A Natural and Controlled Source Seismic Profile
through the Eastern Alps: TRANSALP
J. Kummerow1,*, R. Kind1,2, O. Oncken1,2, P. Giese2, T. Ryberg1, K. Wylegalla1,
F. Scherbaum3 and TRANSALP Working Group
1GFZ Potsdam, Telegrafenberg, 14473 Potsdam, Germany
2Freie Universität Berlin, Fachrichtung Geophysik, Malteser Str. 74-100, 12249 Berlin,
3Universität Potsdam, Institut für Geowissenschaften, POB 601553, 14415 Potsdam,
* Corresponding author. Tel.: +49-331-288-1239, fax: +49-331-288-1277.
E-mail address: email@example.com (J. Kummerow).
The combined passive and active seismic TRANSALP experiment produced an
unprecedented high-resolution crustal image of the Eastern Alps between Munich and
Venice. The European and Adriatic Moho are clearly imaged with different seismic
techniques: near vertical incidence reflections and receiver functions. The European
Moho dips gently southward from 35 km beneath the northern foreland to a maximum
depth of 55 km beneath the central part of the Eastern Alps, whereas the Adriatic Moho is
imaged primarily by receiver functions at a relatively constant depth of about 40 km. In
both data sets, we have also detected first order Alpine shear zones, such as the Helvetic
detachment, Inntal fault and Sub-Tauern ramp in the north. Apart from the Valsugana
thrust, receiver functions in the southern part of the Eastern Alps have also observed a
north dipping interface which may penetrate the entire Adriatic crust (ACI, Adriatic Crust
Interface). Deep crustal seismicity may be related to the ACI. We interpret the ACI as the
currently active retroshear zone in the doubly vergent Alpine collisional belt.
Keywords: Eastern Alps, continent-continent collision, receiver functions
The Alps, commonly considered a typical and exceptionally well studied example of a
doubly vergent collisional orogen, have become a primary object to develop quantitative
physical models and models of the geophysical imaging potential of small-scale
collisional orogens [1-5]. Much of this development is due to a substantially improved
understanding of orogeny based on a series of deep geophysical studies in the Central and
Western Alps (see [6, 7] for overview and references therein). In contrast, the deep
architecture of the Eastern Alps is considered to be different from that of the Western
Alps, mainly owing to significant differences in the near-surface tectonic structure and
kinematic pattern. In particular, the debate about the respective roles of crustal thickening
versus lateral escape , the existence of crustal indentation and the deep structure of the
Eastern Alps etc. are unsolved issues that deep crustal images are hoped to resolve. The
absence of near vertical incidence data in the Eastern Alps has only been overcome since
1998 by the geophysical TRANSALP project. The TRANSALP Working Group 
primarily described the near vertical reflection part of this project and resulting models
of the crust in the central Eastern Alps. In this paper, we present the first high-resolution
study in this region combining receiver function (RF) and active seismic techniques
aiming at better constraining the deep architecture of the crust (cf. [10-13] for similar
Near vertical incidence reflection and receiver function techniques have several
similarities. Both techniques provide information about the topography of seismic
discontinuities, and similar processing steps are applicable (e.g. migration). Differences
include the frequency content and, in consequence, the resolution. Controlled source
reflections typically have frequencies of 10 Hz or higher, while teleseismic conversions
have periods of one or several seconds. Therefore, reflection seismology yields high-
resolution images, whereas receiver functions resolve the most prominent structures
associated to first-order petrophysical discontinuities. Application of reflection
seismology is commonly restricted to the crust, while teleseismic conversions have
practically unlimited depth resolution. Last, but not least, teleseismic studies have the
potential to image discontinuities with a broad transitional character – of up to several km
width - that are virtually invisible for reflection studies. Both techniques complement
each other. Recent applications of the receiver function technique in the Tibetan Plateau
and the Andes [14, 15] have demonstrated the particular potential to highlight the large-
scale architecture of active orogenic belts.
