"Researchers creating numerical models of the micro-mechanics of debris movement in an earthquake fault often produce odd results that do not match laboratory experiments," says Dr. Chris Marone, associate professor of geosciences. "Very little has been done to look at how initial and boundary conditions such as surface characteristics and particle dimensionality affect friction."
Angel hair pasta, glass rods, glass beads and sand have different sizes and surface finishes so researchers can explore the importance of these characteristics in earthquake faults. The researchers use stacks of pasta or glass rods to measure friction created by shear motion. For shear of a single plane or one-dimensional motion, they apply force to the rods so the motion is in the long direction. For two directions or two-dimensional motion, the force is applied across the stack of pasta. The spheres and sand are used to measure three-dimensional motion.
One problem is that numericists model one or two-dimensional smooth particles while experimentalists work with natural materials that have three dimensions and are irregular. Current computer capabilities cannot handle three-dimensional calculations or account for surface irregularities, but Marone, Kevin M. Frye, a former MIT student, and Karen Mair, former MIT postdoctoral fellow, experimented with all three types of motion and both smooth and rough surfaces. They reported their work in two recent papers in Geophysical Research Letters and the Journal of Geophysical Research.
Between the two sides of an earthquake fault lie the wear material that accrues from the two sides slipping past each other. This material, called gouge, can be smaller than sand particles or as large as boulders, although most of the material is small. To simulate gouge in the laboratory, the researchers used smooth glass beads, quartz sand, quartz glass fiberoptic rods and angel hair pasta. They tested the materials in a test rig that allowed them to control the directions of motion applied.
"We began with the pasta because it had the same shape and size as the glass rods, which had not arrived yet," said Marone. "However, they worked not only to test the set up, but in the experiment as well, so we continued with them."
He notes that because the pasta was machine manufactured, it was very uniform. Pasta does compress more than glass, but the researchers can easily account and correct for that.
"Our data show that discrepancies reported in recent (numerical) works are due to differences in initial conditions - rough versus smooth particles -- and particle dimensionality -- 2-D versus 3-D granulated particles," said Marone. "In this case the actual material does not have a significant effect because many rock types have a coefficient of friction of .6 to .62."
The researchers also found that smooth glass beads produce very different results than quartz sand with the sand producing more friction than the smooth beads. To ensure that the rougher surface of the sand caused the difference and not the size or some other characteristic of the sand, the researchers ran the experiment beginning with smooth glass beads. They gradually crushed the beads creating broken beads with rough surfaces, and the final friction recorded was equivalent to the quartz sand.
"Using spheres is not a good approximation of what occurs in the real world," says Marone. "However, their inability to model rough particles limits the numerical models."
The Penn State researcher is continuing this line of investigation with one of his students who will test a variety of pastas to see how rod diameter and layer thickness influences the two-dimensional friction measurements. He will test spaghetti, spaghettini, vermicelli, capellini and angel hair.