Source: Valeri Korneev, Dan Hawkes
In early May 2012, a team of scientists including ESD’s Valeri Korneev and Paul Cook , USGS‘s Andy Snyder and Joe Svitek, and volunteer Stas Bystrichenko installed a novel three-component magneto-acoustic seismic sensor (MAS) at the San Andreas Fault Observatory at Depth (SAFOD) pilot hole at Parkfield, CA, located above the San Andreas Fault. This sensor enables first-of-a-kind measurements of low-magnitude seismic events, which in turn could end up having extremely large implications.
The MAS (Belyakov, 2005) was given to LBNL by its inventor and developer Askold Belyakov, a Russian scientist who devoted himself to this unique tool for most of his long career. The sensitive core of MAS is made of elasto-magnetic material, which changes its magnetization in response to elastic deformation. This tool has a phenomenal peak displacement sensitivity of about 1 femto-meter —an extremely tiny quantity (femto = 10–15 )—and is capable of recording frequencies up to 1600 Hz.
As Korneev explains, there are at least two good reasons why we need to track such high frequencies on seismic records. One is that present earthquake monitoring networks have natural limitations in terms of their highest operating frequencies (and smallest detectable events) they can sense, which restricts their ability to collect enough useful data to monitor changes in seismic activity. Smaller seismic events are much more frequent than larger events (such as discernible, detectable earthquakes), but require probing at higher frequencies to be detected. A sensitive high frequency tool such as MAS is needed to record the more-frequent low-magnitude events in a catalog. While a small magnitude-2 event occurs several times per week at SAFOD, the MAS records up to several dozen smaller events per second there. Most of the current earthquake prediction methods use seismicity change as an indicator of the stress changes that cause earthquakes. The multiplicity of tiny microseismic events at Parkfield should enable seismologists to drastically increase the time resolution of their seismic activity evaluations, and make practically instantaneous measurements of seismicity levels.
The second reason to use the MAS is its ability to literally “listen to” the seismic records. Korneev passed along the following story by way of explanation:
“When my eldest son was at school, he asked me once ‘Dad, what are you doing at work?’ We were in the kitchen, which had a tile floor, so I took some utensils from a drawer and said, ‘Look at these three things— a knife, a spoon, and a fork. All of them have about the same size and a shape—and they are made of the same material. Now, turn your head away. I’ll drop them one after another on the floor, and you tell me what they are.’ He made no mistakes. ‘See,’ I said, ‘I’m trying to teach a computer how to do what you just did.’
Korneev continues: “Hearing is a vitally important component of our everyday life and it has its unique capabilities. For example, we can easily tell the difference between the multiple ways water flows— the sound of water pouring in a cup is very different from that from a shower head, or water cascading down a waterfall. Special studies have shown that a human voice is more unique than a fingerprint. Why is that? It is because we have ears, two very sensitive sound sensors with a wide frequency band, which constantly keep loading our portable supercomputer—our brain—with data. And we also have a long and rich history of using these tools in our everyday life. Conventional seismic sensors record data for frequencies up to 30–40 Hz, just where our acoustic frequency range (20–20,000 Hz) starts. The ability of MAS to be very sensitive above a KHz range makes possible the involvement of human hearing in seismic data processing and interpretation, potentially making a big difference for these procedures.”
Korneev realized this when he was sitting in a recording hut about 50m from the sensor. “From an audio output, I was able to hear and recognize the voices of my collaborators, who were near the sensor. The MAS effectively worked as a microphone; the weak acoustic energy of the sound waves penetrated through the steel cover of MAS without distortion. Then, as soon as the tool was lowered down the hole, I turned on the registration station and there it was, the cracking sound of the San Andreas Fault. That the signal can literally be heard helps to better understand their source and nature.”
Want to hear it yourself? Click here »
ESD scientists plan to explore MAS’s capabilities and find the links between its recordings and other geophysical fields to obtain a new kind of information about the physics of earthquakes.
Belyakov, A.S. (2005), Magnetoelastic sensors and geophones for vector measurements in geoacoustics. Acoustical Physics, 51, 53–65. (pdf)