2. GEODYNAMIC FRAMEWORK
The Alps resulted from collision of the European plate with the Adriatic plate during the
Early Tertiary following southward consumption of the narrow intervening Penninic
Ocean under the Adriatic plate. Most of the present day architecture of the Eastern Alps
has only developed in the subsequent stages (see [2, 8, 16], and references therein)
generating a series of adjoining zones, each with a different internal architecture and
kinematic style (for details see [8, 16-22]): In a north-south section these include the
European foreland with the Molasse basin of southern Germany deepening towards the
Alpine front; a thin-skinned wedge overthrusting the European foreland on top of an
important system of detachments (including the basal thrust of the Northern Calcareous
Alps, NCA, that formed part of the Adriatic microplate and the underlying Helvetic
detachment comprising the shelf deposits of the European shelf); a core zone (comprising
the Tauern Window) of strongly shortened rocks of mostly European affinity confined to
the north and south by steeply dipping post-collisional strike slip faults (Inn Valley fault,
Periadriatic fault system); a thick-skinned backthrust wedge overthrusting the Adriatic
plate and Po basin to the South (including the Dolomite Mountains). The core zone,
moreover, is severely affected by lateral escape to the east driven by indentation of the
Adriatic plate into the central Eastern Alps.
Post-collisional accretion and foreland-directed orogenic growth along the N and S
borders affected the Alpine belt until about 10 Ma ago. Only in the Venetian region
compressional accretion continued to the present [17, 20, 21]. The more internal
Valsugana thrust belt accumulated deformation from the Late Oligocene until the Late
Miocene to Pliocene, while the southward adjoining Bassano-Friuli thrust system
forming the southern margin of the Eastern Alps was initiated in the Late Miocene.
Current deformation of this Eastern Alpine section still shows significant present day
seismicity (e.g. [23-25]). The seismicity is dominated by strike-slip kinematics in most of
the Eastern Alps with additional thrusting confined to the southern margin, the
kinematics being strongly controlled by extrusion of the Alpine core zone eastwards
towards the Pannonian basin.
The deep architecture of the crust and mantle underlying the above orogenic zones is a
matter of debate as are some of the structures related to the crustal shortening. The only
currently available constraints are derived from severalwide-angle seismic studies
through the Eastern Alps, which are summarised by [26-31]. According to these earlier
studies the European Moho dips to the south down to more than 50 km depth beneath the
central Eastern Alps while the Adriatic Moho appears at 25-40 km depth beneath the Po
valley dipping slightly to the North.
3. A CROSS SECTION THROUGH THE EASTERN ALPS
The deep seismic reflection profiling was accompanied by a remarkably dense
earthquake recording network consisting of mostly three-component, short period sensors
(Fig. 1 and ). This configuration provided the rare opportunity to map inner crustal
interfaces by means of the receiver function method and to compare the results with the
seismic reflection data. Whereas  focussed on the TRANSALP controlled source
seismic section, this study accentuates the implications of the high resolution receiver
function profile. New data from 7 stations are added to the earlier data set , which
were deployed in the southern segment of the TRANSALP profile throughout the year
2002 (squares in Fig. 1). Different from the crustal section published in , the short-
period processing of the data moreover highlights the inner crustal structure, resulting in
a finer, better constrained RF section of the Eastern Alpine crust (Fig. 2a).
Time domain RF data were projected to depth domain using a Kirchhoff-approximate
migration. The velocity model is a crust with constant Vp (6.0 km/s) and Vp/Vs ratio
(1.73) and a thickness of 40 km, overlaying a mantle with IASP91 velocities .
For comparison, the line drawing from the TRANSALP near vertical reflection data is
superimposed (Fig. 2b). The reflection data were depth migrated in a velocity model,
which was obtained by a combination of stacking velocities and a tomographic image for
the upper ~15 km and a refraction seismic model by Miller et al.  for the deeper crust
. The RF and the reflection data from the crust agree well in both data sets in spite of
the differences in the wave lengths of the seismic waves (several hundred meters in
reflection seismics versus several kilometres in teleseismic receiver functions). This
consistency documents the potential of the receiver function method to map inner crustal
interfaces provided that the station spacing is adequate (<5 km as in the TRANSALP
The European Moho (EM) and the Adriatic Moho (AM) are easily identified in Fig. 2a.
The European Moho is at about 35 km depth at the northern end of the profile, which is
already deeper than at the permanent broadband station FUR near Munich (32 km, ).
It reaches its maximum depth of about 55 km south of the Periadriatic Line (46.6-
46.9°N). The Adriatic Moho appears south of about 46.6°N at 40 km depth in the
receiver function data and continues flat to the southern end of the profile.
