William Bandy

Seismotectonic constraints on the convergence rate between the Rivera and North American plates

There are two significantly different types of models for the convergence rate between the Rivera and North American plates. The first type, the high-rate model (Bandy, 1992), predicts convergence rates of approximately 5.0 cm/yr near the southern end of the Rivera-North America subduction zone and between 2.0 and 3.0 cm/yr at its northern end. In contrast, the second type, the low-rate model (e.g., DeMets and Stein, 1990), predicts convergence rates of between 2.0 and 3.3 cm/yr near the southern end of the Rivera-North America subduction zone and between 0.6 and 1.7 cm/yr at its northern end. Seismotectonic relationships, which relate seismic characteristics of subduction zones (maximum magnitudes, maximum seismic depths, etc.) to plate tectonic parameters (convergence rates, age of the oceanic lithosphere, etc.) provide a means of distinguishing between the two different models. Three such relationships suggest that the Rivera-North American and Cocos-North American convergence rates should be roughly equal across the Rivera-Cocos plate boundary, favoring the high-rate model. Employing the high-rate model, one can evaluate the magnitude and distribution of the strike-slip component of forearc motion, Vss, produced by oblique convergence between the Rivera and North American plates. The analysis indicates both a progressive increase and clockwise reorientation of Vss northwestward along the plate contact zone of the Rivera-North America subduction zone. Such a distribution in Vss should produce a northwestward movement of and NW-SE oriented extension within the interior of the Jalisco Block, consistent with previous proposals of Jalisco Block motions. Also, such a distribution in Vss should produce a slight clockwise rotation of the Jalisco Block in the vicinity of Bahia de Banderas, consistent with paleomagnetic data.

The subducted Rivera‐Cocos Plate Boundary: Where is it, what is it, and what is its relationship to the Colima Rift?

Gravity data, the geometry of the Wadati-Benioff zone beneath western Mexico, and the seafloor morphology of the Rivera-Cocos plate boundary west of the Middle America trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the Southern Colima rift. Thus, the Southern Colima rift likely formed in response to divergence between the subducting Rivera and Cocos plates due to direct coupling between these two plates and the overriding North American plate. In contrast, the subducted plate boundary lies east of and oblique to the Northern Colima and Central Colima grabens. East of the Central Colima graben a low density zone overlies the boundary and underlies surface exposures of Cretaceous granitoids and associated thermal springs and shallow focus earthquakes; characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted Rivera and Cocos plates. These characteristics along with the adjacent locations of the low density upper mantle and the Central Colima graben are consistent with crustal extension produced by the uniform-sense normal simple shear mechanism.

The October 9, 1995 Colima‐Jalisco, Mexico Earthquake (Mw 8): An aftershock study and a comparison of this earthquake with those of 1932

Data from portable seismographs and a permanent local network (called RESCO) are used to locate the aftershocks of the October 9, 1995 Colima-Jalisco earthquake (Mw 8.0). The maximum dimension of the aftershock area, which is rectangular in shape, is 170 km × 70 km. Our study shows that the mainshock nucleated ∼24 km south of Manzanillo, near the foreshock of October 6, 1995 (Mw 5.8), and propagated ∼130 km to the NW and ∼40 km to SE. The aftershock area lies offshore and is oriented parallel to the coast. The observed subsidence of the coast is a consequence of this offshore rupture area. The aftershocks reach unusually close to the trench (within 20 km). This may be due to lack of sediments with high pore pressure at shallow depth. There are some similarities between this earthquake and the two great earthquakes of 1932 (3 June, Ms 8.1; 18 June, Ms 7.8) which occurred in this region. In both cases the aftershocks were located offshore and the coastline subsided. The sum of seismic moments and the rupture lengths of the 1932 events (1.8×1021 N-m and 280 km, respectively), however, were greater than the 1995 earthquake. Also a comparison of seismograms of 1932 and 1995 earthquakes show great differences. It seems that the 1995 event is not a repeat of either June 3 or June 18, 1932 earthquakes.

