Central American Seismic Center (CASC)

Central American Seismic Center (CASC)
Central American Seismic Center (CASC)
E. Alvarenga Centro de Investigaciones Geot~cnicas, San Salvador, El Salvador
R. earquero, I. aoschini Instituto Costarricenses de Electricidad, San Jose, Costa Rica
J. Escobar Departamento de F|sica, Universidad Nacional Autenoma de Honduras, Tegucigalpa, Honduras
J. Fernandez Escuela Centroamericana de Geologia, Universidad de Costa Rica, San Jos~, Costa Rica
P. Jayol Instituto Nacional de Sismologfa, Vulcanologia, Meteorologia e Hidrologfa, Guatemala
J. Havskov Institute of Solid Earth Physics, University of Bergen, Norway
N. G~lvez Centro de Investigaciones Geotecnicas, San Salvador, El Salvador
Z. Hern~ndez Instituto Nacional de Estudios Territoriales, Managua, Nicaragua
L. Ottem~Jller Institute of Solid Earth Physics, University of Bergen, Norway
J. Pacheco Instituto de Geofisica, Universidad Autenoma de M~xico, Mexico
C. Redondo, W. Rojas Escuela Centroamericana de Geologfa, Universidad de Costa Rica, San Jose, Costa Rica
F. Vega Observatorio Vulcanolegico y Sismolegico, Universidad Nacional, Heredia, Costa Rica
E. Talavera Instituto Nacional de Estudios Territoriales, Managua, Nicaragua
W. Taylor Instituto Costarricenses de Electricidad, San Jos~, Costa Rica
k. Tapia Instituto de Geociencias, Universidad de Panama, Panam~i
C. Tenorio Departamento de Fisica, Universidad Nacional Autenoma de Honduras, Tegucigalpa, Honduras
J. Toral Instituto de Geociencias, Universidad de Panama, Panam~
INTRODUCTION
O n May 1, 1998 the Central American Seismic Center
(CASC) was opened in Costa Rica. The center has the purpose of permanently storing the most important data from
seismic stations in Central America, as well as collecting data
from Central American stations in semirealtime in order to
issue daily bulletins. All data in the database will be available
for access through the Internet.
BACKGROUND
In 1988 a cooperative project in disaster prevention between
all Central American countries was started and organized
through C E P R E D E N A C (Centro de Prevenci6n de Desastres Naturales en America Central). One of the most important components in the program was seismology, and for all
six countries reinstallation and/or improvement o f existing
analog field stations and central recording equipment was
initiated. Data acquisition systems were installed in all the
Central American country networks, and by the end of 1992
95 digital stations were in operation. Today the number is
138. At the same time a joint Central American effort was
made to collect, process, and distribute the seismic data for
Central America. The initial idea was that the data center
would move between the countries, which then would be
responsible for the processing. This was done for 1992 and
1993, but it soon became clear that a permanent data center
was needed. It is estimated that a substantial part of the data
recorded since 1992 has been lost (see below). In 1996 the
installation of broadband stations was started in the countries, and it was decided by C E P R E D E N A C that a permanent seismic center should be created in Costa Rica. This
paper gives an overview o f the current situation o f instrumentation in Central America, status o f the database, and the
daily operation of the center.
394 SeismologicalResearchLetters Volume69, Number5 September/October1998
90~
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9
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9 Figure 1. Seismic stations digitally recording in Central America, May 1, 1998. Triangles represent short-period stations and circles broadband stations. A total of 138 stations is shown. Many of the networks are so small in extensionthat the individual stations cannot be distinguished9
SEISMIC NETWORKS IN CENTRALAMERICA
Seismic networks in Central American countries consist
mostly o f analog transmitting stations varying between three
in the case o f Honduras and 67 in the case of Costa Rica
(Figure 1). In addition there are eight permanent broadband
or extended short-period stations. The data are sent from the
field stations by radio using conventional FM modulation,
or in a few cases (Panamfi and Costa Rica) by telephone. At
the local recording centers, the signals are passed on to the
SEISLOG data acquisition systems (Utheim and Havskov,
1997). Some o f the channels are at the same time recorded
on paper. The processing is done with the SEISAN
(Havskov, 1997) on Sun workstations. At seven of the central recording sites, a three-component accelerometer, Kinemetrics FBA23, is connected directly to the SEISLOG
system and recorded at two gain levels, thus covering a
dynamic range o f 105 db.
