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Seismogenic Characteristics and
Seismic Structure of the Mariana Arc:
Comparison with Central America
Douglas A. Wiens, James Conder, Sara Pozgay, Mitchell Barklage and
Erica Emry
Dept. of Earth and Planetary Sciences
Washington University, St. Louis, MO, USA
Eruption of Anatahan Volcano,
Northern Mariana Islands,
June 10, 2003
 Outline
• Summarize seismological results from the Mariana focus site for
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comparison with Costa Rica
Crustal structure from wide-angle seismics - growth of arc crust
Seismic velocity and attenuation tomography of the mantle wedge
Anisotropy - constraints on mantle flow in arcs
Seismic coupling: Why do island arcs lack large thrust
earthquakes?
 Margins Mariana Subfac Imaging Project
Joint US-Japan project
Active source component:
Long across-arc transect - 2003
100 OBSs - Japan
Takahashi, Kodaira
Along strike profiles - 2002
50 OBSs - US
Klemperer, PI
Passive imaging component:
58 OBSs deployed for 11 months
20 land broadband stations
Recovered in 2004
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Across-arc wide-angle study
•Results
•Interpretation
Takahashi et al (2006)
 Crustal Tomography
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“Tonalitic” mid-crustal layer thickest beneath Eocene frontal arc
Mafic layer and crust thickest along Eocene frontal arc
Individual centers of mafic addition along modern arc - volcanos stable wrt time
Arc production rate ~ 80 km3/km/Ma - similar to Aleutians
Calvert et al, 2007
 Oceanic Arc Velocities
• Modern Mariana arc similar
to Izu-Bonin
• Aleutian oceanic arc is
thicker due to thicker lower
crust
• Both arcs predominantly
more mafic than continental
crust
Calvert et al. (2007)
 Velocity and Attenuation Tomography
P velocity
P wave attenuation
Barklage et al, in prep.
Pozgay et al., in prep. - SEE POSTER
 Mariana Arc Shear Wave Splitting
Rose Diagrams - plotted at station
for sources in upper 250 km
Pozgay et al. [2007] - POSTER
Spatial Averaging - for paths in the
upper 250 km
•Along-strike fast directions from forearc to backarc
•Fast directions rotate to APM-parallel beyond spreading center
•Interpreted as along-strike flow in a low viscosity channel
 Conclusions - Mariana seismic
structure
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P, S, and Q tomographic images show low velocity regions from
40-100 km depth beneath the arc and backarc spreading centers
that may result from melt.
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Arc magma “source region” is separated from backarc source
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Mariana shows along-strike fast shear wave splitting observations
extending from the arc to the backarc spreading center.
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These measurements are best interpreted as along-strike mantle
flow in a low viscosity channel extending from arc to spreading
center
 Plate coupling - why do island arcs lack large
thrust earthquakes?
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To understand seismic coupling it
is necessary to study the extreme
end members
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Coupling parameter () =
seismic slip rate / tectonic slip rate
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Most subduction zones have  =
0.1 to 0.7 (Pacheco et al., 1993)
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Only Mariana and Java have  <
0.005
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Mariana is the type example of a
decoupled seismic zone, lacking
any large thrust earthquakes
Uyeda and Kanamori [1979]
 What causes “decoupling” of
subduction zones?
Geodynamic Forces?
Width of the Seismogenic Zone?
Uyeda and Kanamori [1979]
Sediment Subduction?
Serpentinization of the Mantle?
Hyndman and Peacock [2003]
 Izu-Bonin-Mariana Serpentinite Seamounts
Mariana and Izu-Bonin forearc contains numerous Serpentinite seamounts
Formed by Serpentinite mud volcanism
Provide evidence of geochemistry and petrology at mantle depths
MCS Profile
Seismic Refraction Results
Oakley et al., [2007]
Kamamura et al [2002]
 Mariana Seismicity Profile
• Earthquakes located from P and S
waves picked from land and OBS stations
• Highly seismic region from 20 - 55 km depth
represents shallow thrust zone
• Double seismic zone extends from 60-180 km
depth
• Shallow thrust zone and double seismic zone
begin approximately beneath the Serpentinite
seamounts
• Shallow thrust zone has length ~ 90 km - not
anomalous relative to other subduction zones
 Mariana Forearc Seismicity and Focal Mechanisms
Mariana Thrust Earthquakes
CMT depths
CMT and Regional Waveform Mechanisms
•Little seismicity within or beneath
the seamounts
• Seismicity concentrations
just arcward of both seamounts
• Seismicity gap just to the south
of Big Blue
• Seismicity extends from 10-55 km
depth - lower limit ~ 300ºC ?
Black Mechanisms - Global CMT
Red Mechanisms - Waveform inversion
Seismicity near Big Blue Seamount
Seismicity
Most thrust zone microseismicity occurs
in “patches” arcward of Big Blue smt at
depths of 30-50 km.
Focal mechanisms
Black Mechanisms - Global CMT
Red Mechanisms - Waveform inversion
 Conclusions - Coupling in the Mariana arc
• Sparse seismicity in the outer forearc within and beneath
seamounts
• Most shallow thrust microseismicity begins arcward of the
seamounts at depths of 25-50 km - within the mantle; extent of
thrust seismicity not controlled by crust-mantle transition
• Seismicity occurs in highly seismic “patches” that may be
related to topography on the incoming plate
• Down-dip limit of seismicity occurs at ~ 55 km depth (~ 300°C;
the width of the seismogenic zone is not anomalous
• Our Hypothesis:
– Outermost forearc is serpentinized: Serpentinite causes stablesliding behavior in upper part of shallow thrust zone
(alternative - perhaps it is mostly locked??)
– Seismicity in deeper parts of the thrust zone occurs on small
asperities perhaps controlled by partial serpentinization or
incoming plate topography
– Small size of these asperities prevents large earthquakes