ARTICLE
Seismici y a he Cas o gas ese oi d i en by
po e p essu e di usion and aspe i ies loading
Simone Cesca 1✉, Daniel S ich2,3, F ancesco G igoli 4,5, Alessand o Vuan 6,
José Ángel López-Comino 2,3,7, Pe e Niemz 1,7, Es e anía Blanch 8,9, To s en Dahm 1&
William L. Ellswo h 10
The 2013 seismic sequence a he Cas o injec ion pla o m o sho e Spain, including h ee
ea hquakes o magni ude 4.1, occu ed du ing he ini ial filling o a planned Unde g ound Gas
S o age acili y. The Cas o sequence is one o he mos impo an cases o induced seis-
mici y in Eu ope and a a e example o seismici y induced by gas injec ion in o a deple ed oil
field. He e we use ad anced seismological echniques applied o an enhanced wa e o m
da ase , o esol e he geome y o he aul s, de elop a g ea ly enla ged seismici y ca alog
and eco d de ails o he up u e kinema ics. The sequence occu ed by p og essi e aul
ailu e and unlocking, wi h seismici y ini ially mig a ing away om he injec ion poin s,
igge ed by po e p essu e di usion, and hen back again, b eaking la ge aspe i ies loaded o
highe s ess and p oducing he la ges ea hquakes. Seismici y occu ed almos exclusi ely
on a seconda y aul , loca ed below he ese oi , dipping opposi e om he ese oi
bounding aul .
h ps://doi.o g/10.1038/s41467-021-24949-1 OPEN
1GFZ Ge man Resea ch Cen e o Geosciences Po sdam, Po sdam, Ge many. 2Ins i u o Andaluz de Geo ísica, Uni e sidad de G anada, G anada, Spain.
3Depa amen o de Física Teó ica y del Cosmos, Uni e sidad de G anada, G anada, Spain. 4Depa men o Ea h Sciences, Uni e si y o Pisa, Pisa, I aly.
5Swiss Seismological Se ice, ETH Zu ich, Zu ich, Swi ze land. 6Na ional Ins i u e o Oceanog aphy and Applied Geophysics - OGS, T ies e, I aly.
7Ins i u e o Geosciences, Uni e si y o Po sdam, Po sdam-Golm, Ge many. 8Depa amen de Física-EPSEB, UPC Ba celona Tech, Ba celona, Spain.
9Obse a o i de l’Eb e (OE), CSIC—Uni e si a Ramon Llull, Roque es, Spain. 10 Depa men o Geophysics, S an o d Uni e si y, S an o d, CA, USA.
✉email: simone.cesca@g z-po sdam.de
NATURE COMMUNICATIONS | (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1|www.na u e.com/na u ecommunica ions 1
1234567890():,;
An h opogenic seismici y is s imula ed by s ess pe u ba-
ions, ac u ing p ocesses, o po e p essu e changes in
he subsu ace accompanying fluid and mass mo emen s
d i en by indus ial ac i i ies1,2. Since hese human ac ions only
a ec he uppe mos se e al kilome e s o c us , induced ea h-
quakes a e expec ed o occu a simila dep hs. As a ma e o ac ,
almos all majo cases o injec ion- ela ed induced seismici y wi h
an accu a e hypocen al loca ion occu ed a ~4–8 km dep h
(Supplemen a y Table 1). Due o i s shallow dep h, induced
seismici y poses an impo an seismic haza d, as e en mode a e
magni ude ea hquakes can p oduce s ong shaking and localized
damages3–6. Fo hese easons, an h opogenic seismici y has
become a ma e o g ea socie al conce n and ep esen s a new
ocus o seismic haza d analysis1,7–9. While induced seismici y
accompanied he exploi a ion o di e en na u al esou ces o e
he pas cen u y7,10–14, so-called injec ion-induced seismici y
(IIS) has gained scien ific a en ion in he pas ew decades1,15,16.
The in e es in IIS has been p omp ed by he ecen occu ence o
a numbe o impo an ea hquakes, which ha e been associa ed
wi h fluid injec ion in he ame o hyd aulic ac u ing17–19,
was ewa e disposal4,20, con en ional hyd oca bon ex ac ion7,14,
and geo he mal exploi a ion ope a ions5,6,21.
Unde g ound gas s o age (UGS) acili ies pe mi s o age and
ex ac ion o la ge olumes o na u al gas o manage fluc ua ions
in demand o p o ide a s a egic ese e. They a e ope a ed
ou inely a hund eds o si es globally, ypically using deple ed
oil/gas ese oi s, aqui e s, o ca e ns in ock/sal o ma ions.
This ype o injec ion/ex ac ion ope a ions ypically has a weak
seismic impac , and only a ew cases o seismici y po en ially
ela ed o gas s o age ha e been epo ed. Gas s o age-induced
seismici y may ha e occu ed a he Gazli gas s o age, Uzbekis an,
a e he p e ious deple ion o a gas field, which had al eady
expe ienced la ge damaging ea hquakes in 1976 and 198422.
When he ese oi was used o gas s o age, cycling injec ion/
ex ac ion ope a ions we e accompanied by ea hquakes up o M
523. Ano he case o gas s o age-induced seismici y has been
p oposed o he Hu ubi UGS, Xinjiang, China24, whe e he la -
ges ea hquake eached Mw 3.0 and was loca ed a ~4 km dep h,
sligh ly deepe han he ese oi o ma ion. Mo e ecen cases o
induced mic oseismici y in Eu ope, epo ed a di e en gas
s o age acili ies2,25, did no exceed Mw 1.0. The gene al lack o
induced seismici y obse a ions a UGSs may be pa ially
a ibu ed o poo seismic moni o ing24, bu e en well-moni o ed
gas ese oi s, such as he Collal o ese oi , NE I aly, showed an
almos comple e absence o induced seismici y, wi h no mic o-
seismici y abo e M 0.0 wi hin 3 km om he ese oi 26. Ano he
eason o he low seismogenic po en ial o gas s o age ope a ions
could be ha gas injec ion schedules a e ypically designed and
enginee ed no o exceed he s ess condi ions exis ing p io o o
du ing he o iginal ese oi p oduc ion. To da e, he mos
impo an case o seismici y induced by gas s o age in Eu ope is
he Cas o p ojec , o sho e Spain, which has been conside ed o
ha e igge ed a seismic sequence o >1,000 ea hquakes, peaking
wi h h ee M 4+ea hquakes in Sep embe –Oc obe 201327,28.
