I think earthquakes in Iceland differ somewhat from earthquakes in California in that Iceland has a small population, doesnt have large buildings and scientists are hopeful they can give fair warning. I believe the biggest known was about 7.1. In terms of a volcanic eruption, if there's a big one we'll be feeling the effects in the UK too and it'll be climatic issues rather than lava that's the issue.
So there's not the history of 'The Big One ' in terms of earthquakes in the same way as California.
I was reading up on this the other day, in part because of events in Iceland but also because Mount St Helens has been a bit more noisy than usual and eruptions in this area are known to be triggers for big earthquakes.
California and the Pacific Northwest are overdue for 'The Big One'. San Andreas is over due and will cause 'thousands of lives lost' but by the sound of it they are more concerned not about the San Andreas but up in the Pacific Northwest because it's perhaps less prepared and they think the earthquake may be bigger.
If you read up about the 1700 earthquake it's really frightening. It wasn't documented because it was still tribal land - however they know when it was partly through oral history, tree evidence and documents in Japan that describe a massive tsunami...
https://en.m.wikipedia.org/wiki/1700_Cascadia_earthquake
If you scroll down to future threats for some sobering reading:
The geological record reveals that "great earthquakes" (those with moment magnitude 8 or higher) occur in the Cascadia subduction zone about every 500 years on average, often accompanied by tsunamis. There is evidence of at least 13 events at intervals from about 300 to 900 years with an average of 570–590 years.
As seen in the 1700 quake, the 2004 Indian Ocean earthquake, and the 2011 Tōhoku earthquake and tsunami, subduction zone earthquakes can cause large tsunamis, and many coastal areas in the region have prepared tsunami evacuation plans in anticipation of a possible future Cascadia earthquake. However, the major nearby cities, notably Seattle, Portland, Vancouver, Victoria, and Tacoma, which are located on inland waterways rather than on the coast, would be sheltered from the full brunt of a tsunami. These cities do have many vulnerable structures, especially bridges and unreinforced brick buildings; consequently, most of the damage to the cities would probably be from the earthquake itself. One expert asserts that buildings in Seattle are inadequate even to withstand an event of the size of the M7.9 1906 San Francisco earthquake, let alone a more powerful one.
Kenneth Murphy, who directs FEMA's Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, stated, "Our operating assumption is that everything west of Interstate 5 will be toast."
Recent findings conclude that the Cascadia subduction zone is more complex and volatile than previously believed. In 2010, geologists predicted a 37% chance of an M8.2+ event within 50 years, and a 10% to 15% chance that the entire Cascadia subduction zone will rupture with an M9+ event within the same time frame. Geologists have also determined the Pacific Northwest is not prepared for such an earthquake. The tsunami produced could reach heights of 80 to 100 feet (24 to 30 m).
Look up where Interstate 5 is.
The thing that's scariest to me isn't necessarily the earthquake either. It's the chaos that would follow with looting and break down in civil defence and Americans running around with guns.
I'm not bloody surprised anyone on the West Coast lives in fear of the big one. 37% chance of another Big Cascadia earthquake by 2060... I don't like those odds.
Look at the odds for volcanos and earthquakes in Iceland and you don't see anything in the league.