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/lp/de-gruyter/the-temporal-evolution-of-seismicity-and-variability-of-b-values-along-SmXfJzsgNz
- Publisher
- de Gruyter
- Copyright
- © 2023 Asma Nasir et al., published by Sciendo
- eISSN
- 2072-7151
- DOI
- 10.17738/ajes.2023.0001
- Publisher site
- See Article on Publisher Site
Abstract
A The bst 2200 ract m thick Cretaceous units of well Gänserndorf UeT3 have been biostratigraphically analyzed based on cuttings from The V 3210 ienm na B to as 5140 in Tra m. nsfT ehe r Fa deposits ult System ( from VBthe TFS) i Tir s t olic he m Glinz ost ac endor tive f f S ayncline ult syste (a m i par n tt hof e re the giobur n bied etwNor een t ther he E n a C salcar tern A eous lps, t A h lps) e wcan estebe rn C lar ar gely pathc ia or nrs ela an td t ed h with e Pathe nnoL no ia w n er Ba Gsosau in. Th Subg e spa rt oup ial aof nd the tem Grünbach poral distS riyncline bution o . A f n ea er xtc h eption quakes is athe long basal the f unit ault , s which ystem has shono ws a equiv hete alen roge t n in eo the us p Grünbach attern inclS u yncline ding a l . o Tn his g-tlo im w e d er e unit cay o is f s subdivided eismicity at in th to e n a o non-mar rthern pine art o lo f t w h er e V and BTFa S, lar wh gely ich w mar as i ine nteupper rpreted t par o r t. e No sulage t froc m a l onstr on ain g a ts ftar ere sh ao vc ailable k sequf e or ncthe e sulo bs w eer qupar ent t t, o t wher he 1 eas 906 D the oupper brá Vopar da e t a has rthq a u possible ake (M=5 age .7). Irn t ange his p fra om per w middle e invT eur stonian igate it f o o C th oniacian. er segme Fn or ts o this f tunit he V , B which TFS diis sp documen lay similatr l ed on for g-the term d first ec time lines o from f se the ismGlinz icity t endor hat m f iS gyncline ht indic , a w te e l pr on opose Glinz g aftershoc endor k sequ f F en or cma es ftion as new lithostr ollowing strong, ye a t u tig n rr aphic t ecorde er d, e m. arthquakes in historical times. TIhe n oGlinz rder t endor o ana f lys Fm. e tis he d over ist lain ribub tiy on o the f s Grünbach eismicity, t Fm., he V which BTFS i is s d initv er id cala ed i tn ed to a by rb a itthick rary s unit egmof enc tonglomer s of about 5 ates 0 k . T m l hese eng ar th e e in ac ter hpr . Tet he ed seas gm equiv ents alen are ts chof osthe en tDr o o eist ver ett lap en eac Conglomer h other to at ae voi Mb d . m The issi calcar ng inf eous orma nannof tion fro ossils m neof igh these bouri units ng se suggest gments d a la ue test to a Sr an bit tonian rarily ste o le ear cte ly d s Ce ampanian gment boage und . a Non-mar ries. For e ine ach s conditions egment w pre a evailed nalyse t dur hing e tedeposition mporal evoof lutthe ion o Grünbach f seismicit Fy a m., nbut d camar lcula ine te t incursions he parame ar te e rindica s of tht e c ed ofr or res par pots nd of inthe g Gu Dr te eist nbett erg en -RiC ch onglomer ter (GR) ra et la e tMb ion. . The top of the Grünbach Fm. is formed by an about 50-m-thick The temp unit oral s of ec is oal mi, cr it ich y pin att Char erns r ace eae vealoogonia, ed from t which, he segtm ogether ents cowith verinthe g thDr e D eist ob ett rá V en od conglomer a area con afties rm t ser hv e p e as rot mar ractker ed a la fy te er rsfh or oc ck s orr eela qution ence f with ollothe winout g th cr e 1 ops 90in 6 e the arth Grünbach quake. All b Syncline ut one o . Tf t he hGrünbach e other seg Fm m. eis nto s d ver o n lain ot s bh y o mar w te ls m and porsilt al c y hshales anges o of f s the eism Piesting icity coF m m. pafror abwhich le to th a e l la o tn e g C -ampanian term Dobrá V and od M a a aastr fter ich sho tian ck sage equis endocumen ce. Seismi tc ed ity p . Ma ar tt ine ernc sonditions , however, i pr nedomina clude sho ted rt- t dur erm O ing m this oriin -ty ter pe a valf. tT ehe rsho topmost cks follounit wing m in w oell der G aänser te ear ndor thqu f aUe kes s T3 u is ch a over s t thrust he 20ed 00 E on brthe eichM sd aastr orf e ich art tian hquP ak iesting e (M=4 F.m. 8). T and he s repr egm esen ent c ts oC ve ampanian ring the SW t sandst ip oones f the V and BTFS r conglomer evealed a 2 ates 00 y of ea the rs lo Grünbach ng graduaF l d m. ecT re his ase o Gänser f the l ndor arge f sT t o hrust bserv is ed m detec agtn ed ituand des s biostr tartin a g w tigrit aphically c h the 1794 L onstr eob ained f en (M= or the first time 4.7) earthquak .e. The 1794 event is the oldest earthquake listed in the catalogue for the region under consideration. It therefore remains open if the recorded decay of seismicity results from the 1794 event, or a stronger earthquake before that time. The latter is corroborated by the low magnitude of the 1794 earthquake which would typically not be considered to cause long aftershock sequences. 1. G In R a tro - a duc nd b tion -values, calculated for the individual segments, vary significantly along the VBTFS. Values range from 0.47 to 0 T.he 86 ( Glinz b-vaendor lues) a f n Sd 0 yncline .81 to 2 is .an 54 ( about a-valu70 es)k , r m eslong pectiand vely. D upata shorw a s egion igin nifSlo icav nak t p ia os in itithe ve c W or est reler atn ioC n o arpa f a- a thians nd b(P -vlöchinger alues and a c to oi 8 n kc m ide wide nce o , f t SW h-NE e low tr e ending st b-valu struc es w tur ith f e a in ult s the egV m ienna ents with laet rge s al., e1961; ismic s Slchlag ip defin ict it w s a eit nd v and erW y l ag ow s reich, eism 1992; icity i W n t ag hr e l eich ast a Basin, pproxwhich imately 3 was 00 y br eiefly ars. Tdescr hese p ibed arts o bf t y h W e V essely BTFS w (1984; ere previou and sly iM ntarschalko erpreted a , 1995; s “lockHof ed” f er au et lt s al e ., g2013) ments(F , w ig h . i1). ch h These ave a s 1992) ignific and ant p Hamilt otenton ial tet o ral el. e(1990). ase futu It re s is t par ron t g e of a the rthq Tu ira olic kes, in spit oc e o cur f t renc he