Pure Appl. Geophys. 170 (2013), 2163–2172 Ó 2013 Springer Basel DOI 10.1007/s00024-013-0667-9 Pure and Applied Geophysics Relative Motion Between the Rivera and North American Plates Determined from the Slip Directions of Earthquakes GERARDO SUÁREZ,1 SAID H. JARAMILLO,1 and W. L. BANDY1 Abstract—So far, the direction and rate of relative motion between the Rivera and the North American plates (RIV-NAM) has been determined by the combination of two Euler poles: Rivera (RIV), with respect to Pacific (PAC), and PAC with respect to North America. Here, we estimate the relative motion of this plate pair (RIV-NAM) assuming that the horizontal projection of the direction of slip of the earthquakes occurring on the RIV-NAM boundaries reflect their relative plate motion. A catalog of earthquakes for which focal mechanisms are reported since 1976 is used in the analysis. Earthquakes were considered in the three segments of the RIV-NAM plate boundary: the subduction zone of the Rivera plate beneath the Jalisco block, the Tres Marias Escarpment and the events associated with the Tamayo Fracture Zone. The best fitting Euler pole is determined using a grid search of 64 potential poles. The slip direction predicted for each grid point is compared to the slip direction of the focal mechanisms of the earthquakes on the plate boundary. The best fitting Euler pole, determined in a root mean square sense (RMS), is located at 21.8°N, 107.6°W. A rate of rotation of 5.3°/year is estimated assuming the seismic earthquake cycle of the 1932 and 1995 great earthquakes represents a lower bound of the rate of plate motion in the subduction zone. The best fitting Euler pole shows that the subduction of the Rivera plate takes place in a direction perpendicular to the trench with a relative velocity of 4.3 cm/year, offshore Manzanillo. The rate of relative motion RIVNAM decreases from SE to NW. North of approximately 21°N, the subduction of the Rivera plate becomes oblique to the trench and the relative velocity between the two plates decreases to an average of 1.9 cm/year. This slow rate of convergence may explain the rapid decrease of seismicity in the trench and the apparent absence of large earthquakes in this region. In the Tres Marias Escarpment, our bestfitting pole suggests that subduction stops, giving way to high-angle reverse faulting perpendicular to the Tres Marias Escarpment, in agreement with the reverse faulting earthquakes occurring here. To the north of 22.5°N, the slip predicted by the best-fitting pole suggests right-lateral faulting in a direction parallel to the Tamayo Fracture Zone, at a very low velocity (0.5–1.0 cm/year). The best fitting Euler pole determined here lies very close to the RIV-NAM plate boundary in the vicinity of the Tamayo Fracture Zone. This location of our best fitting Euler pole explains the low relative plate velocity, the relatively low level of seismic activity and the presence of a broad zone of deformation that accommodates the RIV-NAM motion. 1 Instituto de Geofı́sica, Universidad NacionalAutónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico. E-mail: [email protected]; [email protected] 1. Introduction The collision of the Farallon-Pacific spreading center with the western margin of North America at about 28 Ma, marked the beginning of a profound change in the tectonic environment of western North America. The Farallon plate was segmented into a series of smaller plates, one of which is the Rivera plate (ATWATER 1970). The exact location of the plate boundaries and the direction and velocity of relative motion between the Rivera and North America and the Rivera and the Cocos plate have been debated in the literature since the discovery that the Rivera behaved as a separate plate (e.g., LARSON 1972; MINSTER and JORDAN 1979; KLITGORD and MAMMERICKX 1982; MAMMERICKX and KLITGORD 1982; NESS et al. 1985; BANDY and YAN 1989; DEMETS and STEIN 1990; BANDY 1992; LONSDALE 1995; DEMETS and WILSON 1997; BANDY et al. 1997; and SUÁREZ et al. 1999) (Fig. 1). One of the major difficulties in obtaining accurate poles of instantaneous motion for some of these