Loading transfer mechanism of a piled raft subjected to normal faulting in sand

Although different kinds of foundations have been investigated against an earthquake faulting, the interaction between pile group and dip-slip fault has not yet been fully understood. This letter investigates the interaction between piled raft and normal faulting by means of centrifuge and numerical modelling. In centrifuge test, a piled raft was simulated with a half model for a better observation of fault rupture path under the raft. The loading transfer mechanism was further examined using a three-dimensional finite difference software (FLAC3D). The measured and computed results showed that the piled raft displaced and tilted linearly with the magnitude of faulting. The fault rupture bifurcated into two and diverted towards both edges of the raft. Two types of loading transfer mechanism were identified during faulting. Working load transferred from the raft to the underneath piles, and also from the piles on the side of the hanging wall to the piles on the footwall side, resulting in compression failure of the piles on the footwall side.

Fault Geometry and Mechanism of the Mw 5.7 Nakchu Earthquake in Tibet Inferred from InSAR Observations and Stress Measurements

Different types of focal mechanism solutions for the 19 March 2021 Mw 5.7 Nakchu earthquake, Tibet, limit our understanding of this earthquake’s seismogenic mechanism and geodynamic process. In this study, the coseismic deformation field was determined and the geometric parameters of the seismogenic fault were inverted via Interferometric Synthetic Aperture Radar (InSAR) processing of Sentinel-1 data. The inversion results show that the focal mechanism solutions of the Nakchu earthquake are 237°/69°/−70° (strike/dip/rake), indicating that the seismogenic fault is a NEE-trending, NW-dipping fault dominated by the normal faulting with minor sinistral strike-slip components. The regional tectonic stress field derived from the in-situ stress measurements shows that the orientation of maximum principal compressive stress around the epicenter of the Nakchu earthquake is NNE, subparallel to the fault strike, which controlled the dominant normal faulting. The occurrence of seven M ≥ 7.0 historical earthquakes since the M 7.0 Shenza earthquake in 1934 caused a stress increase of 1.16 × 105 Pa at the hypocenter, which significantly advanced the occurrence of the Nakchu earthquake. Based on a comprehensive analysis of stress fields and focal mechanisms of the Nakchu earthquake, we propose that the dominated normal faulting occurs to accommodate the NE-trending compression of the Indian Plate to the Eurasian Plate and the strong historical earthquakes hastened the process. These results provide a theoretical basis for understanding the geometry and mechanics of the seismogenic fault that produced the Nakchu earthquake.

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

<p>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.</p>

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

<p>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.</p>

Seismotectonic Analysis of the 2019–2020 Puerto Rico Sequence: The Value of Absolute Earthquake Relocations in Improved Interpretations of Active Tectonics

We present a new catalog of calibrated earthquake relocations from the 2019–2020 Puerto Rico earthquake sequence related to the 7 January 2020 Mw 6.4 earthquake that occurred offshore of southwest Puerto Rico at a depth of 15.9 km. Utilizing these relocated earthquakes and associated moment tensor solutions, we can delineate several distinct fault systems that were activated during the sequence and show that the Mw 6.4 mainshock may have resulted from positive changes in Coulomb stress from earlier events. Seismicity and mechanisms define (1) a west–southwest (∼260°) zone of seismicity comprised of largely sinistral strike-slip and oblique-slip earthquakes that mostly occurs later in the sequence and to the west of the mainshock, (2) an area of extensional faulting that includes the mainshock and occurs largely within the mainshock’s rupture area, and (3) an north–northeast (∼30°)-striking zone of seismicity, consisting primarily of dextral strike-slip events that occurs before and following the mainshock and generally above (shallower than) the normal-faulting events. These linear features intersect within the Mw 6.4 mainshock’s fault plane in southwest Puerto Rico. In addition, we show that earthquake relocations for M 4+ normal-faulting events, when traced along their fault planes, daylight along east–west-trending bathymetric features offshore of southwest Puerto Rico. Correlation of these normal-faulting events with bathymetric features suggests an active fault system that may be a contributor to previously uncharacterized seismic hazards in southwest Puerto Rico.

