The Performance of Triangular Fault Elements in Earthquake Simulators

Michael Barall, & Terry E. Tullis

Published 2016, SCEC Contribution #6015

An earthquake simulator is a computer program that generates a synthetic earthquake catalog spanning thousands of years, or longer. Most of the computational effort in an earthquake simulator goes into computing how slip on one part of a fault affects stresses on other parts of the fault, and on other faults. The computation is done by discretizing the fault system into a large number of fault elements, and using Greens functions to determine how a pattern of slip on some fault elements affects the stresses on all the fault elements. Traditionally, earthquake simulators have used rectangular fault elements, chosen so that the Okada Greens functions can be used. Recently, due to the development of new Greens functions for triangular dislocations, it has become practical to use triangular fault elements. The purpose of this project is to assess the accuracy of stress calculations performed with triangular fault elements, as compared to the accuracy of the same calculations done with rectangular fault elements. For planar faults, rectangles and triangles can be expected to give the same results. But when a fault is curved, partitioning it into rectangular fault elements will necessarily create gaps and overlaps between adjacent elements. In contrast, partitioning a curved fault into triangular fault elements can be done using a triangular mesh which has no gaps or overlaps between adjacent elements. Because triangles can represent curved fault geometry more accurately than rectangles, one intuitively expects that stress calculations performed with triangles should be more accurate than stress calculations done with rectangles. However, our results are contrary to the intuitive expectation. In our tests, triangles are not superior to rectangles. One or the other may be superior in a particular case, but, overall, rectangles perform as well as or better than triangles. Another unexpected result is that one triangulation of a fault surface may perform significantly better than another triangulation with a different pattern of triangles.

Citation
Barall, M., & Tullis, T. E. (2016). The Performance of Triangular Fault Elements in Earthquake Simulators. Seismological Research Letters, 87(1), 164-170.