Just sitting around this morning when I thought something exploded outside. Big boom, then a sharper cracking sound, then a rumble. Then another boom.
Turns out it was a deep earthquake a few miles from us. Reported as a 4.0, but it felt a lot bigger. Kind of scary, but no damage. Definitely got our attention.
I have a little answer to this from my SAIC geophysics days……. we are used to feeling earthquakes in the waves that travel laterally through the crust, and as they propagate, they get various path lengths due to reflections of the waves in the crust (think of a waveguide or fiber optics, where the crust and the mantle interact). So what starts as a very short impulse (boom) turns into a long, drawn out vibration. The farther the distance from the epicenter, the more dispersed the wave and energy over a longer time. But when right on top of a deep quake, it does not disperse and it happens all at once – boom. It is why the damage at the epicenter is so much worse – there is much more energy in the vertical plane, as compared to a rolling wave. I felt one of those in the 80s when we have a 4.0 (or so) right under San Diego bay – same thing, a big crash. Thank goodness most in San Diego county are low magnitude. The Rose Canyon fault is the one big exception – it is due to have a big one fairly soon, based on the geological record, and it can go to about 7.0 (not so good for La Jolla……).
That makes a lot of sense. And we’re directly down the path of the San Luis Rey river from the epicenter, so that path and aquifer might act as a transmission corridor for the force. We’ve had a bunch of quakes here in the last 20 years, but this one was a little different. For a second I thought that someone at Camp Pendleton had lobbed a mortar shell into our backyard.