Configuración estructural de la falla Teras (Provincia de Cuencas y Sierras) y su ruptura en el temblor sonorense del 3 de mayo de 1887 con magnitud M_w 7.5

Max Suter

Instituto de Geología, Universidad Nacional Autónoma de México, Estación Regional del Noroeste, Apartado Postal 1039, 83000 Hermosillo, Sonora, México. sutermax@aol.com

Manuscript received: June 17, 2007
Corrected manuscript received: December 10, 2007 ]]> Manuscript accepted: December 12, 2007

ABSTRACT

During the great 3 May 1887 Sonoran earthquake (surface rupture end–to–end length: 101.8 km; M_w = 7.5±0.3), an array of three north–south striking Basin and Range Province faults (from north to south Pitáycachi, Teras, and Otates) snapped along the –western margin of the Sierra Madre Occidental plateau. My detailed field survey of the Teras fault and the 1887 earthquake rupture zone along this fault included mapping the rupture scarp and measuring surface deformation at 27 sites.

The Teras fault is –20 km long, strikes N12°E, and has an average dip of62°W; its maximum throw is >1,640 m and its long–term slip rate >0.08 mm/yr. The fault is structurally simple; it is not internally segmented along strike and does not branch. Striation measurements and the style of faulting indicate extensional dip–slip without significant lateral displacement. In the north, a 2.5–km wide, unbreached right step–over separates the Pitáycachi and Teras faults, whereas the southern limit of the Teras fault is a partly breached relay ramp, where the fault trace shows a 60° bend and jogs to the west. Here, the displacement of the Teras fault is transferred onto the Otates fault and the normal faults bounding the Iglesitas horst.

The 1887 surface rupture along the Teras fault (end–to–end length 19.9 km) generally coincides with the mapped trace of the Teras Basin and Range Province fault. Based on 27 measurements, the maximum surface offset is 184 cm and the mean offset 112 cm. The along–rupture surface offset distribution is asymmetric, with the maximum near the southern end of the segment. This suggests that the Teras and Otates segments could be part of a single continuous 1887 rupture that stepped across the structurally complex basement ridge between them. A rough estimate of the average recurrence interval of 1887–size earthquakes on the Teras fault can be obtained from the average slip rate since 23 Ma and the amount of slip on this segment during the 1887 earthquake; the resulting values are 15 to 26 kyr.

Key words: southern Basin and Range Province, 1887 Sonoran earthquake, earthquake surface rupture, seismotectonics, earthquake geology.

RESUMEN

Durante el gran temblor sonorense del 3 de mayo de 1887 (largo de la ruptura: 101.8 km; magnitud M_w = 7.5±0.3), un arreglo de tresfallas con rumbo norte–sur de la Provincia de Cuencas y Sierras—es decir, de norte a sur, las fallas Pitáycachi, Teras y Otates— se reactivaron a lo largo del borde poniente del plateau de la Sierra Madre Occidental. Dentro del estudio de la falla Teras y de la zona de ruptura del temblor de 1887 a lo largo de esta misma falla se cartografía el escarpe de la ruptura y se midieron datos morfológicos–estructurales de la deformación en 27 sitios.

El escarpe de la ruptura sísmica de 1887 a lo largo de la falla Teras mide 19.9 km y coincide en general con la traza geológica de la misma falla. Con base en 27 mediciones, la separación vertical máxima a través del escarpe es 184 cm y el promedio 112 cm. La distribución de estas separaciones a lo largo de la falla es asimétrica, ubicándose el máximo cerca de la terminación sur de la falla. Esto sugiere que los segmentos Teras y Otates pudieran ser partes de una ruptura continua a través del estructuralmente complejo alto de basamento que existe entre los dos segmentos. Una estimación burda del intervalo promedio de recurrencia de temblores de tamaño 1887 se puede obtener a partir de la tasa promedio de deslizamiento de la falla Teras desde 23 May de la cantidad de deslizamiento en el temblor de 1887; los resultados varian entre 15 y 26 kyr.

Palabras clave: Provincia de Cuencas y Sierras, temblor sonorense de 1887, ruptura de temblor, sismotectónica, geología de temblores.

INTRODUCTION

North–south striking, west–dipping Basin and Range Province normal faults and associated half–grabens form the western edge of the Sierra Madre Occidental plateau in northeastern Sonora, Mexico over a distance greater than 300 km. Slip in 1887 within this segmented normal fault array caused the largest historical earthquake of the southern Basin and Range tectonic–physiographic province and produced the world's longest recorded normal–fault surface rupture in historic time (dePolo et al, 1991; Yeats et al., 1997). Field observations indicate that three major range–bounding normal faults ruptured in the 1887 Sonoran earthquake (Suter and Conteras, 2002). The surface rupture dips –74°W, has a maximum vertical displacement of 5.1 m, and is composed (from north to south) of the Pitáycachi, Teras, and Otates segments (Figure 1). These are the only Basin and Range Province faults in Mexico with known historical earthquake surface rupture. Including two isolated minor segments to the north of the Pitáycachi segment (Figure 1), the 1887 rupture trace adds up to 86.7 km, and the distance between the rupture trace extremities is 101.8 km. Based on the end–to–end length of the rupture trace and the length versus magnitude regression for normal faults by Wells and Coppersmith (1994, tab. 2A), M_w is estimated as 7.5±0.3.

