scholarly journals Microseismicity of the Central Alpine Fault Region, New Zealand

2021 ◽  
Author(s):  
◽  
Bronwyn Cherie O'Keefe
Keyword(s):  
New Zealand ◽  
High Stress ◽  
Fluid Pressure ◽  
Time Frame ◽  
B Value ◽  
Point Sources ◽  
Local Magnitude ◽  
Linear Inversion ◽  
Alpine Fault ◽  
Brittle Ductile Transition

<p>This study investigates the spatial and temporal patterns in microseismicity along the central section of the Alpine Fault, South Island, New Zealand. This section, between Harihari and Karangarua, has significantly lower seismicity than the regions to the northeast and southwest. Several hypotheses of mechanisms said to contribute to the anomaly have been proposed over the years including locked fault, slow slip, shallow creep and external fluids affecting the thermal regime and brittle-ductile transition. Focussing on the shallow crust, the contrasting seismic character is compared to the northern and southern sections from seismicity behaviour, focal mechanisms and seismogenic depth. A temporal array of eight seismographs (including three broadband instruments) was augmented with three GeoNet stations bounding the array. This provided an average spacing of 14 km and a magnitude cut-off of ML 1.6 compared to the GeoNet national network cut-off of ML 2.6 and station spacing of 80-100 km. The Gutenberg-Richter distribution for the four month time frame analysed defned a b-value of 0.75 plus or minus 0.06 which may indicate a locked, heterogeneous zone under high-stress from fluid pressure or a predominance of thrust mechanisms over the survey period. Seismicity over the deployment was within the average range of the last 15 years. The 'horseshoe' shaped seismicity pattern observed from long-term national catologue data is similar for smaller magnitudes. While the central portion of the Alpine Fault is quieter with unusually low b-value, the region is not aseismic. Neither does it experience the level of microseismicity seen in creeping faults. The brittle-ductile transition varies laterally along the fault and is estimated at up to 15 km for most of the survey region but closer to 10 km for the region associated with the highest orogenic uplift rates which compares well with past studies. A local magnitude scale was developed from direct linear inversion of the pseudoWood-Anderson amplitudes and event-station distances. A linear inversion of data from the standard New Zealand magnitude equation characterised an attenuation parameter of 0.0167 km minus 1; more than double the value used in national local magnitude calculations (of 0.0067 km minus 1). Swarm clustering dominates the seismicity character of the time frame. Utilising the earthquake relocation program HypoDD, a selection of clusters both near the Alpine Fault and away from it resolve to point sources. Those close to the Alpine Fault are located in what may be the footwall of the Fault which may indicate that the velocity model has located the events too far to the northwest.</p>

scholarly journals Microseismicity of the Central Alpine Fault Region, New Zealand

2021 ◽  
Author(s):  
◽  
Bronwyn Cherie O'Keefe
Keyword(s):  
New Zealand ◽  
High Stress ◽  
Fluid Pressure ◽  
Time Frame ◽  
B Value ◽  
Point Sources ◽  
Local Magnitude ◽  
Linear Inversion ◽  
Alpine Fault ◽  
Brittle Ductile Transition

<p>This study investigates the spatial and temporal patterns in microseismicity along the central section of the Alpine Fault, South Island, New Zealand. This section, between Harihari and Karangarua, has significantly lower seismicity than the regions to the northeast and southwest. Several hypotheses of mechanisms said to contribute to the anomaly have been proposed over the years including locked fault, slow slip, shallow creep and external fluids affecting the thermal regime and brittle-ductile transition. Focussing on the shallow crust, the contrasting seismic character is compared to the northern and southern sections from seismicity behaviour, focal mechanisms and seismogenic depth. A temporal array of eight seismographs (including three broadband instruments) was augmented with three GeoNet stations bounding the array. This provided an average spacing of 14 km and a magnitude cut-off of ML 1.6 compared to the GeoNet national network cut-off of ML 2.6 and station spacing of 80-100 km. The Gutenberg-Richter distribution for the four month time frame analysed defned a b-value of 0.75 plus or minus 0.06 which may indicate a locked, heterogeneous zone under high-stress from fluid pressure or a predominance of thrust mechanisms over the survey period. Seismicity over the deployment was within the average range of the last 15 years. The 'horseshoe' shaped seismicity pattern observed from long-term national catologue data is similar for smaller magnitudes. While the central portion of the Alpine Fault is quieter with unusually low b-value, the region is not aseismic. Neither does it experience the level of microseismicity seen in creeping faults. The brittle-ductile transition varies laterally along the fault and is estimated at up to 15 km for most of the survey region but closer to 10 km for the region associated with the highest orogenic uplift rates which compares well with past studies. A local magnitude scale was developed from direct linear inversion of the pseudoWood-Anderson amplitudes and event-station distances. A linear inversion of data from the standard New Zealand magnitude equation characterised an attenuation parameter of 0.0167 km minus 1; more than double the value used in national local magnitude calculations (of 0.0067 km minus 1). Swarm clustering dominates the seismicity character of the time frame. Utilising the earthquake relocation program HypoDD, a selection of clusters both near the Alpine Fault and away from it resolve to point sources. Those close to the Alpine Fault are located in what may be the footwall of the Fault which may indicate that the velocity model has located the events too far to the northwest.</p>

