Arenal and Miravalles Seismological Network

Arenal-Miravalles Seismological and Volcanological Observatory (OSIVAM)

For more than 20 years, the Costa Rican Institute of Electricity (ICE) has continued to develop seismological research in Costa Rica's hydroelectric and geothermal zones. The goals of the research are to improve both civil work design and post-construction monitoring. In the Arenal/Miravalles region, ICE has developed the most important hydroelectric projects in the country, which generate more than 40% of the country’s electric power. Due to the short distance between the volcano, Sangregado Dam (Lake Arenal Dam), and Arenal Lake, it is very important for ICE to maintain permanent monitoring of the volcano.

The Arenal-Miravalles monitoring network has twelve digital seismological monitoring stations. In operation since 1994, it facilitates the permanent monitoring of the Arenal-Corobicí-Sandillal Hydroelectric Complex and the Miravalles Geothermal Field. The center for recording and processing of the network's data is located at the building of the Arenal Miravalles Seismological and Volcanological Observatory (OSIVAM) in Quebrada Grande, near Tilarán, Guanacaste.

ARENAL SEISMIC NETWORK

Around Arenal, there are four seismic stations (FOR, CAS, MAC and REY,) and two more stations (BAL and CHI) that are located more than 8 kilometers away. There are three large motion equipment machines at the Sangregado Dam (6.5 km west of the volcano, Fig. 2), while the Miravalles Geothermal Field contains six seismic stations and one accelerograph. Signals from these stations are telemetered to the main Observatory facilities (OSIVAM) in Chiripa. However, network operation is also controlled from the ICE-Seismological Laboratory in San José. Stations are equipped with a GPS (Global Positioning System) time signal receiver, solar panels, and buffer batteries
Since 1975, a network of analogic, seismographic (helicorders) stations have been operating around the Arenal Volcano. However, only one analogic station (named Fortuna: FOR) is still under operation. It is located 3.7 km east of the summit crater, and the FOR digital station sits beside it. Additionally, there are six portable seismographic stations available (MQ-800).

MIRAVALLES NETWORK AND GEOTHERMAL PLANT

The Miravalles Geothermal Plant was inaugurated on March 25, 1994, with a first unit generation capacity of 55 MW. The geothermal field is located in a caldera structure where several fault systems have been determined. Through several different periods (1978, 1983-84, 1986 and 1991-present,) the Miravalles area has been monitored with portable seismic equipment. Results of this work have increased our knowledge of the zone’s main seismic characteristics. It has shown relatively low levels of activity (10 to 15 microearthquakes per month) and several seismic alignments, which are probably related to active local faults.

Seismic studies are carried out in many different parts of the world where geothermal resources are utilized, and they have shown certain relationships between an increment of seismic activity, exploitation, and the re-injection of fluids. Therefore, the ICE considers implementation of a modern seismic network necessary for monitoring the geothermal field. The main objectives of the network are:

1. Permanent and continuous real-time monitoring of the geothermal field.

2. Early detection of any anomaly that could be induced by the geothermal exploitation.

3. Determination of seismic parameters of local earthquakes and its classification.

4. Further seismological scientific investigations related to Geothermics.

CONCLUSION

Costa Rica's modern seismological network has replaced old, analogic systems with new digital technology for seismology investigation. The technical instrumentation helps to improve understanding of Costa Rica's natural phenomena, such as earthquakes and volcanic activity.

Sources: http://geohazards.cr.usgs.gov and http://geoscope.ipgp.jussieu.fr