In the time domain, the receiver function data underline the abrupt change from the
European to the Adriatic Moho found in the depth migrated data (Fig. 3a). They represent
a more basic data set than the migrated RF, since they are not biased by a perhaps
incorrect velocity model. The Moho step is not only observed in the direct Moho
conversions (Pms), but also in the vertical offset between the legs of the multiples in Fig.
3b. Multiple energy is suppressed in the short-period data (Fig. 3a), and only appears in
the simulated broadband data (Fig. 3b).
Zhu and Kanamori  presented a method to estimate the crustal thickness and average
crustal Vp/Vs, which uses the direct Moho conversion (Pms) and two crustal multiples
(the positive PpPms and the negative PpSms) and a predetermined crustal Vp. The
resulting Moho depth is not very sensitive to Vp . Crustal multiples are relatively
prominent for the European crust, where the value for crustal Vp/Vs is about 1.73 (Fig.
3b). Vp/Vs apparently does not differ significantly in the Southern Alps, though crustal
multiples are weaker there. Accounting for an uncertainty in Vp/Vs of 0.05, the error for
the Moho depth value is < 3km .
Using a crustal Vp/Vs value of 1.73 and Vp=6.0 km/s, Pms delay time picks from Fig.3
were transformed to Moho depth along the TRANSALP section (green circles in Fig. 2a).
The results agree well with the migrated receiver function data. Even in the deepest part
of the crustal root at ~55 km depth, where the migrated RF data are biased by a mantle
velocity for depths greater than 40 km due to the assumed laterally homogeneous
background velocity model, the differences are minor. Previous deep seismic refraction
measurements [29, 34] and a recent active tomography study  indicate that the
average crustal Vp is higher on the Adriatic side of the section. Accordingly, we also
plotted Moho measurements for the Adriatic crust -this time assuming an increased Vp
value of 6.3 km/s (blue circles in Fig. 2a). This results in a slight deepening of the
Adriatic Moho from 40 km to 42 km depth.
These errors are within the limits of the resolution given by the periods of the teleseismic
phases used in the RF (T >1s, wavelength of Ps phase >4 km). Even if they are accounted
for, the magnitude of the step between EM and AM amounts to 10-15 km. Ryberg and
Weber  also showed in a more general study, that migration of receiver functions
with velocities incorrect by less than 10 per cent results in still good migration results.
The flat Adriatic receiver function Moho seems to be in some disagreement with the line
drawings of the near vertical reflection data in Fig. 2b. In contrast to the well-defined
European reflection Moho, the Adriatic Moho lacks a clear reflection signal and rather
shows a broad gently N-dipping band of weak short reflections that dip steeper to the
north than the band itself. While this observation may be taken to indicate a north-
dipping component of the Adriatic Moho, in particular at its northern end (46.4-46.6°N),
it more probably reflects the deep crustal fabric above or at the Moho. As a consequence,
the transition between the European and the Adriatic Moho would appear more abrupt in
the receiver function data than in the seismic reflection data. For verification, we
separated the receiver function data into events arriving from north or south, respectively
(Figs.2c,d). No significant northerly dip of the Adriatic Moho can be recognised in either
of these two sections. We attribute this possible disagreement to the difference in
frequency content of both data sets, which makes the seismic waves sensitive to features
occurring at different structural scales. Small-scale structures – i.e. a pervasive
deformation fabric or a small-scale imbrication of the crust-mantle boundary - might be
superimposed on the larger-scale crust-mantle contrast seen by the receiver functions.
