Geo-mechanical and rheological modelling of upper crustal faults and their near-surface expression in the Netherlands
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Abstract Despite recent observations of slow earthquakes along the Nankai subduction zone, none have been reported in the central Nankai Trough between the Kii Channel and Cape Shionomisaki. In November 2018, a very dense array of 96 ocean‐bottom seismometers were deployed by JAMSTEC to acquire active‐source seismic refraction dataset (supplemented by a multichannel seismic reflection profile) from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. We applied traveltime tomography to the refraction data to constrain the P wave velocity down to the upper mantle, coordinating with a migrated seismic reflection profile to confirm the depth of the Moho and interpret shallower structural features. From a comparison with a transect across the Kumano basin, we conclude that structural and physical differences between these two locations, especially the geometry of the subducting plate surface, lead to different slow earthquake activities.
Accretionary wedge
Trough (economics)
Seismometer
Seismic Tomography
Seismic refraction
Receiver function
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Bathymetric features on the seafloor often show evidence for crustal deformation by tectonic events. Although the crustal structure obtained from seismic reflection surveys provides important information such as active faults, it is difficult to estimate the lateral continuity from sparse survey lines. In the Nankai Trough in central Japan, great earthquakes with accompanying tsunamis occur with recurrence intervals of 100–200 years. Recent analogue modelling of thrust fault initiation at the toe of a subduction zone, based on the Nankai region, suggests that a new thrust would initiate in the trench axis a few kilometers seaward of the currently active frontal thrust which is the uppermost plate boundary fault at the seaward end of accretionary prism (Dotare, Yamada, Adam, Hori, & Sakaguchi, 2016). In the central Nankai region (off Shikoku), this is the location of a protothrust zone (PTZ) which exists in the seaward region of the frontal thrust, consisting of many incipient thrusts within the trench sediment wedge, as imaged in legacy seismic reflection profiles (e.g. Moore, Mikada, Moore, Becker, & Taira, 2005). The PTZ is an area at the stage of frontal thrust initiation (Figure 2b), where several incipient faults with indistinctive displacement are formed. The PTZ is also recognized in other seismogenic subduction margins associated with incipient deformation by subduction (e.g. Barnes et al., 2010). Not only the splay fault from the seismogenic zone, but also the frontal thrust has the possibility to initiate the tsunamigenic rupture estimated by drilling results (Sakaguchi et al., 2011) Thus, it is important to investigate the PTZ along the trench for understanding the process of the development of a new frontal thrust. In order to extract the micro-scale deformation of the PTZ at the seafloor, we applied a new developed elaborate visualization “Red Relief Image Map (RRIM)” (Chiba, Kaneda, & Suzuki, 2008) which is a new three-dimensional presentation technique to high-quality bathymetric data by the multi-narrow beam echo sounder which have been simultaneously acquired for high-resolution bathymetric data associated with seismic survey area (Figure 1). The RRIM method was developed to image by the multi-layered topographic information such as landform element layers, topographic slope, positive openness and negative openness using high-density, high-resolution Digital Elevation Model (DEM) data on onshore region (Chiba et al. 2008; Chiba, Suzuki, & Hiramatsu, 2007). The new parameter I using the RRIM method is calculated from the two following openness parameters: where Op and On represent positive and negative openness, respectively. The method is known to be an effective tool for the extraction of slight active fault traces hidden under vegetation on land (e.g. Lin, Kaneda, Mukoyama, Asada, & Chiba, 2013). To understand the geomorphologic features in PTZ, we apply this RRIM method to integrate the overall DEM data with a minimum resolution of 50 m using high-quality bathymetric data obtained by legacy seismic surveys (e.g. Park et al., 2002). The RRIM method can be deduced from the deformation of a different scale in the same figure. Figure 1 shows the RRIM image from Izu Arc to Nankai Trough. Clear complicated deformation structures around the volcanic front, such as a volcano-bounded basin between rear arc volcanos (e.g. Yamashita, Takahashi, Tamura, Miura, & Kodaira, 2017), are recognized in Izu Arc. We carried out a dense high-resolution seismic reflection survey with 10–20 km spacing over 1500 km of line length during 2013 and 2014 in order to understand the deformation around the frontal thrust at the trench axis (Figure 1). Clear seismic reflection images from frontal thrusts to PTZ along the trench axis are obtained between off Cape Muroto and off Cape Ashizuri (Figure 2). It is difficult to image the deformation of both the frontal thrust and the protothrust zone by conventional bathymetric mapping due to the different scale of their deformation. To image the detailed characteristics of the seafloor, we use the RRIM method with color depending on the depth scale. Figure 3 shows the expanded edition of conventional bathymetric map and multi-colored relief image map around the Nankai Trough. Not only the visible deformation of the accretionary prism, but also some arrangements of incipient thrust beneath the seafloor within PTZ corresponding to seismic reflection profiles along the trench axis are sharply recognized by the RRIM method. Our findings can provide information about possible location of future frontal thrust within the PTZ. We thank Tatsuro Chiba, Mikako Sano, Iki Shinji, Mimura Koushi, Taisho Kondo of Asia Air Survey Co., Ltd. for supporting to create the red relief image map. The authors are grateful to two anonymous reviewers and associate editor, Ryuichi Shinjo for their valuable comments and editing on the manuscript. The Generic Mapping Tools software was used to execute some bathymetric maps (Wessel & Smith, 1998). This study is partially supported by JSPS KAKENHI Grant Number 16K17824.
Accretionary wedge
Thrust fault
Seafloor Spreading
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Thrust fault
Surface rupture
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Thrust fault
Seafloor Spreading
Earthquake rupture
Trough (economics)
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The Southern Apennines chain is related to the west‐dipping subduction of the Apulian lithosphere. The strongest seismic events mostly occurred in correspondence of the chain axis along normal NW–SE striking faults parallel to the chain axis. These structures are related to mantle wedge upwelling beneath the chain. In the foreland, faulting develops along E–W strike‐slip to oblique‐slip faults related to the roll‐back of the foreland. Similarly to other historical events in Southern Apennines, the I 0 = XI (MCS intensity scale) 23 July 1930 earthquake occurred between the chain axis and the thrust front without surface faulting. This event produced more than 1400 casualties and extensive damage elongated approximately E‐W. The analysis of the historical waveforms provides the chance to study the fault geometry of this “anomalous” event and allow us to clarify its geodynamic significance. Our results indicate that the M S = 6.6 1930 event nucleated at 14.6 ± 3.06 km depth and ruptured a north dipping, N100°E striking plane with an oblique motion. The fault propagated along the fault strike 32 km to the east at about 2 km/s. The eastern fault tip is located in proximity of the Vulture volcano. The 1930 hypocenter, similarly to the 1990 (M W = 5.8) Southern Apennines event, is within the Mesozoic carbonates of the Apulian foredeep and the rupture developed along a “blind” fault. The 1930 fault kinematics significantly differs from that typical of large Southern Apennines earthquakes, which occur in a distinct seismotectonic domain on late Pleistocene to Holocene outcropping faults. These results stress the role played by pre‐existing, “blind” faults in the Apennines subduction setting.
Hypocenter
Transform fault
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Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 "backthrust" sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as 'unsticking' of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults.
Forearc
Accretionary wedge
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Abstract The Nankai Trough, Japan, is a subduction zone characterized by the recurrence of disastrous earthquakes and tsunamis. Slow earthquakes and associated tremor also occur intermittently and locally in the Nankai Trough and the causal relationship between slow earthquakes and large earthquakes is important to understanding subduction zone dynamics. The Nankai Trough off Muroto, Shikoku Island, near the southeast margin of the rupture segment of the 1946 Nankai earthquake, is one of three regions where slow earthquakes and tremor cluster in the Nankai Trough. On the Philippine Sea plate, the rifting of the central domain of the Shikoku Basin was aborted at ~15 Ma and underthrust the Nankai forearc off Muroto. Here, the Tosa‐Bae seamount and other high‐relief features, which are northern extension of the Kinan Seamount chain, have collided with and indented the forearc wedge. In this study, we analyzed seismic reflection profiles around the deformation front of accretionary wedge and stratigraphically correlated them to drilling sites off Muroto. Our results show that the previously aborted horst‐and‐graben structures, which were formed around the spreading center of the Shikoku Basin at ~15 Ma, were rejuvenated locally at ~6 Ma and more regionally at ~3.3 Ma and have remained active since. The reactivated normal faulting has enhanced seafloor roughness and appears to affect the locations of slow earthquakes and tremors. Rejuvenated normal faulting is not limited to areas near the Nankai Trough, and extends more than 200 km into the Shikoku Basin to the south. This extension might be due to extensional forces applied to the Philippine Sea plate, which appear to be driven by slab‐pull in the Ryukyu and Philippine trenches along the western margin of the Philippine Sea plate.
Forearc
Accretionary wedge
Seamount
Seafloor Spreading
Trough (economics)
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Transform fault
Convergent boundary
Seismometer
North American Plate
Eurasian Plate
Pacific Plate
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Paleoseismology
Thrust fault
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