Stress accumulation and release since 1882 in Ometepec, Guerrero, Mexico: implications for failure mechanisms and risk assessments of a seismic gap.

  • Gonzalez-Ruiz J
  • McNally K
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We found evidence that four episodes of large shallow (h < 30 km)
interplate earthquakes ruptured approximately the same segment of
the Middle America subduction zone in Ometepec, Guerrero, Mexico,
during the last century: in 1890 (M = 7.2), 1937 (MS = 7.5), 1950
(MS = 7.3), and 1982 (MS = 6.9 and 7.0). This segment is about 70
km long, which is near the limit of resolution to define seismic
gap dimensions. We synthesized the spatial, temporal, and mechanistic
patterns of regional relocated seismicity (mb ‚â• 4.0) to investigate
the mechanics of rupture within the same seismic gap for the relatively
large number of times (four) offered by the Ometepec case history.
Despite the similarities in source depths and geometries of the shallow
main shocks, we found marked differences in their magnitudes, recurrence
intervals, and rupture modes (e.g., degree of source complexity and
event multiplicity). The episodic amounts of energy release do not
appear to scale with the observed elapsed times between the shallow
main shocks (47.1, 13.0, and 31.5 years in chronological order) according
to time- or slip-predictable models of earthquake recurrence. We
interpret these findings in the context of a variable rupture mode
of the Ometepec region. This variable rupture mode suggests that
probabilistic forecasts of future large earthquakes in the region,
which are based solely on the historic record of main shocks, can
be uncertain by at least ±57% of the average local repeat time of
30.5 years and 1/2 unit of magnitude. Specific predictions of future
strong ground motions are obviated because earthquakes within the
gap do not exhibit a “characteristic” behavior. We also found
that all four episodes of interplate main shocks were preceded by
large downdip normal fault earthquakes at intermediate depths (M
‚â• 6.5, h > 60 km). The time intervals between extensional and subsequent
compressional events range from 1.6 to 9.9 years with an average
of 6.3±3.7 years. This apparent pattern which initiates at depth
was further documented for the final 2/3 of the last seismic cycle.
From the spatiotemporal distribution of seismicity during this final
period, we identified three general stages of stress release. Stage
I, from 1964 to January 1980, was characterized by scattered seismicity.
Stage II, from February 1980 to June 6, 1982, showed a significant
increase in downdip seismic activity while the shallow zone became
quiescent. (There was also a period of apparent seismic quiescence
from mid-1975 to early 1978). Most seismic energy in stage II was
released on October 24, 1980, by a downdip normal fault earthquake
at intermediate depth (MS = 7.0, h = 65 km). Stage III began on June
7, 1982, with the occurrence of two large interplate thrust fault
earthquakes at shallow depths (MS = 6.9, h = 18 km, and MS = 7.0,
h = 19 and 23 km). The documentation of stage III through June 1986
shows subsidence of previous downdip activity and an aftershock zone
which is spatially coincident with the shallow quiet zone identified
in stage II. The spatiotemporal characteristics of the 1982 main
shock-after shock sequence suggests that rupture propagated to the
northwest toward the site of the 1957 (MS = 7.5) Acapulco-San Marcos
earthquake. This propagation is of interest because the latter site
is presently considered to possess high seismic potential. We suggest
that downdip normal fault seismic activity at intermediate depths
reflects a symptomatic state of stress associated with some basic
mechanism of plate interaction prior to interplate slippage in a
large earthquake. This mechanism may have an important bearing on
the generation and possible intermediate-term forecasts of large
shallow interplate earthquakes along the subduction zone in Mexico.

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  • J. R. Gonzalez-Ruiz

  • K. C. McNally

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