Comment on A new estimate for present-day Cocos-Caribbean plate motion: Implications for slip along the Central American volcanic arc by Charles DeMets Marco Guzmán-Speziale Juan Martín Gómez Unidad de Investigación en Ciencias de la Tierra Instituto de Geofísica, UNAM
The Central American volcanic arc is the site of large (up to magnitude 6.5, White and Harlow, 1993) crustal earthquakes with strike-slip focal mechanisms; the faulting plane is either right-lateral parallel to the trend of the arc or left-lateral, perpendicular to the arc. Oblique plate convergence (and hence slip partitioning) has been previously proposed as the driving mechanism for strike-slip faulting along the volcanic arc [e.g., Fitch, 1972; Harlow and White, 1985; Guzmán-Speziale, 1995]. DeMets [2001] (hereafter DM2001) has determined a new pole of rotation for Cocos-Caribbean relative plate motion. He suggests that relative velocity vectors determined using his new Gaussian pole further supports this interpretation. A closer look at available evidence, however, points out to contradictions in this model. Here we comment on these contradictions. SLIP PARTITIONING Partition of the vector of relative plate motion into a component along the earthquake slip direction and a component along the arc depends on the directions of these two vectors and the direction of trench normal [e.g., Fitch, 1972; Beck, 1983; McCaffrey, 1992; Yu et al., 1993]. Using the new pole of DM2001, the azimuth of plate convergence between the Cocos and Caribbean plates is, from west to east, 20 to 25. The average azimuth of earthquake slip vectors is approximately 10 to 12 east of the azimuth of plate convergence [DM2001] (Figure 1). Trench normal, however, is not properly determined or not properly used in DM2001. From visual inspection, it is clear that the southeastern Middle America Trench is not oriented in one direction alone (Figure 1). At least three segments may be recognised: From the gulf of Tehuantepec to the gulf of Fonseca (longitudes -95 to -88.5 ), from here to Nicoya Peninsula (longitude -85.5), and then to the intersection with Cocos Ridge (longitude -83 ). We determined the general orientation of each of the three segments by performing a least-squares fit of a great circle to the digitised deepest part of the trench. The direction of trench normal for the first segment is ~N26 E; for the second segment it is about N36 E, and N29 E for the third.
In the first and third segments, the angle of obliquity, that between the vector of plate motion and trench normal, is very small. Hence, the component of plate motion along the arc (and perpendicular to trench normal) is also very small: about 6 mm/yr. Furthermore, when error limits are taken into account the angle of obliquity tends to zero. In the second segment, on the other hand, ibliquity is non-negligible and speed along the arc is16 to 20 mm/yr. FAULTING AND EARTHQUAKE FOCAL MECHANISMS In Sumatra, slip partitioning parallel to the trench is resolved along the Sumatra (or Semangko) Fault [e.g., Fitch, 1972]. But along the Central America volcanic arc there is no through-going fault that could be associated to strike-slip faulting parallel to the trench. DM2001 states that the existence of dextral-slip faults within the volcanic arc in Guatemala, El Salvador, and Nicaragua [Carr, 1976; Weinberg, 1992] and the concentration of earthquakes along the volcanic arc [White and Harlow, 1993], strongly suggest that the trench-parallel component of motion is concentrated along the volcanic arc. These right-lateral faults are of local extent and conjugate left-lateral faults are just as abundant, even reported in the same papers that DM2001 uses as evidence for along-arc right lateral motion [Carr, 1976; Weinberg, 1992; see also Weyl, 1980]. Most, if not all, earthquakes along the volcanic arc show almost pure strike-slip faulting, either right-lateral parallel to the arc, or left-lateral perpendicular to it [e.g., White, 1991; White and Harlow, 1993]. From focal mechanisms alone, it is not possible to determine which one is the actual faulting plane, although DM2001 implies along-the-arc right-lateral motion for all of them. There is indeed evidence that some of the earthquakes occurred along such faults. Some other earthquakes, however, took place along left-lateral faults perpendicular to the trend of the volcanic arc. The Managua earthquakes of 31 March, 1931 and 23 December, 1972, are known to have occured along the northeast-striking Tiscapa Fault, with documented left-lateral surface motion [Brown et al., 1973; Dewey and Algermissen, 1974; Ward et al., 1974]. From the distribution of