fes act t arh e alinked t histor b ic y asubsur l and in fac stre um out en cr ta ops lly r , e such corde as d s nappe eismicsy ity i stem s veand ry lo rw eaches . We fifr nd t om hH is i imber nterp g re in ta A ti ustr on c ia oin rro the borated b the y thGießhübl e low b-va and lues t Glinz hat s endor ugge f ssynclines t high diffand erenthe tial s P tr rott esses es f SE or t to he Jak seubo faulv t, se Gajar gme y n and ts. Záhorská Ves in Slovakia in the and Studienka Gosau (Wessely, 1992; 1993; 2006; Hofer et NE (Wessely et al., 1993; Ralbovský and Ostrolucký, 1996) al., 2013 and literature therein). According to Wessely et (Fig. 1). The structure is completely covered by Neogene al. (1993), the Glinzendorf Syncline might continue into deposits. the Grünbach Syncline in the SE, but this was doubted by The Glinzendorf Syncline is part of a series of surface Hofer et al. (2013). and subsurface outcrops of the Cretaceous to Paleogene Although the Glinzendorf Syncline is covered by about Gosau Group, which range from the Gießhübl and 3000 m of Neogene sediments, several wells have reached Grünbach-Neue Welt outcrops along the southwestern and drilled the Cretaceous units. Wessely (1984; 1993; margin of the Vienna Basin to the Brezová and Myjava 2006), Wessely et al. (1993) and Hofer et al. (2013) mention 1 1 Open Access. © 2022 Mathias Harzhauser, Stjepan Ćorić, Matthias Kranner, Michael König, Ales Vrsic , published by Sciendo. This work is licensed under the CC BY 4.0 License. The temporal evolution of seismicity and variability of b-values along the Vienna Basin Transfer Fault System 1. Introduction of the VBTFS in the Mur-Mürz Valley, the Vienna Basin and The Vienna Basin, a Miocene pull-apart basin between the adjacent Brezovské Karpaty shows a peculiar pattern the eastern margin of the Alps and the Carpathian fold- with concentrated activity in the south of the Vienna thrust belt, is one of the most seismically active areas in Basin and an apparent lack of earthquakes in its central intraplate Europe. The main active fault system is referred and northern part (Fig. 1; Hinsch and Decker, 2003; 2011). to as the Vienna Basin Transfer Fault System (VBTFS), a Here, we analyze the historical and instrumental records major, about 380 km long left-lateral strike-slip fault sys- of earthquakes in this region in order to better under- tem, that starts from the central Eastern Alps (Mur-Mürz stand the causes for this heterogeneous distribution. Fault, Gutdeutsch and Aric, 1988; Brückl et al., 2010), The spatiotemporal correlation and energy release crosses the entire pull-apart basin from SW to NE (e.g., of earthquake occurrence is complex, but not random Royden, 1985; Wessely, 1988; Kröll and Wessely, 1993; (Hainzl et al., 2003). The Gutenberg–Richter law states Decker, 1996; Decker et al., 2005; Beidinger and Decker, that the number of earthquakes with magnitude great- 2011) and proceeds into the Dobrá Voda area of the Car- er or equal to a certain magnitude occurring in a given pathian fold-thrust belt. The fault system is further traced time decreases logarithmically with increasing magni- into the area around Žilina in the Váh Valley (Schenková tude (Gutenberg and Richter, 1942). The gradient of the et al., 1995; Gutdeutsch and Aric, 1988; Decker and Peres- Gutenberg-Richter (GR) relation, the so-called b-value, is son, 1996; Sefara et al., 1998). commonly close to 1 in seismically active regions (Scholz, The spatial distribution of earthquakes along the part 2002). However, the b-value varies with respect to time, Figure 1: (a) Distribution of earthquakes along the Vienna Basin Transfer Fault System (VBTFS). Blue circles show earthquakes from the Austrian earth- quake catalogue (ZAMG, 2020), brown circles are earthquakes listed in the ACORN (2004) catalogue. Black circles denote earthquake with M≥5. Green polygon indicates the extent VBTFS. Note the uneven distribution of seismicity with very low seismicity along the Lassee (LS) and Zohor (ZS) segments in the Vienna Basin as well as close to the SW termination of the Mur-Mürz-Fault (MM; see inset (b for location of the named segments). White stippled line marks the outline of the Miocene pull-apart basin. (b) Overview maps showing the extent of the VBTFS and locations of fault segments mentioned in the text. 2 Asma NASIR et al. space, and magnitude ranges, since it is related to earth- 7-8 Ma; Peresson and Decker, 1997). Pull-apart basin quake rupture dynamics, or seismic source characteristics formation was associated with about 30 km of sinistral (Senatorski, 2019). In addition, the b-value is observed displacement along the VBTFS, which corresponds to an to decrease with stress (Scholz, 1968; 2015). Therefore, average Miocene sinistral slip rate of 4 mm/a (Linzer et al., Schorlemmer and Wiemer (2004) proposed that spatially 2002; Decker et al., 2005). Since then, tectonic movement varying b-values can be used to forecast future seismicity slowed down to a moderate level of 1-2 mm/a as deter- more accurately than the approach in which one assumes mined by GPS geodesy (Grenerczy et al., 2000; 2005; see a constant b-value equal to the average regional value. also Möller et al., 2011, for discussion). Umnig et al. (2015) Another possible cause of the documented heteroge- reported a slip rate of 0.35-0.43 mm/a based on a 4-years neous earthquake activity might be the occurrence of data series from a local GNSS network. Geological data long aftershock sequences. Aftershock activity is gener- derived from the age and thickness of Quaternary sedi- ally triggered by a strong earthquake. In the aftermath ments that accumulated in a Quaternary pull-apart basin of a large earthquake, this aftershock activity leads to a (Mitterndorf Basin) indicate a slip rate between 1.5 and local increase of seismic activity, which later on decays 2.6 mm/a (Decker et al., 2005). back to a lower level labeled