After a damaging, medium-sized earthquake (Mw 5.8; October 17, 2015) in El Galpón area in the Salta Province, we installed a local seismological network around the estimated epicenter covering also remarkable tectonic and geological orographic structures. The 2015 earthquake took place in the Andean foreland at about 17km depth. The so called Santa Bárbara System is the easternmost morphostructural region of the central Andes. As a part of the broken foreland it is bounded to the north by the Subandean Belt and the Sierras Pampeanas lying in the south; to the east joins the Chaco-Paraná basin.Eje: Estudio del Interior Terrestre.Facultad de Ciencias Astronómicas y Geofísica

Alvarado, P.

Aranda, G.

Hongn, F.

Krüger, F.

Landgraf, A.

Morales, F.

Zeckra, M.

English

SEDICI - Repositorio de la UNLP

   
 
 
 
  
 
172  
STRATEGy  
 Seismic neTwoRk/Array in norThwEstern arGentina:  
Study of the 2015 El Galpón earthquake and its aftershock 
sequence 
 
M. Zeckra1, F. Krüger¹, A. Landgraf¹, G. Aranda², F. Hongn², F. Morales³ y P. 
Alvarado³ 
 
1 University of Potsdam, Germany. zeckra@uni-potsdam.de  
2 Universidad Nacional de Salta (CONICET – IBIGEO). 
3 Dpto. Geofísica y Astronomía, FCEFN Universidad Nacional de San Juan - CONICET 
(GIGEOBIO). 
 
Introduction 
After a damaging, medium-sized earthquake (Mw 5.8; October 17, 2015) in El 
Galpón area in the Salta Province, we installed a local seismological network around 
the estimated epicenter covering also remarkable tectonic and geological orographic 
structures. 
The 2015 earthquake took place in the Andean foreland at about 17km depth. The 
so called Santa Bárbara System is the easternmost morphostructural region of the 
central Andes. As a part of the broken foreland it is bounded to the north by the 
Subandean Belt and the Sierras Pampeanas lying in the south; to the east joins the 
Chaco-Paraná basin.  
There is a north to south segmentation which is reflected in different tectonic styles:  
The Subandean Belt is dominated by thin-skinned deformation mainly observed in its 
sedimentary cover with records of the strongest shortening along the Andes (Kley et al. 
1999). Instead, shortening decreases suddenly in the Santa Bárbara System, where 
basement-cored ranges show a thick-skinned style related to inversion of Cretaceous 
rift normal faults. According to Iaffa et al. (2013) and Barcelona et al. (2014), thick 
sedimentary basins partly which exhibit thin-skinned deformation enclose ridges 
uplifted by high-angle dipping faults. The Metán basin (Fig. 3) recording both 
Cretaceous-Paleogene rifting and Cenozoic foreland sedimentation is an outstanding 
geological feature in the Santa Bárbara System. The Sierras Pampeanas to the south 
show the least amount of shortening with deep rooted basement thrusts just to the east 
of the thin-skinned Precordillera (Jordan and Allmendinger, 1986). From north to south 
the Santa Bárbara system shows a tectonic transition from pure thin-skinned 
   
 
 
 
  
 
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deformation to foreland basement thrusts. This also correlates with changes in the 
subduction zone geometry (Alvarado et al., 2009).  
Deep seated earthquake hypocenters are located along the subducted Nazca plate 
with two depth clusters at around 250 km and 600 km (Fig. 1). Besides the slab 
seismicity, the study area is also affected by shallow crustal earthquakes. Thus, 
releasing of stresses from the foreland deformation can result in strong earthquakes, 
i.e. the Esteco (M>7) 1692 earthquake (INPRES, 2017).  
The Metán basin where the 2015 earthquake has occurred, has been well studied 
using seismic line analyses (Iaffa et al. 2011; 2013). Location and orientation of faults 
are known as well as depocenters and thickness of the sedimentary units in the basin. 
However, much less is known from seismicity and active faults. Hence we installed a 
temporary seismic network around the 2015 epicenter and the major orographic 
structures such as Cerro Colorado and Sierra de la Candelaria.  
 