This article focuses on the Teras fault and its rupture in the 1887 earthquake. The first part of the paper documents the stratigraphy and structural configuration of the fault. These regional geologic data are essential to the documentation and interpretation of the new earthquake–related field observations and the understanding of the structural interaction between the Pitáycachi, Teras, and Otates faults. The second part presents in detail the surface rupture of the 1887 earthquake along the Teras fault. Ultimately, these data should be integrated in regional seismic hazard evaluations and will contribute towards a better kinematic model of complex interactions between earthquake rupture segments. Knowledge of the rupture geometry, slip distribution, and fault zone properties is also necessary to understand the spatial distribution of energy dissipation during coseismic slip (Kanamori and Rivera, 2006; Shipton et al., 2006). Moreover, detailed mapping of the rupture geometry is a prerequisite to characterize the rupture propagation, rupture speed, and seismic wave radiation of this earthquake.

The study is based on field mapping, structural and morphological field measurements, and the structural interpretation of digital elevation models and satellite images. The Teras fault is accessible by vehicle on an improved dirt road that leads from Morelos across the Bavispe River to Vega Azul and from there along the 1887 surface rupture past El Toro Ranch, over Puerto El Aire to Pilares de Teras (Figure 2). Field mapping was based on 1:50,000 scale vertical aerial photographs supplied by Instituto Nacional de Estadística, Geografía e Informática (INEGI); these data were subsequently transferred onto the corresponding INEGI 1:50,000 scale topographic map sheets Morelos (H12–B56, 1981) and Colonia Oaxaca (H12–B66, 1982).

PHYSIOGRAPHY AND VEGETATION

Due to its location within the horseshoe–shaped bend of the Bavispe River, Sierra El Tigre is a sky–island range within the desert (Felger and Wilson, 1995; Fishbein et al, 1995). The vegetation is conifer forest above 1800 m, oak–grassland between 1800 and 1100 m, and mesquite–grass–land and desert scrub below 1100 m (White, 1948; Brown and Lowe, 1994). Differences in vegetation and rates of erosion and sedimentation along the 1887 surface rupture are probably the major reasons for the variable degree of conservation of the rupture scarp. The better–preserved northern (lower) part of the surface rupture is in the desert, whereas the less preserved southern (higher) part, which receives more precipitation, is in grassland and oak forest.

PREVIOUS STUDIES

Previous studies of the 1887 earthquake focused on intensity distribution and attenuation (DuBois and Smith, 1980; Sbar and DuBois, 1984; Bakun, 2006), regional seis–motectonics (Suter and Contreras, 2002), the geomorphol–ogy (Bull and Pearthree, 1988; Pearthree et al, 1990) and microseismicity (Natali and Sbar, 1982) of the Pitáycachi segment (Figure 1), and the 1887 surface rupture along the Otates segment (Suter, in press).

The fault along the western margin of the Sierra Pilares de Teras was first reported by Mishler (1920), who indicated between 450 and 920 m of throw. Imlay (1939, p. 1727 and 1732) introduced the name "Teras" for the fault. Both of these authors also mentioned the 1887 surface rupture along the fault but did not provide details, since Mishler's paper was focused on mining and Imlay's work on stratigraphy. Previously, Goodfellow (1888) had mentioned a possible southern continuation of the surface rupture, ~24 km long, beyond the Pitáycachi segment (Figure 1) into the Teras mountain range, based on observations relayed to him by some prospectors. In their unpublished fieldwork in 1980 and 1981, Darrell G. Herd and Catherine R. McMasters of the U. S. Geological Survey traced the surface rupture for 21 km along the Sierra Pilares de Teras following a linear S12°W trend and measured throws of ~1 m (Herd and McMasters, 1982; USGS, 1982, p. 209). The unpublished rupture trace map by Herd and McMasters was included repeatedly, on a reduced scale, in overview maps of the 1887 rupture (for example, Natali and Sbar, 1982; Nakata et al, 1982; Bull and Pearthree, 1988; Wallace e/ al, 1988; Wallace and Pearthree, 1989; Pearthree et al, 1990; dePolo et al, 1991; Yeats et al, 1997). The trace mapped by Herd and McMasters mostly coincides with the rupture trace documented in this study (Figure 2).

A seismotectonic map of this region including epicenters, focal mechanisms, and the traces of Basin and Range Province faults was provided in Suter and Contreras (2002). A generalized view of the regional geology is given on the INEGI 1:250,000 scale geologic map sheetNacozari (H12–6, 1983), the 1:500,000 scale geological map of Sonora by Fernández–Aguirre et al. (1993), the 1:1,000,000 scale geological map of Sonora by González–León et al. (2006), and the Consejo de Recursos Minerales 1:50,000 scale geologic map sheet Colonia Oaxaca (H12–B66) by Cerecero–Luna and Castro–Escarrega (1996).

STRATIGRAPHY

Lithostratigraphic units in the footwall of the Teras fault include Paleozoic limestone, the Cretaceous Bisbee Group, early Tertiary lakebeds, and Early Oligocene felsic volcanic rocks; lithostratigraphic units in the hanging wall of the fault include Early Oligocene felsic volcanic rocks, Oligocene mafic volcanic rocks, Quaternary piedmont alluvium, scree along the fault scarp, and alluvium of the Bavispe River (Figure 2).

