Spatial variation of the b-value observed for the periods preceding and following the 24 August 2016, Amatrice earthquake(ML 6.0) (Central Italy) ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 Spatial variation of the b-value observed for the periods prece d- ing and follow ing the 24 August 2016, Amatrice earthquake (M L 6.0) (Central Italy) CATERIN A MON TUORI*, MAURA MURRU, GIUSEPPE FALCON E Istitu to N azion ale d i Geofisica e Vu lcan ologia, Rom e, Italy *caterina.montuori@ingv.it Abstract This paper deals with a preliminary spatial and temporal analysis of the b-value variability, observed in the ar- ea where the August 2016 Amatrice earthquake (ML6.0) occurred. With comparison of the pre- and post- periods of the mainshock, an investigation of anomalous zone of b-values was performed aiming to find possi- ble links with barriers and/or asperities in the crustal volume where seismic sequence was developed. Prelimi- nary results show an area with high b-value (b=1.6) where the mainshock originated. Conversely, two low b- value (b=0.8) volumes are located at the border of the seismogenic structure. The location of these two areas is consistent with a preliminary fault slip inversion, suggesting the presence of two highly stressed patches of co-seismic deformation located NW and SE of the mainshock, with a high potentiality to rupture causing a possible moderate or larger event: the first one in the North (Norcia), the second one in South, next to the area of Amatrice and Campotosto. I. IN TRODUCTION On 24 Au gu st 2016 1:36:32 (UTC) a local m a g- n itu d e 6.0 earth qu ake, 8 km d eep , occu rred in Cen tral Italy, close to Accu m oli, a village in th e area located betw een tw o tow n s: N orcia an d L'Aqu ila. It ru p tu red a SW d ip p in g, N N W-SSE tren d in g fau lt segm en t. Th e m ain sh ock h as triggered a stron g sequ en ce of aftersh ocks (blu e colou r in Figu re 1) th at in volved a p o r- tion of th e Ap en n in e ch ain alon g an axis of abou t 50 km (from Ussita v illage till Cam p ot o- sto Lake) an d abou t 15 km d eep . Th e sp atial aftersh ock d istribu tion su ggests th e activation of d ifferen t fau lt segm en ts (Mt. Vettore fau lt segm en t an d several oth er an tith etical stru c- tu res d ip p in g tow ard s N E) follow in g th e m ain sh ock occu rred on 24 Au gu st, 2016. Th e area affected by th e Am atrice earth qu ake sequ en ce is located in a p ortion of th e Cen tral Ap en n in es w ith a com p lex tecton ic settin g. An exten sion al tecton ic regim e, w ith sever al m ajor Qu ater n ar y n or m al fau lts orien ted alon g th e axis of th e w h ole Ap en n in es (N W-SE), is su p erim p osed ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 on an in h erited th ru st sequ en ce. Th is is m ain ly m ad e of th e Um bria-March e Carbon atic- Mesozoic sequ en ce (N orth -w estern h alf) an d of Latiu m -Abru zzi flysch tertiary sequ en ce (Sou th -eastern h alf), sp lit by th e in active th ru st of Sibillin i Mts. (Olev an o – An trod oco lin e). Th e Qu atern ary n orm al fau lts bord er th e Laga Mt., th e Vettore Mt., th e N orcia basin an d th e n earby Qu atern ary in term ou n tain basin s. D e- sp ite its m od er ate exten t, th e Am atrice eart h - qu ake claim ed 309 lives; th e tow n of Am atrice w as d estroyed en tirely as w ell as m an y su r- rou n d in g villages. Th e area h ad been affected by a d estru ctive earth qu ake in 1639 (Am atrice, Io 9-10 MCS, M w 6.2) w h ich cau sed 500 fatali- ties. Recen tly, Cen tral Italy h as been stru ck by tw o stron g m ain sh ocks: on e occu rred in 1997, Colfior ito earth qu ake w ith M w 6.0 (Um bria- March e region ) an d th e oth er on e in 2009, L'Aqu ila earth qu ake w ith M w 6.3 (Abru zzo re- gion ). Figure 1. Shallow seismicity map (M w 1.4) for the Central A pennines region (A pril 16, 2005-September 16, 2016). Historical seismicity (CPTI15 release 1.5, Rovida et al. 2016) is highlighted by black