Monday, July 08, 2019

The July 2019 Ridgecrest Earthquakes - Odd Indeed But Not Surprising

I grew up in southern California and earthquakes have been a part of my life since I felt my very first one in the 1960s. My family still lives there and as a geologist I always remind them to be mindful of the tectonic "gold mine" they sit on top of. I guess geologists may be the only ones (besides home repair stores) that think of earthquakes in not strictly negative terms.

I happened to be in southern California at my cousin's wedding when the June 1992 Landers earthquake hit and when it rattled the windows and sloshed the pool water outside our hotel room, the person laying next to me asked what was happening and then what we should do. The only thing I could think of (naturally) was "earthquake" and "just lay here and enjoy it."

So when I heard about the July 4th earthquake that ruptured the ground near Ridgecrest California, I couldn't wait to see the first images (below).

Photo showing how the pavement cracked during the rupture event, produced during the July 4 M 6.4 quake. Look closely at the painted lines. (Photo courtesy of Emily Guerin/LAList)

This is a view looking straight down on one of the ruptures. Note that no matter which side of the crack you stand on, when you look to the other side across the crack, the line shifts to the left. This is called left-lateral displacement. (Photo courtesy of Emily Guerin/LAList)

Additional ruptures from the July 4th earthquake near Ridgecrest.

When I saw these photos, they caused me to ponder this event more closely knowing that the famous San Andreas system of faults has right-lateral displacement. This breakage didn't seem to follow that pattern. However, there is another fault that abuts the San Andreas in a perpendicular fashion and I immediately thought of it as a possible fault for this earthquake - the Garlock Fault.  (Photo courtesy of Emily Guerin/LAList).

Map used from the California Seismic Safety Commission showing the Garlock Fault system and its relationship to the San Andreas system.

As it turns out, this July 4 event happened just to the north of the Garlock Fault, on a previously unknown fault (but the offset was in the same sense as the Garlock - left lateral). It's likely that the break on this event was related to the larger Garlock Fault system.

Then on July 5, another large quake, this time registering 7.1 occurred on a fault perpendicular to the fault that slipped on previous days quake. And this rupture curiously showed right-lateral displacement (see photos below).

Note here that no matter which side of the fault you are standing, when you look across to the other side the displacement is to the right. (Photo by Beth Hadden)

Aerial view of the same area (Photo by Brian Olsen).

A really good article on LiveScience about the displacements and the faults can be found here.

I often get hopeful when news organizations announce an upcoming story about these earthquake events. But invariably, these stories turn into human interest pieces that really have nothing to do with geology, the crust or how brittle substances behave under stress. I guess most people wouldn't care about that. But then, maybe they should label their stories, "How People Are Affected and Are Reacting to the Recent California Earthquakes."

The asthenosphere, a portion of the upper mantle that is hot and pliable - all while remaining a solid - churns slowly beneath the brittle lithosphere that we all live on. As the asthenosphere slowly creeps, the brittle lithosphere must respond to the tugging motion from below. Occasionally, the brittle lithosphere cracks under stress and a rupture and earthquake occur.

I often use an example that many people can visualize - imaging holding a Snickers candy bar with both hands and moving your hands in the opposite direction. Then look at the chocolate covering the candy bar and watch how subparallel cracks grow as the bar is progressively deformed. The picture above is a great example of this as well!

The "Big One" is coming for southern California. But engineers and geologist have been preparing for it and there may be no better place to experience a big jolt, except for other First World, quake-prone areas like Japan, China and Chile. Quakes rarely kill or injure people but falling objects do. My initial response to my bedmate in the 1992 Landers quake may have been cute in some sense, but it was potentially quite dangerous had we been nearer to the epicenter. When you feel it start to shake, look around and head away from anything that can fall on you. And then, sit back and feel the rumble of a force much greater than us mere humans.

No comments: