Zions Bancorp has reported a $50 million impairment charge, reigniting concerns over the stability of U.S. regional banks. The impairment is linked to a loan underwritten by its subsidiary, California Bank & Trust. This announcement led to a significant drop in Zions Bancorp's stock, which fell by 15%. Western Alliance Bancorp also saw its stock decline by 13%, contributing to a broader downturn in the sector. The KBW Bank Index, a benchmark for the banking sector, closed down 3.6% as investors reacted to the renewed financial instability. The developments highlight ongoing vulnerabilities within regional banks, which continue to face challenges in the current economic climate.