Astiz Delgado, Luciana Maria de Los Angeles (1987) I. Source analysis of large earthquakes in Mexico. II. Study of intermediate depth earthquakes and interplate seismic coupling. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:08222012-144207263
Along the coast of Mexico and Central America several seismic gaps were defined by the timing, location and extent of large earthquakes. Among these regions with high seismic potential, the Ometepec and Michoacan gaps have broken since 1980. The 1982 Ometepec doublet and the 1981-1986 Michoacan sequence are studied in detail in Part I.
The seismic moment of each of the Ometepec doublet events is 2.8X 1026 dyne cm. The first event involved a deeper asperity (at 20 km) that caused an incremental stress change large enough to trigger the second event at shallower depth. The second event is best modeled by two sources at 15 and 10 km depth. T he largest event of the Michoacan sequence occurred on September 19, 1985 (Mw=8.0) and caused extensive structural damage and death to over ten thousand people in Mexico City. The first event of the sequence was the 1981 Playa Azul event, which broke the central part of the gap. It is 27 km deep and has a seismic moment of 7.2X 1027 dyne cm. The seismic moment of the September 19, 1985 earthquake was released in two distinct events with the rupture starting in the northern portion of the seismic gap and propagating 95 km to the southeast with low moment release through the area already broken by the 1981 Playa Azul earthquake. The rupture propagated 125 km further southeast with an Mw=7.5 event on September 21, 1985. Another aftershock occurred on April 30, 1986, 50 km to the northwest of the September 19 mainshock. The most recent Michoacan events are shallower, 17-22 km, than the Playa Azul earthquake, which has a higher stress drop suggesting a higher stress level at greater depths in the Michoacan gap. The slip vectors of these events are consistent with the convergence direction of the Cocos and North American plates.
Part II investigates the relation of intermediate-depth earthquakes to the shallower seismicity, especially since these events may reflect the state of inter-plate coupling at subduction zones. A catalog of earthquake focal mechanisms was gathered, which includes all events listed by NOAA and ISC catalogs with M>6 and depth between 40 to 200 km, that occurred between 1960 and 1984. The final catalog includes a total of 335 events; 47 were determined by this study. Focal mechanism solutions for intermediate-depth earthquakes with M>6.8 can be grouped into four : 1) Normal-fault events (44%), and 2) reverse-fault events (33%), both with a strike nearly parallel to the trench axis. 3) Normal or reverse fault events with a strike significantly oblique to the trench axis (10%) and 4) tear faulting events (13%).
Simple models of plate coupling and geometry suggest that Type 1 events occur at strongly coupled plate boundaries where a down-dip extensional stress prevails in a gently dipping plate. Continental loading may be another important factor. In contrast, large normal fault earthquakes occur at shallow depths in subduction zones that are decoupled. Type 2 events with strike subparallel to the subduction zone, most of them with near vertical tension axis, occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km. In terms of our simple model, the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may induce near vertical tensional stress at intermediate depth and, consequently, the change m focal mechanism from Type 1 to Type 2 events. Events of Type 3 occur where the trench axis bends sharply, causing horizontal extensional or compressional intraplate stress. Type 4 are hinge faulting events.
We also investigate the temporal variation of the mechanism of large intraplate earthquakes at intermediate depths in relation to the occurrence of large underthrusting earthquakes in Chile. Focal mechanisms were determined for three large events (March 1, 1934: M=7.1, d=120 km, April 20, 1949: M=7.3, d=70 km and May 8, 1971: Mw=7.2, d=150 km), which occurred down-dip of the great 1960 Chilean earthquake (Mw=9.5) rupture zone. The 1971 event is down-dip compressional, whereas the 1949 and the 1934 earthquake focal mechanisms are consistent with a down-dip te nsional mechanism. Published fault plane solutions of large intermediate-depth earthquakes (March 28, 1965 and November 7, 1981), which occurred down-dip of the Valparaiso earthquakes of 1971 (Mw=7.8) and 1985 (Mw=8.0) are also down-dip tensional. These results suggest that before a major thrust earthquake, the interplate boundary is strongly coupled and the subducted slab is under tension at intermediate depths; after the occurrence of an interplate thrust event, the displacement on the thrust boundary induces transient compressional stress at intermediate depth in the downgoing slab. This interpretation is consistent with the hypothesis that temporal variations of focal mechanisms of outer-rise events are due to changes of interplate coupling.
The variation of intermediate-depth earthquake focal mechanisms with M≥6 is examined region by region in relation to local variations of the strength of interplate coupling. In summary, regions that are mostly uncoupled present down-dip tensional stresses in a steeply-dipping slab probably induced by the negative buoyancy of the subducted lithosphere (e.g., North Scotia arc). Double seismic zones may be present in partially coupled regions in response to un bending of the downgoing slab (e.g., Northeast Japan). Lateral bending or tearing of the slab influences the stress distribution within the subducted plate (e.g., New Ireland). Subduction of topographic highs may also change the interplate coupling locally (e.g., Louiville ridge in T onga). Regions that are mostly coupled are generally shallow dipping and the observed continuous seismicity is at most 300 km deep. In these regions normal faulting events occur at the base of the coupled region (e. g. South America). In contrast, normal faulting events occur at the trench axis at uncoupled regions where the strains due to bending of the plate are largest. Temporal variations in the interplate coupling due to the occurrence of large thrust events at the plate boundary are suggested in several regions such as Middle America and Chile.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Geological and Planetary Sciences|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||14 May 1987|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Benjamin Perez|
|Deposited On:||23 Aug 2012 17:28|
|Last Modified:||26 Dec 2012 04:44|
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