soil radon time series: surveys in seismic and volcanic areas

6
PERGAMON Radiation Measurements31 (1999) 307-312 Radiation Measuroments SOIL RADON TIME SERIES: SURVEYS IN SEISMIC AND VOLCANIC AREAS N. SEGOVIA*, M. MENA**, P. PENA*, E. TAMEZ*, J.L. SEIDEL***, M. MONNIN*** AND C. VALDES** * IN]N, Ap. Post. 18-1027, 11801 Mexico D.F., Mexico ** IGFUNAM, Ciudad Universitaria, 04510 Mexico D.F., Mexico *** Universite de Montpellier-CNRS, Laboratoire de Geochimie, Montpellier, France ABSTRACT Soil radon surveys have been pertbrmed in a long term mointoring basis with SSNTD (LR 115 type 11), 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 t~mction 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 fimction of local earthquakes and meteorological perturbations. Additionally, complementary results from recent changes in the activity pattern of an active volcano indicate that degassing processes induced anomalous soil radon emanation correlated with the volcanic activity changes. KEYWORDS Soil radon; SSNTD; seismic zones; active volcano; continuous monitoring. INTRODUCTION ~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 volcamc eruptions (Segovia et al., 1993). Five years radon in soil monitoring are actually available in a very intense seismic zone located along the Pacific coast of Mexico. In this zone the subduction of the Cocos Plate under the North America Plate defines a high risk area. A seismic gap has been identified at the State of Guerrero where a large earthquake (up to M = 8.2) is predicted to occur in the next years. Such an earthquake would mainly affect several of the largest cities in the Country (Singh and Mortera, 1991: Maldonado et al., 1996). The existence of a seismic gap does not mean that seismic quiescence occurs, on the contrary, an intense seismic activity occurs characterised by a large number of small and moderate magnitude events (M<5.0). On the other hand, the area under study is periodically affected by typhoons and tropical hurricanes that induce exteriml fluctuations to the soil radon emanation. Additionally, since December 1994 the Popocatepetl volcano located 60 km from Mexico City started an eruptive phase. The response of soil radon was recorded with track detectors and showed an increase that correlated with the starting eruptive phase and stabilised later (Segovia et al., 1997). The soil radon monitoring has continued with passive and active detectors. In this paper some changes observed in the radon patterns are discussed. 1350-4487/99/$ - see front matter© 1999 ElsevierScience Ltd. All fights reserved. PII: S1350-4487(99)00142-0

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Page 1: Soil radon time series: Surveys in seismic and volcanic areas

PERGAMON Radiation Measurements 31 (1999) 307-312

Radiation Measuroments

S O I L R A D O N T I M E SERIES: S U R V E Y S IN S E I S M I C A N D V O L C A N I C A R E A S

N. SEGOVIA*, M. MENA**, P. PENA*, E. TAMEZ*, J.L. SEIDEL***, M. MONNIN*** AND C. VALDES**

* IN]N, Ap. Post. 18-1027, 11801 Mexico D.F., Mexico ** IGFUNAM, Ciudad Universitaria, 04510 Mexico D.F., Mexico

*** Universite de Montpellier-CNRS, Laboratoire de Geochimie, Montpellier, France

ABSTRACT

Soil radon surveys have been pertbrmed in a long term mointoring basis with SSNTD (LR 115 type 11), 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 t~mction 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 fimction of local earthquakes and meteorological perturbations. Additionally, complementary results from recent changes in the activity pattern of an active volcano indicate that degassing processes induced anomalous soil radon emanation correlated with the volcanic activity changes.

KEYWORDS

Soil radon; SSNTD; seismic zones; active volcano; continuous monitoring.

INTRODUCTION

~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 volcamc eruptions (Segovia et al., 1993). Five years radon in soil monitoring are actually available in a very intense seismic zone located along the Pacific coast of Mexico.

