scholarly journals Improved earthquake detection as a probe for active fault structures in New Zealand's central Southern Alps

2021 ◽  
Author(s):  
◽  
Calum Chamberlain
Keyword(s):  
Cross Correlation ◽  
Matched Filter ◽  
Low Frequency ◽  
Southern Alps ◽  
Phase 2 ◽  
Earthquake Detection ◽  
Alpine Fault ◽  
Drill Site ◽  
Python Package ◽  
Drilling Project

<p>This thesis concerns the detection and analysis of micro-seismicity and low-frequency earthquakes in New Zealand's central Southern Alps. We make use of the 6.5 year continuous seismic dataset collected using the Southern Alps Microearthquake Borehole Array (SAMBA), alongside other temporary and permanent seismic deployments nearby. The small station spacing of this deployment allows for high resolution seismic studies near the Alpine Fault, a dextral-transpressive plate boundary fault between the Pacific and Australian plates.  Using this dataset we have documented the rst evidence of low-frequency earthquakes on or near the deep extent of the Alpine Fault. By using a network based crosscorrelation detection method we have generated a 3 year catalogue of 14 low-frequency earthquake families. These low-frequency earthquake families locate close to other indicators and models of the deep extent of the Alpine Fault, and we interpret these low-frequency earthquakes to represent shear failure on or near the deep extent of the Alpine Fault. These low-frequency earthquakes highlight a near-continuous background rate of deformation, punctuated by short periods of tremor. We also observe higher rates of low-frequency earthquake generation after large regional earthquakes. The magnitudes of our low-frequency earthquakes range from Mʟ‒0.8‒1.8, and appear to follow an exponential distribution, implying that there might be a characteristic length-scale of failure.  We have extended the catalogue of low-frequency earthquake templates using the full 6.5 year dataset and an objective synthetic detection methodology. We developed a new methodology for template detection after other methods failed, or were not feasible. This method employs simple synthetic template events, which, rather than trying to capture all of the complexities of the body waves we try to detect, approximate a simple waveform that does not correlate well with background noise. To undertake this method we have developed a multi-parallel Python package, which is highly portable (we have run this on computers ranging from dual-core, 8GB RAM laptops to a 393 node, 6349 CPU cluster computer) and distributed via an open-source model. This package was run through the 6.5 year dataset on the New Zealand E-Science PAN cluster to e fficiently (<48 hours clock-time) generate a spatially and temporally continuous catalogue of low-frequency earthquake templates. Using this method to detect an initial suite of over 25,000 detections grouped into 600 families we have generated 600 good quality, discrete stacked waveforms for use in further matched-filter detection routines. We have shown that, for templates with both P and S-phase picks, these templates locate near to our previously determined low-frequency earthquake family locations.  Using a network matched- filter detection technique we have generated a catalogue of micro-seismicity in a region of low-seismicity near the Whataroa Valley, motivated by the Deep-Fault Drilling Project; Phase-2. We detected 300 earthquakes that include a selection of near-repeating earthquakes. We find that most detected events are not similar enough to be termed repeating. For 106 earthquakes we are able to generate high-precision magnitudes calculated by singular-value decomposition of similar waveforms. We find a high b-value of 1.44 for these earthquakes, with no earthquakes above Mʟ1.6. By generating high precision cross-correlation derived picks for individual detections and employing a double-difference location methodology we show that seismicity does not delineate a single structure; rather we interpret the detected seismicity as temporally-limited earthquake sequences on small asperities adjacent to the Alpine Fault. Focal mechanisms for the best recorded events show dominantly strike-slip mechanisms, with lesser reverse and normal components.  During the drilling of the Deep-Fault Drilling Project: Phase-2 borehole we operated a real-time earthquake detection system around the drill-site. This was a multi-national effort involving 16 seismologists in three countries monitoring the automatic detections in shifts. During the 5 month real-time monitoring period we detected and located 493 earthquakes, none of which occurred within 3km of the drill-site, nor required changes to the drilling operations. We undertook this monitoring using open-source software, which employed a standard energy based detection scheme.  This thesis has contributed four complementary earthquake catalogues, a further three years of continuous seismic data from the central Southern Alps, and an opensource Python package for detection and analysis of earthquakes using cross-correlation techniques. The characteristics of these catalogues highlight deformation modes on and near one of the world's major strike-slip plate boundaries, both at depth, and at the upper extent of the seismogenic zone.</p>

scholarly journals Improved earthquake detection as a probe for active fault structures in New Zealand's central Southern Alps