Each country has at least one permanent broadband or
near-broadband station. Panamfi and Costa Rica have IRIS
stations, while the remaining countries have SEISLOG sys-
tems with 24-bit digitizers and various types o f sensors ranging from LP sensors to Guralp 40T.
Currently 230 channels of digital data are recorded in
Central America in a uniform format (SEISAN) from 138
seismic stations (Figure 1).
DATACENTER OPERATION
The data center has two functions: (1) automatically collect
data for large events (M > 4) from all regional stations with
m o d e m or Internet connection and determine a preliminary
location and magnitude in near realtime, and (2) be a permanent archive for seismic data in Central America.
The near-reahime system is based on the network datacollection system SEISNET (Ottem~511er, 1998). This system
connects to the IRIS or SEISLOG systems in the region
using Internet or modem. At regular time intervals, detection parameters are downloaded to the central computer and
events are declared based on the number o f networks having
detections in a given time window. T h e SEISLOG systems
will give P- and S-trigger times for each channel as well as
duration of the event. Based on this information, a prelimi-
Seismological Research Letters Volume69, Number5
September/October1998
395
nary location and magnitude is calculated. After the event
has been declared, waveform data for the event are automatically downloaded for selected stations and networks. The
intention is that the event detection process shall run continuously so that automatic preliminary epicenters and magnitudes will be available within 15-20 minutes. T h e limitation
in location speed is the Internet capacity, which also limits
the amount of waveform data transferred. At the moment
continuous operation cannot be done because o f the costs
involved in using the modem stations. It is planned that at
least one center in each country will be Internet-connected
within one to two years.
The operator at the center will, on a daily basis, check all
network detections and manually make preliminary regional
locations for events larger than magnitude 4.0. It is estimated that about two to three events will be declared daily,
and these events will be used to make a regional catalog of
earthquakes. Since the center cannot permanently collect
and store waveform data for all events in the region due to
storage limitations, only those center-declared events will be
reprocessed and waveform data collected. The preliminary
locations and magnitudes for these events will be sent daily
to all interested parties.
W h e n individual centers have finished processing their
data, they will send the parameter data to the regional center
together with waveform data for the regional events. The
data center will store all readings and magnitudes made
available by the individual networks in separate network
databases without doing any reprocessing. The regional database will be made using all phase data from the region, which
will be merged and checked with the help of the waveform
files. A monthly bulletin for the complete regional catalog
will be issued. Thus, the aim is that the center will have
nearly all data available from the whole region for larger
events and most of the phase data for the local networks.
Additionally, Nicaragua is currently installing several
tide gauges which transmit signals to Managua in realtime,
and has a program to issue tsunami warnings. T h e intention
is that the near-realtime locations from CASC will be sent
directly to Nicaragua in order to help in determining the tsunami potential o f an event.
DATABASESTRUCTURE AND FORMATS
The data are stored in the SEISAN database system. This is
a simple file-based hierarchical system where the parameters
are stored in ASCII and waveforms are stored in binary formats. Each database for local networks and the regional catalog consists o f yearly and monthly directories in which there
is one ASCII file with parameter data (hypocenters, magnitudes, phase readings, etc.) for each event. The file also gives
the names o f the binary waveform files available either in the
regional center or at the local centers. In addition, the
parameters are also stored in monthly files for easy access to
larger data sets. It is thus very easy to extract one event, one
month o f events, or a whole database o f events. T h e wave-
form files are also stored in a hierarchical system divided into
years and months.
Outside users can get access to the data using anonymous FTP to the address 163.178.105.34 in Costa Rica.
When logging in, the user will get information about the file
structure. If logging in using an Internet browser, the ASCII
files can be displayed and the user can find which waveform
files are associated with a given event. Response information
and station coordinates are also available. For more details on
the database structure, see the SEISAN manual. The SEISAN software and manual are available on ftp.ifjf.uib.n0 or
on ISC C D - R O M , Volume 5. The near-realtime locations
and recent waveform data are also available on the above
FTP address in Costa Rica.
STATUS OF DATABASE
The authors o f this communication held a workshop at the
data center from April 29 to May 8, 1998. Participants
brought available data, and a skeleton o f the database, covering mainly the years 1992 to 1997, was set up (Table 1).
However, the database has data back to 1505 (Rojas et al.,
1993). Although much data were missing, the database now
contains parameter data for more than 120,000 events and
about 10,000 waveform files, o f which only 125 are from
broadband stations.
Data for the regional catalog were preliminarily processed in order to update the regional catalog for 1994-1997.
In this first update, event selection was based on events with
magnitude larger than 4.5 reported by PDE. Figure 2 shows
epicenters of events in the regional database of coda-wave or
body-wave magnitude larger than or equal to 4.5. The aim is
to complete the database for all parameter and waveform
data back to 1992. However, it seems that a substantial
amount of data, which were stored on tape, has been lost. It
is also clear from Figure 2 that the location accuracy is poor,
so a complete revision has to be done. Currently, the events
in the regional database were located using on average two
networks. This number will probably increase to three networks once all currently available data have been included. In
general, about 15% of all events in the region are recorded by
two or more networks. Thus, most events are recorded by
only one network, which is also the reason to keep most of
the parameter data in separate network databases.
It turned out during the workshop that many networks
had no parameter data for large regional events (M > 5). It is
currently not known whether the data have been considered
too distant for some networks to process, the local systems
have not triggered, or the data are lost. However, it is hoped
that the declaration of regional events by the data center will
prevent this from happening in the future. One o f the main
purposes of sending the daily mail o f declared events is to
ensure that data for important events are collected. Most
countries have several days o f data in the ring buffers on the
data-acquisition systems, so it should be possible to retrieve
most of the data.
396 SeismologicalResearchLetters Volume69,Number5 September/October1998
TABLE 1
Content and statistics of CASC database as of May 1, 1998. In addition to the earthquake databases shown below, there is
also information about station coordinates and calibration curves, as well as data from recent locations.
Database code and operator
Time period
# events
# waveforms
Network databases
1984-1997
23968
760
GUA: Guatemala National Seismic Institute
1983-1997
43157
1787
SAL: Geotechnical ResearchCenter,El Salvador
1992-1997
2476
1838
HON: National University of Honduras
1976-1997
21148
1805
NIC: Nicaraguan Institute of Geophysics
1992-1997
19022
1702
RSN: University of Costa Rica
1984-1997
7309
366
UNA: National University of Costa Rica
1994-1997
2372
474
ICE: National electric company of Costa Rica
1992-1997
4866
1307
UPA: University of Panamd
Other databases
1992-1997
1217
879
CAM: Regional database made by CASC
1505-1992
27016
-HIS: Historical seismicity
1934-1997
723
-FOC: Fault plane solutions
1967-1994
107
116
ACC: Accelerometer data
in
the
time
period,
so
in
most
cases,zero
to
1997.
Data
is
available
for
all
agencies
Number of events in the network databasesfor the years 1992
events means that the data has not arrived yet.
1992
454
1029
5349
1596
0
97
83
0
936
CAM
GUA
SAL
HON
NIC
RSN
UNA
ICE
UPA
1993
345
1669
6739
686
1916
3288
2919
0
1342
1994
110
904
6237
0
1359
2027
684
253
1019
1995
119
530
6500
0
1124
3490
0
317
607
1996
89
848
3845
0
2500
2975
0
353
517
CASC will contain all data submitted by the participating countries. However, only data in the area 40-20 ~ N and
940-76 ~ W will be reprocessed for the regional database.
CURRENT PROCESSING PARAMETERS
1997
100
1019
233
194
0
3825
737
1449
445
The model is partly based on experience from the individual
countries and partly on testing using the complete data set.
Generally, coda-wave magnitudes are used, based on a preliminary Central American coda-wave magnitude scale
developed by calibrating the coda lengths with PDE-ma
magnitudes (Marroquin and Ariola, 1992; Vega, 1993):
The regional crustal model used for location is:
M c = -0.72 + 2.4 log (coda) + 0.00046 * dist
P velocity (km/sec)
3.5
5.0
6.0
6.8
8.0
8.3
8.5
Depth to interface (km)
0.0
1.0
6.0
13.0
35.0
200.0
300.0
where coda is the coda length in seconds and dist is the hypocentral distance in km.
Comparing the coda magnitudes to the M c magnitudes
for the years 1994-1997 for the regional database (364
events with m b and Me), the average rnb and M~ are 4.75 and
4.95, respectively. Thus, the M c scale gives reasonably accurate preliminary estimates of magnitude, although slightly
overestimating the magnitudes.
Seismological ResearchLetters Volume69, Number5
September/October1998 397
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9 Figure 2. Regional earthquakes in the CASC database with coda or m b magnitude > 4.5 in the time period 1992-1997.