In this zone the subduction of the Cocos Plate under the North America Plate defines a high risk area. A seismic gap has been identified at the State of Guerrero where a large earthquake (up to M = 8.2) is predicted to occur in the next years. Such an earthquake would mainly affect several of the largest cities in the Country (Singh and Mortera, 1991: Maldonado et al., 1996). The existence of a seismic gap does not mean that seismic quiescence occurs, on the contrary, an intense seismic activity occurs characterised by a large number of small and moderate magnitude events (M<5.0). On the other hand, the area under study is periodically affected by typhoons and tropical hurricanes that induce exteriml fluctuations to the soil radon emanation.

Additionally, since December 1994 the Popocatepetl volcano located 60 km from Mexico City started an eruptive phase. The response of soil radon was recorded with track detectors and showed an increase that correlated with the starting eruptive phase and stabilised later (Segovia et al., 1997). The soil radon monitoring has continued with passive and active detectors. In this paper some changes observed in the radon patterns are discussed.

1350-4487/99/$ - see front matter © 1999 Elsevier Science Ltd. All fights reserved. PII: S 1350-4487(99)00142-0

Page 2: Soil radon time series: Surveys in seismic and volcanic areas

308 N. Segovia et aL /Radiation Measurements 31 (1999) 307-312

25' N GULF OF MEXICO

15' N [~'¢ 'q~

~ ' 110*W 90' W

18 ° N

17 ° N

• LA UMIOM

" ~ PAPAll OA ----NOll SAN LUI$ I..4. LOMA

~ . i EL CAYAC 0

PACIFIC OCEAN ~r.RRO O[ PIEDRA ~1118A14 MAR O 08 - " " * * ~ klAN qU CI..llk rl

lo ow 1,0'ow

MEXICO Cl~j~ TOLUCA • ~ J

Fig. 1. Location of the monitoring stations along the seismic zone of the Pacific coast of Mexico and the Popocatepetl volcano.

Table 1. The radon monitoring stations along the coastal seismic zone of the State of Guerrero.

STATION NAME COORDINATES SOIL TYPE

Marquelia GM

Cerro de Piedra GCP

San Marcos GSM

Acapulco-Sabana GAS

Acapulco-Centro GAC

Coyuca de Benitez-I GCB-I

Coyuca de Benitez-2 GCB-2

El Cayaco GEC

San Luis de la Loma GSL

Papanoa Huerta GPH

Papanoa Escuela GPE

La Union GLU

Guacamayas MG

16o34'45"N ;95o49'00"W

16o46'45"N ;99o37'45"W

16°47'45"N ;99023 , 15"W

16°53'30"N ;99°50'00"W

16°52'50"N ;99050'30 ' ' w

16o58'45"N ;100o06'00"W

16o56'45"N ;100o05'45"W

17002'45"N ;100°I6'00"W

17016'00 ' 'N ; 100053 , 15"W

17019 , 15"N ;101°02'30"W

17o19'15"N ;101o02'30"W

17059 , 15"N ; 101048 , 15"W

18001 , 15"N ;102°12'30"W

Residual soil

Residual soil

Residual soil and quartz diorite

Residual soil and metamorphic rocks.

Granitic rocks

Residual soil

Sand

Residual soil

Granitic rocks

Sand

Metamorphic rocks

Residual soil

Residual soil, igneous rocks

Page 3: Soil radon time series: Surveys in seismic and volcanic areas

N. Segovia et al. / Radiation Measurements 31 (1999) 307-312 309

EXPERIMENTAL

The monitoring was performed in 13 stations located in a coastal line parallel to the Mesoamerican trench and in the flank of the Popocatepetl volcano (Fig. 1). The nomenclature of the stations located around the seismic gap of Guerrero and the soil type at each station are shown in Table 1.

Radon detection

Long term determination of radon in soil, has been systematically performed with SSNTD, The detection material consists of cellulose nitrate foils, LRII5 type II, manufactured by Dosirad Co., France. They are exposed in the field for periods of one month, at a shallow depth of 70 cm. The alpha tracks are etched in a 2.5 N NaOH solution at 60°C during a pre-established time to reach a fixed residual thickness of 6.5 l~m. The detectors are then spark counted.

