Tonatiuh Domínguez

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

Comparative characteristics of the 1997-1998 seismic swarms preceding the November 1998 eruption of Volcán de Colima, México

Abstract The recent eruption of Volcan de Colima, Mexico, began on 20 November 1998 and was preceded by a 12-month period of seismic activity that included five swarms of microearthquakes. The swarm activity occurred mainly within the southern part of Colima Volcanic Complex. We located about 600 events of magnitude Mw from −0.5 to 2.7 within a 50 km2 area including the active crater of Volcan de Colima and the region between it and the Pleistocene volcano Nevado de Colima 5.5 km to the north. The majority of hypocenters within this area did not exceed 5 km depth below sea level (BSL) and were mainly distributed within the volcanic edifices of Volcan de Colima and Nevado de Colima. We investigated the spatial distribution and spectral properties of events that occurred during three main seismic swarms: in November–December 1997, June–July 1998 and October–November 1998. We identified a temporal change in the depths of events from November–December 1997 and June–July 1998 (from 4 km BSL to 4 km above sea level (ASL)) to October–November 1998 (from 0 to 4 km ASL). Two groups of events with mean P-wave corner frequencies of 5 Hz (LF) and 10 Hz (HF) were distinguished. HF events dominated during the first swarm whereas the LF events were dominant during the June–July 1998 swarm. During October–November 1998, the numbers of HF and LF events were comparable, but very low-frequency low-stress drop events disappeared two days before the eruption. We consider the LF events to have resulted from fracturing along two systems of local faults, whereas the HF events could have been produced by dike movement along these same faults. We propose a model for construction of a magma passageway by the opening of two systems of local faults triggered by magma intrusion.

Site Effects in a Volcanic Environment: A Comparison between hvsr and Array Techniques at Colima, Mexico

Colima city is the capital of the Mexican federal state of the same name. It is located close to the Pacific coast and is subjected to a large seismic risk. We present a microzonation study in this city, based on microtremors using single-station and array measurements. We applied horizontal-to-vertical spectral ratios (hvsr) analysis to single-station measurements at 310 sites within the city, concentrating measurements in zones that were damaged by the January 2003 ( M 7.4) earthquake. The results show that a seismic zonation based exclusively on single-station microtremor measurements is not a reliable alternative when the local geology is complex and site effects are not the result of a single-impedance contrast. For this reason, we applied two independent analysis techniques to array measurements of microtremors: the spatial autocorrelation (spac) method and the refraction microtremor (ReMi) method. We used linear arrays to record 25-sec microtremor windows at eight sites within the city, which were analyzed with those two techniques. The result of both techniques of analysis is a phase-velocity dispersion curve, which can be inverted to obtain a shallow S -wave velocity profile. Two of the sites were the location of shallow (50 m) boreholes, where P - and S -wave velocity profiles were measured using a P-S suspension log. The phase-velocity dispersion curves obtained from the ReMi and spac analyses of the microtremor records showed very good agreement. The velocity profiles inverted from the phase-velocity dispersion curves showed good agreement with the suspension logging measurements at one of the two sites where they were available and poor agreement at the other site. The transfer functions computed from the inverted soil profiles are in good agreement with previous estimates of local amplification from spectral ratios analysis of earthquake records. Our results are compatible with previous indications of site effects and explain the failure of single-station microtremor measurements when the concept of dominant frequency loses its meaning. Finally, we propose an estimate of local site amplification at the city of Colima, which will be useful for future predictions of ground motion at this city.

Near real-time satellite monitoring during the 1997-2000 activity of Volcán de Colima (México) and its relationship with seismic monitoring

Abstract The 1997–2000 activity of Volcan de Colima provided a unique opportunity for the joint analysis of satellite and seismic monitoring techniques. Three stages of volcanic activity were observed: pre-lava stage, stage of lava eruption, stage of explosive eruptions. Advanced Very High Resolution Radiometer satellite data provided observations of both summit thermal anomalies and ash emission. Daily monitoring of Volcan de Colima was performed using early morning or late night NOAA 12 and 14 satellite passes gathered in real time at our satellite ground receiving station. Seismic observations were carried out by the Red Sismica del Estado de Colima. Good correlations are observed between seismic activity and parameters monitored by satellite during the three stages. Five seismic swarms were recorded during the reactivation period. Four of the seismic swarms coincided with periods of increase in summit temperature and strong ash emissions. It is important to note that the ash emissions were observed a few days before the beginning of seismic swarms. The thermal saturation at the summit was reached a few hours after the beginning of lava effusion on November 20, 1998, and remained at this level until May 1999 even after the lava flow stopped in early February 1999. The lava front was also clearly identified as a source of thermal anomalies and ash plumes. Study of the summit temperature–ash emission relationship allowed us to classify the strong explosions of 1999–2000. The strongest explosion, on February 10, 1999, produced the largest, ash-rich plumes that were detected over a wide area (∼150 km 2 ) and near-saturation temperature anomalies (47.2, 48.4°C). The main limitation of volcano satellite monitoring occurs during overcast conditions. Clouds prevented us from monitoring the seismic swarm of July 1998 and the explosion of July 1999. This study shows that satellite monitoring on a near real-time basis is a powerful tool not only for research in volcanology but also as an aid for scientific committees to define probable eruption scenarios during volcanic crisis and for Civil Protection Authorities for risk mitigation due to ash emission and sudden heating of the crater without seismicity.