Richter local magnitudes M E are also calculated using
the modified Richter attenuation relation given by H u t t o n
and Boore (1987). The scale seems to give reasonable values
(Vega, 1993) using the regional calibration information
(Escobar et al., 1993). However, the practice of using only
coda magnitude is dominant, since only 404 single network
locations out of 44,000 in the period 1 9 9 4 - 1 9 9 7 reported
M E. The regional center will calculate coda magnitude, ML,
and moment magnitudes based on spectral analysis for the
regional events.
Clearly, current regional magnitudes scales and crustal
structure are preliminary. However, it is reasonable to expect
that improvements will be made once the large regional database is available.
DISCUSSION
Station quality and spacing are very uneven (Figure 1). This
means that in some cases relatively important events are only
recorded in one country. In terms o f rapid and accurate location, the local networks will often provide the fastest and
most accurate solution, particularly if the event is within the
local networks. However, the many subduction zone events,
398
Seismological Research Letters
Volume 69, Number5
as well as events in the Caribbean, will often be badly located
by local networks since the events are too far outside the networks or the networks are too small. A regional solution
could be better, and if available within a short time could
also be important in case of large earthquakes. However, it is
also clear that the current regional database must be completely revised, since there are too many bad locations. The
broadband data have been little used in the region, and only
few data are available at CASC. There is obviously room for
large improvement both in terms o f data collection and processing of broadband data.
In Central America and the Caribbean area there is currently a network of broadband stations cooperating in the
MIDAS (Middle America Seismographic Network) project
in which the Panamanian and Costa Rican IRIS stations participate. MIDAS is planning to set up a data center in Puerto
Rico at the end of 1998. MIDAS and CASC cover overlapping areas, and there are therefore intentions to cooperate
closely in data collection and processing (D. Novello, personal communication).
Despite the unevenness o f the Central American Network and the incompleteness o f the CASC database, the creation of the Central American Seismic Center represents a
September/October1998
very significant improvement in practical seismology for
Central America, particularly by collecting the data in one
place in a standard format. It is obvious that CASC has a
good potential for uniformly defining the regional seismicity
in Central America and for providing a good basis for future
research in seismology for the region. El
ACKNOWLEDGEMENTS
C E P R E D E N A C is supported by the Swedish and Norwegian governments through development grants given by the
Swedish Development Agency (SIDA) and the Norwegian
Agency for Development (NORAD). We also thank governments in all Central American countries for supporting
cooperation through CEPREDENAC.
CASC's address:
att. Carlos Redondo
Central American Seismic Center
School o f Geology
University o f Costa Rica
Costa Rica
Phone 506 253 8407
Fax 506 253 2586
email: [email protected]
[email protected]
REFERENCES
Escobar, J., B.M. Storheim, C. Aranda, and J. Havskov (1993). Calibration of Central American seismic stations. Report #11 under
the project Reduction of Natural Disasters in Central America,
Institute of Solid Earth Physics, University of Bergen, Norway,
52 pp.
Havskov, J. (1997). The SEISAN earthquake analysis software for the
IBM PC and Sun Version 6.0 manual, Institute of Solid Earth
Physics, University of Bergen, 236 pp.
Hutton, L.K. and D.M. Boore (1987). The M Lscale in Southern California, Bull Seism. Soc. Am. 77, 2074-2094.
Marroquin, G. and L.A. Arriola (1992). Estudio de magnitud coda
para la regi6n centromearicana. Report under the project Reduction of Natural Disasters in Central America, Institute of Solid
Earth Physics, University of Bergen, Norway, 14 pp.
Ottem611er, L (1998). SEISNET user manual, version 1.1, Institute of
Solid Earth Physics, University of Bergen, 30 pp.
Rojas, W., H. Bungum, and C. Lindholm (1993). A catalog of historical and recent earthquakes in Central America. Report under the
project Reduction of Natural Disasters in CentralAmerica, NORSAR, Norway, 76 pp.
Utheim, T. and J. Havskov (1997). The SEISLOG data acquisition system version 7.0 user manual, Institute of Solid Earth Physics,
University of Bergen, 101 p.
Vega, E (1993). Large earthquakes (mb > 5.0) recorded by the Central
American Seismic Network in the period January 1992 to April
1993. Report #9 under the project Reduction of Natural Disasters
in CentralAmerica, Institute of Solid Earth Physics, University of
Bergen, Norway, 192 pp.
Institute o f Solid Earth Physics
University of Bergen
Bergen, Norway
(].H.)
Seismological ResearchLetters Volume69,Number5 September/October1998 399
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