If you think the earth is shaking below you — you may be right. In the Lake Tahoe-Truckee region the Sierra Nevada mountain range is moving northwest with the North American Tectonic Plate, about one half-inch per year. This movement is paired with its transform plate boundary: the Pacific Tectonic Plate, moving southeast about 2-3 inches per year. Most of us know that when the plates grind together, the fault lines in certain areas begin to crack and fissure farther. But it does not occur all at once.
A local series of earthquakes caused by the earth’s crust movement is known as an earthquake swarm. Recently, earthquake swarms have become commonplace for the Sierra Nevada. In August 2008, Mogul, a suburb west of Reno, was hit with weeks of small-magnitude quakes, which surrounded a 5.0 temblor and caused around $1,000,000 of damage. In July 2014, a swarm in the northwest region of Nevada, near the Oregon/California border, again lasted weeks with a total of around 800 quakes. The two largest measured 4.7 magnitude. This swarm fortunately occurred in a low population area and didn't cause sizable damage.
The Truckee area is in a very active fault zone. On June 27 – 29 of this year, Truckee experienced an earthquake swarm which surrounded a 4.0 magnitude quake. Most of these temblors occurred on the intersection of the Polaris Fault and the Dog Valley Fault. (See diagram.) Twenty-eight small quakes were recorded on just June 27, while the largest hit at 2:09 a.m. that day.
A second possibility for earthquake swarms is also related to water. Earthquake trigger studies have been done measuring the depression of soil caused by water content during winter months; called seasonal water loading. In a typical winter, the soil may condense to about one centimeter. The trouble begins when the soil starts to dry out and expand: rebounds that cause stress on the faults leading to upticks in small magnitude earthquakes regionally. "It's not like we're seeing an earthquake season, but the timing of this water unloading is when we're (historically) getting more earthquakes," said Chris Johnson, a graduate student at the Berkeley Seismological Laboratory.
As someone who studies earthquake triggers, Nicholas van der Elst, a research geophysicist for the U.S. Geological Survey in Pasadena summed it up by saying, "We have a pretty good understanding of earthquakes on a basic level and why they happen, however, we don't know things like how quickly stress accumulates and what is the breaking point of a fault.”
It seems we also don’t know how many factors are at play when an earthquake occurs, but the research continues.