1
Piecewise delamination of Moroccan lithosphere from beneath the
2
Atlas Mountains
3
M. J. Bezada, E. D. Humphreys, J. M. Davila, R. Carbonell, M. Harnafi, I.
4
Palomeras, A. Levander.
5
6
Supplementary Information
7
8
Squeezing test
9
10
The first step of the squeezing test is to obtain a model that best accounts for the
11
observed delays without including any fast anomalies deeper than 200 km, other
12
than those belonging to the Alboran slab. It is thus necessary to selectively damp
13
those nodes where fast anomalies are produced by the inversion. To impose this
14
constraint, we first define the set of non-slab model nodes. The first iteration
15
proceeds without any additional constraint. Non-slab nodes that are assigned a
16
fast velocity anomaly in the first iteration receive a very high damping value in all
17
subsequent iterations so their total accrued anomaly tends to zero, i.e. they are
18
“squeezed”. Additional non-slab nodes may become squeezed if fast anomalies
19
are assigned to them in later iterations. In this manner, only those model nodes
20
where the inversion attempts to place fast anomalies receive extraordinary
21
damping.
22
23
Purported Intermediate Depth Earthquakes
1
24
25
Intermediate-depth earthquakes have been reported to occur beneath the
26
Moroccan Atlas region by several authors [e.g. Buforn et al., 1995; Hatzfeld and
27
Frogneux, 1981] . These earthquakes hold particular interest given that they
28
seem to be unrelated to a subduction process and pose a challenge to the
29
current understanding of intermediate-depth seismicity. They have been
30
associated with delamination [Ramdani, 1998] but their occurrence in what we
31
image as asthenosphere-filled lithospheric cavities produced by delamination
32
makes them even more puzzling.
33
34
Our array provides unprecedented coverage of the region, and the seismic
35
tomography models provide a high-resolution 3D velocity structure of the area
36
that can help produce accurate locations of these events. We selected for
37
relocation two events located in the Middle Atlas and eastern Moroccan Meseta
38
(Figure S1) with catalog depths ≥90 km that occurred during our deployment.
39
The magnitude of these events (ml 2.5 and 2.6) is large enough that they very
40
well recorded by our stations, allowing for confident picking of P and S arrivals.
41
We picked the arrival times for P and S phases and locate the events using a
42
three-dimensional grid-search approach. For hypothetical hypocenters on a
43
mesh surrounding the catalog locations, we calculate travel-times through our
44
velocity model and find the point that minimizes travel-time misfits.
45
2
46
We find that the locations of the best-fitting hypocenters differ laterally from the
47
reported locations by as much as a few tens of kilometers. More importantly,
48
superficial (<5 km) depths fit the arrival times significantly better than the
49
reported catalog depths (Table S1). Given that the depths of these relatively
50
high-quality events were greatly overestimated, we conclude that there has been
51
a systematic miss-location of events in this area, and events thought to be of
52
intermediate-depth are probably crustal in nature.
53
54
References
55
56
57
58
Buforn, E., C. Sanz de Galdeano, and A. Udías (1995), Seismotectonics of the
Ibero-Maghrebian region, Tectonophysics, 248(3–4), 247–261,
doi:10.1016/0040-1951(94)00276-F.
59
60
61
Hatzfeld, D., and M. Frogneux (1981), Intermediate depth seismicity in the
western Mediterranean unrelated to subduction of oceanic lithosphere,
Nature, 292(5822), 443–445, doi:10.1038/292443a0.
62
63
64
65
Ramdani, F. (1998), Geodynamic implications of intermediate-depth earthquakes
and volcanism in the intraplate Atlas mountains (Morocco), Physics of the
Earth and Planetary Interiors, 108, 245–260, doi:10.1016/S00319201(98)00106-X.
66
Event #1
Event #2
catalog
preferred
catalog
preferred
latitude
33.038
33.275
33.89
33.72
longitude
-4.785
-4.468
-5.98
-5.86
depth (km)
91
0
99
4
RMS misfit (s)
4.1
0.8
6.3
3.3
3
67
Table S1. Catalog locations and preferred locations of the two events analyzed.
68
Our preferred locations are much shallower than the reported ones, residing in
69
the upper crust, and provide a significantly better fit to the observations (Figure
70
S2).
71
72
73
74
Fig. S1. Map of relocated events. Open symbols show catalog locations, solid
75
symbols indicate locations using 3D velocity model. In each case the depth is
76
shown next to the symbol. Inverted triangles show the location of stations that
77
recorded at least one of the events. Thin gray lines show tectonic boundaries for
78
spatial reference.
79
80
4
81
82
Fig. S2. Calculated and observed travel-times as a function of epicentral
83
distance for the two relocated events, and histograms of misfits. Travel-times
84
were calculated through the 3D velocity structure for the original catalog locations
5
85
and our preferred locations. For both events the preferred (shallow) locations
86
provide a significantly better fit to the observations.
6