NEW_Accomplishments.indd - IRIS

2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES
Seismic Imaging of the Himalayan Collision Zone
Sheehan, Anne F., Vera Schulte-Pelkum, Gaspar Monsalve, Tom de la Torre • University of Colorado at Boulder
Francis Wu • SUNY Binghamton
The Himalayan Nepal Tibet PASSCAL Seismic Experiment (HIMNT) included the deployment of 28 broadband seismometers
throughout eastern Nepal and southern Tibet in order to better understand the mountain building processes of this
region. Our new images from receiver functions and tomography show evidence
of the basal decollement of the Himalaya and an increase in Moho depth from
45 km beneath Nepal to 75 km beneath Tibet. We find strong seismic anisotropy
above the decollement, which may develop in response to shear processes taken
up as slip in great earthquakes at shallower depths. Many local earthquakes
were recorded during the deployment, and the large contrast in crustal thickness
and velocity structure over a small lateral distance makes the use of a 3D
velocity model important to determine accurate hypocenters. Seismicity shows
strong alignment of shallow (15-25 km depth) events beneath the region
of highest relief along the Himalayan Front, and a cluster of upper
mantle earthquakes beneath southern Tibet (70-90 km depth).
The upper mantle earthquakes are not expected by
weak-mantle models. Focal mechanisms of
these upper mantle earthquakes are almost
all strike-slip, markedly different from the
normal faulting mechanisms observed for
earthquakes in the mid and upper crust beneath
Tibet. This change in the orientation of
the major horizontal compression axis from
vertical in the upper crust to horizontal in
the upper mantle suggests a transition from
deformation driven by body forces in the
crust to plate boundary forces in the upper
mantle. Several lines of evidence point to a
decoupling zone in the Tibetan mid or lower
crust, which may be related to the presence
of a previously suggested flow channel in the
Tibetan mid crust. Surface wave tomography
results reveal a strong east-west lithospheric
thickness variation across the Himalayan
arc, with thicker lithosphere to the west of
Kathmandu and thinner to the east. This
lithospheric thickness variation seems to
affect the nature of subduction across the
Himalayan arc, and likely has implications
for seismogenesis and the strength of the
lithosphere. Our preliminary results indicate
that upper mantle depth Tibetan earthquakes
are only found in the regions with thinner
lithosphere.
b
85
SAGA
Tibetan Plateau
(Tethyan
Himalaya)
85
86
86
(Top) Figure Overview 1 map with topography. Study area outlined in red. Location of
INDEPTH profiles shown in blue. (bottom) Topography map showing HIMNT
seismic stations (black diamonds). Hypocenters located within our network are
color coded by depth.
75
75
80
80
85
85
90
90
95
a
INDEPTH
35 profiles 35
Tibet
30 30
25 India
25
20 20
LAZE
29 29
SAJA
XIXI MNBU
NAIL DINX MAZA
High
Himalaya
YALA
RC14
NLMU
RBSH
28
BUNG
28
Kathmandu
NAMC
SUKT
THAK JIRI
PHAP
Lesser
Himalaya
SIND
RUMJ TUML
PHID
27 HILE
27
ILAM
JANA
GAIG
event depth [km]
Ganges Plains
87
SSAN
Indus-Tsangpo Suture Zone
BIRA
87
Nepal
ONRN
88
0 50 100
88
95
111