plate boundaries is the fact that the Rivera plate has deformed and some of the plate boundaries have changed locations and their style of deformation over time (e.g., DEMETS and TRAYLEN 2000; BANDY et al. 2011). Furthermore, the RIV-NAM pole is obtained by a combination of two Euler poles. The PAC-RIV pole runs into the unavoidable problem of having a big uncertainty because the Pacific-Rivera boundary is very short. The errors in this pole are compounded in the calculation of the Rivera-North American pole of motion. This paper discusses the relative motion between the Rivera and the North American plates (RIVNAM). The Euler poles proposed to date, that describe the motion between RIV-NAM, lie very 2164 G. Suárez et al. close to the Rivera plate. Thus small changes in the location of the pole induce large variations in both the direction and in the rate of motion between RIV-NAM. Therefore, the poles of relative motion of RIV-NAM have been calculated combining the Rivera-Pacific Euler pole with the one describing the motion of the Pacific relative to North America. As discussed above, this process compounds the errors of each of the Euler poles involved. An alternative method to estimate the pole of relative motion between RIV-NAM, is to assume that the horizontal projection of slip vectors of earthquakes occurring along the RIV-NAM plate boundary reflects the instantaneous relative motion of these plates. The purpose of this work is to calculate the RIV-NAM best-fitting Euler pole, using the slip directions of all earthquakes with published focal mechanisms on the boundaries of the Rivera and North American plates: earthquakes where the Rivera Pure Appl. Geophys. plate subducts the Jalisco block, events beneath the Tres Marias Escarpment (TME) to the northwest of the subduction zone, and earthquakes on the Tamayo Fracture Zone (TFZ). This last plate boundary is not clearly defined (Fig. 1). Some authors suggest that here, the RIVNAM boundary coincides with two fractures zones extending to the southeast from the East Pacific Rise at 22°N and 22.5° (LONSDALE 1995; DEMETS and WILSON 1997). However, practically all of the seismicity takes place in the vicinity of the TFZ (Fig. 1) suggesting that a large fraction of the relative plate motion takes place along this plate boundary. This region has been defined as a diffuse plate boundary where deformation takes place over a broad area. Here, the slip direction was measured on the nodal planes oriented nearly east west showing right-lateral strike-slip motion (Fig. 2). We believe that it is valid to use slip vectors in the area of the TFZ. Although Figure 1 Black dots represent the epicentral locations of earthquakes reported by the ISC from 1976 to 2007. The area enclosed by a dashed line and the shadowed zone represents the rupture length of 3 June and 18 June, 1932 earthquakes, respectively. The oval enclosed by a continuous line represents the aftershocks zone of the 9 October 1995. The boundaries of the Rivera plate are shown as: TTF-Tamayo Transform Fault; TFZTamayo Fracture Zone (dashed line); TME-Tres Marias Escarpment (solid line); EPR-East Pacific Rise; RTF-Rivera Transform Fault; MSSMoctezuma Spreading Segment; MAT-Middle American Trench (line with triangles) Vol. 170, (2013) Relative Motion Between the Rivera and North American Plates 2165 Figure 2 Focal mechanisms of the earthquakes used in this work are shown in low-hemispheric projections, where dark areas represent compressional arrivals. The earthquakes are grouped as: 1) events on the subduction zone (5, 7, 8, 11, 13, 14 and 16); 2) earthquakes on the TME (1, 2 and 15); and 3) events in the vicinity of the TFZ (3, 4, 6, 9, 10 and 12) the earthquakes here may not represent the full motion between the plates (RIV-NAM), it has been shown in other areas of diffuse deformation, like the plate boundary between the Australian and Indian plates, that the general direction of motion coincides with the orientation of the slip vectors (GORDON et al. 1990; GORDON, 1998; ROYER et al. 2007). The slip vectors of the published focal mechanisms are used in a grid search of potential Euler poles. The pole of rotation that best fits the direction of slip of all earthquakes in a root mean squares sense (RMS) is assumed to be the best fitting pole for RIV-NAM. 2. Tectonic Boundaries and Poles of Relative Motion of the Rivera Plate 2.1. Plate Boundaries of the Rivera Plate The limits between the Rivera and Pacific plates are the Rivera Transform Fault (RTF) to the south and the East Pacific Rise (EPR) to the northwest (MINSTER and JORDAN 1979; EISSLER and MCNALLY 1984; DEMETS and STEIN 1990) (Fig. 1). The Euler pole defining the relative motion between these two plates is fairly well constrained, although changes in its location since 0.78 Ma are still being debated. The plate boundary and the style of tectonic deformation between Rivera and Cocos are controversial. There are no bathymetric features that can be clearly associated with the Rivera-Cocos plate boundary. The relative plate motion has been interpreted as strike-slip, a rift, or a compressional environment (e.g., KLITGORD and MAMMERICKX 1982; NIXON 1982; EISSLER and MCNALLY 1984; MAMMERICKX and CARMICHAEL 1989; DEMETS and STEIN 1990; BOURGOIS and MICHAUD 1991; LONSDALE 1995; DeMETS and WILSON 1997; MICHAUD et al. 1997; BANDY et al. 1995, 1998a, b, 2000; KOSTOGLODOV and BANDY 1995; SUÁREZ et al. 1999; SERRATO-DÍAZ et al. 2004). The Middle American Trench (MAT), between approximately 18.5° North and the Islas Marias, 2166 G. Suárez et al. marks the boundary between the Rivera and the North American plate. Although this segment of the MAT is a plate boundary with relatively low seismicity, some of the larger subduction earthquakes in the MAT have originated in this segment of the trench: the great Jalisco earthquake of 1932 and, more recently, the 1995 Colima-Jalisco earthquake (e.g., SINGH et al. 1985; PACHECO et al. 1997) (Fig. 1). To the northwest of the MAT, the plate boundary appears to be delimited by the TME and the TFZ. EISSLER and MCNALLY (1984) suggested that the TFZ acts as a right-lateral transform fault between these two plates. Other authors, however, have suggested that in this region the Rivera plate has been accreted to the North American plate since 1.5 Ma (LONSDALE 1995; DEMETS and WILSON 1997; BIRD 2003). Relative Motion Between Rivera and North American Plates. There is a debate regarding the location of the Euler pole and the rate of rotation between the Rivera-North American (RIV-NAM) plates (e.g., KOSTOGLODOV and BANDY 1995). However, all of these models agree that for the past 3 Ma, the location of the pole lies very close to the Rivera plate. Thus small changes in the location of the pole Pure Appl. Geophys. result in drastic changes in the rate and direction of relative plate motion. 3. Methodology Used to Determine RIV-NAM Pole of Rotation 3.1. Earthquakes Selected for the Analysis The seismicity of western Mexico is dominated by the subduction of the Cocos and Rivera plates beneath the North American plate. Along the MAT, the seismicity varies drastically along the strike-slip. To the north of 20°N, the seismicity in the subduction zone decreases rapidly both in the number and in the magnitude of the earthquakes, with respect to that observed in the south. The seismicity beneath the TME and along the TFZ is sparse and diffuse (Fig. 1). All published source mechanisms earthquakes in the region encompassing the RIV-NAM plate boundary, occurring from 1976 to 2010, were collected from the Global Centroid Moment Tensor Catalog (GCMT) and other sources (Fig. 2; Table 1). The epicentral locations of the earthquakes are those reported by the International Seismological Centre. Table 1 Epicentral Locations and Focal Mechanisms of the Earthquakes Id Date year-month-day Latitude (°) Longitude (°) Depth (km) Strike (°) Dip (°) Slip (°) Azimuth of slip vector projected (°) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1948-12-04a 1976-02-09b 1982-12-08 1989-01-31 1989-12-08c 1991-04-01 1995-10-09 1995-10-18 1999-03-12 2000-12-08 2001-04-29 2001-11-13 2003-01-22 2003-02-17 2007-02-11 2007-09-06 21.6 21.63 22.71 22.28 19.25 22.37 19.11 19.42 22.27 22.72 18.76 