The Trembling Earth

now at blogs.agu.org/tremblingearth

Tag Archives: seismology

40 years of earthquakes on one seismogram

Want to see the pulse of the Earth?

Last year marked the 50th anniversary of the Albuquerque Seismological Laboratory (ASL), a facility in the quiet mountains of New Mexico that is used by the USGS as a hub to maintain both the instruments and data transmission capabilities of the global (GNS) and national (ANSS) seismic networks. The ASL is used to develop new seismograph technology and to test and calibrate instruments in the absence of urban noise and in close proximity to several long-running reference stations.

In honor of the golden anniversary in June 2011 the Incorporated Research Institutions for Seismology put together the [nearly] continuous record of seismograms from these long-running stations in a plot that’s simple, beautiful, and fascinating. I’m going to hang this poster somewhere.

40 years of global earthquakes as recorded at a site outside of Albuquerque, New Mexico. Check out IRIS’s website for the full size version and info about how it was created.

You can read how the plot was generated and details of the more prominent earthquakes at IRIS’s website, but there are some striking things the keen observer will find right off the bat, at least after a brief orientation:

Each of the 465 lines represents a single month of seismic record, squished to fit on the page. The numbers at the bottom represent days of the month, and for months with fewer than 31 days the tail ends of the lines (the final 1-3 days) are filled with zeroes, making them flat lines. Read it from top to bottom and left to right like a book, and you’ll see a rich mixture of small local or regional earthquakes sprinkled among large, long-duration teleseismic earthquakes–ones of substantial magnitude that occurred far around the Earth but were recorded at these stations in Albuquerque thanks to the immense energy they released.

The two largest events are conspicuous in the final years. These of course are the M9.1 Sunda megathrust earthquake that hit Sumatra, Indonesia in 2004, and the M9.0 Tohoku earthquake that hit Japan last year. The bottom right corner is full of other megaquakes, mostly centered in Indonesia, save for the M8.8 Maule, Chile earthquake of 2010.

Pick your most significant historical earthquake from the last 4 decades and try to find it on this plot! Or just marvel at this continuous record of our rocky planet’s pulse.

Advertisements

Musings on seismology and earthquake hazard

A seismogram of the M6.8 “Nisqually” earthquake that struck central Washington state in 2001.

Phew, I’ve been a neglectful blogger… probably to the benefit of my dissertation research. I’ve got a few interesting posts in the pipeline, so I think I’ll take what time I can to finish them, then set them up so they’re nice and distributed for you to read.

In the meantime, you should all check out two cool blogs maintained by earthquake aficionado Arne Christensen, in which he compiles media and eyewitness accounts from two of the U.S.’s most notable contemporary earthquakes: Loma Prieta and Nisqually.

Arne recently asked me a few pressing questions he had about seismology and seismic risk on the U.S. west coast. The questions were very provocative and very relevant, so I crafted some responses that were as informative as I could make them without stepping on anyone’s toes. They’re good distillations of my thoughts on earthquakes, and I’ll vouch for my own synopsis of current earthquake research (although I advise anyone seeking formal advice to seek more direct sources, like the USGS, the Pacific Northwest Seismic Network, and contributing researchers). Caveats and citations aside, I think you should all go have a read of my “interview” for my personal take on modern seismic hazard “hot topics,” then you should peruse the wealth of accounts Arne has collected of recent significant quakes.

Part 1: Living with Earthquakes on the West Coast

http://nisquallyquake.wordpress.com/2012/04/27/living-with-earthquakes-on-the-west-coast/

Part 2: Seismology Issues and Changes in the State of the Art

http://nisquallyquake.wordpress.com/2012/04/29/the-work-of-seismology-explaining-a-few-issues-and-changes-in-the-state-of-the-art/

I welcome any discussion of the points I made, and any corrections by more credible seismologists than myself.

Japan quake felt over >2500km radius

One of the many remarkable features about this planet’s largest quakes (like the one that just happened in Japan) is their truly global effect. Be it the tsunami or seismic waves perceptible and imperceptible, most parts of the planet have been touched significantly by the 8.9.

