Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 266

The main characteristics of soil (velocity and density) are also considered in the
analysis. In order to have an image of the dimensions of this computational
problem, it must be mentioned that, during the modeling of the fault rupture, the
discretization contains 9 million elements, 1.7 million nodes and 5.1 million
degrees of freedom. Using this numerical approach, the Northridge (USA), Chi-
Chi (Taiwan) and Denali (Alaska) earthquakes were modeled (Aagaard et al,
2001c, Aagaard et al, 2004, Anderson et al, 2003). The numerical modelling was
used also to analyze the characteristics of different source types.
Another methodology for the simulation by computer of strong ground motions
near the fault was presented by Miyatake (2000). This method is based on fourth-
order 3-D finite-difference simulation method with staggered grids, which allows
determining the spatial pattern of ground velocity vectors near the fault.
In the future, the ground motion numerical simulations will likely play an
increasingly important role in the Engineering Seismology and Earthquake
Engineering.
7.5.5 Characterization of Near-source Ground Motions with Finite Element
Simulation
Due to the different influencing factors affecting the recorded ground motions by
alteration of values caused by the source features, it is very difficult in practice to
determine these values and to state the differences between the primary
characteristics of the different source types. Additionally, the scarce coverage of
recording stations limits the ability to capture ground motions close to fault
ruptures, where the influence of these factors is reduced. Therefore, when in the
future the ground motion numerical simulation will be able to eliminate the
parasite influences, it will likely play an increasingly important role in the
structural design, helping to understand the nature of these ground motions,
including the variability caused by the changes in the seismic parameters.
Using the facilities offered by the supercomputers Centre for Advanced
Computing Research at the California Institute of Technology and the
implementation of parallel processing, Aagaard and his team (2000 to 2004)
performed a systematic analysis of the influence of the source parameters on
earthquakes, such as:
- Source type: thrust (including subduction and collision faults) and strike-
slip faults;
- Source parameters: fault dip and rupture surface dimensions (for thrust
faults), rupture speed, peak slip rate, hypocenter location, distribution of
slip, average slip, fault depth, etc.
The numerical simulations refer to the characteristics of long period near source
ground motions for thrust (blind and surface rupture types) fault, with M 6.6-7.0
magnitudes and strike-slip fault, with M 7.0-7.1 magnitudes.
Blind thrust faults (Fig. 7.44) . The first analyzed problem is the effect of
forward directivity (Aagaard, 2000). The examined Earth block is a region of
60x60 km down to a depth of 24 km. The rupture surface is burying one with the
dimensions of 28 km long and 18 km wide and 8.6 km below the surface and a