The View of Japan's 8.9 Earthquake from Modesto, California

First off, I am sorry for the quality of the photographs. We have a seismometer, yes, but it is a simple teaching model, and the computer program is something like 15 years old (and has never been updated), so the system barely works at all. But it was working well enough to record the shaking of the ground in Modesto, California for several hours after the 8.9 magnitude earthquake in Sendai, Japan. The quake, if the 8.9 magnitude estimate remains unchanged, is the 5th largest ever recorded (I'm hearing that it might be upgraded to 9.0 or 9.1). It has killed hundreds and unleashed a horrific tsunami that has spread across the Pacific Ocean basin, causing damage even in Crescent City and Santa Cruz, California. The simple pendulum-based seismometer records ground motions in my geology laboratory on the campus of Modesto Junior College.

The first photo (above) shows the onset of the waves from the quake. Earthquakes produce a series of different waves which move in different ways, and at different velocities. The first waves arrived in Modesto at 9:58 PM local time, about 12 minutes after the quake began shaking about 80 miles offshore of Sendai, Japan (that is if the computer clock is right, not a safe assumption actually). These are Primary Waves, compressional waves that are analogous to sound waves. Obviously they travel faster than the other waves. They are an example of body waves, those which travel through the earth, not just on the surface. The second set of waves that would have arrived were Secondary Waves, a slower set of body waves that are generated by shearing motions. The closest analogy I can think of is a whipping motion, like that of shaking a stretched "slinky" up and down (this is the demonstration I use in classes). The record on the seismograph doesn't allow me to pinpoint the S-wave arrival.

A third group of waves arrive several minutes later. These are surface waves, and as their name suggests, they don't pass through the earth, they travel at the surface. One type of surface wave is analogous to ripples on a pond after a pebble has been tossed in. The surface of the water rises and falls, but fish in the water do not bob up and down. They are unaffected by the passage of the ripples. A second form of surface wave shakes side to side rather than up and down. Surface waves have a lot to do with the damage caused by earthquakes, given the way that energy is concentrated at the surface, and the distortions of the ground caused by the waves. Surface waves continued to be recorded at Modesto Junior College for hours after the quake (as well as reflecting and refracting body waves). In the picture above, 26 minutes had elapsed, and the monitor was off the scale for minutes at a time.

The seismometer is automatically set to record a two hour time period after being tripped, and at the end of two hours, the waves were still immense (above). They probably continued for three or four hours. It's important to realize that the earthquake didn't last this long. From what I've read, the shaking went on for around three minutes at Sendai, Japan (and that was more than enough to cause terrible damage). If the peal of a bell is an earthquake, the waves being recorded at Modesto are the reverberations of the bell. They waves travel back and forth around the planet for many, many hours. The refraction and reflection of these many waves provide seismologists with a picture of the Earth's interior.

A note about the magnitude scale: it is open-ended scale that originally measured the size of the biggest wave to determine if a quake was big, medium or small. It is not a 1-10 scale as is often stated in the media, although rocks can only store enough seismic energy to produce about a magnitude 9.5 event. A giant asteroid impact could produce a much larger event, well beyond a ten. The media often report that a rise of one magnitude represents a quake that is 10 times larger, but this is not exactly accurate. For smaller quakes, the increase of 10 times represents a 10-fold increase in the size of the waves. The waves of quakes at around magnitude 6.5 don't get higher, though, so the magnitude scale loses meaning.

Quakes obviously get bigger, though. The total energy of a quake increases by a factor of just over 30 times per magnitude number. Thus, a magnitude 7.0 like the one that devastated Haiti last year was 1/30th the size of a magnitude 8 quake, and roughly 1/1,000th the size of the Sendai quake. The horrible death toll of the Haiti quake (despite the smaller energy release) was the result of many other factors, such as the shallow nature of the quake, and the location of the focus directly underneath an urban area. The Sendai quake was 80 miles offshore of Japan.

