This week, we’ve seen an uptick in aftershock frequency following the May 15, 2020 M6.5 Monte Cristo event. You may be wondering: which fault is rupturing in these events? Spoiler alert: there are several. Buckle up for this twisty turvy edition of #FaultFriday.

Let’s look at this Google Earth satellite image with known and mapped active faults in the area around the Monte Cristo earthquake. The red lines are the faults, and the yellow star is the epicenter of the Monte Cristo event.

No, your eyes aren’t fooling you. The star doesn’t plot on a fault. Wait…what?

Here’s the twist: this earthquake wasn’t on one big, known fault. This earthquake showed itself on the surface as lots of little cracks.

This is what one such crack that together makes up the earthquake rupture zone at the surface. The first image shows what geologists saw in the field hiking around this rupture, then the arrows point to the crack, and then the line *really* shows the location of the fault.

Here’s a close-up view of a different crack. Jagged, right?? This crack fractured through an interlocked mosaic of pebbles on the surface, called desert pavement. It’s hard, very hard. LeBron could break ankles on this stuff.

You may not be impressed, thinking…"THIS is all the Monte Cristo earthquake did?” … yup, tiny but mighty indeed! The landscape was littered with these small faults, all moving together to move the Earth. Teamwork makes the (tectonic) dream work.

Determining which fault, or faults, ruptured below the surface in the Monte Cristo earthquake is still an open research question. USGS and other scientists are hard at work trying to figure this out using seismological data.

Ok so: puny cracks at the surface and not on a previously-mapped fault. Does that mean there haven’t been earthquakes here before, or that this is a new fault? Not exactly. This may mean that the faults are cozy and covered up by sediments eroding off neighboring mountain ranges.

Or, maybe every time there is an EQ in this system, the expression at the surface is so small, it barely leaves a mark in the landscape. These small cracks observed immediately after the Monte Cristo EQ in mid-May were already eroding from wind and other weather by early June.

But WHY?!

Why do earthquakes happen in this part of Nevada?

You’ll have to wait until next #FaultFriday to learn more about the faults of Walker Lane. -