as ‘normal’ background seis- Two studies have been carried out to evaluate the seis- micity (Stein and Liu, 2009). The length of an aftershock mic energy release of the fault and compare it to geo- sequence can vary from a few months at plate margins logically and geodetically derived slip rates, considering to several years and even decades and centuries in in- both, pre-instrumental and instrumental earthquake traplate regions (Stein and Liu, 2009), depending not only data (Hinsch and Decker, 2003; 2011). The cited studies re- on the magnitude of the causing earthquake, but appar- vealed significant seismic slip deficits for the VBTFS as a ently also on the regional level of background seismicity whole, and several fault segments in particular. The larg- and the fault-loading rate. At the northeastern end of the est slip deficits were recorded for the Lassee and Zohor Vienna Basin, the 1906 Dobrá Voda mainshock (M=5.7) Segments (see Fig. 1 (b) for location) and the SW part of has caused elevated seismicity in its near vicinity which the Mur-Mürz Fault which released virtually no seismic is still visible today (Nasir et al., 2020). Therefore, we will energy in historical times. These parts of the VBTFS were check whether the seismic activity on other parts of the interpreted as currently locked segments. VBTFS can be explained as the result of a (pre-)historic Earthquakes of magnitude 5 tend to happen on av- strong mainshock. erage every 25 years along the VBTFS in the last about In this publication, we will address the questions raised 250 years (Fig. 1). The strongest recorded events are the above using a newly compiled earthquake catalogue us- earthquakes of 1267 Kindberg (I =VIII/M=5.4) and 1907 ing the data by ZAMG (2020) and ACORN (2004) to cover Dobra Voda (I =VII-IX/M=5.7). These magnitudes are well the whole extent of the VBTFS. As the major part of the below the magnitude of Maximum Credible Earthquakes 754-years-long earthquake catalogue is based on inten- (MCE) estimated from the length and fault area of geo- sity, we apply an empirical intensity-magnitude conver- logically defined fault segments by knick points of the sion formula, which is derived from the catalogues. strike-slip fault and branchlines of normal faults (Decker and Hintersberger, 2011; Hinsch and Decker, 2011). The latter reveal MCE magnitudes between about M=6.0-6.8 2. Tectonic setting for the different fault segments. The MCE estimates are The Vienna Basin Transform Fault is an active fault sys- supported by paleoseismological evidence (Hintersberg- tem extending over a distance of some 380 km from the er et al., 2014). Eastern Alps through the Vienna Basin into the West Car- pathians. Active sinistral movement is indicated by mod- erate seismic activity in a NE striking zone paralleling 3. Earthquake data the fault system, focal plane solutions and recent stress Earthquake catalogues are one of the most important measurements (Decker et al., 2005). The Vienna Basin is products of seismology. Before any scientific analysis it is regarded as one of the most prominent seismic active necessary to assess the quality, consistency, and homo- regions of Austria (Fig. 1). The pull-apart basin is orient- geneity of the data (Woessner and Wiemer, 2005). As the ed NE-SW and extends from the Semmering mountain Vienna Basin is located partly in Austria and Slovakia, two range to the little Carpathians in Slovakia (Gutdeutsch earthquake catalogues have to be considered in order to and Aric, 1988). The VBTFS passes between the capitals cover the entire length of the VBTFS. The Austrian earth- of Austria (Vienna) and Slovakia (Bratislava), which are quake catalog includes both historical and instrumental situated to the west and to the east of the fault system, data from 04.05.1201 to 06.05.2020 having a magnitude respectively. range of M =1-6.1 (ZAMG, 2020). In comparison to an old- The VBTFS and the Vienna pull-apart basin started er version that we used in earlier completeness studies to develop in the Middle Miocene. Extension and basin (Nasir et al., 2013), one historical earthquake (27.08.1668; subsidence initiated in the Badenian (15.5 Ma) as dated M=4.6) at Wiener Neustadt and one instrumental earth- by the growth strata of the basin fill (Royden, 1985; Wes- quake (05.07.1973; M=0.7) have been removed from the sely, 1988) and terminated in the Late Miocene (about new Austrian earthquake catalogue. 3 The temporal evolution of seismicity and variability of b-values along the Vienna Basin Transfer Fault System The ACORN (2004) earthquake catalogue covers a magnitudes that are higher than the values listed in the rectangular region encompassing the Eastern Alps, West Austrian earthquake catalogue (Fig. 2a; ZAMG, 2020). The Carpathians, and Bohemian Massif (Czech Republic, Slo- same is true for comparison of earthquakes recorded in vakia, Hungary and Austria; Lenhardt et al., 2007b). The the instrumental era 1906-2020 (Fig. 2b). temporal span of the ACORN earthquake catalogue is Comparison of the intensity-magnitude correlations between 1267 to 2004 with a magnitude range from 0.5 in the ACORN catalogue for historical earthquakes to 5.7. (M=0.6899I ) and instrumental earthquakes (M=0.6864I ) 0 0 Both, the Austrian earthquake catalogue (ZAMG, 2020) reveals very little differences. Both correlations differ as well as the ACORN earthquake catalogue, are domi- from intensity-magnitude correlations for historical nated by pre-instrumental earthquake data. Generally, earthquakes (M=0.667I ) and instrumental earthquakes historical earthquake data are reported with intensity, (M=0.6549I ) used in the Austrian earthquake catalogue. which is then converted into magnitude. For this conver- (Fig. 2). Intensity-magnitude correlations in the Austrian sion different formulas and empirical correlations were catalogue therefore reveal systematically lower magni- suggested for countries in Central Europe (Grünthal et al., tude values for earthquakes, which were only recorded 2009). The conversion of intensity into magnitude for the by their epicentral intensity. For intensity VII earthquakes Austrian catalogue is obtained by the formula M= 2/3I the difference is about 0.2 magnitude scales. Neverthe- (Lenhardt, 2007a). Conversion for the ACORN earthquake less, we used the magnitudes provided in the catalogues catalogue is given by the following formula (Grünthal et for further calculations. al., 2009). M = 0.682I + 0.16 (not considering focal depth) 4. Complications, delustering and completeness w 0 M = 0.667I + 0.3log(h) + 0.1 (considering focal depth) analysis of the compiled catalogue covering the w 0 VBTFS where h is the focal depth and I is the macroseismic The recognition and removal of fore- and aftershocks intensity. from the raw catalogues is mandatory for further work In the region of the VBTFS, the majority of earthquakes based on the earthquake data such as the calculation of are listed with hypocenter depths up to 10 km. The max- GR parameters and the assessment of catalogue com- imum depth is 35 km. Comparison of both catalogues in pleteness because it is generally assumed that earth- the overlapping region (Fig. 2a) shows the effect of the quakes are poissonian-distributed and therefore in- use of different intensity-magnitude conversions. Inten- dependent of each other (Gardner and Knopoff, 1974; sity to magnitude conversions for historical earthquakes Shearer and Stark, 2011). for intensities 3 ≤ I ≤ 6 in the ACORN catalogue reveal The earthquake data used in the current study were Figure 2: Intensity vs magnitude plot for earthquakes in the overlapping region of the ACORN (2004, blue triangles) and ZAMG (2020, orange dia- monds) earthquake catalogues for historical earthquakes covering the time of 1267-1905 (a), and instrumental data covering the length of 1906-2020 (b). The ACORN earthquake catalogue stops in 2004. 4 Asma NASIR et al. compiled from the Austrian and ACORN earthquake cata- equal to 5 (Fig. 3). Nine of these strong earthquakes were logues to cover the whole extent of the VBTFS (Fig. 1). The recorded in the last 135 years. compilation became necessary as existing regional cata- The compiled catalogue was subjected to complete- logues such as the catalogue by Grünthal and Wahlström ness checks using two different methods. The reasons (2003) set the lower magnitude level for the catalogue for repeating the completeness analyses reported by entries at M =3.50. For the area of the VBTFS such a lower Nasir et al. (2013) are the use of an updated catalogue cut-off removes a large part of the recorded seismicity. for Austria, which now extends up to 2020 and includes The seismic data for the VBTFS in ZAMG (2020) and revisions made according to the historical earthquake ACORN (2004) comprises a magnitude range M=0.5-5.7 research by Hammerl and Lenhardt (2013). Secondly, the and cover a nominal time period of 754 years from 1267- region analyzed in this study is considerably larger than 2020 (738 years for the ACORN data). The clustered com- the area examined by Nasir et al. (2013), which did not piled earthquake catalogue records 1739 seismic events. extend to the Mur Mürz Fault. Duplicate earthquakes have been removed manually for the area shown in Figure 1. For the overlap area the Aus- trian earthquake catalogue has been given priority and 4.1 TCEF completeness analysis duplicate earthquakes were removed from the ACORN TCEF (Temporal Course of Earthquake Frequency) is the catalogue. most widely used method for completeness analyses in Figure 3 shows the distribution of earthquakes at Central Europe (e.g., Lenhardt, 1996; Grünthal et al., 1998). VBTFS over time. The figure clearly shows that, although In this method, the cumulative number of earthquakes of the nominal time coverage of the combined catalogue is a magnitude class is plotted versus time. Slope changes about 750 years, only eight earthquake records are avail- in the plot illustrate changes of the completeness of the able from the period prior to 1800. Remarkable increases catalogue (Nasir et al., 2013, Grünthal et al., 1998). It is of the numbers of recorded earthquakes are observed common presumption that the latest steepening of the th around 1900 and close to the end of the 20 century. slope occurred when the data became complete for the Using the same method as in Nasir et al. (2013), the magnitude class under consideration (Gasperini and Fer- compiled data set was declustered manually using fault rari, 2000). Completeness-corrected recurrence intervals length-magnitude correlations to determine the maxi- for each magnitude class are then calculated from the mum distance of aftershocks from the magnitude of the time interval corresponding to the latest linear segment preceding mainshock (Wells and Coppersmith, 1994) and of the curves and the number of records in this time in- standard time windows after the mainshock according to terval (Nasir et al., 2013). Gardner and Knopoff (1974). To account for possible inac- The overall analysis in Figure 4 shows significantly curately determined earthquake locations the minimum steepening slopes for all magnitude classes M<4 around distance is set to 10 km for the spatial window. For more 1900. These changes relate to the onset of regular earth- detail we refer to Nasir et al. (2013; 2020). The earthquake quake records in the former Austro-Hungarian Empire in data after declustering comprises 1603 earthquakes in- the aftermath of the 1895 Ljubljana earthquake. TCEF re- cluding 12 earthquakes with magnitudes greater than or sults further show that records of magnitude class 0≤M<1 Figure 3: Magnitude vs. time plot from the combined catalogue (ZAMG, 2020; ACORN 2004). 