Seismicity and state of stress in Guerrero segment of the Mexican subduction zone

Abstract

[1] We take advantage of a relatively dense network of seismic stations in the Guerrero segment of the Mexican subduction zone to study seismicity and state of stress in the region. We combine our results with recent observations on the geometry of the subducted Cocos plate imaged from receiver function (RF) analysis, an ultraslow velocity layer mapped in the upper crust of the subducted slab, and episodic slow slip events (SSEs) and nonvolcanic tremors (NVTs) reported in the region to obtain a comprehensive view of the subduction process. Seismicity and focal mechanisms confirm subduction of the Cocos plate below Mexico at a shallow angle, reaching a depth of 25 km at a distance of 65 km from the trench. The plate begins to unbend at this distance and becomes horizontal at a distance of $120 km at a depth of 40 km. Some of the highlights of the inslab seismicity are as follows: (1) A cluster of earthquakes in the depth range of 25-45 km, immediately downdip from the strongly coupled part of the plate interface, revealing both downdip compressional and extensional events. This seismicity extends from $80 to 105 km from the trench and may be attributed to the unbending of the slab. (2) The slab devoid of seismicity in the distance range of $105-160 km. (3) Sparse inslab seismicity revealing downdip extension in the distance range of 160-240 km. The NVTs are also confined to this distance range. The episodic SSEs occur on the horizontal segment, in the distance range of $105-240 km, where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted SP phases. Thus, in the distance range of $105-160 km, SSEs occur but NVTs and inslab earthquakes are absent. This suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of $160 km, giving rise to both the inslab earthquakes and NVTs. No inslab earthquake occurs beyond 240 km. The receiver function images and P wave tomography suggest that the slab begins a steep plunge at a distance of $310 km, reaching a depth of 500 km around 340 km from the trench. The negative buoyancy of such a slab should give rise to large extensional stress in the slab. Yet inslab seismicity is remarkably low, which may be explained by a slab that is not continuous up to a depth of 500 km, but is broken at a shallower depth. The resulting slab window may permit subslab material to flow through the gap. This may provide an explanation for the recent rift-related basalts found near Mexico City. The fore-arc, upper plate seismicity, which during the period of study (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) consisted of a moderate earthquake (M w 5.8) near the coast (H = 12 km), its numerous aftershocks, and two shallow events farther inland, demonstrates a trench-normal extension in the upper plate near this convergent margin, a state of stress that may be explained by tectonic erosion and/or seaward retreat of the trench. Seismicity, location of the ...

Origin, evolution, and tectonic setting of the eastern part of the Mexican Volcanic Belt and comparison with the Central American Volcanic Arc from conventional multielement normalized and new multidimensional discrimination diagrams and discordancy and significance tests

“…These include fracture-related origin (e.g., von Humboldt, 1808; Mooser and Maldonado-Koerdell, 1961;Mooser, 1969;De Cserna, 1971), transtension or intraplate transform (e.g., Shurbet and Cebull, 1984;Cebull and Shurbet, 1987), plume-related origin (e.g., Moore et al, 1994;Márquez et al, 1999a), extension or rift-related origin (e.g., Sheth et al, 2000;Márquez et al, 2001;Verma, 2002Verma, , 2004Verma, , 2009, and a slabdetachment model (e.g., Ferrari, 2004). Thus, its origin has been highly debated (e.g., Ferrari and Rosas-Elguera, 1999;Márquez et al, 1999b;Blatter et al, 2001;Ferrari et al, 2002;Verma, 2002Verma, , 2004Verma, , 2009Gómez-Tuena et al, 2007;Pérez-Campos et al, 2008;Pacheco and Singh, 2010;.…”

Present state of knowledge and new geochemical constraints on the central part of the Mexican Volcanic Belt and comparison with the Central American Volcanic Arc in terms of near and far trench magmas

“…There are practically no deep earthquakes beneath the C-MVB (Figure 1; Pacheco and Singh, 2010). On the other hand, in Central America this depth contour of 60 km lies totally in the Pacific Ocean, and most of the front-arc volcanoes in Central America are located above the area where the subducted slab reaches depths of 100 to 200 km ( Figure 1).…”

“…The deepest trace of the slab is only about 60 km at about 250 km horizontal distance from the MAT (Figures 1 and 2; Pacheco and Singh, 2010) and still away from the volcanic front of the C-MVB. There are practically no deep earthquakes beneath the C-MVB (Figure 1; Pacheco and Singh, 2010).…”

“…For the latter comparison, Pacheco and Singh (2010) are also shown resulting from the subduction of the Cocos plate beneath the North American plate; note that practically no deep earthquakes are registered beneath the C-MVB because the depth contours of only 20-60 km can be traced (see dotted curves marked by 20, 40, and 60); 250 km and 350 km are the horizontal distances from the MAT; one transect for the dense seismic network in southern Mexico used by the UNAM-Cal Tech collaborative project (Pérez-Campos et al, 2008;Pacheco and Singh, 2010) is also shown schematically by a thick irregular curve.…”

“…The application of discordancy tests prior to the significance tests is recommended in order to comply with the basic assumption of normally distributed variables (Jensen et al, 1997;Miller and Miller, 2005;Verma, 2005). The statistical inference from these computer programs is made through the new highly precise and accurate critical values obtained from Monte Carlo simulations (Verma Arc (CAVA) along with the three oceanic plates (Pacific, Cocos, and Rivera) and two continental plates (North American and Caribbean; figure was modified after Verma, 2002Verma, , 2015a; the abbreviation are: Pu-Puerto Vallarta; V-Veracruz; W-western; Ccentral; E-eastern; G-Guatemala; S-El Salvador; H-Honduras; N-Nicaragua; C-Costa Rica; and P-Panama; schematic locations of samples are shown by small crosses; the numbers 487, 488, 495, and 1256 are Deep Sea Drilling Project Sites in the Cocos plate; the numbers 60 and 200 represent the approximate depth in km of the subducted slab; QSC-Quesada Sharp Contortion in Costa Rica defines the southern limit of the CAVA; the two transects for the dense seismic network in southern Mexico used by the UNAMCal Tech collaborative project (Pérez-Campos et al, 2008;Pacheco and Singh, 2010;Kim et al, 2011) are also shown schematically by thick irregular curves.…”