Short term radon monitoring is performed with Clipperton type continuous monitoring (set-up at 1 m depth) based on a silicon diode detector associated with electronic data processing and storage units of low energy consumption (Segovia et al., 1997).

RESULTS AND DISCUSSION

Figure 2 shows records from the LR 115 response of the variations of soil radon along the seismic coast of the Guerrero state.

The seismicity in the region has been large and it is quite difficult to assess the radon response in the soil to such an intense local seismicity. However some peaks appear at the same moment in different stations, indicating regional perturbations. In 1997, when several earthquakes (M>6) occurred, at radon monitoring stations GLU, GPH, GSL, GEC,GAS GSM GCP and GM, an increase was observed. On the other hand in 1995, when a 7.3 earthquake occured, a previous peak was observed at 5 stations. Results by Virk and Sharma (1997) have shown that high seismic activity can bring a confused pattern of soil radon behaviour and that meteorological and porosity changes induce espurious signals The geological characteristics of the soil where the station is settled induce differences in the radon levels. In Fig. 2, the mean radon values in the recorded period together with the standard deviation and the maxim and minima values obtained are indicated. The stations with higher radon emanation potential are GAC and GPE, which are settled in granitic and metamorphic rock soil. The lowest are those in the sandy coastal soil, GCB-2, GLU and GPH.

An example of the behaviour of the Clipperton probe settled in Aeapulco in shown in Fig. 3. The continuous radon behaviour shows a depletion after a 7.3 earthquake, that occurred at the beginning of 1997. The depletion recuperates in April, coinciding with a series of earthquakes in March-April. The seismicity during the year was extremely intense and no recuperation seems to occur for the radon emanation, with such an intense seismicity it is quite difficult to establish a biunivocal relation between the earthquakes and tile soil radon response. However groups of earthquakes can elevate the radon levels. The variations observed from June to November, 1997, correspond to a period of particularly intense meteorological perturbations produced as a consequence of El Ni~o, culminating with the formation of the tropical hurricane Paulina that stroke the Guerrero coast from October 6-10, 1997. A radon peak occurred on October 9, 1997.

Page 4: Soil radon time series: Surveys in seismic and volcanic areas

310 lq.. Segovia et al. / Radiation Measurements 31 (1999) 307-312

0

0

0

m

E

0

,m

o - ~ ' ~ x J ' ~ , - - - a ~ , . ~ . ~ - - ~ , .

~I ~ -.~ ~.~

0 ~

15 2"4 'JI *0'4 0~a

m

~ ~ , _ ~ -

i i 8

Fig. 2. Soil radon time series at the monitoring stations along the Pacific coast of Mexico obtained with track detectors. The seismicity of the zone (Mc > 4) is also shown.

Me

0

4 O

~E 30 ~" 20 ta 10 , X

0 0'~ ¢T~ O~ O) 03 O) 03 03 0'~ 03 03 O) • - - o, i 0 4 ~ ~ ¢6 ~ 06 oi d , - O O O O O O ¢~ O O v-- ~ v~.

O O O O O O ¢~ O O O O O

Fig. 3. A short term radon behavior example at the Acapulco Clipperton monitoring station (1997).