2021 ◽  
Author(s):  
◽  
Calum Chamberlain
Keyword(s):  
Cross Correlation ◽  
Matched Filter ◽  
Low Frequency ◽  
Southern Alps ◽  
Phase 2 ◽  
Earthquake Detection ◽  
Alpine Fault ◽  
Drill Site ◽  
Python Package ◽  
Drilling Project

<p>This thesis concerns the detection and analysis of micro-seismicity and low-frequency earthquakes in New Zealand's central Southern Alps. We make use of the 6.5 year continuous seismic dataset collected using the Southern Alps Microearthquake Borehole Array (SAMBA), alongside other temporary and permanent seismic deployments nearby. The small station spacing of this deployment allows for high resolution seismic studies near the Alpine Fault, a dextral-transpressive plate boundary fault between the Pacific and Australian plates.  Using this dataset we have documented the rst evidence of low-frequency earthquakes on or near the deep extent of the Alpine Fault. By using a network based crosscorrelation detection method we have generated a 3 year catalogue of 14 low-frequency earthquake families. These low-frequency earthquake families locate close to other indicators and models of the deep extent of the Alpine Fault, and we interpret these low-frequency earthquakes to represent shear failure on or near the deep extent of the Alpine Fault. These low-frequency earthquakes highlight a near-continuous background rate of deformation, punctuated by short periods of tremor. We also observe higher rates of low-frequency earthquake generation after large regional earthquakes. The magnitudes of our low-frequency earthquakes range from Mʟ‒0.8‒1.8, and appear to follow an exponential distribution, implying that there might be a characteristic length-scale of failure.  We have extended the catalogue of low-frequency earthquake templates using the full 6.5 year dataset and an objective synthetic detection methodology. We developed a new methodology for template detection after other methods failed, or were not feasible. This method employs simple synthetic template events, which, rather than trying to capture all of the complexities of the body waves we try to detect, approximate a simple waveform that does not correlate well with background noise. To undertake this method we have developed a multi-parallel Python package, which is highly portable (we have run this on computers ranging from dual-core, 8GB RAM laptops to a 393 node, 6349 CPU cluster computer) and distributed via an open-source model. This package was run through the 6.5 year dataset on the New Zealand E-Science PAN cluster to e fficiently (<48 hours clock-time) generate a spatially and temporally continuous catalogue of low-frequency earthquake templates. Using this method to detect an initial suite of over 25,000 detections grouped into 600 families we have generated 600 good quality, discrete stacked waveforms for use in further matched-filter detection routines. We have shown that, for templates with both P and S-phase picks, these templates locate near to our previously determined low-frequency earthquake family locations.  Using a network matched- filter detection technique we have generated a catalogue of micro-seismicity in a region of low-seismicity near the Whataroa Valley, motivated by the Deep-Fault Drilling Project; Phase-2. We detected 300 earthquakes that include a selection of near-repeating earthquakes. We find that most detected events are not similar enough to be termed repeating. For 106 earthquakes we are able to generate high-precision magnitudes calculated by singular-value decomposition of similar waveforms. We find a high b-value of 1.44 for these earthquakes, with no earthquakes above Mʟ1.6. By generating high precision cross-correlation derived picks for individual detections and employing a double-difference location methodology we show that seismicity does not delineate a single structure; rather we interpret the detected seismicity as temporally-limited earthquake sequences on small asperities adjacent to the Alpine Fault. Focal mechanisms for the best recorded events show dominantly strike-slip mechanisms, with lesser reverse and normal components.  During the drilling of the Deep-Fault Drilling Project: Phase-2 borehole we operated a real-time earthquake detection system around the drill-site. This was a multi-national effort involving 16 seismologists in three countries monitoring the automatic detections in shifts. During the 5 month real-time monitoring period we detected and located 493 earthquakes, none of which occurred within 3km of the drill-site, nor required changes to the drilling operations. We undertook this monitoring using open-source software, which employed a standard energy based detection scheme.  This thesis has contributed four complementary earthquake catalogues, a further three years of continuous seismic data from the central Southern Alps, and an opensource Python package for detection and analysis of earthquakes using cross-correlation techniques. The characteristics of these catalogues highlight deformation modes on and near one of the world's major strike-slip plate boundaries, both at depth, and at the upper extent of the seismogenic zone.</p>

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

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

<p>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.</p>

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

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

<p>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.</p>

scholarly journals Detecting Real Earthquakes Using Artificial Earthquakes: On the Use of Synthetic Waveforms in Matched-Filter Earthquake Detection

