4 June 2013

Return to Tohoku – Taking a big quake’s temperature

Posted by pweiss

By Paul Gabrielsen

Guest blogger Paul Gabrielsen, a former hydrologist and current graduate student in science communication at the University of California at Santa Cruz, has been tracking the progress of an experiment off the Japanese coast, where scientists recently took measurements from deep inside the fault that lurched and brought destruction to Japan in March 2011. Stay tuned for future posts as the scientists further analyze their data and tell Paul more about what they’ve learned.

There’s a hole in the bottom of the ocean near Japan, the deepest ever drilled for science, that leads to the heart of one of the world’s most dangerous faults. Two years ago this

Patrick Fulton, a seismology researcher at the University of California, Santa Cruz, holds temperature sensors retrieved from an earthquake fault more than seven kilometers under water to learn more about the devastating March 2011 Japan quake and tsunami. Photo by Paul Gabrielsen.

fault, which marks the spot where one tectonic plate grinds past another, unleashed the Tohoku earthquake and subsequent tsunami, which devastated the east coast of Japan. The earthquake’s power astonished geologists, who didn’t think the fault was capable of such destruction. So why such a massive quake?

To find the answer, Patrick Fulton of the University of California, Santa Cruz (UCSC), and an international team drilled through more than 800 meters of rock, seven kilometers beneath the waves, to take the fault’s temperature. When plates slip during an earthquake, the friction of rock sliding past rock creates heat. Bigger earthquake, more heat. Fulton, a UCSC seismology researcher, measured the leftover heat in the fault with temperature sensors, strung along a rope like pearls on a necklace. He placed the sensors in the well last summer and sailed away, hoping the fault wouldn’t slip again and snap the string in half. (see previous post about this research on Out of the Fog – a blog of the UCSC Science Communication program.)

On April 26, 2013, Fulton stood on deck of the research vessel Kaireiand watched the string of cigar-sized sensors (the temperature “observatory,” in his words) return to the surface. The moment was particularly emotional for Fulton, who had been planning this experiment for years and sailed with four research cruises to the drilling site. He’s come away from the experience with a deep understanding of the scientific and emotional impacts of the Tohoku quake

Last month, Fulton took an afternoon to share his thoughts on the project with me:

You found the residual heat you were looking for on the Tohoku fault. How much heat are we talking about?

Less than a half of a degree Celsius. Temperature increases linearly when you go down to deeper depths. We can see, as we get closer to the fault, that the temperature gets even hotter than it should be just based on the depth that it’s at. There is an extra little bump in temperature from the frictional heating on the fault zone.

What does that bump tell you about the fault?

It suggests that the fault was really, really weak during the earthquake. We’re trying to figure out what the friction was during the earthquake, which tells us what the forces on the fault were. It’s going to change our understanding of the forces that allow earthquakes to grow into very large earthquakes.

How did you get involved in this project?

A lot of my research as a PhD student at Penn State was related to this question of friction. So I did a lot of work on the San Andreas Fault, looking for signatures of friction on that fault over millions of years of sliding. But then, in 2007, I was talking with [UCSC geology professor] Emily Brodsky, and we had this idea: What if we could measure the amount of heat and friction on a fault for a single earthquake? Is it possible? If we’re really trying to answer this big question in earthquake physics, what do we need to do to make that happen? I started running computer simulations to see what it was going to take. Over many years we continued to make preparations, and when this earthquake happened off of Japan, the amount of slip was about ten times as much as we were prepared for.

So when you first heard of the Tohoku earthquake, did you say to yourself “This is it”?

No, I was so focused on the humanitarian disaster. But a few weeks later we all started to think, “well, this is huge, and as a community we need to try to do something to get as much science out of it as we can.” But when we were preparing, we always were thinking of big earthquakes on land. There were a lot more challenges that came up with having to drill underwater.

Did you ever think this experiment might not work?

Many times. We determined that it was on the edge of what was doable. If we were patient and tried our best at it, it was a high risk, high reward project. Luckily, it all came together.

You weren’t able to recover the sensors on your first attempt. How did you feel when you returned home without the observatory?

It seemed very scary. It was possible that a giant landslide had covered our well. If that were the case, it would be impossible, no matter how hard we tried, to recover the sensors. We had already had many successes in terms of finding the fault, getting good logging data, and getting core samples. So I still felt the project was a success, but I was a little disappointed that we may not be able to finish this temperature experiment. But I knew that we would try again.

What’s life like on these research ships?

We used two different boats. I was on each of them twice. One is the deep-sea vessel Chikyu, which is the drilling ship. That’s a big boat with an international crew and science party. Most everything is in English.

The other boat was the research vessel Kairei, and it’s a much smaller boat; it hosts the Kaiko 7000-II ROV that let us search for the sensors. It’s more susceptible to seas and bad weather. It’s mainly a Japanese crew and science party and the food is a little different — very Japanese. They have fish for breakfast, lunch, and dinner. I’m not the biggest fan of eating fish.

What made the Tohoku earthquake personal for you?

When we cored through the fault and got a sample of the actual fault zone, many of the Japanese crew that usually aren’t around the science part of the ship were able to come and look and see what the fault really looked like. That was a moving experience, to see them see what caused that devastation.

We all have a science curiosity about what causes earthquakes and tsunamis, but there is a societal relevance to the work that we do. The questions of why faults slip and how they slip a lot, and whether faults can create a tsunami, is one that has a lot of impact for people living along the coast.

How did you celebrate recovering the sensors?

I went up into the mountains and just had a day of hiking to relax, be away from everything, and reflect on what we accomplished. That was my celebration.

Fulton's research in Japan may lead to a better knowledge of how much damage a fault can unleash. Photo by Paul Gabrielsen.

This post also appears on the UCSC Science Communication program blog Out of the Fog. Paul Gabrielsen also writes for ScienceNOW and will soon join the communications team at NASA Goddard Space Flight Center.