5 The temporal evolution of seismicity and variability of b-values along the Vienna Basin Transfer Fault System Figure 4: Cumulative number of earthquakes vs. time (TCEF) for the Figure 5: Stepp completeness analysis of the combined catalogue combined catalogue covering the VBTFS. The steepest slope in the covering the VBTFS. The graph shows the standard deviation of mean plot for an individual magnitude class indicates that the catalogue is rate of earthquakes occurrences plotted vs. time. Colours indicate in- complete for that period of time. Note that the magnitude class 0≤M<1 dividual magnitude classes. The catalogue is regarded complete for never reaches completeness. Changes in slope for the intensity class- the time period for which the standard deviation (σ) of the mean re- es of 1≤M<2 and 2≤M<3 in 2004 are due to the ending of the ACORN currence rate (λ) of earthquakes of a given magnitude class follows the catalogue in this year. dashed 1/√T trend line. Where it deviates from that line, the catalogue is considered incomplete. Note that the magnitude class of 0≤M<1 never reached completeness. cannot be regarded complete at any time because the tude class 5 ≤ M < 6, the catalogue is regarded complete slope of the corresponding curve is much lower than the since 1875. The intensity-based completeness analysis for one of intensity class 1≤M<2 indicating that by far not all VBTFS (Nasir et al., 2013) in comparison revealed 109 years events with M<1 were recorded in recent years. Periods to 209 years completeness time windows for the intensi- of complete records along the VBTFS range from 24 years ty classes III<I ≤IV and VI<I ≤VII. Intensity class VII<I ≤VIII 0 0 0 for magnitude class 1≤M<2 to 223 years for magnitude did not include enough earthquakes to calculate a stable class 4≤M<5. For magnitude class 5≤M<6, which contains recurrence interval. only 12 earthquakes, complete records are estimated to start in 1884 (Table 1). M Completeness period (TCEF) 4.2 Stepp completeness analysis 1<M≤2 1997-2020 Stepp (1972) proposed a statistical approach to ana- 2<M≤3 1997-2020 lyze catalogue completeness. The test relies on the sta- 3<M≤4 1885-2020 tistical property of the Poisson distribution highlighting 4<M≤5 1830-2020 time intervals during which the recorded earthquake oc- 5<M≤6 1885-2020 currence rate is uniform (Stepp, 1972). The method was described in detail by Nasir et al. (2013). The magnitude classes are analyzed for completeness using time win- Table 1: TCEF derived periods of complete earthquake records along dows of different length in the time period between 1267 the VBTFS. and 2020 (754 years; Fig. 5). The calculation uses ten time windows of 10 years (1901-2020), two time windows of Magnitude Completeness period 50 years (1801-1900) and one time-window covering 534 (Stepp Test) years (1267-1800). The corresponding completeness of all 1<M≤2 2001-2020 magnitude classes are estimated manually from the parts 2<M≤3 1911-2020 of the calculated curves that follow a linear trend paral- 3<M≤4 1801-2020 lel to 1/√T line. For the corresponding time intervals, the mean rate of occurrence of earthquakes of the analyzed 4<M≤5 1801-2020 magnitude class is stable. 5<M≤6 1875-2020 The completeness period for 1≤M<2 is 20 years (2001- 2020), for 2≤M<3 110 years (1911-2020), for 3 ≤ M < 4 and Table 2: Periods of complete earthquake records derived from the 4 ≤ M < 5 220 years (1801-2020). For the highest magni- Stepp test (Stepp, 1972). 6 Asma NASIR et al. 5. Long and short-duration aftershock sequences 1906 Dobrá Voda earthquake (Fig. 6a) was described in Large earthquakes are typically followed by aftershock detail by Nasir et al. (2020). The Dobrá Voda earthquake activity, which generally is assumed to decay hyperbol- of 09.01.1906 (M=5.7) at the VBTFS in Slovakia initiated ically as stated by Omori’s law (Ogata, 1983). Based on earthquake activity in vicinity of the mainshock (< 13 km) empirical data, Stein and Liu (2009), on the other hand, which is gradually decaying since 1906. The pattern suggested that large earthquakes may have much lon- shown in Figure 6a suggests that aftershock activity ex- ger aftershock sequences, which depend on the slip tends to the present. Figure 6b, on the other hand, shows rate of faults and may last for decades or even centuries. the seismicity in the time before and after the 11.07.2000 At the VBTFS, the seismicity following the 1906 Dobrá (M=4.8) Ebreichsdorf earthquake. In this case the after- Voda earthquake was identified as an example of such shock activity decayed hyperbolically and reached the a long aftershock sequence (Nasir et al., 2020). Deciding level of background seismicity within about 300 days af- whether the earthquake activity in a defined region can ter the mainshock. be related to a long aftershock activity subsequent to a In order to identify seismicity patterns that may be in- strong earthquake is challenging because the definition dicative of long-term aftershock sequences comparable of aftershock activity depends on numerous parameters to the 1906 Dobrá Voda example, we divide the VBTFS such as the definition of the area treated as aftershock into arbitrarily selected segments. Seismicity recorded zone and the level of background seismicity before the within each of these segments is further analyzed to mainshock (Stein and Liu, 2009). check whether it shows distinctive patterns of decaying Figure 6 illustrates two examples of short- and long- earthquake activity which may be related to long after- term patterns of decaying earthquake activity forming shock sequences. The purpose is to identify possible aftershock sequences. Seismicity subsequent to the strong earthquakes which occurred before the start of records in the earthquake catalogue, and which may be- come “visible” due to their aftershock sequences. 