squares (the numbers are referred to the years of occurrence). The main shocks of the 1997 Colfiorito, 2009 L’A quila an d the recent 2016 A matrice are shown as green, red and blue stars, respectively. The seismicity of three ana- lyzed time periods is reported in different colors: the first on e in red , th e secon d on e in yellow an d th e last in blue (see for details Table 1). The black box highlights the area under study (i.e., 12.6°E–13.8°E, 42.2°N –43.0°N ). Black dots are the main towns: A QU=L' A quila, CM P=Campotosto, A M T=A matrice, A CC=A ccumoli, A RQ=A rquata del Tronto, N RC= N orcia, USS=Ussita. ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 Sp atial an d tem p or al v a riation s of b-v alu e of th e Gutenberg-Richter (G-R) relation (1944), h ave been exten sively stu d ied for variou s tecton ic regim es. Th is p ar am eter d ep en d s on th e stress regim e, th e tecton ic ch ar acter of th e region , th e h eterogen eities of m aterials, an d th e tem p er atu re [i.e., Sch oltz, 2015]. Th ese factors can cau se, locally, ch an ges in th e b-valu e com p ared to th e global average valu e (assu m ed equ al to 1). Low b-valu es h ave been correlated w ith areas of asp erity, locked p art of a fau lt w h ere th e n u cleation of earth qu akes is likely to h ap p en [Schorlemmer et al., 2004, Schorlemmer an d W iemer, 2005, Torman et al., 2012]. Figure 2. Plot of M c with time for all the analyzed period (A pril 16, 2005-September 16, 2016). In the upper right corner, the M c zoom for the 2016 A matrice sequence is depicted in the inset box (A ugust 24-September 18, 2016). Recen tly a few au th or s h ave fou n d th at b- valu e in th e con tin en tal cru st d ecreases d ow n to a d ep th corresp on d in g to th e tran s i- tion betw een br ittle an d d u ctile m aterial; b e- low th is d ep th th e b-valu e w as observed stron gly in creasin g [i.e., M ori and A bercrom- bie, 1997; Spada et al., 2013; Scholtz, 1968 an d 2015]. H igh b-v alu es h av e been correlated w ith th e h igh est slip d u rin g large eart h - qu akes [Görgün et al., 2009; Sobiesiak et al., 2007]. Moreover, th ey w ere even associated w ith th e p resen ce of flu id s in th e aftersh ock areas, facilitatin g th e evolu tion of th e s e- qu en ce itself. Th e occu rren ce of a stron g a f- tersh ock m ay be d u e to th e u p w ellin g of d eep flu id s w h ich can red u ce th e effective stress an d th en trigger an earth qu ake [W and and M anga, 2009]. A few au th ors h igh ligh ted th at m an y aftersh ock sequ en ces w ere m ain ly d istribu ted in h igh p ore p ressu re areas an d th eir sp atio tem p oral d istr ibu tion cou ld h ad been related to th e tran sp ort of flu id s. Su ch an alysis w as con d u cted in Italy for b oth se- qu en ces like th e on e in 1997 (Colfiorito) an d th e sequ en ce in 2009 (L’Aqu ila) [M alagnini et al., 2012; De Gori et al., 2012; M iller et al., 2004] an d in som e oth er cou n tr ies: 1992 Lan d er s earth qu ake [Bosl and N ur, 2002], th e after- sh ocks of n or th ern C h ile’s 1995 An tofagasta earth qu ake [Shapiro et al., 2003] an d after- sh ocks follow in g th e 2008 Wen ch u an M s 8.0 earth qu ake occu rred in Ch in a [Liu et al., 2013]. ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 Figure 3. M aps of completeness magnitude for each period (I-II-III in Table1). W hite squ ares are the main towns shown in Figure 1 as black dots. Th e sp atial variation of b-v alu e of th e Guten- berg-Richter (G-R) relation in th e area h it by th e sequ en ce, before an d after th e Am atrice earth qu ake, is in vestigated in th e p resen t w ork. Th e aim is to u n d er stan d th e m ean in g- fu l variation s of b-valu e th at cou ld be lin ked w ith th e stress accu m u lated w ith in an asp e r- ity p laced alon g or arou n d a fau lt su rface. Th e p eriod before th e Am atr ice m ain sh ock in clu d es th e seism ic sequ en ce of L'Aqu ila earth qu ake in 2009 Th e