About 5 km² of Paleozoic marine limestone crop out in the footwall near Pilares de Teras, where they are faulted against Tertiary felsic volcanic rocks and scree (Figure 2). The limestone was studied by Imlay (1939) along Arroyo El Cinco de Mayo (Figure 2), where the section is —750 m thick and mostly thick–bedded. In this area, the limestone contains diagenetic stylotites, chert, brachiopods, and fu–sulinids, and is overlain by siliciclastic rocks of the Glance Conglomerate and La Monta Formation (Figure 2). Imlay (1939) correlated the Arroyo El Cinco de Mayo section with the lower part of the section cropping out in La Pita (Santa Rosa) canyon (Figure 1), 15 km farther southeast (Devery, 1979). Paleozoic limestone also crops out (–0.2 km²) in the footwall of the Otates fault (Figure 1), where it is faulted against Tertiary mafic volcanic rocks and talus (Suter, in press, fig. 4). This limestone is coarse to medium bedded, has diagenetic stylotites, and contains recrystallized fusulinids. It is lithologically similar to the Paleozoic marine carbonates reported from La Pita (Santa Rosa) canyon and the Pilares de Teras region.

Previously unmapped outcrops of the Cretaceous Bisbee Group exist along Arroyo de los Estrada (Figure 2). In this section, the Paleozoic limestone is overlain by a 26°E dipping, gray–purplish basal conglomerate (Glance Conglomerate), >50 m thick, with poorly sorted and poorly rounded clasts that are mostly quartzite and partly limestone. The Glance Conglomerate is overlain by –600 m of felsic lava, with a porphyritic texture, a glassy matrix, –10% quartz, –20% feldspars, and no mafics. The lava is overlain by 48°E dipping, well–stratified red mudstone and conglomerate, –300 m thick; 80% of the components are from the underlying lava, 10% are quartzite, and 10% are limestone. These siliciclastic rocks belong to either the Glance Conglomerate or the La Morita Formation. Up–section follows green–gray felsic lava, –50 m thick, which may be a Tertiary sill, and which in turn is overlain by more red mudstone and conglomerate of the La Morita Formation, –150 m thick. These uppermost siliciclastic rocks are overlain by the Aptian–Albian Mural Limestone (Figure 2), which is here composed of oolitic–bioclastic grainstone with chert lenses and 1–m–thick banks of shell hash.

Volcanic rocks interstratified in the Glance Conglomerate and La Morita Formation, similarly to the ones in the Estrada section, have also been observed elsewhere in northeastern Sonora (Cerro La Muella section southeast of Naco; Araujo–Mendieta and Estavillo–González, 1987), northwestern Sonora (Nourse, 2001) and southeastern Arizona (Dickinson and Lawton, 2001, and references therein). The Glance Conglomerate and La Morita Formation in the Estrada section are lithologically similar to the sections described by Montaño–Jiménez (1988) from La Carabina canyon (Figure 1; southernmost east–west drainage on Figure 2) and farther south (his "Conglomerado Carabinas" unit). Large local topographic relief must have existed during the deposition of the Glance Conglomerate and La Morita Formation. In the Estrada section, these rocks are — 1,200 m thick and overlie Permian limestone, whereas 15 km farther southeast, in La Pita canyon, the Glance Conglomerate and La Morita Formation are missing, and the Mural Limestone overlies directly Paleozoic limestone (Imlay, 1939).

On the northern slope of Arroyo de Los Estrada, this Permian–Mesozoic section is overlain with an angular unconformity by horizontally oriented green–yellow greywacke (Figure 2), most likely lake deposits, –100 m thick, followed up–section by >500 m of Early Oligocene rhyolite of the Sierra Madre Occidental volcanic province (see below). These lakebeds also crop out at two places between Arroyo de Los Estrada and Arroyo El Cinco de Mayo (Figure 2). They are probably of Late Cretaceous and/or early Tertiary age —younger than the underlying Cretaceous Bisbee Group but older than the overlying Early Oligocene rhyolite— and may belong to the Tarahumara Formation (McDowell et al. 2001).

Overlying the Paleozoic limestone and the Cretaceous Bisbee Group with an angular unconformity and locally overlying the Late Cretaceous–early Tertiary lakebeds are ignimbrites, rhyolite, and rhyolitic tuff of the Sierra Madre Occidental volcanic province, which is part of the Cordilleran magmatic–volcanic arc system (Ferrari et al, 2005). This volcanic sequence, which is mapped as single unit in this study (Figure 2), is up to –900 m thick in the Sierra El Tigre and occupies by far the largest part of the footwall of the Teras fault. These rocks are also exposed in the hanging wall along the southern part of the Teras fault. Detailed stratigraphic and petrographic descriptions of these rocks are provided by Mishler (1920) and Montano–Jiménez (1988) for the region of El Tigre (Figure 1), and by Piguet (1987) and Montigny et al. (1987) for the western margin of the Sierra Madre Occidental plateau in the Huásabas – Nacori Chico – Mesa Tres Ríos region, –100 km farther south. The upper part of this sequence yielded a K–Ar age of 31± 1.3 Ma (Early Oligocene) near El Tigre mine (Montaño–Jiménez, 1988). Stratigraphically equivalent ignimbrites of the Boot Heel volcanic field in the southern Peloncillo Mountains, –70 km north of Sierra Pilares de Teras, yielded ⁴⁰Ar/³⁹Ar ages of 26.8–35.1 Ma (Mclntosh and Bryan, 2000).

The Early Oligocene felsic volcanic sequence of the study area is overlain by locally erupted basalt and mafic scoria. In the Nacori Chico – Mesa Tres Ríos region, stratigraphically equivalent basalts have Oligocene K–Ar ages of 26–30 Ma (Montigny et al, 1987; Demant et al, 1989). These rocks crop out in the hanging wall of the Teras fault and reach a thickness >500 m where the Bavispe River is carved into them. They are overlain by remnants of Quaternary alluvial fans (Figure 2). Well–indurated, poorly stratified conglomerate and sandstone of the Miocene syntectonic Báucarit Formation crop out in the very southwestern corner of Figure 2; they are part of the fill of the Angostura basin (Figure 3).

