Preliminary analysis of radon time series before the Ml=6 Amatrice earthquake: possible implications for fluid migration ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7166 Preliminary analysis of radon time series before the Ml=6 Amatrice earthquake: possible implications for fluid migration Valentina Cannelli, Antonio Piersanti, Elena Spagnuolo, Gianfranco Galli Istituto Nazionale di Geofisica e Vulcanologia valentina.cannelli@ingv.it Abstract On August 24, 2016 a Ml=6.0 earthquake occurred in Central Apennines, Italy, between the towns of Norcia and Amatrice, causing severe destruction and casualties in a wide area around the epicenter. We present a preliminary analysis of continuous radon concentration data collected from the second half of 2012 to the day after the earthquake by a long term radon monitoring station, installed at Cittareale (RI, Italy), about 11 km south-west of the epicenter. We combine the field data analysis with the outcome of dedicated laboratory experiments, aimed to study real time radon emission dynamics from rock samples subject to normal and shear stress loads in absence of fluid transport and migration phenomena. Our results suggest the possibility of a minor role played by phenomena related to fluid migration for the Amatrice seismic event with respect to other recent Apennine earthquakes. I INTRODUCTION C urrent research on soil radon emana- tion in terms of analyses of long time series has revealed its potential infor- mative power regarding the link between tem- poral variation of this noble gas concentration and seismogenic processes [Stefansson(2011), Piersanti et al.(2016)]. In fact, the radioactive nature of radon makes it a potentially ef- ficient marker to study and monitor fluid flows. In recent years, new laboratory ex- periments gave unambiguous evidence of the link between the rock state of stress and variations in the radon emanation properties [Tuccimei et al.(2010), Mollo et al.(2011)]. How- ever, the analysis is complicated by the sus- ceptibility of radon emissions to meteoro- logical parameters and site-specific features [Jaishi et al.(2014), Piersanti et al.(2015)]. We had the possibility of analyzing a three-years long time series of radon concentration ac- quired by a monitoring station installed at about 11 km south-west of the 24 August, 2016, Ml=6.0 Amatrice earthquake epicenter. We present the preliminary results obtained lim- iting the impact of meteorological effects on the measured radon time series and combin- ing them with the results of a series of labo- ratory experiments to study radon concentra- tion variations in connection with a process of gradual deformation in shear. II METHODS II.1 Soil radon observations Radon data were collected by a high sensi- tivity, high efficiency active radon monitor- ing station based on a Lucas cell [Lucas(1957)], installed at Cittareale (CTTR, 42◦37′3.0′′N 13◦9′33.5′′E) in May 2010, at about 960 m 1 mailto:valentina.cannelli@ingv.it ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7166 above sea level. In August 2012 a new radon concentration detector replaced the previous one. In this work we decided to employ only data from the latest (and currently work- ing) detector. The station is located in a basement hosting the municipal archive of the city (occasionally accessible to technical staff only) and measures radon concentra- tion with an adjustable acquisition time (cur- rently is 115 minutes); its efficiency is 0.06 count min−1/Bq m−3 and the minimum de- tectable concentration is as low as 6 Bq m−3. CTTR acquires simultaneously local temper- ature values by means of a specific sensor co-located with the radon one. All other daily values of meteorological parameters em- ployed in our analysis (external temperature, pressure, precipitation) are approximated as short term (12-24h) weather forecast by an Ital- ian weather website (http://www.ilmeteo.it/). The complete time series of radon concen- tration data recorded at station CTTR from August 1, 2012 to August 25, 2016, together with the time series of local temperature are shown in Figure 1a and Figure 1b, respec- tively. CTTR radon concentration data show a marked seasonal signal, ascribable to a major correlation with temperature (see blue monthly moving-average of radon time se- ries in Figure 1a and daily average of lo- cal temperature in Figure 1b), as labora- tory tests [Iskandar et al.(2004)] and long term radon monitoring studies [Jaishi et al.(2014), Piersanti et al.(2015)] indicate. The meteorolog- ical corrected radon concentration values that we will show and discuss in the following sec- tions have been obtained applying the proce- dure described in [Piersanti et al.