6. Seismicity of arbitrarily selected segments of the VBTFS The VBTFS is divided into eight segments of approxi- mately 50 km length. Segments are selected to overlap each other for not missing possible aftershock sequences which followed earthquakes that occurred close to one of the segment boundaries (Fig. 7). The arbitrarily select- ed segment boundaries do not agree with the kinematic fault segments of the VBTFS defined based on fault ge- ometry (Hinsch and Decker, 2011; Beidinger and Decker, 2011). The selection of segment lengths of about 50 km is driven by the MCE estimates of M=6.0-6.8 for the VBTFS (Decker and Hintersberger, 2011). Earthquakes with such Segment GR a- and b-values calculated for: no. Total catalogue length Completeness period a b a b 1 2.08 0.68 2.31 0.67 2 1.05 0.50 1.20 0.49 3 - - 1.34 0.49 4 1.48 0.63 1.99 0.66 5 1.86 0.70 2.43 0.76 6 1.47 0.63 2.39 0.69 7 1.06 0.60 2.12 0.71 8 0.81 0.47 2.54 0.86 Figure 6: Different pattern of decaying earthquake activity. (a) Long Table 3: a- and b-values of the Gutenberg-Richter relations calculated aftershock sequence of 09.01.1906 (M=5.7) Dobrá Voda earthquake. for eight segments of the VBTFS. See Figure 8 for segment location. Note that time is provided in years. (b) Short (Omori-type) aftershock In segment 3 the total catalogue length is equal to the completeness sequence subsequent to the 11.07.2000 (M=4.8) Ebreichsdorf earth- period (earthquake records start in 1890). quake. The inset in Fig 6(b) shows aftershocks in the first 7 days after the mainshock. 7 The temporal evolution of seismicity and variability of b-values along the Vienna Basin Transfer Fault System Figure 7: Subdivision of the VBTFS into 8 arbitrary overlapping segments of about 50 km length (S-1 to S-8) used for the analysis of aftershock sequences and calculation of GR-parameters. See text for further explanation. magnitudes are broadly associated with slip on <50 km mainshock, (Nasir et al., 2020). The time-magnitude plot long faults (Wells and Coppersmith, 1994). It is therefore includes a second strong earthquake in 1914 (M=5.1) assumed that for each possible strong earthquake the which is regarded as an aftershock of the 1906 event. full length of the slipped fault is contained by one of the The distance between the epicenters of the two events is segments. The selection of segment length and overlaps 33 km. The slow regular decrease of the largest observed is further guided by the consideration of inaccuracies of magnitudes over time until 2004 (end of the ACORN cat- epicenter locations of historical earthquake data and the alogue) suggests that the aftershock sequence lasted necessity of a minimum number of earthquakes for de- much longer than predicted by the Omori law. The level fining GR relations. of background seismicity with M=2.6-4.3 (11 earthquakes) Segments 1 and 2 are located in Slovakia and include recorded before the 1906 Dobrá Voda earthquake be- the strongest earthquake recorded at the VBTFS (1906 tween about 1800 and 1900 is only reached about 100 Dobrá Voda, M=5.7). The segments contain 207 and 96 years after the mainshock. earthquakes with M=0.5-5.7, respectively. Time coverage Segment 3 covers the border region between Austria is 1805-2004 (segment 1) and 1794-2017 (segment 2). The and Slovakia. It includes 78 earthquakes with M=1-5.2 for first data entries in segment 1 (1805, M=4.3) and segment the time period 1890 to 2017 (Fig. 8). The time plot shows 2 (1794, M= 2.6) are followed by a data gap until 1852 4 earthquakes in 1890 including a mainshock (M=4.5) and (M=4.3). Time series of both overlapping segments 1 and three aftershocks with different epicenter locations with- 2 show a general decay of seismicity, which starts with in distances of 5 km from mainshock (Fig. 8). The 1927 the 1906 Dobrá Voda (M=5.7) earthquake (Fig. 8). The Schwadorf earthquake (M=5.2) is followed by a short temporal evolution of seismicity in segment 1 around aftershock sequence. The apparent long-term decay of Dobrá Voda reveals a level of background seismicity with seismicity in the temporal window cannot be regarded M=2.6-4.3 (11 earthquakes) recorded between about to be related to aftershock activity due to the distance of 1800 and 1906. While the significant earthquakes (M=4- events from the Schwadorf mainshock (Fig. 8). 5) in the time before 1906 are scattered over the entire Magnitudes recorded in segment 4 range from M=1- region, seismicity concentrates within a distance of less 5.2 with data length from 1590-2019 and a total number than 13 km from the epicenter after the 1906 Dobrá Voda of 487 earthquakes (Fig. 8). This segment shows no tem- 8 Asma NASIR et al. Figure 8: Left column: Location of segments 1-8. Center: time vs. magnitude plots. Green line shows the visually estimated start of catalogue completeness with respect to earthquakes with M≥3. Right column: Gutenberg-Richter diagrams for segments 1-8 of the VBTFS. 9 The temporal evolution of seismicity and variability of b-values along the Vienna Basin Transfer Fault System Figure 8: Continued. 10 Asma NASIR et al. poral seismicity pattern that could indicate gradual de- earthquake must, however, remain uncertain because of cays of seismicity except for the 1927 Schwadorf (M=5.2) the poor earthquake record and the inaccuracies inher- earthquake (see above). ent in the determination of the historical epicenters. For segment 5 magnitudes between M=1-5.3 were re- The GR-trendlines consider data from the time window corded for 673 events between 1668 and 2020. Records between the first recorded earthquake at the segment start from 1668 with M=4.6 at Wiener Neustadt (ACORN, and 2020 (blue) and the period for which the record of 2004) followed by another earthquake in 1712 having M ≥ 3 events is estimated to be complete as indicated M=4 at the same epicenter (Fig. 8). It must be noted that in the corresponding time-magnitude plots (green), re- the 1668 event has been excluded from ZAMG (2020) spectively. Segment 1 and the overlapping segment 2 based on the assessment by Hammerl and Lenhardt show the 1906 Dobrá Voda long aftershock sequence. (2013). The