22.38 18.94 18.88 21.41 19.54 –106.7 –106.59 –106.82 –107.30 –105.14 –107.00 –104.20 –104.93 –107.35 –107.55 –104.58 –106.95 –104.30 –104.96 –106.27 –105.02 0d 11 ± 2 62 20.3 15 10 42.3 51.6 6.6 4.6 29 17.7 45.7 11.1 47.3 63.4 316 276 160 276 13 303 302 273 274 89 292 321 299 282 314 310 54 36 90 74 22 10 9 25 83 87 18 78 23 34 29 25 116 93 180 173 162 132 92 49 –179 178 77 –169 85 64 114 96 20 ± 4 3±2 340 283 31 351 30 44 273 271 35 310 34 38 20 34 a JARAMILLO and SUAREZ (2011) b GOFF et al. (1987) c Global Centroidal Moment Tensor (CMT) d ISS Vol. 170, (2013) Relative Motion Between the Rivera and North American Plates The database also includes the largest instrumental earthquake on the TME. This event, which took place on 4 December 1948, had a magnitude Ms 6.9; Mw 6.4 (JARAMILLO and SUAREZ 2011). The focal mechanism of the 1948 earthquake indicates high angle reverse faulting on NW to SE oriented nodal planes (Fig. 2). 3.2. Grid Search for the Best Fitting Euler Pole In order to determine the Euler pole that best describes the relative motion between the Rivera and North American plates, a grid search was conducted. At each point of the grid, the direction of relative motion was estimated and compared with the slip direction of the focal mechanisms of the earthquakes on the RIVNAM plate boundaries. A grid spacing of 0.2° was selected. This value is consistent with the estimated errors in the epicentral locations of most of the earthquakes used in the analysis. Furthermore, the locations of the existing, best-fitting Euler poles describing the motion of this plate pair have error estimates that are far greater than the proposed grid spacing. The focal mechanisms used in the grid search are: (1) Events that occur along the MAT reflecting Rivera plate subduction; (2) Earthquakes that take place beneath the TME; (3) Events with epicenters 2167 that lie in the vicinity to the TFZ (Fig. 2; Table 1). The process followed in the grid search analysis is summarized as follows: 1. A grid was generated from 21° to 22.4°N and 107° to 108.4°W, with a spacing of 0.2°. This area yields 64 potential Euler poles. 2. For each point on the grid, a velocity vector was calculated for all earthquakes on the RIV-NAM plate boundaries (Table 1). The velocity vector is defined as the horizontal projection of the slip vector of all published focal mechanisms. 3. The differences between the slip vectors of the earthquakes and the direction of motion predicted by each point on the grid were computed. 4. A root mean square value (RMS) was calculated of the difference between the predicted and the observed slip direction for each potential Euler pole of the grid. 4. The Best-Fitting RIV–NAM Euler Pole Before analyzing which is the best-fitting pole, it is illustrative to compare independently the errors in each of the three segments of the RIV-NAM plate boundary (Fig. 3). Figure 3 The graph shows the RMS errors between the observed and estimated direction of slip for the 64 poles on the grid. RMS errors are shown separately for earthquakes along the subduction zone (MAT) (set 1); the TME (set 2), and the TFZ (set 3) (see inset) 2168 G. Suárez et al. The errors between observed and predicted slip directions in the MAT subduction zone are the smallest. On this plate boundary, the RMS errors are relatively constant and are the smaller of the three plate boundaries. In contrast, the TME and the TFZ show consistently larger errors. For all poles, the errors are larger on the TFZ than on the TME (Fig. 3). In the case of the TME, minima in RMS errors are observed for poles 20–40. The errors rise sharply for poles 41–64. For the rest of the poles, the RMS errors on the TME plate boundary show large variations and local minima (Fig. 3). In the TFZ, not only are the errors larger compared to the other two segments, but they also show a large variability, which makes it difficult to estimate a minimum. On average, poles numbers 0–19 show the largest errors (Fig. 3). The relatively large RMS errors in the TFZ may be due to the fact that this plate Pure Appl. Geophys. boundary is probably a broad zone of