Beside alluvial fans, the Quaternary deposits consist of alluvium along the Bavispe River and the arroyos that drain Sierra Pilares de Teras, and scree along the Teras fault south of Puerto El Aire (Figure 2). The scree is typically composed of coarse (0.5–40 cm diameter) unstratified rock debris and covered by desert grassland. In some sections, the lower part of the scree is cemented by carbonate.

STRUCTURAL CONFIGURATION OF THE TERAS FAULT

The Teras fault delimits Sierra Pilares de Teras on its western side. Similarly to the Otates fault (Suter, in press), the Teras fault is characterized in cross section by a steep footwall escarpment (Figures 1 and 3) that has not retreated with time; no pediment exists either along the Teras or along the Otates fault. The highest elevations of Sierra Pilares de Teras are located along this escarpment due to long–term footwall uplift. The length of the Teras fault is –20 km and its average strike is N12°E (Figure 2). The average dip of the fault is 62°W based on measurements at six outcrops of the fault plane in Paleozoic limestone (Figure 2). The maximum throw of the Teras fault is > l,640 m as indicated by the offset of the boundary between the felsic and mafic volcanic sequences. This value is based on the elevation of the lowermost outcrops of the mafic sequence in the Bavispe River valley (800 m asl) and the highest elevation of the erosional surface of the felsic sequence in the Sierra Pilares de Teras (2,440 m asl). The footwall is not significantly rotated along the Teras fault; in the Arroyo de los Estrada outcrops (Figure 2), the Late Cretaceous–early Tertiary lakebeds are still horizontal.

In map view the Teras fault is structurally simple; it is not internally segmented along strike and does not branch. The entire fault ruptured in the Sonoran earthquake; the fault trace is practically identical to the 1887 surface rupture (Figure 2). The trace extends from the piedmont alluvium southeast of Morelos in the north to Arroyo La Bota in the south, where the trace shows a 60° bend and jogs to the west. In the north, a 2.5–km–wide right step separates the Teras from the Pitáycachi fault across an unbreached relay ramp (northernmost part of Sierra Pilares de Teras) and the Bavispe River (Figures 1 and 2).

A major transverse basement ridge exists south of the Teras fault (Figure 3). The dogleg at the southern termination of the Teras fault is located at the northern margin of this basement ridge. Structurally, the basement ridge coincides with a fault relay zone where the displacement of the Teras fault is transferred onto the Otates fault and the normal faults bounding the Iglesitas horst. The north–south striking fault separating the Iglesitas horst from the Angostura basin (Figure 3) dips 56° to 78° W; its throw is a minimum of 1,420 m and likely to be –2,000 m. The fault separating the Iglesitas horst from the Higueras basin (Figure 3) dips subvertically and has a minimum throw of 1,080 m (Suter, in press). There is a 6–km–wide right step in the range–bounding normal fault segments between the Teras fault and the fault delimiting the Iglesitas horst from the Angostura basin (Figures 1 and 3). The step is partly breached by the dogleg of the Teras fault and by a major bend in the trace of the fault delimiting the Iglesitas horst from the Angostura basin (Figure 3).

The region northwest of the basement ridge is a sediment sink characterized by large relict alluvial fans and the valleys of three major arroyos incised into them (Figure 3). The southern two of these arroyos drain the transverse basement ridge, whereas the northern arroyo, La Bota, which has by far the largest catchment area, drains the footwall of the Teras fault (Figure 3). Major sediment accumulation in form of alluvial fans is typical at relay ramps of en echelon fault arrays (Jackson and Leeder, 1994; Gawthorpe and Leeder, 2000) and has also resulted from numerical experiments of the development of segmented normal fault systems using a model integrating tectonics and surface processes (Contreras and Scholz, 2001).

The hanging walls of the Otates and Pitáy cachi faults (Figure 1) are characterized by major alluvial basins. The Higueras basin (hanging wall of the Otates fault; Figure 3) is filled with the syntectonic Báucarit Formation, which consists of as muchas 250 m of well–indurated, poorly stratified conglomerate and sandstone (Suter, in press). The basin in the hanging wall of the Pitáy cachi fault (Figure 1) is filled with alluvial fan deposits overlying siltstone, thin sandstone, and conglomerate typical of braided steam deposits (Biggs et al, 1999), initially described by Aguilera in 1888 (Suter, 2007). Conversely, the Teras fault is a range–front fault with only minor alluviation. The deposits may already have been eroded, since this area is dissected by the Bavispe River (Figures 1 and 3). However, at places where remnants of alluvial fan surfaces are preserved locally (Figure 2), it can be inferred that these fans were < 100 m thick. Alternatively, the syntectonic depression formed by the hanging wall of the Teras fault may have been filled by locally erupted basalt and mafic scoria, which cover a large area (Figure 2) and reach a thickness >500 m.