A he Cas o p ojec , cushion gas was injec ed om a pla -
o m, loca ed ~22 km o sho e he coas o Spain, in o a o me ly
deple ed hyd oca bon ese oi (Fig. 1). The ese oi i sel
ex ends along he NNE–SSW di ec ion and deepens eas wa d.
The geology is cha ac e ized by a ka s ed, ac u ed limes one.
The ese oi is sealed on he NW by he Ampos a aul , a no mal
aul ac ing as he ese oi oo . The o he sides o he ese oi
a e sealed by an aqui e 29. When gas is injec ed, i emains
apped a he op o he ese oi , and as he injec ion con inues,
his will displace he gas–oil–wa e in e aces downwa ds.
A se ies o injec ion es s we e pe o med a Cas o s a ing in
201329. Seismici y was fi s obse ed close o he Cas o p ojec
pla o m on Sep embe 5, 2013, 3 days a e he s a o a la ge-
scale gas injec ion in he deple ed Ampos a oil ese oi 27,29. The
close p oximi y o he hypocen e s o he injec ion wells, he
empo al co ela ion be ween injec ion ope a ions and seismic
un es , and he low na u al backg ound seismici y in he epi-
cen al egion we e a s ong a gumen o ea ly e idence o
igge ed o induced seismici y27–32. The seismici y inc eased in
a e and maximum magni ude, eaching a magni ude MbLg 3.0
on Sep embe 13, 2010 (Ins i u o Geog afico Nacional, IGN,
ca alog). Injec ion ope a ions we e s opped on Sep embe 17,
201329. Ne e heless, as obse ed o o he induced seismici y
cases4,5,33, seismici y con inued in he ollowing days. La ge
ea hquakes occu ed, peaking in h ee ea hquakes wi h mag-
ni ude Mw >4 in ea ly Oc obe 27, be o e finally ading away. The
Cas o p ojec was pe manen ly closed in 2019.
Seismici y pa e ns and hei ela ionship o gas injec ion
ope a ions ha e been discussed in many ea ly publica ions and
open epo s on he Cas o p ojec , assessing epicen al
loca ions27–29, hypocen al dep hs27–29,31,32, ocal mechanisms
and momen enso s27,29,31, s a is ical seismici y pa ame e s27,30,
he local eloci y s uc u e28, and po en ial mechanisms o aul
eac i a ion27,31,32,34. Un o una ely, he g owing numbe o
scien ific publica ions has no been accompanied by a clea e
unde s anding o he seismogenic p ocesses a Cas o , as some
seismological esul s emain deba ed, and ul ima ely he e is no
common ag eemen on which aul (s) was ac i a ed. The fi s
majo ques ion conce ns he o e all spa ial dis ibu ion o he
epicen al loca ions, po en ially p o iding in o ma ion on he
o ien a ion o ac i a ed aul (s). In one s udy, he o ien a ion was
ound o be oughly pa allel o he coas 27 and pe pendicula o i
in ano he 28. As o he hypocen al dep h, he e is some ough
ag eemen on he iden ifica ion o shallow c us al sou ces bu
wi h a b oad ange o dep h es ima es, mos ly a ying be ween
1–527 and 6–8km
28,32 bu including e en deepe es ima es28,29.
Such la ge unce ain y p e en s de elopmen o eliable models
o aul ac i a ion. Finally, he e is consensus on he ocal
mechanism, showing s ike-slip mechanisms wi h a s eep NW–SE
plane and a SE dipping NE–SW plane, wi h some a iabili y on
he dip angles27,29,31. Un o una ely, each o he possible wo
plane o ien a ions fi s one o he p oposed seismici y dis ibu-
ions, so ha he aul geome y emains un esol ed. B inging
oge he p e ious esul s is challenging, gi en he a ie y o da a,
eloci y models, and seismological me hods and he lack, in mos
p e ious s udies, o eliable unce ain y es ima ions.
He e we ecollec a b oad da ase om 31 seismic s a ions
(Supplemen a y Fig. 1), including one ocean bo om seismome e
(OBS), subs an ially inc easing he da ase and/o he azimu hal
co e age, in compa ison o p e ious wo ks, ei he using
<10 s a ions27,29 o <20 wi h a s ongly asymme ic ne wo k
geome y31. We model seismic da a by a combina ion o mode n
seismological echniques, including a p obabilis ic momen enso
in e sion, empi ical G een’s Func ion, a - egional and ele-
seismic a ay analysis, and empla e ma ching, which a e used o
he fi s ime o analyze his seismic sequence. While he assess-
men o pa ame e unce ain ies allows explaining p e ious pa -
ially con adic o y esul s, ou new esul s significan ly imp o e
he loca ion accu acy and sou ce pa ame e esolu ion, which
allows us o econs uc he complex, mul i-s age e olu ion o
seismici y and p opose a sel -consis en up u e scena io o he
seismic un es .
Resul s
We adop a ange o mode n echniques (see de ails in he
“Me hods”sec ion), which p o ide subs an ial new esul s.
Templa e ma ching is used o enhance he ca alog size, ela i e
ARTICLE NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1
2NATURE COMMUNICATIONS| (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1 | www.na u e.com/na u ecommunica ions
loca ion me hods o es ablish he con o e sial dis ibu ion o
epicen e s, a ay echniques o in e accu a e dep h es ima es
based on independen da a, momen enso and up u e di ec-
i i y analysis o cons ain he up u e plane o ien a ions, and
finally an accu a e assessmen o wa e o m simila i y o confi m
he o e all simila i y in loca ion and ocal mechanism. Resul s
s abili y is assessed using he local eloci y model G28, es ing
al e na i e models (Supplemen a y Fig. 2), and p o iding sou ce
pa ame e s unce ain ies, which help o explain some incon-
sis encies among p e ious s udies. The new esul s esol e de ails
o he e olu ion o seismici y wi h ongoing injec ion and shed
ligh on he physical causes o induced seismici y and i s
magni ude.
Templa e ma ching de ec ion. We ep ocess con inuous
eco dings o enhance he seismic ca alog wi h he aim o be e
acking he seismici y e olu ion in space and ime. Using
wa e o m empla e ma ching (examples o h ee de ec ions a
di e en magni ude le els a e shown in Supplemen a y Figs. 3–5),
we a e able o conside ably augmen he de ec ed ea hquakes
om 536 (IGN ca alog) and 982 (Eb o ca alog) o 3437 ea h-
quakes (Supplemen a y Fig. 6 and Supplemen a y Da ase 1). The
augmen ed ca alog includes he 148 empla es used. The mini-
mum magni ude in ou ex ended ca alog is Mw −0.2. The
magni ude o comple eness imp o ed om Mc ~2.0 (IGN)35 and
Mc 1.3 (Eb o)27 o Mc 0.6 and 0.3 in he injec ion and pos -
injec ion pe iods, espec i ely (Supplemen a y Fig. 7), using
magni udes o he Eb o ca alog as e e ence. The analysis o he
ex ended ca alogs confi ms a subs an ial change in he seismici y
occu ed on Sep embe 17, 2013, when injec ion ope a ions we e
in e up ed (Supplemen a y Fig. 7). The b- alue d opped om
0.99 ± 0.04 o 0.77 ± 0.01 be ween he injec ion and pos -injec ion
seismici y phases, suppo ing p e ious findings based on ca alog
alone27.
Ea hquake eloca ion. We use wo independen , ad anced
ela i e ea hquake localiza ion me hods, based on wa e o m
c oss-co ela ion and
S
-
P
di e en ial imes combined wi h dis-
ance geome y echniques. Bo h me hods ake ad an age o he P
and S wa e o ms and ha e a highe esolu ion han he con-
en ional, absolu e localiza ion me hods used in p e ious s udies
o he Cas o sequence27–29. These echniques a e especially
sui ed o he analysis o spa ially clus e ed seismici y, as obse ed
a Cas o , and less sensi i e han absolu e loca ion echniques o
s uc u al he e ogenei ies ou side he seismogenic olume and o
ne wo k asymme y. Wa e o m-based ela i e hypocen e loca-
ions ob ained o a subse o 51 e en s wi h magni udes >2.0
(Supplemen a y Da ase 2) demons a e ha he sequence was
es ic ed o a single, ~4 km long, N42°E ending lineamen
(Fig. 2a), e en a e accoun ing o unce ain ies (Supplemen a y
Fig. 8). P ecise loca ions esul om c oss-co ela ion ela i e
iming, and achie ed an a e age oo mean squa e a el ime
e o s o 0.02 s. Hypocen e s concen a e wi hin an ~1 km wide
dep h in e al (Fig. 2c, d), al hough ela i e dep hs a e di ficul o
esol e due o he asymme ic ne wo k geome y (Supplemen a y
Fig. 1). As discussed below, he absolu e dep h o seismici y is less
well esol ed and likely a a dep h o ~3 km.
Loca ions de eloped using an independen and al e na i e
app oach36 confi m he o ien a ion o he seismici y. The me hod
elies on he solu ion o a seismological dis ance geome y
Fig. 1 O e iew map o he s udy egion o sho e Spain. The map shows he loca ion o he Cas o pla o m (whi e squa e), closes seismic s a ions
ALCN and ALCX (whi e iangles), and he app oxima e ace o he Ampos a aul ( ed do ed lines)47,48. The uppe igh inse shows he geog aphical
loca ion o he s udy a ea, he lowe igh inse a pho o o he Cas o pla o m (cou esy Ál a o González). The map has been plo ed wi h GeoMapApp
(www.geomapapp.o g), using he Global Mul i-Resolu ion Topog aphy (GMRT)70 and egional ba hyme y a e he EMODNET 500 m compila ion.
NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1 ARTICLE
NATURE COMMUNICATIONS | (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1| www.na u e.com/na u ecommunica ions 3
p oblem36 using di e en ial S-P a el imes a he closes seismic
s a ions, ALCN and ALCX. These wo s a ions, he closes o he
Cas o pla o m, a e op imally si ua ed o his me hod, ha ing
almos pe pendicula azimu hs wi h espec o he seismici y
(Fig. 2b and Supplemen a y Fig. 9). While mo e sca e ed,
loca ions we e ob ained o a la ge da ase o 408 ea hquakes
(Supplemen a y Da ase 3). The ea hquakes loca ed wi h he
wa e o m co ela ion app oach ha e a la ge ange o S-P
di e en ial imes a ALCX (SW), han a ALCN (NW), which
confi ms he NE–SW end o he seismici y (Supplemen a y
Fig. 9). Loca ion esul s a e s able and no subs an ially biased by
he assumed eloci y model (Supplemen a y Fig. 10). Bo h
ca alogs a e included in he Supplemen a y Ma e ial.
Momen enso in e sion. Momen enso solu ions o 11
ea hquakes wi h Mw >3.0 (Supplemen a y Table 2) a e based on
a p obabilis ic in e sion app oach37 and conside s di e en one-
dimensional (1D) eloci y models and wa efield a ibu es
(Supplemen a y Figs. 11–13). The esul s a e s able and inde-
penden o he eloci y models used. All s udied e en s display a
simila , p edominan ly s ike-slip aul ing s yle (Fig. 3). The le -
la e al nodal plane ends NE–SW, oughly pa allel o he coas ,
and dips owa d SE (s ike 42° ± 3°, dip 48° ± 11°, ake −1° ± 8°,
es ima ed om he dis ibu ion o bes quali y A and B solu ions
o 9 ea hquakes assuming model G, Supplemen a y Table 2).
The igh -la e al nodal plane, ending NW–SE (s ike 313° ± 4°),
is sub- e ical. The enhanced ca alog o momen enso s esem-
bles he gene al ocal mechanism p oposed in p e ious
wo ks27,28,31. Cen oid loca ions depend on he chosen model,
wi h he a e age loca ions closes o he injec ion pla o m o
model G. Cen oid dep hs a e age 3.4 ± 1.6 km. The la ges
magni udes o Mw 4.09 ± 0.04 a e ound o model G. Simila
esul s a e ob ained o o he eloci y models, wi h a ew cases
ha ing sligh ly g ea e dep hs and magni udes (Supplemen a y
Table 2).
Focal dep h es ima ion om dep h phases a egional and
eleseismic dis ances. We conduc ed an independen analysis o he
ocal dep h o he h ee la ges e en s using dep h phases obse ed a
egional and eleseismic dis ances on di e en a ays, using he
abede o ool (h ps://gi hub.com/He Muelle luedenscheid/abede o).
G een’s unc ions compu ed using local eloci y models a bo h he
sou ce and a ay loca ions (de ails in he Supplemen a y No e 8)
o m syn he ic beams ha also accoun o he momen enso s
es ima ed in his s udy. Measu ed ime delays be ween di ec P and
he nea -su ace eflec ed pP phase a e o he o de o 1.5–1.8 s, as
es ima ed a he GERES a ay38, Ge many (~1400 km dis ance). We
ob ained simila esul s using he ILAR, US a ay a eleseismic
Fig. 2 Loca ions based on wa e o m co ela ions and
S
-
P
me hod. a,c,dLoca ions based on wa e o m co ela ions in map iew and c oss-sec ion.
bLoca ions based on
S
-
P
me hod in c oss-sec ion. Ea hquake loca ions (ci cle, colo s deno ing o igin ime in a,c,d, and S-P di e en ial ime a s a ion
ALCX in b, acco ding o colo ba s in aand b, espec i ely) a e plo ed in map iew (a,b) and along AB (c) and CD (d) c oss-sec ions (p ofiles AB and CD
a e shown in a). Black ci cles in bdeno e hose e en s used in he wa e o m co ela ion loca ion (a), o which S-P ime es ima es a e a ailable. A
pen agon deno es he loca ion o he Cas o pla o m, double g ay solid lines ske ch he injec ion wells29.
ARTICLE NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1
4NATURE COMMUNICATIONS| (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1 | www.na u e.com/na u ecommunica ions
dis ances (~8000 km). These delays a e compa ed o he mo eou
and wa e o m pa e ns syn hesized om heo e ical seismog ams a
each a ay loca ion. Syn he ic wa e o ms and delaysa econsis en
wi h cen oid dep hs o 3–4km(Fig.4and Supplemen a y Fig. 14).
Rup u e di ec i i y. The up u e p ocess o he wo la ges
ea hquakes o he sequence, he Oc obe 1, 2013, 03:32 Mw 4.1
(EQ1) and Oc obe 2, 2013, 23:06 Mw 4.1 (EQ2) ea hquakes,
e ie ed by applying empi ical G een’s unc ion (EGF)
echniques39, show simila appa en sou ce ime unc ions
(ASTFs), wi h du a ions o 0.25–0.72 s (Fig. 5and Supplemen a y
Fig. 15). A clea azimu hal pa e n o he ASTFs is iden ified o
bo h e en s, om fi ing S wa e-based appa en du a ions
(Fig. 5). The esul ing sou ce pa ame e s (Supplemen a y Table 3)
show ha bo h ea hquakes sha e simila up u e cha ac e is ics.
We es ima e up u e leng hs o ~1.1 km, in ag eemen wi h he
size app op ia e o a c ack model (see “Discussion”), wi h a
up u e du a ion o ~0.5 s. The in insic ade-o be ween ise
ime and up u e eloci y40,41 leads o high unce ain ies;
assuming a ise ime o 0.15–0.25 s, he es ima ed up u e eloci y
would be ~2.7 km/s, simila o he shea wa e eloci y a 3 km
dep h in model G. Rup u es om bo h ea hquakes a e ound o
be asymme ic bila e al, wi h mos o he up u e (66 and 75% o
he o e all up u e leng h o he wo ea hquakes, espec i ely)
p opaga ing owa d NNE om he ini ial hypocen e . These
esul s be e co ela e wi h he N42°E-o ien ed aul , dipping
SW, which is esol ed independen ly om he band o seismici y
and he momen enso solu ions, and hey a e inconsis en wi h
a up u e along he al e na e plane o he momen enso , which
is sub- e ical and s ikes o NW.
