing field data, post-earthquake LiDAR-derived Digital Elevation Models (DEMs), and post-earthquake ortho-rectified aerial photography. Ground deformation data is most detailed near the Marlborough coast where the 2016 rupture trace is well-exposed on agricultural grassland on the Kekerengu fault. In the southwest, where surface fault rupture traversed the alpine slopes of the Seaward Kaikoura ranges, fault mapping relied heavily on the LiDAR-derived DEMs.   At 24 sites along the Kekerengu fault, I document co-seismic wear striae that were formed during the earthquake and were preserved on free face fault exposures. Nearly all of these striae were distinctly curved along their length, demonstrating that the direction of near-surface fault slip changed with time during rupture of the Kekerengu fault. Co-seismic displacement on the Kekerengu fault initiated as oblique-dextral (mainly dextral-reverse), and subsequently rotated to become nearly-pure dextral slip. These slip trajectories agree with directions of net displacements derived from offset linear features at nearby sites. Temporal rotation of the slip direction may suggest a state of low shear stress on the Kekerengu fault before the earthquake, and a near-complete reduction in stress during the earthquake, as has been inferred for other historic earthquakes that show evidence for changing slip direction with time.</p>

scholarly journals Effect of the Kaikōura Earthquake on Velocity Changes In and Around the Ruptured Region: A Noise Cross-Correlation Approach

2021 ◽  
Author(s):  
◽  
Megan Kortink
Keyword(s):  
Ambient Noise ◽  
Seismic Velocity ◽  
Earthquake Sequence ◽  
Large Magnitude ◽  
Velocity Increase ◽  
Before And After ◽  
Seismic Velocity Changes ◽  
Velocity Changes ◽  
Kaikoura Earthquake ◽  
Velocity Decrease

<p>Seismic velocity changes before and after large magnitude earthquakes carry information about damage present within the faults in the surrounding region. In this thesis, temporal velocity changes are measured before and after the 2016 Kaikōura earthquake using ambient noise interferometry between 2012 - 2018. This period contains the Mw 7.8 2016 Kaikoura earthquake as well as the 2013 Cook Strait earthquake sequence and a few deep large magnitude earthquakes in 2015 - 2016. Three primary objectives are identified: (1) investigate seismic velocity changes in the Kaikōura region and their connection to the 2016 Kaikōura earthquake to try and determine if there was a change before/after the earthquake, (2) determine how this change varied across the region, and (3) consider if ambient noise can lead to improved detection and understanding of geological hazard.   The primary approach used to measure velocity changes in the Kaikōura region involved cross correlating noise recorded by seismic stations across the region. Velocity changes are sought by averaging the best result from multiple onshore station pairs. A secondary approach was also used, in which specific station pairs were averaged to determine if there were more localised velocity changes over more specific regions. This was to determine if the velocity changes observed following the 2016 Kaikōura earthquake occurred over the entire ruptured region.   Following the 2016 Kaikōura earthquake a velocity decrease of 0.24±0.02% was observed on the average of the vertical-vertical components for eight stations. The remaining eight cross-component pairs showed a smaller seismic decrease with an average value of 0.22±0.05%. After the decrease following the Kaikōura earthquake, there is a steady velocity increase of 0.13±0.02% over a one-and-a-half-year period. This indicates that prior to the earthquake, seismic velocity was at a steady state until it was perturbed by the Kaikōura earthquake, and seismic velocities rapidly decreased over all stations. Across the region, stations with a longer interstation distance and further away from ruptured faults had a smaller decrease in velocity than station pairs with a smaller interstation distance that were closer to ruptured faults. We interpret the velocity decrease following the Kaikōura earthquake as a result of cracks opening during the earthquake. The velocity increase following the earthquake is indicative of the cracks slowly healing.   The Cook Strait earthquake sequence that occurred in 2013 did not cause any velocity changes at the stations used in this thesis. This has been interpreted to be because the changes were too small compared to the background noise or the stations were not recording during the time of the earthquake sequence. Two other decreases were also observed in the region following two deep earthquakes in April 2015 (Mw 6.2, depth = 52km) and February 2016 (Mw 5.7, depth = 48km). Both of these events resulted in a small seismic decrease of 0.1±0.02%. Although these earthquakes were close to seismic stations when they occurred, they were much deeper and had a smaller magnitude than the Kaikōura earthquake so did not cause a large velocity decrease. By understanding what causes velocity changes it is possible to have an improved understanding of the geological hazard in the region.</p>

Detection of tremors in the Marlborough region and its relationship with the 2016 Mw 7.9 Kaikoura (New Zealand) earthquake

2021 ◽  
Author(s):  
Pierre Romanet ◽  
Florent Aden-Antoniow ◽  
Ryosuke Ando ◽  
Stephen Bannister ◽  
Calum Chamberlain ◽  
...  
Keyword(s):  
New Zealand ◽  
Current Knowledge ◽  
Strike Slip ◽  
Tectonic Activity ◽  
North East ◽  
South West ◽  
Using Data ◽  
Kaikoura Earthquake ◽  
Dextral Strike Slip ◽  
Over Time

&lt;p&gt;Seismic tremor has previously been reported in the Marlborough (New Zealand) region, with detections made using the national GeoNet network. However, because of the sparsity of that network, only 40 tremors were detected using 6 stations. We conducted a similar analysis again, but this time using data from 4 stations from the GeoNet network as well as 16 stations from a local campaign network, bringing the total number of stations to 20. Our new tremor catalog contains 4699 tremors (around 100 times more events than the previous catalog) and spans the period 2013-2019 which include the major 2016 Mw7.9 Kaikoura earthquake. Based on our current knowledge, that makes the Marlborough region the most active region for tremors in New Zealand.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The observed tremor in the region are split into two clusters, separated by a gap of around 20 km. The South-West cluster has an elongated shape in the direction of the upper-plate dextral strike-slip (Hope and Clarence) faults. The occurrence of tremor before the Mw 7.9 Kaikoura earthquake is fairly constant over time. After the earthquake however we observe &amp;#160;a strong acceleration in the rate of tremor, that slowly recovers over time. At the end of the analysis (May 2019), more than 2 years after Kaikoura earthquake, the tremor burst rate has still not recovered to the previous rate before the earthquake. We also observe several episodes of tremor migration, with a migration velocity of around ~50km/day, most of the migration being from South-West to North-East.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;This new tremor catalog provides a unique opportunity to better understand possible interaction of a major earthquake with the tremor activity and will help to better understand the local tectonic activity of the Marlborough region.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

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