Yang
Hongfeng
Yang
Hongfeng
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ArticleDeep outer-rise faults in the Southern Mariana Subduction Zone indicated by a machine-learning-based high-resolution earthquake catalog(American Geophysical Union, 2022-06-06) Chen, Han ; Yang, Hongfeng ; Zhu, Gaohua ; Xu, Min ; Lin, Jian ; You, QingyuOuter-rise faults are predominantly concentrated near ocean trenches due to subducted plate bending. These faults play crucial roles in the hydration of subducted plates and the consequent subducting processes. However, it has not yet been possible to develop high-resolution structures of outer-rise faults due to the lack of near-field observations. In this study we deployed an ocean bottom seismometer (OBS) network near the Challenger Deep in the Southernmost Mariana Trench, between December 2016 and June 2017, covering both the overriding and subducting plates. We applied a machine-learning phase detector (EQTransformer) to the OBS data and found more than 1,975 earthquakes. An identified outer-rise event cluster revealed an outer-rise fault penetrating to depths of 50 km, which was inferred as a normal fault based on the extensional depth from tomographic images in the region, shedding new lights on water input at the southmost Mariana subduction zone.
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ArticleMechanism of progressive broad deformation from oceanic transform valley to off-transform faulting and rifting(Elsevier, 2022-01-25) Zhang, Fan ; Lin, Jian ; Zhou, Zhiyuan ; Yang, Hongfeng ; Morgan, Jason P.Oceanic transform faults (TFs) are commonly viewed as single, narrow strike-slip seismic faults that offset two mid-ocean ridge segments. However, broad zones of complex deformation are ubiquitous at TFs. Here, we propose a new conceptual model for the progressive deformation within broad zones at oceanic TFs through detailed morphological, seismic, and stress analyses. We argue that, under across-transform extension due to a change in plate motion, plate deformation occurs first along high-angle transtensional faults (TTFs) within the transform valleys. Off-transform normal faults (ONFs) form when across-transform deviatoric extensional stresses exceed the yield strength of the adjacent oceanic lithosphere. With further extension, these normal faults can develop into off-transform rift zones (ORZs), some of which can further develop into transform plate boundaries. We illustrate that such progressive complex deformation is an inherent feature of oceanic TFs. The new conceptual model provides a unifying theory to explain the observed broad deformation at global transform systems.
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ArticleContinental interior and edge breakup at convergent margins induced by subduction direction reversal: a numerical modeling study applied to the South China Sea margin(American Geophysical Union, 2020-10-06) Li, Fucheng ; Sun, Zhen ; Yang, Hongfeng ; Lin, Jian ; Stock, Joann M. ; Zhao, Zhongxiang ; Xu, Hehua ; Sun, LongtaoThe dynamics of continental breakup at convergent margins has been described as the results of backarc opening caused by slab rollback or drag force induced by subduction direction reversal. Although the rollback hypothesis has been intensively studied, our understanding of the consequence of subduction direction reversal remains limited. Using thermo‐mechanical modeling based on constraints from the South China Sea (SCS) region, we investigate how subduction direction reversal controls the breakup of convergent margins. The numerical results show that two distinct breakup modes, namely, continental interior and edge breakup (“edge” refers to continent above the plate boundary interface), may develop depending on the “maturity” of the convergent margin and the age of the oceanic lithosphere. For a slab age of ~15 to ~45 Ma, increasing the duration of subduction promotes the continental interior breakup mode, where a large block of the continental material is separated from the overriding plate. In contrast, the continental edge breakup mode develops when the subduction is a short‐duration event, and in this mode, a wide zone of less continuous continental fragments and tearing of the subducted slab occur. These two modes are consistent with the interior (relic late Mesozoic arc) and edge (relic forearc) rifting characteristics in the western and eastern SCS margin, suggesting that variation in the northwest‐directed subduction duration of the Proto‐SCS might be a reason for the differential breakup locus along the strike of the SCS margin. Besides, a two‐segment trench associated with the northwest‐directed subduction is implied in the present‐day SCS region.
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ArticleGeometrical effects of a subducted seamount on stopping megathrust ruptures(John Wiley & Sons, 2013-05-30) Yang, Hongfeng ; Liu, Yajing ; Lin, JianWe have numerically simulated dynamic ruptures along a “slip-weakening” megathrust fault with a subducted seamount of realistic geometry, demonstrating that seamounts can act as a barrier to earthquake ruptures. Such barrier effect is calculated to be stronger for increased seamount normal stress relative to the ambient level, for larger seamount height-to-width ratio, and for shorter seamount-to-nucleation distance. As the seamount height increases from 0 to 40% of its basal width, the required increase in the effective normal stress on the seamount to stop ruptures drops by as much as ~20%. We further demonstrate that when a seamount is subducted adjacent to the earthquake nucleation zone, coseismic ruptures can be stopped even if the seamount has a lower effective normal stress than the ambient level. These results indicate that subducted seamounts may stop earthquake ruptures for a wide range of seamount normal stress conditions, including the case of the thrust fault being lubricated by seamount-top fluid-rich sediments, as suggested from observations in the Japan and Sunda Trenches.
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ArticleDeep seismic structure across the southernmost Mariana trench: Implications for arc rifting and plate hydration(American Geophysical Union, 2019-04-05) Wan, Kuiyuan ; Lin, Jian ; Xia, Shaohong ; Sun, Jinlong ; Xu, Min ; Yang, Hongfeng ; Zhou, Zhiyuan ; Zeng, Xin ; Cao, Jinghe ; Xu, HuilongThe southernmost Mariana margin lacks a mature island arc and thus differs significantly from the central‐north Mariana and Izu‐Bonin margins. This paper presents a new P wave velocity model of the crust and uppermost mantle structure based on a 349‐km‐long profile of wide‐angle 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 5.5–6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0–7.3 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 7.5–7.8 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 near‐trench crustal zone of ~20–30 km in the subducting plate is interpreted as a result of faulting‐induced porosity changes and fracture‐filling 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.
