We record earthquakes through the use of seismographs. Seismographs are instruments that create an electrical signal when the ground moves. The ground can move and create a signal because a truck drives by, a sonic boom, wind pushing tree roots or an earthquake. By comparing the signal from many seismographs at different locations we can determine if an earthquake occurred and by timing the waves that travel from the earthquake, determine where and when it occurred. The Seismological Laboratory of Caltech and the U. S. Geological Survey together operate a network of over 300 seismographs called the Southern California Seismographic Network.
The shaking one feels during an earthquake is not the direct movement of the fault but rather the movement carried by the waves generated because of that slip. If you compare an earthquake to snapping your fingers, the fault movement is the fingers while the shaking you feel is the snapping sound - the air moving in your ear drum. This shaking, and not direct fault motion, is the primary cause of damage in an earthquake. Both thrust and strike-slip earthquakes (vertical and horizontal fault motion) produce both vertical and horizontal shaking. Contrary to rumor, the ratio of vertical to horizontal shaking in the Northridge earthquake was the same as seen in other earthquakes. However, because many people were directly on top of the fault, the level of both types of shaking were high.
Strike-slip faults are fault planes where the plane of the fault is vertical - perpendicular to the Earth's surface - and the rock blocks on either side move horizontally during an earthquake. The San Andreas, San Jacinto, Garlock, Elsinore and Newport-Inglewood are some of the strike-slip faults in southern California.