Thread by @JudithGeology

How do we measure shaking in an earthquake?

Magnitude measures the size of the quake - but how much shaking you feel depends on how close you are to the fault, the materials under your feet, and more.

Here is a map of shaking in the #TurkeySyriaEarthquake

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Mostly, shaking is decribed by its "intensity". Intensities communicate what the shaking "feels like" and how it could affect structures in the area.

Explore intensities in the Feb 6 M7.8 earthquake here:

earthquake.usgs.gov/earthquakes/ev…

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Intensities are calculated by measuring shaking with seismometers. When shaking is very strong, only certain kinds of seismometers - those designed to record strong ground motions - will work: more sensitive ones clip, or go off scale.

en.wikipedia.org/wiki/Accelerog…

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The most common number seismologists report for shaking is PGA, or Peak Ground Acceleration. This is the highest acceleration the seismometer experienced during the shaking. The units are a fraction of g, the acceleration due to gravity.

en.wikipedia.org/wiki/Peak_grou…

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Since PGA is measured in units of g, if PGA = 1, then the station accelerated sideways as much as it usually accelerates down due to gravity, at least for a moment in time!

PGA can be matched to intensity.

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Matching PGA to intensity is very important.

1st, it allows us to understand the impact of modern events.

2nd, it allows us to interpret historical records of shaking to quantify past quakes. Since earthquakes are infrequent, understanding historical events is critical.

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Why does shaking vary so much in a single earthquake?

The biggest issue is distance. Monday's M7.8 earthquake was caused by slip on a fault that is >250 km long. On average, areas close to high-slip zones shook more than those further away.

earthquake.usgs.gov/earthquakes/ev…

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But the materials beneath your feet can dramatically change groundshaking too - soft sediments amplify ground shaking.

In Southern California, shaking in the basins may be amplified 5x compared to nearby mountains!

pubs.usgs.gov/of/2001/of01-1…

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Site amplification might explain why, in Turkey's M7.8 earthquake, stations within a couple of km of each other recorded wildly different peak ground accelerations.

Here, stations range from 0.37g to 1.37g!

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Geotechnical engineers can assess the potential for site amplification in advance. This is a great tool for reducing earthquake vulnerability, because it side-steps the fact that we can't know which earthquake will happen next - amplification will happen for any earthquake.

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But peak ground acceleration (PGA) on its own isn't sufficient.

There's a lot that PGA does not measure, including

-how long the shaking lasted, and
-frequencies of shaking

These factors can dramatically change the damage of the event.

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The frequency of shaking can have a big impact on building damage, because different height buildings resonate at different frequencies!

High frequencies resonate with short buildings.
Low frequencies resonate with tall buildings.

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iris.edu/hq/inclass/ani…

So the damage can change depending on the shaking frequency & built environment.

Example: In the 2015 M7.8 Nepal quake, there was less high frequency shaking - & because most of the buildings in Kathmandu are short, damage was lower than expected!

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science.org/content/articl…

In contrast, in 1985, shaking from a M7.5 earthquake almost 400 km from Mexico City resonated within ancient lake sediment below the city. The resonance frequency matched that of many buildings, leading to widespread damage.

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nytimes.com/interactive/20…

The shaking in Mexico City also lasted longer (up to nearly 3x longer), because the waves continued to resonate within the basin it sits on!

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nature.com/articles/srep3…

Since frequencies are important, the USGS also reports the Peak Spectral Acceleration - the peak acceleration experienced for different frequencies. This is also reported as a fraction of g, but only represents part of the total shaking.

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earthquake.usgs.gov/earthquakes/ev…

Planning for earthquake safety therefore requires more than just knowing possible earthquake magnitudes. Where are the faults, exactly? How much slip might we see, on which parts? What is underneath the buildings? How will those buildings resonate? How might they fail?

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Every year geologists & engineers make progress on these questions; every earthquake teaches us new lessons.

We need public support to keep learning, and to keep implementing lessons learned.

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