Synthesized investigation of seismotectonic vertical processes in the forearc coastal area of the plate convergence zone

利用第四纪地貌学方法，研究了地震构造抬升与非地震构造沉降的动态组合模式，以了解太平洋板块与欧亚板块汇聚形成的构造过程。得到的结果如下：1.研究结果。发现了一个连续的九个海洋阶地，它们至少在MIS21(约800ka)之后形成于间冰期阶段。平均抬升速率估计为0.2~0.4m/ka。沿着三陆海岸120公里长的北部海岸，至少在3到5米的高度可以辨认出全新世的海洋梯田。羽化年龄可能在7ka至5ka之间。13个短钻孔和2个长钻孔的钻孔岩心资料显示了构造历史：8ka至7ka之间以7~9 mm/a的速度连续下沉，2.9ka左右可能出现幕式隆升，产生浅海地貌，2.9ka后持续下沉，速率超过1 mm/a。这五个港口过去50年的潮汐记录表明，三陆海岸正在持续下沉。下沉速度在2到11毫米/年之间，在靠近日本海沟的港口有增加的趋势。实际估计在全新世晚期至少有两次同震抬升事件，可能发生在7-5KA和3KA。这一地震情景预测重现时间约为3000年，抬升超过10米。长达100多公里的沿海大隆起的极端同震事件需要广泛的板间断裂或从板块边界开始的分支断裂，以达到板块会聚释放应力的目的。这种灾难性的断层活动可能与8级地震有关。

We investigated the dynamic combination model of seismotecotonic uplift and non-seismotectonic subsidence, in order to understand tectonic processes originated from the plate convergence between the Pacific Plate and the Eurasia Plate, using Quaternary and geomorphological techniques. The obtained results are as follows.1. A succession of nine marine terraces is recognized, which were formed at interglacial stages at least after MIS21 (ca.800ka). Average uplift rate is estimated to be 0.2 to 0.4m/ka.2. At least Holocene marine terrace is recognizable at the height in 3 to 5 meters along the 120km long, northern coast of the Sanriku Coast. The emergence age is probably between 7ka and 5ka.3. Boring core data of 13 short drilling and 2 long drilling show the tectonic history : successive subsidence in rate of 7-9mm/yr between 8ka and 7ka, a possible episodic uplift producing the emergence of shallow marine topography around 2.9ka, and continuous subsidence with the rate of more than 1mm/yr after 2.9ka.4. Tidal records in the past 50 years at the five ports indicate that the Sanriku Coast is continuously subsiding. The subsidence rate ranges in 2 to 11mm/yr and tend to increase at the port closer to the Japan Trench.5. At least two co-seismic uplift events are practically estimated in the late Holocene, which likely occurred at 7-5ka and 3ka. This seismic scenario prevides that the recurrence time is about 3000 years and the uplift exceeds 10 meters.6. The extreme coseismic event of large uplift along the more than 100km long coast requires the extensive interplate faulting or the branching faulting from the plate boundary for the stress release by plate convergence. Such catastrophic faulting is probably associated with the M8-class earthquakes.