(Greenberg, 1974). Though unsuccessful it developed
critical technology and led directly to the Deep Sea Drilling Project.
Observational techniques
Studies of the Mohorovičić discontinuity have largely
focused on seismic techniques. The analysis of
seismograms for the P n and other arrivals was used to
map the Moho and the presence of other intracrustal discontinuities (Prodehl et al., 2013), particularly on continents. In the oceans, studies of seismic refraction were
pioneered by Ewing and Ewing (1959) with the use of
controlled-source seismology (active sources). Following
these first studies (e.g., Shor, 1963; Sutton et al., 1971),
marine multichannel reflection studies helped define the
oceanic Moho (e.g., Hasselgreen and Clowes, 1995).
Active-source seismology has proved very effective in
continental regions through the use of both deep seismic
reflection and refraction experiments. Reflection studies
provide the highest resolution images of the Moho,
whereas seismic refraction studies provide the best velocity control, and hence relation to the standard Moho definition. More recently the use of earthquake receiver
functions has revealed the richness of intracrustal structure
such as double and inverted Mohos in subduction zones
(Bostock et al., 2002). Receiver functions allow for the
removal of earthquake source complexity and image local
structure. Stacking and migration produce depth sections
and illuminate the variability of the Moho, though generally with lower resolution than active-source studies,
except across dense networks. Novel techniques like
inversions of ambient noise (Shapiro and Campillo,
2004) and surface waves (Lebedev et al., 2013) further
illustrate Moho complexity. Surface wave waveforms
and dispersion are strongly influenced by Moho depth,
but combined with a priori knowledge of seismic velocities; receiver functions and gravity can be jointly inverted
to create regional maps of the Moho.
It is possible to use gravity data to study crustal structure and detect the Moho as the density increases across
the interface (e.g., Oldenburg, 1974). However, gravitational inversions are nonunique and require additional
information. It is possible to overcome the nonuniqueness
by a joint inversion of gravity data with point information
on crustal structure from seismology. High-resolution
global gravity data from satellite missions (GRACE and
GOCE) has led to global and regional Moho models
(Reguzzoni et al., 2013; van der Meijde et al., 2013).
Current knowledge
The Mohorovičić discontinuity is not fully mapped in the
oceanic basins, and the distribution of observations is
irregular (Mutter and Carton, 2013). Often we refer
instead to the reflection Moho, the deepest, highamplitude, laterally extensive reflection or group of reflections present at depths commensurate with other estimates
of crustal thickness (Cook et al., 2010). However, current
knowledge puts its average depth below the seafloor at
6–7 km reaching thicknesses >25 km at oceanic plateaus
like Ontong Java (Gladczenko et al., 1997). Initial observations implied a sharp interface and little change towards
the ridge crest. Recent work has shown that the Moho
ranges from a sharp interface to a gradual transition
(Mutter and Carton, 2013). Moho variability in the oceans
Mohorovic ˇic ´ Discontinuity (Moho), Figure 1 A seismogram from a M 4.3 earthquake in Central California recorded at station PFO,
280 km away, shows the early arrival of P n . - Inset: Cartoon showing the direct P and the headwave, P n , along the interface.
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MOHOROVIC ˇ IC ´ DISCONTINUITY (MOHO)
critical technology and led directly to the Deep Sea Drilling Project.
Observational techniques
Studies of the Mohorovičić discontinuity have largely
focused on seismic techniques. The analysis of
seismograms for the P n and other arrivals was used to
map the Moho and the presence of other intracrustal discontinuities (Prodehl et al., 2013), particularly on continents. In the oceans, studies of seismic refraction were
pioneered by Ewing and Ewing (1959) with the use of
controlled-source seismology (active sources). Following
these first studies (e.g., Shor, 1963; Sutton et al., 1971),
marine multichannel reflection studies helped define the
oceanic Moho (e.g., Hasselgreen and Clowes, 1995).
Active-source seismology has proved very effective in
continental regions through the use of both deep seismic
reflection and refraction experiments. Reflection studies
provide the highest resolution images of the Moho,
whereas seismic refraction studies provide the best velocity control, and hence relation to the standard Moho definition. More recently the use of earthquake receiver
functions has revealed the richness of intracrustal structure
such as double and inverted Mohos in subduction zones
(Bostock et al., 2002). Receiver functions allow for the
removal of earthquake source complexity and image local
structure. Stacking and migration produce depth sections
and illuminate the variability of the Moho, though generally with lower resolution than active-source studies,
except across dense networks. Novel techniques like
inversions of ambient noise (Shapiro and Campillo,
2004) and surface waves (Lebedev et al., 2013) further
illustrate Moho complexity. Surface wave waveforms
and dispersion are strongly influenced by Moho depth,
but combined with a priori knowledge of seismic velocities; receiver functions and gravity can be jointly inverted
to create regional maps of the Moho.
It is possible to use gravity data to study crustal structure and detect the Moho as the density increases across
the interface (e.g., Oldenburg, 1974). However, gravitational inversions are nonunique and require additional
information. It is possible to overcome the nonuniqueness
by a joint inversion of gravity data with point information
on crustal structure from seismology. High-resolution
global gravity data from satellite missions (GRACE and
GOCE) has led to global and regional Moho models
(Reguzzoni et al., 2013; van der Meijde et al., 2013).
Current knowledge
The Mohorovičić discontinuity is not fully mapped in the
oceanic basins, and the distribution of observations is
irregular (Mutter and Carton, 2013). Often we refer
instead to the reflection Moho, the deepest, highamplitude, laterally extensive reflection or group of reflections present at depths commensurate with other estimates
of crustal thickness (Cook et al., 2010). However, current
knowledge puts its average depth below the seafloor at
6–7 km reaching thicknesses >25 km at oceanic plateaus
like Ontong Java (Gladczenko et al., 1997). Initial observations implied a sharp interface and little change towards
the ridge crest. Recent work has shown that the Moho
ranges from a sharp interface to a gradual transition
(Mutter and Carton, 2013). Moho variability in the oceans
Mohorovic ˇic ´ Discontinuity (Moho), Figure 1 A seismogram from a M 4.3 earthquake in Central California recorded at station PFO,
280 km away, shows the early arrival of P n . - Inset: Cartoon showing the direct P and the headwave, P n , along the interface.
516
MOHOROVIC ˇ IC ´ DISCONTINUITY (MOHO)