Seismic deployment and processing of seismic data 
The network extends 110 km from north to south and 60 km from east to west (see 
Fig. 3). Each station consists of a Lennartz LE-3D/5s seismometer and a DATA-CUBE³ 
digitizer. Power supply is provided by a battery, which is recharged through a solar 
panel. This system is recording with 100 Hz on all three components and running fully 
autonomously. The data is stored on the digitizer. Maintenance is done every 5 months 
in order to download the data and release memory. It is planned to obtain seismic 
records continuously for up to 15 months. 
Detection and location of all recorded earthquakes is the first step of data 
processing. Thus, a first step consisted of using an automatic detector (Heimann, pers. 
Comm) based on synthetic seismograms and doing a grid search through the whole 
dataset. Then, an automatic autoregression picker searches for P and S wave arrival 
times in combination with a network coincidence trigger to select events only with 
similar arrival time picks at four different stations minimum (Leonard and Kennett, 
1999). All picks are manually controlled and repicked if necessary. In the end, the 
events are located using HYPOSAT a FORTRAN routine to calculate hypocenters of 
seismic events (Schweitzer, 1997, 2001). 
Up to now, a global seismic velocity model is used to locate seismic events. The first 
months of seismological records contain earthquakes of different epicentral distances. 
Local earthquakes within the network are the most frequent observations (Fig. 2). Many 
of these events are supposed to be categorized as aftershocks of the 2015 El Galpón 
earthquake. For some events residents noticed macroseismic activity and reported it. A 
second category of earthquakes with an emergent onset in the arrival of their seismic 
records are events located at regional distances with several 100 km of epicentral 
   
 
 
 
  
 
174  
distance. Less frequent events are local very deep earthquakes as well as teleseisms. 
The latter are difficult to identify due to the limited frequency bandwidth of the 
instruments.  
 
Results 
Preliminary results of earthquake epicenters are shown in figure 3. Within the first 
month of recording approximately 230 events have been registered and 160 
earthquakes have been located. Although using the IASP91 global seismic velocity 
model (Kennet, 1991), the epicenter errors are small for earthquakes located inside the 
network. We also observed a small cluster of earthquakes in between the two mountain 
ranges Cerro Colorado and Sierra de la Candelaria. 
In order to improve the seismic location and reduce its errors a local velocity model 
is needed. This will be derived from ambient noise cross correlation. This technique 
assumes when calculating the cross correlation of the waveform at two seismic stations 
that one of the sensor is a virtual source. The result of the cross correlation is the 
Green's function, which only contains information of the path in between both stations. 
After correlating all possible pair-station combinations (forward and backward) a local 
shallow underground model can be interpolated.  
Another analysis of deeper structures can be done using receiver functions. We plan 
to use conversion of teleseismic P to s wave to derive information about the depth and 
the orientation of the Mohorovičić crust-mantle discontinuity. We will try to identify the 
Nazca slab conversion phases. It is worth to note that the subducted plate seems to be 
aseismic underneath the network (Fig. 1). Some of the deep neighbor earthquakes with 
focal depths deeper than 200km will be tested fo a local receiver function analysis. This 
can produce steep incidence angles of the seismic ray path at the STRATEGy seismic 
stations and help to identify the Moho and mid-crustal discontinuities. A similar 
technique has been applied in the eastern Sierras Pampeanas by Perarnau et al. 
(2012). 
Earthquake locations can be improved using the double difference method 
(Waldhauser and Ellsworth, 2000). This relocation method relates the residuals 
between observed and theoretical travel-time differences of pairs of close hypocenters 
at two stations. This will help to create images of the local seismicity and improve the 
ability to relate it to the seismotectonics of the region of study. 
In the end, all the derived information will be used for moment tensor inversion. 
These results are necessary to identify the mechanisms of deformation at different 
parts of the fault system beneath the Metán basin. Furthermore, we expect to gain 
more details about kinematic and orientation of faults which led to the uplift of the 
surrounding mountains ranges. 
   
