Active faults sources for the Pátzcuaro–Acambay fault system (Mexico): fractal analysis of slip rates and magnitudes <i>M</i>_w estimated from fault length

The Pátzcuaro–Acambay fault system (PAFS), located in the central part of the Trans-Mexican Volcanic Belt (TMVB), is delimited by an active transtensive deformation area associated with the oblique subduction zone between the Cocos and North American plates, with a convergence speed of 55&thinsp;mm yr−1 at the latitude of the state of Michoacán, Mexico. Part of the oblique convergence is transferred to this fault system, where the slip rates range from 0.009 to 2.78&thinsp;mm yr−1. This has caused historic earthquakes in Central Mexico, such as the Acambay quake (Ms = 6.9) on 19 November 1912 with surface rupture, and another in Maravatío in 1979 with Ms = 5.6. Also, paleoseismic analyses are showing Quaternary movements in some faults, with moderate to large magnitudes. Notably, this zone is seismically active, but lacks a dense local seismic network, and more importantly, its neotectonic movements have received very little attention. The present research encompasses three investigations carried out in the PAFS. First, the estimation of the maximum possible earthquake magnitudes, based on 316 fault lengths mapped on a 15&thinsp;m digital elevation model, by means of three empirical relationships. In addition, the Hurst exponent Hw and its persistence, estimated for magnitudes Mw (spatial domain) and for 32 slip-rate data (time domain) by the wavelet variance analysis. Finally, the validity of the intrinsic definition of active fault proposed here. The average results for the estimation of the maximum and minimum magnitudes expected for this fault population are 5.5 ≤ Mw ≤ 7. Also, supported by the results of H at the spatial domain, this paper strongly suggests that the PAFS is classified in three different zones (western PAFS, central PAFS, and eastern PAFS) in terms of their roughness (Hw = 0.7, Hw = 0.5, Hw = 0.8 respectively), showing different dynamics in seismotectonic activity and; the time domain, with a strong persistence Hw = 0.949, suggests that the periodicities of slip rates are close in time (process with memory). The fractal capacity dimension (Db) is also estimated for the slip-rate series using the box-counting method. Inverse correlation between Db and low slip-rate concentration was observed. The resulting Db = 1.86 is related to a lesser concentration of low slip-rates in the PAFS, suggesting that larger faults accommodate the strain more efficiently (length  ≥ 3&thinsp;km). Thus, in terms of fractal analysis, we can conclude that these 316 faults are seismically active, because they fulfill the intrinsic definition of active faults for the PAFS.</p

Estudio preliminar de las caracteristicas morfologicas y geotecnicas de los flujos de avalancha del volcan el Tancitaro, en Michoacan, Mexico

Estudio preliminar de las caracteristicas morfologicas y geotecnicas de los flujos de avalancha del volcan el Tancitaro, en Michoacan, Mexic

The debris avalanche and debris flow from the Tancitaro volcano

In the SW sector of the Tarascan corridor we can observe two important volcanic activities, a composite collapsed volcano (Tancitaro) and a big concentration of monogenetical volcanoes. Both edified over granitic an andesitic bedrock. The NE-SW faults are the principal structures that control the distribution of the monogenetical volcanoes, but there is also NNW-SSE faulting with volcanic alineation. The Tancitaro volcano is located in the intersection of the NW-SE and NE-SW structures. The Tancitaro Volcano is one of the highest volcanoes in the central part of the Mexican Volcanic Belt (3840 msnm), is an andesitic to dacitic composite cone., its morphology is characterized also by U-shaped, glacially incised valleys, which are cut by an east-facing horseshoe-shaped crater. A big fan was deposited on the Tepalcatepec depression. The fan from the Tancitaro collapse is formed by fluvial, debris avalanche and debris flow deposit. It is 60 km large and has an area of approximately 176 km2. If we consider that the last glacial period was 10000 and 6000 years ago and that the Tancitaro collapse structures cut the glacial valleys the debris avalanche belongs to an Holocene deposit. A Morphological and mapped study was performed in the fan deposit and show two important flows, the first is formed by a debris avalanche and the second corresponds to a debris flow

Landslide impact on the archaeological site of Mitla, Oaxaca

Some large landslide deposits are recognized within and around the Mitla archaeological site, which is located in a very seismically active region cut by the Tehuacan‐Oaxaca fault system. Previous studies have shown that this fault system has produced coseismic ruptures and associated earthquakes in the past. Our work on the Mitla landslide deposits clearly demonstrates that their morphology and nature are typical of dry‐rock avalanches. Thus, collapses of rock already weakened by alteration and/or weathering, and unstably mobilized by intense rainfall, are discarded as possible triggers of the avalanche events. We instead propose that the landslides were triggered by earthquake activity. Moreover, our data indicate that part of the original Mitla settlement lies buried under a large rock‐avalanche deposit produced sometime during the post‐Classic period (900–1520 AD). At that time, the city of Mitla was inhabited by over 10,000 people as estimated from archaeological reconstructions. This devastating landslide almost entirely obliterated Mitla, which at the time of the Spanish arrival still existed as a city, but much reduced in area and populations

The Mitla Landslide, an Event That Changed the Fate of a Mixteco/Zapoteco Civilization in Mesoamerica

Before the arrival of the Spanish conquerors, Mitla was the second most important city in the valleys of Oaxaca, México. However, the ruins that are visible today do not seem to match the size of a city of more than 10,000 inhabitants. Geological and geophysical studies suggest that part of the city was covered by the deposits of a dry landslide likely to have been caused by an earthquake with a magnitude that could vary between 6 and 7. This landslide is monolithological, which is why two geophysical methods were used in order to evaluate its geometrical characteristics and to suggest the possible existence of archeological remains under the landslide

