Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone

Abstract

The regional distribution of stresses associated with the subduction of the Cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms, body and surface wave analyses of large earthquakes, and seismicity distributions. Broad patterns of consistent behavior are found across the region, from the Rivera Plate boundary in the northwest to the Guatemala/El Salvador border in the southeast, and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges, the Tehuantepec Ridge and the Orozco Fracture Zone. Information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large (Ms ≥ 7.5) earthquakes have occurred historically. We identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity. The first, closest to the trench and reflecting the mechanical interaction of the converging plates, is a zone of shallow thrust earthquakes extending 100–150 km inland from the trench. The second is a zone of normal faulting, beginning at about 200 km inland from the trench, h ≥ 60 km, which extends continuously along the entire length of the descending plate throughout the region. The third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150–200 km inland from the trench. This zone extends from the Rivera plate boundary in the northwest to the Guatamala region in the southeast. At this point, the quiet region pinches out, and the thrust and normal faulting zones abut and overlap. Superimposed on this overall pattern, we find locally only minor changes in areas of aseismic ridge subduction, aside from the prominent seismic slip gaps. Furthermore, on October 25, 1981, the Playa Azul earthquake (Ms = 7.3) occurred in the midregion of the Orozco Fracture Zone. Body and surface wave analyses of this event show a simple source rupture and shallow thrust fault mechanism, as are found elsewhere in the region. The seismic moment is MO = 1.3 × 1020 N m; the calculated stress drop is 4.5 MPa, not extraordinarily high, as might be expected in these pervasive seismic gaps. An event in the Tehuantepec Ridge region, on January 24, 1983, Ms = 6.7, was a large normal faulting event, but further interpretation is ambiguous due to the proximity of other normal faulting. We conclude that while aseismic slip may be occurring in the areas of ridge subduction, the possibility of large thrust earthquakes cannot be ruled out, due to the overall similarities with adjacent regions.

Seismic Subduction of the Nazca Ridge as Shown by the 1996–97 Peru Earthquakes

“…The marked lack of great earthquakes that are associated with the subduction of major oceanic ridges (KELLEHER and MCCANN, 1976;VOGT et al, 1976;LEFEVRE and MCNALLY, 1985) has led to numerous papers which hypothesize that such ridges may have mechanical properties that allow strain to be dissipated aseismically. The November 12, 1996, M w 8.0, earthquake is due to the subduction of the Nazca Ridge beneath coastal Peru, and is an important exception to the earlier observation.…”

Rupture Process During the Mw 8.1 2017 Chiapas Mexico Earthquake: Shallow Intraplate Normal Faulting by Slab Bending

“…There is a well-known trade-off between the focal depth and the source time function when determining the focal parameters for the shallow large earthquakes, and estimates of focal depth can be deceptive when it is difficult to distinguish the differences in synthetic waveforms (Christensen & Ruff, 1985). Moreover, the geological setting of the source region is probably complex because the Tehuantepec Fracture Zone (TFZ) on the Cocos plate is subducting near the source region (Manea et al, 2003;Manea & Manea, 2008;Menard & Fisher, 1958), and the geometry of the subducting plate differs across the TFZ (Bravo et al, 2004;LeFevre & McNally, 1985;Ponce et al, 1992;Rebollar et al, 1999a). Thus, a detailed analysis of the rupture process of the 2017 Chiapas earthquake is critical to overcome the uncertainty of the focal depths seen in the CMT analyses, and to understand the event's driving mechanism in its tectonic context.…”

Tectonic Setting and Source Parameters of the September 19, 1985 Michoacan, Mexico Earthquake

“…Previous to the September 1985 earthquake, the Michoacan area, with its seismic quiescence and subducting fracture zone, was similar to the southern Oaxaca area, where the Tehuantepec Ridge is subducting, and where there are no large earthquakes in the historic record. One possibility suggested to explain the seismic quiescence in these areas was that features such as the Orozco fracture zone and the Tehuantepec Ridge may be locally affecting the subduction process, such that the area is subducting aseismically, or more slowly than adjacent regions of the plate boundary [Singh et al, 1980;McNally and Minster, 1981;Lefevre and McNally, 1985). Alterations of subdue- [Singh et al, 19851.…”