Martha Savage - Academia.edu (original) (raw)

Papers by Martha Savage

Geochemistry Geophysics Geosystems, Jun 1, 2019

Fluid injection into the Earth's crust can induce seismic events that cause damage to local infra... more Fluid injection into the Earth's crust can induce seismic events that cause damage to local infrastructure but also offer valuable insight into seismogenesis. The factors that influence the magnitude, location, and number of induced events remain poorly understood but include injection flow rate and pressure as well as reservoir temperature and permeability. The relationship between injection parameters and injection-induced seismicity in high-temperature, high-permeability reservoirs has not been extensively studied. Here we focus on the Ngatamariki geothermal field in the central Taupō Volcanic Zone, New Zealand, where three stimulation/injection tests have occurred since 2012. We present a catalog of seismicity from 2012 to 2015 created using a matched-filter detection technique. We analyze the stress state in the reservoir during the injection tests from first motion-derived focal mechanisms, yielding an average direction of maximum horizontal compressive stress (S Hmax) consistent with the regional NE-SW trend. However, there is significant variation in the direction of maximum compressive stress (1), which may reflect geological differences between wells. We use the ratio of injection flow rate to overpressure, referred to as injectivity index, as a proxy for near-well permeability and compare changes in injectivity index to spatiotemporal characteristics of seismicity accompanying each test. Observed increases in injectivity index are generally poorly correlated with seismicity, suggesting that the locations of microearthquakes are not coincident with the zone of stimulation (i.e., increased permeability). Our findings augment a growing body of work suggesting that aseismic opening or slip, rather than seismic shear, is the active process driving well stimulation in many environments. Plain Language Summary When industries inject fluid into the Earth, oftentimes earthquakes are the result. Although this presents a hazard to nearby infrastructure and populations, studying these events helps us understand how fluid is moving underground and the processes that create earthquakes. In this paper, we describe over 9,000 earthquakes that occurred over a 4-year period at a geothermal field on the North Island of New Zealand. The details of where and when they occurred and how big they were give us clues about fractures ∼3,000-m underground and therefore how easily fluid can move from point A to point B underground. We focus on two wells where operators injected cold water (∼20 • C) at high pressure to try to open fractures in the reservoir. This would allow them to extract and reinject more fluid and therefore produce more power. We found that, while some of these operations triggered earthquakes, the amount of fluid that could be pumped into the ground was not related to the number of earthquakes. This means that, in the future, companies may be able to design injection operations so that they achieve their goal of opening fractures underground, without also generating large earthquakes.

Journal of Geophysical Research: Solid Earth

We use 12 temporary and 9 permanent broadband seismometers that were operating for ∼400 days from... more We use 12 temporary and 9 permanent broadband seismometers that were operating for ∼400 days from October 2018 to November 2019 to generate the first published earthquake catalog and local magnitude function for southern New Caledonia (SNC). Local hypocenters mostly have depths <20 km, but east of the New Hebrides‐Vanuatu subduction zone they deepen to >100 km. Our local magnitude estimates ML for 107 earthquakes in the subduction zone are consistently 1.1 units smaller than Mw and mb over a range of Mw from 4.5 to 7.5, as determined by the United States Geological Survey. Our catalog has 460 earthquakes with Mw≥3.7 in the subduction zone and the largest event in SNC has ML 3.8. Seismicity rates in SNC are low, but ML> 5 earthquakes are 2.7 times more frequent than elsewhere in the northern Australian plate. The probability of an ML> 5 event in 50 years is 0.6 in SNC. The hazard of damaging seismic shaking in SNC is dominated by local moderate‐magnitude earthquakes, rath...

Journal of Geophysical Research: Solid Earth

Seafloor pressure sensor data is emerging as a promising approach to resolve vertical displacemen... more Seafloor pressure sensor data is emerging as a promising approach to resolve vertical displacement of the seafloor in the offshore reaches of subduction zones, particularly in response to slow slip events (SSEs), although such signals are challenging to resolve due to sensor drift and oceanographic signals. Constraining offshore SSE slip distribution is of key importance to understanding earthquake and tsunami hazards posed by subduction zones. We processed seafloor pressure data from January to October 2019 acquired at the Hikurangi subduction zone, offshore New Zealand, to estimate vertical displacement associated with a large SSE that occurred beneath the seafloor array. The experiment included three self‐calibrating sensors designed to remove sensor drift, which, together with ocean general circulation models, were essential to the identification and correction of long‐period ocean variability remaining in the data after applying traditional processing techniques. We estimate th...

