Local Magnitude Study of the Seismic Activity on Itacarambi, MG

doi:10.4236/ijg.2012.31025

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International Journal of Geosciences, 2012, 3, 222-225

http://dx.doi.org/10.4236/ijg.2012.31025 Published Online February 2012 (http://www.SciRP.org/journal/ijg)

Local Magnitude Study of the Seismic Activity on

Itacarambi, MG

Ítalo Lopes de Oliveira, George Sand França, Cristiano Naibert Chimpliganond

Seismological Observatory, University of Brasília, Brasília, Brazil

Email: [email protected]

Received November 5, 2011; revised December 14, 2011; accepted January 6, 2012

ABSTRACT

The importance of studying the local magnitude related to seismic activity occurred recently in the region of Ita-

carambi, state of Minas Gerais, is due to the fact that these were earthquakes of intraplate origin. From the study of

[1] and the relation between local magnitude and seismic signal duration, was performed a data analysis obtained in

the same region, on the period between October/2007 and June/2008, in which we can estimate the equation

MD = 2.153(±0.072) LogD – 1.925(±0.132) to calculate the magnitude of local duration. We can also estimate one value

for the b parameter using the equation LogN = a – bMD from a frequency-magnitude study. It was found the value of b

= 0.826(±0.020) for the general activity of Itacarambi, MG, that is within the universal range proposed by [2].

Keywords: Intraplate Earthquakes; Local Magnitude; Duration of Seismic Signal; Parameter b

1. Introduction

The city of Itacarambi-MG, especially the district of Car-

aíbas, has been affected by a continuous occurrence of

earthquakes in recent years, some of considerable pro-

portions even in the case of intra-plate events. In this

study we hope to determine a formula to calculate the

local magnitude of the region, but what comes to be

magnitude and what its significance?

The magnitude is correlated with the amount of energy

released by the earthquake in the source, and calculating

it gives us an absolute value which helps in comparing

the relative size of earthquakes, in other words, give us a

better estimate of how destructive may be an earthquake.

Reference [1] proposed an equation to calculate local

magnitude of events occurred up to 100 km of distance

from the recorder. Based on a logarithmic scale and

seismic signals amplitude, according to Richter local

magnitude is given by:

ogA LogAM =L (1)

A represents the maximum amplitude of the signal in

μm and A0 a pre-established value. The [1] study, al-

though known and used worldwide, was carried out with

earthquakes from the west coast of North America,

however it is common ground that both the area and re-

corder are completely different as it portrays the formula

described by Richter for our region. Thus we sought to

obtain a similar equation obtained by Richter, but using

the local parameters and registers today.

The Itacarambi-MG region is located at the north of

Minas Gerais state, near São Francisco river, about 660

km from the metropolis, Belo Horizonte (Figure 1). In

October 2007 was installed a network with 10 seismo-

meters of short period in the region (Figure 2), to moni-

tor recent earthquakes occurred in this region [3]. This

work will be done with data collected through this network.

Initially, studies will be conducted in the region to de-

termine the magnitude of events occurred between Oc-

tober/2007 and June/2008 from the Equation (1). And in

a second stage of the study, a relationship between local

magnitude and duration of the signal will be estimated, in

order to improve the calculation of small tremors.

Figure 1. Location of Itacarambi, MG region. Square rep-

resents cities and districts, star represents the key event

occurred in the region, of magnitude equal to 4.9 mb.

Í. L. DE OLIVEIRA ET AL. 223

Figure 2. Arrangement of local seismographic stations. The

rectangles represent the stations by which it was not possi-

ble to measure the duration of a given seismic signal.

2. Methods

We have many examples of local and regional magnitude

in intra-plate areas [4-7].

The analyzed seismic data were collected in digital

form by the Seismological Observatory at the University

of Brasília. The SAC software was used for the reading

and processing of these digital seismic records [8].

A total of 451 seismic events were analyzed with the

objective of obtaining the values related to the seismic

signal duration and maximum length of the P-wave in the

vertical component record’s. The duration of the sig-

nal-D was obtained directly out of the original record. An

arbitrary time of ten seconds was chosen, beginning from

P-wave’s first arrival (Figure 3). Three out of the 10

available stations were unable to get the data about the

duration (Figure 2). Either a malfunctioning of the sta-

tion instruments or the noise signal might have been the

cause. In order to get the maximum amplitude, caused by

a gain in the efficiency of the instrument’s response, it

was necessary canceling the effects to get a response

equal to Wood-Anderson Seismometer (a seismometer

used by Richter to obtain the Equation (1)). Only then

was it possible to calculate ML. Such effects were can-

celled primarily through a Transference Function, which

converts data from the velocity × time’s original instru-

ment into Wood-Anderson instrument data, in which we

have some data presented in terms of displacement ×

time.

After the data’s conversion and obtaining seismic sig-

nal’s maximum amplitude on the arrival of the first

P-wave, we used the Equation (1) to calculate the local

Figure 3. Seismic event recorded by the Jan09 station, 0802

08_1529_jan09. T0 represents the beginning of a seismic

signal (P-wave) and T5 represents obtained signal’s dura-

tion.

magnitudes. 140 events, of seven different stations, were

selected out of a 451 seismic occurrences. In the end,

three magnitude ML > 3 were discarded so a more accu-

rate formula could be obtained for the calculation of the

local magnitude, such calculation based on the maximum

amplitude of the P-wave and on the seismic signal as

well, since these signals are usually pretty much satu-

rated due to the stations’ being too close to one another.

