Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust

Punctuated growth of an accretionary prism and the onset of a seismogenic megathrust in the Nankai Trough

Abstract Ocean drilling in the Nankai Trough forearc suggests a new scenario for the evolution of the Nankai subduction zone. Continuous subduction since the Late Cretaceous has been a common tectonic scenario, although the plate subduction was transferred from the Pacific Plate to the Philippine Sea Plate during the Miocene. Seismic reflection studies coupled with drilling have demonstrated that two episodes have controlled the recent evolution of the Nankai forearc: a resurgence of subduction at ~ 6 Ma after cessation since ~ 12 Ma and rapid growth of the accretionary prism since ~ 2 Ma because of the influx of large amounts of terrigenous sediments from the Japan Alps in central Japan. Both episodes were synchronous with large-scale plate reorganizations. The westward subduction of the Philippine Sea Plate initiated both in the Ryukyu and the Philippine trenches at ~ 6 Ma. Rifting in the Okinawa and Mariana troughs started at ~ 6 Ma. Compressive tectonics in northeast Japan started at ~ 3–2 Ma, and resultant mountain building with active surface erosion commenced in central Japan at ~ 2 Ma. This recent compressive tectonic phase might be due to the initiation of convergence of the Amurian Plate with the Okhotsk or North American Plate along the eastern margin of the Japan Sea. In addition to this event, the strong collision and indentation of the Izu-Bonin Arc since ~ 2.5 Ma was also enhanced in central Japan

Dynamic formation process of thick deformation zone on the shallow plate boundary fault of the Japan Trench: insight from analog experiments of half-graben subduction

Abstract The 2011 Tohoku-oki earthquake unexpectedly ruptured to the shallowest portion of the plate boundary fault and triggered a large tsunami. The shallow portion had generally been regarded as a seismically stable zone until this event, but its significance has now been dramatically revealed for future disaster mitigation. This research approaches the shallow portion, especially the formation process of its structure and plate boundary faults. Scientific drilling conducted near the Japan Trench after the earthquake reported a thin plate boundary fault (~ 7 m) and thick deformation zone (~ 100 m). This thin fault would be expected given the relatively small displacement near the trench (~ 3.2 km), but the deformation zone thickness is anomalously wide given this small magnitude of slip. To understand the dynamic deformation processes that lead to the development of a thick deformation zone surrounding a thin fault core, we conducted forward modeling of an analog experiment with the technique to visualize fault activity. Sandbox experiments are effective for the approximation of the geological phenomenon and structure. The seismic profile of the largest slip region in the 2011 earthquake shows that a half-graben structure has been subducted underneath the frontal wedge, thus we focused on this structural architecture. As a result, we found a new fault formation pattern, i.e., the frontal thrust (the most frontal part of décollement) periodically partitioned into pieces, which connect again to form a large-continuous fault. The fault also oscillates up and down during this process, which we call “dancing,” and a thick shear zone is formed in a relatively short time where this occurs, even though the fault only has a small displacement. By analogy, the thick deformation zone observed at the Japan Trench could be formed by such fault dancing. The energy of the fault activity is commonly estimated from the fault displacement derived from the thickness of the shear zones. Applying the thickness-displacement law without considering the effect of the dancing may cause overestimation. The architecture of the shear zone formed is similar to that of a mélange, and the origin of tectonic mélanges may be explained by this mechanism

A case of pelvic squamous cell carcinoma of unknown primary origin that responded well to radiotherapy and nivolumab

Squamous cell carcinoma of unknown primary origin in the pelvis is rare. We report a case of a 64-year-old woman with a large osteolytic squamous cell carcinoma of unknown primary origin in the pelvis that presented with p16 expression. The patient presented with leg pain and swelling and was admitted to our hospital. Computed tomography scans of the pelvis revealed a large osteolytic tumor. A computed tomography-guided needle biopsy was performed, and pathological examination revealed neoplastic cells with metastatic squamous cell carcinoma presenting with p16 expression. Despite a whole-body examination, tumor origin remained undetected. The patient was treated for this metastatic squamous cell carcinoma of unknown primary using palliative radiotherapy for hip pain and nivolumab. Remarkable reduction in the tumor marker levels and tumor size were obtained after therapy. Finally, partial remission and progression-free survival for more than 7 months were achieved. In conclusion, we experienced a rare case with a large p16-positive squamous cell carcinoma of unknown primary in pelvis, which responded well to radiotherapy and nivolumab