Answer 1:
This is a good question. To start, let me first give an introduction to the setting that produces the largest Earthquakes and Volcanoes.
The Earth’s crust broken up into tectonic plates that move around. In some places, the plates are spreading apart (diverging). These are mostly at mid ocean ridges where magma comes up to the surface to create new oceanic crust. As these plates move away from each other they also run into each other at convergent boundaries. As the thinner, denser oceanic crust meets continents, they end up going beneath the thicker, less dense continental crust. This is called a subduction zone. As the oceanic plate sinks below the continent it can get “stuck” and build up stress. Once this stress and energy is released, it produces an Earthquake. The oceanic crust also melts as it dives deeper, producing more magma that rises and is erupted at the surface. The region of plate boundaries with the most subduction zones is called “the ring of fire” (check out this map ).
Notice how the plate boundaries, earthquakes, and volcanoes are all related! Now that we know where we can expect the most earthquakes and volcanic activity, the answer to your question. Actually, the answer to this question is not as straightforward as it may seem. So, in order to most accurately answer it, I will rephrase the question a few different ways. Let’s start with earthquakes…
In which country do we locate the most earthquakes? Japan.
The whole country is in a very active seismic area, and they have the densest seismic network in the world, so they are able to record many earthquakes.
Which country actually has the most earthquakes? Indonesia is in a very active seismic zone, also, but by virtue of its larger size than Japan, it has more total earthquakes.
Which country has the most earthquakes per unit area? This would probably be Tonga, Fiji, or Indonesia since they are all in extremely active seismic areas along subduction zones. The sparse seismic instrumentation in those areas doesn't allow us to actually record all the smaller earthquakes.
Which country has the most Volcanoes?
After a quick search, here are the top 10 countries with the most volcanoes:
United States – 173; Russia – 166; Indonesia – 139; Japan – 112, Chile – 104; Ethiopia – 57; Papua New Guinea – 53; Philippines – 50; Mexico – 43; Argentina – 39
Notice that most of these are along the ring of fire. However, some are not, BUT, they are still near major tectonic plate boundaries. So, in general, anywhere there is a major plate boundary we can expect to see the largest Earthquakes and the most Volcanic activity.
I hope that helps, and stay curious!! | 