next M=5 earthquake occurred at a distance The time-magnitude diagram for segment 3 shows de- of 5 km from Wiener Neustadt (Neudörfl) in 1736. For caying seismicity after the 1927 Schwadorf earthquake. the next stronger event on 1768 (M=5) ZAMG (2020) also Segment 4 is including the short aftershock sequence lists an epicenter at about 5 km distance from Wiener following the 2000 Ebreichsdorf mainshock (compare Neustadt. In sum, 5 out of 7 earthquakes recorded be- Fig. 6b). Note that segments 3 to 7 do not show temporal tween 1668 and 1841 are listed with epicenters in close seismicity patterns comparable to the long-term decay proximity to each other. It can, however, not be judged after the 1906 Dobrá Voda earthquake (segment 1 and if the observed clustering correctly reflects the spatial 2). Segment 8 shows a regular decrease of the largest distribution of seismicity, or it is due to inaccurate loca- recorded magnitudes after the 1794 Leoben earthquake tion of the historical events (compare Gangl and Decker, that is comparable to the patterns subsequent to 1906 2011). In the time before 1898 only 7 earthquakes were Dobrá Voda and might indicate a long aftershock se- recorded in a period of 230 years. Seismicity patterns quence. indicative of aftershock sequences are not evident. Dis- In addition to the temporal distribution of seismicity tances between the epicenters of earthquakes between along the VBTFS, we analyzed the frequency-magnitude 1898 and 1964 that might indicate an apparent increase correlation of earthquakes for the whole fault system and of the maximum recorded magnitudes from about M=4 the 8 selected fault segments. The correlation is stated by to M=5.3 are too large to identify the events as fore- or the GR relation which can be written as aftershocks. logN(Mc) = a - bM Segment 6 includes 506 earthquakes with M=1-5.4 where N is the number of events with magnitudes larg- that occurred between 1267 and 2020. The significant er than the magnitude of completeness Mc and a is the recorded earthquakes are 1267 Kindberg (M=5.4), 1811 corresponding level of seismic activity (Gutenberg and Krieglach (M=4.4), 1830 Mürzzuschlag (M=4.4) and 1837 Richter, 1942). Mürzzuschlag (M=4.7; Fig. 8). The magnitude-time plot Figure 9 shows the results of calculating GR relations does not include any pattern indicative of aftershocks. All earthquakes are randomly distributed over the approxi- mately 50 km long segment. The nominal record length for segment 7 covers 754 years (362 earthquakes). Records, however, contain no data for the 527 years between 1267 (Kindberg, M=5.4) and 1794 (Leoben, M=4.7). The next notable earthquakes followed in 1811 (Krieglach, M=4.4), 1830 (Leoben, M=4.4) and 1847 (Kindberg, M=4.1). ZAMG (2020) places the earthquakes in 1794 and 1830 within 1.3 km distance (Leoben). The other three events mentioned above oc- curred at distances of 10km from each other. The dis- tance of 40 km between the two groups, however, does not suggest linkage. For segment 8, ZAMG (2020) lists 259 earthquakes with the first data entry of the 1794 Leoben earthquake. From the time between 1794 and 1899 only two earthquakes are secured by historical records. In spite of the poor data coverage, the time-magnitude plot shows a gradual decrease of the largest observed magnitudes between 1794 and the 1980ies. The strongest events of this row (1794 Leoben; 1830 Leoben; 1899 St. Stefan) occurred at Figure 9: Comparison of Gutenberg-Richter relations obtained for a nominal distance of some 10 km not contradicting an the VBTFS from the declustered compiled catalogue without applying interpretation as aftershocks. Interpreting the declin- completeness correction (black), after TCEF completeness correction ing seismicity as a long aftershock sequence of a strong (red) and after Stepp correction (blue). See text for discussion. 11 The temporal evolution of seismicity and variability of b-values along the Vienna Basin Transfer Fault System for the entire VBTFS. The graph shows that the actual correction to be more reliable for extrapolating the re- values of the GR a- and b-values differ when using dif- currence periods of large magnitude earthquakes. ferent completeness corrections. Without applying any The GR relations calculated for the 8 arbitrarily select- correction, the b-value obtained from the declustered ed segments of the VBTFS are shown in Figure 8. The GR catalogue of the VBTFS is 0.57 which is low for a seismi- values were calculated from the uncorrected clustered cally active region. Applying the Stepp and TCEF correc- catalogue as it was not possible to apply completeness tions leads to a significant increase of the a-values. The correction on individual segments. The numbers of re- b-values after TCEF and Stepp correction are 0.85 and corded earthquakes in the individual segments proved 0.74 (Fig. 9). The b-value calculated for the VBTFS from insufficient for applying the TCEF or Stepp method for the catalogue without completeness correction leads to each magnitude class. To assess the resulting errors, we underestimate the number of earthquakes with smaller calculate GR-values of all segments for two time periods: and medium magnitudes. The frequencies of small mag- (a) the nominal time coverage of the catalogues for each nitude earthquakes resulting from TCEF and Stepp cor- segment starting from the oldest recorded earthquake; rection is almost identical. The TCEF correction, however, and (b) the time period for which visual inspection of the seems to underestimate the frequency of earthquakes of time-magnitude graphs in Fig. 8 suggest completeness higher magnitudes by the higher b-value because TCEF for M≥3 earthquakes, i.e., starting from 1885 or 1895. includes the highest magnitude earthquakes based on These estimated completeness periods are corroborated the assumption that records of these earthquakes are by the TCEF and Stepp