Fine Structure of a Postfailure Wadati‐Benioff Zone

Abstract

The hypocentral distribution of locally recorded aftershocks of the great (Ms=8.1) Michoacan, Mexico, earthquake of September 19, 1985, defines a narrow Wadati‐Benioff zone structure, roughly 10 km thick, dipping 14° at N23°E. This is in good agreement with the source geometry obtained by waveform modeling of the 1985 Michoacan mainshock and the large 1979 Petatlán earthquake in the adjoining region. We inverted for the crustal velocity structure in the epicentral region by applying the Levenberg‐Marquardt non‐linear least squares algorithm to our local aftershock data. The velocity model consists of a layer with linearly increasing velocity in depth overlying a dipping, constant velocity halfspace. Our hypocentral location program uses a velocity model of the same form together with ray tracing. The earthquake hypocentral resolution obtained with this program is significantly better than that from conventional approaches (HYPO) and looks very promising for use in velocity structures with an important dipping interface like subduction zones.

Coseismic slip of two large Mexican earthquakes from teleseismic body waveforms: Implications for asperity interaction in the Michoacan Plate Boundary Segment

“…For both events, point source responses are calculated using the chosen fault mechanism and the crustal structure adopted by Mendoza and Hartzell [ 1988a] from the gradient velocity model of Stolte et al [1986]. A triangular source time function of 1-s duration is used for each point source.…”

“…This value is very similar to the strike observed in the centroidal source mechanisms computed for the 1985 Michoacan earthquake [e.g., Dziewonski et al, 1986;Priestley and Masters, 1986;Riedesel et al, 1986;Astiz et al, 1987]. The fault dip is fixed at 14 ø , which corresponds to the dip of the Wadati-Benioff zone inferred by Stolte et al [1986] from the hypocentral distribution of the aftershocks following the 1985 Michoacan and Zihuatanejo earthquakes. Table 1 lists the centroidal source parameters that have been previously computed by other investigators for the Playa Azul and Zihuatanejo earthquakes.…”

“…Mendez Masters, 1986; Riedesel et al, 1986]. In addition, aftershocks located in the source region map a zone elongated to the southeast [e.g., Stolte et al, 1986; Universidad Nacional Autonoma de Mexico (UNAM) $eismology Group, 1986]. These observations would additionally suggest that the September 21 earthquake represents the southeast continuation of interplate rupture that had been initiated during the 8.1 M s earthquake two days earlier [e.g., Astiz et al, 1987].…”

Thermal models of the Mexico subduction zone: Implications for the megathrust seismogenic zone

“…Open squares are in‐slab earthquakes [ Singh et al , 2000]; open circles are relocated earthquakes [ Pardo and Suarez , 1995]; solid triangles are seismic reflection data (Jalisco [ Michaud et al , 2000]); open triangle is from seismic refraction data (Oaxaca [ Nava et al , 1988]). Locations of megathrust main shocks (open diamonds) and their aftershocks (small circles) were given by Pacheco et al [1997] for Jalisco], Stolte et al [1986] for Michoacan, and Singh et al [2000] for Oaxaca. Bathymetry data were used to constrain the plate surface seaward of the trench [ Prol‐Ledesma et al , 1989; Pardo and Suarez , 1995].…”

“…This was followed 2 days later by an M W 7.7 earthquake to the southeast. The majority of aftershocks for both events were located between 40 and 120 km from the trench [ UNAM Seismology Group , 1986; Stolte et al , 1986]. The width of the aftershock area is slightly narrower in the northwest part of the 19 September rupture area, with the landward limit of the aftershocks ∼80 km from the trench.…”