Personally I really don't think that 500 kg is enough to cause a 2.3 seismic event.

I have too little knowledge about this, but haven't heard anyone with expertise mentioning this.

And it really doesn't matter how much was used it was so excessive that it puts out its own message.

If it was Russia, then they did send a powerful message. They could destroy all sorts of infrastructure in international waters and cause a complete meltdown of the global economy with Europe going down the most. In the worst case, we'd end with 10$ per day or roughly the mid 1950s again, but without a war hardened generation and no social or cultural cohesion.

On top comes the motive for Russia that it is a massive land power connecting various global regions. Irrespective of who the perpetrator was, the explosions will give them a massive advantage over projects in open waters like pipelines, communication cables, oil drilling, or even shipping. All of that will have to be reinforced with no end in sight. Costs will quadruple to protect everything sufficiently against such attacks.