analyses (Tab. 1,2). complete for the whole length of the catalogue. Stepp Comparison of the GR values obtained for the different completeness analysis, however, shows that the number time windows shows that a- and b-values are calculated of earthquakes of the highest magnitude class (5<M≤6) from the nominal time coverage of the catalogue are is too small to calculate a reliable recurrence rate. The systematically smaller than the values obtained from the corresponding data is therefore not considered in the time window corresponding to the assumed complete GR calculation (see Nasir et al., 2013, for a more detailed records. This is due to the widely incomplete record of discussion). We regard the results obtained after Stepp small and medium earthquakes before about 1885. The Figure 10: Gutenberg-Richter b-values calculated for eight arbitrarily selected overlapping fault segments and the time period for which time-mag- nitude graphs in Fig. 8 suggest completeness for M≥3 earthquakes. Note the variation of b-values ranging from 0.49 to 0.86. Fault segments with low levels of historical and instrumental seismicity and low seismic slip rates (S2 and S3) are characterized by low GR b-values. Seismic slip rates from Hinsch and Decker (2011). 12 Asma NASIR et al. results further show that both, GR a- and b-values vary velocities in the range of 1-2 mm/a. The high b-values of significantly between the segments. For the assumed 0.67 and 0.76 are calculated for segments 1 and 5 which, completeness period since about 1885 a-values reach on the other hand, include parts of the VBTFS with seis- from 1.20 to 2.54 and b-values from 0.49 to 0.86 (b-val- mic slip rates of about 0.5 mm/a (Dobrá Voda area) and ues; Fig. 10). The smallest b-values (0.49) are calculated 0.7-1.1 mm/a (northeastern part of the Mur-Mürz fault for segments 2 and 3. Visual inspection of the seismicity and southwestern Vienna Basin). Data therefore indicate along the VBTFS shows that these segments are char- that GR b-values correlate negatively with the seismic acterized by the lowest earthquake activity in historical slip deficit of the VBTFS. A similar negative correlation of times (Fig. 10). b-values and the slip deficit rate was reported by Nanjo and Yoshida (2018). Variations of the GR b-value are commonly related to 7. Discussion and conclusions different states of stress in the deformation zone (Scholz, The analysis of time sequences of earthquakes record- 1968; 2015) with low b-values indicating high differential ed in 8 arbitrarily selected segments of the VBTFS was stress (Farrell et al., 2009; Scholz, 2015). GR b-values are performed to identify possible long aftershock sequenc- therefore considered to be proxies of stress which iden- es subsequent to major earthquakes. The analyses were tify highly stressed fault segments or asperities that re- stimulated by Stein and Liu (2009) who suggest after- sist slip (Schorlemmer and Wiemer 2005; Nuannin, 2006). shock durations of tens to several hundreds of years for Low b-values were also related to knickpoints and chang- slow moving faults with fault loading rates of few mm/ es of fault strike (Öncel et al., 1996). It is assumed that such year. Nasir et al. (2020) show that this model is at least highly stressed segments are locations where future rup- applicable to the 1906 Dobrá Voda (M=5.7) earthquake, tures are likely to occur (Schorlemmer and Wiemer, 2005; which is followed by a century-long aftershock sequence. Bayrak and Bayrak, 2012; Hussain et al., 2020). At least for Assessments of earthquake time sequences along the plate boundaries, this assumption is corroborated by the VBTFS is strongly limited by the length and the complete- finding that small b-values characterized the focal areas ness of available earthquake records. TCEF and Stepp of strong earthquakes prior to fault rupture (Nanjo et al., completeness tests show that tolerably complete records 2012; Nanjo and Yoshida, 2021). only exist for the last about 200-300 years. For the VBTFS, fault segments with high seismic slip With exception of segment 8, time sequences obtained deficits such as the ones next to the Lassee- and Zohor for fault segments apart from the 1906 Dobrá Voda segments (see Fig. 10 for location) were previously inter- (M=5.7) earthquake did not reveal long-term decays of preted as “locked” fault segments which have a signifi- seismicity that might be interpreted as long aftershock cant potential to release future strong earthquakes, in sequences. Segment 8, covering the southwestern tip of spite of the fact that historical and instrumentally record- the VBTFS, revealed a 200 years long gradual decrease of ed seismicity is very low (Hinsch and Decker, 2003; 2011). the largest observed magnitudes starting with the 1794 This interpretation is corroborated by the low b-values Leoben (M=4.7) earthquake. Epicentral distances of the that suggest high differential stresses for these segments. largest events in the row allow an interpretation as after- shocks. The 1794 event is the oldest earthquake listed in the catalogue for the region under consideration. It must Acknowledgements therefore remain open if the recorded decay of seismic- We thank Seth Stein for his stimulating discussion on ity results from the 1794 event, or a still older, possibly aftershock sequences during the Fragile Earth confer- stronger earthquake before. The latter is corroborated by ence in Munich. We gratefully acknowledge the care- the low magnitude of the 1794 earthquake which would ful and very constructive reviews by Ewald Brückl and typically not be considered to cause long aftershock Christoph von Hagke. Their comments and suggestions sequences. 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Journal
Austrian Journal of Earth Sciences – de Gruyter
Published: Jan 1, 2023
Keywords: Vienna Basin Transfer Fault System; seismicity; aftershock; Gutenberg-Richter relations