Further evidence for the step between EM and AM is given by the calculation of
theoretical seismograms. We used a finite difference algorithm described in . The 2-
D input model (Fig. 4a) reproduces the Moho topography as derived from the migrated
section (Fig. 2a). We neglected inner crustal structures to avoid interferences of the Moho
signal with multiple energy. The relevant display windows, which contain the Moho
conversion, are given in Fig. 4b,c for calculated and observed receiver functions and
seismic wave fronts incident from north and south, respectively. The step in Moho depth
is more apparent for southern back azimuths (Fig. 4c), whereas for events from the north
a phase identified as a diffracted wave obscures the step (Fig. 4b). According to this
interpretation, the apparent overlap of Moho conversions in Figures 2c and 4b (at 46.3-
46.7°N) may be a consequence of the steep transition between the European and the
Adriatic Mohos rather than an indication of continuing subduction of European crust
beneath the Adriatic Moho. But under the aspect of a phase transition in the European
lower crust towards high-pressure petrology (i.e. eclogite facies), which is expected in
that depth range, an overlapping between the Adriatic Moho and the European Moho may
Both, the northern as well as the southern flank of the orogen, exhibit important features
of smaller magnitude than the respective Mohos in the receiver function data, all of them
dipping toward the core of the Alps. These features partly correlate with the reflection
image. In the north, they include the converters I and II, which line up with parallel
reflections. In the reflection section, they correlate with the base of the Mesozoic, the
Helvetic detachment and the basal thrust of the Northern Calcareous Alps (I) and the base
of the more internal Austroalpine nappes (upper gently dipping part of II) respectively [9,
22]. The small vertical distance of the three features related to convertor I of less than 2
km defies a more specific assignment of this low-resolution signal to either of these
features. Its brightness may well stem from the intercalation of clastics and carbonates
with evaporates that particularly localized detachment formation in this part of the
Eastern Alps . Both, the Helvetic and the Austroalpine detachments, merge in the
middle crust to a moderately S-dipping feature termed the Sub-Tauern ramp (convertor
II). This dipping feature is better imaged by the reflection data than by receiver functions.
While the core of the orogen does not show well-defined conversions, the southern flank
again shows up with two north-dipping features. Convertor III can be linked to the
Valsugana thrust system and to associated strong near vertical incidence reflections that
probably image parts of the sedimentary sequence. It dips north to a depth of 15-18 km
where the convertor flattens linked to some subhorizontal inconspicuous reflections that
continue to about 46.7°N. South of the surface exposure of the Valsugana thrust belt,
these reflections may be seen to correlate with bright short near surface converters, again
suggesting that features near the interface of the sediments with the underlying basement
may be controlling the receiver function image.
A new feature within the Adriatic crust is revealed by the receiver functions and the near
vertical reflection data: it is located well within the pre-Mesozoic Alpine basement, dips
northward and may extend to a depth of about 30 km beneath the Dolomites (ACI,
Adriatic Crust Interface in Figs. 2a,b). The Adriatic Crust Interface is defined by large
amplitudes of the Ps converted phases in a depth interval between 20 km and 25 km (Fig.
2a), while its possible deeper continuation is indicated by the reflectivity pattern of the
seismic reflection section (Fig. 2b).
Comparison with previous studies
Our observations of the European Moho, the Adriatic Moho and ACI show a complex
relation to the results of earlier controlled source seismic experiments in the same region.
The new results of TRANSALP and revised informations from previous studies are
compiled in Fig. 5 which displays the main morphological features of the European and
Adriatic Moho. Information about the Moho depth is spatially restricted to the locations
of long-range seismic refraction profiles parallel and perpendicular to the strike of the
Eastern Alps. Regarding the European Moho, the TRANSALP observations fit very well
the existing image of the seismic refraction Moho (within ±3 km) and confirm the map-
view mismatch between the Periadriatic Line and the transition between the EM and the
AM (Fig. 5). This mismatch underlines the upper crustal nature of this important feature
as already suggested by the earlier seismic reflection interpretation . Because it is very
difficult to detect offsets of a few km of the EM, resp. the AM, in the seismic refraction
records, the contours in Fig. 5 do not show such possible displacements. Considerable
orogen parallel variations in Moho depth between 30 km and 45 km are known to exist in
the Adriatic plate (Fig. 5) [27, 28, 30]. Scarascia and Cassinis  divide the Adriatic
region into two distinct units, a region with a thick crust (35-45 km Moho depth) and a
region with a thin crust (25-30 km Moho depth). The thin crust can be asssociated with
the gravity high of Verona/Vicenza (minimum Moho depth 30 km or even less). The
southernmost section of the TRANSALP profile is situated at the NE-flank of this gravity
high in the transitional zone between the shallow and deep AM. As described in the
previous chapter, the receiver functions image the Adriatic Moho at a constant depth of
40 km along the southern TRANSALP transect. The covered area is an about 50 km wide
strip along the TRANSALP line, which is the distribution of the piercing (conversion)
points of the incoming rays at 40 km depth (Fig.6a). Hence, we suggest that the interface
at 20-30 km depth (ACI) is an inner crustal boundary of the Adriatic domain. In contrast
to the well-defined image by the receiver functions and the near vertical reflection data,
the ACI is difficult to recognise in the seismic refraction data. Only very few profiles
show distinctly developed ACI phases (yellow streak in Fig. 5). Thus, a spatially coherent
contour mapping of the ACI is not feasible.