scholarly journals Microseismicity, tectonic stress, and exhumation rates near the central Alpine Fault, New Zealand

2021 ◽  
Author(s):  
◽  
Konstantinos Michailos
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Large Earthquake ◽  
Southern Alps ◽  
Focal Mechanisms ◽  
P Wave ◽  
Data Set ◽  
Alpine Fault ◽  
Brittle Ductile Transition ◽  
Average Maximum

<p>This thesis documents a detailed examination of the seismic activity and characteristics of crustal deformation along the central Alpine Fault, a major obliquely convergent plate-boundary fault. Paleoseismic evidence has established that the Alpine Fault produces large to great (M7−8) earthquakes every 250−300 years, in a quasi-periodic manner, with the last surface-rupturing earthquake occurring in 1717. This renders the fault late in its typical earthquake cycle, posing substantial seismic risk to southern and central New Zealand. Understanding the seismic and tectonic character of this fault may yield information of both societal and scientific significance regarding seismic hazard and late-interseismic processes leading up to a large earthquake. However, the central Alpine Fault is currently seismically quiescent when compared to adjacent regions, and therefore requires detailed, long-duration observations to study seismotectonic processes. The work in this thesis addresses the need for a greater understanding of along-strike variations in seismic character of the Alpine Fault ahead of an anticipated large earthquake.  To achieve observations with high spatial and temporal resolution across the length of the central Alpine Fault, I use 8.5 years of continuous seismic data from the Southern Alps Microearthquake Borehole Array (SAMBA), and data from four other temporary seismic networks and five local GeoNet permanent sites. Incorporating all of these temporary and permanent seismic sites provides us with a dense composite network of seismometers. Without such a dense network, homogeneous examination of the characteristics of low-magnitude seismicity near the Alpine Fault would be impossible.  Using this dataset, I have constructed the most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes and covering the time between late 2008 and early 2017. To construct this catalog I created an objective workflow to ensure catalog uniformity. Overall, 7,719 earthquakes were successfully relocated with location uncertainties generally ≤ 0.5 km in both the horizontal and vertical directions. The majority of the earthquakes were found to occur southeast of the Alpine Fault (i.e. in the hanging-wall). I observed a lack of seismicity beneath Aoraki/Mount Cook that has previously been shown to be associated with locally high uplift rates (6–10 mm/yr) and high geothermal gradients (∼60◦C/km). Seismogenic cut-off depths were observed to significantly vary along the strike of the Alpine Fault, ranging from 8 km beneath the highest topography to 20 km in the adjacent areas.  To quantify the scale of the seismic deformation, a new local magnitude scale was also derived, corrected for geometric spreading, attenuation and site terms based on individually calculated GeoNet moment magnitude (Mw) values. Earthquake local magnitudes range between ML –1.2 and 4.6 and the catalog is complete above ML 1.1.  To examine the stress regime near the central Alpine Fault, I built a new data set of 845 focal mechanisms from earthquakes in our catalog. This was achieved by manually determining P wave arrival polarity picks from all earthquakes larger than ML 1.5. In order to determine the orientations and characteristics of the stress parameters, I grouped these focal mechanisms and performed stress inversion calculations that provided an average maximum horizontal compressive stress orientation, SHmax, of 121±11◦ , which is uniform within uncertainty along the length of the central Southern Alps. I observed an average angle of 65◦ between the SHmax and the strike of the Alpine Fault, which is consistent with results from similar previous studies in the northern and southern sections of the Alpine Fault. This implies that the Alpine Fault is misoriented for reactivation, in the prevailing stress field.  Using a 1-D steady-state thermal structure model constrained by seismicity and thermochronology data, I investigated the crustal thermal structure and vertical kinematics of the central Southern Alps orogen. The short-term seismicity data and longer-term thermochronology data impose complementary constraints on the model. I observed a large variation in exhumation rate estimates (1–8 mm/yr) along the length of the Alpine Fault, with maximum calculated values observed near Aoraki/Mount Cook. I calculated the temperature at the brittle-ductile transition zone, which ranges from 440 to 457◦C in the different models considered. This temperature is slightly hotter than expected for crust composed by quartz-rich rocks, but consistent with the presence of feldspar-rich mafic rocks in parts of the crust.</p>

scholarly journals Crustal Thermal Structure and Exhumation Rates in the Southern Alps Near the Central Alpine Fault, New Zealand