Gravity and seismicity over the Guerrero Seismic Gap, Mexico

Four detailed (average station interval = 5 km) gravity transects were recently conducted in the Pacific coastal region of Mexico. A differential GPS technique was used to determine the elevation and coordinates of the gravity stations. The profiles are oriented northeast-southwest and extend from the coast up to ∼60 km inland. The Bouguer gravity anomaly is decreasing consistently along every profile from 60–80 mGal at the coast with an approximately constant regional gradient of −2.2 mGal/km normal to the trench. A plot of the gravity anomaly against the distance from the trench axis demonstrates that the regional slope in the gravity anomaly is shifting gradually (20–25 mGal) inland along the coast of Guerrero from the southeast (Atoyac) to the northwest (Petatlan - Zihuatanejo). A model cross section of the Mexican subduction zone (MSZ) based on the tomography inversion for the Guerrero region shows that the gravity anomaly values and the regional anomaly trend can be explained mostly by the effect of the density contrast between the slab and the continental crust. The upper surface of the subducted slab (USS) and the seismogenic contact zone between the upper plate and the slab is traced clearly in several seismicity cross sections based on the data of the regional seismic network in Guerrero. The depth and shape of the USS revealed from the seismicity and gravity anomaly data for the same profiles are in good agreement. This correlation may be fairly useful when applied to gravity profiles in order to estimate the depth of the USS and the seismogenic contact in other parts of the MSZ which lack reliable seismicity data.

Implications of the October 1995 Colima‐Jalisco Mexico earthquakes on the Rivera—North America Euler vector

Rivera-North America Euler vectors fall into three groups based on the predicted motion of the Rivera plate relative to the North America plate along the Middle America Trench north of 18.5°N. Namely, those that predict (1) a slow, more easterly directed motion, (2) a slow, more northerly directed motion, and (3) a faster, more northerly directed motion. The focal mechanism solution, rupture area and seismic moment of the great (M w =8.0) Colima-Jalisco, Mexico, earthquake of October 9, 1995 and its foreshocks and aftershocks provide additional constraints on the Rivera-North America Euler vector. The slip direction (N30°E) of the main event clearly favors those Euler poles which predict a more northerly direction of motion. This direction lies within 5° of those predicted by these poles, whereas, it differs by between 9° and 18° from those poles which predict a more easterly motion. The amount of slip which occurred during the main event suggests a minimum convergence rate of between 4.3 and 6.5 cm/yr. This favors Euler vectors which predict higher convergence rates, however, a firm conclusion cannot be made due to the uncertainty in the recurrence time estimation. A new Rivera-North America Euler pole, calculated incorporating the information provided by the 1995 events, lies at 21.87°N, 109.58°W. It predicts a N30.3°E directed motion of the Rivera plate relative to North America at the location of the main event; a difference of only 0.3° from the slip direction of the main event.

Morphology and magnetic survey of the Rivera-Cocos plate boundary of Colima, Mexico

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of RiveraCocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during cam paigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3oN) of the present day Rivera Transform at about Chron 2A 3 (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3oN and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3oN the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.

The Colima, Mexico, Earthquake (MW 5.3) of 7 March 2000: Seismic Activity Along the Southern Colima Rift

