Earth Planets Space, Vol. 57 (No. 10), pp. 973-985, 2005
S. I. Franco1, V. Kostoglodov1, K. M. Larson2, V. C. Manea3, M. Manea3, and J. A. Santiago1
1Institudo de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Del. Coyoacan, 04510 Mexico D.F., Mexico
2Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309-0429, USA
3California Institute of Technology, Seismological Laboratory, Pasadena, CA, MS 252-21, USA
(Received November 18, 2004; Revised September 30, 2005; Accepted September 30, 2005)
The aseismic slow slip event of 2001-2002 in Guerrero, Mexico, with an equivalent magnitude MW ~ 7.5, is the largest silent earthquake (SQ) among many recently recorded by GPS in different subduction zones (i.e. Japan, Alaska, Cascadia, New Zealand). The sub-horizontal and shallow plate interface in Central Mexico is responsible for specific conditions for the ~100 km long extended transient zone where the SQs develop from ~80 to ~190 km inland from the trench. This wide transient zone and relatively large slow slips of 10 to 20 cm displacements on the subduction fault result in noticeable surface displacements of 5-6 cm during the SQs. Continuous GPS stations allow one to trace the propagation of SQs, and to estimate their arrival time, duration and geometric attenuation. These propagation parameters must be accounted in order to locate source of slow slips events and to understand the triggering effect that they have on large subduction earthquakes. We use longbaseline tiltmeter data to define new time limits (onset and duration) for the SQs and continuous records from 8 GPS stations to determine the propagation of the 2001-2002 SQ in Central Mexico. Data from the CAYA and IGUA GPS stations, separated by ~170 km and located along the profile perpendicular to the trench, are used to determine that the surface deformation from the 2001-2002 SQ started almost instantaneously. It propagated parallel to the coast at ~2 km/day with an exponential attenuation of the horizontal surface displacement and a linear decrease of its duration with distance. Campaign data obtained yearly from 2001 to 2005 at the Oaxaca GPS network have been modeled according to a propagation of the 2001-2002 SQ step-like displacement anomaly. This modeling shows that the SQ ceased gradually in the central part of the Oaxaca segment of the subduction zone (west of Puerto Angel, PUAN) and then it apparently triggered another SQ in SE Oaxaca (between PUAN and Salina Cruz, SACR). The estimated horizontal velocities for inter-event epochs at each GPS site are used to assess an average interplate coupling in the Central Oaxaca subduction zone.
Key words: Subduction, silent earthquakes, propagation, coupling ratio, GPS, Mexico.