Discovering Ancient SuperQuakes: Why Large Earthquakes Occur and Where

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Today we talk Earthquakes! 
The list of EQ dating back to antiquity is shocking.  The amplitude and deaths caused are estimated, but a glimpse of this list is very sobering.  Below is a list of some of the important and more recent.

• Medieval Europe (italy, spain, portugal) were all very aware of the risks of Earthquakes and were starting to apply new engineering specifically for earthquake mitigation (anchor plates in buildings, exaggerated buttresses). 
• 1700: The Cascadia subduction zone. Research by Brain Atwater has revealed when this EQ occurred.  Estimated to be 8.7 - 9.2 moment magnitude.  Atwater has discovered ghost forests of red cedars that were killed by being lowered and submerged in salt water as a result of a massive EQ.  The tree rings date back to 1699 - the last growing season before the EQ.  Records in Japan confirm a tsunami that caught them offguard.  
• December, 1811 to February 1812: 3 large EQ in New Madrid, MO 8.1,7.8 8.0.  Caused by old stress from a failed rift zone.
• 1906: San Francisco EQ 7.9
• 1964: The Great Alaskan EQ. 9.2 the shaking lasted 4 min and 38 seconds.  The Richter scale put these two as the same strength.  
• 1976: China.  7.6 EQ that unlike the year before, no foreshocks or warnings.  Estimated up to 750,000 people died.
• 2004: Sumatra 9.1  227,000 people died
• 2009: L’Aquila, italy, 5.8, 309 people died; Italian seismologist went on trial for incorrectly reassuring the population that no earthquake would occur. Earthquake was predicted by one researcher a month before, he was called alarmist. Then the earthquake occurred and 7 people (6 scientists and one official) were convicted of giving false or misleading statements that downplayed the risks. This result was criticized by most major scientific organizations and outlets.  These verdicts were later overturned by the Italian Supreme Court 2014
• 2011: Japan 9.0 Almost 21,000 people died

It’s also appropriate to talk about the exponential scale.  The richter scale is outdated and really measures the amplitude and distance the waves travelled.  It’s not accurate in discerning major EQ.  The moment magnitude scale measures the amount of movement along a fault and the force required to make that movement.  It also measures all of the seismic waves produced by an EQ.

The scales vary logarithmically.  In terms of the amount of ground shaking, a difference of 1 varies by an exponent of 10.  But in terms of energy released, it varies by powers of 30 (We round to 30.  It’s actually 31.6.  That means that for each increase in .2, the energy released is doubled) 

Why do Earthquakes occur?

1 - Think back to the plate tectonics episode.  The movement of tectonic plates causes stress along the plate boundaries. 

Use N. America as a starting point (more on the west coast, few on the east coast) but then we compare them to other countries like Germany and the UK (which have very few) Italy (lots of earthquakes with >10,000 killed) and western Pacific (same). 

 
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