How do scientists develop an understand of fault behavior, and estimate the average long term rate of earthquakes on faults? The most direct way is paleoseismology: digging trenches (typically) across faults to identify evidence of historic and prehistoric earthquakes
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https://twitter.com/pm_figueiredo/status/1633465678995812352

Paleoseismology dates back to the 1970s or thereabouts, with early work by McCalpin, Sieh, and others. Like the rest of geology it was initially a (white) man's game. But as .@pm_figueiredo says, the playing field has changed.
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So what's the deal with site response? Is earthquake shaking stronger in places underlain by sediments, or not? The answer is, it's complicated. To deal with this, seismologists have developed empirical site factors -- empirical meaning derived mostly from observations. These site factors go with modern models that predict shaking as a function of magnitude and distance. Site factors predict local effects as a function of shaking amplitude, sediment properties, and frequency of shaking.

The .@EarthScope_sci animation that Judith retweeted is a terrific illustration of basin response & site response: earthquake shaking is amplified by layers of "soft sediments." My 1989 Nimitz Freeway study turned into a textbook example.
But wait, there's more...
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https://twitter.com/JudithGeology/status/1631790271032709121

Turns out that, at strong shaking levels, sediments don't always transmit energy effectively -- in the parlance they behave nonlinearly. Soft sediments can slump or produce sand blows, grossly nonlinear behavior that can exacerbate damae
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The late Nicholas Ambraseys, whom I regard as the godfather of modern historical seismology, talked about "zero-intensity collapse" of structures and landforms
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https://twitter.com/NWSLosAngeles/status/1631336133752918016

The point being, observations of landslides/rockfalls and even structural collapse during earthquakes are not reliable indicators of shaking severity because these failures can occur without any earthquake shaking. Even weak shaking can be the straw that breaks the 🐫's back.
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Earthquake prediction has been called, by people including me, the holy grail of seismology.
In fact the holy grail is more prosaic, and more attainable: understanding how the ground will shake in future earthquakes so buildings and infrastructure can be built appropriately
A 🧵 Forecasts of shaking from future earthquakes, what we call ground motion models, are one of the key ingredients for hazard maps, which in turn inform building code development
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Did you know that the USGS now issues operational aftershock forecasts for significant events? Start with the event page, e.g., URL below, and scroll down to the aftershock forecast link. These forecasts are based on well-established aftershock stats 1/
earthquake.usgs.gov/earthquakes/ev… although, like snowflakes, no two aftershock sequences are entirely alike, so forecasts can only give statistical likelihood. Initial forecasts use average parameters for a region, but are updated over time based on how productive a given sequence is 2/

Aftershocks commonly bunch up near the ends of an initial earthquake rupture, in this case suggesting that yesterday's M7.7 broke about 200 km of the plate boundary. Statistically, if another significant quake occurs, it is most likely to be near the most active aftershock zone. That's what happened in Nepal in 2015: The Dolakha aftershock occurred near where the Gorkha rupture ended to the east, in an area that lit up with aftershocks right after the mainshock.

Let's talk about earthquake shaking. Ppl who feel earthquakes sometimes talk about "rollers" versus "shakers." The shaking you feel in any earthquake depends to some extent on geology: if you live in a valley, you tend to feel waves sloshing, or rolling. But shaking also depends on earthquake magnitude and distance. Earthquakes release energy with a range of frequencies, like music has a range of tones. The bigger the quake, the stronger the booming low tones. That's thing one.

*THREAD* Okay let's talk about magnitude. For non-specialists, the important point is that magnitude reflects overall earthquake size, whereas the shaking that you feel at any given place depends a lot on what happens to waves after they leave the fault. The question is, what do we mean by "size." The seismology community has adopted moment magnitude as the "best measure" on an earthquake size; whether we say so or not, we're almost always reporting moment magnitudes, at least for big earthquakes.