Geological Society of America Abstracts with Programs

Science Advances

New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthen... more New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthenosphere boundary (LAB) is more structured than previously thought. Three distinct layers are interpreted within a 10- to 12-km-thick LAB zone beginning at a depth of ≈70 km: a 3 (±1)–km-thick layer at the bottom of the lithosphere with a P-wave ( V P ) azimuthal anisotropy of 14 to 17% and fast azimuth subparallel to the direction of absolute plate motion and a 9 (±2)–km-thick, low V P channel with a P-wave–to–S-wave velocity ratio ( V P / V S ) of >2.8 in the upper 7 km of the channel and 1.8 to 2.6 in the lower 2 km of the channel. The high V P / V S ratios indicate that this channel may contain 3 to 20% partial melt that facilitates decoupling of the lithosphere from the asthenosphere and reduces resistance for plate motion. Furthermore, the strong azimuthal anisotropy above the low-velocity layer suggests localization of strain due to melt accumulation.

AGU Fall Meeting Abstracts, Dec 1, 2018

Geophysical Journal International, 2021

SUMMARY Seismic velocity changes before and after large-magnitude earthquakes carry information a... more SUMMARY Seismic velocity changes before and after large-magnitude earthquakes carry information about damage present in the surrounding region. This study presents temporal velocity changes detected prior to and following the 2016 November Mw 7.8 Kaikōura earthquake in Canterbury, New Zealand. We use continuous waveform data from 11 short-period seismometers within the Kaikōura region with an average interstation distance of 83 km. Nine-component day-long empirical Green’s functions were computed for frequencies between 0.1 and 0.9 Hz for continuous seismic records from 2012 January 1 to 2018 February 28, which also include the 2013 Cook Strait and Lake Grassmere earthquakes. Using the moving-window cross-spectral method, seismic velocity changes were calculated. Immediately following the 2016 Kaikōura earthquake, a decrease in seismic velocity averaged across all component pairs of approximately 0.2 per cent was observed. An increase in seismic velocity of approximately 0.1 per cen...

Geochemistry, Geophysics, Geosystems, 2020

We investigate orogenic uplift rates and the thermal structure of the crust in the hanging wall o... more 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 a...

The use of coercion in mental healthcare has long been subject to controversy, and the call for v... more The use of coercion in mental healthcare has long been subject to controversy, and the call for viable alternatives is growing both within the profession and among people with lived experience of coercion in mental healthcare. • This Discussion Paper outlines recent developments in practice, research and international human rights law concerning coercion in mental health settings with the aim of supporting psychiatrists and other mental health professionals in their work towards improving the quality and safety of mental health services and putting sound alternatives to coercion in placeVytauto Didžiojo universiteta

Geophysical Journal International, 2019

Harrat Rahat is a volcanic field located in west-central Saudi Arabia and is the site of the most... more Harrat Rahat is a volcanic field located in west-central Saudi Arabia and is the site of the most recent eruption in the country (1256 CE). An earthquake swarm at a nearby volcanic field in 2009 prompted the need for new hazard models for this region, which includes the holy city of Medina. Tomography studies can be used to infer material properties of the subsurface such as partial melt, and are instrumental for volcanic hazard assessment. Regional earthquakes have been used to determine mantle structure, but such crustal models are often hindered by an insufficient number of earthquakes in the plate interior. We use ambient seismic noise to compute Rayleigh and Love surface-wave dispersion maps between 5 and 12 s for northern Harrat Rahat. The surface-wave maps are inverted to produce shear-wave velocities using a neighbourhood algorithm and interpolated into a pseudo-3-D model. The distributions of surface-wave and shear-wave velocities are heterogenous, varying between ±3 and 8 ...

Journal of Geophysical Research: Solid Earth, 2021

The Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in... more The Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in its typical earthquake cycle. Understanding the subsurface structures is crucial to understand the tectonic processes taking place. A unique seismic survey including 2D lines, a 3D array, and borehole recordings, has been performed in the Whataroa Valley and provides new insights into the Alpine Fault zone down to ∼2 km depth at the location of the Deep Fault Drilling Project (DFDP)‐2 drill site. Seismic images are obtained by focusing prestack depth migration approaches. Despite the challenging conditions for seismic imaging within a sediment filled glacial valley and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. A set of several reflectors dipping 40°–56° to the southeast are identified in a ∼600 m wide zone that is interpreted to be the minimum extent of the damage zone. Different approaches image one d...

