Unusual Patterns of Seismicity during Eruptive and Non-eruptive Periods at the Persistently Restless Telica Volcano, Nicaragua

Telica Volcano, Nicaragua, is a persistently restless volcano with high rates of seismicity that can vary from less than ten events to over a thousand events per day. Low-frequency (LF) events dominate the seismic catalogue and seismicity rates at Telica show little clear correlation with periods of eruption. As such, traditional methods of forecasting of volcanic activity based on increases in seismicity and recognition of LF activity are not applicable. A single seismic station has been operating at Telica since 1993, and in 2010 we installed a broadband seismic and continuous GPS network (TESAND network) at Telica. In this study we investigate the seismic characteristics surrounding a nine-month period of phreatic to phreatomagmatic explosions in 1999, and also from the initial three-and-a-half year deployment of the TESAND network, including a three-month phreatic vulcanian eruptive period in 2011. We demonstrate that pertinent information can be obtained from analysis of single-station data, and while large seismic networks are preferable when possible, we note that for many volcanoes this is not possible. We find unusual patterns of seismicity before both eruptive periods; rather than a precursory increase in seismicity as is observed prior to many volcanic eruptions, we observe a decrease in seismicity many months prior to eruption. We developed a new program for cross-correlation of large seismic data catalogues and analysed multiplet activity surrounding both eruptive periods. We observed that the formation of new multiplets corresponds to periods of high event rates (during inter-eruptive periods) and high percentages of daily events that belong to a multiplet. We propose a model for the seismicity patterns observed at Telica, where changes in seismicity are related to a cyclic transition between open-system degassing and closed-system degassing. Periods of open-system degassing occur during non-eruptive episodes and are characterised by high event rates, a broad range of frequency content of events and high degrees of waveform correlation. A transition to closed-system degassing could be due to sealing of fluid pathways in the magmatic and/or hydrothermal system, or due to magma withdrawal. Periods of closed-system degassing are characterised by low event rates, higher frequency contents and low degrees of waveform correlation. Eruptive periods may then represent a transition from closed-system degassing to open-system degassing, however the system must also be capable of transitioning to open-system degassing without eruption. These observations have important implications for volcano monitoring and eruption forecasting at persistently restless volcanoes. Rather than a precursory increase in seismicity as is often observed prior to eruption at other volcanoes, our observations indicate that phreatic eruptions at Telica occur after a decrease in seismicity, a corresponding change in the frequency content of events, and a decrease in waveform correlation. These changes may represent a period of closed-system degassing that could culminate in phreatic eruptions. The inclusion of real-time analysis of variations in frequency content and multiplet activity provides critical information for volcano monitoring institutions