deformation and not a discrete fault accommodating the relative plate motion here. In a zone of diffuse deformation, a single pole would not fit all earthquakes. Pole number 28 is at the center of a broad minimum and shows the smallest RMS value when the three plate boundaries are considered jointly. This point was selected as the best fitting Euler pole (21.8°N, 107.6°W). The locations of the poles and their associated RMS errors are shown on Fig. 4. In plane view, pole number 28 clearly shows that the errors increase for all poles that are farther away from this point (Fig. 4). It is worth noting that the location of the best-fitting Euler pole determined from the grid search lies about 20 km to the east of the Euler pole suggested by LONSDALE (1995) (Fig. 4). Statistically, the slip predicted by the points inside the contour with a value of RMS \ 20 have a 95 % confidence level. Figure 4 Map view of the grid points used in the search of the best-fitting RIV-NAM Euler pole. The grid points are colored according to the observed RMS values obtained from the difference between observed and predicted slip directions. The best-fitting Euler pole (blue point) lies in the center of the minimum of the RMS errors. Errors increase away from this minimum. Other Euler poles proposed are shown as: square (DEMETS and STEIN 1990); a pentagon (MINTER and JORDAN 1979); black circles (BANDY and PARDO 1994); a triangle (DEMETS et al. 1994); a star (LONSDALE 1995); a diamond (BANDY et al. 1997); an asterisk (DEMETS and WILSON 1997); and an inverse triangle (DEMETS et al. 2010). Red arrows represent the direction of slip predicted by the best fitting Euler pole and the green arrows the observed direction of slip of the focal mechanisms. The bathymetry of the TME is shown in the inset from SANDWELL and SMITH (1997) Vol. 170, (2013) Relative Motion Between the Rivera and North American Plates The analysis presented here allows determining only the geographic location of the best-fitting Euler pole. It is not possible, however, to estimate the rate of rotation of the estimated Euler vector. In order to estimate the angular velocity of the pole, a calculation is made following the approach of BANDY et al. (1997). These authors propose that a lower bound of the slip rate of the Rivera plate beneath the North American plate may be obtained by summing the slip observed during the most recent earthquake cycle in this segment of the subduction zone. Summing the slip of the 3 and 18 June 1932 and the 9 October 1995 great earthquakes, an angular velocity of rotation of 5.3°/year is determined. 5. Discussion of Results The rate and direction of motion predicted by the best fitting Euler pole indicate that south of 2169 approximately 20°N subduction of the Rivera plate beneath the North America takes place in a direction N30°E–N50°E, at a rate of approximately 4.31 cm/year. This direction is almost perpendicular to the trench and agrees with the slip direction of the focal mechanisms of the major earthquakes in the area (Fig. 5). North of 20°N, the convergence becomes oblique to the subduction zone and the rate of relative motion decreases sharply to about 1.9 cm/year (Fig. 5). This change in direction of relative motion and the slower slip rate may explain the decrease in the rate of seismicity observed here (Fig. 1). RUTZ-LÓPEZ and NUÑEZ-CORNÚ (2004) on the basis of locally recorded earthquakes, suggested a subduction direction that is oblique to the trench in agreement with the direction of slip suggested here. In the TME, the relative motion RIV-NAM is perpendicular to the sharp bathymetric escarpment that delineates the TME (Figs. 4, 5). The subduction Figure 5 The arrows on the map indicate the average direction of slip along the various RIV-NAM plate boundaries predicted by the best-fitting Euler pole. The number indicates the predicted rate of motion. Solid contour indicates the value of RMS \ 20 for the best-fitting Euler pole proposed here (black point). Dashed ellipses indicate 95 % confidence areas for previous poles proposed. Shaded zone indicates region of diffuse deformation proposed