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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 • • • • 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 Across-arc wide-angle study •Results •Interpretation Takahashi et al (2006) Crustal Tomography • • • • “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 • 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. • Arc magma “source region” is separated from backarc source • Mariana shows along-strike fast shear wave splitting observations extending from the arc to the backarc spreading center. • 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? • To understand seismic coupling it is necessary to study the extreme end members • Coupling parameter () = seismic slip rate / tectonic slip rate • Most subduction zones have = 0.1 to 0.7 (Pacheco et al., 1993) • Only Mariana and Java have < 0.005 • 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