Plate Supduction and Prediction of Earthquakes Along the Middle America Trench

Abstract

Results of detailed studies of seismic slip, tectonic structures, fault mechanisms, and rupture patterns along various segments of the Middle America Trench provide a tectonic framework for analyzing long, intermediate, and short term patterns of seismicity preceding large (M_s ≥ 7), shallow earthquakes. Long term. Average repeat times of 33±8 yrs for earthquakes M_s ≥ 7.5 since 1898 and 35±24 yrs since 1540 indicate that the 81 yr history (1898 to 1979) may be adequate for estimating temporal deficiencies in seismic slip. Seismic slip rates are spatially nonuniform on a scale of 100 km and forecasts are only valid when local conditions (eg. subduction of seafloor topography) are considered. The current temporal deficiency in seismic slip offshore Mexico and Guatemala is equivalent to 5 earthquakes of M_s = 8 in the next 5 years, or 6 in the next 10 years. The most likely areas for the next large shallow earthquakes (M_ s≥ 7.5) are between longitudes 98.5°–99.7°W, 97.2°–97.7°W, and 90° – 92° W. A major episode of 6 earthquakes with 7.9 ≤ M_s ≤ 8.2 occurred between 1899 and 1907. At least 3 of these events were in the regions listed above, where average repeat times have been 31.7±9.2 yrs (M_s ≥ 7.5). Intermediate term. Five of the six largest earthquakes (M_s ≥ 7) since 1965 were preceded by between 19 to 46.5 months of relative seismic quiescence (m_b ≥ 4), with probabilities of 0.8% to 2.6% of this quiescence having occurred due to chance. Between 98.5° to 99.7°W, a conspicuous seismic quiescence has persisted for 48 months; this quiescence has a 0.05% probability of occurring due to chance. Short term. A local field array of seismographs documented the following patterns during the 3 weeks prior to the 29 November 1978 earthquake (M_s = 7.8) in Oaxaca, Mexico: • (1) The mainshock location was surrounded by a seismically quiet area of 2800 km2, which was broken by only a single cluster of activity 2 weeks before the final foreshock sequence. • (2) At 1.8 days before the mainshock, the foreshock sequence began migrating NE to SW through the quiescent zone toward the eventual rupture point; activity then subsided for 12 hours until failure. • (3) The foreshock sequence was identifiable, on a statistical basis, relative to previous activity during the 3‐week period. A significant result of this study was the documentation of a foreshock sequence at energy levels below standard WWSSN detection thresholds, but otherwise similar to other sequences observed worldwide. This observation suggests the possibility that many large earthquakes may have foreshock activity, which has not been detected at teleseismic distances, and that small earthquakes are at least as sensitive as indicators of the level of stress as are larger (m_b ≥ 4) earthquakes.

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