Wa e o m-based classifica ion. We employ ne wo k-based e en
simila i y clus e ing42 o iden i y e en s wi h high wa e o m
simila i y a mul iple s a ions, which implies high simila i y in
hei loca ions and ocal mechanisms. The subse o 51 eloca ed
e en s, wi h magni udes >2.0, shows e y simila wa e o ms
ac oss he s a ion ne wo k. Su ace wa es a e highly simila bu
only isible o la ge magni ude e en s, so we analyzed P and S
Fig. 3 O e iew o he momen enso in e sion esul s. a O e lay o de ia o ic momen enso solu ions ob ained o he Oc obe 2, 2013, 23:06 UTC
ea hquake (Supplemen a y Table 2) ou o he boo s ap app oach (g ay ocal sphe es) and bes solu ion using all da a ( ed hick line ocal sphe e).
bP obabili y densi y unc ions o momen magni ude (Mw) and dep h o he same e en ( ed do ed and solid line indica e bes and mean solu ions, da k
o ligh ed backg ound confidence le els o 68, 90, and 95%). cExamples o wa e o m fi ( ed and black lines deno e syn he ic and obse ed displacemen
wa e o ms, espec i ely) a selec ed s a ions o he same e en (s a ion name, azimu h, dis ance, weigh , and maximal displacemen a e epo ed).
dO e lay o momen enso solu ions o 9 s udied e en s (quali y A and B, Mw 3.4–4.1), assuming h ee di e en eloci y models: model I (blue),
G (pu ple), and V ( ed ocal sphe e); hick black lines deno e he ocal sphe e o he cumula i e momen enso .
NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1 ARTICLE
NATURE COMMUNICATIONS | (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1| www.na u e.com/na u ecommunica ions 5

wa e o ms (Fig. 6). The ne wo k-based clus e ing iden ifies six
compac , spa ially sepa a ed clus e s (Fig. 6and Supplemen a y
Fig. 16) wi h wa e o ms changing g adually along he lineamen .
A supplemen a y analysis o P/S and Rayleigh/Lo e (R/L)
ampli ude a ios e eals smoo h ends along he NE s iking
lineamen (Supplemen a y Fig. 17), which can be modeled using
syn he ic seismog ams. The esul s indica e ha he loca ion
change is he eason o he obse ed wa e o m a ia ions among
neighbo ing clus e s, no a ying ocal mechanisms. All o hese
esul s imply ha he aul geome y emains s able o e he
cou se o he sequence, wi h seismici y occu ing in neighbo ing
clus e s a di e en imes, suppo ing he p og essi e ailu e o
di e en pa ches along one common, plana s uc u e. Only
clus e s a he NE and SW bounda ies o he cloud show some
di e ences, which may be a ibu ed o he la ge sepa a ion o
hese e en s, o possibly changes in aul geome y and local
he e ogenei y a he e mina ions o he ac i e aul segmen .
Discussion
The cu en deba e abou seismological esul s a Cas o p inci-
pally e ol es a ound he hypocen al loca ions, hei spa ial
dis ibu ion, and he dep h ange o he seismici y. Absolu e
loca ions a e indeed poo ly esol ed, as demons a ed by la ge
changes in loca ion when using di e en eloci y models28.
Absolu e loca ion unce ain ies o a single e en 28 show a simila
pa e n, wi h poo esolu ion in he NW–SE di ec ion and a
ade-o wi h he hypocen al dep h. Such loca ion unce ain ies
a e a ibu ed o he s ongly une en ne wo k geome y, in
combina ion wi h la e al s uc u al he e ogenei ies in he Valen-
cia T ough43. Mul i-e en s ela i e loca ion me hods, based on
di e en ial phase a i al imes om phase picks and/o wa e o m
c oss-co ela ions, can imp o e he loca ion esolu ion and
sha pen seismici y pa e ns44–46. Ou esul s, and specifically
hose o he c oss-co ela ion-based eloca ion, esol e how
seismici y is dis ibu ed along an elonga ed olume, ex ending
sub-pa allel o he coas along a N42°E di ec ion (Fig. 2and
Supplemen a y Fig. 8). The o ien a ion, shape, and ex en o he
imaged ac i e egion ma ch he ese oi ’s ou line29,47. The
hickness o he laye a ec ed by seismici y is confined o 2 km
only, wi h good esolu ion. As ea hquakes did no occu abo e o
below his laye , and epicen al unce ain ies a e well below 1 km,
he dip o he aul ac i a ed ac oss he laye canno be esol ed
om seismici y. Shallow hypocen al dep hs, wi hin he uppe
c us , a e ound in o he s udies27,28,32, bu hei absolu e alues
a e inconsis en . The delay imes be ween di ec and su ace
eflec ed sub- e ical wa es is only 1.5–1.8 s. We model bo h he
delay and he wa e o ms (model G) o pP e sus P phases o he
h ee la ges ea hquakes o he Cas o sequence using indepen-
den da a and can cons ain he absolu e dep hs a 3–4 km. The
cen oid dep hs om ou p obabilis ic momen enso in e sions
a e also consis en be ween 0 and 4.7 km (Supplemen a y
Table 2). Deepe sou ces a 6–8 km ha e been ecen ly sugges ed,
based on he modeling o sho pe iod c us al e e be a ion in a
shallow, low eloci y laye 32. Such an app oach also has he
po en ial o accu a e dep h es ima e; howe e , dep h
Fig. 4 Summa y o seismic dep h es ima ions o he Oc obe 1, 2013, 03:32:44 UTC, ea hquake based on seismic a ay beam modeling.
a–cCompa ison o he obse ed ( hick blue line) and syn he ic ( hin black lines) beams a he GERES a ays (Ge many). Syn he ics a e compu ed o h ee
eloci y models (models G, I, and V in a–c, espec i ely), he p e e ed ocal mechanism de i ed using model G (s ike 41°, dip 56°, ake 4°), and dep hs
be ween 1 and 7 km. Obse ed beams a e plo ed a he dep hs o 3–4 km, o which wa e o ms and P and pP pulses a e bes modeled. Bo h obse ed and
syn he ic beams ep esen no malized displacemen s, bandpass fil e ed in he ange 0.25–1.80 Hz. dTheo e ical sou ce dep h as a unc ion o he pP delay
o he h ee models a he sou ce (model I in blue, model G in pu ple, model V in ed): gi en he pP-P delay, and assumed a eloci y model, we can
es ima e he dep h. We conside delays o 1.5–1.8 s, as picked om beams a GERES, Ge many and ILAR, US a ays, and an a e age o 1.65 s. Th ee
scena ios a e shown (dashed lines), leading o a dep h o 2.4 ( ed line), 3.8 (pu ple line), and 5.4 (blue line) km.