Regional variations of seismic attenuation of Lg waves in southern Mexico

Attenuation of Lg waves is estimated using 61 events located in the subduction zone of the Middle American trench and recorded by the seismic network of Laguna Verde, Veracruz, Mexico. We estimate the anelastic attenuation coefficient γ of Lg waves or, equivalently, QLg by calculating the rate of acceleration spectra decay with distance. We consider paths from two regions: Guerrero-Veracruz (NE-SW direction) and Oaxaca-Veracruz (North-South direction). Assuming a frequency dependence of γ of the form γ(ƒ) = γ0ƒη, we find that η=0.175±0.05 for both regions and that γ0 was lower for paths from Guerrero to Veracruz (γ0=0.0071±0.002) than for paths from Oaxaca to Veracruz (γ0=0.0161±0.003) in the frequency range from 2 to 7 Hz. In terms of the quality factor QLg, values of QLg= 134±30ƒ0.83±0.2 and QLg=59±10ƒ0.81±0.2 were found for the Guerrero-Veracruz and the Oaxaca-Veracruz paths, respectively. This difference in attenuation may be due to the state of stresses that prevails in both regions including the density and fluid content of fractures, which are attenuation mechanisms also suggested for other regions [Mitchell, 1995].

Regional variations of seismic attenuation from coda and L g waves in northern Baja California

Attenuation of coda and Lg waves was estimated from events recorded by the seismic network of northern Baja California, Red Sismica del Noroeste de Mexico (RESNOM), in the frequency band 1 to 12 Hz in order to find a regional variation of Qc (crustal quality factor for S waves) in northern Baja California, Mexico. We used the single scattering model of Sato [1977] in the time domain to estimate quality factor Qc and a spectral approach to relate acceleration spectra with distance to estimate the anelastic attenuation coefficient γ of Lg waves. We calculated Qc for short (<10 s) and long lapse times (<70 s) using local events and the events that originated at the main tectonic features of the area. Assuming a frequency dependence of Qc of the form Qc(ƒ)=Qcƒα we observed for short lapse times a regional variation of Q0 with low values from about 53 in the Inner Continental Borderland (ICB) province to about 115 in the Peninsular Ranges (PR) and 144 in the Gulf of California Depression (GCD) province. The α values range from 0.70 and 0.85 for events in the ICB and PR to 0.44 in the GCD region. For long lapse times and for events located in the PR we found Q0 values from 49 at stations within the ICB to 101 at stations within the GCD. Attenuation coefficient γ of Lg waves between 1 and 6 Hz was also estimated using events located in southern California with source-station distances up to 420 km. We found that frequency dependence of γ was higher for source-receiver paths going from a typical continental structure to a pull-apart basin (the GCD province) than for paths within the PR. Using all data in a linear inversion process and assuming a group velocity (VLg) of 3.4 km/s, we obtained a power law dependence of the form QLg(ƒ) = 288 ± 20ƒ0.32±.02

Real‐Time AVHRR Thermal Monitoring and Ash Detection: The Case of Colima Volcano (Mexico)

In this report we examine the application of the Advanced Very High Resolution Radiometer (AVHRR) data for real-time surveillance of volcanic thermal phenomena and eruption plumes. The 14 active volcanoes of Mexico are monitored in real-time, additionally a further 18 volcanoes across Central America. This paper concentrates on the thermal monitoring and ash plume detection at Volcan de Colima, considered one of Mexicos most active volcanoes. The results reported here were selected from 296 nocturnal satellite passes acquired during 18:00 and 8:00 hours (local time). Ash clouds were detected in forty-seven (16%) passes. Frequent thermal anomalies were observed in sequential satellite passes together with ash emission. These were correlated with explosion-related waveforms apparent in seismic data. Two time periods are identified: the first period (March to May 2002) was characterized by very strong thermal anomalies with thermal pulses lasting 5 to 7 days. These anomalies were associated with incandescent pyroclastic and lava flows on the flanks of the volcano. During the second period (June to August 2002) the thermal anomalies had a much lower magnitude, with minor ash emission observed only on 19 July. During the middle of August 2002 a slight increase in the magnitude of the thermal anomalies was observed, with ash emission only on 21 August. Although there were no observations of ash fall in the city of Colima, or damage to human health, infrastructure or economic activities during this period, such real-time satellite monitoring may prove to be a very useful tool for volcano surveillance.