S. De la Cruz-Reyna

Overview of the 1997^2000 activity of Volcan de Colima, Mexico

Abstract This overview of the 1997–2000 activity of Volcan de Colima is designed to serve as an introduction to the Special Issue and a summary of the detailed studies that follow. New andesitic block lava was first sighted from a helicopter on the morning of 20 November 1998, forming a rapidly growing dome in the summit crater. Numerous antecedents to the appearance of the dome were recognized, starting more than a year in advance, including: (1) pronounced increases in S/Cl and δD values at summit fumaroles in mid-1997; (2) five earthquake swarms between November–December 1997 and October–November 1998, with hypocenters that ranged down to 8 km beneath the summit and became shallower as the eruption approached; (3) steady inflation of the volcano reflected in shortening of geodetic survey line lengths beginning in November–December 1997 and continuing until the start of the eruption; (4) air-borne correlation spectrometer measurements of SO2 that increased from the background values of

Chemical variations of tephra-fall deposit leachates for three eruptions from Popocatépetl volcano

Abstract Leachates from ash samples of the Popocatepetl eruptions of April 30, 1996, May 12, 1997, and October 17, 1998 settled at different distances from the crater were analyzed for anions (SO 4 2− , Cl − , F − ) and some metals. This study is aimed at determining the causes of the compositional variations of the leachates, to assist the assessment of water, soil and crop contamination due to ash deposits. Different behavior was observed in the ion concentrations with distance for the three eruptions. On April 30, 1996, SO 4 2− and F − concentrations increased with distance, and Cl − remained almost constant. On May 12, 1997, concentrations of the three anions decreased with distance. On October 17, 1998, F − , Cl − and SO 4 2− increased more than three-fold with distance. Tephra size distributions were also different for the three eruptions. The observed trends of the leachates’ anion concentrations may have different causes: the type and intensity of the eruptions, the distribution of the tephra sizes, the degree of interaction of the tephras with volcanic gases, humidity, static charge, the original characteristics of the solid material, the transport time from the crater to the site of settling, and the relative angle between the wind direction and the sampling line. Enrichment factors and concentration trends for metals with distance suggest that Co, Ni, Cu and Pb in the leachates resulted mostly from volcanic gas adsorption.

Chemical characteristics of the crater lakes of Popocatetetl, El Chichon, and Nevado de Toluca volcanoes, Mexico

Abstract Three crater lakes from Mexican volcanoes were sampled and analyzed at various dates to determine their chemical characteristics. Strong differences were observed in the chemistry among the three lakes: Nevado de Toluca, considered as dormant, El Chichon at a post-eruptive stage, and Popocatepetl at a pre-eruptive stage. Not surprisingly, no influence of volcanic activity was found at the Nevado de Toluca volcano, while the other volcanoes showed a correlation between the changing level of activity and the evolution of chemical trends. Low pHs ( SO 4 2− and F− decreased irregularly at El Chichon from 1983 until 1997. Major cations concentrations also diminished at El Chichon. A 100% increase in the SO 4 2− content was measured at Popocatepetl between 1985 and 1994. An increase in the Mg/Cl ratio between 1992 ( Mg/Cl =0.085) and 1994 ( Mg/Cl =0.177) was observed at Popocatepetl, before the disappearance of the crater lake in 1994. It is concluded that chemical analysis of crater lakes may provide a useful additional tool for active-volcano monitoring.

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.

Energy partition in the 1982 eruption of El Chichon volcano, Chiapas, Mexico

Abstract The 1982 eruption of El Chichon volcano, Chiapas, Mexico has been extensively investigated from various standpoints, including seismicity, tephra geology, atmospheric effects and tectonic structure. In this paper, the six largest of a series of explosions are isolated, considering various sources of information. Then, the seismic energy released by volcanic earthquakes and tremor is estimated using seismograms of low magnification in order to avoid saturation of seismic wave amplitudes. The explosion kinetic energy is evaluated by two methods: one by the initial velocity of the ejecta and the other by the air wave data recorded at a distance. Both the results are roughly coincident. The thermal energy is classified into two components: that transferred by volcanic ejecta and that consumed by convective clouds. The thermal energy released by the explosions is very large compared with that of mechanical energy, such as the seismic energy and explosion kinetic energy. After the last major explosion of April 4, A-type volcanic earthquakes swarmed till the end of April. Their occurrence rate decreased with time in an aftershock-like sequence. These earthquake swarms may have been due to post-eruptive readjustment of the stress field within and near the volcano. Such seismic activity contrasts with other examples of similar major eruptions, which were followed by growth of lava domes in the new craters after the paroxysmal eruptions. The 1982 eruption of El Chichon volcano has not been followed by the growth of a lava dome within the newly formed crater.

