acoustic emission in cerro blanco, argentina · at cerro blanco (cb), san juan province, argentina,...

Acoustic Emission in Cerro Blanco, Argentina

Cynthia HUCAILUK1, María ARMEITE

1,

Isabel LOPEZ PUMAREGA1, José RUZZANTE

1, 2,

Bernardo VECA3, Dino FILIPUSSI

1, 2, 4, Miguel A. SABIO MONTERO

5

1 ICES (“International Center for Earth Sciences”), National Atomic Energy Commission,

Av. General Paz 1499, Buenos Aires, Argentina.

Phone: +54 11 6772 7766; [email protected], [email protected], [email protected], ruz-

[email protected], [email protected], [email protected] 2 U.T.N., Delta Regional Faculty, Buenos Aires, Argentina.

3 ICES Mendoza

4 UNSAM, National University of General San Martín, Buenos Aires, Argentina.

5 Material and Soil Institute, San Juan National University, San Juan, Argentina [email protected]

Abstract

At Cerro Blanco (CB), San Juan Province, Argentina, the Second Acoustic Emission (AE) Station in “Los An-

des” area was established since 2005. The AE activity is recorded continuously, taking the values of the AE

signal parameters and their RMS value, one point each 30 seconds. The purpose of the use of AE in geological

structures is to detect the stress changes showing the current condition of the structure. To understand their be-

haviour, the variations of the AE with seismic activity in the area are analysed. Two AE sensors are used: low

resonant frequency (25 kHz) and high resonant frequency (150 kHz). The period analysed in this paper is from

2005 to January 2012. During this period, 1858 earthquakes produced in San Juan province are related with the

AE RMS values taking account of their depth and magnitude.

Keywords: Acoustic Emission, Microseismic, Seismic Precursor, Cracks, Signal Analysis.

1. Introduction

1.1. Acoustic Emission

Acoustic Emission (AE) is the process of elastic waves generation inside a material as a result

of stress field changes when it undergoes some kind of external solicitation. AE is a dynamic

phenomenon. This can be detected by piezoelectric sensors distributed on the structure, giving

account of the increasing of cracks. The work range of AE is generally between 20 kHz and 1

MHz. Its relation with geological structure has been investigated for several years in order to

study its applications as one of the possible precursor technique of earthquake and volcanic

eruptions [1-13].

During 2003, an Argentine research group in collaboration with Italians began the installation

of the First AE Station in the Andes Mountains, on the Peteroa volcano in Mendoza Province,

Argentina [14, 15]. The Second AE Station was set up on Cerro Blanco (CB), San Juan, Ar-

gentina, beginning the infrastructure construction during 2005 [16]. Both installations are part

of a wider research project, which includes two more AE Stations during the next years, one

in Cacheuta (Mendoza Province) and the other in Tierra del Fuego Island.

In this paper, the analysis of the CB AE Station is presented. A small building contains the

AE system (see figure 1). Crack propagation creates discontinuous waves, pulses type or

events; after a quick increase they have a maximum and then they decay. The signal is then

amplified and processed electronically. Taking into account a voltage reference, called

“threshold”, it has been defined the following parameters that characterise the AE signal:

Amplitude (A), Duration (D), Rise Time (RT) and number of counts or Ring Down (RD). The

Root Mean Square (RMS) value of the signals, which is the most relevant in this work, is also

determined.

30th European Conference on Acoustic Emission Testing & 7th International Conference on Acoustic Emission University of Granada, 12-15 September 2012

www.ndt.net/EWGAE-ICAE2012/

mailto:[email protected]








Figure 1. AE Laboratory, CB, San Juan province. Figure 2. CB’s photograph, Argentina.

1.2 Cerro Blanco

CB is located in San Juan Province by the side of the Provincial Route Nº 12, at 31º30’ South

Latitude and 68º50’ West Longitude (see figure 2). It is 32 km from San Juan City, (Capital of

the province). This region is subjected to stresses product of sub horizontal subduction of

Nazca plate under the South American plate and is a very active seismic area. The rock that

constitutes CB has been considered by different authors, as dacite composition. Its body is a

laccolith composed of dacites and andesites, corresponding to an event of mesolicico volcan-

ism. On floor, the body of the CB is elliptical in shape with its major axis of 3000 meters

long, oriented North-South. The outcrop southern width is 800 m, growing to the North where

it reaches 2200 m high; its depth is estimated at 2.4 km. CB is asymmetric, with its higher

portion at western end, 800 m above the level of the San Juan River, decreasing the height

gently eastwards [17].