aftershocks, the San Salvador earthquake of 10 October, 1986, also took place along a left-lateral fault perpendicular to the trend of the volcanic arc; however, the largest aftershock of the sequence, three days after the main shock, had a clear right-lateral mechanism, parallel to the arc [Harlow et al., 1993]. By contrast, the 3 May, 1965 main shock, which ocurred in the same area as the 1986 event, had a right-lateral along-the-arc mechanism, judging from the very elongated character of isoseismal contours [White et al., 1987]. Other earthquakes for which there is evidence for right-lateral, arc-parallel faulting in El Salvador are the 1917-1919 sequence, as well as the 1951 event, because their isoseismal contours show distinct elongation along the arc. In Costa Rica, the isoseismals of the April and May, 1910, events are also elongated in a NW-SE direction [Montero and Dewey, 1982]. Aftershocks of the recent El Salvador earthquake of February 13, 2001 (as reported in the internet page of Centro de Investigaciones Geotécnicas), also aligned parallel to the volcanic arc. BUTTRESSING The forearc sliver being detached may be free to move, as in the case of Sumatra [e.g., Fitch, 1972; Beck, 1991], or buttressed, like the Kuril arc [e.g., Kimura, 1986; Wang, 1996]. In fact, Beck [1991] points out that most subduction zones with oblique subduction and along-arc strike-slip faulting are buttressed. In such cases, the leading edge of the sliver shows compression perpendicular to the arc [Kimura, 1986; Beck, 1991; Wang, 1996]. The Central America forearc sliver is buttressed at its leading edge, offshore Guatemala. Northwest of here, the Chiapas forearc is either not moving with respect to North America or being displaced along the strike-slip faults of southeastern Mexico, in a left-lateral strike-slip sense [e.g., Guzmán-Speziale et al., 1989]. If indeed the southeastern forearc is being sheared off along the volcanic arc, as proposed by DM2001, the head-on collision with the Chiapas forearc should be reflected as compression somewhere offshore the Mexico-Guatemala border region. This is not the case; neither free-air gravity anomaly maps [Couch and Woodcock, 1981] nor seismic reflection profiles [Sánchez-Barreda, 1981] in the area show any indication of
compressive structures. Earthquake focal mechanisms that would suggest such a state of stress are also absent. It is difficult to imagine how the forearc sliver would be displaced being buttressed and at such low velocities as 7 mm/yr. CONCLUSIONS Contrary to what is proposed by DM2001, there are several evidences that suggest that oblique subduction and slip partitioning is not the mechanism responsible for strike-slip faulting along the Central America volcanic arc: Along-arc components of relative plate motion are small and variable along the forearc because the angle of obliquity is also small. For some earthquake faulting planes are right-lateral, parallel to the volcanic arc whereas for others, left-lateral faulting perpendicular to the volcanoes is observed. The forearc is buttressed at its northwestern (leading) edge but there are no signs of compression, as would be expected if it were being displaced by oblique plate convergence.
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FIGURE CAPTIONS Fig. 1. a) Central American volcanic arc (triangles). Also indicated are the Middle America Trench and directions perpendicular to it (white arrows). b) Solid line: azimuth of Cocos- Caribbean plate convergence; dashed line: Trench-normal azimuth; circles: azimuth of earthquake slip vectors. c) Arrows: direction of relative plate convergence, size is proportional to speed; dashed line: component of relative plate motion along direction of trench normal; solid line: component of relative plate motion along the arc; dotted line: average direction of earthquake slip vectors.