Rocks usually shake when they break, but sometimes they don't (Seismic and aseismic slip of oceanic strike-slip earthquakes)

Friday, 18 December 2015: 16:45
310 (Moscone South)
Kasey Aderhold, Incorporated Research Institutions for Seismology, Seattle, WA, United States
Rocks make waves and shake when they break along faults, but sometimes the fault sides move along without making waves and shaking. We don't know what controls this, but it could be what the rocks are made of, how hot they are, how much they are pressed from above, how often they move, or how fast they are moved by one another. Straight-down faults under water are usually younger and more simple than straight-down faults on land, so it is easier to tell what might make one part of the fault break and shake and one part of the fault move without shaking. There are also a lot more under water faults than land faults, so we don't have to wait around for a large one on land to break. That could cause a lot of serious problems and deaths, and we want to understand where it will break so we can warn people.
To figure out what makes straight-down faults shake or not shake, I use the first couple of waves that are immediately sent through the world. When a fault breaks it sends out waves in rings and when they arrive at computers sensing in the ground very far away numbers are written down to show what the computer felt. The waves look different if the part of the fault that moved was deep or close to the top, if the slip was fast or slow, if there was a lot of slip or not very much, and if the slip was in one direction or another. I make up pretend numbers and see if they are the same as the numbers written by the computers. We can use the computers far away on land to understand what happened at the faults under the water. By studying many of these faults, we can track down what is the same about the parts of the faults that shake and what is different about the parts of the faults that don't shake.