  Solar active region (AR) 12673 produced 4 X-class, 27 M-class, and numerous
lower class flares during its passage across the visible solar disk in
September 2017. Our study is to answer the questions why this AR was so
flare-productive and how the X9.3 flare, the largest one of the last decade,
took place. We find that there was a sunspot in the initial several days, and
then two bipolar regions emerged nearby it successively. Due to the standing of
the pre-existing sunspot, the movement of the bipoles was blocked, while the
pre-existing sunspot maintained its quasi-circular shaped umbra only with the
disappearance of a part of penumbra. Thus, the bipolar patches were
significantly distorted, and the opposite polarities formed two semi-circular
shaped structures. After that, two sequences of new bipolar regions emerged
within the narrow semi-circular zone, and the bipolar patches separated along
the curved channel. The new bipoles sheared and interacted with the previous
ones, forming a complex topological system, during which numerous flares
occurred. At the highly sheared region, a great deal of free energy was
accumulated. On September 6, one negative patch near the polarity inversion
line began to rapidly rotate and shear with the surrounding positive fields,
and consequently the X9.3 flare erupted. Our results reveal that the
block-induced complex structures built the flare-productive AR and the X9.3
flare was triggered by an erupting filament due to the kink instability. To
better illustrate this process, a block-induced eruption model is proposed for
the first time.
