Title page for ETD etd-385113359611541


Type of Document Master's Thesis
Author Norabuena, Edmundo O.
URN etd-385113359611541
Title Velocity Structure of the Subducting Nazca Plate beneath central Peru as inferred from Travel Time Anomalies
Degree Master of Science
Department Geophysics
Advisory Committee
Advisor Name Title
Bollinger, Gilbert A.
James, David E.
Snoke, J. Arthur Committee Chair
Keywords
  • none
Date of Defense 1993-12-01
Availability unrestricted
Abstract
Arrival times from intermediate-depth

(110-150 km) earthquakes within the region

of flat subduction beneath central Peru

provide constraints on the geometry and

velocity structure of the subducting Nazca

plate. Hypocenters for these events, which

are beneath the sub-andean and eastern

Peruvian basins, were determined using a

best-fitting one-dimensional velocity-depth

model with a 15-station digitally-recording

network deployed in the epicentral region.

For that model, P-wave travel times to

coastal stations, about 6-degrees

trenchward, exhibit negative residuals of up

to 4 seconds and have considerably more

complexity than arrivals at the network

stations. The residuals at coastal stations are

conjectured to result from travel paths with

long segments in the colder, higher velocity

subducting plate. Travel time anomalies

were modeled by 3-D raytracing.

Computed ray paths show that travel times

to coastal stations for the eastern Peru

events can be satisfactorily modeled if

velocities relative to the surrounding mantle

are 6% lower within the uppermost slab (a 6

km thick layer composed of basaltic oceanic

crust) and 8% higher within the cold

peridotitic layer (which must be at least 44

km thick). Raytracing runs for this plate

model show that "shadow zones" can occur

if the source-slab-receiver geometry results

in seismic rays passing through regions in

which the slab undergoes significant changes

in slope. Such geometries exist for seismic

waves propagating to some coastal stations

from sources located beneath the eastern

Peruvian basin. Observed first-arrival times

for such cases do in fact have less negative

residuals than those for geometries which

allow for "direct" paths. Modeling such

arrivals as trapped mode propagation

through the high-velocity part of the plate

produces arrival times consistent with those

observed.

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