Yesterday afternoon, just before 4 pm local time, a M6.0 earthquake occurred at the California-Nevada border. Let’s dive deeper into some of the regional geology on this edition of #FaultFriday.
East of the San Andreas fault, the plate boundary doesn’t stop moving. Even though ~70% of the relative motion of the Pacific-North American plates occurs within the San Andreas fault system, that leaves ~30% to be accommodated elsewhere.
Moving east from the San Andreas fault from ~San Francisco, more plate motion is accommodated at the eastern rangefront of the Sierra Nevada. If you’ve ever wondered why the Sierra look even more impressive from the east than the west, enter: active tectonics.
The Sierra Nevada are pretty impressive because, well, earthquakes. If we think of big scale plates of plate tectonics, and then think a little bit smaller, the Sierra Nevada act as a ‘microplate’ in the larger tectonics picture.
The Sierra are solid, made mostly of volcanic rocks & granite. This not only changes how we feel an earthquake, but also means the Sierra are acting as a tectonic block. This block moves NW at ~12 mm/yr, in comparison to the total relative plate motion of ~50 mm/yr.
The eastern edge of the Sierra Nevada block marks a change in relative plate motion directions. While the Sierra are moving largely north, the block east of the Sierra-the Great Basin-is extending, with plate motion oriented more westerly. Hm. Seems like cause for a traffic jam.
This traffic jam gets worse to the east of the Sierra in the Walker Lane. The Sierra block moves northward comparatively unbothered. Sierra is on a mission in the HOV Lane while Walker Lane is asking, what’s the rush?, sitting in the right-hand lane.
What this means for faulting is: We would expect to see normal & right-lateral faulting here generally oriented north, following the eastern boundary of the Sierra, right where this earthquake occurred.
From seismological observations, a focal mechanism shows two possible fault planes—a north-striking normal/right-lateral fault dipping shallowly to the east, or a south-striking normal/left-lateral fault dipping moderately to the west. Lots of words: let’s unpack a map.
The north-striking fault seems at first to fit the description of the Antelope Valley fault, which is right at the Sierran rangefront. But, given that the hypocenter is currently understood to be west of the east dipping fault trace at the rangefront, that doesn’t add up.
A perhaps better fitting potential culprit is the Slinkard Valley fault, west of the mapped trace of the Antelope Valley. The hypocenter is in the right dip direction here, but is a bit close to the mapped trace given the low dip indicated by the focal mechanism.
This could mean that there is another back-valley fault that was activated in this event, perhaps slightly into the range itself. As more aftershocks occur, illuminating the causative fault, we will have a better sense of the fault plane that actually ruptured in this event.
For more information, be sure to check the event summary page:…. We expect aftershocks to continue. In the case of shaking, remember to Drop, Cover and Hold On!

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More from @USGS_Quakes

9 Jul
Update on the M 6.0 #earthquake that occurred at 3:29 pm Pacific Time July 8, in Antelope Valley, California, near the Nevada border. #califearthquake You can find up-to-date details about the earthquake here:…
Shaking of intensities up to VII was reported nearest the epicenter. The quake was felt all the way to the Pacific Coast and throughout CA’s Central Valley as well as strongly in Reno and Carson City.
Did You Feel It? Report here:…
Our @USGS_ShakeAlert earthquake early warning system sent out some ShakeAlert-powered alerts through phone apps and @Google @Android. Check out… for more information about ShakeAlert’s performance.
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6 Jul
An M5.2 earthquake occurred 12 km (7.5 mi) NNW of Kukuihaele, Hawaii yesterday at 1:34 PM local time. So far >1,300 people have reported feeling shaking. If you felt this quake (or not!), click the link below to tell us about your experience. For more on EQs in Hawaii, see 🧵👇.
Thousands of earthquakes occur every year in the State of Hawaii most of which are small & caused by eruptive processes w/in the active volcanoes on & around the Island of Hawaii, especially in the southern districts where the Kilauea, Mauna Loa & Loihi volcanoes are most active. Image
Other EQs can occur in Hawaii along tectonic faults in the crust & upper mantle & can be related to deep structural adjustments from the weight of the lava-built islands on Earth's underlying crust. This is the most common source of damaging EQs north of the Island of Hawaii. Image
Read 7 tweets
4 Mar
Two large earthquakes have occurred beneath the southwest Pacific today. The earlier M7.3 was strongly felt across much of New Zealand. The more recent M7.4 occurred ~4 hr later, ~900 km away. Given the large distance between them, the events are probably not directly related. 🧵 map showing earthquake epicenters for March 5 in the south P
The @USGS-determined focal mechanism and origin location for the earlier M7.3 near New Zealand suggest that the earthquake resulted from complex faulting within the subducting Pacific Plate.…
@USGS The M7.4 much farther north along the Kermadec Trench has a focal mechanism and origin depth consistent with slip along the subduction interface between the Pacific and Australia Plates.…
Read 5 tweets
2 Oct 20
Yes, swarms aside, it is still Friday, and on this channel, that means #FaultFriday. To learn more about the area around the Westmorland swarm, let’s look at the Imperial fault of southern California today.
The Imperial fault is a right-lateral strike-slip fault ( that runs south of the Salton Sea, across the US-Mexico border, and into Mexico. The Imperial fault had two significant earthquakes in the 20th century, a M6.9 in 1940 and a M6.5 in 1979. Map showing Imperial fault and Westmorland Swarm in southern
Before the 1940 El Centro event, this fault probably did not have an earthquake in the previous 300 years. Geologists can literally uncover the history of past earthquakes along a fault by digging a shallow trench across a fault and observing and dating the offset layers.
Read 6 tweets
1 Oct 20
An #earthquake swarm has kicked off southwest of the #SaltonSea today, producing 240 earthquakes as of 8pm Pacific. The largest earlier this evening was M4.9.
So what does this mean? We’ve put together some scenarios to explain what could happen next.…
The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderate-sized earthquakes (M4.5 to 5.4) may occur. Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.
A less likely scenario is a somewhat larger earthquake could occur (up to a M6.9). Earthquakes of this size could cause damage around the area close to the swarm and would be followed by aftershocks that would increase the number of smaller earthquakes per day.
Read 7 tweets
30 Sep 20
What do subduction zones do? They do everything! They make earthquakes and volcanos and tsunamis. 😸
When one plate slides under another, it sinks into the hotter parts of the Earth and starts a melting process. The melt floats up creating a line of volcanoes (called a chain or arc) like the Cascades volcanoes in California, Oregon, and Washington. Oceanic-continental converg...Map of Cascades volcanosMount St. Helens (left) and...
The process of sliding one plate under another creates earthquakes. When this happens underwater, if the conditions are right, the vertical motion can spawn a tsunami. Stuck area ruptures, releas...
Read 6 tweets

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