Surface deformation and geologic structures associated with active blind faults

在日本东北部和中部选定的活动断层上进行了地震反射剖面和重力剖面分析，以阐明与盲活动断层相关的地表变形和地质构造机制。得到了以下结果：1。日本东北部现今活动断裂多起源于中新世正断层，上新世以来在压应力作用下被逆冲断层重新活化。对2003年北宫城6.4级地震震源区进行了地震反射和重力测量。虽然震源机制解为逆冲型2，但中新世地质构造表明震源断层上存在大量正滑。2004年中越地震的震源区位于褶皱冲断带。地表和地下地质资料表明，该褶皱冲断带下部有大规模的滑脱断裂，该断裂在中新世造成了该地区的伸展（裂谷）和厚沉积物的沉积。大多数余震发生在分离层以下。因此，2004年地震的震源断裂很可能与薄皮构造无关，而代表了分离下中古生代基底的内部变形。与日本东北部相反，日本中部逆构造作用不明显。认为伊纳谷断裂带是在第四纪早期新生的。然而，地震反射和重力测量结果表明，在伊纳河谷盆地充填物下存在一条隐蔽的活动逆冲断层。在这条盲断层的上盘一侧是一层厚厚的沉积岩，表明这条断层可能在中新世发生了正常滑动。这表明日本西南部的地壳结构也受到中新世伸展的影响。

We performed seismic reflection profiling and gravity profiling across selected active faults in northeast and central Japan, in order to clarify the mechanism of surface deformation and geologic structures associated with blind active faulting. The following results were obtained :1.Most of presently active faults in northeast Japan were originated from Miocene normal faults, and have been reactivated as thrust faults under compressive stress regime since Pliocene time. Seismic reflection and gravity surveys were performed across the source area of the 2003 Northern Miyagi earthquake of Mw 6.4. We found Miocene geologic structures indicative of a large amount of normal slip on the source fault, although focal mechanism solutions of the earthquake are of thrust type.2.The source region of the 2004 Chuetsu earthquakes is located in a fold-and-thrust belt. Surface and subsurface geologic data suggest that this fold-and-thrust belt is underlain by a large-scale detachment fault, which was responsible for the stretching (rifting) of, and the deposition of thick sediments in, this area in Miocene time. Most of aftershocks occurred below the detachment horizon. Therefore it is likely that the source faults of the 2004 earthquakes were not related to the thin-skinned structure but represent internal deformation of Mesozoic-Paleozoic basement beneath the detachment.3.In contrast to northeast Japan, inversion tectonics is not obvious in central Japan. It has been believed that the Ina Valley fault zone was newly born in early Quaternary time. Our seismic reflection and gravity survey, however, revealed that a blind active thrust fault exists beneath basin-fill sediments in the Ina Valley. On the hanging-wall side of this blind fault is a thick pile of sedimentary rocks, indicating normal slip on this fault possibly in Miocene time. This suggests that crustal structures of southwest Japan also have been affected by Miocene extension.