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

Abstract

Teleseismic body waves recorded for the October 25, 1981, Playa Azul and September 21, 1985, Zihuatanejo earthquakes in western Mexico were inverted to derive the distributions and depths of coseismic slip. Broadband P wave displacements and intermediate‐period records were included to incorporate a wide frequency band in the analysis. For the Zihuatanejo earthquake, digital SH records located away from the nodal directions were used in the inversion process. Nonnodal SH waveforms were not available for the Playa Azul earthquake, but additional azimuthal coverage was obtained by including selected worldwide analog P wave records. The results for the Playa Azul earthquake indicate that rupture occurred in two separate zones both updip and downdip of the point of initial nucleation with the majority of the slip concentrated in a circular region (15‐km radius) downdip from the hypocenter. The maximum slip in this downdip region exceeds 3.6 m and is at a depth of about 16 km. The computed seismic moment is 7.14 × 1026 dyn cm. Coseismic slip is observed to occur entirely within the area of reduced slip separating the two main shallow sources of the Michoacan earthquake that occurred almost 4 years later on September 19, 1985. For the Zihuatanejo earthquake the P and SH data suggest rupture over a larger area (30‐km radius) with a lower peak slip (2 m) and encompassing depths between 12 and 26 km. Slip is concentrated in an area adjacent to one of the major sources of the Michoacan earthquake and represents the southeastern continuation of rupture along the Cocos‐North America plate boundary. The corresponding seismic moment is 1.35 × 1027 dyn cm. The zones of peak slip observed for the Playa Azul, Zihuatanejo, and Michoacan earthquakes are interpreted as asperity regions that control the cessation and generation of large earthquakes within the Michoacan segment of the plate boundary. The stress drops within each of these asperities are observed to be very similar and may reflect a characteristic asperity strength for this portion of the subduction zone.

Extreme wave deposits on the Pacific coast of Mexico: Tsunamis or storms? — A multi-proxy approach

“…an earthquake potentially capable of producing a larger tsunami. Moreover, the observed land-level change interpreted from environmental indicators distinguished in the stratigraphic and microfossil record agrees with our co-seismic deformation modeled using Mendoza's (1993) co-seismic slip data for the September 21st, 1985 earthquake and our model for vertical co-seismic displacement (Fig. 4b).…”

“…Another possible tsunami to correlate Event 2 with is the large Mw 7.6 tsunamigenic earthquake that occurred two days later on September 21st, 1985 immediately to the SE of Zihuatanejo (Mendoza, 1993) (Fig. 4).…”

“…Seismic activity along the Pacific coast of Mexico is mostly related to subduction of the Rivera-Cocos plates under the North America plate. This megathrust faulting caused the 19th September 1985 Mw 8.1 earthquake and tsunami and its Mw 7.6 aftershock on 21st September (Mendoza, 1993) that also triggered a tsunami. Before this event, on March 14th, 1979 a tsunami was triggered by a Mw 7.6 earthquake that occurred near Ixtapa-Zihuatanejo.…”

Tsunami Source Inversion Using Tide Gauge and DART Tsunami Waveforms of the 2017 Mw8.2 Mexico Earthquake

“…The earthquake is known as the Chiapas earthquake or the Pijijiapan earthquake of 2017, and it was felt from the Oaxaca and Chiapas regions in the south to the center of Mexico in the capital. This event is the most powerful earthquake in Mexico since the 1985 Michoacan earthquake in Mexico City (Mendoza 1993) and the 1932 Jalisco earthquake (Farreras and Sanchez 1991;Ramírez-Herrera et al 2014). Along the Pacific coast of Mexico, there are two distinctly defined seismic gaps: the Tehuantepec and Guerrero gaps.…”

Shallow seismicity patterns in the northwestern section of the Mexico Subduction Zone

“…Map of the broadband seismic station distribution, indicated by black triangles, across the states of Jalisco, Colima, and Michoacán along the Middle America Subduction Zone. Major earthquake rupture zones as defined by previous studies Reyes et al, 1979;Schmitt et al, 2007;Mendoza, 1993) are outlined in black. Red circles indicate an area of afterslip following the 1995 Mw 8.0 earthquake.…”