M. Mena

Volcán El Chichón, Mexico: Pre-1982 S-rich eruptive activity

Abstract Mapping and sampling of the interior of El Chichon Volcano was made possible by the exposures of the 1982 explosion crater. The 1-km-diameter, 200-m-deep crater exposes rocks produced during several eruptive episodes: volcanic domes, talus breccias, plinian airfall deposits and pyroclastic-flow deposits of compositionally similar alkalic hornblende-trachyandesites with phenocrysts of anhydrite. All of the rocks, except those leached of anhydrite, have extremely high S contents. El Chichon consists of a volcanic dome complex, airfall debris, and pyroclastic outflow sheets. Prior to 1982, its last major activity was about 650 ± 100 yr B.P.

Soil radon time series: Surveys in seismic and volcanic areas

Abstract Soil radon surveys have been performed in a long term monitoring basis with SSNTD (LR 115 type II), in order to observe possible fluctuations due to high magnitude seismic events and volcanic eruptions. Five-year radon time series are available in stations located in an intense seismic zone located along the Pacific coast of Mexico. The series analyses have been performed as a function of the local seismicity and geological characteristics. A discussion is intended to explain the lack of biunivocal relation between single radon peaks and earthquakes for the long term monitoring data using SSNTDs. Examples of short term radon anomalies obtained with continuous probes are also discussed as a function of local earthquakes and meteorological perturbations. Additionally, complementary results from recent changes in the activity pattern of an active volcano indicate that degassing process induced anomalous soil radon emanation correlated with the volcanic activity changes.

Radon emanometry in soil gases and activity in ashes from El Chichon Volcano

Radon (222Em) emanation measurements in soil gases are reported in connection with the 1982 eruptions of El Chichón Volcano.222Em detection is performed with LR-115 cellulose nitrate track detectors. Results show a general decreasing pattern of222Em concentration in soil with time after the eruptions. Activity measurements of radon daughters in ashes show that a large amount of222Em and220Em was released from magma degassing during the eruptions, and that only a fraction of the degassed magma was erupted. An important fluid interaction between the magma chamber and the surrounding sedimentary rock is also suggested.

Radon in soil anomaly observed at Los Azufres geothermal field, Michoacan: a possible precursor of the 1985 Mexico earthquake (Ms = 8.1)

A radon anomaly in a nuclear track detector placed on a fault was detected prior to the destructive (Ms = 8.1) Mexican earthquake of 19 September 1985. The fault is a structural feature of the geothermal field ‘Los Azufres’, 260 km NE of the epicentral area. Since no other phenomenon can be found as a possible cause for the radon anomaly, it is proposed that preseismic regional stress perturbations may produce changes in the fluid transport patterns at the fault, suggesting that radon measurements in similar conditions could provide a good precursor in high seismic risk areas.

A model of the Chicxulub impact structure (Yucatan, Mexico) based on its gravity and magnetic signatures

Abstract The separating power of the first and second vertical derivatives have enabled us to infer from the Bouguer gravity anomaly the major morphological features of the Chicxulub impact structure. We infer a structural high of a composite nature (a twin peak?). We corroborated the existence of a first concentric ring, which is continuous to the west and SW of the structural high. The existence of more rings is not supported by our analysis. We were also able to constrain the diameter of the structure to about 200 km (this is also supported by the gravity and magnetic modelling). We elaborated a model for the subsurface structure that is robust in the sense that it accounts for the gravity and magnetic anomalies. Our study suggests that the Chicxulub impact structure corresponds to a crater with a central structural high and surrounded by a ring. According to crater formation theories, it would have been formed in a moderately thick lithosphere that only permits the formation of one or few rings.