ARTICLE NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1
6NATURE COMMUNICATIONS| (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1 | www.na u e.com/na u ecommunica ions
unce ain ies we e no epo ed he e, and i is unclea o which
ex en his ype o modeling is a ec ed by he adop ion o a
simplified 1D model. Hypocen e s a such dep hs would imply a
delay o he pP phase o 2.6–3.3 s, when assuming he local model
G, which a e oo la ge compa ed o he delays o 1.5–1.8 s
obse ed o he la ges e en s om egional and eleseismic a ay
analysis. Thus, we conclude ha ea hquakes we e shallowe ,
mos likely in he ange 3.0–4.0 km bu no exceeding 5.3 km
(Fig. 4d). Ou new Bayesian dep h es ima es imply ha ea h-
quake oci would be only ~1–2 km deepe han he injec ion poin
a ~2 km dep h a he op o he gas s o age laye . As o he
momen enso s, ou esul s ag ee wi h p e ious findings27,31,
which a e cha ac e ized by p edominan ly s ike-slip mechan-
isms, wi h aul planes o ien ed N42°E and N47°W, espec i ely.
We p o ide explici unce ain y es ima es o all esol ed sou ce
pa ame e s, which accoun o bo h da a- and model- ela ed
unce ain ies. This allows a obus join in e p e a ion o he
esul s om di e en echniques, aking in o accoun he eso-
lu ion o each me hod.
Ou esul s ha e impo an implica ions o he iden ifica ion
o he ac i a ed aul (s). P e iously, i has been hypo hesized ha
he seismici y a Cas o migh ha e occu ed along mapped aul s,
such as he la ge N25°E-o ien ed48,49 Ampos a aul 29 and he
Mon sia aul sys em a he ese oi si e, which has mul iple sub-
pa allel aul s o ien ed NW27. Al e na i ely, unknown aul s wi h
he wo possible o ien a ions iden ified by he aul planes o he
s ike-slip ocal mechanisms ha e been in oked: ei he a NE
s iking aul , gen ly s eeping owa d SE27, o NW ending
aul (s)28. Ou esul s, bo h conside ing he spa ial elonga ion o
ela i e loca ions, simila i y o momen enso s, he high co e-
la ion o wa e o ms and wa e o m a ibu es along he end, and
he up u e di ec i i y o he la ges e en s, sugges he ac i a ion
o a single NE–SW-o ien ed aul . Ou esul s also exclude he
ac i a ion o he shallow Mon sia aul sys em o an unknown
deepe aul wi h simila NW–SE o ien a ion28, below he
Ampos a aul . Simul aneous ac i a ion o he whole Mon sia
sys em, which could po en ially explain he spa ial dis ibu ion o
seismici y27, can be ejec ed as incompa ible wi h he up u e
(a.1) (b.1)
(a.2) (b.2)
Fig. 5 Rup u e di ec i i y in e ed o he wo la ges ea hquakes o he sequence. a Mw 4.1 Oc obe 1, 2013, 03:32 UTC and bMw 4.1 Oc obe 2, 2013,
23:06 UTC ea hquakes a he Cas o pla o m. a.1, b.1. The du a ion o appa en sou ce ime unc ions a e plo ed so ed by azimu h, showing appa en
du a ions (ci cles) and momen a es (g ay a ea) a each s a ion (see labels), and he asymme ic bila e al up u e model om he di ec i i y analysis
(dashed lines). a.2, b.2. Appa en du a ions as in a.1, b.1 (ci cles) along wi h he syn he ic p edic ions o he in e ed model (dashed lines) show he
p edominan up u e di ec ion in a pola plo , whe e appa en sou ce du a ions a e plo ed along he adial axis (axis icks co espond o 0.2 s). The
unce ain y in he di ec ion o he up u e di ec ion ( ange o angles deno ed wi h blue and ed a eas) a e es ima ed om he esiduals.
NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1 ARTICLE
NATURE COMMUNICATIONS | (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1| www.na u e.com/na u ecommunica ions 7
di ec i i y esul . The spa ial dis ibu ion o epicen al loca ion
esembles qui e well he o ien a ion o he Ampos a aul , which
o ms he oo o he ese oi , and, as so, o he ese oi
geome y. Howe e , he Ampos a aul dips o he NW, wi h a
~60° dip angle a shallow dep hs48,49, ~40° a he ese oi le el,
and ~30° below i 29, which is inconsis en wi h p e iously p o-
posed ocal mechanisms27,29,31, as well as wi h all ou bes quali y
(A and B) momen enso solu ions. We conclude ha he
Ampos a aul did no pa icipa e in he seismic sequence, con-
fi ming p e ious esul s27,28.
Only one up u e scena io emains consis en wi h all esul s:
ac i a ion o a p e iously unde ec ed aul an i he ic o he
Ampos a aul , wi h a simila s ike, bu dipping owa d he SE.
This hypo hesis has been p e iously p oposed27 and can now be
efined. The la e al dis ibu ion o he seismici y, which esembles
he end o he Ampos a aul , and he shallow dep h o he
hypocen e s, jus 1–2 km below he injec ion, poin s o a aul
wi hin o a he below he ese oi , dipping SE, and bounded o
he NW by he Ampos a aul ; such aul o ien a ion and
mechanism ype a e consis en wi h he local s ess o ien a ion27.
The spa ial associa ion o he hypocen e s and injec ion poin s
and he clea empo al co ela ion among injec ion ope a ions
and seismici y make a clea case o induced seismici y50, which
indeed has ne e been deba ed.