Figure 4 shows an exhumed part of the Teras fault plane in a large natural outcrop at site 227 (Figure 2). The photograph is approximately 15–30 mwide. The 43–56°W dipping fault cuts off Paleozoic limestone and is partly mantled by fault gouge and scree. The fault plane is characterized by major subvertical wear groove striations. Four striations measured at this site and two more from sites 231 and 233 (Figure 2) are shown on a lower hemisphere, equal–area stereoplot (Figure 5). These six striations (Table 1) are a proxy for the 1887 earthquake slip vector, since it is unknown whether they formed during the 1887 event, and since no striations have been observed on the Teras segment of the 1887 surface rupture in surficial deposits. The striations indicate consistently extensional dip–slip motion with a very minor left–lateral strike–slip component. Also shown on Figure 5 are the P (shortening) and T (extension) kinematic axes corresponding to each measurement, the average P and T kinematic axes (heavy black squares), and the corresponding great circles in form of earthquake focal mechanism nodal planes (T quadrant shaded in grey). They resulted from a kinematic analysis of these fault slip data using the program FaultKinWin developed by Richard W. Allmendinger (version 1.2, 2001, courtesy of http://www.geo.cornell.edu/pub/rwaAVindows); see also Marrett and Allmendinger (1990). The fault plane of the focal mechanism strikes N24°E and dips 55°W, whereas the auxiliary plane strikes N1°W and dips 38°E (Figure 5). The result is practically identical to the focal mechanism of the 25 May 1989 M 4.2 earthquake (Wallace and Pearthree, 1989), which had its epicenter ~20 km northwest of the trace of the Teras fault, and is similar to the composite focal mechanism obtained by Natali and Sbar (1982) for well–located microearthquakes near the southern tip of the Pitáycachi segment of the 1887 rupture. The similarity of the focal mechanism presentation of the striations measured on the Teras fault (Figure 5) and the focal mechanism of the 25 May 1989 M 4.2 earthquake (Suter and Conteras, 2002, fig. 2) suggests that this earthquake may have had its source on the Teras fault plane.

From the age of basalt flows intercalated with the lowermost fill of nearby basins (McDowell et al., 1997; González–León et al., 2000; Paz–Moreno et al, 2003), it canbe inferred that the activity of the Teras fault, and Basin and Range Province faulting in the epicentral region of the 1887 earthquake in general, started ~23 Myr ago. The average slip rate of the Teras fault is >0.08 mm/yr based on 23 Myr of fault activity, a maximum throw of 1,640 m, and an average dip of 62°.

Pre–Basin and Range Province structures within the footwall of the Teras fault

In the Arroyo de los Estrada valley, southeast of Puerto El Aire, the top of the Paleozoic limestone is vertically displaced by 600 m across a NW–SE striking normal fault (Figure 2). The same fault does not displace either the Teras fault or apparently the overlying Oligocene felsic volcanic rocks (Figure 2). Several normal faults of the same orientation were mapped within Sierra El Tigre farther south where they do displace the Oligocene felsic volcanic rocks (Suter, in press; Figure 3).

The Paleozoic limestone and the Cretaceous Bisbee Group within the footwall of the Teras fault dip 20–52°E southeast of Puerto El Aire and in El Cinco de Mayo canyon and up to 70°W in Las Carabinas canyon (Figure 2). The strata in the El Cinco de Mayo – Carabinas region can be interpreted as forming a fold with a NNW–SSE trending axis (Figure 2). Since the lakebeds overlying these rocks southeast of Puerto El Aire are horizontal, it follows that this rotation cannot be due to motion along the Teras fault but is probably entirely the result of Late Cretaceous – early Tertiary Laramide shortening.

THE 1887 EARTHQUAKE RUPTURE ALONG THE TERAS FAULT

Here I provide a detailed account of the surface rapture of the 1887 earthquake along the Teras fault based on field observations. I mapped the surface rapture as well as the nearby geology at a 1:50,000 scale (Figure 2) and studied the rapture scarp at 27 sites, where I measured the scarp width, scarp height, and the height of the free face (Figure 6, Table 2). At sites with a basal fissure (Table 2), the height and width of the scarp were measured across the fissure. I also noted at each site the material exposed by the fault and, in the case of rock debris, the fragment size, and whether the material is cemented or not. Furthermore, I photographed the 120–year old rapture scarp at several sites (Figures 7, 8, 9 and 10, from north to south).

The 1887 surface rapture along the Teras fault generally coincides with the mapped trace of the fault (Figure 2). The endpoints of the documented surface rapture are located in the south at 109.259°W/ 30.635°N, at an elevation of 1,130 masl and in the north at 109.212°W/30.810°N at an elevation of 860 m (Figure 2). The length along the rapture trace of the Teras segment is –20.6 km (Figure 11 and Table 2), whereas the endpoint–to–endpoint distance is 19.9 km. In the northernmost part of the Teras fault, the 1887 surface rapture passes within alluvial fan deposits (sites 64 and 65) or displaces alluvial fans against Tertiary volcanic rocks of the Sierra El Tigre (sites 10 and 11; Figure 2). In the central part of the Teras fault, the rapture passes within Tertiary volcanic rocks in a fault–parallel, long, and narrow valley —erroneously labeled El Tigre valley in Natali and Sbar (1982), Wallace et al. (1988), and Wallace and Pearthree (1989)— which passes through El Toro ranch and Puerto el Aire (Figure 2; Suter, in press, fig. S2). Here rhyolite is faulted against basalt, which is partly overlain by relict alluvial fans (Figure 2). Erosion of this tectonically controlled valley and down cutting of the Bavispe River canyon must have occurred after deposition of the alluvial fans. The 1887 rapture was most likely continuous along this part of the fault, but the amount of displacement can only be documented at some few places where the fault–parallel valley is intersected by alluviated east–west valleys draining Sierra El Tigre (Figures 2 and 7). In its southern part (south of Puerto el Aire), the 1887 rapture passes within scree slopes as steep as 22° (Table 2, Figure 9) at the base of the talus formed by the Teras fault, which delimits Permian carbonates and subordinately Mesozoic siliciclastic rocks from Tertiary volcanic rocks (Figure 2). The southern endpoint of the 1887 surface rapture on the Teras fault is north of Arroyo La Bota (Figure 2, site 33), where the fault makes a 60° bend to the west. At several locations, the surface rapture passes only a few meters from the stratigraphic contact between bedrock and scree (Figures 2 and 8–10). Therefore, the earthquake rapture likely coincides with the Teras fault at depth.

