Southern California Regional Rocks and Roads

On July 5, 2019 at 2013 hrs, a Mw 7.1 earthquake occurred near Ridgecrest, California. At the time, I was at my home in San Diego, about 220 miles away from the epicenter. Both my husband and I felt the quake quite strongly in our building, owing to the local geology, direction of the seismic waves, and the type of structure. It was the first time since I’ve lived here that I started to feel a bit of fear that something may get damaged or worse due to an earthquake. The shaking lasted about 30 seconds or so here and caused many things inside to sway.

Being a geologist, I was rather excited afterward. First, though, I was a bit concerned that something bad just happened to Los Angeles. I knew the quake was a distance away and large just by how it felt. Long period waves, such as the ones felt here, are only from a large event like this one. I was happy to hear that it was where it was, though I knew that even that would cause a tremendous amount of damage to the region. It was not long after that I decided that it was certain where my birthday trip would go. See, whenever possible, I try to take my birthday off of work. I figure, everyone has their own “special day”, so this one was mine. In this case, I couldn’t think of a better thing to do and see than a surface rupture of a fault line from an earthquake I personally felt.

On August 21, I headed up to Palmdale to stay with a friend that I was going to go the rest of the way to Ridgecrest. On August 22, we headed up to Ridgecrest via Mojave and Garlock. Along the way, we stopped to investigate an old alignment of US 395 near the Searles Station turnoff. I had seen the roadway in aerial photography and wanted to verify if it was indeed an alignment. Sure enough, it was and had some older paving still intact.

Old structures in Garlock with the El Paso Mountains in the background.

From Searles Station, we headed down to State 178 east of the town of Ridgecrest. There, I finally saw what I had been waiting for – the fault rupture. I was, well, giddy with excitement. This was only the first time I’d ever experienced a major earthquake that had surface rupture. Sure, I’d been through the 1994 Mw 6.7 Northridge earthquake when I was living in Santa Clarita, but it didn’t really have surface rupture. I also got to see the rupture from the 1992 Mw 7.3 Landers earthquake. I was asleep for that one but woke up for the Mw 6.5 Big Bear quake two hours later. Long story.

Map of the area. Section A covers the fault rupture. Section B covers the rockfalls in Poison Canyon. Section C covers the ridge shatter visible along State 178. Sections B and C will be covered in a subsequent page.

I felt like a kid in a candy store. Or… more appropriately, a geologist in a mineral store. We stopped alongside State 178 where the rupture had been. By this time, more than a month after the event, the road was repaired as well as the pipeline adjacent. The rupture was still very obvious. Even the change in grade was still very apparent on the highway. At the east end of the new pavement, there was an abrupt rise, which it seems Caltrans didn’t quite work out when they resurfaced. The fence alongside the road, marking the boundary of the military base, was offset a couple of feet, making for some interesting photos. We simply followed the rupture south from there.

Alongside the offset fence. You can see the vertical offset too. Hwy 178 is to the right.

South of State 178, the rupture was greatly disturbed by so many feet.

Fractured ground. Here, the rupture exhibits more extension than vertical offset.

After crossing back over State 178, we followed the rupture for quite a while. As the fault was effectively buried by a large amount of Recent alluvium and Pleistocene lacrustine deposits, the surface expression of the rupture varied quite a bit over such a small area. At this end of the fault, offset was much less. I measured 2.5 feet of right-lateral motion and 1.5 feet of vertical motion. This oblique movement added to the intrigue of the field trip. The rupture could really be categorized in three different ways. One was a simply scarp. I interpreted this to only be in areas where the rock below was a little more competent. The second was a stepped rupture. This seemed to be in areas where the subsurface geology was less competent and likely underlain by weaker alluvium, such as a small dry stream along the alluvial fan / bajada we were crossing. The third, to be honest, was the most interesting to me, which was a small graben. These formed in a few sections and were usually a foot or so wide.

One of the grabens, stepped on a few times for good measure it seems.

After hiking a ways along the rupture, we came upon two old roads. The first, likely from the 1910’s, showed a nice stepped offset. This was the original alignment of the Trona Road, which was realigned and paved in the 1940’s. This newer alignment was, of course, also offset but the pavement allowed for a better measurement.