catalogo de terremotos para america del sur catalog of earthquakes ...

128
R. Repeated area corresponding to some other event of the catalog. The
events of categories F and R are not included in the catalog.
[n general the epicentral determinations of earthquakes of category A
and some of category B are quite precise, better than the instrumental determinations
with data of the international net of seismographs (NEIS and ISC).
The epicentral data extracted from the international bulletines were
mostly abandoned, principally those with great imprecision in the determination
of the parameters and others because they correspond to PmKP phases of
large magni tude events which have occurred in other regions of the Earth
(Assumpcao, 19S3 a). Other epicenters were relocated principally for those
events recorded by many stations by the international seismic net.
T'iere sti l1 exist some earthquakes with epicenters located very distant
from tne areas with larger member of seismographs; in these cases, they
have been onlv dlefined by the macroseismic reports. Some nets of local or
regional chna rcter, present problems concerning the time control, lowering
the precision of the determination when it is necessary to use that informa­
:ion. These situations are taken into account in order to define the error
which is introduced in the dete:mination of the epicenters.
METIOI)S ISED IN THE PREPARATION OF THE CATALOGUE
Hypocentral IParmneters
The earthquakes with macroseismic data, have their times of origin defined
with a precision of minutes, obtained from the existing descriptions
for each event. In the case of events with instrumental data the precision
is in the order of a few seconds. with better precision when the epicenter
is within a seismological net. In this case the HYPO 71 (Lee and Lahr, 1975)
is used with tke preliminar'y crustal models existing for Brazil (Dias et al,
1980) or the model of lerrin (1968) which gives better results than the model
of Jeffrey,'s & Bullen (1940).
The coordinates of the epicenter for the earthquakes with macroseismic
data are obt:ained from the map of isoseismal curves or the affected areas
with a precision in the order of tens of kilometers of less for those events
for which there is abundant data. Thme events with instrumental data have
their coordinates automatically determined by means of the IIYPO 71 program,
as has already been mentioned. In this case there is also a value for the
focal depth. In the case of the events with macroseismic data this parameter
is not calculated. For some earthquakes, outside of Brazil, such as that
occurred on 5 August 1983 in Codajfs-Amazonas (Assumpcao et al, 1983), the
r'idin.s of the pP phase are used to calculate the depth of the focus.
For the earthquakes with macroseismic data, in densely populated zones,
the maximum intensity observed corresponds to the maximum epicentral intensity.
When the data is scanty, an extrapolation is carried out when necessary.
The magnitude for the events with data that allow the drawing of a map
with isoseismal curves or of the affected area, is calculated by the following
equation: