Upper plate deformation and its relationship to the underlying Hikurangi subduction interface, southern North Island, New Zealand

<p>At the southern Hikurangi margin, the subduction interface between the Australian and Pacific plates, beneath the southern North Island of New Zealand, is ‘locked’. It has previously been estimated that sudden slip on this locked portion of the interface could result in a subduction zone or ‘megathrust’ earthquake of Mw 8.0-8.5 or larger. Historically, however, no significant (>Mw 7.2) subduction interface earthquake has occurred at the southern Hikurangi margin, and the hazard from subduction earthquakes to this region, which includes New Zealand’s capital city of Wellington, remains largely unknown. Patterns of uplift at active margins can provide insight into subduction processes, including megathrust earthquakes. With the objectives to i) contribute to the understanding of partitioning of margin-parallel plate motion on to upper plate faults, and ii) provide insight into the relationship of permanent vertical deformation to subduction processes at the southern end of the Hikurangi margin, I investigate flights of late Pleistocene fluvial and marine terraces preserved across the lower North Island. Such geomorphic features, when constrained by numerical dating, provide a valuable set of data with which to quantify tectonic deformation - be they locally offset by a fault, or collectively uplifted across the margin. Fault-offset fluvial terraces along the Hutt River, near Wellington, record dextral slip for the southern part of the Wellington Fault. From re-evaluated fault displacement measurements and new Optically Stimulated Luminescence (OSL) data, I estimate an average slip rate of 6.3 ± 1.9/1.2 mm/yr (2σ) during the last ~100 ka. However, slip on the Wellington Fault has not been steady throughout this time. During the Holocene, there was a phase of heightened ground rupture activity between ~8 and 10 ka, a period of relative quiescence between ~4.5 and 8 ka, and another period of heightened activity during the last ≤ 4.5 ka. Moreover, these results agree with independent paleoseismological evidence from other sites along the Wellington Fault for the timing of ground rupture events. The time-varying activity observed on the Wellington Fault may be regulated by stress interactions with other nearby upper plate active faults. Net tectonic uplift of the southern Hikurangi margin is recorded by ancient emergent shore platforms preserved along the south coast of the North Island. I provide a new evaluation of the distribution and age of the Pleistocene marine terraces. Shore platform altitudes are accurately surveyed for the first time using Global Navigational Satellite Systems (GNSS). From these data I have determine the shore platform attitudes where they are preserved along the coast. The terraces are also dated, most for the first time, using OSL techniques. The most extensive Pleistocene terraces formed during Marine Isotope Stages (MIS) 5a, 5c, 5e and 7a. Because the ancient shorelines are now obscured by coverbed deposits, I use shore platform attitudes to reconstruct strandline elevations. These strandline elevations, corrected for sea level during their formative highstands, have been used to quantify rates of uplift across the southern Hikurangi margin. In the forearc region of the Hikurangi margin, within ~70 km of the trough, uplift observed on the marine terraces along the Palliser Bay coast monotonically decreases away from the trough. The highest uplift rate of 1.7 ± 0.1 mm/yr is observed at the easternmost preserved terrace, near Cape Palliser, about 40 km from Hikurangi Trough. Further to the west, at Lake Ferry, uplift is 0.8 ± 0.1 mm/yr. The lowest rate of uplift, 0.2 ± 0.1 mm/yr, is observed at Wharekauhau, the westernmost marine terrace preserved on the Palliser Bay coast. Overall, the terraces are tilted towards the west, away from the trough, with older terraces exhibiting the most tilting. This long-wavelength pattern of uplift suggests that, in this forearc region of the margin, deep-seated processes, most likely subduction of a buoyant slab in combination with megathrust earthquakes, are the main contributors to permanent vertical deformation. West of Palliser Bay, at a distance of >70 km from the Hikurangi Trough, vertical offsets on the marine terraces are evident across upper plate faults, most notably the Wairarapa and Ohariu Faults. The uplift rate at Baring Head, west and on the upthrown side of the Wairarapa Fault, is as much as 1.6 ± 0.1 mm/yr. At Tongue Point, where the Ohariu Fault offsets the marine terraces preserved there, uplift calculated from the western, upthrown side of the fault is 0.6 ± 0.1 mm/yr. These uplift rates suggest that, in the Axial Ranges, in addition to sediment underplating, movement on the major active upper plate faults also contributes to rock uplift.</p>

Upper plate deformation and its relationship to the underlying Hikurangi subduction interface, southern North Island, New Zealand