Fig.6b shows the seismicity of the study region superimposed on the receiver function
image. Depth determinations of earthquakes have been found to vary significantly in
different studies. The use of depth phases at regional distances, as in teleseismic studies,
results in more accurate depth determinations [40, 41]. Seismograms of the Gräfenberg
array  in northern Bavaria of a magnitude 5.1 event in the Trento-Vicenza region
demonstrate the existence of sPn depth phases (Fig. 7). The time delay between sPn and
Pn phase is converted to a source depth of 23 km for this event (September 13, 1989). For
a second magnitude 5.1 event in the same region (July 21, 1983, 45.84°N-11.32°E) a
large source depth (18 km) is confirmed by Gräfenberg sPn depth phases. The revised
depth determination of the two largest events in the region puts them directly at the depth
of the Adriatic Crust Interface, although their epicenters are more than 50 km west of
TRANSALP. However, it must be noted that at surface this part of the Alpine backthrust
system is strongly affected by N-S trending strike slip faults rather than the more linearly
ENE-WSW trending thrust belt next to the TRANSALP line. Despite this near-surface
kinematic pattern, the fault plane solution of the 1983 event  indicates its thrust
Above the ACI, significant – so far non-relocated - active seismicity occurs to as far
south as the current geomorphological limit of the Alps. Seismicity is particularly strong
in the hangingwall of the ACI while scattered seismicity of the overlying Valsugana
thrust belt indicates minor deformation of the latter as well. Published fault plane
solutions include strike-slip as well as thrust-type events [24, 43] underscoring active
shortening in the hangingwall of the ACI. At surface, current uplift and shortening has
been observed for the Bassano fault system  or Grappa System (, see also ) as
well as for the southward adjoining Monticelli Hills, where the underlying fault has not
yet broken to the surface. From these observations and from the coincidence of the larger
relocated events with the ACI near Trento we suggest that the Adriatic Crust Interface
coincides with the active thrust fault that accommodates current shortening of the
Adriatic plate. In contrast to most earlier sections through the Eastern Alps, including the
interpretation by the TRANSALP Working Group , these results would support
Schönborn’s  model of stacked detachments: These formed progressively deeper in
the Adriatic crust as the Eastern Alpine retrowedge developed rather than having a single
joint detachment active throughout Southern Alpine shortening. Elastic modelling of
lithospheric strength by Willingshofer and Cloetingh  would indicate that the ACI is
located at or slightly below the brittle-plastic transition in the Adriatic crust. It is not
surprising, therefore, that nearly all of the seismic deformation is located in the
hangingwall of the active detachment with exception of the largest events that typically
nucleate near this transition. The receiver function visibility of the ACI may again be
controlled by inherited lithological contrasts in the pre-Alpine basement that are
preferentially reactivated as Alpine faults.
In contrast to the Southern Alps, active seismicity in the northern Alps is not focused at
the orogenic front, but is localised along the Inn Valley fault system and its subsidiary
faults with nearly exclusive strike slip deformation (e.g. ). This deformation is
probably confined to the hangingwall of the Helvetic and NCA detachment (converter I
in Fig. 2a) since neither the reflection nor the receiver function data indicate its disruption
or offset. Only weak seismicity is observed along the Periadriatic fault system and even
less within the Alpine core zone, in both cases at shallower depths than along the
southern deformation front. This observation has also been made in the Central and
Western Alps. It is usually attributed to the current thermal structure of the orogen with a
warmer and weaker core of the orogen juxtaposed against cooler and stronger margins
(e.g. ). Moreover, the lateral continuity of the deep structures, in particular of the
Moho, would indicate that the current strike slip kinematics related to the active extrusion
of the core zone of the Eastern Alps to the east – as suggested by Ratschbacher et al.  –
is restricted to the crust and does not affect deeper parts of the orogenic wedge.