The 1887 surface rapture along the Teras is structurally simple; it is not internally segmented along strike and, unlike the Pitaycachi segment, it has no significant branches or subsidiary faults (Figure 2). An exception is the step faulting observed at site 37 (Table 2), where a minor scarp, with a height of 20 cm, is located 30 m upslope of the main scarp. Small–scale surficial complications could be observed in an exceptionally well–exposed cross section (Figure 8) of the rapture zone directly south of site 35 (Figure 2). The 1887 motion is partitioned there along two west–dipping normal faults, which seem to merge at a shallow depth, and two minor antithetic normal faults. Most of the displacement was along the eastern one of the two west–dipping normal faults, which is in this section partly a shear zone rather than a discrete fault plane. Coarse rock fragments in the lower part of the hanging wall section terminate against this fault, which suggests prior movement along the same fault.

Surface offset and coseismic slip

The surface offset (vertical separation of the ground surface) was calculated from the scarp height and the slope angle above and below the scarp (Figure 6) (Bucknam and Anderson, 1979; McCalpin, 1996; Yeats et al., 1997). The surface offset remains constant, whereas the scarp width and height increase as the scarp broadens with age (Figure 6). Because of the steep, up to 22° inclined slopes adjacent to the 1887 rupture, the surface offset deviates significantly from the scarp height (Table 2). For three of the sites (5, 6, and 36), the calculated surface offset is smaller than the (vertically) measured free face (Table 2) and therefore not useful. This problem did not arise with any of the numerous surface offsets calculated for the Pitáycachi segment but was frequent with the surface offsets computed for the Otates segment (Suter, in press). Because of the steep slopes along the southern part of the Teras fault (Table 2) and especially along the Otates fault, the base of the rupture scarp is often not well defined unless marked by a basal fissure. The transition from the small debris apron at the base of the scarp (Figure 6) to the steep hillslope below is often gradual. Furthermore, the hillslope values used to calculate the surface offset have an uncertainty of ±2°; they were taken from the corresponding INEGI 1:50,000 scale topographic map sheets, which have 20–m contour intervals. The most reliable scarp parameter is the free face (Figure 6), which gives a minimum estimate for the surface offset, whereas the scarp height should give a maximum estimate. For the three sites where the computed surface offset turned out to be smaller than the measured free face (Table 2), I assumed the offset was not more than the free face value (Figure 11). This assumption provides a minimum value for the vertical separation of the ground surface.

A distribution of the surface offsets measured at 27 sites (Table 2) along the rupture trace is shown on Figure 11. The maximum vertical separation is 184 cm at site 15, the minimum vertical separation 51 cm, and the arithmetic average (mean) of the vertical separations is 112 cm. The dashed line on Figure 11 is a linear best fit to the surface offset y values. The regression line corresponds to the function y = ax + b, where a= 3.939 and b = 65.15 cm. The correlation coefficient is 0.6020. The along–scarp distribution is asymmetric, with the maximum offset near the southern end of the segment. The high surface offset values at the southern end of the mapped surface rupture of the Teras segment and at the northern end of the mapped surface rupture of the Otates segment suggest that the two segments could be part of a single continuous rupture that stepped across the structurally complex basement ridge between them (Suter, in press). The median of the predicted values (106 cm; y–coordinate forthe midpoint of the regression line) is somewhat smaller than the arithmetic average of the individual vertical separation measurements (112 cm).

The ground surface offset distribution can be used to estimate the coseismic slip distribution at depth. The transform from surface offset y to slip s is s = y / sin , where is the average dip of the rupture surface. Since no striations were observed on the surface rupture scarp, the 62° average dip measured on the Teras fault plane (Figure 2) is taken as a proxy. Based on this assumption, the maximum slip along the Teras fault in the 1887 earthquake was 208 cm, the slip resulting from the arithmetic average of the vertical separation measurements was 127 cm, and the slip resulting from the average value of the regression function was 120 cm.

Magnitude

Based on the above slip values and the rupture length, we can estimate the moment magnitude M_w for the rupture of this individual segment from the corresponding empirical scaling laws for normal faults by Wells and Coppersmith (1994). Their maximum displacement–versus–magnitude regression indicates a magnitude of 6.84±0.34 forthe 1887 rupture of the Teras fault. Assuming an average displacement of 127 cm, their average displacement–versus–magnitude regression indicates the practically identical magnitude of 6.85±0.33. Based on the 19.9 km endpoint–to–endpoint distance of the rupture trace of the Teras segment, the length–versus–magnitude regression for normal faults by Wells and Coppersmith (1994, tab. 2A) yields a lower M_w of 6.57±0.34 for the 1887 rupture of the Teras fault. These calculations indicate that the aspect (displacement/length) ratio of the 1887 rupture of the Teras fault is somewhat higher than for the average normal fault rupture of the Wells and Coppersmith (1994) data set. According to their regressions, the typical length of a magnitude M_w 6.85 normal fault rupture is 26.0 km, whereas the maximum and average displacements of a M_w 6.57 normal fault rupture are 160 and 74 cm, respectively.

