Teleseismic body‐wave analysis of the 9 October, 1995 (M_w = 8.0), Colima‐Jalisco, Mexico Earthquake, and its largest foreshock and aftershock

Abstract

Abstract. The source process of the October 9, 1995, ColimaJalisco, Mexico, earthquake (Mw=8.0), and its largest foreshock and aftershock were determined from teleseismic body waves, using a least-squares inversion scheme. The three events are shallow-dipping, thrust-fault earthquakes, in agreement with the relative plate motions for Rivera-North America and CocosNorth America plate boundaries. Neither the foreshock nor the largest aftershock show significant differences in focal mechanism nor centroidal depth with respect to the average mechanism of the mainshock. The rupture of the mainshock began near the foreshock hypocenter and propagated, with a variable rupture velocity, towards the NW. The source-time function for this event shows four distinct episodes of energy release. About 50% of the moment was released at the northern end of the rupture, between 90 and 110 km from the epicenter. This rupture geometry is in agreement with crustal deformation observed inland using GPS measurements. However, the scalar seismic moment of the main shock obtained in this study Subruction zones with slow convergence rates, similar to the Rivera margin, exhibit long periods of interseismic activity between large earthquakes [Heaton and Hartzell, 1986]. This does not appear to be the case along the Jalisco-Colima margin. On October 9, 1995, a great earthquake hit the coastal towns of Colima and Jalisco, causing severe damage from both the shaking and the tsunami that followed the earthquake [Borrero et al., 1997]. The damage during this earthquake, unlike the 1932 earthquakes, was limited to the coast. This fact, in addition to the large discrepancy observed between the reported magnitudes (Ms=7.4 by PDE and Mw=8.0 by Harvard CMT), calls for a detailed source study of this event. Here we present a teleseismic body-wave analysis of the mainshock, the foreshock, and the largest aftershock.

Slip kinematics and dynamics during and after the 1995 October 9Mw=8.0 Colima-Jalisco earthquake, Mexico, from GPS geodetic constraints

“…This earthquake was the first significant rupture of the Middle America Trench northwest of the Manzanillo Trough since the 1932 June 3 ( M w =8.2) and 1932 June 18 ( M w =7.8) earthquakes (Singh et al 1985). Earthquake focal mechanisms for the 1995 October 6 ( M w =5.8) foreshock, the main shock, and the 1995 October 12 ( M w =6.0) aftershock (Dziewonski et al 1997, Escobedo et al 1998) are consistent with shallow thrusting in a direction 5–10° anticlockwise from both the N40°E Rivera–North America convergence direction (DeMets & Wilson 1997) and the direction normal to trench. Inversions of surface and body waves recorded at local and teleseismic distances (Courboulex et al 1997; Escobedo et al 1998; Mendoza & Hartzell, 1999) indicate that the rupture initiated at a depth of 15–20 km near the northwest edge of the Manzanillo Trough and propagated ∼150 km to the northwest.…”

“…Earthquake focal mechanisms for the 1995 October 6 ( M w =5.8) foreshock, the main shock, and the 1995 October 12 ( M w =6.0) aftershock (Dziewonski et al 1997, Escobedo et al 1998) are consistent with shallow thrusting in a direction 5–10° anticlockwise from both the N40°E Rivera–North America convergence direction (DeMets & Wilson 1997) and the direction normal to trench. Inversions of surface and body waves recorded at local and teleseismic distances (Courboulex et al 1997; Escobedo et al 1998; Mendoza & Hartzell, 1999) indicate that the rupture initiated at a depth of 15–20 km near the northwest edge of the Manzanillo Trough and propagated ∼150 km to the northwest. The rupture consisted of several subevents, the largest of which began 35–40 s after the initial rupture and affected shallow regions of the subduction fault ∼100 km northwest of the Manzanillo Trough (Courboulex et al 1997, Escobedo et al 1998).…”

“…Inversions of surface and body waves recorded at local and teleseismic distances (Courboulex et al 1997; Escobedo et al 1998; Mendoza & Hartzell, 1999) indicate that the rupture initiated at a depth of 15–20 km near the northwest edge of the Manzanillo Trough and propagated ∼150 km to the northwest. The rupture consisted of several subevents, the largest of which began 35–40 s after the initial rupture and affected shallow regions of the subduction fault ∼100 km northwest of the Manzanillo Trough (Courboulex et al 1997, Escobedo et al 1998). The coseismic displacements of 11 GPS sites (Melbourne et al 1997) are best fit by a model in which the majority of the seismic moment was released in two patches, one near the northwest edge of the Manzanillo trough and the other ∼80–120 km farther northwest.…”

Source tomography by simulated annealing using broad-band surface waves and geodetic data: application to theMw= 8.1 Chile 1995 event

“…Failure to account for it may lead to overestimated values. body-wave inversions (using synthetics) yielded a lower value around 2.2 km s-' (Escobedo et al 1997;S. Leborgne, personal communication, 1996).…”

Rapid postseismic transients in subduction zones from continuous GPS

“…This proves important for differentiating between postseismic afterslip along the main shock asperities and postseismic slip triggered distal, in this case, down dip, to the main shock rupture. Three different methods of teleseismic analysis consistently show two primary asperities at depths above 15 km, shallower than the geodetic inversion (above 18 km) but at similar distances along strike from the centroid moment tensor (CMT) centroid [ Dziewonski et al , 1997; Mendoza and Hartzell , 1999; Escobedo et al , 1998; Courboulex et al , 1997]. The seismological estimates of the shallower slip distribution are probably the better estimate, given that the GPS inversion is hindered by having only mainland coastal stations and therefore becomes increasingly insensitive to variations in progressively shallower faulting.…”