Effect of Structural Heterogeneity and Slip Distribution on Coseismic Vertical Displacement from Rupture on the Seattle Fault

Workshops in 2001 and 2002 were convened to determine critical issues in the
development of tsunami inundation maps for the Puget Sound region. The Tsunami
Inundation Mapping Effort (TIME) is conducted under the multi-agency National
Tsunami Hazard Mitigation Program (NTHMP). The Puget Sound Tsunami/Landslide
Workshop in 2001 focused on integrated tsunami research involving a wide range
of research studies and tsunami hazard mitigation issues. The 2002 Puget Sound
Tsunami Sources workshop (González et al., 2003) made specific recommendations
for tsunami source modeling and improving our state of knowledge for sources in
the Puget Sound region. One of the recommendations stated in González et al.
(2003) is "Develop methods to assess the sensitivity of coastal areas to
tsunami inundation, based on multiple simulations that reflect the possible
range of variations in the source parameters." Tsunami inundation models rely
heavily on the imposed initial conditions which, for an earthquake source, is
the coseismic vertical displacement field. For example, Koshimura et al. (2002)
use the geologic uplift observations (Buknam et al., 1992) to constrain the
slip distribution for the event that occurred 1100 years ago, resulting in an
average slip of 3.7 m and a magnitude of 7.6. Walsh et al. (2003) develop a
tsunami inundation map for Elliot Bay based on a M 7.3 earthquake and the
geologic uplift observations from the 1100 y.b.p. event as in Koshimura et al.
(2002), though they use a constant fault dip of 60° rather than different dips
for deep and shallow segments. The objective of this report is to examine how
coseismic vertical displacement from a smaller M 6.5 Seattle Fault earthquake
(as in Hartzell et al., 2002) is affected by structural heterogeneity and
different slip distribution patterns.

The three-dimensional crustal structure of the Puget Sound region has recently
been defined using shallow seismic reflection data (Pratt et al., 1997; Johnson
et al., 1999) and reflection and wide-angle recordings from the large-scale
SHIPS experiments (e.g., Brocher et al., 2001; ten Brink et al., 2002). The
presence of a deep sedimentary basin (Seattle Basin) adjacent to the Seattle
Fault has led to the question of whether structural heterogeneity has an effect
on our estimate of vertical displacement for earthquake scenarios in the
region. We use a three-dimensional elastic finite-element model (Yoshioka et
al., 1989) to calculate vertical displacements from rupture on a two-segment
(deep and shallow) Seattle fault using a heterogeneous crustal structure.
Similar studies by Geist and Yoshioka (1996) and Masterlark et al. (2001) used
three-dimensional, finite-element models (FEM) to study the effect of
structural heterogeneity on coseismic displacement fields. Results for the
Puget Sound study are compared to calculations using a homogeneous structure as
assumed with conventional elastic dislocation solutions. Effects of slip
distribution patterns on vertical displacement is computed using the stochastic
source model adopted for tsunami studies by Geist (2002). Finally, we examine
an alternate model for shallow faulting proposed by ten Brink et al. (2002) and
Brocher et al. (submitted) and its effect on the vertical displacement field.

[Summary provided by the USGS.]