John Tromp

High-frequency simulations of global seismic wave propagation using SPECFEM3D_GLOBE on 62K processors

SPECFEM3D_GLOBE is a spectral-element application enabling the simulation of global seismic wave propagation in 3D anelastic, anisotropic, rotating and self-gravitating Earth models at unprecedented resolution. A fundamental challenge in global seismology is to model the propagation of waves with periods between 1 and 2 seconds, the highest frequency signals that can propagate clear across the Earth. These waves help reveal the 3D structure of the Earths deep interior and can be compared to seismographic recordings. We broke the 2 second barrier using the 62K processor Ranger system at TACC. Indeed we broke the barrier using just half of Ranger, by reaching a period of 1.84 seconds with sustained 28.7 Tflops on 32K processors. We obtained similar results on the XT4 Franklin system at NERSC and the XT4 Kraken system at University of Tennessee Knoxville, while a similar run on the 28K processor Jaguar system at ORNL, which has better memory bandwidth per processor, sustained 35.7 Tflops (a higher flops rate) with a 1.94 shortest period. Thus we have enabled a powerful new tool for seismic wave simulation, one that operates in the same frequency regimes as nature; in seismology there is no need to pursue periods much smaller because higher frequency signals do not propagate across the entire globe. We employed performance modeling methods to identify performance bottlenecks and worked through issues of parallel I/O and scalability. Improved mesh design and numbering results in excellent load balancing and few cache misses. The primary achievements are not just the scalability and high teraflops number, but a historic step towards understanding the physics and chemistry of the Earths interior at unprecedented resolution.

Present-day secular variations in the low-degree harmonics of the geopotential: Sensitivity analysis on spherically symmetric Earth models

[1]xa0We predict present-day rates of change of the long-wavelength components of the Earths geopotential due to the late Pleistocene glacial cycles (hereinafter referred to as glacial isostatic adjustment, or GIA). These predictions are generated using spherically symmetric, self-gravitating, (Maxwell) viscoelastic Earth models. Previous studies have generally considered only zonal (i.e., azimuthally independent) harmonics, or the so-called coefficients. We extend these efforts to focus on the nonzonal harmonics and explore the sensitivity of our predictions to changes in a variety of model inputs. As an example, we examine the influence on the GIA predictions of ignoring rotational perturbations and assuming an elastically incompressible model and find that these assumptions can have a significant impact on the degree-2, order-1 coefficients, while the other harmonics are generally affected by <10%. Though the GIA predictions are generally insensitive to variations in lithospheric thickness and upper mantle viscosity, reasonable variations in either lower mantle viscosity or the late Pleistocene ice history will have a more significant effect. Mindful of upcoming dedicated gravity missions, we show that predictions of the GIA-induced changes of the geoid can vary by at least 0.5 mm yr−1 over a plausible range of Earth and ice models.