There doesn't need to be a run, a bank will be closed by FDIC if it is insolvent.
And in SVB's case, a very small amount of withdrawals was enough to trigger insolvency. (As it forced the bank to sell, and thereby mark-to-market it's long-term treasuries.)
SVB had $45B withdrawals on their $175B deposit base. That is not a small amount of withdrawals.
JPM has $500B of cash and cash equivalents on their $3T deposit base. Of which only ~$34B is in actual cash. Even JPM would fall (without Fed backstop) if JPM had that much withdrawals as a percentage of deposit.
> SVB had $45B withdrawals on their $175B deposit base. That is not a small amount of withdrawals.
Almost all of those withdrawals happened after the bank was insolvent.
SVB had to sell all of their available securities to cover day-to-day customer withdrawals (The problem with being the bank of choice for startups is that they aren't making any money, aren't getting any new investments, but are still spending money.)
The bank run started after SVB started dipping into its underwater long-term securities, and borrowing money, and doing emergency fundraising.
A more diversified bank (Like any of the big four) would avoid this problem, because their regular day-to-day activity would be a ~net-zero balance of withdrawals and deposits. SVB was uniquely vulnerable because of its undiversified customer base, where normal customer activity pushed it towards insolvency.
And in SVB's case, a very small amount of withdrawals was enough to trigger insolvency. (As it forced the bank to sell, and thereby mark-to-market it's long-term treasuries.)