<p>At the southern Hikurangi margin, the subduction interface between the Australian and Pacific plates, beneath the southern North Island of New Zealand, is ‘locked’. It has previously been estimated that sudden slip on this locked portion of the interface could result in a subduction zone or ‘megathrust’ earthquake of Mw 8.0-8.5 or larger. Historically, however, no significant (>Mw 7.2) subduction interface earthquake has occurred at the southern Hikurangi margin, and the hazard from subduction earthquakes to this region, which includes New Zealand’s capital city of Wellington, remains largely unknown. Patterns of uplift at active margins can provide insight into subduction processes, including megathrust earthquakes. With the objectives to i) contribute to the understanding of partitioning of margin-parallel plate motion on to upper plate faults, and ii) provide insight into the relationship of permanent vertical deformation to subduction processes at the southern end of the Hikurangi margin, I investigate flights of late Pleistocene fluvial and marine terraces preserved across the lower North Island. Such geomorphic features, when constrained by numerical dating, provide a valuable set of data with which to quantify tectonic deformation - be they locally offset by a fault, or collectively uplifted across the margin. Fault-offset fluvial terraces along the Hutt River, near Wellington, record dextral slip for the southern part of the Wellington Fault. From re-evaluated fault displacement measurements and new Optically Stimulated Luminescence (OSL) data, I estimate an average slip rate of 6.3 ± 1.9/1.2 mm/yr (2σ) during the last ~100 ka. However, slip on the Wellington Fault has not been steady throughout this time. During the Holocene, there was a phase of heightened ground rupture activity between ~8 and 10 ka, a period of relative quiescence between ~4.5 and 8 ka, and another period of heightened activity during the last ≤ 4.5 ka. Moreover, these results agree with independent paleoseismological evidence from other sites along the Wellington Fault for the timing of ground rupture events. The time-varying activity observed on the Wellington Fault may be regulated by stress interactions with other nearby upper plate active faults. Net tectonic uplift of the southern Hikurangi margin is recorded by ancient emergent shore platforms preserved along the south coast of the North Island. I provide a new evaluation of the distribution and age of the Pleistocene marine terraces. Shore platform altitudes are accurately surveyed for the first time using Global Navigational Satellite Systems (GNSS). From these data I have determine the shore platform attitudes where they are preserved along the coast. The terraces are also dated, most for the first time, using OSL techniques. The most extensive Pleistocene terraces formed during Marine Isotope Stages (MIS) 5a, 5c, 5e and 7a. Because the ancient shorelines are now obscured by coverbed deposits, I use shore platform attitudes to reconstruct strandline elevations. These strandline elevations, corrected for sea level during their formative highstands, have been used to quantify rates of uplift across the southern Hikurangi margin. In the forearc region of the Hikurangi margin, within ~70 km of the trough, uplift observed on the marine terraces along the Palliser Bay coast monotonically decreases away from the trough. The highest uplift rate of 1.7 ± 0.1 mm/yr is observed at the easternmost preserved terrace, near Cape Palliser, about 40 km from Hikurangi Trough. Further to the west, at Lake Ferry, uplift is 0.8 ± 0.1 mm/yr. The lowest rate of uplift, 0.2 ± 0.1 mm/yr, is observed at Wharekauhau, the westernmost marine terrace preserved on the Palliser Bay coast. Overall, the terraces are tilted towards the west, away from the trough, with older terraces exhibiting the most tilting. This long-wavelength pattern of uplift suggests that, in this forearc region of the margin, deep-seated processes, most likely subduction of a buoyant slab in combination with megathrust earthquakes, are the main contributors to permanent vertical deformation. West of Palliser Bay, at a distance of >70 km from the Hikurangi Trough, vertical offsets on the marine terraces are evident across upper plate faults, most notably the Wairarapa and Ohariu Faults. The uplift rate at Baring Head, west and on the upthrown side of the Wairarapa Fault, is as much as 1.6 ± 0.1 mm/yr. At Tongue Point, where the Ohariu Fault offsets the marine terraces preserved there, uplift calculated from the western, upthrown side of the fault is 0.6 ± 0.1 mm/yr. These uplift rates suggest that, in the Axial Ranges, in addition to sediment underplating, movement on the major active upper plate faults also contributes to rock uplift.</p>

Water contents of nominally anhydrous orthopyroxenes from oceanic peridotites determined by SIMS and FTIR

This study presents new secondary ion mass spectrometry (SIMS) reference materials (RMs) for measuring water contents in nominally anhydrous orthopyroxenes from upper mantle peridotites. The enstatitic reference orthopyroxenes from spinel peridotite xenoliths have Mg#s between 0.83 and 0.86, Al2O3 ranges between 4.02 and 5.56 wt%, and Cr2O3 ranges between 0.21 and 0.69 wt%. Based on Fourier-transform infrared spectroscopy (FTIR) characterizations, the water contents of the eleven reference orthopyroxenes vary from dry to 249 ± 6 µg/g H2O. Using these reference grains, a set of orthopyroxene samples obtained from variably altered abyssal spinel peridotites from the Atlantic and Arctic Ridges as well as from the Izu-Bonin-Mariana forearc region was analyzed by SIMS and FTIR regarding their incorporation of water. The major element composition of the sample orthopyroxenes is typical of spinel peridotites from the upper mantle, characterized by Mg#s between 0.90 and 0.92, Al2O3 between 1.66 and 5.34 wt%, and Cr2O3 between 0.62 and 0.96 wt%. Water contents as measured by SIMS range from 68 ± 7 to 261 ± 11 µg/g H2O and correlate well with Al2O3 contents (r = 0.80) and Cr#s (r. = -0.89). We also describe in detail an optimized strategy, employing both SIMS and FTIR, for quantifying structural water in highly altered samples such as abyssal peridotite. This approach first analyzes individual oriented grains by polarized FTIR, which provides an overview of alteration. Subsequently, the same grain along with others of the same sample is measured using SIMS, thereby gaining information about homogeneity at the hand sample scale, which is key for understanding the geological history of these rocks.

Present-day vertical land movement in San Fernando (La Union) and Currimao (Ilocos Norte), northwest Luzon, Philippines

&lt;p&gt;The northwestern coast of Luzon Island is located within the forearc region of the Manila Trench where emergent coral reef platforms have been reported; and an uplift rate of 0.5 m/kyr has been estimated for the past 7,000 years in San Fernando and Currimao. This study examined the present-day vertical land movement (VLM) in both sites using tide gauge records and retracked Jason satellite altimeter missions. Both the tide gauge and satellite data were corrected for tides using the T_Tide algorithm and the difference between the tide gauge sea level (TGSL) and sea surface heights (SSH) from the satellite were calculated. The influence of VLM was inferred from the differences between the TGSL and SSH, then validated using available GNSS data.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Hourly TGSL for San Fernando is available from 2002 to 2018 with a completeness index (CI) of 37%. The satellite products used were the 20 Hz MLE4 and 1Hz ALES retracked Jason satellite series downloaded from AVISO+ and OpenADB, respectively. The MLE4 product indicates subsidence with a rate of 0.43 &amp;#177; 0.10 mm/yr, while ALES indicates uplift at 1.93 &amp;#177; 0.42 mm/yr. GNSS observations at the San Fernando TG benchmark (TGBM) from 2017 to 2019 shows subsidence at 0.74 &amp;#177; 0.40 mm/yr, which agrees well with the VLM estimate from the difference between TGSL and MLE4 SSH.&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Currimao TG station has a CI of 90% from 2008 to 2016. Satellite products used were the 20 Hz MLE4 and 20 Hz ALES retracked Jason-2 downloaded from AVISO+, and both indicate uplift with a rate of 7.30 &amp;#177; 0.17 and 6.24 &amp;#177; 0.25 mm/yr, respectively. The present-day uplift agrees with the geological records, however, there are no GNSS data at the TGBM to validate the present-day vertical motion.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The differences between the present-day vertical motion of San Fernando and Currimao may indicate the influence of other fault systems associated with the Philippine Fault or segmentation of the forearc. Subsidence in San Fernando could imply stress accumulation in the area and the observed uplift in the geological records are cumulative co-seismic vertical displacements.&amp;#160;&amp;#160;&lt;/p&gt;

Attenuation contrast in mantle wedge across the volcanic front of northeastern Japan that controls propagations of high-frequency S-wave later phases

Distinct later phases of waves with rich high-frequency (> 8 Hz) components were observed for intraslab earthquakes that occurred at intermediate depths, particularly at depths exceeding 100 km, in the northeastern (NE) Japan subduction zone. These high-frequency later phases (HFLPs) showed anomalously large peak-amplitude delays, up to ~ 50 s after direct S-wave arrivals at stations in the backarc region. Using a source-scanning algorithm, we investigated the locations of passing points affecting the propagation of HFLPs. The passing points were estimated to be in the forearc region in the entire NE Japan, indicating that HFLPs are scattered waves that pass through the forearc region. The propagating HFLPs seem to bypass the backarc mantle wedge, as a consequence of the distinct attenuation contrast in the mantle wedge across the volcanic front in NE Japan. These HFLP observations suggest that the high-attenuation zone in the backarc mantle wedge controls propagations of the high-frequency waves of intraslab earthquakes, in addition to the scatterers possibly locate in the forearc region.