2. Scope of the Study

The AE system has two channels, two Deci sensors with 25 kHz, low frequency (LF, channel

1), and a 150 kHz high frequency (HF, channel 2), both resonant. They are positioned on 60

cm glass bars, employed as wave guides and fixed on the rock (see figure 3). After the pre-

amplification stage, the signals are conducted to the AE system. The system has an amplifier

and a signal conditioner for each channel. It is driven by a PC and it was developed and con-

structed at National Atomic Energy Commission. The AE is registered through their RMS

values on each sensor and measuring the signal parameters (A, D, R) only on channel 2. The

RMS values are recorded each 30 seconds (2880 samples every day) on each channel. In this

work the daily AE information is averaged during 24 hours, so only one data is obtained for

each day.

The AE system was installed in November 2006, but not always it could be possible to record

the values. During certain opportunities it was necessary to make adjustments in the AE sys-

tem due to occasional interruptions of electrical supply or difficulties on the PC performance,

and then there is not complete AE information during all time. The period studied begins at

2007 and goes up to January 2012. The AE data are transmitted to Buenos Aires (more than

1200 km) by Internet e-mail. The daily AE RMS values were related with the earthquakes

occurred at San Juan province, considering only those with Magnitude (M) higher than or

equal to 4 (Richter Scale, from the lighter earthquake up to the greater ones). A criterion to

correlate the AE with earthquakes was settled. If at least “one day before” or “during” the

earthquake, the daily RMS AE was higher than the selected analysis threshold (0.2 V), the

earthquake is labelled as “detected with AE”, in another way, it is labelled as “not detected

with AE”. The channel 2 (HF) showed approximately constant values over all the period con-

sidered; so, only the information provided for channel 1 (LF) is here considered. The seismic

information of San Juan Province was provided by National Institute of Seismic Prevention

(INPRES) [18].

Previous papers have presented several studies and analysis of AE values in CB [19- 23]. Sta-

tistical analysis of different parameters of AE signals and their arrival times was analysed

with different probability distributions [21].

Figure 3. Two AE sensors set up on glass bars fixed.

Figure 4. Satellite image of earthquakes epicentres (M ≥ 4) in San Juan Province. The position of CB

is indicated with a green pin, January 2005-January 2012.

3. Results and discussion

The borders of the province, taking into account latitude and longitude, are:

- Northern boundary: 28º23’ South Latitude.

- Southern boundary: 32º37’ South Latitude.

- Eastern boundary: 66º43’ West Longitude.

- Western boundary: 70º35’ West Longitude.

To study some features of the seismicity of San Juan province, the earthquakes inside the pe-

riod: January 2005 to January 2012, was considered. The earthquakes with Magnitude (M)

lower than 4 (Richter Scale, lighter earthquake), are neglected. Related with EA records, only

the period 2007-January 2012 is analysed.

3.1. Seismic Activity at San Juan Province

A satellite image of San Juan province is shown in figure 4. The epicentres of all the earth-

quakes (M ≥ 4) during the time interval 2005-2012 are marked. As can be seen, the zone of

CB has a very high seismic activity. We may say that the geographic position of CB corre-

sponds to the “geometrical centre” of that point’s distribution.

In figure 5, the M values of the earthquakes are represented by their sequence of occurrence

(2005-January 2012). A total of 1858 earthquakes have been considered. The end of each year

is indicated by different coloured arrows. Most of them occurred during 2011 year.

Figure 5. Graph of M vs. earthquakes sequence, San Juan Province, 2005-2012 (all M values).

In the figure 6, the same graph is represented but earthquakes with M ≥ 4 are only considered.

The end of each year is indicated by different coloured arrows. The earthquakes are distribut-

ed evenly in time.

Figure 6. Graphic of earthquakes with M ≥ 4, San Juan Province, 2005-2012.

Figure 7. Earthquakes depth related with their sequence of occurrence (all M values), 2005-2012.

Figure 7 presents a graphic of earthquakes depth (all M values) related with their sequence of

occurrence. It is possible to see that a greater quantity of earthquakes deeper than 70 km occur

than those with lesser depths. Figure 8 shows the earthquakes depth (M ≥ 4) related with their

sequence of occurrence. A very important quantity of highest depth earthquakes can be seen.

A mean value of 3.5 deeper earthquakes occurred before a shallow one.

