midsection

noun [专属名词]上腹部；中央部分；胴体

Earth's midsection bulges and the catastrophic land displacement caused a small reduction in the bulge, making the planet more round.
地球中部是稍稍膨胀的，历经灾难的陆地位移导致了隆起的轻微减少，让地球变得更圆。