Smithsonian Institution Global Volcanism Network Bulletin v. 20, no. 4, April 1995 Popocatepetl (Mexico) Located seismic events and summit crater observations Popocatepetl central Mexico 19.02N, 98.62W; summit elev. 5,465 m A new episode of explosive activity began at Popocatepetl volcano on 21 December 1994. The eruption followed increases in seismicity, SO2 flux, and fumarolic activity seen in the 13 months proceeding it (Bulletin v. 18, no. 11; v. 19, nos. 1-4, 6, 8, 10, 12). More recent observations have included ash plumes, SO2 flux, seismicity, and temperature and chemistry of springs around the volcano (Bulletin v. 20, nos. 1-3). The following report was submitted by Carlos Valdes-Gonzalez, Guillermo Gonzalez-Pomposo, and A. Arciniega-Ceballos (UNAM). "We report on Popocatepetl seismic activity during the interval 21 December 1994 to 2 May 1995. Activity was monitored using 7 seismic stations located around to the volcano above 2,600 m elevation (figure 8). These stations are part of the Popocatepetl Seismic Network. Beginning 21 December, the volcano changed dramatically in its seismic and fumarolic activity. Several explosions emitted ash that fell on Puebla City, an area located about 50 km away [Bulletin v. 19, no. 11; v. 20, nos. 1-3]. About 22 hours after this activity, seismic tremor was observed for the first time at several stations. "In the 21 December-2 May interval we located 75 seismic events in the vicinity of the volcano (figure 8). We used arrival times from digital records from at least three stations and located the events using Hypocenter software. The average standard location errors in the horizontal and vertical directions do not exceed 1 km with a standard deviation of 0.14 km (figure 9). Earthquake magnitudes (calculated using a coda length magnitude for tectonic events in Mexico) ranged between 1.4 and 3.4 (as represented by different sized dots on figure 9). The E-W cross section of the hypocenters (figure 9) shows a concentration of seismic events in a circle of 3.0 km diameter and in a conduit that connects to the overlying crater. These results crudely suggest a magma chamber located below sea level and connected to the volcano crater. A N-S cross section suggests the same findings. "During the first four days (21-24 December) seismic tremor was continuous and of high amplitude. During the following 20 days (25 December-13 January) tremor was also continuous, but the amplitude diminished five-fold compared to the first four days. After that, in the next 45 days (14 January-28 February), tremor turned sporadic with durations of about 10 minutes and with amplitudes comparable to those in the first four days. During the last 60 days, tremor became more sporadic with smaller durations, but it still had amplitudes similar to, and in some cases exceeding, those of the first four days. "On 12 March an expedition lead by Enrique Chaves-Popuard reached the volcano's summit. The meteorological conditions allowed the team to videotape the interior of the crater. The following observations were made: a) the crater lake disappeared, b) three new craters appeared at the foot of the main crater's E wall, c) most of the fumarolic emissions came from these new craters, d) the number of small fumarolic vents has increased in the older inner crater, and e) several fumarolic vents were observed in the S and E walls of the main crater." Information Contacts: Carlos Valdes-Gonzalez, Guillermo Gonzalez-Pomposo, and A. Arciniega-Ceballos, Departamento de Sismologia y Volcanologia, Instituto de Geofisica, UNAM, Ciudad Universitaria 04510 D.F., Mexico (Email: email@example.com). Figure 8. Stations of the Popocatepetl Seismic Network (triangles) and epicenters for located events detected 21 December to 2 May 1995 (dots). Courtesy of Instituto de Geofisica, UNAM. Figure 9. An E-W cross section of the hypocenters beneath Popocatepetl for the interval 21 December 1994 to 2 May 1995. Earthquake magnitudes are shown by dot sizes; the size of error bars are discussed in the text. Courtesy of Instituto de Geofisica, UNAM.