1 Shallow seismicity, triggered seismicity, and 2 ambient noise tomography at the long- 3 dormant Uturuncu volcano, Bolivia 4 Jennifer A. Jay1, Matthew E. Pritchard1, Michael E. West2, Douglas Christensen2, 5 Matthew Haney3, Estela Minaya4, Mayel Sunagua5, Stephen R. McNutt2, Mario 6 Zabala4 7 1. 3162 Snee Hall, Department of Earth and Atmospheric Science, Cornell 8 University, Ithaca, NY 14853 9 2. Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 10 3. Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, AK 11 4. Observatorio San Calixto, La Paz, Bolivia 12 5. SERGEOTECMIN, La Paz, Bolivia 13 Phone (714) 403-3480 14 Fax (607) 254-4780 15 Email [email protected] 16 17 Abstract 18 Using a network of 15 seismometers around the inflating Uturuncu volcano from April 19 2009 to 2010, we find an average rate of about 3 local volcano-tectonic earthquakes per day, and 20 swarms of 5-60 events a few times per month with local magnitudes ranging from -1.2 to 3.7. The 21 earthquake depths are near sea level, more than 10 km above the geodetically inferred inflation 22 source and the Altiplano Puna Magma Body. The Mw 8.8 Maule earthquake on 27 February 2010 23 triggered hundreds of earthquakes at Uturuncu with the onset of the Love and Rayleigh waves and 24 again with the passage of the X2/X3 overtone phases of Rayleigh waves. This is one of the first 25 incidences in which triggering has been observed from multiple surface wave trains. The 26 earthquakes are oriented NW-SE similar to the regional faults and lineaments. The b-value of the 27 catalog is 0.49, consistent with a tectonic origin of the earthquakes. We perform ambient noise 28 tomography using Love wave cross-correlations to image a low velocity zone at 1.9 to 3.9 km 29 depth below the surface centered slightly north of the summit. The low velocities are perhaps 30 related to the hydrothermal system and the low velocity zone is spatially correlated with 31 earthquake locations. The earthquake rate appears to vary with time –a seismic deployment from 32 1996-1997 reveals 1-5 earthquakes per day, whereas 60 events/day were seen during 5-days using 33 1 seismometer in 2003. However, differences in analysis methods and magnitudes of 34 completeness do not allow direct comparison of these seismicity rates. The rate of seismic activity 1 35 at Uturuncu is higher than at other well-monitored inflating volcanoes during periods of repose. 36 The frequent swarms and triggered earthquakes suggest the hydrothermal system is metastable. 37 Keywords: seismology, triggered earthquakes, earthquake swarms, South America 38 Introduction 39 Uturuncu is a 6008 m high Pleistocene dacite stratovolcano located in the 40 Altiplano-Puna volcanic complex (APVC) of the central Andes in SW Bolivia 41 (22o16' S, 67o11' W), southeast of the town of Quetena Chico. 39Ar/40Ar ages of 42 the dacite lavas have shown that the volcano was active between 890 ka and 271 43 ka, with the lavas becoming younger and less extensive at higher elevations 44 (Sparks et al. 2008). Although there are no recent eruptions, Uturuncu has been 45 showing signs of unrest in recent years through surface deformation and fumarolic 46 activity. InSAR studies have discovered a region of uplift that covers an area of 47 about 70 km in diameter with a central uplift rate of approximately 1 to 2 cm/yr 48 between 1992 and 2006 (Pritchard and Simons 2004; Sparks et al. 2008). The 49 source of this deformation is at least 17 km deep below local relief, indicating a 50 magmatic rather than hydrothermal cause, and suggesting that the deformation is a 51 result of the intrusion of new magma into the Altiplano-Puna Magmatic Body 52 (APMB) (Sparks et al. 2008). Also, two active sulfur-producing fumarole fields 53 have been documented near the summit (e.g. Kussmaul et al. 1977; Sparks et al. 54 2008). In May 2003 and May 2009, the temperature of these fumaroles was 55 measured to be slightly below 80o C (Sunagua 2004; Sparks et al. 2008; J. 56 Gottsmann, pers. comm.), which is nearly the boiling point of water at the 57 elevation of Uturuncu. 58 These signs of activity have sparked interest in further studies of Uturuncu 59 to investigate the depth and location of potential zones of partial melt and whether 60 that melt is moving. This paper will focus on using local earthquake locations, 61 rates, and magnitudes, the volcano’s response to large regional and global 62 earthquakes, and ambient noise tomography to probe the subsurface magma 63 plumbing system. 64 Background 65 Uturuncu is within the APVC, an area of volcanism which covers about 66 50,000 km2 between 21o to 24o S in Bolivia, Argentina, and Chile (de Silva 1989). 