What causes an increase in the number and intensity of small earthquakes before an eruption?

Quakes move nearer to the surface as an eruption approaches. And these "microearthquakes" happen more frequently before an eruption, until all the seismometer can register is a distinctive vibration, slight but continuous, known as a harmonic tremor. The seismic activity associated with volcanism is usually as a result of pressure increases in the magma chamber....