Why do earthquakes and volcanoes occur at the edges of plates?

Most earthquakes happen at or near the boundaries between Earth's tectonic plates because that's where there is usually a large concentration of faults. Some faults crack through the Earth because of the stress and strain of the moving plates. ... Movement along those faults can cause earthquakes too.

Explanation:

when the chinese communist party took power in 1949, it began to build new di and levees to hold the yellow and yangtze rivers back.