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Dr. Lucy Jones @DrLucyJones
, 3 tweets, 1 min read Read on Twitter
USGS | M 4.4 tonight 7km NE of Aguanga, CA In the pluton between Elsinore an San Jacinto faults. Thousands of tiny quakes have been occurring here for 2 years. earthquake.usgs.gov/earthquakes/ev…
Magnitude is controlled by the area of fault that moves in a quake. These tiny quakes near Cahuilla are moving very small areas and release only a small amount of strain. Caltech's Hauksson has looked for and not found evidence for associated fluid motion.
Many areas have swarms of small quakes. Sierra Nevada, Imperial Valley, Owens Valley but usually shorter duration. Oregon-Nevada border region had a long swarm 2014-15. This Cahuilla swarm is longest duration in SoCal since we've been able to see small quakes (since 1980)
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Now building codes.
A building is built to withstand some shaking - not some magnitude. Any building can withstand a M8 if it is far enough away.
The code says that a building should not collapse in the worst shaking that has a 10% chance of happening in 50 years. 1/4
There is not one standard for all of California. At every location, @USGS calculates what is the expected shaking from known faults and a new building is built for that. A building in San Bernardino has to be stronger than a building in Orange County. 2/4
The building code doesn't try to prevent damage. It just says the building shouldn't kill people. Building "to code" means the weakest building that will not collapse in the pretty bad quake. It gives us disposable buildings. 3/4
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I'm seeing lots of confusion between earthquake magnitude, shaking and damage. Here's a simple outline:
Earthquakes happen because of slip across a fault that produces shaking as one of its effects. The total energy released is the magnitude. One number per quake. 1/7
Every point on the quake's fault produces energy, so the bigger the fault, the bigger the quake. Magnitude depends on the fault length, width, and the amount of slip. Both width and slip tend to scale with length, so you can guess the length of the fault from the magnitude. 2/7
The fault starts to move at a hypocenter and then moves down the fault at ~3 km/sec. A 3 km fault (M~5) breaks in 1 sec. A 300 km fault (~M8) takes 100 sec to break. So the duration of the quake depends on magnitude. Bigger quakes last longer and affect a larger area. 3/7
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1/3: There is increased risk after a big earthquake, but it depends on location. Increase is highest at the fault that produced 1st quake & dies off with distance, time and magnitude. So immediately after a M3 (on a 10m fault) there is a tiny increase right at the site.
2/3: Immediately after a M8 on a 500 km fault, there is a big increase at the fault, that dies off with distance and approaches background around 3-4 times the fault length or something like 2000 km away. The Venezuela earthquake increased the risk in the southern Caribbean.
3/3: The M8.2 Fiji earthquake increased the risk for a section of the South Pacific - and we saw M6s in Indonesia, Vanuatu and New Guinea. The increase in number of quakes you saw does have a pattern - it just doesn’t extend to the US.
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M 4.4 - 11km NNE of Decatur, Tennessee earthquake.usgs.gov/earthquakes/ev…
These colored lines show the intensity of ground shaking from the #earthquake. How hard the ground shakes is related to the magnitude of event, the distance from the event and the local geology earthquake.usgs.gov/earthquakes/ev…
So far, 6000+ people have reported feeling this #earthquake via the @USGS. Did you feel it? Let them know 👉 earthquake.usgs.gov/earthquakes/ev…
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(1) #Earthquake near #Anchorage, Alaska in the last hour.

Moderately deep & strong, expect some damage, but loss of life unlikely. Magnitude is usually downgraded from initial level.

Best info source:

earthquake.usgs.gov/earthquakes/ev…
(3) Some photos from around Anchorage that I've seen on Twitter. In New Zealand we have an earthquake of this type about once or twice a year. It is scary, but thankfully little damage is done.

#DropCoverHold is the best advice. More: ready.gov/earthquake
(4) If you feel an earthquake when you're on a beach or within a few miles of the coast, be aware of #tsunami risk.

"If it's long or strong, get gone." There is no time for warning sirens or messages.

If you don't feel it, that means any tsunami is hours away. You have time.
Read 36 tweets
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Good morning from a UK with slightly less dull weather! I want to discuss two topics which are often confused & misunderstood, but first I wanted to clarify a few things from yesterday's threads #earthquake #Stress #faults 1/n
Referring back to the block diagram for dip-slip faults from yesterday, @CriticalStress_ noted that the stress arrows at the top were wrong. All faults have a confining (inward) compressional stress, but stresses from other directions can be larger 2/n
This diagram from @SEELeeds shows that in the example block diagram I gave the smallest stress (sigma-3, min) is inward perpendicular to the fault. The primary stress - the largest amount of force acting on the rock to produce a normal fault - is vertical: GRAVITY! 3/n
Read 31 tweets
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(1) I will look into the details of this earthquake, since depth and location affect how much damage it did.

The last thing the starving, oppressed people of Venezuela need is a major earthquake.
(2) The first reports of an earthquake's magnitude always gets downgraded about 15-20 minutes later. Currently this one looks to be about 6.8.

Crucially, it's quite deep, at 76 miles (123km).

The USGS website model estimates 10-100 deaths & USD $10 million to $100 million.
(3) If this had been a shallow earthquake, the carnage would be massive. Thankfully it was deep.

There will be many aftershocks. People will be very afraid.

Here's the USGS impact modelling:
earthquake.usgs.gov/earthquakes/ev…
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Shallow M7.2 #quake strikes north of Lombok, #Indonesia, according to @USGS.
M7.0, according to #Indonesia government agency. bmkg.go.id/warning.bmkg
Revised to M6.9 by @USGS after seismologist review. earthquake.usgs.gov/earthquakes/ev… #Lombok #Indonesia #quake
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M8.2 #quake reported in South Pacific. earthquake.usgs.gov/earthquakes/ev…
No #tsunami threat per @NWS_PTWC: tsunami.gov/events/PHEB/20…
Update on the South Pacific big #quake:
Read 4 tweets

Trending hashtags

#tsunami #yellowstone #Harvey #UAE #Anchorage #RealTruth #AS #Pluton #AIQ #Bali #MisspellingsCount #marr #AstroturfersGonnaAstroturf #faults #Hurricanes #usgs #TheStormIsHere #HurricaneSeason #here #Maranatha #PrayforLombok #TheBeginningIsNear #FreePress #DefyHistory #EarthQuakes

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