Muddy Waters

Do you know Muddy Waters?  He was a famous Blues Musician, largely of Chicago in the 1940’s and 1950’s, but finishing strong through the 1970’s.  “Hoochie Coochie Man” and “I’ve Got My Mojo Working” his hand penned. 

For today’s blog,  Muddy’s song  -- Rollin’ and Tumblin’ --  might best fit.

Seismologists like myself are always asked, “When will the next ‘big one’ strike?” Lacking a crystal ball and (as yet) any observable iron clad earthquake precursor, we resort to indirect stabs at reply.  One accepted approach logs the occurrences of past ‘big ones’ and capsulizes the result something like this –

“Previous large earthquakes have happened here on average every XX years plus or minus YY years.”  

If YY is not too large compared to XX, we can usually quell the question with a response based on probability –

“There is a QQ percent chance of this area experiencing a large earthquake in the next NN years”. 

 Nothing wrong with this process but truth to say, to pin XX and YY down to the extent to make QQ meaningful, we need lots of  ‘big ones’ in that log book – about ten really.  In the instrumental era of the last century and half we might have one or two ‘big ones’ in any place, but more often we have none. 

Where earthquake faults are on land and accessible, it might be possible to “dig in the dirt” to expose the slippage and recover information on pre-historic earthquakes. Such paleo-seismic data extends the log.

In other places like the Pacific Northwest, traditional paleo-seismic digs are impossible because the master fault of the Cascadia Subduction Zone lies tens of km below the surface.  There, scientists have to look to the seabed to find traces of past quakes.  Yes --- “Muddy Water” takes stage here in the form of turbidity flows and turbidite deposits.

Turns out that big earthquakes in Cascadia shake loose quantities of silt and sand from the steeper parts of the continental slope offshore. This material runs out for tens or hundreds of kilometers in systems of submarine canyons as muddy flows or turbidity currents. Bits of each current are left behind as the flow passes and due to scant erosion in the deep ocean, these residues are preserved in a sequence of thin layers. By taking cores of the seabed, experts can recognize and date them. Because each layer stands proxy for a major quake, turbidite analysis telescopes the earthquake log back many thousands of years and helps increase faith in that “QQ” above.

Please take a look at this video.  It explains the process fairly well. 

So, thanks to Muddy Waters, seismologists can quantify the probability of Rollin’ and Tumblin’ in the Pacific Northwest.  He’d be proud.

Steven N. Ward   Santa Cruz