c http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S0016-71692009000300006


Fault segments of the SW MAFS and their seismic potential
MAFS contains by eight active normal fault segments, mainly oriented NE–SW or E–W, with a secondary striking to N–S.
Querendaro–Indaparapeo segment (QI)
QI segment begins at Los Azufres geothermal field. It is oriented to ENE–WSW. Two dacitic domes were extruded and subsequently crossed by the fault. Towards Querendaro, we find andesitic–basaltic lava flows dated at 700 ka (Pradal and Robin, 1994), A >100 m scarp forms the southern boundary of the Querendaro lacustrine depression. The segment shows a SRL value of 20 km. The Mw value obtained from equation [1] ranges between 6.6 and 6.9 and the slip–rate is 0.14 mm/yr. By applying the Slemons'' (1982) plot, the recurrence time period (PR) for this interval of earthquake magnitude is between 9 and 15 ka.Small graben basins and "in echelon" normal faults are associated to the main fault segment in the vicinity of Querendaro. These grabens configure a pull–apart basin indicating that QI presents secondary left–lateral fault movement.
Charo segment (Ch)
This segment is also within Los Azufres geothermal field and oriented NE–SW. The youngest deposit affected by the movement of this segment is the rhyolitic dome of El Rosario (300 ka). Around Querendaro this segment exhibits 10 cinder cones extruded through the Miocene rocks of Sierra Mil Cumbres. In the vicinity of Charo, this segment develops triangular facets that affected Miocene volcanic rocks of Sierra de Mil Cumbres and recent fluvio–lacustrine sediments around La Golera area. Ch presents a SRL of 30 km and from Ec (1), the Mw value varies between 6.8 and 7.1. The fault slip–rate is 0.2 mm/yr and the interval period of recurrence (PR) is between 6.7 and 10 ka.
Taimbaro and Alvaro Obregon segment (TAO)
Magnetometry and georadar techniques were used to study the geometry of this segment, mainly oriented to NE–SW and secondary E–W. TAO is 35 km long and exhibits a surface scarp greater than 100 m. At its eastern part, TAO crossed lava flows aged 8 Ma whereas at its western tip point, this segment displaces basaltic lava flows of El Quinceo – Las Tetillas volcano, lava flows containing corn cob moulds (Martinez and Hobson, 1907). The most recent fault movement generated surface displacements from a few up to tens of meters (Garduno–Monroy et al., 2001). Assuming that maize existed during the last 3.6 ka, the presence of maize in this fault segment means that was reactivated after 3.6 ka (Watts and Bradbury, 1982).The western termination of this segment can be observed in the zone of Las Joyitas (NW of Morelia), where the fault acted as a vent for the extrusion of two cinder cones and subsequently crossed them. The garbage dump of the city of Morelia is located there. Mw ranged between 6.9 and 7.1. Hence, TAO segment presents a non–uniform fault slip–rate throughout time. The oldest materials Upper Miocene–Pliocene show a vertical displacement of 200m; thus the slip–rate is calculated 0.025 mm/yr. Best the youngest detected movement of 3.6 ka generated a fault scarp of ca. 10 m, suggesting a fault slip–rate of 2.78 mm/yr, two orders of magnitude higher than the precedent value. We suggest that the slip–rate value depends on the time scale, i.e; the short–term is the Holocene slip–rate and the long–term is the Pliocene to Present–day slip–rate value. More data are needed. The PR interval for the long–term case is estimated at 60 to 100 ka.
Central Morelia segments (CM)