Journal of Geophysical Research: Solid Earth, 2021

Many types of slow earthquakes have been discovered at subduction zones around the world. However... more Many types of slow earthquakes have been discovered at subduction zones around the world. However, the physical process of these slow earthquakes is not well understood. To monitor offshore slow earthquakes, a marine seismic and geodetic experiment was conducted at the Hikurangi subduction margin from May 2014 to June 2015. During this experiment, a large slow slip event (Mw 6.8) occurred directly beneath the ocean bottom seismometer (OBS) network. In this study, S‐wave splitting and polarization analysis methods, which have been previously used on onshore data to investigate tremor and anisotropy, are applied to continuous OBS waveform data to identify tremors that are too small to detect by the envelope cross correlation method. Continuous tremor activity with stable polarization directions is detected at the end of the 2014 slow slip event and continued for about 2 weeks. The tremors are generated around a southwest bend in the slow slip contours and at the landward edge of a sub...

Earth and Planetary Science Letters, 2020

Controlled-source seismic studies at most subduction zones show that bending of the subducting pl... more Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (V p) in the upper mantle under oceanic plateaus. Onshoreoffshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (V p ≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (V p ≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle V p ≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered V p in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in V p is ≈10%, and a similar drop in V p is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bendinginduced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the "regular mantle" (V p ≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle V p under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle V p under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting.

Earth and Planetary Science Letters, 2020

Geophysical Journal International, 2019

SUMMARY Fluid injection for geothermal production has the potential to produce subsidence and mic... more SUMMARY Fluid injection for geothermal production has the potential to produce subsidence and microseismicity that can incur heavy financial cost or hazard. Due to this, novel ways to monitor subsurface deformation to supplement existing methods are highly sought after. We use seismic ambient noise to obtain time-dependent measurements of shear velocity within the geothermal reservoirs of Rotokawa and Ngatamariki, two producing geothermal fields in the Taupō Volcanic Zone located in the central North Island of New Zealand and operated by Mercury Energy. We investigate the relationship between shear wave velocity changes and geothermal injection by selecting time periods at the fields when injection and production volumes were significantly altered: 2009–2010 at Rotokawa, when geothermal injection was quadrupled due to the start-up of a new power station, and 2012–2013 at Ngatamariki, the beginning of geothermal injection for electricity production at that field. Shear wave velocity ...

New Zealand Journal of Geology and Geophysics, 2018

ABSTRACT Double Benioff Zones (DBZ) are ubiquitous in subduction systems worldwide, but the stres... more ABSTRACT Double Benioff Zones (DBZ) are ubiquitous in subduction systems worldwide, but the stress systems that give rise to them are not well known. We characterise stress orientations in the upper and lower bands of a DBZ in the dipping subducted Pacific plate beneath the southern Hikurangi margin, New Zealand. Stress orientations were calculated from focal mechanisms with 10 or more polarity picks, using a Bayesian stress inversion technique. The stress orientations in the upper band of seismicity are consistent with a down-dip extensional stress regime, whereas stress orientations within the lower band of seismicity are consistent with a down-dip compressional stress regime. This demonstrates that bending stresses are dominant within the Pacific plate beneath the southern North Island, possibly due to subduction beneath thickened mantle lithosphere of the Australian plate.

Geophysical Journal International, 2018

Geochemistry, Geophysics, Geosystems, 2018

The Alpine Fault is an oblique strike‐slip fault that is known to fail in large magnitude (M7–8) ... more The Alpine Fault is an oblique strike‐slip fault that is known to fail in large magnitude (M7–8) earthquakes, yet it is currently seismically quiescent. We examine the low‐magnitude earthquake activity occurring along the central portion of the Alpine Fault using seismic data from five temporary seismic networks deployed for various lengths of time between late 2008 and early 2017. Starting from continuous seismic data, we detect earthquake arrivals and construct the longest and most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes. This enables us to study the distribution and characteristics of the seismicity in unprecedented detail. Earthquake locations are constrained by high‐quality automatic and manual picks, and we perform relocations using waveform cross‐correlation to better constrain hypocenters. We have derived a new local magnitude scale calibrated by Mw values. Magnitudes range between ML−1.2 and 4.6, and our ca...

