Seismic attenuation structure in central Mexico: Image of a focused high‐attenuation zone in the mantle wedge

Abstract

[1] Velocity spectra from moderate-sized earthquakes were used to investigate the P wave attenuation structure in central Mexico. In particular, we included regional events with magnitudes in the range of 4.5 to 6.1 recorded from 2005 to 2007 on the Middle American Subduction Experiment (MASE) array, which consists of 100 broadband sensors across central Mexico from Acapulco to Tempoal, near the Gulf of Mexico. By assuming a Brune-type source, a frequency-independent t* value was obtained for each seismogram in the frequency band 1 to 30 Hz. These measurements were then inverted for twodimensional spatial variations in Q p in the cross section along the MASE array, perpendicular to the trench. The model has uniform 20 km vertical grid spacing down to a depth of 200 km and 50 km or 100 km horizontal grid spacing depending on ray coverage. The inversion results show low attenuation in the subducting slab and high attenuation in the mantle wedge and the crust below and to the north of the volcanic belt. The focused high-attenuation zone (Q p < 200) in the mantle wedge lies away from the top of the slab, between depths of 80 km and 120 km beneath the volcanic belt, and is likely to be related to relatively high temperature, fluids, and partial melts produced in subduction process. The high-attenuation region in the lower crust correlates with the low-resistivity and low-velocity region and could be caused by partial melts and fluids from dehydration and magmatic processes.

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico

“…6). Zones of high conductivity (Jödicke et al 2006), low velocity (Iglesias et al 2010) and high attenuation (Chen & Clayton 2009) are consistent with the anomalously high-anisotropy percentage zone found in the crust below the TMVB. This last correlation leads us to believe that the vertical fracturing produced by the ascent of lower density materials is an important mechanism that contributes to the observed high shear wave time splitting within this volcanic province.…”

“…Within the range of observations for this layer, we were able to distinguish four major ones: (1) fast azimuthal orientations quasiparallel with the convergence direction of the Cocos and North America plates, for the first 80 km of the array, (2) two subregions with relative low-anisotropy percentage (∼1.3 per cent) confined to 80-105 and 140-170 km from the coast, (3) a dominant parallelism between the fast polarization directions and the ENE-WSW and E-W trending of the extensional faults within the TMVB and (4) high-anisotropy percentages in central Mexico (∼2 per cent) which we attribute to the vertical fracturing of the crust caused by fluid ascent and partial melts (Jödicke et al 2006;Chen & Clayton 2009;Iglesias et al 2010).…”

Distribution of hydrous minerals in the subduction system beneath Mexico

“…The Cocos plate is steeply subducting beneath two volcanic regions (SCVF and ZVB), and the magmatic H 2 O contents in the monogenetic volcanoes in these regions range between 2.0 and 6.0 wt% (Johnson et al, 2009). The active slab dewatering process inferred from our inverted data may also well correlate with highly attenuating region in the overlying mantle wedge beneath the TMVB at a depth of 80-120 km (Chen and Clayton, 2009). This highly attenuated area was previously suggested to be related to fluids and partial melts produced in the subduction process (Chen and Clayton, 2009).…”

“…The active slab dewatering process inferred from our inverted data may also well correlate with highly attenuating region in the overlying mantle wedge beneath the TMVB at a depth of 80-120 km (Chen and Clayton, 2009). This highly attenuated area was previously suggested to be related to fluids and partial melts produced in the subduction process (Chen and Clayton, 2009). Furthermore, at a depth of 20-40 km, the slab dewatering process followed by the slab rollback was seismically evidenced as a low-resistivity anomaly at the lower continental crust directly beneath the entire arc (Jödicke et al, 2006).…”

“…Furthermore, at a depth of 20-40 km, the slab dewatering process followed by the slab rollback was seismically evidenced as a low-resistivity anomaly at the lower continental crust directly beneath the entire arc (Jödicke et al, 2006). This anomaly has also been shown as high seismic attenuation (Chen and Clayton, 2009) and low shear wave velocities (Iglesias et al, 2010).…”