Let’s start with perceptible seismic waves, i.e., ones people don’t need sensitive instruments to detect. The elastic energy released by this earthquake was enough to set the ground rippling at frequencies and amplitudes people can percieve at distances of greater than 2,500 kilometers (over 1,500 miles).

Shaking intensity as reported online by citizens of east Asia. Data points are colored by intensity and sized by city population. The USGS Did-You-Feel-It page (click image for link) contains the original data and plots of response time and attenuation with distance.

Note the felt reports in Taiwan and Beijing. This affected radius is quite comparable to the huge swaths of the globe that felt both the M8.8 Maule, Chile event last year (most of South America) and the 2004 9.1 Sumatra, Indonesia quake (most of southeast Asia and India):

Felt reports from the other M~9 earthquakes that have occurred since the advent of the USGS's Did You Feel It surveys.

Not only did the perceptible shaking stretch far around the globe in all of these cases, but sizeable elastic waves detected only by sensitive seismometers rippled through the solid crust (and bounced around deep in the mantle), observably circling the globe several times.

Seismic events larger than M~6.5 tend to be recorded globally on broadband seismometers–those that can detect a broad range of frequencies, from sharp local jolts to the long, slow undulations of large, distant quakes. I’ll have another post on seismic waves and attenuation later, for now suffice it to demonstrate the following: Below is a seismic record from northern California showing waves from the 7.2 “foreshock” in Japan on March 9.

Northern California seismic recording of a 7.2 in Japan on March 8 (9th in Japan). Read this like a book; each line represents 15 minutes of recording, so where the oscillations last >15 minutes the records overlap, making it a bit messy. Basically, the ground oscillated detectably in California from the distant quake. The amplitude of ground motion is small, and the frequency of the largest waves is 1 per ~30 seconds, meaning it takes the ground 30 seconds to oscillate one way then back the other, slower than your bedroom fan... no wonder we can't sense it without seismometers.

Compare that to the Mar 11 M8.9 in virtually the same location recorded on the same instrument:

Magnitude 8.9 earthquake in Japan recorded in northern California. The amplitude of ground motion is literally off the charts, but since the frequency is relatively low, we couldn't feel the waves passing though they certainly were.

Now, check out yesterday’s:

March 11 seismic record section from northern California

At the top of the record section is the messy, high-amplitude disruption from the initial passage of the 8.9’s surface waves. Just before 12:00 UTC you can see the dense green squiggle that represents the arrival of seismic waves from a 7.1 aftershock in Japan, with the early high-frequency waves superimposed on the low frequency oscillations continuing from the 8.9.

Several large Japanese aftershocks are apparent throughout the next few hours (in the low- to mid- M6 range, represented by occasional dense, high-frequency squiggles). The record in the background continues to undulate irregularly as elastic waves reverberate through the Earth, bouncing off of the dense metal core, refracting and reflecting off of every boundary or irregularity they come to. Imagine someone jumping into an undisturbed swimming pool–or try it yourself!–and watch as the initial coherent rings of waves bounce off the walls and turn the pool surface into a sloshy, ripply mess. That’s what has happened to the planet.

At around 02:00 UTC another extended period of oscillation begins. This likely represents the (much-diminished) seismic waves from the original earthquake re-circling the globe. Hopefully someone with more seismology expertise than myself will soon put together a nifty stacked plot like this one from 2004 illustrating this repetition.

Of course, the tsunami still has the Pacific ocean sloshing about, roiling in turbulent currents where it reaches the shallow topography of coastlines.

You can see just how turbulent and dangerous this ocean-wide surge of water can be in this video from the coast of northern California, taken as the tsunami arrived. Pay attention to the speed with which the water rises and falls.

Many thanks to Aron Meltzner for doing some superb video sleuthing and pointing me to many of the tsunami clips. I may post a compilation of more footage from all around the Pacific in the near future.

%d bloggers like this: