Mw 6.4 and 7.1, Ridgecrest, Southern California earthquakes

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davenn

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The first decent SoCal quake for some years.
A Mw 6.4 approx 200km (~130 miles) NE of Los Angeles, in the Ridgecrest area.


This was a shallow crustal quake @ 10.7 km depth

the seismogram from my recorder …..

190704  UT M6.2 offshr BC, 6.4 sthrn California zhi.jpg


The first event is a Mw6.2 offshore British Columbia, Canada
The lower event is the Mw6.4 from California


Cheers
Dave
 
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Tom.G

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I live about 160 miles (edit: that's road miles. 130 air miles) from that and felt it at 10:35AM. I'm in an apartment building that is a 100 year old converted church (foot-thick concrete; had a major rebuild after the big quake here in 1933). It was a long rolling motion here as is typical of distant quakes. I was sitting at the computer when it struck. The initial displacement (from the first shock on the seismogram?) was mild, and I wondered if I was imagining something or having a stroke.

While I was sorting that out, the second wave hit and made things obvious. The maximum movement was perhaps an inch, all horizontal with a period around one second. Total duration here was about 6-10 seconds with a steep fall-off in amplitude.

Cheers,
Tom
 
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davenn

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It was a long rolling motion here as is typical of distant quakes. I was sitting at the computer when it struck. The initial displacement (from the first shock on the seismogram?) was mild, and I wondered if I was imagining something or having a stroke.

cool, thanks for the first hand report, Tom :smile:
 

davenn

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davenn

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A seismogram of the Mw7.1 just before it starts overwriting itself

1 hour's worth of recording

190706  UT M7.1 Southern Calif zhi sm.jpg
 

Tom.G

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Hmm... some folks here in Southern California refer to this as "Another Shake-n-Bake season." Many know there is no correlation or association, but it may relieve a bit of psychological stress.
 

jim mcnamara

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@davenn How does this mess show up your seismograph? The USGS map is pretty fuzzy, - due to limits of resolution?

246193


"This isn't going to stop in the near future," he said. "The aftershocks, they haven't slowed down since the 7.1 (magnitude earthquake). For a period of time there was constant vibration."
 

Klystron

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I felt the Friday night 'quake in southwest Las Vegas valley; nearest landmark Bishop Gorman high school https://www.bishopgorman.org/directions

While performing seated leg stretches in a ground floor condominium, the chair started rocking and dipping followed by the floor rippling slightly. As stated by members in Southern California, the 'quake felt distant, lasting ~1 minute. A digital clock read 20:21 PDT as the sensation dissipated.

No outages or damage in my area that I know. Reported observation on USGS website.
 

Astronuc

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I caught some the news on CNN. They had Caltech seismologist Lucy Jones, who mentioned a 50/50 chance of a stronger earthquake within a week, and it was less than two days later.


CNN had one of their meteorologists (!) discussing talking about it and he was getting stuff wrong, e.g., first saying 8 km depth was 10 miles (it's 5 miles), which he later corrected. Then he went on about how unusual the number of aftershocks were. USGS reports earthquakes in US 2.5 mag and above, while they typically use 4 mag and above outside the US. Obviously, we seen earthquakes with numerous aftershocks or small tremors here on PF. Take a look at the earthquakes in Canterbury/Christchurch area in NZ.
 

nsaspook

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I caught some the news on CNN. They had Caltech seismologist Lucy Jones, who mentioned a 50/50 chance of a stronger earthquake within a week, and it was less than two days later.

This is a clip starring Dr. Lucy Jones from a few years back. Love it.
 

davenn

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@davenn How does this mess show up your seismograph? The USGS map is pretty fuzzy, - due to limits of resolution?
Being on the other side of the Pacific, I only recorded the Mw's 6.4 and 7.1, so the other much smaller events
didn't have any effect :smile:

A zoomed in version of the map that @jim mcnamara posted above. I have annotated it and only included
the M4.6 and above events ......
( Sorry for the poor quality, I had to heavily compress this for PF)

190704-10 UT Mw6.4, 7.1 Southern California events.jpg


Ridgecrest is just south of that "A" shaped runway feature

cheers
Dave
 

Astronuc

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Crack near Ridgecrest visible from low earth orbit.


Another interesting observation that I need to verify
A recent state-by-state ranking by the U.S. Geological Survey showed Nevada fourth in seismic activity, behind Alaska, Wyoming and Oklahoma, and just ahead of California.
from
 
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Astronuc

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Update: Magnitude 7.1 Earthquake in Southern California

An interesting plot of magnitude as a function of time

I'd love to see one in 3D as a function of time.

Also -
** The URL https://earthquake.usgs.gov/ will continue to bring folks to the EHP homepage, and near-realtime earthquake information will continue to be available at the same URL as always. Most other webpages will be migrated into the new U.S. Geological Survey website at https://www.usgs.gov/. We’ll provide temporary redirects to the new website from all the webpages with new URLs.**
 

davenn

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BillTre

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Here is a NY Times article on these two earthquakes and 1,000's of other earthquakes that followed them.
This article has some great visualizations, showing the location/size of earthquakes on a map, during and after the recent two big ones, rolling out in the order that they occurred in.
The earthquakes are shown on a map of the affected area, as well as a larger scale version for the whole state (not showing the size-to-strength relationship).

Here is a non-motion screen capture of the graphic:
Screen Shot 2019-07-20 at 11.59.44 PM.png


Watching these earthquakes appear on the maps, makes one think of how earlier quakes locations might be triggering later ones.
The article says:
“Once a large earthquake has happened, the odds of another large earthquake are as high as you’re ever going to see,” said Susan Hough, a seismologist at the United States Geological Survey.

That’s because powerful earthquakes often trigger more quakes nearby. Each of the thousands of aftershocks triggered by the July 5 earthquake has a tiny chance of being strong but is much more likely to be weak. More aftershocks mean more rolls of the dice, increasing the odds that at least one aftershock will be severe.
One thing the visualizations did not show was the direction of movement where the quakes were located (little arrows on the circles would have been informative for example).

To me, it looks like as the earthquakes occur (presumably releasing crustal tension) the tension might build up in neighboring areas, where (there might be an increased likelihood of) another earthquake being occurring, releasing its local tension.
The area of these two quakes looks to me like a zone of interacting fissures getting mashed between the neighboring plates, kind of like a bunch of rubble being pushed around between tow large blocks.

How does this compare to "normal" geological thought?
Is there deeper research on this? I have not heard of it.
 

davenn

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Watching these earthquakes appear on the maps, makes one think of how earlier quakes locations might be triggering later ones.
How does this compare to "normal" geological thought?
What you need to know/understand is that the rupture lengths of the 6.4 and 7.1 rupture areas are not
limited to within the area of the circles on the mapped faults.
In particular, rupture length of the M7.1 would have been from the Y junction just south of where it is
located on the map and right along to the northern end of the aftershock zone.
The aftershocks then occur along that ruptured length of fault and within the region immediately
surrounding it. You can see how there were other smaller faults that ruptured as part of the stress
field realignment.
The M6.4 ruptured the faults in the southern quarter of the zone. The M7.1 the top three quarters but
note that it extended to the SE through the M6.4 area as the full length of the main NW/SE trending
fault unzipped.

6.4 rupture area.jpg



The M6.4 was primarily 2 unilateral ruptures
1) mainly on the couple of faults that run SW to NE
2) and then in an area to the NW .... this initiated the M7.1 which was a bilateral rupture
centred on where the M7.1 circle is located

Complex ruptures like this are quite common. There's been 2 major quakes in New Zealand in recent
years where many smaller faults close to the main fault ruptured.


Dave
 
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