tech n iqu e em p loyed in th is p ap er h as been alread y ap p lied by th e au th ors in d if- feren t tecton ic areas [M urru et al., 1999, 2004, 2005; M ontuori et al., 2010; De Gori et al., 2012]. II. DATA AN D AN ALYSIS Th e d ata u sed in th is stu d y (Ap ril 16, 2005 to Sep tem ber 18, 2016) are d raw n from th e Ita l- ian Seism ic In stru m en tal an d Par am etr ic D a- tabase (ISIDE) p rovid ed by IN GV. An area con tain in g 27,223 even ts h avin g m agn itu d e M L ≥1.4 an d d ep th ≤30 km w as selected (see black box in Figu re 1). Th e first step w as to calcu late th e m agn itu d e of com p leten ess (M c ) w h ich is an im p ortan t p ar am eter w h en est i- m atin g b-valu es [W iemer an d W yss, 2000]. M c is com p u ted as a fu n ction of tim e by slid in g th e tim e w in d ow s, each con tain in g 150 earth qu akes an d step p in g by 5 even ts (Fig- u re 2). M c valu es, for th e w h ole p eriod an a- lyzed , ch an ge from 1.4 to 1.9, excep t for tw o p eriod s of tim e correlated w ith tw o stron g even ts occu rred in Ap ril 2009 (L'Aqu ila) an d in Au gu st 2016 (Am atrice), w h ere M c reach es th e valu e of abou t 3.0. In th e early h ou rs of a seism ic sequ en ce m ay be very d ifficu lt to d e- tect sm all sh ocks; th is m ay lead , as a con se- qu en ce, to an in crease of th e m agn itu d e of ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 Figure 4. M aps of b-values for the three periods shown in Table 1. a) I period (M c 1.7); b) II period (M c 1.9); c) III period (M c 2.1). The A A ' and BB' lines indicate the b- value cross-sections of Figures 5a and 5b, respectively. In Figure 4b the red star is related to the 2009 L' A quila earthquake, while the white stars indicate the strongest aftershocks of the sequence. Figure 4c shows the 2016 A matrice aftershocks (white stars) with M4.2 and the location of the mainshock (M 6.0) (red star). Black squares are the main towns. com p leten ess [Enescu et al., 2007]. Afterw ard s th e catalo g w as sp lit in to th ree p eriod s (Table 1). Th e first on e r an ges from ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 2005 to th e tim e ju st before th e L'Aqu ila m ain sh ock th at occu rred in Ap ril 2009. Th e secon d on e in clu d es th e aftersh ocks se- qu en ce of L’Aqu ila earth qu ake (th e first m on th is n ot in clu d ed in th e an alysis) an d th e seism icity occu rred ju st before th e Am a- trice earth qu ake Th e last tim e p eriod sp an s from th e Am atrice aftersh ocks sequ en ce (a few h ou rs follow in g th e m ain sh ock) to th e en d of th e d ataset (Sep tem ber 16, 2016). Th e begin n in g of th e last tw o p er iod s d oes n ot coin cid e w ith th e occu rren ce of tw o m ain sh ocks becau se it h as been p referred to take in to con sid er ation th e p eriod s w h en th e com p leten ess m agn itu d e reach es stable v a l- u es. M c h as been th en recalcu lated as a fu n ction of sp ace for each con sid ered p eriod (Figu re 3), by u sin g th e best com bin ation m eth od in softw are p ackage Zm ap [W iemer, 2001]. By m ap p in g M c , th e ran ge of m agn itu d es, as w ell as th e sp atial exten t in w h ich th ese m agn itu d es are rep orted com p letely, can be d eterm in ed [Wyss an d Stefan sson , 2006]. In Table I h as been also rep orted th e M c an d th e n u m ber of even ts w ith M >M c con tain ed in each p eriod . On ce th e M c valu e h as been calcu lated , th e m ap p in g of th e b-v alu e in sp ace h as been d raw n for th e th ree catalogs. A grid of 2 km sp acin g an d a fixed n u m ber of even ts (N =80) for each n od e h as been em p loyed . Th e sam - p lin g volu m es are cylin d er -sh ap ed w ith h or- izon tal axes an d r ad ii in versely p rop or tion al to th e seism icity d en sity. We on ly p lot even ts w ith in a m axim u m cylin d er rad iu s ran g in g from 1.0 to 6 km for th e first an d secon d p e- riod s, from 1.0 to 5 km for th e th ird on e (Fig- u re 4). Th