Reconnaissance study of Colola and Maruata, the nesting beaches of marine turtles along the Michoacan coast in southern Mexico

Las tortugas marinas de la especie Chelonia agassizi navegan anualmente cientos de kilómetros en mar abierto para llegar a sus áreas de reproducción y anidación. Se cree que esta tarea se realiza mediante el uso de información magnética terrestre y de aquella derivada de los atributos físicos de dichas áreas. Así, la geomorfología, topografía y geofísica de las playas natales pudieran facilitar a las tortugas el identificarlas. Este estudio describe estos elementos en las playas de Colola y Maruata, sitios de anidación de C. agassizi en México. Colola, en el oeste, es una playa elongada de 4.8 km de longitud y 150 m de ancho, limitada al norte por depósitos fluviales costeros. Maruata, en el este, es una playa curva de 2.3 km de longitud y 40 m de ancho bordeada por depósitos fluviales costeros al noreste y por pendientes coluviales de depósito volcano-sedimentarias en el sureste. Levantamientos topográficos y observaciones de campo realizados en Colola (otoño 1998/primavera 2000), documentaron que los perfiles de playa cambian durante y entre estaciones reproductivas. En Maruata las anomalías magnéticas tienen un rango de valores de 50 nT a nivel del mar a > 350 nT en el Cerro Vigía. El vector de campo geomagnético varia en inclinación e intensidad con la latitud a partir de la horizontal en Islas Galápagos hasta cerca de los 45° en el sur de México y de 30,000 nT a > 40,000 nT, respectivamente. Localmente la inclinación e intensidad en Colola y Maruata muestran valores de 44.87° y 40364 nT y 44.85° y 40353 nT, respectivamente. Estos parámetros pudiesen imponer limitaciones a la ecología sensorial de las tortugas. doi: https://doi.org/10.22201/igeof.00167169p.2010.49.4.12

Paleomagnetic and rock-magnetic survey of Brunhes lava flows from Tancitaro volcano, Mexico

Este estudio presenta los resultados paleomagnéticos del Volcán Tancitaro, (perteneciente al Campo Volcánico Michoacán Guanajuato), en el oeste de México, como una contribución a la base de datos de promediados en el tiempo del campo global. Se realizó el estudio paleomagnético y de magnetismo de roca en once flujos de lava. Se colectaron 120 núcleos orientados en el volcán Tancitaro y sus áreas aledañas. Todos los sitios fueron previamente fechados por 40Ar –39Ar (Ownby et al., 2007,) y van desde 793 ka hasta el presente. Se realizaron experimentos de magnetismo de roca, curvas de susceptibilidad e histéresis magnética y en la mayoría de los casos la remanencia magnética fue por la presencia de titanomagnetita pobre en Titanio con estructura magnética de pseudodominio simple. Ocho de los once flujos de lava dieron polaridad normal, mientras que los tres restantes produjeron paleodirecciones inconsistentes y no se tomaron en cuenta para el análisis y se debió probablemente por efectos de rayos. La paleodirección principal obtenida de los ocho flujos es Inc=39.5°, Dec= 356.4°, k=29, αc95=9.1° lo cual corresponde a la posición del polo con Plat=84.4°, Plong=219.9°, K=33 y αc95=8.5° Esta dirección es prácticamente consistente con la paleodirección esperada del Plio Cuaternario, derivada del polo de referencia para el Craton Norteamericano. La variación paleosecular coincide con otros estudios de la misma latitud y con modelos estadísticos recientes. La inclinación media cae dentro de la incertidumbre del Eje Dipolar Geomagnético más 5% de contribuciones cuadrupolares. doi: https://doi.org/10.22201/igf.00167169p.2009.4.1

The 27 May 1937 catastrophic flow failure of gold tailings at Tlalpujahua, Michoacán, Mexico

On 27 May 1937, after one week of sustained heavy rainfall, a voluminous flood caused the death of at least 300 people and the destruction of the historic El Carmen church and several neighborhoods in the mining region of Tlalpujahua, Michoacán, central Mexico. This destructive flood was triggered by the breaching of the impoundment of the Los Cedros tailings and the sudden release of circa 16 Mt of water-saturated waste materials. The muddy silty flood, moving at estimated speeds of 20–25 m s⁻¹, was channelized along the Dos Estrellas and Tlalpujahua drainages and devastated everything along its flow path. After advancing 2.5 km downstream, the flood slammed into El Carmen church and surrounding houses at estimated speeds of ~ 7 m s⁻¹, destroying many construction walls and covering the church floor with ~ 2 m of mud and debris. Revision of eyewitness accounts and newspaper articles, together with analysis of archived photographic materials, indicated that the flood consisted of three muddy pulses. Stratigraphic relations and granulometric data for selected proximal and distal samples show that the flood behaved as a hyperconcentrated flow along most of its trajectory. A total volume of the Lamas flood deposit was estimated as 1.5 × 10⁶ m³. The physically based bidimensional (2-D) hydraulic model FLO-2D was implemented to reproduce the breached flow (0.5 sediment concentration) with a maximum flow discharge of 8000 m³ s⁻¹ for a total outflow volume (sediment + water) of 2.5 × 10⁶ m³, similar to the calculations obtained using field measurements. <br><br> Even though premonitory signs of possible impoundment failure were reported days before the flood, and people living downstream were alerted, authorities ordered no evacuations or other mitigative actions. The catastrophic flood at Tlalpujahua provides a well-documented, though tragic, example of impoundment breaching of a tailings dam caused by the combined effects of intense rainfall, dam weakness, and inadequate emergency-management protocols – unfortunately an all-too-common case scenario for most of the world's mining regions