Joining he in o ma ion o ela i e loca ions and empla e
ma ching, we can econs uc he spa io empo al e olu ion o
seismici y (Fig. 7). No e ha in Fig. 7we cons ain absolu e
loca ions in he way ha ea lies e en s ma ch he injec ion
loca ion. We can iden i y h ee phases. The fi s begins on Sep-
embe 2, oge he wi h he s a o he injec ion, and con inues
un il he injec ion s ops. The second phase begins on Sep embe
17 and con inues un il he end o Sep embe and is associa ed
wi h he e olu ion o po e p essu e and seismici y. These wo
phases a e cha ac e ized by di e en b- alues, which d ops om
~1.0 (la ge p edominance o small ea hquakes) o 0.8 (highe
a e o la ge magni ude e en s). The hi d phase, om he end o
Sep embe o ea ly Oc obe , includes all o he la ges e en s o
he sequence. O e all, wa e o ms o ea hquakes occu ing in
di e en phases a e e y simila (Fig. 6); mino wa e o m
changes can be a ibu ed o sligh ly changing loca ions.
Du ing bo h phases 1 and 2, seismici y mig a es unila e ally
owa d he SW (Fig. 7c), a an a e age eloci y o ~180 m/day,
consis en wi h a unila e al p essu e di usion model51. In he
hypo hesis ha he di usion is confined o a linea s uc u e, he
empo al mig a ion o he seismici y on can be econs uc ed
(Fig. 7b) assuming a di usi i y o app oxima ely 0.5 m2/s51.I is
no ewo hy ha he gas is injec ed close o he pla o m loca ion
and o he SW o i h ough 4 wells spanning abou 600 m, whe e
he ese oi oo is shallowes 29,47. The ho izon al dis ibu ion o
epicen e s esembles he ese oi shape29,47. The empo al
mig a ion o he seismici y can be explained as ollows (Fig. 8).
The gas was injec ed in o he highes a ea o a pen oo -shaped
ese oi , elonga ed NNE–SSW, bounded on he NW by he
Ampos a Faul and becoming p og essi ely deepe owa d SSW
and NNE47. I has been in e p e ed ha he no he n end o he
Ampos a aul e mina es a ~2 km dep h in o Te ia y laye s,
sugges ing ha his po ion o he aul is ec onically inac i e29.
Due o he sligh inclina ion o he idge egion o he sealed
ese oi in he SW and NE di ec ion, he gas sa u a ed olume
would slowly ex end o dep h du ing phase 1, o occupy a na ow
olume elonga ed NNE–SSW29,47. As he injec ion p oceeded,
mode a e magni ude seismici y g adually mig a ed o he SW o
he injec ion poin bu no owa d he NE (Fig. 7). The lack o
seismici y o he NE sugges s di e ences in he pe meabili y o in
aul (s) p ope ies, such as a aul bending o o se . The seismi-
ci y mig a ion con inued du ing phase 2, ollowing he end o
injec ion, p obably eflec ing ongoing po e p essu e
edis ibu ion.
The hi d phase began ab up ly by Sep embe 28–29, 2013,
cha ac e ized by a as (~1 km/day) backwa d p opaga ion o he
seismici y om he ex eme SW end owa d he injec ion poin ,
accompanied by all o he la ges e en s o he sequence,
including he h ee M 4+ea hquakes (Fig. 7). Phase 3 does no
ma k a subs an ial change in he b- alue (0.8). In de ail, he ini ial
ac i i y appea ed a a ew spo s loca ed on he edges o o be ween
he up u e a eas o he la e , la ge e en s (Fig. 8). Es ima ed
up u e a eas o la ge e en s, excep he wo la ges ones, a e
gene ally non-o e lapping. The assump ions he e a e a cons an
s ess d op (3 MPa), which leads o up u e sizes consis en wi h
leng hs up o 1.0–1.2 km in e ed om ASTFs, and ha hey a e
on a single plane. Howe e , as mig a ion and simila wa e o ms
and ocal mechanisms o ea hquakes sugges a single aul
ailing p og essi ely, wi h homogeneous geome y, poin ing o a
SE dipping, shallow (~3 km) aul plane. The NE unila e al
di ec i i y esol ed o he la ges ea hquakes in ea ly Oc obe
suppo s he spa ial e olu ion o he up u e p ocess om SW o
NE. The las e en s o he sequence, loca ed a he NE ip o he
ac i a ed aul , a e mos likely a e shocks o he la ges ea ly
Oc obe e en s and a e loca ed in he o wa d di ec i i y di ec-
ion. O e all, he seismici y e olu ion ma ks a kind o ound- ip,
whe e a slow di usion-d i en mig a ion away om he injec ion
wells is ollowed by a sudden e u n by he seismic igge ing o
unb oken aspe i ies.
Based on he space– ime pa e ns, se e al scena ios a e possible
o explain he complex up u e mig a ion. One is simply po e
p essu e di usion, as he ea lies ac i i y occu s a he No h,
whe e he pla o m is loca ed, and close o he injec ion well
bo om. Seismici y mig a es o he SW a a eloci y o abou 180
m/day. This is a ypical ange o po e p essu e di usion51 and
may ep esen he mo emen o he p essu e on , causing c i-
ically s essed a eas o up u e fi s , as he p essu e inc eased.
A e he injec ion s ops, he SW mig a ion con inues as po e
Fig. 6 Wa e o m simila i y o eloca ed e en s. P and S wa e o ms (he e
shown o s a ion EMOS) a e e y simila , e en when fil e ed wi h a
passband o 1–8 Hz. The signals change g adually along he SW–NE
ending lineamen (equi alen o p ofile CD in Fig. 2a). By applying a
wa e o m-based e en simila i y clus e ing algo i hm, we find six spa ially
sepa a ed clus e s o e en s (colo -coded, clus e numbe ing based on
o igin ime o he fi s clus e ed e en ); no e ha he clus e ing is no only
based on he wa e o ms o he exempla y s a ion EMOS, shown he e, bu
also on se e al o he s a ions. Wa e o ms a e scaled by he maximum
ampli ude wi hin he ime window (bold) and aligned by he c oss-
co ela ion ime shi o emphasize wa e o m simila i y.