Furthermore, the station close to center of the array

wasn’t used in calculations and considering that the epi-

central distance is on the order of 5 km, thus we don’t

why to incorporate distance dependence in our duration

magnitude formula and derive duration magnitude using

the following equation:

LD12

MMccLogD (2)





3. Results

On acquiring data for the maximum amplitude and the

duration of the seismic signal, we performed an analysis

to each station correspondent data. These generated ML ×

LogD graphs for all stations used, as shown in Figures 4

and 5 for stations Jan03 and Jan06 respectively.

Making some calculations to get all the linear regres-

sion equations averages, we were able to determine a

general equation which allows us to make an estimate of

the region’s local magnitude and any possible errors as-

sociated to it:





M2.1530.072 LogD1.9250.132  (3)

From the Equation (3), we calculated the magnitude

for all 137 events selected and by performing an analysis

with the aid of the graphics, the linear regression equa-

tions for each station, along with the errors related to

them (the stations), we noticed that the stations present

some variation in the record of both the maximum

Í. L. DE OLIVEIRA ET AL.

224

Figure 4. Graph ML × LogD at station Jan03.

Figure 5. Graph ML × LogD at station Jan06.

amplitude and the duration of the signal for a same-nat-

ured event. An magnitude analysis makes this fact pretty

obvious, this discrepancy in the records also produces

some variation in the magnitude of an event recorded by

several stations. Some examples in the Table 1 below

represent such variations, and allow us to sort out the

stations used for our study into three groups which share

similar behaviors: Jan03 and Jan05; Jan02 and Jan07;

Jan06 and Jan09.

These different behaviors could be explained by some

factors such as the distance between the group of stations

and the event’s epicenter. This situation applies very well

for the contrast among the first group and the others.

Moreover, another factor that may justify that difference

of behavior, mainly between the last two groups, is the

geological setting. Stations Jan02 and Jan07 are localized

nearby a carbonate geology river while stations Jan06

and Jan09 are in a more complex geology region. Never-

theless, the station Jan01 doesn’t fit into either groups.

4. Discussion

Table 1. Comparison of the magnitudes obtained from dif-

With the analysis of each station result, we arrived at a

(s) LogDMD

ferent stations for the same seismic event.

Date_Hour_StationAmp. P (m)ML Dur.

041107_1529_jan011.64E–051.216 50.746 1.7051.747

041107_1529_jan031.02E–051.007 25.966 1.4141.120

041107_1529_jan051.14E–051.059 33.837 1.5291.368

041107_1529_jan065.32E–051.726 61.203 1.7871.922

080208_1529_jan021.38E–051.139 30.993 1.4911.286

080208_1529_jan064.37E–051.640 51.077 1.7081.753

080208_1529_jan074.66E–051.668 25.758 1.4111.113

080208_1529_jan091.22E–042.088 48.924 1.6901.713

Amp. P: P-wave amplituhte m; Dr

eneral equation that enables us to make some estimate

de; ML: Ricr’s localagnitudeur.: Duation of

seismic signal; LogD: Logarithm of duration; MD: Magnitude of local dura-

tion.

as to the local magnitude, based on the duration of each

seismic signal for the earthquakes in the Itaracambi-MG

region.





M2.1530.072 LogD1.9250.132  (3)

D is the signal’s duration in seconds. It’s worth

tio

[7], we can draw a

men-

ning that all the seismometers and registers used were

of the DM24 type (GURALP brand).

Having as a reference the works of

mparison between the Richter’s ML magnitude and the

MD magnitude obtained by Equation (3). In Figure 6

these magnitude’s data have been plotted and indicate

that the adjustments made are sufficiently reliable, the

standard deviation from MD to ML for the seismic activi-

ties of our choice being equal to:







i137

0.341σ

(4)

From the magnitudes estimated by the Equation (3)

can estimate, on the basis of a frequency-

 

LogNa bM

LogN0.8260.020 M2.9880.034

Σi MMσ

d from the date on which the events took place, we can

come up with a magnitude × time histogram which can

be used to make an estimate of the region’s seismic oc-

currences.

Also, we

agnitude study, a value for the parameter b using the

Equation (5) according to [9], establishing a correlation

between the number of earthquakes (N) and the duration

of their magnitude (Figure 7). The result obtained for the

parameter b ended up being within the universal range

[2].





 (5)

As next steps of this study, we will estimate the mag-

nitude through the frequency domain [10].

Í. L. DE OLIVEIRA ET AL.

225

Figure 6. Correlation between the ML magnitude and the

MD magnitude.

Figure 7. Logarithm of the number of events (N) × magni-

5. Acknowledgements

ish to thank Ibama (Parque

and the

G.S.F. wishes to thank CNPq-3003529/2010-5.

gical Society of America,

ophysical Research,

. 641-644.

tude (M). Dashed line re presents the Equation (3), b = 0.826

(±0.020).

The authors of this study w

das Cavernas Peruaçu), Mr. Evandro and the city-hall of

Itacarambi-MG for their support in the course of the field

work, Ruan R. Alves and Cesar G. Pavão for several

suggestions that improvement the manuscript. I.L.O.

wishes to thank CNPq for the funding via PIBIC

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