2021 ◽  
Author(s):  
Calum Chamberlain ◽  
John Townend
Keyword(s):  
Focal Mechanism ◽  
Matched Filter ◽  
A Priori ◽  
Low Frequency ◽  
Aftershock Sequence ◽  
Filter Method ◽  
Detection Rates ◽  
Earthquake Detection ◽  
American Geophysical ◽  
American Geophysical Union

©2018. American Geophysical Union. All Rights Reserved. Matched-filters are an increasingly popular tool for earthquake detection, but their reliance on a priori knowledge of the targets of interest limits their application to regions with previously documented seismicity. We explore an extension to the matched-filter method to detect earthquakes and low-frequency earthquakes on local to regional scales. We show that it is possible to increase the number of detections compared with standard energy-based methods, with low false-detection rates, using suites of synthetic waveforms as templates. We apply this to a microearthquake swarm and an aftershock sequence, and to detect low-frequency earthquakes. We also explore the sensitivity of detections to the synthetic source's location and focal mechanism. Source-receiver geometry has a first-order control on how sensitive matched-filter detectors are to variations in source location and focal mechanism, and this likely applies to detections made using both synthetic and real templates.

scholarly journals Detecting Real Earthquakes Using Artificial Earthquakes: On the Use of Synthetic Waveforms in Matched-Filter Earthquake Detection

2021 ◽  
Author(s):  
Calum Chamberlain ◽  
John Townend
Keyword(s):  
Focal Mechanism ◽  
Matched Filter ◽  
A Priori ◽  
Low Frequency ◽  
Aftershock Sequence ◽  
Filter Method ◽  
Detection Rates ◽  
Earthquake Detection ◽  
American Geophysical ◽  
American Geophysical Union

©2018. American Geophysical Union. All Rights Reserved. Matched-filters are an increasingly popular tool for earthquake detection, but their reliance on a priori knowledge of the targets of interest limits their application to regions with previously documented seismicity. We explore an extension to the matched-filter method to detect earthquakes and low-frequency earthquakes on local to regional scales. We show that it is possible to increase the number of detections compared with standard energy-based methods, with low false-detection rates, using suites of synthetic waveforms as templates. We apply this to a microearthquake swarm and an aftershock sequence, and to detect low-frequency earthquakes. We also explore the sensitivity of detections to the synthetic source's location and focal mechanism. Source-receiver geometry has a first-order control on how sensitive matched-filter detectors are to variations in source location and focal mechanism, and this likely applies to detections made using both synthetic and real templates.

scholarly journals LOW-FREQUENCY NOISE MEASUREMENTS OF IR PHOTODETECTORS WITH VOLTAGE CROSS CORRELATION SYSTEM

Measurement ◽  
2021 ◽  
pp. 109867
Author(s):  
Krzysztof ACHTENBERG ◽  
Janusz MIKOŁAJCZYK ◽  
Carmine CIOFI ◽  
Graziella SCANDURRA ◽  
Krystian MICHALCZEWSKI ◽  
...  
Keyword(s):  
Cross Correlation ◽  
Low Frequency ◽  
Frequency Noise ◽  
Low Frequency Noise ◽  
Ir Photodetectors ◽  
Noise Measurements ◽  
Correlation System

scholarly journals QuakeMigrate: a Modular, Open-Source Python Package for Automatic Earthquake Detection and Location

2021 ◽  
Author(s):  
Tom Winder ◽  
Conor Bacon ◽  
Jonathan Smith ◽  
Thomas Hudson ◽  
Tim Greenfield ◽  
...  
Keyword(s):  
Open Source ◽  
Earthquake Detection ◽  
Python Package

Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate

2021 ◽  
Author(s):  
Duna Roda-Boluda ◽  
Taylor Schildgen ◽  
Hella Wittmann-Oelze ◽  
Stefanie Tofelde ◽  
Aaron Bufe ◽  
...  
Keyword(s):  
New Zealand ◽  
Strike Slip ◽  
Southern Alps ◽  
Fault System ◽  
Tectonic Deformation ◽  
Seismic Cycle ◽  
Erosion Rates ◽  
Alpine Fault ◽  
Oblique Convergence

&lt;p&gt;The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.&lt;/p&gt;&lt;p&gt;To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and &gt;10 mm/yr, but we find that erosion rates of &gt;10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~10&lt;sup&gt;3&lt;/sup&gt; yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our &lt;sup&gt;10&lt;/sup&gt;Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.&lt;/p&gt;&lt;p&gt;Our results highlight the potential for &lt;sup&gt;10&lt;/sup&gt;Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

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