e m axim u m likelih ood estim ation m eth od is u sed to d eter m in e b-valu es [A ki, 1965]. Table I: Time periods choose for the analysis. M c and events in each period are also reported Period s M c N even ts I 16/ 04/ 2005 06/ 04/ 2009 1.7 1187 II 08/ 05/ 2009 23/ 08/ 2016 1.9 2803 III 24/ 08/ 2016 18/ 09/ 2016 2.1 1234 In ord er to an aly ze th e b-v alu e in d ep th , th e earth qu akes are p rojected on to vertical cross - section s alon g th e strike of th e fau lt (Figu re 4 an d Figu re 5). Th e n od e sep aration for th ese cross section s w as 2 km an d sam p les of N =60 w ere con sid ered . Th e m axim u m r ad iu s con sid ered for th ese cross section s is 5 km for both tim e p eriod s. To estim ate th e statis- tical m ean in gfu l of th e b-v alu e, th e m eth od of Shi and Bolt (1982) w as em p loyed . To m easu re th e con fid en ce lim its of th e b- valu e an om alies for th e section s, th e Utsu 's test [Utsu, 1992] w as th en em p loyed . III. RESULTS AN D CO N CLUSION S Map s of th e sp atial d istr ibu tion of th e b- valu e are sh ow n in Figu re 4. For th e fir st p eriod (Figu re 4a) th e low e st b valu e (b abou t 0.8) is fou n d in th e area w h ere th e L'Aqu ila m ain sh ock w as n u cleated . Th e d ecrease in b-valu e cou ld be correlated w ith fau lt asp erity (Görgün, 2013). Sim ilar low b valu es (~0.8) h as been observed in a region SW of Sp oleto (SPT). Th e b p aram eter, be- tw een N orcia (N RC) an d Cam p otosto (CMP), varies from 1 to 1.4 an d it is very h igh (~2) in th e n orth ern p art of th e area. ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 Table II: M ainshock and aftershocks occurred during the 2016 A matrice sequence (24/08/2016- 18/09/2016). The time is reported in UTC. Tim e yyyy/ m m / d d h h :m m Lon (°E) Lat (°N ) M L 2016/ 08/ 24 01:36 13.234 42.698 6.0 2016/ 08/ 24 01:37 13.253 42.712 4.5 2016/ 08/ 24 01:56 13.275 42.614 4.4 2016/ 08/ 24 02:33 13.154 42.794 5.3 2016/ 08/ 24 04:06 13.125 42.769 4.4 2016/ 08/ 24 11:50 13.146 42.819 4.5 2016/ 08/ 24 17:46 13.222 42.663 4.2 2016/ 08/ 25 03:17 13.208 42.753 4.3 2016/ 08/ 25 12:36 13.290 42.596 4.4 2016/ 08/ 26 04:28 13.290 42.600 4.8 2016/ 08/ 28 15:55 13.238 42.820 4.2 2016/ 09/ 03 01:34 13.13 42.775 4.2 2016/ 09/ 03 10:18 13.215 42.866 4.3 Figu re 4b sh ow s th e after sh ocks sequ en ce of L'Aqu ila earth qu ake an d th e seism icity b e- fore th e m ain sh ock of th e Am atrice ear t h - qu ake. In th e sam e figu re, th e ep icen ter of th e L’Aqu ila m ain sh ock is also p lotted . With resp ect to th e p reviou s p eriod , an in crease of th e b-v alu e in an area betw een Cam p otosto (CMP) an d Accu m u li (ACC) is clearly seen ; at th e sam e tim e, a d ecreasin g of b-valu e is fou n d to th e West of Cam p otosto an d Am a- trice (b~0.9), w h ere m ost of seism icity is co n - cen trated . Figu re 4c sh ow s th e d istribu tion of b-valu e calcu lated u sin g Am atrice aftersh ocks (Table II). Th e b-valu es in aftersh ock zon e var y b e- tw een 0.8 an d 1.6. Th e h igh est b-valu es, are fou n d n ear th e m ain sh ock ep icen ter, w h ich is rep orted on ly for com p arison . Low est b- valu es are fou n d in th e SE (Am atrice an d Cam p otosto) an d in th e N -N E (betw een Ar- qu ata d el Tron to an d Ussita) of th e m ain sh ock ep icen ter. To d efin e th e d istribu tion of b-v alu e an om a- lies in d ep th , before an d follow in g th e m ain sh ock of Am atrice earth qu ake, th e b- valu e an alysis h as been p erform ed on th e cross section s AA’ an d BB', p rojectin g th e seism icity w ith in 10 km on each sid e of th e cross section al p lan e (Figu res 4b an d 4c). Section AA' (Figu re 5a) sh ow s an area of an om alou s h igh b-valu e (2.1±0.25) located arou n d Accu m oli area, betw een 6 an d 12 km d ep th . A w ell-d efin ed v ariation from h igh to low b-valu e, both vertically an d later ally can be observed in th e area betw een Accu m oli (ACC) an d Cam p