S waves shake the ground in a shearing, or crosswise, motion that is perpendicular to the direction of travel. These are the shake waves that move the ground up and down or from side to side. S waves are called secondary waves because they always arrive after P waves at seismic recording stations.

S waves move rock particles up and down, or side-to-side–perpendicular to the direction that the wave is traveling in (the direction of wave propagation).

There are two types of surface waves: Love and Rayleigh waves. Love waves move back and forth horizontally. Rayleigh waves cause both vertical and horizontal ground motion. These can be the most destructive waves as they roll along, causing the ground to rise and fall as they pass.

S waves are indicated by an abrupt change in wave amplitude. In the seismogram below, we see that the S wave arrived at about 34 sec after the P wave arrived. This time difference is called the S-P interval and is the lag time between the P and S wave.

Measure the difference in arrival times between the first shear (s) wave and the first compressional (p) wave, which can be interpreted from the seismogram. Multiply the difference by 8.4 to estimate the distance, in kilometers, from the seismograph station to the epicenter.

Seismic Wave Motions—4 waves animated

• Body Waves – Primary (P) & Secondary (S) Waves.
• Surface Waves – Rayleigh & Love Waves.

Scientists use triangulation to find the epicenter of an earthquake. When seismic data is collected from at least three different locations, it can be used to determine the epicenter by where it intersects. Every earthquake is recorded on numerous seismographs located in different directions.

A large earthquake far away will feel like a gentle bump followed several seconds later by stronger rolling shaking that may feel like sharp shaking for a little while. A small earthquake nearby will feel like a small sharp jolt followed by a few stronger sharp shakes that pass quickly.

Though there is no way to pinpoint the exact arrival of an earthquake, scientists can examine sediment samples to get an idea of when major earthquakes occurred in the past. By measuring the amount of time between events, they can come up with a rough idea of when a major quake might hit.

CoreLogic, a business analysis service, estimated a Southern San Andreas fault rupture will cause 3.5 million homes to be at risk with $289 billion in reconstruction value. Water, electricity and gas lines cross the San Andreas fault in Los Angeles. They break during the quake and remain unfixed for months.

According to USGS there is a 70% chance that one or more quakes of a magnitude 6.7 or larger will occur before the year 2030. Two earthquakes have previously been data-classified as big ones; The San Francisco quake in 1906 with a magnitude of 7.8 and the Fort Tejon quake in 1857 that hit 7.9.