The recent Bitcoin Cash (BCH) block reward halving has wreaked havoc on the cryptocurrency’s hash rate — with an apparent miner exodus plummeting hash power more than 80% over two days.

As a result, launching a 51% attack on BCH currently costs less than $10,000 per hour, raising concerns regarding the security of the network.

BCH Hash rate plummets 80%

In response to BCH’s block rewards dropping from 12.5 BCH to 6.25 BCH, many miners appear to have migrated their hash power to the Bitcoin (BTC) network.

According to f2pool, BCH hash rate dropped from nearly 4,200 pentahashes per second (PH/s) to as low as 720 PH/s within two days of the halving taking place. Over the past five hours, the Bitcoin Cash hash rate has more than doubled from the local low to hover at 1,600 PH/s.

BCH hashrate over seven days. Source: f2pool

Mining data site fork.lol reported a more than 30% increase in Bitcoin hashrate amid BCH’s aggressive decline.

BTC and BCH hashrate over 30 days. Source: fork.lol

Price of 51% attack on BCH drops below $10,000

The sudden dip in hashrate has also left BCH highly exposed to the threat of 51% attacks — with Crypto51 estimating that it would cost $9,130 per hour to launch an attack on the Bitcoin Cash network using rented hashpower.

On May 24, 2019, the two largest mining pools on the BCH network — BTC.top and BTC.com — executed a white-hat 51% attack to prevent an entity from exploiting a vulnerability that would have had left some coins “up for grabs.”

Will the Bitcoin hash rate follow suit post-halving?

The crypto community is speculating as to whether the crash in BCH hashrate is a sign of things to come for Bitcoin as the network prepares for a block reward reduction to be implemented in roughly 33 days. On Reddit, one user posted:

“[T]he BTC miners are more f***** than the BCH miners if the hash rate suddenly drops 75% in a single day. BCH adjusts its difficulty every 24 hours. BTC adjusts its difficulty every 14 days. So miners must mine at a loss for a much longer period until the network can operate as normal again (unless there are some last minute panic changes in the difficulty adjustment protocol).”

Bitcoin is not the only cryptocurrency with a looming block reward reduction, with concerns extending to how the hash rate underpinning the Bitcoin Gold (BTG), Bitcoin Diamond (BCD), Zcash (ZEC), and ZCoin (XZC) will be impacted the projects’ respective upcoming halvings.

While Bitcoin SV (BSV) halved in the past 12 hours, the network does not appear to have suffered a loss in hash rate.