In subduction zones worldwide, seafloor pressure data are used to observe tectonic deformation, particularly from megathrust earthquakes and slow slip events (SSEs). However, such measurements are also sensitive to oceanographic circulation‐generated pressures over a range of frequencies that conflate with tectonic signals of interest. Using seafloor pressure and temperature data from the Alaska Amphibious Community Seismic Experiment, and sea surface height data from satellite altimetry, we evaluate the efficacy of various seasonal and oceanographic pressure signal proxy corrections and conduct synthetic tests to determine their impact on the timing and amplitude prediction of ramp‐like signals typical of SSEs. We find that subtracting out the first mode of the complex empirical orthogonal functions of the pressure records on either the shelf or slope yields signal root‐mean‐square error (RMS) reductions up to 73% or 80%, respectively. Additional correction with proxies that exploit the depth‐dependent spatial coherence of pressure records provides cumulative variance reductions up to 83% and 93%, respectively. Our detectability tests show that the timing and amplitude of synthetic SSE‐like ramps can be well constrained for ramp amplitudes ≥4 cm on the shelf and ≥2 cm on the slope, using a fully automated detector. The principal limits on detectability are residual abrupt changes in pressure that occur as part of the transition to and from summer to winter conditions but are not adequately characterized by our seasonal corrections, as well as the inability to properly account for instrumental drift, which is not readily separated from the seasonal signal.
Using Seafloor Geodesy to Detect Vertical Deformation at the Hikurangi Subduction Zone: Insights From Self‐Calibrating Pressure Sensors and Ocean General Circulation Models
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 that long‐period oceanographic signals that were not synchronous between pressure sensors and reference sites influenced our inferred displacements by 0.3–2.6 cm, suggesting that regionally deployed reference sites alone may not provide sufficient ocean noise correction. After incorporating long‐period ocean variability corrections into the processing, we calculate 1.0–3.3 cm of uplift during the SSE offshore Gisborne at northern Hikurangi, and 1.1–2.7 cm of uplift offshore the Hawke's Bay area at central Hikurangi. Some Hawke Bay displacements detected by pressure sensors near the trench were delayed by 6 weeks compared to the timing of slip onset detected by onshore Global Navigation Satellite System sites, suggesting updip migration of the SSE.
more » « less- NSF-PAR ID:
- 10388463
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 127
- Issue:
- 12
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
null (Ed.)International Ocean Discovery Program (IODP) Expedition 372 combines two research topics, slow slip events (SSEs) on subduction faults (IODP Proposal 781A-Full) and actively deforming gas hydrate–bearing landslides (Proposal 841-APL). Our study area on the Hikurangi margin east of New Zealand provides unique locations for addressing both research topics. Gas hydrates have long been suspected of being involved in seafloor failure; not much evidence, however, has been found to date for gas hydrate–related submarine landslides. Solid, icelike gas hydrate in sediment pores is generally thought to increase seafloor strength, as confirmed by a number of laboratory measurements. Dissociation of gas hydrate to water and overpressured gas, on the other hand, may destabilize the seafloor, potentially causing submarine landslides. The Tuaheni Landslide Complex on the Hikurangi margin shows evidence for active, creeping deformation. Intriguingly, the landward edge of creeping coincides with the pinchout of the base of gas hydrate stability (BGHS) on the seafloor. We therefore hypothesize that gas hydrate may be linked to creeping by (1) repeated small-scale sliding at the BGHS, in a variation of the conventional model linking gas hydrates and seafloor failure; (2) overpressure at the BGHS due to a permeability reduction linked to gas hydrates, which may lead to hydrofracturing, weakening the seafloor and allowing transmission of pressure into the gas hydrate stability zone; or (3) icelike viscous deformation of gas hydrates in sediment pores, similar to onshore rock glaciers. The latter two processes imply that gas hydrate itself is involved in creeping, constituting a paradigm shift in relating gas hydrates to submarine slope failure. Alternatively, creeping may not be related to gas hydrates but instead be caused by repeated pressure pulses or linked to earthquake-related liquefaction. We have devised a coring and logging program to test our hypotheses. SSEs at subduction zones are an enigmatic form of creeping fault behavior. At the northern Hikurangi subduction margin (HSM), they are among the best-documented and shallowest on Earth. They recur about every 2 y and may extend close to the trench, where clastic and pelagic sediments about 1.0–1.5 km thick overlie the