Short and long term radon in soil monitoring for geophysical purposes

Abstract The subduction of the Cocos Plate under the North America Plate defines a high risk seismic zone along the Pacific coast of Mexico. A seismic gap has been identified at the Guerrero State as a place where a large earthquake, up to MS=8.2, can occur within the next years. In this area an in-soil radon network, consisting of fixed SSNTD monitoring stations, has been operated during several years on a monthly exposure basis. A continuous radon probe that registers data on a short term basis is also operating since recently. The results of the two radon detection systems are compared and discussed as a function of the local seismic activity observed at the Guerrero coastal zone.

Radon-in-soil variations related to volcanic activity

Two active volcanoes, the Popocatepetl in Mexico, and the Piton de la Fournaise on the island of La Reunion, France, have been monitored for soil radon for several years. The Popocatepetl, located 60 km from Mexico City, started an eruptive period in December, 1994. Two radon-in-soil monitoring stations are operated there, since 1993. The behaviour of soil radon showed an increase as a function of the eruptive phase. The Piton de la Fournaise volcano has also been under survey for several years. A radon network based on 20 Clipperton II automatic probes was set on July, 1994. Since then, the volcano has entered a quiescence phase. Thus it has been possible to draw the pattern of the radon behavior under the influence of atmospheric conditions.

Volcanic monitoring for radon and chemical species in the soil and in spring water samples

Abstract Soil radon has been monitored at two fixed stations in the northern flank of Popocatepetl Volcano, a high risk volcano located 60 km SE from Mexico City. Water samples from three springs were also studied for radon as well as major and trace elements. Radon in the soil was recorded using track detectors. Radon in the water samples was evaluated using the liquid scintillation method and an Alphaguard. The major elements were determined through conventional chemical methods and trace elements using an ICP-MS equipment. Soil radon levels were low, indicating a moderate diffuse degassing through the flanks of the volcano. Groundwater radon had almost no relation with the eruptive stages. Water chemistry was stable in the reported time (2000–2002).

Structure of La Primavera caldera, Jalisco, Mexico, deduced from gravity anomalies and drilling results

Abstract Previous studies of La Primavera caldera have mostly been based on surface geology and topography. Since 1980, many wells, exploring for geothermal energy, have reached depths of about 2 to 3 km at the center of the caldera. The results of the drillings, together with those of the gravity surveys, provide information about the subsurface structure of the caldera, and shed light on its formation. The drilling results and gravity anomalies at La Primavera caldera and San Marcos, located at about 40 km distance from the caldera, suggest that regional gravity anomalies can be interpreted in terms of depths of the granitic basements: the basement beneath La Primavera caldera is about 3 km deep and consists of roughly the same horizon as that beneath San Marcos. The drilling results within the caldera reveal that the depth of the caldera fills ranges from 0.3 to 1 km at the drilling sites. The andesite basement, about 1 km deep, remains approximately horizontal, and the granitic basement has a depth of about 3 km. The surface topographies, such as the postcaldera domes, scarcely disturb the subsurface strata. The local gravity anomalies show two lows within the caldera reflecting the configuration of caldera bottom, two funnel-shaped depressions, one of which corresponds to a vent of the Tala tuff deduced from geological observations. The mass deficiency within the caldera estimated from the gravity anomaly, satisfies the general relationship that the mass deficiency is proportional to the caldera diameter cubed. This means that caldera structure is three-dimensional: the larger the diameter, the deeper the funnel-shape. At present this argument may be limited to funnel-shaped calderas.

Soil radon response around an active volcano

Abstract Soil radon behavior related to the volcanic eruptive period 1997–1999 of Popocatepetl volcano has been studied as a function of the volcanic activity. Since the volcano is located 60 km from Mexico City, the risk associated with an explosive eruptive phase is high and an intense surveillance program has been implemented. Previous studies in this particular volcano showed soil radon pulses preceding the initial phase of the eruption. The radon survey was performed with LR-115 track detectors at a shallow depth and the effect of the soil moisture during the rainy season has been observed on the detectors response. In the present state of the volcanic activity the soil radon behavior has shown more stability than in previous eruptive stages.