The 7 March 2000 earthquake ( M W 5.3), located within the southern Colima rift in southwestern Mexico, is the largest crustal earthquake ever recorded within the state of Colima, Mexico. The mainshock was located at 8 km depth, and the aftershocks are distributed at depths between 5 and 10 km. Aftershock distribution follows a normal decay as measured by the modified Omoris law, with a p value of 0.96. Both aftershock locations and focal mechanisms (determined for the mainshock and some of the aftershocks) delineate a fault dipping toward the northwest and striking west-southwest. This fault segment is parallel to the southern end of the Tamazula fault, which ends at Manzanillo Bay. Regional waveform modeling for the mainshock situates the earthquake on a normal fault with a significant left-lateral strike-slip component on a plane striking 260° and dipping toward the northwest at 46°. We use a small aftershock with similar focal parameters as the mainshock to deconvolve P and S waves recorded at the permanent station COIG. From the deconvolution we obtained a source duration of 0.75 sec from the P waves and 0.9 sec from the S waves for the main event and a stress drop of 320 bars. The aftershock area is much larger than the area measured from the apparent source time function. From the aftershock area and the seismic moment obtained by Harvard University (Centroid Moment Tensor Catalog), we compute a static stress drop of 20 bars. Fault-plane solutions from the mainshock and some of the aftershocks indicate that the northern, and larger, segment of the Tamazula fault is perpendicular to the extensional axis (directed east-southeast). This study confirms the change in the direction of the extensional axis of the southern Colima rift with respect to the northern Colima rift (extensional axis oriented almost east-west). Here we show that the Tamazula fault zone, at least its eastern segment, is active, and a re-evaluation of the seismic hazard posed by this fault to the state of Colima and the cities of Colima and Manzanillo is urgent. Manuscript received 13 September 2002.

The 11 December, 1995 earthquake (Mw=6.4): Implications for the present-day relative motion on the Rivera-Cocos Plate Boundary

The 11 December, 1995 earthquake is the largest and best constrained instrumentally recorded event which has occurred on the Rivera-Cocos plate boundary. The reported focal mechanism for this event indicates almost pure strike-slip faulting with nodal planes oriented north-south and east-west. A visual inspection shows that the seismograms recorded world-wide strongly suggest a directivity effect indicative of a rupture propagating eastward from the epicenter. This observation is confirmed by a directivity analysis which shows a sharp reduction in the difference between observed and synthetic seismograms when the rupture direction is at an azimuth of approximately 90°. These results indicate that the east-west trending nodal plane of the earthquake of 11 December, 1995 is the actual fault plane. Considering that this is the largest instrumentally recorded earthquake in the region, it strongly suggests that the relative motion of the Rivera plate with respect to the Cocos plate takes place along east-west oriented faults, and that the sense of motion is right-lateral, strike-slip.

Geophysical-Archaeological Survey in Lake Tequesquitengo, Morelos, Mexico

In August 2009, a marine geophysical survey was conducted in Lake Tequesquitengo (located in the state of Morelos, Mexico) to delineate the extent of the remains of a small town that has been submerged since the mid 19th century. The survey consists of the acquisition and mapping of magnetic, single beam bathymetric and sidescan sonar data. A dual receiver marine GPS navigation system was used to position the boat during the survey. Except for the larger structural remains that are visible on the side scan sonar images, the magnetic anomaly map proved to be most useful in delineating the extent of the town. These anomalies exhibit short wavelength components in the area surrounding a submerged church, with the shortest wavelength components being confined to the area immediately east of the church. These short wavelength components are only observed near the church; therefore, we propose that they delineate the buried remnants of the submerged town.

Analysis of the unusual earthquake of 13 August 2006 in Michoacán, México

The moderate earthquake of 13 August 2006 which occurred in the coastal area of Michoacán, México, offered the first opportunity to study an earthquake that has a focal mechanism oriented practically perpendicular to the vast majority of the earthquakes occurring along the subduction zone of the Mexican Pacific continental margin. The location and focal mechanism estimated in this study are in close agreement with those estimated by the Global Centroid Moment Tensor (CMT) project and the US Geological Survey, National Earthquake Information Center (NEIC) and place the earthquake in a complex tectonic region where 3 lithospheric plates converge. Our review shows that for the most severe historical earthquakes in the area the seismic recurrence period has expired, consequently the seismic hazard of this region is high and the analysis of the unusual event must be considered important. The main purposes of this study are (i) re-estimate the location and focal mechanism of the unusual event by using available seismic records close to the source, (ii) conduct a tectonic analysis of the area in relation with the previous fault plane estimated, (iii) evaluate the peak ground accelerations generated for this particular thrust event relative to those occurring during the more common events and (iv) generate the isoseismal map. The analysis of the intensities of this event together with a tectonic analysis of the area where this event occurred, attest to an unexpected behavior of this event in this region.