Based on : Villani, Fabio, and Vincenzo Sapia. "The shallow structure of a surface-rupturing fault in unconsolidated deposits from multi-scale electrical resistivity data: The 30 October 2016 Mw 6.5 central Italy earthquake case study." Tectonophysics 717 (2017): 628-644. Available at : https://www.researchgate.net/publication/318896772_The_shallow_structure_of_a_surface-rupturing_fault_in_unconsolidated_deposits_from_multi-scale_electrical_resistivity_data_The_30_October_2016_Mw_65_central_Italy_earthquake_case_study

1. Title : The shallow structure of a surface-rupturing fault in
unconsolidated deposits from multi-scale electrical resistivity
data: The 30 October 2016 Mw 6.5 central Italy earthquake
case study
Journal : Tectonophysics 717, 628-644
Year of Publication : 2017
Authors : Fabio Villani, Vincenzo Sapia
(National Institute of Geophysics and Volcanology, Italy)
Presented by,
Adithya Shashidhara Shettar
17411001

2. Introduction
In general, only limited portions of active brittle fault zones are exposed at the
surface, due to the combined result of erosion and tectonic rock exhumation. In many
cases, active normal faults trigger rapid feedback response of surface processes, such
as gravitational instability and erosional dismantling of the uplifted footwall blocks,
coupled with enhanced sedimentation in the downthrown hanging-wall counterpart.
This results in a complex setting of the uppermost structural levels, particularly in
the case of tectonically active regions. For these reasons, geophysical investigation
of active fault zones can provide unique insight (Everett, 2013). Electrical resistivity
methods proved successful in imaging the subsurface of such complex structures.

3. The Problem
The main processes that lead to low electrical resistivity within fault zones are:
1) a higher degree of fracturing within the damage zone, which increases
permeability thus facilitating fluid circulation
2) the development of clayey minerals along the slip surfaces, within damage zone
3) reaction-driven changes in porosity of the protolith
So, the problem boils down to the identification of a Low resistivity region in the
subsurface.

4. Study Area
The study area is
a profile across
an exposed
coseismic rupture
of a normal fault
in the central
Appenines of
Italy.

5. Data and Methods
1. In order to recover the shallow subsurface electrical resistivity
image of the fault zone, the subsoil was investigated over one
transect and nearly orthogonal to the trace of the coseismic
surface ruptures
2. All ERT profiles are purposely centred on the rupture
3. Instrument used : Syscal R2 (IRIS Instrument) with a set of
64 stainless steel electrodes
4. Both Dipole-Dipole (DD) and Wenner-Schlumberger (WS)
array data were collected (To reach different depths of
investigation, and obtain good compromise between horizontal
and vertical resolution

6. Data Processing and other details
1. Injected a square-wave signal for 250 ms into the
ground, using 100, 200 and 400 Volts for the
P1_2m, P1_5m, and P1_10m profiles, respectively.
2. Resistance contact check allowed us to avoid
relatively high resistance values.
3. Used fresh water to enhance coupling between the
electrodes and the ground.
4. To account for noisy data, obvious outliers were
manually edited and apparent resistivity data
derived from very low voltage and/or current level
were removed before running the inversion.
5. Acquiring reciprocal records was not possible from
their instrument

7. Results (2m electrode spacing)
The western part of the ERTs exhibits a very shallow layer (unit A; <1 m thick on average)
characterized by ρ < 170 Ωm, and locally being <100 Ωm: the bottom of this layer is defined
by a sharp resistivity gradient. Below unit A, both the western and eastern parts of the ERT
models are characterized by a pack of high-resistivity material (ρ > 400 Ωm, unit B).
B,B’ and B’’ are named so because they are
different regions of same layer.
Below the scarp, a relatively-low resistivity
region (ρ ~100–170 Ωm) is evident at
x = 60–66 m (unit FZ).

8. Results (5m electrode spacing)
A nearly horizontal pack of relatively uniform low-resistivity (ρ <130 Ωm), which
is named unit C. This has no counterpart in the eastern part of both models, where a
heterogeneous and patchy resistive body is found down to the bottom (ρ ~ 200–370
Ωm; unit D). As for results of profile ERT P1_2m, these models still image below
the scarp a steeply-dipping and narrow zone of relatively low-resistivity (unit FZ),
which displays some continuity with the
easternmost portion of unit C.

9. Results (10m electrode spacing)
Data for array DD are quite noisy and the RMS residuals pretty high (Table 1),
however the unit FZ is very clear, being 10–12 m wide, and it preserves a sub-
vertical attitude down to the model bottom. Data for the WS array are of higher
quality, and the obtained resistivity model suggests that unit FZ continues at depth
(down to about 120 m below the ground surface) with comparable width and
attitude. This model provides us with
useful information at depth to definitely
constrain the thickness of the low-
resistivity unit C to about 25 m.

10. Interpretation
1. The main electrical units are referred to coarse alluvial fan bodies and interbedded
fine-grained fluvio-lacustrine sediments, and the fault zone FZ is interpreted as a
narrow, sub-vertical band of relatively-low resistivity developed up to the very near
surface.
2. The average shift along the fault surface is mapped in the figure, by estimating the
base of feature B, in the western and eastern sides of the fault.

11. Conclusion
1) Following the 30 October 2016 quake, the primary surface faulting within the
basin occurred in correspondence of a steeply-dipping to sub-vertical fault zone,
clearly detected down to a depth of about 100–120 m;
2) The fault zone appears as a narrow and elongated relatively low-resistive
(100–150 Ωm) region that we interpreted to be the result of the migration of
water from the shallow aquifer squeezed by seismic waves; then, the increase of
permeability is due to the fracturing of the damage zone;
3) The fault zone also shows a nearly constant width (20–30 m) at depths>40 m,
as imaged by the 5-m and 10-m spaced ERT profiles;

12. Thank You
!?

13. Appendix