subducting, seamount-studded Hikurangi Plateau. The northern HSM thus provides an excellent setting to use IODP capabilities to discern the mechanisms behind slow slip fault behavior, as proposed in IODP Proposal 781A-Full. The objectives of Proposal 781A-Full will be implemented across two related IODP expeditions, 372 and 375. Expedition 372 will undertake logging while drilling (LWD) at three sites targeting the upper plate (midslope basin, proposed Site HSM-01A), the frontal thrust (proposed Site HSM-18A), and the subducting section in the trench (proposed Site HSM-05A). Expedition 375 will undertake coring at the same sites, as well as an additional seamount site on the subducting plate, and implement the borehole observatory objectives. The data from each expedition will be shared between both scientific parties. Collectively, the LWD and coring data will be used to (1) characterize the compositional, structural, thermal, and diagenetic state of the incoming plate and the shallow plate boundary fault near the trench, which comprise the protolith and initial conditions for fault zone rock associated with SSEs at greater depth, and (2) characterize the material properties, thermal regime, and stress conditions in the upper plate above the SSE source region. These data will be used during Expedition 375 to guide the installation of CORK observatories at the frontal thrust and in the upper plate above the SSE source to monitor temporal variations in deformation, fluid flow, seismicity, and physical and chemical properties throughout the SSE cycle (Saffer et al., 2017). Together, these data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface.more » « less
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null (Ed.)International Ocean Discovery Program (IODP) Expedition 372 combined two research topics, slow slip events (SSEs) on subduction faults (IODP Proposal 781A-Full) and actively deforming gas hydrate–bearing landslides (IODP Proposal 841-APL). Our study area on the Hikurangi margin, east of the coast of New Zealand, provided unique locations for addressing both research topics. SSEs at subduction zones are an enigmatic form of creeping fault behavior. They typically occur on subduction zones at depths beyond the capabilities of ocean floor drilling. However, at the northern Hikurangi subduction margin they are among the best-documented and shallowest on Earth. Here, SSEs may extend close to the trench, where clastic and pelagic sediments about 1.0–1.5 km thick overlie the subducting, seamount-studded Hikurangi Plateau. Geodetic data show that these SSEs recur about every 2 years and are associated with measurable seafloor displacement. The northern Hikurangi subduction margin thus provides an excellent setting to use IODP capabilities to discern the mechanisms behind slow slip fault behavior. Expedition 372 acquired logging-while-drilling (LWD) data at three subduction-focused sites to depths of 600, 650, and 750 meters below seafloor (mbsf), respectively. These include two sites (U1518 and U1519) above the plate interface fault that experiences SSEs and one site (U1520) in the subducting “inputs” sequence in the Hikurangi Trough, 15 km east of the plate boundary. Overall, we acquired excellent logging data and reached our target depths at two of these sites. Drilling and logging at Site U1520 did not reach the planned depth due to operational time constraints. These logging data will be augmented by coring and borehole observatories planned for IODP Expedition 375. Gas hydrates have long been suspected of being involved in seafloor failure; not much evidence, however, has been found to date for gas hydrate–related submarine landslides. Solid, ice-like gas hydrate in sediment pores is generally thought to increase seafloor strength, as confirmed by a number of laboratory measurements. Dissociation of gas hydrate to water and overpressured gas, on the other hand, may weaken and destabilize sediments, potentially causing submarine landslides. The Tuaheni Landslide Complex (TLC) on the Hikurangi margin shows evidence for active, creeping deformation. Intriguingly, the landward edge of creeping coincides with the pinch-out of the base of gas hydrate stability on the seafloor. We therefore hypothesized that gas hydrate may be linked to creep-like deformation and presented several hypotheses that may link gas hydrates to slow deformation. Alternatively, creeping may not be related to gas hydrates but instead be caused by repeated pressure pulses or linked to earthquake-related liquefaction. Expedition 372 comprised a coring and LWD program to test our landslide hypotheses. Due to weather-related downtime, the gas hydrate-related program was reduced, and we focused on a set of experiments at Site U1517 in the creeping part of the TLC. We conducted a successful LWD and coring program to 205 mbsf, the latter with almost complete recovery, through the TLC and gas hydrate stability zone, followed by temperature and pressure tool deployments.more » « less
