State seismologist Michael West wrote that an earthquake’s magnitude is much less important than how close people are living to its epicenter.
“Alaskans often feel the distant rumbles of very large earthquakes, but that is nothing like the violent shaking that occurs close to the epicenter, even during earthquakes with unimpressive magnitudes,” West wrote in an opinion piece for Alaska Dispatch News last year.
There are several different types of waves associated with earthquakes and they move through and effect the earth’s crust differently. Pressure waves and sheer waves rumbled the Chilkat Valley, U.S. Geological Survey geologist Rob Witter said this week.
Because rock is elastic, it behaves like a spring, Witter said. Plate tectonics, the constant collision and sliding under and across earth’s plates, “is strong enough to bend and squeeze rock as if they’re elastic plates.”
Pressure waves, or P-waves, behave similar to sound waves and move through the earth faster. P-waves squeeze the rock like a spring and rock bounces back.
“You feel it as a subtle bump,” Witter said. “Shortly after you feel the big shaking, that’s the side to side motion caused by a sheer wave.”
Sheer waves, or S-waves, behave like their name. They shear rock and that’s the wave you feel as they shake and rumble the earth.
“S-waves cause most of the damage,” Witter said. “It’s the side-to-side motion that topples books off of bookshelves.”
Seismologists can measure where earthquakes originate based on the separation in time of P-waves and S-waves.
According to Witter, Love and Rayleigh waves are two other types of seismic waves that travel more slowly than P- or S-waves.
Rayleigh waves are trapped on the Earth’s surface and are what people observe when they report seeing the surface undulate like waves in the ocean.