5. THE UPPER MANTLE
Migrated receiver function data down to 800 km depth are displayed in Fig. 8 (complete
data set in Fig. 8a, events from the west only in Fig. 8b, events from the east only in Fig.
8c). The map of piercing points (where the P to S conversion occurs) at 410 and 660 km
depth (Figs. 9a,b)shows that the covered area extends to the Central Alps and the
foothills of the Eastern Alps towards the Pannonian basin. Strong Moho conversions and
Moho multiples dominate the sections down to about 300km depth. They obscure
potential structures within the upper mantle. Thus, no direct image of the slab is visible.
The global discontinuities at 410 and 660 km depth in the IASP91 model  are quite
clear. They appear at only slightly shallower depth than in the IASP91 model in the
western section (Fig. 8b), but appear to be significantly raised in the eastern section (Fig.
8c). This would indicate that the upper mantle below the Eastern Alps and its northern
foreland is faster than the global average (cf. Figs. 8, 9). This is in agreement with cooler
temperatures there caused by former subduction and with higher than normal velocities
observed in seismic tomography [45, 46]. Lippitsch et al.  hypothesised that a
reversal in dip direction from southeast subducted European lower lithosphere in the
Central Alps to northeast dipping Adriatic lower lithosphere occurs in the Eastern Alps
near the TRANSALP profile. Our mantle RF are in accordance with this assumption for
the upper mantle northeast of the TRANSALP line. The upper mantle RF do not provide
constraints on the slab polarity west of TRANSALP because data coverage from
southwestern directions is insufficient (Fig. 9). The crustal TRANSALP reflection
seismic and RF section (Fig. 2b), however, indicates that the subduction is directed
southward at ~12° longitude and similar to the subduction style reported for the Central
Alps . A change in subduction polarity could only take place east of the TRANSALP
The differential time between the 410 and 660 discontinuities remains as predicted by the
IASP91 model, indicating that the cold and fast upper mantle does not extend to the
mantle transition zone. This is opposed to large-scale fast anomalies in the transition zone
beneath central Europe from P wave tomography .
In conclusion, the receiver function data of the Alpine crust confirm the seismic
discontinuities obtained from controlled source seismic data (near vertical incidence and
wide angle seismology). More generally, they underscore the potential of the receiver
function method to image the gross structure of orogenic belts. Beaumont and Quinlan
 have first suggested, that the seismic reflection pattern of orogenic belts images shear
zones, which are a direct consequence of the strain distribution and, hence, of the
rheological architecture of colliding lithospheres. Obviously, the receiver function data
also image this property by highlighting the key shear zones of the eastern Alpine system.
This is especially clear on the northern flank - the prowedge side of the orogen with
respect to former subduction polarity - where the shelf material of the downgoing
European plate was accreted above a detachment (I in Fig. 2a) under the older thrust
stack of the Austroalpine nappes (II in Fig. 2a). On the southern flank of the Alps - the
retrowedge side in the upper plate - the backthrust belt related to the Middle to Late
Tertiary Valsugana thrust belt (III in Fig. 2a) shows up in conversions from incident
waves from a northerly backazimuth (Fig. 2c). The north dipping ACI across parts of the
Adriatic crust may represent the currently active retrowedge shear zone system. This
zone links to the fault system, which was suggested for the deep continuation of the still
active Bassano-Friuli thrust wedge [17, 20, 21] that marks the southernmost end of the
topographic surface expression of the Alps. Below the Southern Alps thrust stack, the
Adriatic Moho appears to be flat at 40 km depth. At lithospheric scale, the receiver
function data clearly show the same style of indentation as has been observed in the
Central Alps .
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This research was supported by the German Bundesministerium fuer Bildung und
Forschung. Seismic stations have been provided by the GFZ Potsdam, the
University of Potsdam and the ETH Zuerich, the University of Genova and OGS
Trieste. Additional members of the TRANSALP working group are: H. Gebrande,
E. Lueschen, M. Bopp, F. Bleibinhaus, M. Stiller, K. Millahn, H. Grassl, F.
Neubauer, L. Bertelli, D. Borrini, R. Fantoni, C. Pessina, M. Sella, A. Castellarin,
R. Nicolich and A. Mazotti. We wish to thank Jim Mechie for reading the
manuscript and for helpful comments. We also thank two anonymous referees for
their constructive reviews.