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null (Ed.)The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a coordinated, multiexpedition International Ocean Discovery Program (IODP) drilling project designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies. The fundamental scientific objectives of the NanTroSEIZE drilling project include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system. IODP Expedition 365 is part of NanTroSEIZE Stage 3, with the following primary objectives: (1) retrieval of a temporary observatory at Site C0010 that has been monitoring temperature and pore pressure within the major splay thrust fault (termed the “megasplay”) at 400 meters below seafloor since November 2010 and (2) deployment of a complex long-term borehole monitoring system (LTBMS) that will be connected to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) seafloor cabled observatory network postexpedition (anticipated June 2016). The LTBMS incorporates multilevel pore pressure sensing, a volumetric strainmeter, tiltmeter, geophone, broadband seismometer, accelerometer, and thermistor string. Together with an existing observatory at Integrated Ocean Drilling Program Site C0002 and a possible future installation near the trench, the Site C0010 observatory will allow monitoring within and above regions of contrasting behavior of the megasplay fault and the plate boundary as a whole. These include a site above the updip edge of the locked zone (Site C0002), a shallow site in the megasplay fault zone and its footwall (Site C0010), and a site at the tip of the accretionary prism (Integrated Ocean Drilling Program Site C0006). Together, this suite of observatories has the potential to capture deformation spanning a wide range of timescales (e.g., seismic and microseismic activity, slow slip, and interseismic strain accumulation) across a transect from near-trench to the seismogenic zone. Site C0010 is located 3.5 km along strike to the southwest of Integrated Ocean Drilling Program Site C0004. The site was drilled and cased during Integrated Ocean Drilling Program Expedition 319, with casing screens spanning a ~20 m interval that includes the megasplay fault, and suspended with a temporary instrument package (a “SmartPlug”). During Integrated Ocean Drilling Program Expedition 332 in late 2010, the instrument package was replaced with an upgraded sensor package (the “GeniusPlug”), which included pressure and temperature sensors and a set of geochemical and biological experiments. Expedition 365 achieved its primary scientific and operational objectives, including recovery of the GeniusPlug with a >5 y record of pressure and temperature conditions within the shallow megasplay fault zone, geochemical samples, and its in situ microbial colonization experiment; and installation of the LTBMS. The pressure records from the GeniusPlug include high-quality records of formation and seafloor responses to multiple fault slip events, including the 11 March 2011 Tohoku M9 and 1 April 2016 Mie-ken Nanto-oki M6 earthquakes. The geochemical sampling coils yielded in situ pore fluids from the splay fault zone, and microbes were successfully cultivated from the colonization unit. The complex sensor array, in combination with the multilevel hole completion, is one of the most ambitious and sophisticated observatory installations in scientific ocean drilling (similar to that in Hole C0002G, deployed in 2010). Overall, the installation went smoothly, efficiently, and ahead of schedule. The extra time afforded by the efficient observatory deployment was used for coring in Holes C0010B–C0010E. Despite challenging hole conditions, the depth interval corresponding to the screened casing across the megasplay fault was successfully sampled in Hole C0010C, and the footwall of the megasplay was sampled in Hole C0010E, with >50% recovery for both zones. In the hanging wall of the megasplay fault (Holes C0010C and C0010D), we recovered indurated silty clay with occasional ash layers and sedimentary breccias. Some of the deposits show burrows and zones of diagenetic alteration/colored patches. Mudstones show different degrees of deformation spanning from occasional fractures to intervals of densely fractured scaly claystones of up to >10 cm thickness. Sparse faulting with low displacement (usually <2 cm) is seen in core and exhibits primarily normal and, rarely, reversed sense of slip. When present, ash was entrained along fractures and faults. On one occasion, a ~10 cm thick ash layer was found, which showed a fining-downward gradation into a mottled zone with clasts of the underlying silty claystones. In Hole C0010E, the footwall to the megasplay fault was recovered. Sediments are horizontally to gently dipping and mainly comprise silt of olive-gray color. The deposits of the underthrust sediment prism are less indurated than the hanging wall mudstones and show lamination on a centimeter scale. The material is less intensely deformed than the mudstones, and apart from occasional fracturation (some of it being drilling disturbance), evidence of structural features is absent.more » « less