I am almost always disapointed in the commentary of news readers on the cable networks. A few minutes of education would make a world of difference during fast moving events like the Sendai quake. Wikipedia has a reasonably good description of how magnitude is measured. And by the way, I am not the world's greatest authority on seismology, and I am open to correction if I got some of this wrong! That's what the comments section is for...

Postscript: Silver Fox at Looking for Detachment has a list of the geoblogosphere's response to the Japan earthquake here.

Update: Both the USGS and Japanese geologists have upgraded the magnitude of the quake to 9.0, making it the 4th largest ever recorded.

8.9 Earthquake in Japan

We are learning more about a massive quake in Japan, with an estimated magnitude of 8.9 offshore of Sendai, Honshu. A major tsunami has been generated and the cable news shows are showing aerial shots of an extraordinary wave spreading across agricultural fields, overwhelming roads and villages. From the maps, it appears that the quake was generated in the subduction zone on the Pacific side. There is a good chance that a tsunami could be spreading through the Pacific, but I have not heard any confirmation. I would only say that if the authorities issue a warning, take it seriously and get to a safe place.

The most stunning aspect so far has been the aerial perspective of the tsunami spreading across the landscape. The 2004 tsunami in Indonesia was documented better than any previous event, but I don't recall seeing any aerial shots like this. Tsunamis are not really waves, they are surges of water that rush inland at speeds sometimes approaching 100 mph. The surge picks up debris and uses it as a battering ram. It is terrifying to see.

The TV commentators are starting to get up to speed, but for the best information from seismologists, check out the U.S. Geological Survey here. They have links to the current tsunami conditions in the Pacific basin.

My night class watched the 7.2 quake on Tuesday on our teaching seismograph, and I assume it was a foreshock to this event. Ironically, the subject of the day in my physical geology class was the types of damage caused by quakes. We mentioned tsunamis...

I pray for those in the path of the devastation.

UPDATE #1: A tsunami watch has been issued for the Hawaiian Islands. If a wave has been generated, it will arrive about 3 am Hawaii time.

UPDATE #2: The tsunami watch has been upgraded to a tsunami warning in Hawaii. Please take it seriously. The waves, if any, will reach the coast of California and Oregon around 7-7:30 am local time. At that distance, I don't expect much, but things can change overnight.

UPDATE #3: If 8.9 magnitude becomes official, this is the 5th largest quake in the world since 1900. Chile 1960, 9.5; Alaska 1964, 9.2; Sumatra 2004, 9.1; Kamchatka, Russia, 1952, 9.0. I finally hear an MSNBC commentator get the energy numbers right: the difference in energy between this and the Haiti quake at 7.0 is closer to 1,000 times, not 100 times. The equivalent of 1,000 Haiti quakes just hit offshore of Japan.

UPDATE #4: A tsunami watch has been announced for the coast of California. Take it seriously if you live in coastal areas. If it comes, it will arrive around 7:30-7:45 am in the morning. Listen for official warnings and obey them! Don't do the California thing and wander down to the beach to look for it.

UPDATE #5: The Pacific Coast is now under a tsunami warning, with expected wave heights of 3 feet or so, arriving around 8 in the morning depending on location. Take it seriously; if you are on the beach, you could easily be swept away by such waves. Follow official warnings; emergency personnel know what they are doing!

UPDATE #6: Some damage is now being reported in California as surges of water hit Crescent City and other coastal towns. Damage is "significant", not major, mostly along the lines of boats being jostled and ripped from moorings. I'm hearing of waves 5-6 feet high along parts of the California coast, and of 8 foot waves in Hawaii. 11 people were killed at Crescent City in 1964 from a tsunami of 12 feet, in part because they didn't realize the waves come in groups, and the first wave is not necessarily the largest. It is a cautionary tale for all the people rushing out to the beach to "see the tsunami". It might not be a good idea to become, literally, part of the story. Stay out of coastal areas until an "all clear" signal is given.

UPDATE #7: Silver Fox at Looking for Detachment has a list of the geoblogosphere's response to the Japan earthquake here.

UPDATE #8: Both the USGS and Japanese geologists have upgraded the magnitude of the quake to 9.0, making it the 4th largest ever recorded.