11 research outputs found
Along-strike variation in slab geometry at the southern Mariana subduction zone revealed by seismicity through ocean bottom seismic experiments
Author Posting. © The Authors, 2019. This article is posted here by permission of The Royal Astronomical Society for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 218(3), (2019): 2122-2135, doi: 10.1093/gji/ggz272.We have conducted the first passive Ocean Bottom Seismograph (OBS) experiment near the Challenger Deep at the southernmost Mariana subduction zone by deploying and recovering an array of 6 broad-band OBSs during December 2016âJune 2017. The obtained passive-source seismic records provide the first-ever near-field seismic observations in the southernmost Mariana subduction zone. We first correct clock errors of the OBS recordings based on both teleseismic waveforms and ambient noise cross-correlation. We then perform matched filter earthquake detection using 53 template events in the catalogue of the US Geological Survey and find >7000 local earthquakes during the 6-month OBS deployment period. Results of the two independent approaches show that the maximum clock drifting was âŒ2 s on one instrument (OBS PA01), while the rest of OBS waveforms had negligible time drifting. After timing correction, we locate the detected earthquakes using a newly refined local velocity model that was derived from a companion active source experiment in the same region. In total, 2004 earthquakes are located with relatively high resolution. Furthermore, we calibrate the magnitudes of the detected earthquakes by measuring the relative amplitudes to their nearest relocated templates on all OBSs and acquire a high-resolution local earthquake catalogue. The magnitudes of earthquakes in our new catalogue range from 1.1 to 5.6. The earthquakes span over the Southwest Mariana rift, the megathrust interface, forearc and outer-rise regions. While most earthquakes are shallow, depths of the slab earthquakes increase from âŒ100 to âŒ240 km from west to east towards Guam. We also delineate the subducting interface from seismicity distribution and find an increasing trend in dip angles from west to east. The observed along-strike variation in slab dip angles and its downdip extents provide new constraints on geodynamic processes of the southernmost Mariana subduction zone.We express our appreciation to the science parties and crew members of the R/V Shiyan 3 for deployment and collection of the OBS instruments during the Mariana expeditions. This study is supported by the Hong Kong Research Grant Council Grants (No. 14313816), Faculty of Science at CUHK, Chinese Academy of Sciences (No. Y4SL021001, QYZDY-SSW-DQC005, 133244KYSB20180029), the National Natural Science Foundation of China (No. 41890813, 91628301, 41676042, U1701641, 41576041, 91858207 and U1606401), the National Key R&D Program of China (2018YFC0309800 and 2018YFC0310100). Generic Mapping Tools (Wessel & Smith 1991) and PSSAC (developed by Prof Lupei Zhu) are used for data analysis and figure preparation in this study. Constructive comments from Dr Lidong Bie and two anonymous reviewers are helpful in improving the manuscript
Crustal plumbing system of post-rift magmatism in the northern margin of South China Sea: New insights from integrated seismology
Deep seismic structure across the southernmost Mariana trench: Implications for arc rifting and plate hydration
Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 124(5), (2019): 4710-4727, doi:10.1029/2018JB017080.The southernmost Mariana margin lacks a mature island arc and thus differs significantly from the centralânorth Mariana and IzuâBonin margins. This paper presents a new P wave velocity model of the crust and uppermost mantle structure based on a 349âkmâlong profile of wideâangle reflection/refraction data. The active source seismic experiment was conducted from the subducting Pacific plate to the overriding Philippine plate, passing through the Challenger Deep. The subducting plate has an average crustal thickness of ~6.0 km with Vp of 7.0 ± 0.2 km/s at the base of the crust and low values of only 5.5â6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0â7.3 km/s. The overriding plate has a maximum crustal thickness of ~18 km beneath the forearc with Vp of ~7.4 km/s at the crustal bottom and 7.5â7.8 km/s in the uppermost mantle. A zone of slight velocity reduction is imaged beneath the Southwest Mariana Rift that is undergoing active rifting. The observed significant velocity reduction in a nearâtrench crustal zone of ~20â30 km in the subducting plate is interpreted as a result of faultingâinduced porosity changes and fractureâfilling fluids. The velocity reduction in the uppermost mantle of both subducting and overriding plates is interpreted as mantle serpentinization with fluid sources from dehydration of the subducting plate and/or fluid penetration along faults.Data acquisition and sample collections were supported by the Mariana Trench Initiative of the Chinese Academy of Sciences (CAS). We are grateful to the science parties and crews of R/V Shiyan 3 of the South China Sea Institute of Oceanology, CAS, for contributions to data acquisition. Constructive reviews by Robert Stern, Martha Savage, and anonymous reviewers significantly improved the manuscript. We thank Gaohua Zhu, Fan Zhang, Chunfeng Li, Zhen Sun, Zhi Wang, and Minghui Zhao for helpful discussion. The bathymetric maps were plotted using GMT (Wessel & Smith, 1995). Digital files of the velocity models and selected raw data are deposited and accessible online (at https://pan.baidu.com/s/1AbDJOgLZhYn1Câ3sg7S9Xw). This work was supported by the Strategic Priority Program of CAS (XDA13010101), CAS (Y4SL021001, QYZDYâSSWâDQC005, and 133244KYSB20180029), Key Laboratory of Ocean and Marginal Sea Geology, CAS (OMG18â03), National Natural Science Foundation of China (41890813, 41676042, U1701641, 91628301, 41576041, and U1606401), and HKSAR Research Grant Council grants (14313816).2019-10-0
Travel-Time Inversion Method of Converted Shear Waves Using RayInvr Algorithm
The detailed studies of converted S-waves recorded on the Ocean Bottom Seismometer (OBS) can provide evidence for constraining lithology and geophysical properties. However, the research of converted S-waves remains a weakness, especially the S-wavesâ inversion. In this study, we applied a travel-time inversion method of converted S-waves to obtain the crustal S-wave velocity along the profile NS5. The velocities of the crust are determined by the following four aspects: (1) modelling the P-wave velocity, (2) constrained sediments Vp/Vs ratios and S-wave velocity using PPS phases, (3) the correction of PSS phasesâ travel-time, and (4) appropriate parameters and initial model are selected for inversion. Our results show that the vs. and Vp/Vs of the crust are 3.0â4.4 km/s and 1.71â1.80, respectively. The inversion model has a similar trend in velocity and Vp/Vs ratios with the forward model, due to a small difference with âVs of 0.1 km/s and âVp/Vs of 0.03 between two models. In addition, the high-resolution inversion model has revealed many details of the crustal structures, including magma conduits, which further supports our method as feasible
Can Glacial SeaâLevel DropâInduced Gas Hydrate Dissociation Cause Submarine Landslides?
We conducted twoâdimensional numerical simulations to investigate the mechanisms underlying the strong spatiotemporal correlation observed between submarine landslides and gas hydrate dissociation due to glacial seaâlevel drops. Our results suggest that potential plastic deformation or slip could occur at localized and small scales in the shallowâwater portion of the gas hydrate stability zone (GHSZ). This shallowâwater portion of the GHSZ typically lies within the area enclosed by three points: the BGHSZâseafloor intersection, the seafloor at âŒ600 m below sea level (mbsl), and the base of the GHSZ (BGHSZ) at âŒ1,050 mbsl in lowâlatitude regions. The deep BGHSZ (>1,050 mbsl) could not slip; therefore, the entire BGHSZ was not a complete slip surface. Glacial hydrate dissociation alone is unlikely to cause largeâscale submarine landslides. Observed deepâwater (much greater than 600 mbsl) turbidites containing geochemical evidence of glacial hydrate dissociation potentially formed from erosion or detachment in the GHSZ pinchâout zone.
Plain Language Summary
Many submarine landslides spatiotemporally correlate with gas hydrate dissociation. However, direct mechanical evidence supporting whether the overpressure and deformation due to glacial seaâlevel dropâinduced hydrate dissociation are adequate for triggering submarine landslides is lacking. Here, we present twoâdimensional thermalâhydraulicâchemical and geomechanical models of a gasâhydrate system in response to glacial seaâlevel drops and conduct sensitivity analyses of the model behavior under a wide range of key conditions from a global perspective. Our simulations suggest that glacial hydrate dissociation might induce plastic deformation or slip at localized and small scales only possibly within the shallowâwater portion of the hydrate stability zone. The deep part (>1,050 m below sea level) of the bottom boundary of the hydrate stability zone could not slip; therefore, the entire bottom boundary of the hydrate stability zone was not a complete slip surface. We demonstrate that glacial hydrate dissociation alone is unlikely to trigger largeâscale submarine landslides. Our work highlights the vicinity of the upper limit of the hydrate stability zone (where the base of the hydrate stability zone intersects the seafloor) as an important area for investigating overpressure and focused fluid flow, localized plastic deformation or slip, and downslope sediment transport related to glacial hydrate dissociation.
Key Points
Glacial hydrate dissociation might cause potential plastic deformation or slip at localized and small scales in shallow parts of the GHSZ
The large deformation surface at the BGHSZ boundary of the potential plastic deformation zone was not a complete slip surface
Glacial seaâlevel dropâinduced gas hydrate dissociation alone is unlikely to have caused largeâscale submarine landslide