Fig. 1: Location map of the seismic stations within the TRANSALP project and the near
vertical incidence reflection line in the eastern Alps.
IF = Inn Valley Fault, PL = Periadriactic Line, VBT = Valsugana Thrust Belt, BFT =
Fig. 2: a) Depth migrated short-period receiver functions along the TRANSALP line with
simplified geological section (0.08-1 Hz). EM = European Moho, AM = Adriatic Moho, I
= base of Mesozoic sediments and/or flysch sediments or Helvetic detachment, II = Sub-
Tauern Ramp reaching the surface at the Inn valley, III = Sub-Dolomites Ramp reaching
the surface at the tip of the Valsugana thrust belt, ACI = Adriatic Crust Interface, a north
dipping discontinuity in the middle and lower Adriatic crust. Green and blue circles select
values for crustal thickness, obtained from picking Pms times and their multiples in Fig.
3. Times are converted to depth values using a constant Vp/Vs ratio of 1.73 and Vp=6.0
km/s (green circles) and Vp=6.3 km/s (blue circles), respectively. See discussion in the
text. b) Superposition of receiver functions and seismic reflection line drawings. c)
Migrated data subsets for events with northern and d) southern backazimuths to
demonstrate stability of the results.
Fig. 3: Stacked receiver functions of the TRANSALP profile in the time domain after
moveout correction and bandpass filtering (0.08-1 Hz).a) short-period data, b)
broadband data. For the short-period instruments, the data are simulated broadband data.
Amplitudes of traces are balanced. The Moho step is clearly seen, where the European
and Adriatic Moho meet. Traces are sorted for piercing (=conversion) point latitude at 40
Pms = Moho conversion, PpPms = first Moho multiple, which has two P and one S paths
in the crust (darkly filled = positive amplitude), PpSms = second Moho multiple, which
has one P and two S paths in the crust (negative amplitude). Travel time curves of crustal
multiples are indicated for several Vp/Vs ratios (1.65, 1.73 and 1.80).
Fig. 4: Theoretical receiver functions for comparison with the data in Fig. 3b. a) Two-
dimensional crust-mantle model used for the computation of theoretical seismograms. b)
Theoretical receiver functions for events from the north, and c) for events from the south.
Vp/Vs ratio is assumed constant (1.73).
Fig. 5: Network of deep seismic refraction sounding experiments surrounding the
TRANSALP profile [22-25].
The coloured streaks mark the position of the reflecting points of the corresponding
discontinuity along the profiles under discussion. The numbers in the streaks exhibit the
depth of the reflecting points, mainly based on the record sections published by the
authors mentioned above. The two main discontinuities observed in the TRANSALP
receiver function and reflection seismic data (EM, blue colored streaks and AM, green
colored streaks) are present in the seismic refraction record sections. The ACI feature is
only recognisable in very few sections (yellow color). The grey streak displays the
contact zone between the European and Adriatic Moho.
Fig. 6: a) Epicenter distribution of events M>3.0 in the eastern Alps since 1975. Data are
compiled from ISC (International Seismological Centre) and CRS (Centro di Ricerche
Sismologiche Trieste). b) Focal depths are projected parallel to the strike of the Alps onto
the TRANSALP section. Stars represent the two largest earthquakes in the Trento-Schio
region with source depth determined in this study.
Fig. 7: Gräfenberg recordings of the event from 13 September 1989 in the Trento-Schio
region (location 45.9°N-11.3°E, magnitude 5.1). Depth phases sPn are clearly visible,
permitting exact depth determination.
Fig. 8: Migrated receiver functions along the TRANSALP profile down to 800 km
depth. a) all data, b) only events from the west, c) only events from the east. Data are
broadband and simulated broadband recordings for frequencies 0.08-0.5Hz. Mantle
velocities are according to the IASP91 global reference model.
Fig. 9: Location of piercing points at a) 410 km depth and b) at 660 km depth.
N. Calcareous Alps
48.0 47.547.046.5 46.0
Valsugana Thrust Bassano−Friuli Thrust
N. Calcareous Alps
4446 48 50
44 4648 50
44 4648 50
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