2 67 From the late Tertiary to the late Miocene, activity in the APVC was dominated 68 by effusive andesitic volcanism, and from the late Miocene to the earliest 69 Pleistocene, at least 10,000 km3 of dacitic ignimbrite were erupted (de Silva 70 1989); these eruptions occurred in 4 pulses with extrusion rates of 0.004 to 0.012 71 km3/yr (de Silva and Gosnold 2007). This ignimbrite flare-up is associated with 72 the creation of several large caldera complexes and composite volcanoes. 73 The APVC is underlain by the Altiplano-Puna magmatic body (APMB), 74 the largest known active continental crustal magma body (Chmielowski et al. 75 1999). The APMB was identified using seismic receiver functions as a zone of 76 very low seismic velocities at a depth of ~20 km below local relief, with a vs < 1.0 77 km/s and a thickness of ~1 km (Zandt et al. 2003). Zones of very high electrical 78 conductivity have also been observed in the region beneath the APVC and are 79 most likely caused by the existence of partial melts in the crust (Schilling et al. 80 1997). Electrical conductivity investigations of the APMB have quantified the 81 percentage of interconnected partial melt to be at least 14-27 vol.% (Schilling et 82 al. 1997). 83 Although seismology has been used extensively to detect and characterize 84 the APMB, very little research has been conducted regarding shallow seismicity at 85 Uturuncu specifically or other volcanoes of the Central Andes (although see 86 Hellweg 2000; Clavero et al. 2005). From 1-6 April 2003, Sparks et al. (2008) 87 carried out a seismic reconnaissance study at Uturuncu, setting up a single vertical 88 component portable seismic recording system at five different sites at different 89 times. While this study gave some indication of background activity (average of 90 2.6 earthquakes per hour), the present study is the first robust study of seismicity 91 at this volcano using multiple 3-component seismometers. 92 Methods 93 The persistent ground deformation and high rates of seismicity at Uturuncu 94 revealed in 2003 motivated a more complete seismic study of the region. A 95 temporary seismic network was deployed at Uturuncu from April 2009 to April 96 2010 and consisted of 15 stations surrounding the volcano, with the farthest 97 station located about 25 km from the summit (Figure 1). The network consisted 98 of nine Mark Products L22 short period and six Guralp CMG-40T intermediate 99 period 3-component seismometers, and a GPS clock at each station. Data were 3 100 recorded with a Reftek 130 Datalogger at a sample rate of 50 samples/second. 101 Seismic data were later displayed and analyzed using the Antelope software 102 package. Also, we have re-analyzed data from the 5 broadband stations ranging in 103 distance from 15 to 76 km from the summit (Figure 1) from the 1996-97 APVC 104 dataset (Chmielowski et al. 1999) to search for shallow earthquakes. 105 Earthquake Locations 106 We performed a manual review of the waveforms focusing on identifying 107 various types of seismic events and locating local earthquakes. Virtually all of the 108 events recorded are typical earthquakes, which volcano seismologists sometimes 109 call volcano-tectionic VT events. These earthquakes demonstrate high frequency 110 content ranging up to 25 Hz, and clear P and S arrivals. A bandpass filter of 0.8- 111 25 Hz was applied when the P and/or S arrivals could not be clearly distinguished 112 from the noise. Hypocenters of events that had ten or more P and/or S phase picks 113 were located using Antelope's location program genloc (Pavlis 2004). Two 114 different velocity models were used in the earthquake locations. In the first, the 115 seismic velocity structure of the top 40 km was based on Model A of Figure 10 of 116 Leidig and Zandt (2003), which incorporates a 1-km thick low velocity zone at 17 117 km depth. Layers deeper than 40 km are based on Table 1 of Schurr et al (2006). 118 Here, we assume a vp/vs ratio of 1.75 (Table 1). A second velocity model of 119 typical continental crust was also used to locate some of the earthquakes in the 120 catalog. The difference in locations using the two different velocity models was 121 less than typical errors in depth location (1-2 km, with the Leidig and Zandt 122 velocity model giving deeper depths) and negligible differences in latitude and 123 longitude. 124 Local Richter magnitudes were calculated based on the peak amplitude on 125 horizontal components of the waveforms in a time window starting from the P 126 arrival time and ending at two times the S-P time and correcting for the distance 127 between earthquake and station.