- 3534 - EARTHQUAKES - do we understand our future? Global seismology is making progress, with more seismological probes becoming rapidly available, and we hope to constrain some of the key parameters determining geophysical models of the inner core of our Earth in this coming decade.
--------------- 3534 - EARTHQUAKES - do we understand our future?
- Living in northern California is living with earthquakes. Smaller ones are felt or reported on a monthly basis. But further south at the middle section of the San Andreas Fault may have the capacity to host much larger earthquakes..
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- Between the towns of Parkfield and Hollister, the famous California fault undergoes something called “aseismic creep“. Instead of building up strain and then slipping in one earth-rattling moment, the two sections of fault move imperceptibly, releasing stress without causing large quakes.
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- Looking back millions of years in time, researchers have found that this section of fault may have experienced earthquakes of magnitude 7 and higher. That is larger than the magnitude-6.9 Loma Prieta temblor that killed 63 people in the Bay Area in 1989.
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- The San Andreas Fault has three sections. The southern section runs from the Salton Sea to Parkfield, California, and has the capacity for large quakes. In 1857 the magnitude-7.9 Fort Tejon quake shifted the ground at the fault a whopping 29.5 feet.
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- The northern section of the fault runs from the town of Hollister, through the Bay Area up to Cape Mendocino, California. This section of the fault is most famous for the great 1906 San Francisco earthquake, which had an estimated magnitude of 7.9. That is the one we live on in Santa Rosa, California.
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- In between Parkfield and Hollister the fault hasn't given rise to any recorded quakes larger than a magnitude 6. Geoscientists have dug into the fault, looking for signs in the shape of the sediment layers of long-ago earthquakes, and they haven't found any large quakes in the last 2,000 years.
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- Even if the central San Andreas doesn't build up enough stress to start a large earthquake, it could act as a conduit for quakes originating on the northern or southern section of the fault.
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- When a fault slips, it generates friction, which generates heat. This heat can spike the temperature of the rocks in the fault by more than 1,800 degrees Fahrenheit. And those temperature changes can change the structure of organic molecules that accumulate within sediments.
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- Researchers analyzed a sediment core from the central San Andreas that was drilled as part of the “San Andreas Fault Observatory at Depth” (SAFOD) project. Deep in the core, about 1.9 miles down, the researchers found a spot where the biomarkers showed signs of heating.
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- That patch of the fault also consisted of these really highly deformed siltstones, mudstones. It had lots of these small slip layers, so lots of scaley surfaces and shiny surfaces, which is what we would think of as rocks that had hosted lots of earthquakes
This zone of the fault may have hosted more than 100 quakes.
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- The researchers analyzed the quake-deformed section of rock with a method called “potassium-argon dating“. This method takes advantage of the fact that a naturally radioactive variation of potassium, potassium-40, slowly decays into argon gas.
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- When something happens to heat the rock, this gas is released, resetting the "potassium-argon clock" to zero. By looking at the accumulation of argon, the researchers could determine how long it had been since the rocks were heated.
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- The magnitude of the heating indicates that the central San Andreas can indeed undergo a lot of shaking. It's likely that the earthquakes recorded in this section of the fault ranged from magnitudes in the mid-6s to low-7s.
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- The quakes probably started on the southern portion of the fault and sped along the faultline like an unzipping zipper. Knowing that the fault has this capacity is important for understanding the earthquake hazard in central California.
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- The researchers plan to apply the potassium-argon method to other faults, including in the New Zealand bedrock, where there isn't any organic material for traditional carbon-14 dating which works back to 55,000 years, and where there are no sedimentary layers to show the marks of very old quakes.
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- Going beyond earthquakes and much deeper inside the earth we find the Earth's inner core may be filled with a weird substance that is neither solid nor liquid. Scientists believe that Earth's deepest recesses consist of a molten outer core surrounding a densely compressed ball of solid iron alloy.
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- New computer simulations suggest that Earth's hot and highly pressurized inner core could exist in a "superionic state". That is a whirling mix of hydrogen, oxygen and carbon molecules, continuously sloshing through a grid-like lattice of iron.
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- Hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the inner core conditions, showing high diffusion coefficients like a liquid. This suggests that the inner core can be in a superionic state rather than a normal solid state.
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- The planet's core is subject to bone-crushing pressures and scorching temperatures as hot as the surface of the sun. Since the 1950s, advances in the study of earthquake-generated seismic waves, which travel through the core, have enabled researchers to make more refined guesses as to what's inside the heart of the planet.
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- A 2021 study of how a type of seismic wave called a ‘shear (or "s") wave” moved through our planet's interior revealed that Earth's inner core isn't solid iron, as was once believed, but is instead composed of various states of a "mushy" material, consisting of an iron alloy of iron atoms and lighter elements, such as oxygen or carbon.
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- In a regular solid, atoms arrange themselves into repeating grids, but the core simulations suggest instead that in Earth's core, atoms would be transformed into a superionic alloy. This creates a framework of iron atoms around which the other elements, driven by powerful convection currents, are able to freely swim.
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- This constant swilling of the mushy superionic materials could help to explain why the inner core's structure seems to change so much over time, and even how the powerful convection currents responsible for creating Earth's magnetic field are generated.
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- Global seismology is making progress, with more seismological probes becoming rapidly available, and we hope to constrain some of the key parameters determining geophysical models of the inner core of our Earth in this coming decade.
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- So the next time you feel the Earth shaking you are reminded that there is a lot more going on beneath your feet that we still do not understand. Certainly not understood well enough to predict the future. Hang in there.
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April 5, 2022 EARTHQUAKES - do we understand our future? 3534
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