Figure 8. Earthquakes depth vs. Sequence of occurrence (M ≥ 4), 2005-2012.

3.2. Seismic Activity and AE records

As it was mentioned before, the AE was analysed through the daily average of its RMS value,

on Channel 1 (LF) and Channel 2 (HF), that is to say, AE activity was reduced to one point

each day for each channel. During this long period, some AE records could not be registered

because the difficulties of keeping the equipment in working order so far from Buenos Aires

city.

The AE records considered reliable correspond to the period January 2007-January 2012.

Then, only the mentioned period is analysed here. Data analysis up to September 2011 was

presented in previous papers [19]. The example of figure 9 represents a typical evolution of

the daily AE RMS mean value during a month. The occurred earthquakes are indicated men-

tioning the epicentre Longitude and Latitude, depth and M value (Richter scale). Their corre-

sponding dates are indicted by arrows. The LF and HF values were graphed. The earthquakes

indicated in red rectangles, were classified as “detected with AE” because the AE is higher

than the threshold value.

Another example is considered in figure 10, showing the AE evolution during June 2011. The

AE records during the first 5 days were missed due to the failure in the AE system. At the end

of the month, four earthquakes happened; they are indicated in green colour because they

were classified as “not detected with AE” because the AE is below the threshold level.

Figure 9. Evolution of daily AE RMS mean values during November 2010. The earthquakes (all M

values) are indicated.

Figure 10. Daily RMS evolution that shows lack of AE information in the first days of the month and

the earthquakes (M ≥ 4) relation at the end of June 2011.

The three-dimensional graph in figure 11 shows the Latitude, Longitude and depth of earth-

quakes (all M values) occurred in San Juan from 2007 to 2012, distinguishing those that could

be related with AE data (“detected with AE”) and those that could not be related (“not detect-

ed with AE”). It is possible to see the distribution in two clusters well separated according to

their depths.

Figure 12 indicates earthquakes with M ≥ 4 on a three-dimensional graph. It could be seen

129 earthquakes, 27 of them are shallower ones.

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

RM

S a

ve

rag

e (

V)

Days

November 2010

Channel 1

Channel 2

Lat: 31º32'2'' Long: 68º54'43'' Depth: 117 km. Magn:

Lat: 31º24'39'' Long: 69º13'1'' Depth: 107 km Magn: 4,6

Lat: 32º17'53'' Long: 70º15'50'' Depth: 123 km

Lat: 31º18'47'' Long: 68º35'17'' Prof: 31 km Lat: 31º32'2''

Long: 68º54'43'' Depth 117 km. Magn: 3,7




0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

RM

S a

vera

ge(V

)

Days

June 2011 Channel 1

Channel 2




Lat: 31º19'52'' Long: 68º49'44'' Depth: 35 km Magn: 4

In table I, the occurrence of earthquakes from 2007 through January 2012 and their classifica-

tions as “detected with AE” or “not detected with AE” is summarized. It is worth noting that

during 2011 the amount of earthquakes increased very much.

Figure 11. Earthquakes (all M values) locations (Latitude, Longitude and depth), related with the AE,

2007-2012.

Figure 12. Three-dimensional graph with 129 earthquakes (M ≥ 4). 2007 to January 2012.

Table I. Earthquakes reported by INPRES in San Juan Province and their relation with AE

(2007-2012).

Year 2007 2008 2009 2010 2011 2012

(January) Earthquakes

(all M values) 20 28 24 31 1696 322

Earthquakes

(M ≥ 4) 9 14 14 19 47 14

“Detected with

AE” 4 6 9 22 1236 188

“Not detected with

AE” 10 1 3 3 135 0

Earthquakes (all

M values) with-

out AE records

6 21 10 5 265 134

4. Conclusions

The seismic activity at San Juan province during the period 2005-2012 was analysed. During

the years analysed, the CB area showed a great seismic activity with a considerable growth

during the last year.

There was an important occurrence of earthquakes at depths higher than 70 km. On average,

after 3.5 deeper earthquakes occurs one shallower one.

It was possible to study and to correlate the AE data registered in CB from 2007 through Jan-

uary 2012 with the seismic information.

The earthquakes classified as “detected with AE” were independent of their magnitudes and

depths.

Acknowledgements

To National Agency for Scientific and Technological Promotion, Argentina, ANPCyT, PICT

2007-001769: “Emisión Acústica y Precursors Símicos”.

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