Guerrero GAP Leveling

Fig. 1. Location of leveling lines on the Pacific coast of Guerrero. Leveling lines are: PA - Playa Azul, BP - Barra de Potosí, At - Atoyac, Ac - Acapulco. Shaded areas annotated with a year are rupture zones of recent large thrust earthquakes. Thin arrows located inland, over the Guerrero gap indicate an approximate extent along the coastal area of the rupture of the large earthquakes in 1899, 1907, 1909, and 1911. Solid arrows are vectors of the Cocos - NA convergence velocities according to the NUVEL 1A model (DeMets at al., 1994). MAT - Middle America trench. Diamonds indicate location of the continuous GPS stations.

     In order to investigate the interseismic crustal deformations in the southern Mexico the IGF UNAM installed in 1995 four leveling lines of 10-15 km long, starting from the coastline, and oriented perpendicular to the coast (Figure 1). The line Playa Azul located in the Michoacan state represents a complementary ~10 km segment to the leveling line used by Suárez, and Sanchez [1996]. Next three lines are in the Guerrero state. The line Barra de Potosi occupies a site in the middle of northern part of Guerrero, where three recent thrust earthquakes of Ms = 7.5-7.6 took place in 1943, 1979 and 1985. The line Atoyac is in the center of the Guerrero gap and the line Acapulco is in the border between the Guerrero and Acapulco gaps. The location of these leveling lines are selected in a way to cover distinct areas of seismotectonic regime inside and nearby to the Guerrero seismic gap. The existing leveling lines will be extended more inland as soon as an additional support for the project would be available.
     Three of those lines were leveled in 1995. Since that time the lines at Atoyac and Acapulco were reoccupied three and four times respectively. The Playa Azul line was reoccupied in 1996. The Barra de Potosí line was leveled in 1997 and 1998. The observed changes in relative height show a significant rate of crustal uplift along the Barra de Potosí (8-9 mm/12 km per year), Atoyac (~ 7-8 mm/14 km per year) and Acapulco (~7-8 mm/15 km per year) lines. On the other hand, coastal uplift along the Playa Azul line is less than 2 mm/10 km per year, probably due to the release of deformation by the 1985 event.
     The observed uplift along the leveling lines can be compared with rates of vertical surface deformation predicted by models of thrust fault in an elastic half-space (e.g., Savage, 1983) assuming steady-state deformations throughout the entire interseismic period, and homogeneous coupling on the seismogenic contact (Kostoglodov et al., 2000). The geometry of the contact zone is constrained by local seismicity data and gravity anomaly models (Kostoglodov et al., 1996). Tighter constraints on the interseismic coupling in Guerrero will be provided by the GPS measurements at the end of 2000.


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