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ArticleAlong-strike variation in slab geometry at the southern Mariana subduction zone revealed by seismicity through ocean bottom seismic experiments(Oxford University Press, 2019-06-10) Zhu, Gaohua ; Yang, Hongfeng ; Lin, Jian ; Zhou, Zhiyuan ; Xu, Min ; Sun, Jinlong ; Wan, KuiyuanWe 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 2016–June 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 ∼2 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 ∼100 to ∼240 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.
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ArticleEffects of subducted seamounts on megathrust earthquake nucleation and rupture propagation(American Geophysical Union, 2012-12-19) Yang, Hongfeng ; Liu, Yajing ; Lin, JianSubducted seamounts have been linked to interplate earthquakes, but their specific effects on earthquake mechanism remain controversial. A key question is under what conditions a subducted seamount will generate or stop megathrust earthquakes. Here we show results from numerical experiments in the framework of rate- and state-dependent friction law in which a seamount is characterized as a patch of elevated effective normal stress on the thrust interface. We find that whether subducted seamounts generate or impede megathrust earthquakes depends critically on their relative locations to the earthquake nucleation zone defined by depth-variable friction parameters. A seamount may act as a rupture barrier and such barrier effect is most prominent when the seamount sits at an intermediate range of the seamount-to-trench distances (20–100% of the nucleation-zone-to-trench distance). Moreover, we observe that seamount-induced barriers can turn into asperities on which megathrust earthquakes can nucleate at shallow depths and rupture the entire seismogenic zone. These results suggest that a strong barrier patch may not necessarily reduce the maximum size of earthquakes. Instead, the barrier could experience large coseismic slip when it is ruptured.
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ArticleIntra- and intertrench variations in flexural bending of the Manila, Mariana and global trenches : implications on plate weakening in controlling trench dynamics(Oxford University Press, 2017-11-08) Zhang, Fan ; Lin, Jian ; Zhou, Zhiyuan ; Yang, Hongfeng ; Zhan, WenhuanWe conducted detailed analyses of a global array of trenches, revealing systematic intra- and intertrench variations in plate bending characteristics. The intratrench variations of the Manila and Mariana Trenches were analysed in detail as end-member cases of the relatively young (16–36 Ma) and old (140–160 Ma) subducting plates, respectively. Meanwhile, the intertrench variability was investigated for a global array of additional trenches including the Philippine, Kuril, Japan, Izu-Bonin, Aleutian, Tonga-Kermadec, Middle America, Peru, Chile, Sumatra and Java Trenches. Results of the analysis show that the trench relief (W0) and width (X0) of all systems are controlled primarily by the faulting-reduced elastic thickness near the trench axis (Tme) and affected only slightly by the initial unfaulted thickness (TMe) of the incoming plate. The reduction in Te has caused significant deepening and narrowing of trench valleys. For the cases of relatively young or old plates, the plate age could be a dominant factor in controlling the trench bending shape, regardless the variations in axial loadings. Our calculations also show that the axial loading and stresses of old subducting plates can vary significantly along the trench axis. In contrast, the young subducting plates show much smaller values and variations in axial loading and stresses.
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ArticleSeismic structures of the Calico fault zone inferred from local earthquake travel time modelling(John Wiley & Sons, 2011-06-16) Yang, Hongfeng ; Zhu, Lupei ; Cochran, Elizabeth S.We analysed high-frequency body waves of local earthquakes to image the damage zone of the Calico fault in the eastern California shear zone. We used generalized ray theory and finite difference methods to compute synthetic seismograms for a low-velocity fault zone (FZ) to model the direct and FZ-reflected P and S traveltimes of local earthquakes recorded by a temporary array across the fault. The low velocity zone boundaries were determined by apparent traveltime delays across the fault. The velocity contrast between the fault zone and host rock was constrained by the traveltime delays of P and S waves and differential traveltimes between the direct and FZ-reflected waves. The dip and depth extent of the low velocity zone were constrained by a systematic analysis of direct P traveltimes of events on both sides of the fault. We found that the Calico fault has a ∼1.3-km-wide low velocity zone in which the P- and S-wave velocity decreased 40 and 50 per cent, respectively, with respect to the host rock. The low velocity zone dips 70° northeast and extends 3 km in depth.
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ArticleUpper mantle hydration indicated by decreased shear velocity near the Southern Mariana Trench from Rayleigh wave tomography(American Geophysical Union, 2021-07-26) Zhu, Gaohua ; Wiens, Douglas A. ; Yang, Hongfeng ; Lin, Jian ; Xu, Min ; You, QingyuReduction of seismic velocities has been employed to study the hydration of incoming plates and forearc mantle in recent years. However, few constraints have been obtained in the Southern Mariana Trench. We use an ocean bottom seismograph (OBS) deployment to conduct Rayleigh wave tomographic studies to derive the SV-wave velocity structure near the Southern Mariana Trench. Measured group and phase velocities as a function of period are inverted to determine the SV-wave velocity using a Bayesian Monte Carlo algorithm. The incoming Pacific Plate is characterized by low velocities (3.6–4.1 km/s) within the upper ∼25 km of the mantle near the trench, indicating extensive mantle hydration of the incoming plate in southern Mariana. The velocity reduction in the forearc mantle is not as large as in central Mariana, most likely indicating a lower forearc serpentinization in this region, which is consistent with the absence of serpentinite mud volcanoes.