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The Decollement Depth of Active Thrust Faults in Italy: Implications on Potential Earthquake Magnitude
Language
English
Obiettivo Specifico
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
11/38 (2019)
Pages (printed)
3990-4009
Issued date
2019
Subjects
Compressional Seismicity in Italy
Abstract
Thrust fault ruptures during earthquakes do not often propagate down to the brittle‐ductile
transition. Lithological variations control the behavior and depth of regional basal thrusts and
decollement planes. Thrust fronts may be discontinuous along strike, limiting the dimension of single
coseismic ruptures. These factors control the maximum expected magnitude in one region. This is the case of
Italy where the convergence of few millimeter per year in the Apennines accretionary prism and along the
retrobelt of the Alps generates compressional earthquakes with moderate to strong magnitudes. Here, using
geological and geophysical data, we first compile a map of the undulated active basal thrust decollement for
Italy that occurs from 1 to 17‐km depth. Then, we verify the relationship between the length of seismogenic
ruptures in thrust faults (Lf) and the maximum depth of thrust faulting (zmax) of related earthquakes and
find that their ratio (Lf/zmax) ranges between 2 and 4. Finally, we compute the potential seismogenic volume
and estimate the maximum magnitude using an empirical relationship that multiplies the decollement
depth and the Lf/zmax ratio. Maximum calculated magnitude is 6.7 ± 0.37 (depending on Lf/zmax and fault
dip angle), consistent with the largest magnitude of thrust‐related earthquakes recorded in Italy (6.5–7.0).
Lower magnitudes are predicted in the Ionian Seas at the external front of the Apennines where smaller
crustal volumes are involved, whereas higher magnitudes are expected in the southern Po Basin, the western
Adriatic Sea, Sicily offshore, and the Southern Alps where the decollement is deeper and the brittle volumes
are far greater.
transition. Lithological variations control the behavior and depth of regional basal thrusts and
decollement planes. Thrust fronts may be discontinuous along strike, limiting the dimension of single
coseismic ruptures. These factors control the maximum expected magnitude in one region. This is the case of
Italy where the convergence of few millimeter per year in the Apennines accretionary prism and along the
retrobelt of the Alps generates compressional earthquakes with moderate to strong magnitudes. Here, using
geological and geophysical data, we first compile a map of the undulated active basal thrust decollement for
Italy that occurs from 1 to 17‐km depth. Then, we verify the relationship between the length of seismogenic
ruptures in thrust faults (Lf) and the maximum depth of thrust faulting (zmax) of related earthquakes and
find that their ratio (Lf/zmax) ranges between 2 and 4. Finally, we compute the potential seismogenic volume
and estimate the maximum magnitude using an empirical relationship that multiplies the decollement
depth and the Lf/zmax ratio. Maximum calculated magnitude is 6.7 ± 0.37 (depending on Lf/zmax and fault
dip angle), consistent with the largest magnitude of thrust‐related earthquakes recorded in Italy (6.5–7.0).
Lower magnitudes are predicted in the Ionian Seas at the external front of the Apennines where smaller
crustal volumes are involved, whereas higher magnitudes are expected in the southern Po Basin, the western
Adriatic Sea, Sicily offshore, and the Southern Alps where the decollement is deeper and the brittle volumes
are far greater.
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article
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2019_Tectonic M max contraction Italy.pdf
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