Ambient noise correlation analysis of S-net records: extracting surface wave signals below instrument noise levels

SUMMARY We applied ambient noise cross-correlation analysis to the cabled ocean bottom seismic network offshore northeast Japan (Seafloor observation network for earthquakes and tsunamis along the Japan Trench: S-net) to extract surface waves propagating in the ocean area of the forearc region. We found two types of peculiar pulses in the cross-correlation functions (CCFs) of ambient seismic noise records: periodic pulses mainly every minute and sharp pulses around the lag time zero. These pulses strongly contaminate the surface wave signals in the CCFs at frequencies below ∼0.1 Hz. The periodic pulses originate from periodic instrument noises, while the zero-lag pulses originate from random instrument noises which are coherent within station pairs. By developing solutions to remove the periodic and zero-lag pulses based on the characteristics of the pulses, we succeeded in extracting Rayleigh and Love wave signals from the S-net records at 0.03–0.3 Hz, while the surface wave signals at 0.03–0.1 Hz were not visible without the application of these solutions. These solutions widen the frequency range of analysis, and may be applicable to other seismic networks, particularly to recent dense but non-broad-band networks. We identified the fundamental and first higher modes of Rayleigh waves and the fundamental mode of the Love wave. The extracted surface wave signals can constrain the shear wave velocity structure from the sediment to seismogenic zone around the megathrust plate boundary in the forearc region.

Serpentine alteration as source of high dissolved silicon and elevated δ30Si values to the marine Si cycle

Serpentine alteration is recognized as an important process for element cycling, however, related silicon fluxes are unknown. Pore fluids from serpentinite seamounts sampled in the Mariana forearc region during IODP Expedition 366 were investigated for their Si, B, and Sr isotope signatures (δ30Si, δ11B, and 87Sr/86Sr, respectively) to study serpentinization in the mantle wedge and shallow serpentine alteration to authigenic clays by seawater. While serpentinization in the mantle wedge caused no significant Si isotope fractionation, implying closed system conditions, serpentine alteration by seawater led to the formation of authigenic phyllosilicates, causing the highest natural fluid δ30Si values measured to date (up to +5.2 ± 0.2‰). Here we show that seafloor alteration of serpentinites is a source of Si to the ocean with extremely high fluid δ30Si values, which can explain anomalies in the marine Si budget like in the Cascadia Basin and which has to be considered in future investigations of the global marine Si cycle.

The 2020 M 5.9 Falam Earthquake the Subduction-Induced Strike-Slip Earthquake, West Myanmar

An earthquake with magnitude 5.9 occurred in the east of Falam on 16th April, 2020 at 11:45:23 (UTC). The epicenter is situated at latitude 22.789°N, longitude 94.025°E, 38 km ESE of Falam, at the depth of 10 km. Focal mechanism solution for this event is normal faulting (USGS). The epicentral location is in the Kabaw Valley along which Kabaw fault runs through in N-S direction. The Kabaw fault is situated in forearc region at the eastern base of N-S trending Rakhine Western Ranges under which the India oceanic plate is obliquely subducting beneath the Burma continental plate. The 2020 M 5.9 Falam earthquake occurred along two closely linked tectonic settings: north-eastward oblique subduction zone and north-south trending Kabaw fault zone system in the forearc region.The Falam earthquake ruptured the Tripura segment, one of the segments of India subduction zone, located approximately between latitude 22°-24°N according to the geographical location. This event is a rare intraplate earthquake and a subduction-induced strike-slip earthquake that ever occurred for the recent time in Myanmar. The shock was felt by cities of Gangaw, Kalemyo, Kalewa, Mandalay, Kyauk-se, Monywa. This earthquake was preceded by a loud sound and shaking lasts 1 minute. A few aftershocks of magnitude >3.5 followed the main shock in the vicinity of the epicenter. The vibration spread a wide area along Rakhine Yoma and Myanmar lowland area. The investigation of field survey from social media was found that the event reaches Modified Mercalli Intensity scale VIII based on people’s perception, indoor effects and damaged buildings. Damage is severe in some poorly built structures and upper parts of stupa and pagodas.