 
 
 
  
 
175  
 
Figure 1: Hypocenters (blue dots) between 1900 – 2015 in the 23 – 28°S latitude region from 
the ISC Bulletin plotted as a function of depth and longitude; elevation profile (red line) and 
location of volcanoes (red – inactive, green – active, triangles) exaggerated by a factor of 10; 
black lines indicate the region of the temporary installed seismological network. 
Figure 2: A local earthquake on September 1, 2016 with epicenter location within the seismic 
network; note it is very close to station 02A. P and S wave propagation throughout the network 
can be observed. Only vertical component displayed, bandpass filter 1 – 20 Hz. 
   
 
 
 
  
 
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Conclusions 
Analyses of one month of data using the 13 temporary seismic Strategy stations 
shows around 60 crustal earthquakes, none slab events beneath the El Galpón area 
and approximately 120 outside the network.  
We are on the way to obtain and refine a seismic velocity model that could improve 
characterization of seismic sources and estimate pattern of deformation in the area. 
Figure 3: Preliminary epicentral locations (blue stars) considering the first month of recording of 
the STRATEGy seismic network (green triangles). Location error ellipses are black. Elevation 
background map is derived from SRTM data. Superimposed is the focal mechanism information 
for the 2015 El Galpón Mw 5.7 earthquake (GEOFON Data Centre). Some cities are indicated 
by squares and circles. Former position of station 01A (grey triangle) indicates location of the 
village El Galpón. 
   
 
 
 
  
 
177  
References 
Alvarado et al. 2009. Flat-slab subduction and crustal models for the seismically active 
Sierras Pampeanas region of Argentina. In The Geological Society of America 
204.12: 261-278. 
 
Barcelona et al. 2014. Tectonic activity revealed by morphostructural analysis: 
Development of the Sierra de la Candelaria range, northwestern Argentina. J. of 
South Am. Earth Sciences 56: 376-395. 
 
Iaffa et al. 2011. The role of inherited structures in a foreland basin evolution. The 
Metán Basin in NW Argentina. J. of Structural Geology 33.12: 1816-1828. 
 
Iaffa et al. 2013. Basin fragmentation controlled by tectonic inversion and basement 
uplift in Sierras Pampeanas and Santa Bárbara System, northwest Argentina. 
Geological Society of London, Special Publications 377.1: 101-117. 
 
Kennet 1991. IASPEI 1991 seismological tables. Terra Nova 3.2: 122-122. 
 
Kley et al. 1999. Along-strike segmentation of the Andean foreland: Causes and 
consequences. Tectonophysics 301.1-2: 75-94. 
 
Leonard and Kennett 1999. Multi-component autoregressive techniques for the 
analysis of seismograms. Physics of the Earth and Planetary Interiors 113.1: 247-
263. 
 
Perarnau et al. 2012. Crustal structure of the Eastern Sierras Pampeanas of Argentina 
using high frequency local receiver functions. Tectonophysics 580: 208-217. 
 
Schweitzer 1997. HYPOSAT - A new routine to locate seismic events.  NORSAR 
Scientific Report 1.98: 94-102. 
 
Schweitzer 2001. HYPOSAT— An enhanced routine to locate seismic events. 
Monitoring the Comprehensive Nuclear-Test-Ban Treaty: Source Location.  277-
289. 
 
Waldhauser and Ellsworth 2000. A double-difference earthquake location algorithm: 
Method and application to the northern Hayward fault, California. Bulletin of the 
Seism. Society of America 90.6: 1353-1368. 


STRATEGy Seismic neTwoRk/Array in norThwEstern arGentina: Study of the 2015 El Galpón earthquake and its aftershock sequence

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STRATEGy Seismic neTwoRk/Array in norThwEstern arGentina: Study of the 2015 El Galpón earthquake and its aftershock sequence

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