Normal Faulting and Volcanism in the Kora 3D Seismic Volume

<p>The spatial and temporal relationship between normal faulting and volcanism in offshore Western North Island, New Zealand can be used to gain insight into basin formation, hydrocarbon resources, regional tectonics, and large subduction processes. It is hypothesised that there is a causal relationship between volcanic activity and faulting, however, within the Taranaki Kora 3D seismic volume (survey) this relationship has not yet been explored. The overall aim of this thesis was to map and identify whether there is a relationship between volcanism and normal faulting within the Kora 3D survey. A causal relationship in location and timing between volcanic processes and fault activity was discovered in this study. Two novel models were created to explain the creation of the local stress leading to this causal relationship. The first model uses intrusive magma build up and the second extrusive cone building to explain the changes in local stress. These models not only support the causal relationship between volcanism and faulting activity but also provide a new understanding into how Kora volcanic cone activity may have influenced active faulting in the Kora 3D survey. Application of this new information will allow innovative insights into basin formation, regional and local tectonics, and subducting plate geometry in the Taranaki Basin. This research could be utilized to increase knowledge for prospecting and reduce geologic uncertainty, which is of importance for the New Zealand petroleum industry at this northern end of the Taranaki Basin.</p>

Normal Faulting and Volcanism in the Kora 3D Seismic Volume

<p>The spatial and temporal relationship between normal faulting and volcanism in offshore Western North Island, New Zealand can be used to gain insight into basin formation, hydrocarbon resources, regional tectonics, and large subduction processes. It is hypothesised that there is a causal relationship between volcanic activity and faulting, however, within the Taranaki Kora 3D seismic volume (survey) this relationship has not yet been explored. The overall aim of this thesis was to map and identify whether there is a relationship between volcanism and normal faulting within the Kora 3D survey. A causal relationship in location and timing between volcanic processes and fault activity was discovered in this study. Two novel models were created to explain the creation of the local stress leading to this causal relationship. The first model uses intrusive magma build up and the second extrusive cone building to explain the changes in local stress. These models not only support the causal relationship between volcanism and faulting activity but also provide a new understanding into how Kora volcanic cone activity may have influenced active faulting in the Kora 3D survey. Application of this new information will allow innovative insights into basin formation, regional and local tectonics, and subducting plate geometry in the Taranaki Basin. This research could be utilized to increase knowledge for prospecting and reduce geologic uncertainty, which is of importance for the New Zealand petroleum industry at this northern end of the Taranaki Basin.</p>

Interactions between the prograding Giant Foresets Formation and a subsiding depocentre: Insights from the Parihaka 3D & ES89 2D seismic surveys

<p>This seismic interpretation project provides new insights into the interaction between the Pliocene-aged Giant Foresets Formation and the faults bounding the Northern Graben. A newly named fault-bounded depocentre within the North Taranaki Graben, the Arawa Sub-Basin, has subsided during the Pliocene, attracting volumes of sediment across the Parihaka Fault within large-scale channels. The study images kilometer-scale channels and explores the interplay between the progradation of the Giant Foresets Formation and normal faulting along the Cape Egmont Fault Zone. A focus is placed on imaging the provenance and depositional facies of sedimentary packages throughout the foresetting sequence of the Giant Foresets Formation. Mapping of the Waipipian-Nukumaruan-aged foresetting sequence within the offshore northern Taranaki Basin has previously shown the primary sediment transport direction is primarily NNW. This is contradicted by sediment-transport features mapped within the study area showing the sediment transport direction fluctuates between NE and SE. The primary mechanism of sediment redirection is faulting along the Cape Egmont Fault Zone and subsidence within the North Taranaki Graben, an elongate SW-NE graben within the northern Taranaki Basin. Smaller (˜10s m-scale) channels concentrate into much larger (˜100s m- to km-scale) mega-channels that travel E/NE into the subsiding Arawa Sub-Basin. Volcanic intrusions of the Mohakatino Volcanic Formation have also influenced the evolution of the mega-channels in the study area, via uplift and doming of the seafloor which provided a barrier to the transport of sediment. The Parihaka 3D and ES89 2D seismic surveys are interpreted using the IHS Kingdom software package to create a basic framework of horizons and faults over the Pliocene-Recent interval. Depth grid maps are produced from the grid of horizon picks. Isochore maps are produced which span key intervals between depth grids. A coherency cube of the Parihaka 3D is generated from the 3D seismic volume using OpendTect. Using the framework of faults and horizons within the coherency cube, imaging sediment transport and deposition features in the vicinity of normal faulting is made possible by flattening on a top foresets horizon and horizontally slicing the data at regular intervals. This recreates past conditions by removing the effects of fault-slip and differential compaction. These “time-slices” contain clear images of channels, canyons and fan-deposits allowing sediment provenance and transport direction to be mapped and interpreted. Finally, seismic section images from the Parihaka 3D and ES89 2D seismic surveys are generated along paths intersecting key geological features within the study area.</p>