Long-Term Probabilistic Analysis of Future Explosive Eruptions

The analysis of the patterns of explosive eruption occurrences helps volcanologists and civil defense authorities to make quantitative estimations of the levels of hazard. An important first step in an assessment of volcano hazards is to classify the eruptions by size or magnitude, and to recognize the statistical distributions of the eruption occurrences and repose times between them. For a single volcano, such distributions may be stationary or time dependent. In the latter case, the volcano under study may have different regimes or rates of activity and their distribution should be looked for. The current and future hazard level or probability of eruption occurrence in a certain magnitude range may thus be estimated by Bayesian methods. This type of analysis could prove helpful in assigning priorities for volcano surveillance during repose periods, in decision making during volcanic crises, and in land-use planning. When large populations of volcanoes are considered, as in the global case, the distribution of explosive eruption occurrences seems to approach a stationary pattern, at least for longer times, during which the rate of occurrence of a given class of eruptive magnitude is inversely proportional to the volcanic energy released by eruptions in that magnitude range.

Precursory earthquakes of the 1943 eruption of Paricutin volcano, Michoacan, Mexico

Abstract Paricutin volcano is a monogenetic volcano whose birth and growth were observed by modern volcanological techniques. At the time of its birth in 1943, the seismic activity in central Mexico was mainly recorded by the Wiechert seismographs at the Tacubaya seismic station in Mexico City about 320 km east of the volcano area. In this paper we aim to find any characteristics of precursory earthquakes of the monogenetic eruption. Though there are limits in the available information, such as imprecise location of hypocenters and lack of earthquake data with magnitudes under 3.0. The available data show that the first precursory earthquake occurred on January 7, 1943, with a magnitude of 4.4. Subsequently, 21 earthquakes ranging from 3.2 to 4.5 in magnitude occurred before the outbreak of the eruption on February 20. The (S - P) durations of the precursory earthquakes do not show any systematic changes within the observational errors. The hypocenters were rather shallow and did not migrate. The precursory earthquakes had a characteristic tectonic signature, which was retained through the whole period of activity. However, the spectra of the P-waves of the Paricutin earthquakes show minor differences from those of tectonic earthquakes. This fact helped in the identification of Paricutin earthquakes. Except for the first shock, the maximum earthquake magnitudes show an increasing tendency with time towards the outbreak. The total seismic energy released by the precursory earthquakes amounted to 2 × 10 19 ergs. Considering that statistically there is a threshold of cumulative seismic energy release (10 17–18 ergs) by precursory earthquakes in polygenetic volcanoes erupting after long quiescence, the above cumulative energy is exceptionally large. This suggests that a monogenetic volcano may need much more energy to clear the way of magma passage to the earth surface than a polygenetic one. The magma ascent before the outbreak of Paricutin volcano is interpretable by a model of magma-filled crack formation proposed by Weertman, based on seismic data and other field observations.

Chemical changes in spring waters at Tacaná volcano, Chiapas, Mexico: A possible precursor of the May 1986 seismic crisis and phreatic explosion

Abstract Local seismic activity consisting of sharp earthquakes accompanied by thunderous noise was reported starting in late December 1985 around Tacana volcano (15.13°N, 92.10°W). Portable seismic stations were established in the area by late January 1986 and sampling of the only known thermal spring on the volcano flanks started at the same time. A marked increase in SO42− concentration in the spring water preceded by two months the occurrence of a seismic swarm crisis and a small phreatic explosion. A model involving a crystalline basement fractured by tectonic stresses is proposed to explain the chemical and seismic anomalies, and the consequences on risk of volcanic activity are briefly discussed in terms of the observed behaviour.

Radon variations in active volcanoes and in regions with high seismicity: internal and external factors

Abstract The results of 4 years of observations of radon concentrations in soils of active volcanoes of Costa Rica and a highly seismic region in Mexico are discussed. A distinction is made on external (mostly meteorological) and internal (magmatic or tectonic) factors of variation in radon levels. The geological meaning of radon data can be thus enhanced if the external factors are excluded.