otosto (CMP). It m ay be in - terp reted as a tr an sition from brittle to d u c- tile beh aviou r. Th e low b an om aly (abou t 0.9) observed below Am atrice (AMT) cou ld be con sid ered as a asp erity zon e located at th e bord er of th e area w h ere th e Am atr ice eart h - qu ake w as n u cleated . Th e p test [Utsu , 1992] h as been em p loyed to qu an tify th e statistical sign ifican ce of th e an om alou s areas. Th e r e- su lt of th is test su ggests th at th e h yp oth esis th at th e exam in ed sam p les h ave th e sam e b- valu es, can be rejected as it is n ot statistically sign ifican t at th e 99% level (left bottom p an el in Figu re 5a). Figu re 5b sh ow s a region at th e SE of th e Am atrice m ain sh ock h yp ocen ter of h igh b - valu e (1.65±0.2) arou n d 10 km . Above Am a- trice an d N orcia areas are clearly ev id en t tw o zon es w ith low b-valu es (0.8±0.1) w h ere th e m ajority of aftersh ocks w ith M>4.2, occu rred in th is p eriod , are con cen tr ated . https://en.wikipedia.org/wiki/Statistically_significant https://en.wikipedia.org/wiki/Statistically_significant ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 Figure 5. Cross-sections of the distribution of b-values along the profiles shown in Figures 4b and 4c. (a) A A ' cross-section. (b) BB’ cross-section. The red and grey circles represent the volumes of high and low b- values used to calculate the FM Ds. The white stars represent the aftershocks occurred during the A matrice sequence. The red star is related to the 2016 A matrice mainshocks. Th e p robability th at th e tw o sam p les w ith h igh an d low b-valu es belon g to th e sam e p op u lation is P= 7.7E-003, accord in g to th e Utsu test.Th ese resu lts w ell m atch a p relim i- n ary fau lt slip in version th at su ggests th e p resen ce of tw o h igh ly stressed p atch es of co-seism ic d efor m ation located SE an d N W of th e h yp ocen ter w h ich ru p tu red w ith th e m ain sh ock of 24 Au gu st [Gruppo di lavoro IN GV sul terremoto di A matrice, 2016]. Su ch ANNALS OF GEOPHYSICS, 59, Fast Track 5, 2016; DOI: 10.4401/ag-7273 p atch es, w h ich are correlated to low b-valu es m ay h ave h igh p oten tiality to ru p tu re cau s- in g a p ossible m od erate or larger even t (Görgün, 2013; Tormann et al., 2013 an d refer- en ce th erein ). Du rin g th e review p rocess a stron g even t (M w 6.5) occu rred on 30 October 2016 (06:40:17 UTC) 5 km from N orcia (42.84°N - 13.11°E) an d 9 km d ep th . Th is even t w as p receed ed by tw o oth er stron g sh ocks (m agn itu d e M w 5.4 an d M w 5.9) on 26 October 2016, betw een N orcia an d Ussita. Th e M w 6.5 N orcia earth qu ake is th e m ost p ow erfu l earth qu ake strikin g Italy sin ce th e 1980 M w 6.9 Irp in ia even t. In terestin gly, th e N orcia earth qu ake is located at th e bord er w h ich d e- lim its th e low b-v alu e area h igh ligh ted by ou r an alysis below N orcia (Figu re 4c an d 5b),Th e an alysis p erform ed so far an d d e- scribed in th is p ap er is still in p rogress an d w ill be im p roved by exten d in g th e p eriod of aftersh ocks to m ap th e d istribu tion of th e b- valu e alon g th e fau lt p lan es resp on sible both th e M L 6.0 Am atr ice an d M w 6.5 N orcia earth - qu akes. Th e fu tu re resu lts w ill be com p ared w ith oth er m u ltid iscip lin ar y stu d ies to better u n d erstan d th e earth qu ake occu rren ce in th is area. Th is an alysis su ggests th at m on itorin g th e evolu tion of b-v alu e m igh t be u sefu l for th e evalu ation of seism ic h azard an d eart h - qu ake forecastin g. ACKN OWLEDGMEN TS We th an k R. Con sole for h is u sefu l com - m en ts w h ich h elp ed u s to im p rove th e m an - u scrip t an d D. 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Am ., 96, 599–608, d oi:10.1785/ 0120040056. *Corresponding author: Caterina Montuori, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy; email: caterina.montuori@ingv.it; tel: (+39)0651860496