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:

D

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

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