2021 ◽  
Author(s):  
K Michailos ◽  
Rupert Sutherland ◽  
John Townend ◽  
Martha Savage
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Hanging Wall ◽  
Southern Alps ◽  
Seismogenic Zone ◽  
Fixed Temperature ◽  
Alpine Fault ◽  
Brittle Ductile Transition ◽  
Orogenic Uplift ◽  
Exhumation Rates

© 2020. American Geophysical Union. All Rights Reserved. We investigate orogenic uplift rates and the thermal structure of the crust in the hanging wall of the Alpine Fault, New Zealand, using the hypocenters of 7,719 earthquakes that occurred in the central Southern Alps between late 2008 and early 2017, and previously published thermochronological data. We assume that the base of the seismogenic zone corresponds to a brittle-ductile transition at some fixed temperature, which we estimate by fitting the combined thermochronological data and distribution of seismicity using a multi-1-D approach. We find that exhumation rates vary from 1 to 8 mm/yr, with maximum values observed in the area of highest topography near Aoraki/Mount Cook, a finding consistent with previous geologic and geodetic analyses. We estimate the temperature of the brittle-ductile transition beneath the Southern Alps to be 410–430°C, which is higher than expected for Alpine Fault rocks whose bulk lithology is likely dominated by quartz. The high estimated temperatures at the base of the seismogenic zone likely reflect the unmodeled effects of high fluid pressures or strain rates.

scholarly journals Microseismicity, tectonic stress, and exhumation rates near the central Alpine Fault, New Zealand

2021 ◽  
Author(s):  
◽  
Konstantinos Michailos
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Large Earthquake ◽  
Southern Alps ◽  
Focal Mechanisms ◽  
P Wave ◽  
Data Set ◽  
Alpine Fault ◽  
Brittle Ductile Transition ◽  
Average Maximum

<p>This thesis documents a detailed examination of the seismic activity and characteristics of crustal deformation along the central Alpine Fault, a major obliquely convergent plate-boundary fault. Paleoseismic evidence has established that the Alpine Fault produces large to great (M7−8) earthquakes every 250−300 years, in a quasi-periodic manner, with the last surface-rupturing earthquake occurring in 1717. This renders the fault late in its typical earthquake cycle, posing substantial seismic risk to southern and central New Zealand. Understanding the seismic and tectonic character of this fault may yield information of both societal and scientific significance regarding seismic hazard and late-interseismic processes leading up to a large earthquake. However, the central Alpine Fault is currently seismically quiescent when compared to adjacent regions, and therefore requires detailed, long-duration observations to study seismotectonic processes. The work in this thesis addresses the need for a greater understanding of along-strike variations in seismic character of the Alpine Fault ahead of an anticipated large earthquake.  To achieve observations with high spatial and temporal resolution across the length of the central Alpine Fault, I use 8.5 years of continuous seismic data from the Southern Alps Microearthquake Borehole Array (SAMBA), and data from four other temporary seismic networks and five local GeoNet permanent sites. Incorporating all of these temporary and permanent seismic sites provides us with a dense composite network of seismometers. Without such a dense network, homogeneous examination of the characteristics of low-magnitude seismicity near the Alpine Fault would be impossible.  Using this dataset, I have constructed the most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes and covering the time between late 2008 and early 2017. To construct this catalog I created an objective workflow to ensure catalog uniformity. Overall, 7,719 earthquakes were successfully relocated with location uncertainties generally ≤ 0.5 km in both the horizontal and vertical directions. The majority of the earthquakes were found to occur southeast of the Alpine Fault (i.e. in the hanging-wall). I observed a lack of seismicity beneath Aoraki/Mount Cook that has previously been shown to be associated with locally high uplift rates (6–10 mm/yr) and high geothermal gradients (∼60◦C/km). Seismogenic cut-off depths were observed to significantly vary along the strike of the Alpine Fault, ranging from 8 km beneath the highest topography to 20 km in the adjacent areas.  To quantify the scale of the seismic deformation, a new local magnitude scale was also derived, corrected for geometric spreading, attenuation and site terms based on individually calculated GeoNet moment magnitude (Mw) values. Earthquake local magnitudes range between ML –1.2 and 4.6 and the catalog is complete above ML 1.1.  To examine the stress regime near the central Alpine Fault, I built a new data set of 845 focal mechanisms from earthquakes in our catalog. This was achieved by manually determining P wave arrival polarity picks from all earthquakes larger than ML 1.5. In order to determine the orientations and characteristics of the stress parameters, I grouped these focal mechanisms and performed stress inversion calculations that provided an average maximum horizontal compressive stress orientation, SHmax, of 121±11◦ , which is uniform within uncertainty along the length of the central Southern Alps. I observed an average angle of 65◦ between the SHmax and the strike of the Alpine Fault, which is consistent with results from similar previous studies in the northern and southern sections of the Alpine Fault. This implies that the Alpine Fault is misoriented for reactivation, in the prevailing stress field.  Using a 1-D steady-state thermal structure model constrained by seismicity and thermochronology data, I investigated the crustal thermal structure and vertical kinematics of the central Southern Alps orogen. The short-term seismicity data and longer-term thermochronology data impose complementary constraints on the model. I observed a large variation in exhumation rate estimates (1–8 mm/yr) along the length of the Alpine Fault, with maximum calculated values observed near Aoraki/Mount Cook. I calculated the temperature at the brittle-ductile transition zone, which ranges from 440 to 457◦C in the different models considered. This temperature is slightly hotter than expected for crust composed by quartz-rich rocks, but consistent with the presence of feldspar-rich mafic rocks in parts of the crust.</p>