ARTICLE NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1
8NATURE COMMUNICATIONS| (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1 | www.na u e.com/na u ecommunica ions
p essu e g adien s a e s ill p esen o u he d i e he po e
p essu e on in he ese oi . The e e se pa e n in phase 3 is
di e en . The occu ence o he la ges e en s p og essi ely
up u ing p e iously unb oken pa ches om SW o NE. The
mig a ion eloci y is abou fi e imes as e , in he ange o 1 km/
day. Ou analysis indica es ha la ge aspe i ies successi ely
up u e in la ge magni ude ea hquakes. The aspe i ies may ha e
been loaded o mo e c i ical s ess du ing phases 1 and 2, whe e
s ess in be ween neighbo ing aspe i ies was ans e ed. A pos-
sible cause o he apid backwa d mig a ion o he ea hquakes
could be he g adien o e ical s ess de eloped by he con-
inuous exchange o wa e by gas in he uppe mos pa o he
ese oi . Because o he inclina ion o he cap ock, which dips
gen ly o he SW, he e ical gas column a he injec ion poin is
highe han a he sou hwes e n end o he injec ed gas laye .
No mal s ess is smalle a he injec ion poin han a he
sou hwes e n ip. A up u e p og ession owa d highe Coulomb
s ess change has been sugges ed as he inducing mechanism o
he seismici y52. The sequen ial up u ing o aspe i ies could be
addi ionally a o ed by a p og essi ely educed aul ic ion, as
wa e displaced by he injec ed gas would flow away in o ac u es
and pe meable pa hs, such as he ac i a ed aul . Faul al ing,
in ol ing p essu e changes and uns eady fluid mig a ion along
ac u ed egions and aul s53, can also pa ially explain he
al e na ion o seismici y bu s s and quiescence pe iods. This
could also imply he occu ence o aseismic slip, o which we
ha e no di ec e idence so a . The clus e ing analysis shows ha
weak ea hquakes in phases 1 and 2 ha e he same mechanism as
he la ge ones in phase 3 (Fig. 6), and occu ed along he same
SE dipping s uc u e. I aseismic slip on he Ampos a aul
accompanied he ini ial phases o seismici y, as has been ecen ly
sugges ed34, i did so comple ely aseismically.
Induced seismici y is a complex issue o public in e es , whe e
he exploi a ion o na u al esou ces needs o be pe o med in a
sus ainable way, while also ca e ully assessing he isk. Wi hin he
esea ch field o induced seismici y, scien ific ou comes a e
di ec ly ele an o egula o s, indus y, and socie y. In his
amewo k, scien ific esul s need o be explained wi h a igo ous,
non- echnical and b oadly unde s andable language6. A scien ific
discussion is desi able and in insic o science, bu he commu-
nica ion o incomple e and con adic o y esul s may impede
explana ion o scien ificfindings o a b oad communi y. A pa -
icula p oblem is a lack o communica ion abou unce ain ies,
bo h epis emic and alea o y, which limi ou scien ific answe s o
socie al ques ions. This was p obably he case o he Cas o
p ojec , whe e di e en scien ific publica ions claimed appa en ly
obus answe s o ques ions ha emained deba ed.
Besides p o iding new analysis and esul s on he p ocesses
con olling he Cas o seismic sequence, he p esen wo k aims o
ha monize p e ious esul s and hei appa en disc epancies.
Using ad anced, wa e o m-based seismological me hods and
combining local and egional seismic da a, plus seismic a ays a
la ge dis ances, allows us o econs uc he up u e p ocess o
mode a e ea hquakes o sho e and hus o shed ligh on he
Cas o seismic sequence, p o ing ha his is possible despi e he
lack o a dense local ne wo k. Seismic moni o ing egula ions
Fig. 7 Timeline o injec ion and seismici y. Tempo al e olu ion o amagni udes, bepicen al dis ances om he injec ion well, cepicen al loca ion
p ojec ions along p ofile AB, as in Fig. 2, and dcumula i e momen elease and cumula i e numbe o ea hquakes o he ime pe iod om 2013-09-02
00:00 un il 2013-10-06 00:00 UTC. The onse o h ee main phases a e ma ked by indigo e ical ba s, deno ing injec ion s a (
1
, 2013-09-02), injec ion
s op (
2
, 2013-09-17), and app oxima e fi s igge ing o he aul hos ing la ges ea hquakes (
3
, 2013-09-28, 16:00). The colo scale used in a–c e e s o
e en la i udes ( op igh ). A dashed line deno es he di usion cu e om he injec ion poin a ime
1
, assuming a di usi i y o 0.5 m2/s, which explains
he mig a ion o seismici y du ing phases 1–2, and a dashed-do ed line he as e ea hquake igge ing backwa d p opaga ion wi h a eloci y o ~ 1 km/
day du ing phase 3. No e ha plo ed seismici y co esponds o 964 e en s: 51 accu a ely eloca ed e en s (Fig. 2a) and hose de ec ed by empla e
ma ching using he 51 e en s as empla es; we a ibu ed o de ec ed e en s he same loca ions as hei empla es.
NATURE COMMUNICATIONS | h ps://doi.o g/10.1038/s41467-021-24949-1 ARTICLE
NATURE COMMUNICATIONS | (2021) 12:4783 | h ps://doi.o g/10.1038/s41467-021-24949-1| www.na u e.com/na u ecommunica ions 9