To date, no Quaternary slip rate has been calculated for the Teras fault. Basin and Range Province faults that separate bedrock from internally unfaulted basin fill, such as the Teras fault, typically have vertical slip rates >0.1 mm/yr (dePolo and Anderson, 2000). As mentioned previously, the estimated long–term geologic slip rate of the Teras fault is 0.08 mm/yr based on an estimated throw of 1,640 m along a 62°–dipping fault over a period of 23 Myr. A rough estimate of the average recurrence interval of 1887–size earthquakes on the Teras fault can be obtained from this slip rate and the amount of slip on this segment during the 1887 earthquake; the resulting values are 26 kyr based on the maximum slip, 16 kyr based on the slip resulting from the arithmetic average of the vertical separation measurements, and 15 kyr based on the slip resulting from the average value of the regression function. These estimates are within the range of recurrence intervals documented for normal faults of the southern Basin and Range Province and the Rio Grande rift (10 to 100 kyr; Menges and Pearthree, 1989; Machette, 1998).

Conversely, the Quaternary slip rate of the Pitáy cachi fault, obtained from the fault scarp morphology and the estimated age of soils formed on alluvial surfaces displaced by the fault, is only 0.015 mm/yr (Bull and Pearthree, 1988; Pearthree et al., 1990), twelve times slower than its long–term rate of ~0.18 mm/yr (Suter and Conteras, 2002). Such a decrease of slip rate with time is also characteristic for many faults of the Rio Grande rift; the Socorro fault zone, for example, slowed from 0.18–0.20 mm/yr in the latest Miocene, to about 0.05 mm/yr in the Pliocene, and 0.02–0.04 mm/yrin the past 750 kyr (Machette, 1998). Given this decrease of long–term slip rate with time, the above estimate of the average recurrence interval of 1887–size earthquakes on the Teras fault is likely to be a lower bound.

The change in Coulomb failure stress caused by the rupture of individual segments of a fault zone may advance or delay the rupture of adjacent segments (King et al., 1994; Crider and Pollard, 1998; Stein, 1999; Gupta and Scholz, 2000). This suggests that the various segments of the fault zone on the western edge of the Sierra Madre Occidental plateau may have failed in the past in segment combinations that are different from the one that ruptured in 1887 (Pitáycachi–Teras–Otates), which probably results in major fluctuations of the recurrence intervals for the individual fault segments.

Structural interaction between the Teras and Otates rupture segments

A 15–km–long gap separates the southern end of the Teras segment from the northern end of the Otates segment (Figure 3). Assuming the rupture propagated from north to south, the 1887 rupture was arrested along the southern Teras fault where the fault trace has a 60° bend and jogs to the west. A model of changes in Coulomb failure stress resulting from the 1887 rupture indicates lobes of stress increase at the tips of the rupture segments, especially in the step–over between the Teras and Otates faults (Suter and Conteras, 2002). The northern part of the Otates fault and the northern parts of the normal faults bounding the Iglesitas horst (Figure 3) are located in this zone of stress concentration. Therefore, the 1887 rupture could have either jumped from the Teras to the Otates fault or triggered slip along the faults bounding the Iglesitas horst. However, field–checking the fault traces along the Iglesitas horst did not reveal any obvious morphological or stratigraphic evidence of Quaternary fault activity.

The Teras surface rupture segment has lower surface offset (vertical separation) values than the Otates segment to the south, even though the Teras segment is slightly longer (Suter and Conteras, 2002; Suter, in press). This suggests that the Otates rupture segment may be longer than mapped. This hypothesis is also supported by the high surface offset values at the northern end of the mapped surface rupture of the Otates segment (Suter, in press, fig. 9). Furthermore, the high surface offset values at the southern end of the mapped surface rupture of the Teras segment (Figure 11) and at the northern end of the mapped surface rupture of the Otates segment suggest that the two segments could be part of a single continuous rupture that stepped across the structurally complex basement ridge between them. However, there is no clear evidence of young surface ruptures supporting this hypothesis.

Structural interaction between the Teras and Pitáycachi rupture segments

According to observations by Goodfellow (1887), the Pitáycachi and Teras rupture segments overlap. Based on his rupture map (facsimile in Suter, 2006, fig. 3), the Pitáycachi segment continues in the south across the Bavispe River into the Sierra Pilares de Teras for at least 8 km. The same rupture configuration is shown on the map by Aguilera (1888) (facsimile in Suter, 2006, fig. 5). According to Goodfellow (1888), the scarp heights along the Pitáycachi rupture segment in the Sierra Pilares de Teras are ~0.9 m and do not exceed 1.5 m. Both, Goodfellow (1888) and Aguilera (1888) noticed the scarp height attenuation to near zero, approaching either side of the Bavispe River valley, and Goodfellow (1888) observed locally reverse faulting, 3.2 km south of the river. At his southernmost observation point, 8 km south of the river, the scarp height was one foot or less (<30 cm) and the rupture branched. Since the rupture passes in the Sierra Pilares de Teras within volcanic rocks and is not expressed by a multi–event rupture scarp, I have not been able to confirm Goodfellow's observations. A photograph by C.S. Fly (Suter, 2006, fig. 10) documents the 1887 rupture trace directly south of the river where it separates Tertiary volcanic rocks from scree.

According to independent observations by both Aguilera (1888) and Goodfellow (1888), the aftershocks of the 1887 earthquake were concentrated in the northernmost part of Sierra Pilares de Teras, where the step–over between the Teras and Pitáycachi rupture segments is located. Based on its intensity distribution, the 26 May 1907 Colonia Morelos, Sonora (I_max= VIII, M_I = 5.2±0.4) earthquake also seems to have had its origin in this region (Suter, 2001, fig. 4). These earthquakes may have been caused by an increase of static Coulomb stress at the tips of the Pitáycachi and Teras rupture segments (Suter and Conteras, 2002).

