A: Landslides can happen for a variety of reasons. Everything from heavy rains to volcanic activity and even human activities like mining operations can cause hillsides and mountain slopes to collapse.
Earthquakes can definitely increase the likelihood of landslides. Many times they are the proverbial straw that breaks the camel's back. When an earthquake causes a landslide, it is because it has exploited instability in a landmass that was there before the quake happened. That unstable condition could have been caused by forces such as erosion or even by previous earthquakes.
But landslides don’t necessarily happen every time there’s an earthquake. Tectonic activity in the Earth’s crust means the ground below us is moving all the time — especially in places like California that rest at the margin of two tectonic plates! Most of the earthquakes that happen here and around the world, though, are too small to be felt or to leave any physical changes like cracks in the ground.
Most earthquakes also don’t shake hard enough to set off landslides but the cumulative effect of many small earthquakes could eventually cause rock or mud to slide. It’s like a house of cards set up on a tabletop. A good shake of the table will certainly knock it down but it might take several small taps to cause enough small slips to bring the cards tumbling down.
— Graham Kent, geophysicist, Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics