As earthquakes rumbled California the day after Christmas, some people may be wondering how these tremors happen.

Under a list of the latest earthquakes around the world, the United States Geological Survey showed two incidents in the San Jose area from Boxing Day, including one that reached nearly magnitude 4.0 around 10:30 p.m. PST. That quake jolted the region from between five and six miles underneath the surface, with the USGS recording an epicenter around the Alum Rock neighborhood.

Another earthquake three hours earlier was slightly stronger than a magnitude 3.0 and was centered about four miles below San Martin, a location to the south of San Jose in Santa Clara County.

Californians in the area may have felt a light tremor, roughly a magnitude 2.0, that vibrated a few minutes after the stronger of the Dec. 26 pair, according to the Los Angeles Times. That publication noted there had been a couple of earthquakes reaching a magnitude greater than 3.0 with epicenters in the area in the last 10 days.

It’s probably not lost on soccer fans that there is an MLS team called the San Jose Earthquakes.

Earthquakes are linked to movement in the planet’s crust, its outermost solid layer. The tremors can rattle out when two tectonic plates collide with one another and when one plate is forced underneath another, a process known as subduction.

When they are strong enough, earthquakes are natural disasters on their own, crumbling or toppling structures and injuring people. But they can be linked to other natural phenomena as well, either signaling an impending volcanic eruption or triggering a massive tidal wave called a tsunami.

Before a volcano blows, there can be seismic activity from the ground uplifting to make more room for the molten rock called magma in the space underneath the volcano. Although small earthquakes might signal an impending eruption, tremors occur around volcanoes all the time without immediate eruptions.

In the ocean, powerful earthquakes can displace water, causing a massive wave. As one tectonic plate on the ocean floor moves underneath another, it could create a vertical drop anywhere from a few miles to hundreds of miles and the change in the floor’s topography moves the water. When water gushes in to fill a void or suddenly gets thrown out of its place, the massive ripple effect could lead to tsunamis that grow hundreds of feet into the air.

Although the rattling of an earthquake can be extreme, the Earth is constantly vibrating, even if we can’t feel it. Scientists have referred to the frequency of those consistent tremors as “the Earth’s hum.”