(2016)]. Jul12 Oct12 Jan13 Apr13 Jul13 Oct13 Jan14 Apr14 Jul14 Oct14 Jan15 Apr15 Jul15 Oct15 Jan16 Apr16 Jul16 Oct16 50 100 150 200 250 300 350 400 450 a) R a d o n C o n c e n tr a ti o n ( B q /m 3 ) Jul12 Oct12 Jan13 Apr13 Jul13 Oct13 Jan14 Apr14 Jul14 Oct14 Jan15 Apr15 Jul15 Oct15 Jan16 Apr16 Jul16 Oct16 6 8 10 12 14 16 18 20 22 24 b) In te rn a l te m p e ra tu re ( ° C ) Figure 1: a) Radon concentration (Bq m−3) at CTTR for the period August 1, 2012 - August 25, 2016 both as daily average (yellow dots) and as monthly moving-average (blue line). b) Daily averaged time series of local tempera- ture for the same period as in (a). II.2 Laboratory We conducted four laboratory experiments to study radon concentration variations in con- nection with a process of gradual deformation in shear. The use of controlled conditions of loading and ambient conditions (temperature and humidity), helps reducing the number of implicated variables affecting the natural radon emanation process and simplifies the in- terpretation of the results. We use the rotary shear apparatus SHIVA (Slow to High Veloc- ity Apparatus) installed in the High Pressure- High Temperatures (HP-HT) laboratory of the INGV of Rome simulating close to natural seismic deformation condition at depth of the upper-crust [Di Toro et al.(2010)]. The sample assembly is made of two cylinders 50 mm of external diameter sandwiched in frictional contact under a constant normal load of 5 MPa on tuffs and 15 MPa on tonalities, within a pressure-vessel. The vessel [Violay et al.(2013)] is a device made of stainless steel equipped with sealing O-rings that ensure isolation of the sample pair and guarantee fluid confine- ment. Rock type selection (tuff and tonalite) 2 ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7166 was driven by the radon initial concentration, porosity and shear modulus. We used the same instrument employed in the field to con- tinuously acquire radon/thoron concentration variations during the progress of the experi- ment, with an acquisition time of 1 sec. We use a simple pump to flux air in closed loop between the inlet valve of the vessel and the outlet flange of the radon detector to allow air circulation within the vessel and from the vessel to the detector (Figure 2). Variations of radon concentration are referred to an ini- tial condition set at the achievement of secu- lar equilibrium. Secular equilibrium was en- sured pre-stressing the sample pair in a uni- axial press for four days. Experiments con- sisted in a step-wise increase in shear-stress (τ ≈ 0.2MPa, dt ≈ 5 min) under constant nor- mal load until sample failure, resulting in a fast rotation of the rotary column at prescribed velocity and pronounced wearing and axial shortening of the contact surfaces. The shear- stress was then re-established and increased again, for several cycles, until the initial bare rock is crushed to powder. Deviation from the prescribed experimental conditions were possible due to the onset of mechanical os- cillations in shear stress or a fast increase in the deformation rates, requiring a manual in- tervention. All experiments were well repro- ducible in both the mechanical behaviour and recorded radon emission variations. Figure 2: The experimental apparatus (SHIVA, on the left modified from [Di Toro et al.(2010)]) es- sentially made of a rotary axes, a stationary axis and a sample chamber. The experimental assembly consists of the radon detect. (labelled with a), the connection (b) allowing for close loop circulation of fluids from the vessel (c) to the radon detect.; the air pump for air flux (d). III RESULTS III.1 Soil radon observations Our analysis is based on the phenomenolog- ical observation of the trend of radon con- centration time series during the months of July and August for the four years from 2013 to 2016 and successively on their correlation with variations of meteorological parameters measured in the same periods, through an empirical correction procedure aimed at lim- iting the impact of their variations on the measured radon concentration levels (see for details [Piersanti et al.(2016)]). In Figure 3a daily averaged time series of radon con- centration for the period July-August (2013- 2014-2015-2016) are shown. Radon concen- tration data acquired at station CTTR show for these four time windows values variabil- ity comparable with the one of the com- plete time series but with higher absolute con- centration values, as expected for summer months [Iskandar et al.(2004), Jaishi et al.