Geochemistry Geophysics Geosystems, Jun 1, 2019

Fluid injection into the Earth's crust can induce seismic events that cause damage to local infra... more Fluid injection into the Earth's crust can induce seismic events that cause damage to local infrastructure but also offer valuable insight into seismogenesis. The factors that influence the magnitude, location, and number of induced events remain poorly understood but include injection flow rate and pressure as well as reservoir temperature and permeability. The relationship between injection parameters and injection-induced seismicity in high-temperature, high-permeability reservoirs has not been extensively studied. Here we focus on the Ngatamariki geothermal field in the central Taupō Volcanic Zone, New Zealand, where three stimulation/injection tests have occurred since 2012. We present a catalog of seismicity from 2012 to 2015 created using a matched-filter detection technique. We analyze the stress state in the reservoir during the injection tests from first motion-derived focal mechanisms, yielding an average direction of maximum horizontal compressive stress (S Hmax) consistent with the regional NE-SW trend. However, there is significant variation in the direction of maximum compressive stress (1), which may reflect geological differences between wells. We use the ratio of injection flow rate to overpressure, referred to as injectivity index, as a proxy for near-well permeability and compare changes in injectivity index to spatiotemporal characteristics of seismicity accompanying each test. Observed increases in injectivity index are generally poorly correlated with seismicity, suggesting that the locations of microearthquakes are not coincident with the zone of stimulation (i.e., increased permeability). Our findings augment a growing body of work suggesting that aseismic opening or slip, rather than seismic shear, is the active process driving well stimulation in many environments. Plain Language Summary When industries inject fluid into the Earth, oftentimes earthquakes are the result. Although this presents a hazard to nearby infrastructure and populations, studying these events helps us understand how fluid is moving underground and the processes that create earthquakes. In this paper, we describe over 9,000 earthquakes that occurred over a 4-year period at a geothermal field on the North Island of New Zealand. The details of where and when they occurred and how big they were give us clues about fractures ∼3,000-m underground and therefore how easily fluid can move from point A to point B underground. We focus on two wells where operators injected cold water (∼20 • C) at high pressure to try to open fractures in the reservoir. This would allow them to extract and reinject more fluid and therefore produce more power. We found that, while some of these operations triggered earthquakes, the amount of fluid that could be pumped into the ground was not related to the number of earthquakes. This means that, in the future, companies may be able to design injection operations so that they achieve their goal of opening fractures underground, without also generating large earthquakes.

Journal of Geophysical Research: Solid Earth

We use 12 temporary and 9 permanent broadband seismometers that were operating for ∼400 days from... more We use 12 temporary and 9 permanent broadband seismometers that were operating for ∼400 days from October 2018 to November 2019 to generate the first published earthquake catalog and local magnitude function for southern New Caledonia (SNC). Local hypocenters mostly have depths <20 km, but east of the New Hebrides‐Vanuatu subduction zone they deepen to >100 km. Our local magnitude estimates ML for 107 earthquakes in the subduction zone are consistently 1.1 units smaller than Mw and mb over a range of Mw from 4.5 to 7.5, as determined by the United States Geological Survey. Our catalog has 460 earthquakes with Mw≥3.7 in the subduction zone and the largest event in SNC has ML 3.8. Seismicity rates in SNC are low, but ML> 5 earthquakes are 2.7 times more frequent than elsewhere in the northern Australian plate. The probability of an ML> 5 event in 50 years is 0.6 in SNC. The hazard of damaging seismic shaking in SNC is dominated by local moderate‐magnitude earthquakes, rath...

Journal of Geophysical Research: Solid Earth

Seafloor pressure sensor data is emerging as a promising approach to resolve vertical displacemen... more Seafloor pressure sensor data is emerging as a promising approach to resolve vertical displacement of the seafloor in the offshore reaches of subduction zones, particularly in response to slow slip events (SSEs), although such signals are challenging to resolve due to sensor drift and oceanographic signals. Constraining offshore SSE slip distribution is of key importance to understanding earthquake and tsunami hazards posed by subduction zones. We processed seafloor pressure data from January to October 2019 acquired at the Hikurangi subduction zone, offshore New Zealand, to estimate vertical displacement associated with a large SSE that occurred beneath the seafloor array. The experiment included three self‐calibrating sensors designed to remove sensor drift, which, together with ocean general circulation models, were essential to the identification and correction of long‐period ocean variability remaining in the data after applying traditional processing techniques. We estimate th...

