下列地质灾害主要由外力导致的有         (   )

阅读刘禹锡的《秋风引》一诗。

                                                                                                                         《秋风引^①》刘禹锡

                                                                                                                      何处秋风至？萧萧送雁群。
                                                                                                                      朝来入庭树，孤客最先闻。
       注：①引：古代的一种诗体。

已知数列$\left\{a_n\right\}$满足_{$a_{n+1}=\left\{\begin{array}{c}2a_n\left(0\le a_n<\frac{1}{2}\right)\\ 2a_n-1\left(\frac{1}{2}\le a_n<1\right)\end{array}\right.$，}若$a_1=\frac{6}{7}$，则$a_{2008}$的值为(      )