Download Poster

Abnormal Earthquake Behavior in the
Velocity Zones Beneath Italy
Laura Nichele Raines and Heather Lehto, Ph.D.
Angelo State University, Department of Physics and Geosciences
Slice Stacks
Introduction
There is a section of Italy that has sparked a great
deal of interest when looking at the shear wave
velocities. Italy’s upper island, Sardinia, and a
portion of Italy above the heal host this puzzling
data. One location has a section of earthquakes
along the Wadati-Benioff Zone that disappear. The
earthquakes, which are within a subduction zone,
are absent when there is a higher shear wave (swave) zone present. In this Wadati-Benioff Zone
where the earthquakes do not occur, it is almost as
if the higher velocities inhibit the earthquakes,
because after the high velocity zone has ended, the
earthquakes resume. It could be possible that the
earthquakes aren’t occurring in these areas because
of special conditions along the Wadati-Benioff Zone.
There is also a location, in Sicily, where there is a
very low velocity zone that has all the earthquakes
above or surrounding it. In Sicily, these earthquakes
that happen above the low velocity zone could be
related to the volcanic activity on the island.
Discussion
Map of Italy
• The earthquakes follow the curve of Italy.
• Earthquakes surround the low velocity zone possibly due to magma
formation in that area.
• Earthquakes are in the slab but not in the low velocity zone.
Cross Section A-A’
• From the data collected, there is a section of earthquakes that disappear
along the subduction zone beneath the lower portion of Sardinia and a small
section of Italy (Figure 4).
• These earthquakes along the Wadati-Benioff Zone that disappear could be
caused from a change in pore fluid pressure along the plate (Davis and
Reynolds, 1996).
• Davis and Reynolds (1996) stated that elevated pore pressure can decrease
ultimate and rupture strength along with a decrease in ductility.
• Disappearing earthquakes could possibly be caused from fluid pressure
which counteracts the stress that comes from volatiles, typically water, which
is driven off the subducting plate (Davis and Reynolds, 1996).
• There is a higher velocity area, represented by the blue on the cross sections,
that appears to stop the earthquakes.
• After the higher velocity zone has ended, the earthquakes of all sizes resume
to normal levels.
• The reason this section is so interesting is because there is an undefined gap
of missing earthquakes along the subduction zone.
Cross Section B-B’
• There is also a subduction zone that occurs in Sicily, although it is not as
prominent as the one in Sardinia and portions of Italy (Figure 5).
• Most of the earthquakes occurring along this section of Sicily in the shallow
crust, but not the slab, could be related to fault zones that run through the
area or volcanic activity.
• The s-wave velocity in Sicily is significantly lower than that of Sardinia and
portions of Italy.
Figure 1: This map represents
the study area, which is Italy. A
map is included so that you can
specifically see what areas I
have studied.
Methods
Figure 3: These slice stacks represent the overall swave velocity of Italy from depths of 25km to
200km. Note the area of low velocity is centered
between Sardinia and mid Italy. Below 200 km the
low velocity zone disappears.
• Used a program from Incorporated Research Institute for Seismology
(IRIS) called IRIS Earth Model Collaboration (EMC) to generate the cross
sections, slice stacks, and horizontal views of Italy using the
tomographic velocity model Kustowski, Ekstrom, Dziewonski (2008),
and also used it for the earthquake locations from the IRIS global
catalog.
• Tomographic Models that have the most useful information are cross
sections, stacks, and horizontal slice viewer.
• Used data from existing studies to help explain the problems.
Conclusion
Results
Cross Sections
B
A
B’
A’
There are two portions of Italy that raise curiosity. The main section has
missing earthquakes along the Wadati-Benioff Zone. Disappearing
earthquakes could be caused from a change in pressure or lubrication of
the subducting tectonic plate. The other location with the relatively low
s-wave velocity could have the earthquakes caused from either volcanic
activity or because of where it sits on the faults of Italy.
Acknowledgements
Figure 2: Map of the seismic
behavior seismic across Italy.
A-A’ and B-B’ represent the
locations of cross sections in
figures 4 and 5. Note that the
earthquakes surround the low
velocity zone.
1
Dr. Heather Lehto, Michael Raines, Josue Rodriguez, Pierfrancesco Burrato, Angelo
State University, Angelo State University Undergraduate Research Grant, Angelo State
University Dept. of Physics and Geosciences.
References
Figure 4: A-A’ cross section is from
the lower portion of Sardinia and a
small section of Italy. A subduction
zone appears to be within this
cross section. In addition to the
subduction zone, there is an
absence of earthquakes that
coincides with the high velocity
zone.
Figure 5: B-B’ cross section is
across the upper part of Sicily.
Rather shallow earthquakes seem
to be linked to crustal faults and
volcanoes. A subduction zone is
also located here.
Badal, José, et al. "S-Wave Velocity Images Of The Dead Sea Basin Provided By Ambient Seismic Noise." Journal Of Asian Earth Sciences 75.(2013): 26-35. ScienceDirect. Web. 27 Apr.
2016.
Civetta, L., et al. "Thermal And Geochemical Constraints On The ‘Deep’ Magmatic Structure Of Mt. Vesuvius." Journal Of Volcanology And Geothermal Research 133.The Neapolitan
Volcanoes: Vesuvius, Campi Flegrei and Ischia (2004): 1-12. ScienceDirect. Web. 27 Apr. 2016.
Correale, Alessandra, et al. "A Two-Component Mantle Source Feeding Mt. Etna Magmatism: Insights From The Geochemistry Of Primitive Magmas." Lithos 184-187.(2014): 243-258.
Science & Technology Collection. Web. 27 Apr. 2016.
Davis, George H., and Stephen J. Reynolds. Structural Geology of Rocks and Regions. 2nd ed. New York: John Wiley, 1996. Print.
Hudnut, K. W., and J. J. Taber. "Transition From Double To Single Wadati-Benioff Seismic Zone In The Shumagin Islands, Alaska." Geophysical Research Letters 14.2 (1987): 143. Publisher
Provided Full Text Searching File. Web. 27 Apr. 2016.
"Influence Of Water On Earthquake Generation Along Subduction Zones." (2002): OAIster. Web. 27 Apr. 2016.
IRIS EMC: http://ds.iris.edu/dms/products/emc/horizontalSlice.html#options
Kustowski, B., G. Ekström, and A. M. Dziewoński. "The Shear-Wave Velocity Structure In The Upper Mantle Beneath Eurasia." Geophysical Journal International 174.3 (2008): 978-992.
Science & Technology Collection. Web. 27 Apr. 2016.
Labrousse, L., T. Duretz, and T. Gerya. "H2O-Fluid-Saturated Melting Of Subducted Continental Crust Facilitates Exhumation Of Ultrahigh-Pressure Rocks In Continental Subduction
Zones." Earth & Planetary Science Letters 428.(2015): 151-161. Science & Technology Collection. Web. 27 Apr. 2016.
Špičák, Aleš, Václav Hanuš, and Jiří Vaněk. "Seismotectonic Pattern And The Source Region Of Volcanism In The Central Part Of Sunda Arc." Journal Of Asian Earth Sciences 25.(2005):
583-600. ScienceDirect. Web. 27 Apr. 2016.
Špičák, Aleš, Václav Hanuš, and Jiřı́ Vaněk. "Seismicity Pattern: An Indicator Of Source Region Of Volcanism At Convergent Plate Margins." Physics Of The Earth And Planetary Interiors
141.Structure and tectonics of convergent plate margins (2004): 303-326. ScienceDirect. Web. 27 Apr. 2016.