Geological Society of America Abstracts with Programs

Science Advances

New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthen... more New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthenosphere boundary (LAB) is more structured than previously thought. Three distinct layers are interpreted within a 10- to 12-km-thick LAB zone beginning at a depth of ≈70 km: a 3 (±1)–km-thick layer at the bottom of the lithosphere with a P-wave ( V P ) azimuthal anisotropy of 14 to 17% and fast azimuth subparallel to the direction of absolute plate motion and a 9 (±2)–km-thick, low V P channel with a P-wave–to–S-wave velocity ratio ( V P / V S ) of >2.8 in the upper 7 km of the channel and 1.8 to 2.6 in the lower 2 km of the channel. The high V P / V S ratios indicate that this channel may contain 3 to 20% partial melt that facilitates decoupling of the lithosphere from the asthenosphere and reduces resistance for plate motion. Furthermore, the strong azimuthal anisotropy above the low-velocity layer suggests localization of strain due to melt accumulation.

AGU Fall Meeting Abstracts, Dec 1, 2018

Geophysical Journal International, 2021

SUMMARY Seismic velocity changes before and after large-magnitude earthquakes carry information a... more SUMMARY Seismic velocity changes before and after large-magnitude earthquakes carry information about damage present in the surrounding region. This study presents temporal velocity changes detected prior to and following the 2016 November Mw 7.8 Kaikōura earthquake in Canterbury, New Zealand. We use continuous waveform data from 11 short-period seismometers within the Kaikōura region with an average interstation distance of 83 km. Nine-component day-long empirical Green’s functions were computed for frequencies between 0.1 and 0.9 Hz for continuous seismic records from 2012 January 1 to 2018 February 28, which also include the 2013 Cook Strait and Lake Grassmere earthquakes. Using the moving-window cross-spectral method, seismic velocity changes were calculated. Immediately following the 2016 Kaikōura earthquake, a decrease in seismic velocity averaged across all component pairs of approximately 0.2 per cent was observed. An increase in seismic velocity of approximately 0.1 per cen...

Geochemistry, Geophysics, Geosystems, 2020

We investigate orogenic uplift rates and the thermal structure of the crust in the hanging wall o... more 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 a...

The use of coercion in mental healthcare has long been subject to controversy, and the call for v... more The use of coercion in mental healthcare has long been subject to controversy, and the call for viable alternatives is growing both within the profession and among people with lived experience of coercion in mental healthcare. • This Discussion Paper outlines recent developments in practice, research and international human rights law concerning coercion in mental health settings with the aim of supporting psychiatrists and other mental health professionals in their work towards improving the quality and safety of mental health services and putting sound alternatives to coercion in placeVytauto Didžiojo universiteta

Geophysical Journal International, 2019

Harrat Rahat is a volcanic field located in west-central Saudi Arabia and is the site of the most... more Harrat Rahat is a volcanic field located in west-central Saudi Arabia and is the site of the most recent eruption in the country (1256 CE). An earthquake swarm at a nearby volcanic field in 2009 prompted the need for new hazard models for this region, which includes the holy city of Medina. Tomography studies can be used to infer material properties of the subsurface such as partial melt, and are instrumental for volcanic hazard assessment. Regional earthquakes have been used to determine mantle structure, but such crustal models are often hindered by an insufficient number of earthquakes in the plate interior. We use ambient seismic noise to compute Rayleigh and Love surface-wave dispersion maps between 5 and 12 s for northern Harrat Rahat. The surface-wave maps are inverted to produce shear-wave velocities using a neighbourhood algorithm and interpolated into a pseudo-3-D model. The distributions of surface-wave and shear-wave velocities are heterogenous, varying between ±3 and 8 ...

Journal of Geophysical Research: Solid Earth, 2021

The Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in... more The Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in its typical earthquake cycle. Understanding the subsurface structures is crucial to understand the tectonic processes taking place. A unique seismic survey including 2D lines, a 3D array, and borehole recordings, has been performed in the Whataroa Valley and provides new insights into the Alpine Fault zone down to ∼2 km depth at the location of the Deep Fault Drilling Project (DFDP)‐2 drill site. Seismic images are obtained by focusing prestack depth migration approaches. Despite the challenging conditions for seismic imaging within a sediment filled glacial valley and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. A set of several reflectors dipping 40°–56° to the southeast are identified in a ∼600 m wide zone that is interpreted to be the minimum extent of the damage zone. Different approaches image one d...