CM is composed by two ENE–WSW normal fault sub–segments, located within the City of Morelia. They have downthrown blocks to the north and scarp heights of 20 to 60 m. One of them is located in the northern part of the city (El Realito, Guadalupe and La Colina districts) and the other is the downtown area next to the bus station. Assuming total co–seismic rupture of both sub–segments SRL reaches ca. 5.5 km, Mw ranges between 5.8 and 6.5. The fault slip–rate is 0.12 mm/yr and the PR from 4.5 to 8 ka.Surface subsidence occurs along this fault at a rate of 4 to 6 cm/yr two orders of magnitude faster than the seismic slip–rate. It could cause severe damage to buildings. This process may be a result of over exploitation of groundwater.
La Paloma segment (LP)
Morelos segment (Mo)
Mo segment is 6 km long south of Morelia City. The main orientation is to ENE–WSW affecting Upper Miocene to Pliocene deposits (Morelia ignimbrite). The accumulated vertical fault displacement is ca. 50m Mw ranges between 5.9 and 6.5 and this segment exhibits the lowest slip–rate in the studied area (0.009 mm/yr). By applying the Slemons'' (1982) curve, the PR varies between 40 and 100 ka. Mo may be considered as a bifurcation of the Cointzio segment (Ctz).
Cointzio segment (Ctz)
Ctz segment cuts Miocene rocks, Morelia ignimbrite and, towards the west, displaces the eastern flank of the Cerro El Aguila shield volcano. It is a normal fault oriented towards NE–SW, with a downthrown block to the north and with a scarp of 100 m height. It forms the northern border of the Cointzio dam at Cerro El Aguila, it displaces lavas of 500 ka. Considering the scarp height, the estimated slip– rate is 0.2 mm/yr. Ctz is a 15 km long and may generate earthquakes with Mw between 6.4 and 6.8 and with a period of recurrence 30 < PR < 60 ka.
Huiramba segment (Hu)
This fault segment is discontinuous and extends from the southern part of Huiramba to Cerro El Aguila. The main orientation of Hu is to N–S and with a SRL value of 10 km. Towards the SW of the Cerro El Aguila volcano, the orientation of Hu changes NNE–SSW. Near of the town of Hiuramba, this segment shows normal faulting with hanging walls to the W and surface throw of about 50 m. A small graben with the same orientation N–S is developed on the southern side of the Cerro El Aguila volcano. Hu affects andesiticdacitic domes with an age lesser than 500 ka. The earthquake magnitude interval is (6.2 – 6.7) with a fault slip–rate of 0.1 mm/yr. The time period of recurrence varies between 4.5 and 10 ka.
Patzcuaro segment (Pa)
Pa segment extends from Erongaricuaro to the City of Patzcuaro, and is oriented to E–W. This segment develops the graben of Jaracuaro–Patzcuaro, which is related to the almost 40m uplift of the island of Jaracuaro. The volcano La Taza (8 Ka) (19° 31'' 35''N; 101° 43'' 29 ''W) and the avalanche deposits and collapse of volcano El Estribo, are related to the Pa segment. These avalanche deposits and collapse were generated by M>6 earthquakes.
The SRL value suggests earthquakes ranging between 6.4 and 6.8. The tectonic uplift of Jaracuaro Island and the age of the last paleoseismic event, suggest a fault slip–rate greater than 2.5 mm/yr. This value exceeds slip–rates for the precedent segments of MAFS, by one order of magnitude, but is similar to the short–term slip–rate for the TAO segment. We interpret this value as the short–term slip–rate for the Pa segment (Holocene).
The main result of the seismotectonic analysis is a estimated M7.1 the potential maximum expected earthquake, with PR values ranging between 60 and 100 ka (TAO segment). The minimum PR interval is (4.5–8) ka for earthquake sizes of (5.8–6.5) in the CM segment. The Ch segment presents the same potential maximum size M7.1, but PR is between 6.7 and 10 Ka. All segments are near Morelia City, thus the seismicity is relevant for future hazard studies.