(2014)]. Namely, we registered sharp peaks ranging 3 ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7166 from a few tens up to 400 Bq m−3. The aver- age values of radon concentration evaluated for the period July-August and for the month of August alone are reported in Table 1, in the second and in the fourth column respectively. It is worth noting that 2016 average value is the lowest of all the four years and it is lower than the average values registered during the previous three years by more than 3 standard deviations (see last two rows of Table 1). Table 1: Mean values of radon concentrations in (Bq m−3). Jul-Aug Jul-Aug Aug Aug (uncorr.) (corr.) (uncorr.) (corr.) 2013 227 149 212 96 2014 228 129 222 90 2015 233 158 216 179 2016 214 71 196 63 avg. 229 145 217 122 std.dev. (± 3) (± 15) (± 5) (± 50) 25−Jun 05−Jul 15−Jul 25−Jul 04−Aug 14−Aug 24−Aug 03−Sep 50 100 150 200 250 300 350 400 450 a) R a d o n C o n c e n tr a ti o n ( B q /m 3 ) 2013 2014 2015 2016 25−Jun 05−Jul 15−Jul 25−Jul 04−Aug 14−Aug 24−Aug 03−Sep 0 200 400 600 800 1000 1200 b) R a d o n C o n c e n tr a ti o n C o rr e c te d ( B q /m 3 ) 2013 2014 2015 2016 Figure 3: a) Daily-average time series of radon con- centration for the period July-August for 2013,2014,2015,2016. The black vertical line marks the occurrence of the Ml=6 earthquake. b) The same as in (a) but with daily average time series corrected for meteorological param- eters (CTmax , CRmax = 5; CPmax = 1). In Figure 3b we show the time series of Figure 3a corrected for meteorological param- eters, according to the empirical correction procedure developed by [Piersanti et al.(2016)] in order to remove the effect of meteorolog- ical phenomena on measured radon concen- trations. As described in cited work, the cor- rection parameters are determined through a numerical optimization scheme whose results, in agreement with results obtained for the Pollino range stations (South Italy, Calabrian arc), indicate temperature and precipitations as the most impacting variables on radon con- centration data (CTmax , CRmax = 5), while the role of the atmospheric pressure variations is not well constrained (CPmax = 1). The features of all the considered time series change no- ticeably after the correction. We focus our at- tention on the 2016 July-August corrected one (cyan solid line in Figure 3b) that is signifi- cantly flattened with respect to all the others (see also the average values of corrected radon concentration in the third and fifth column of Table 1). Indeed, the 2016 July-August uncor- rected time series show three low concentra- tion peaks, the latest occurring two days be- fore the earthquake (Figure 3a). The peculiar behavior of 2016 July-August radon timeseries is confirmed also by the observation of rela- tive variations of radon concentration during the period selected for our analysis. Figure 4 shows that the largest relative variations of the whole radon time series occur just in the 90 days preceding the earthquake. 4 ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7166 Nov−12 Jun−13 Jan−14 Jul−14 Feb−15 Aug−15 Mar−16 Sep−16 −0.5 −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 0.5 R n r e la ti v e v a ri a ti o n 14 Aug 21 Jun 23 May Figure 4: Relative variations of radon concentrations for the period January 1, 2013 - August 25, 2016 III.2 Laboratory We conducted two experiments on tonalite and two on tuff to test the instrument sensitiv- ity and the reproducibility of radon concentra- tion variations in case of laboratory faults. In the following we will focus on tonalite since we consider it more representative of a seis- mogenic setting with respect to tuff. Exper- iments conducted at normal load of 15 MPa (experiments n. 1063 and s1095) show that the experimental fault responds to a gradual increase in shear stress (blue solid curve in Figure 5) by sliding at slip rates (black solid curve) varying from less than a few mm s−1 up to 4 cm s−1. These episodic slip events are concomitant to a large increase in the ax- ial shortening which typically ranges from 0 to 2 mm (red solid curve). After the appli- cation of the normal load and with the pro- gressive increase in shear stress the number of radon counts (converted in counts per hour in Figure top panel) was gradually decreas- ing (Figure 5 top panel) until the onset of the largest event where we observed the largest decrease in radon counts. It is worth noting that this large negative radon concentration variation occurred, in the case of both the ex- periments, before sample failure and still far from a condition of seismic sliding (slip rates > 0.1m s−1). Moreover, the correlation be- tween the state of loading if the sample pair and the radon concentration variation follow almost instantaneously. The negative radon concentration variation prior sample failure was also reported in previous static analysis ([Tuccimei et al.