Analysis of the Petatlan aftershocks: Numbers, energy release, and asperities

Abstract

The Petatlan earthquake of March 14, 1979 (Ms = 7.6), occurred between the Middle America trench and the Mexican coast, 15 km southwest of Petatlan, Guerrero, Mexico. From seismograms recorded on smoked paper, FM, and digital tapes, we have identified 255 aftershocks with coda lengths greater than 60 s that occurred 11 hours to 36 days after the main shock. Based on these events, the aftershock epicentral area defined during the period be, tween 11 and 60 hours was about 2000 km2; between 11 hours and 6 days it was about 2400 km 2. Although the area grew to 6060 km 2 in 36 days, most of the activity was still confined within the area defined after 6 days. This suggests that the smaller aftershock area might represent an asperity. The distribution of events and energy release per unit area confirm the existence of heterogeneity in the aftershock area. Thus our data support the concept of an inhomogeneous rupture area that includes an asperity, as suggested by Chael and Stewart (1982) to account for the differences they computed for the body and surface wave moments from WWSSN data. However, the combination of the moments Reichle et al. (1982) report for body and surface waves from IDA data and the rupture areas reported in this paper results in a solution that is most physically realizable in terms of stress drop and slip. We calculate stress drops of 5 and 15 bars, the former for the average over the entire area, the latter for the asperity, and an average slip of 60 cm for the entire area and 120 cm for the asperity. These values for slip are 30% and 60%, respectively, of the convergence of the Cocos plate relative to the North America plate during the 36-year period between the last two major earthquakes in the Petatlan area. Hypocenters of the aftershocks define a zone about 25 km thick, dipping 15 ø with an azimuth of N20øE, which is perpendicular to the Middle America trench. Most aftershocks are below the main shock. The b value estimated for aftershocks in the range 3.0 -< MoL --< 4.5 is 60% higher than one determined similarly by Hsu (1981) for the foreshocks.

Correlation between the fault rupture pattern and associated microseismic activity of the Petatlan earthquake

“…We compare the fault rupture pattern for the Petatlan earthquake obtained by recent P-wave modelling studies (Singh & Mortera 1991;Ruff & Miller 1994;Mendoza 1995) with that inferred from the study of the foreshock-aftershock sequence (Valdes et al 1982;Novelo-Casanova 1983;Novelo-Casanova et al 1984;Hsu et al 1985). The solution provided by the locally recorded data agrees well with the source properties estimated by Singh & Mortera (1991) and Mendoza (1995).…”

“…Studies of the local data of this major earthquake include analysis of the seismicity pattern preceding the mainshock (Gettrust et al 1981;Hsu et al 1983); space-time distribution and properties of the aftershocks (Valdes et al 1982;Novelo-Casanova 1983;Novelo-Casanova et al 1984) and the spatial and temporal variations of seismic activity Novelo-Casanova ( 1983) suggested that two asperities were broken in the rupture area, on the basis of comparing the spatial distributions of local seismicity and energy release before and after the mainshock. Hsu et al (1985) analysed these observations in more detail, and confirmed the heterogeneity along the fault plane.…”

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

“…4a) (Meyer et al, 1980;Valdés et al, 1982;Hsu et al, 1983;Mendoza, 1995). The 1200 km 2 epicentral area was located entirely in the ocean (Sanchez and Farreras, 1993).…”

Aftershocks Identification and Classification

“…Kisslinger [3] qualitatively defines three types of aftershocks: a) Class 1-events occurring in the rupture area of the fault plane or on a thin band around it; b) Class 2-events that occur on the same fault but outside of the co-seismic rupture area c) Class 3-events happening elsewhere, on faults that are different from the one that has generated the mainshock; these events, whether in the same region or not, will not be considered herein as aftershocks, but will be classified as triggered earthquakes. The aftershocks occurring within 24 to 48 hours after a strong earthquake mainly in the co-seismic rupture area, indicate that seismicity is predominantly of Class 1; over longer times the aftershocks area increases [35] [36] [37] [38] [39], and seismicity is predominantly of Class 2.…”