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null (Ed.)The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a coordinated, multiexpedition International Ocean Discovery Program (IODP) drilling project designed to investigate fault mechanics and seismogenesis along subduction megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies. The fundamental scientific objectives of the NanTroSEIZE drilling project include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system. IODP Expedition 365 is part of NanTroSEIZE Stage 3, with the following primary objectives: 1. Retrieval of a temporary observatory at Site C0010 that began monitoring temperature and pore pressure within the major splay thrust fault (termed the “megasplay”) at 400 meters below seafloor in November 2010. 2. Deployment of a complex long-term borehole monitoring system (LTBMS) designed to be connected to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) seafloor cabled observatory network postexpedition. The LTBMS incorporates multilevel pore pressure sensing, a volumetric strainmeter, tiltmeter, geophone, broadband seismometer, accelerometer, and thermistor string. Together with an existing observatory at Integrated Ocean Drilling Program Site C0002 and a planned future installation near the trench, the Site C0010 observatory allows monitoring within and above regions of contrasting behavior of the megasplay fault and the plate boundary as a whole. These include a site above the updip edge of the locked zone (Site C0002), a shallow site in the megasplay fault zone and its footwall (Site C0010), and a site at the tip of the accretionary prism (possible future installation at Integrated Ocean Drilling Program Site C0006). Together, this suite of observatories has the potential to capture deformation spanning a wide range of timescales (e.g., seismic and microseismic activity, slow slip, and interseismic strain accumulation) across a transect from near-trench to the seismogenic zone. Site C0010 is located 3.5 km along strike to the southwest of Integrated Ocean Drilling Program Site C0004. The site was drilled and cased during Integrated Ocean Drilling Program Expedition 319, with casing screens spanning a ~20 m interval that includes the megasplay fault, and suspended with a temporary instrument package (a “SmartPlug”), which included pressure and temperature sensors. During Integrated Ocean Drilling Program Expedition 332 in late 2010, the instrument package was replaced with an upgraded sensor package (the “GeniusPlug”), which included a set of geochemical and biological experiments in addition to pressure and temperature sensors. Expedition 365 achieved its primary scientific and operational objectives, including recovery of the GeniusPlug with a >5 y record of pressure and temperature conditions within the shallow megasplay fault zone, geochemical samples, and its in situ microbial colonization experiment; and installation of the LTBMS. The pressure records from the GeniusPlug include high-quality records of formation and seafloor responses to multiple fault slip events, including the 11 March 2011 Tohoku M9 and 1 April 2016 Mie-ken Nanto-oki M6 earthquakes. The geochemical sampling coils yielded in situ pore fluids from the splay fault zone, and microorganisms were successfully cultivated from the colonization unit. The complex sensor array, in combination with the multilevel hole completion, is one of the most ambitious and sophisticated observatory installations in scientific ocean drilling (similar to that in Hole C0002G, deployed in 2010). Overall, the installation went smoothly, efficiently, and ahead of schedule. The extra time afforded by the efficient observatory deployment was used for coring in Holes C0010B–C0010E. Despite challenging hole conditions, the depth interval corresponding to the screened casing across the megasplay fault was successfully sampled in Hole C0010C, and the footwall of the megasplay was sampled in Hole C0010E, with >50% recovery for both zones. In the hanging wall of the megasplay fault (Holes C0010C and C0010D), we recovered indurated silty clay with occasional ash layers and sedimentary breccias. Mudstones show different degrees of deformation spanning from occasional fractures to intervals of densely fractured scaly claystones of up to >10 cm thickness. Sparse faulting with low displacement (usually <2 cm) is seen in core and exhibits primarily normal and, rarely, reversed sense of slip. When present, ash was entrained along fractures and faults. In Hole C0010E, the footwall to the megasplay fault was recovered. Sediments are horizontally to gently dipping and mainly comprise silt of olive-gray color. The hanging wall sediments recovered in Holes C0010C–C0010D range in age from 3.79 to 5.59 Ma and have been thrust over the younger footwall sediments in Hole C0010E, ranging in age from 1.56 to 1.67 Ma. The deposits of the underthrust sediment prism are less indurated than the hanging wall mudstones and show lamination on a centimeter scale. The material is less intensely deformed than the mudstones, and apart from occasional fracturation (some of it being drilling disturbance), evidence of structural features is absent.more » « less