Rupture kinematics

The structural configuration of the 1887 rupture along the Teras fault does not indicate where the rupture originated and how it propagated. Conversely, the surface rupture along the Pitáycachi segment has a well–developed branching pattern (five north–facing bifurcations in the northern part of the segment, two south–facing bifurcations in its southern part), which indicates that the rupture of the Pitáycachi segment initiated in its central part where the polarity of the rupture bifurcations changes (Suter and Conteras, 2002).

It is therefore very likely that the rupture first propagated bilaterally along the Pitáycachi fault, from where the southern rupture front jumped across the step–over to the Teras fault and from there to the Otates fault (Figure 1). This scenario is supported by the macroseismic observations made by Aguilera (1888), which indicate that this event was a composite earthquake with the sub–events separated by several seconds. Geological observations indicate that historical Basin and Range Province earthquakes with large magnitudes involved sequential rupture of two or more discrete segments (dePolo et al, 1991; Zhang et al, 1999). Furthermore, in a study of the source parameters of Basin and Range Province earthquakes, Doser and Smith (1989) concluded that earthquakes of magnitude >7.0 have complex ruptures best modeled by multiple sub–events. Moreover, the directivity effect of a north–to–south propagating rupture along the southern part of the Pitáycachi fault and the Teras and Otates faults also explains why the isoseismals of this earthquake (Aguilera, 1888; Suter, 2006) are so much elongated in a SSE direction.

CONCLUSIONS

The Basin and Range Province Teras fault, which forms the steep western flank of Sierra Pilares de Teras in northeastern Sonora, ruptured in the 1887 Sonoran earthquake. Rocks exposed along the Teras fault include Paleozoic marine limestone, a lower part of the Cretaceous Bisbee Group (Glance Conglomerate, La Morita Formation, Mural Limestone), early Tertiary lakebeds, Oligocene volcanic rocks, and Quaternary piedmont alluvium and scree. In a previously undocumented section along Arroyo de los Estrada, volcanic rocks are interstratified in the Glance Conglomerate and La Morita Formation. A large topographic relief must have existed locally during the deposition of the Glance Conglomerate and La Morita Formation; in the Estrada section, these rocks are –1,200 m thick and overly Permian limestone, whereas 15 km farther southeast, in La Pita canyon, the Glance Conglomerate and La Morita Formation are missing, and the Mural Limestone overlies directly Paleozoic limestone. The dips measured in the Paleozoic limestone and the Cretaceous Bisbee Group within the footwall of the Teras fault define a NNW–SSE trending fold axis, which must be due to Laramide shortening, since horizontal lakebeds of Late Cretaceous and/or early Tertiary age overly these folded rocks.

The 1887 surface rupture along the Teras fault generally coincides with the mapped trace of the fault. The rupture is structurally simple; it is not internally segmented along strike and, unlike the Pitáycachi segment, it has no branches or subsidiary faults. The vertical separation of the ground surface by the rupture scarp (surface offset) was measured at 27 sites; the scarp free face still exists at 23 of these sites. The maximum vertical separation is 184 cm, and the mean of the vertical separations is 112 cm. A linear best fit to the surface offset values shows the along–scarp distribution to be asymmetric, with the maximum offset near the southern end of the segment. The high surface offsets at the southern end of the Teras and at the northern end of the Otates rupture segments suggest that the two segments could be part of a single continuous rupture that stepped across the structurally complex basement ridge between them.

An unbreached right step–over, 2.5–km wide and with an 8–km overlap (Goodfellow, 1887, 1888), separates the northern end of the mapped surface rupture of the Teras segment from the Pitáycachi segment. This step–over is also defined by a minimum in the 1887 slip distribution, which suggests that the Pitáycachi and Teras are independent rupture segments that do not merge at depth. In their approach to the step–over, the traces of the Pitáycachi and Teras rupture segments are characterized on a smaller length scale by right–stepping en echelon patterns indicating a distributed left–lateral component of motion within this transfer zone.

No striations have been observed on the Teras segment of the 1887 surface rupture in surficial deposits. Striations measured on the Teras fault, which are a proxy for the 1887 earthquake slip vector, indicate consistently extensional dip–slip motion with a very minor left–lateral strike–slip component. The similarity of the focal mechanism presentation of these striations and the focal mechanism of the 25 May 1989 M 4.2 earthquake (Suter and Conteras, 2002, fig. 2) suggests that this earthquake may have had its source on the Teras fault plane.

The surface rupture along the Pitáy cachi segment has a well–developed branching pattern (five north–facing bifurcations in the northern part of the segment, two south–facing bifurcations in its southern part), which indicates that the rupture of the Pitáy cachi segment initiated in its central part where the polarity of the rupture bifurcations changes. It is therefore very likely that the rupture first propagated bilaterally along the Pitáycachi fault, from where the southern rupture front jumped across the step–over to the Teras fault and from there to the Otates fault.

ACKNOWLEDGMENTS

I am thankful to Philip A. Pearthree (Arizona Geological Survey) and Craig dePolo (Nevada Bureau of Mines and Geology) for their careful reviews of this manuscript. Financial and logistic support for this work came in part from Universidad Nacional Autónoma de México (UNAM) and Consejo Nacional de Ciencia y Tecnología (CONACYT, grant G33102–T).

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