Page 5: Soil radon time series: Surveys in seismic and volcanic areas

N. Segovia et al. / Radiation Measurements 31 (1999) 307-312 311

In Fig. 4 the radon in soil behaviour at a station located at the Popocatepetl volcano indicates two main peaks. The first one corresponds to the initiation of the volcano's eruptive phase. The soil radon indicates the response of the gases to ash ejection and to a complex seismic pattern occurred from January to May 1995. The increase in the precipitation and the volcanic activity, probably combined to generate an increase in the soil radon curve in the second half of 1995. The activity slightly decreased from December 1995 to March 1996 where a small step in radon is observed. Between March and July 1996 a lava dome was formed at the bottom of the crater which coincides whith a radon concentration increase. At the end of September the lava dome and the ashes ejection decreased. This depletion produced a sudden fall of radon concentration at the beginning of 1997, The second peak occurred in the second trimester of 1997. In June 30, 1997, the largest event in the present eruptive episode took place. The eruption was preceded by a series of volcano-tectonic earthquakes. A significant event was a relatively shallow eartquake-explosion preceded by tremor signals (Fig. 5). The eruption generated a plume reaching 13 km high and aslffalls were reported. For the first time in this activity period starting in December 1994 the ashes arrived and covered with a 5 mm layer the south west part of Mexico City, located 60 km from the volcano. The airport of Mexico City had to be closed for about 12 hours, until the ash was washed away from the runways. The intensity of the eruption was estimated (CENAPRED, 1997) to have reached a VEI index of 2 to 3. The overall level of activity of the volcano decreased afterwards and in August 12 a moderately large exhalation, accompanied by a 5 km high ash emission occurred. For the rest of the observation time the activity has been lower. This explosion was preceded by a high radon in soil value indicating an excess pressure that permitted the movement of gases through the volcanic cone.

5 ...................................................................................................................................................................... : ...............................................................................................

r n

1 9 9 3 1 9 9 4 1 9 9 5 1 9 9 6 1 9 9 7 1 9 9 8

Fig. 4. The behavior of soi radon with SSNTD at the Paso de Cortes station, Popocatepetl volcano.

In conclusion it was possible to recognise in the variations of the soil radon series the different stages of the activity of the volcano showing a clear increase when the system was closed, avoiding the pressure liberation through the crater. It is worth mentioning that the radon anomaly observed during several months at the beginning of 1997 had a predictive character for the June 30 eruption.

Page 6: Soil radon time series: Surveys in seismic and volcanic areas

312 N. Segovia et al. / Radiation Measurements 31 (1999) 307-312

b~2. ~c~

~ o -

-4

0 o

|

, • = ISeismicMomen

I d3rne / c~t I I I I I

-98.66 -98.64 -98.62 -98.60 -98 58 Lonqitude

Fig. 5. Volcanic seismicity from 30 June, 1997.

A c k n o w l e d g m e n t - The authors acknowledge T. Santamaria, S. Ceballos and D. Cruz for technical assistance. We also acknowledge financial support from CONACYT project 3791P-T and IAEA research contract 8361/R2.

R E F E R E N C E S

CENAPRED (1997) Summary of the activity of the Popocatepetl volcano during 1994 to 1997. The eruption of June 30, 1997, the largest in the present eruptive episode. http:\\www.cenapred.unam.mx/jfg/mvolcan/resumen9497.html.

Maldonado S.C., Monnin M., Segovia N. and Seidel J.L. (1996) A radon measurement network to study radon anomalies as precursors of strong earthquakes in the Guerrero seismic gap. 11 WCEE, CD-Rom, ISBN: 0 08 0428223, Pergamon. Paper 1762, p.6.

Segovia N., Mena M., Pefla P., Tamez E., Seidel J.L. and Monnin M. (1997) Radon in soil variations related to volcanic activity. Radiat. Meas. 28, 745-750.

Segovia N., Mena M. and Tamez E. (1993) Radon monitoring related to a subduction zone in Mexico. Nucl. Tracks Radiat. Meas. 22, 435-440.

Singh S.K. and Mortera F. (1991) Source time functions of large Mexican subduction earthquakes, morphology of the Benioff zone, age of the plate and their tectonic implications. J. Geophys. Res. 96, 21487-21502.

Virk H.S. and Sharma A.K. (1997) ~4icroseismicity trends in N-W Himalaya using radon signals. In Rare Gas Geochemistry (H.S. Virk ed.) Amritsar, pp. 117-135.