(2010)]) on tuff where the radon concentration variation was measured as a cu- mulative value prior and after the application of a normal load under an uniaxial press. 0 5 1 h , 2 4 ', 5 0 .8 '' 0 0.02 0.04 )s / m( et ar pi l S S h e a r S tr e ss ( M P a ) Time 9 A p ri le . 2 0 1 5 0 8 .5 6 :1 6 ( G M T ) Axial shortening 2mm 09:00 10:00 11:00 08:00 10:00 12:00 14:00 700 650 600 550 compression compression + shear 12:00 13:00 14:00 # / h o u r Figure 5: Radon variation concentration (top panel) as a function of the shear stress increase (blue solid line, bottom panel) in case of experiment number s1095. The mechanical response of the system made of fault + experimental appa- ratus is also shown (bottom panel) in terms of slip rate (black solid curve) and axial displace- ment (shortening, red solid line). The num- ber of radon counts (averaged in counts/hour in top panel) decreases with increasing shear stress at constant normal load (compressive load of 15 MPa) and has a maximum negative slope corresponding to the highest recorded slip pulse with slip-rate of 0.04 m/s. The ax- ial shortening (generally associated to sample grinding and wearing) was of the order of 0.1 mm. IV DISCUSSION We have presented the results of an analysis of radon concentration data acquired at CTTR station from 2013 to the day after the Ml=6.0 5 ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7166 Amatrice earthquake supplemented with data obtained from dedicated, original laboratory experiments. The field observations confirm the strong impact of meteorological parame- ters variations on observed radon time series, especially temperature and precipitations. At the same time, both daily average time series and the ones corrected for meteorological pa- rameters evidence for July-August 2016 a dif- ferent behavior with respect to the same time window of the previous years, showing over- all lower average values of radon emanations and an increase of relative variations among different detections. When a meteorological parameter correction is applied, the previous behaviors are confirmed and a flattened trend in the days preceding the earthquake can be also evidenced. Data obtained from labora- tory experiments, aimed to study real time radon concentration variations in connection with a process of gradual deformation in shear and in absence of fluid transport and migra- tion phenomena, give results compatible with the CTTR radon time series behavior during the 2016 July-August period. These combined observations could pave the way to the hy- pothesis of a minor role played by processes associated with fluid transport and migration for the Amatrice seismic event with respect to other recent Apennine earthquakes. Nev- ertheless, the available data, being limited to a single station, do not allow us to rule out the possibility that different portion of the seismogenic volume could have been subject to different styles of fluid dynamics related phenomena. In this respect, a multi-station monitoring of seismogenic areas would repre- sent an important evolution of the presented investigative approach. ACKNOWLEDGMENT The experimental part of this work was supported by the ERC CoG NOFEAR project 614705 (P.I. Giulio Di Toro) References [Di Toro et al.(2010)] Di Toro, G., Niemeijer, A.R., Tripoli A.; et al. (2010), From field geology to earthquake simulation: a new state-of-the-art tool to investigate rock- friction during the seismic cycle (SHIVA), Rendiconti Lincei, 21(Supp.1), S95-S114, pp. 1-20, doi:10.1007/s12210-010-0097-x. [Iskandar et al.(2004)] Iskandar, D., Yamazawa, H., and T. Iida (2004), Quantification of the dependency of radon emanation power on soil temperature, Appl Radiat Isot, 60, 971–973. [Jaishi et al.(2014)] Jaishi, H., Singh, S., Ti- wari, R. P.; et al. (2014), Analysis of soil radon data in earthquake precursory studies, Annals Of Geophysics, 57(5) S0544, doi:10.4401/ag-6513. [Lucas(1957)] Lucas, Henry F. (1957), Im- proved Low-Level Alpha-Scintillation Counter for Radon, Review of Sci- entific Instruments, 28(9), 680–683, doi:10.1016/j.cageo.2003.08.011. 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(2010), Radon and thoron emission from lithophysae-rich tuff under increasing deformation: An ex- perimental study, Geophysical Research Let- ters, 37(5), doi:10.1029/2009GL042134. [Violay et al.(2013)] Violay,M., Nielsen, S., Spagnuolo, E.; et al. (2013), Pore fluid in experimental calcite-bearing faults: Abrupt weakening and geochemical signature of co-seismic processes, EPSL, 361, 74-84. 7 INTRODUCTION METHODS Soil radon observations Laboratory RESULTS Soil radon observations Laboratory DISCUSSION