scholarly journals Crustal Thermal Structure and Exhumation Rates in the Southern Alps Near the Central Alpine Fault, New Zealand

2021 ◽  
Author(s):  
K Michailos ◽  
Rupert Sutherland ◽  
John Townend ◽  
Martha Savage
Keyword(s):  
New Zealand ◽  
Thermal Structure ◽  
Hanging Wall ◽  
Southern Alps ◽  
Seismogenic Zone ◽  
Fixed Temperature ◽  
Alpine Fault ◽  
Brittle Ductile Transition ◽  
Orogenic Uplift ◽  
Exhumation Rates

© 2020. American Geophysical Union. All Rights Reserved. We investigate orogenic uplift rates and the thermal structure of the crust in the hanging wall of the Alpine Fault, New Zealand, using the hypocenters of 7,719 earthquakes that occurred in the central Southern Alps between late 2008 and early 2017, and previously published thermochronological data. We assume that the base of the seismogenic zone corresponds to a brittle-ductile transition at some fixed temperature, which we estimate by fitting the combined thermochronological data and distribution of seismicity using a multi-1-D approach. We find that exhumation rates vary from 1 to 8 mm/yr, with maximum values observed in the area of highest topography near Aoraki/Mount Cook, a finding consistent with previous geologic and geodetic analyses. We estimate the temperature of the brittle-ductile transition beneath the Southern Alps to be 410–430°C, which is higher than expected for Alpine Fault rocks whose bulk lithology is likely dominated by quartz. The high estimated temperatures at the base of the seismogenic zone likely reflect the unmodeled effects of high fluid pressures or strain rates.

Determining hydrological and soil mechanical parameters from multichannel surface-wave analysis across the Alpine Fault at Inchbonnie, New Zealand

2013 ◽  
Vol 11 (4) ◽  
pp. 435-448 ◽  
Author(s):  
L.A. Konstantaki ◽  
S. Carpentier ◽  
F. Garofalo ◽  
P. Bergamo ◽  
L.V. Socco
Keyword(s):  
New Zealand ◽  
Surface Wave ◽  
Mechanical Parameters ◽  
Wave Analysis ◽  
Alpine Fault

scholarly journals The fluid budget of a continental plate boundary fault: Quantification from the Alpine Fault, New Zealand

2016 ◽  
Vol 445 ◽  
pp. 125-135 ◽  
Author(s):  
Catriona D. Menzies ◽  
Damon A.H. Teagle ◽  
Samuel Niedermann ◽  
Simon C. Cox ◽  
Dave Craw ◽  
...  
Keyword(s):  
New Zealand ◽  
Plate Boundary ◽  
Boundary Fault ◽  
Continental Plate ◽  
Alpine Fault

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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