Journal of Geophysical Research: Solid Earth, 2021

Many types of slow earthquakes have been discovered at subduction zones around the world. However... more Many types of slow earthquakes have been discovered at subduction zones around the world. However, the physical process of these slow earthquakes is not well understood. To monitor offshore slow earthquakes, a marine seismic and geodetic experiment was conducted at the Hikurangi subduction margin from May 2014 to June 2015. During this experiment, a large slow slip event (Mw 6.8) occurred directly beneath the ocean bottom seismometer (OBS) network. In this study, S‐wave splitting and polarization analysis methods, which have been previously used on onshore data to investigate tremor and anisotropy, are applied to continuous OBS waveform data to identify tremors that are too small to detect by the envelope cross correlation method. Continuous tremor activity with stable polarization directions is detected at the end of the 2014 slow slip event and continued for about 2 weeks. The tremors are generated around a southwest bend in the slow slip contours and at the landward edge of a sub...

Earth and Planetary Science Letters, 2020

Controlled-source seismic studies at most subduction zones show that bending of the subducting pl... more Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (V p) in the upper mantle under oceanic plateaus. Onshoreoffshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (V p ≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (V p ≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle V p ≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered V p in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in V p is ≈10%, and a similar drop in V p is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bendinginduced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the "regular mantle" (V p ≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle V p under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle V p under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting.

Earth and Planetary Science Letters, 2020

Geophysical Journal International, 2019

SUMMARY Fluid injection for geothermal production has the potential to produce subsidence and mic... more SUMMARY Fluid injection for geothermal production has the potential to produce subsidence and microseismicity that can incur heavy financial cost or hazard. Due to this, novel ways to monitor subsurface deformation to supplement existing methods are highly sought after. We use seismic ambient noise to obtain time-dependent measurements of shear velocity within the geothermal reservoirs of Rotokawa and Ngatamariki, two producing geothermal fields in the Taupō Volcanic Zone located in the central North Island of New Zealand and operated by Mercury Energy. We investigate the relationship between shear wave velocity changes and geothermal injection by selecting time periods at the fields when injection and production volumes were significantly altered: 2009–2010 at Rotokawa, when geothermal injection was quadrupled due to the start-up of a new power station, and 2012–2013 at Ngatamariki, the beginning of geothermal injection for electricity production at that field. Shear wave velocity ...

New Zealand Journal of Geology and Geophysics, 2018

ABSTRACT Double Benioff Zones (DBZ) are ubiquitous in subduction systems worldwide, but the stres... more ABSTRACT Double Benioff Zones (DBZ) are ubiquitous in subduction systems worldwide, but the stress systems that give rise to them are not well known. We characterise stress orientations in the upper and lower bands of a DBZ in the dipping subducted Pacific plate beneath the southern Hikurangi margin, New Zealand. Stress orientations were calculated from focal mechanisms with 10 or more polarity picks, using a Bayesian stress inversion technique. The stress orientations in the upper band of seismicity are consistent with a down-dip extensional stress regime, whereas stress orientations within the lower band of seismicity are consistent with a down-dip compressional stress regime. This demonstrates that bending stresses are dominant within the Pacific plate beneath the southern North Island, possibly due to subduction beneath thickened mantle lithosphere of the Australian plate.

Geophysical Journal International, 2018

Geochemistry, Geophysics, Geosystems, 2018

The Alpine Fault is an oblique strike‐slip fault that is known to fail in large magnitude (M7–8) ... more The Alpine Fault is an oblique strike‐slip fault that is known to fail in large magnitude (M7–8) earthquakes, yet it is currently seismically quiescent. We examine the low‐magnitude earthquake activity occurring along the central portion of the Alpine Fault using seismic data from five temporary seismic networks deployed for various lengths of time between late 2008 and early 2017. Starting from continuous seismic data, we detect earthquake arrivals and construct the longest and most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes. This enables us to study the distribution and characteristics of the seismicity in unprecedented detail. Earthquake locations are constrained by high‐quality automatic and manual picks, and we perform relocations using waveform cross‐correlation to better constrain hypocenters. We have derived a new local magnitude scale calibrated by Mw values. Magnitudes range between ML−1.2 and 4.6, and our ca...