by DEMETS and WILSON (1997) and the fractures zones proposed by LONSDALE (1995). Other symbols indicate previous proposed poles as in Fig. 4 2170 G. Suárez et al. seems to stop in this segment of the trench, giving way to high-angle reverse faulting earthquakes with compressional axes oriented NE–SW, such as the 1948 and 1976 quakes (GOFF et al. 1987; JARAMILLO and SUAREZ 2011). The sharp bathymetric relief of the TME coincides with the presence of reverse faulting as shown by the focal mechanisms (Figs. 2, 4). The convergence rate on this compressional boundary is approximately 0.9 cm/yr. This rate of relative motion is considerably lower than that predicted by KOSTOGLODOV and BANDY (1995) (2.0–3.0 cm/year) based on seismotectonic arguments. It is possible that the rate of rotation used here based on a seismic cycle of only a few tens of years is a lower bound for the rate of relative motion. Nevertheless, it should be pointed out that the low level of seismic activity observed in the TME is more consistent with the slow convergence rate observed from our best fitting Euler pole. On the northwestern RIV-NAM plate boundary, the best-fitting Euler pole lies within the zone of apparent diffuse deformation and explains some of the seismic and bathymetric features in this region. The location of this Euler pole close to this diffuse plate boundary explains why the deformation is very slow here. The rate of relative motion predicted by our best fitting Euler pole for this plate segment is only 0.5–1.0 cm/year. In an area of slow deformation the seismicity is low in both the number of earthquakes and in magnitude, as observed in this region (Figs. 1, 5). Furthermore, the location of our best fitting Euler pole explains the very complicated bathymetry observed in this area. LONSDALE (1995) mapped some east–west features that coming off the East Pacific Rise that are sharply truncated to the east. The termination of these features to the east appears to be controlled by the location the proposed Euler pole. The focal mechanisms of earthquakes in the vicinity of the TFZ show almost pure right-lateral, strike-slip motion parallel to the general orientation of the TFZ (Fig. 2). This is in agreement with the slip vectors predicted by the proposed new Euler pole (Figs. 4, 5). It is possible that the deformation here is taken up by a series of en-echelon, right-lateral faults on a relatively broad zone of deformation. The Euler vector proposed here would predict that some of the east–west features mapped by LONSDALE (1995) to be Pure Appl. Geophys. normal faults. However, there are no focal mechanisms showing this type of motion. 6. Summary A new Euler vector describing the relative motion between the Rivera and North American plates was determined using the horizontal projection of the slip vectors of all the published focal mechanisms along its plate boundaries. The best fitting Euler vector obtained by fitting the horizontal direction of slip of the focal mechanisms determined along the RIV-NAM plate boundaries explains the style of deformation and relative rate of motion along the three segments of this boundary. The decrease in the rate of seismicity from south to north on the MAT is explained by the gradual decrease of the rate of relative motion. Furthermore, the direction of relative motion becomes more oblique in the northern segment of the MAT. There is, however, no evidence of forearc deformation due to this oblique subduction. On the TME, the reverse faulting mechanisms shown by the earthquakes in this region, and predicted by our best fitting Euler vector, may explain the sharp bathymetric step here. The relatively low rate of deformation is explained by the decrease in the relative rate of motion predicted by the Euler pole. The location of the preferred Euler pole describing the RIV-NAM motion on the northwestern boundary of the Rivera helps to understand some of the problems encountered by previous studies. The northwestern boundary of the Rivera plate appears to be a zone of slow and very diffuse deformation. 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