What happened on Oct 16th?
Why such a hashpower drop?
Because of the Byzantium upgrade? Block time went from 30s to around 13s. They made these upgrades:
Addition of ‘REVERT’ opcode, which permits error handling without consuming all gas (EIP 140)
Transaction receipts now include a status field to indicate success or failure EIP 658)
Elliptic curve addition and scalar multiplication on alt_bn128 (EIP 196) and pairing checks (EIP 197), permitting ZK-Snarks and other cryptographic mathemagic™
Support for big integer modular exponentiation (EIP 198), enabling RSA signature verification and other cryptographic applications
Support for variable length return values (EIP 211)
Addition of the ‘STATICCALL’ opcode, permitting non-state-changing calls to other contracts (EIP 214)
Changes to the difficulty adjustment formula to take uncles into account (EIP 100)
Delay of the ice age / difficulty bomb by 1 year, and reduction of block reward from 5 to 3 ether (EIP 649)