Discussion
Age of the Morelia– Acambay Fautling
The age of the TMVB has been controversial. Recent dates have established that volcanism in this arc began ~ 12 Ma ago.I n the study area, the oldest lacustrine sequence has Miocene–Pliocene age. In the Charo region the oldest lacustrine deposits were dated at 7 Ma. At Los Azufres, Pradal and Robin (1994) dated monogenetic volcanism of Querendaro at 700 ka and the lavas were cut by the Morelia–Acambay fault. Further north, in the Araro region, proximal lacustrine deposits with Pirinda–Purhepecha pottery shards were raised 20 m by the E–W faults. In the Morelia region, the MAFS structures displace Upper Miocene (12 Ma) volcanic sequences while further north, in the Tarimbaro region, E–W faults cut the lavas of the Holocene Quinceo volcano, that contain maize cob casts. Fallout deposits of the Quinceo–Las Tetillas volcanoes containing pottery shards were also displaced by E–W faults.

Paleontological studies performed between Acambay and Cuitzeo suggest the following dates:
1. In Tierras Blancas and San Bartola Lanzados, Acambay, Mercer et al. (2002) reported interbedded lacustrine sequences with fallout layers of ash and pumice dated at 1.2 Ma (Late Pliocene–Early Pleistocene). These lacustrine layers contain Equus simplicidens, c.f. Rhynchotherium ?, Camelops sp., Mammuthus sp., Bison sp. and Antilocarpa sp. This fauna confirms the of Pliocene–Pleistocene age of this lacustrine basin and an important tectonic activity synsedimentary.
2. The Pastores fault south of Tierras Blancas and San Bartola Lanzados basin displaces lavas dated by Suter et al. (1995) at 0.4 Ma (Pleistocene).
3. In a small E–W basin in Chincua, a lacustrine column revealed molars of Equus sp. and Cuvieronis sp., which assigns a Miocene–Pliocene age to the deposits. The lacustrine sequence is covered by a gray ignimbrite deposit dated 4 Ma, which agrees with a Miocene–Pliocene age the basin.
4. In Zaragoza, Michoacan, remains of Mammuthus columbi (Falconer) 1857 was found in a small basin bounded by E–W faults, dated Middle to Late Pleistocene.
5. Several authors have made stratigraphic and paleontological studies in a lacustrine sequence in Charo that is tilted and cut by E–W and NNW–SSE faults. They found recorded a rich vertebrate fauna of Proboscidea, Artiodactyla, Peris sodactyla, Lagomorpha, Rodentia and Birds from Early Pliocene to Pleistocene, as corroborated by a date of 3.6 Ma a cinder cone. Israde (1995) reported a date from the lacustrine sequence base of 7 Ma (Upper Miocene).
The evidence suggests that MAFS structures originated evidence suggests that MAFS structures originated in the Upper Miocene. By 7 Ma, the lacustrine basins were depressions that were accumulating sediments, their origin must date back to 8–10 Ma ago as does inception of the E–W Morelia–Acambay faults.
Activity of some segments of MAFS is demonstrated at La Paloma where historical paleosols dated at 1.6 ka are displaced. Fallout contains pottery shards and the Araro lacustrine sediments contain prehispanic ceramics
Four small earthquakes occurred on October 17, 2007, around the City of Morelia. Three of these events were located by the National Seismological Service. They had magnitudes varying from 3.5 to 3.8, with depths between 1–6 km. Such shallow events were not related to subduction of the Cocos Plate, they originated along E–W segments of MAFS. The epicenters of were located along the Central Camionera fault. The length of this fault (8 km), suggests that it originate an earthquake of magnitude >3 and accelerations of 1–2 gales. Such an event can be felt by the population, as in the case of the October 2007 events. The focal mechanism of the earthquakes agrees with the geometry of the Central Camionera fault and possible segments to the west.
This earthquake activity corroborates that E–W segments of MAFS are subjected to a regional tensional field of the TMVB, and that they may be reactivated in the future. At present, only a small segment of MAFS has been activated; if the regional tensional field activates a larger segment (i.e. La Paloma fault) a stronger earthquake could occur with damage to the urban area of Morelia, as in the 1858 earthquake.
Fault inception and evolution of lacustrine depressions
The NNW–SSE faults have often been considered to be structures inherited from the north Mexican Basin and Range province. However, our study suggests that these faults may have been reactivated during the formation of the lacustrine depressions of the TMVB, since the orientation of these basins correspondsextension with a NW–SE orientation that is still active in the central part of the TMVB, Therefore the inherited NW–SE structures have a right–lateral component. This may be observed in the Tzitzio–Valle de Santiago structure, which must be a northern component of the Taxco–Queretaro Fault System that strongly controls the distribution of volcanism, causing displacement towards the SE.
The scarps of MAFS in the study region have heights ranging between 20 and 200 m. They are often larger at the eastern end of the fault. For example, the eastern tips of La Paloma and Tar&#1085;mbaro segments terminate abruptly with scarps higher than 200 m. Toward the west the scarp height decays gradually to ~20 m. These faults must be linked at depth to the eastern segments a cross relay ramps. The Tarimabro fault would be linked to the Alvaro Obregon fault and the La Paloma fault to the Jaripeo fault. If so, the Morelia–Acambay fault system may be coalescence stage (see also Suter et al., 2001), with individual faults linked at depth. Collapse scars are found in Patzcuaro and throughout the region (e.g. at Patzcuaro lake islands or La Paloma fault). Therefore, it appears that the Patzcuaro and La Paloma faults are seismically active, with Holocene and historical earthquakes as recorded. The Araro segment is also active as suggested by displacements in historical lacustrine units plus recent seismicity.
Earthquakes in the central TMVB are either subduction–related (deep interplate or intermediate–depth normal faults), shallow MAFS–related earthquakes. Both types of earthquakes might respond to regional structure of MAFS. For the western MAFS segments, the historic seismicity (two major earthquakes in 1845 and 1858) felt in Morelia and P&#1073;tzcuaro, could not be related to E–W striking faults. Instead, the shape of the 1845 and 1858 earthquakes isoseismals reveals that even in subduction–related earthquakes, the distribution of lacustrine sediments related to E–W faults controls the intensity of damage. The response of the terrain depends of the underlying material. In the studied area, there are two different types of substrate:
(1) In the Patzcuaro region, lava flows overlie lacustrine clays oversaturated with. The 1858 earthquake caused a water level rise in P&#1073;tzcuaro Lake, possibly in response to pressure exerted on oversaturated clays. A tsunami in the lake caused the collapse of over 120 adobe houses. El Estribo, El Metate and Janitzio volcanoes have suffered major collapses during seismic events over the last 30 ka.
(2) In Morelia, ancient lacustrine plains are vulnerable to earthquakes. A seismic microzonification study revealed that some quarters (Prados Verdes, Policia, and Transito) responded unfavorably to earthquakes, A larger earthquake may cause sever losses.

Conclusions
Prior

Paleoseismology of the southwestern Morelia–Acambay fault system, central Mexico