The Tobosi Fault: source of the 2011–2012
Tobosi Earthquake Swarm in Central Costa Rica
María Cristina Araya1-2, Lepolt Linkimer1-2, Walter Montero3, Wilfredo Rojas1-2
1 Red Sismológica Nacional (RSN: UCR-ICE), Apdo. 214-2060, San Pedro, Costa Rica
2 Escuela Centroamericana de Geología, Universidad de Costa Rica
3 Centro de Investigaciones Geológicas, Universidad de Costa Rica
Introduction
From 9 of December, 2011 to 28 of February, 2012, an earthquake swarm took
place in Central Costa Rica near the town of Tobosi, at ~14 km from San Jose,
the capital city of Costa Rica.
On a regional scale, this earthquake swarm is located on a portion of the Central
Costa Rican Deformed Belt (CCRDB), a ~100 km broad zone of deformation that
represents the boundary between the Caribbean Plate and the Panama Microplate
(i.e., Marshall et al., 2000; Montero, 2001, Figure 1A). On local scale, the Tobosi swarm
ocurred between two major left-lateral strike-slip fault systems: Aguacaliente
and Navarro (i.e., Montero, 2001; Fernández & Montero, 2002, Figure 1B).
This research intends to describe the Tobosi earthquake swarm in terms of its
time and space distribution and determine its source. We accomplish these goals
by relocating earthquakes and calculating focal mechanisms using data recorded by
the RSN (UCR-ICE) and analyzing the tectonic geomorphology of the region.
Earthquakes are selected from the RSN (UCR-ICE) data catalog using the following criteria:
a) Between the dates of December 9th 2011 and February 28th 2012
b) With initial locations in the vicinity of the Tobosi region.
c) With no less than ten P-wave phases and one S-wave observations.
d) With GAP less than 180º of the station coverage.
e) With a root mean square (RMS) of less than 0.5.
Methodology
B)
68
64
60
56
52
48
44
40
36
32
28
24
20
16
12
8
4
P wave arrival
Amplitude
Distance (km)
A)
0
8
16
24
32
Time (s)
40
N
SJS1
ICR2
CGA2
IRAZ
The software used for earthquake processing and analysis is SeisAn (Havskov & Ottemöller, 1999).
The time windows used for P-wave and S-wave picking are 5 and 10 s, respectively.
P-wave arrival was usually very clear for the stations near to the source (Figure 2A-B).
2
Station LCR2
Figure 1. A) Location and tectonic setting. Doted line marks
the area shown in Figure 1B. White triangles represent
seismic stations used in this study. Light gray area
represents the approximate location of the CCRDB based on
Montero et al. (2013).
B) Zoom to the study area. Tobosi swarm epicenters are
shown as open circles and historical earthquakes as stars.
Historical earthquakes are taken from Ambraseys and
Adams (2001) and from Morales (1985). Doted line marks
the area shown in Figures 5 and 7.
3
4
5
Time (s)
6
7
8
48
Figure 2. A) Record section of an earthquake on December 23, 2011
(3.4 Mw) registered at stations located less than 70 km from the source.
B) Zoom to the P-wave arrival at the nearest station LCR2 located at 8 km
from the source.
The first motion focal mechanisms are calculated using the program FOCMEC (Focal Mechanisms
Determinations, Snoke, 1984) for the events with more than 7 polarities (Figure 3). The quality of the
focal mechanisms are categorized into two groups: A and B. Quality A (QA) with at least 13 polarities
that were mapped with no polarity errors on the focal sphere. Quality B (QB) corresponds to focal
mechanisms with at least seven polarity observations mapped with no polarity errors.
T
PAR1
URSC
P
LCR2
DRK0
Figure 3. Example of a high quality focal mechanism for the
3.2 Mw event on December 26, 2011.
Finally, field trips were made to the region to characterize the local Tectonic Geomorphology and
complement the earthquake analysis.
Table 1: Focal Mechanism planes strike, dip and rake.
Results
Twenty two earthquakes from the swarm were relocated, the majority occurred between 22 and 27 of December 2011 (Figure 4A); most of the earthquakes had
magnitudes (Mw) that ranged between 2.6 and 3.5 (Figure 4B). Hypocenters ranged between 0.6 and 8.2 km in depth (Figure 4C).
The fault that bounds the Tobosi Transtension structure to the northwest is called the Tobosi Fault. This fault strikes ENE-WSW and has a longitude of about 10 km
(Figure 5). We were able to compute 14 focal mechanisms: seven of QA and seven of QB (Figure 6, Table 1).
1
2
N
5
N
6
N
N
P
T
T
T
P
P
P
7
8
N
11
N
T
P
16
N
P
P
T
P
22
N
18
N
T
T
N
T
P
14
N
N
T
P
13
12
N
T
P
19
T
T
P
N
P
T
P
T
Figure 4. Histograms of the earthquake swarm showing earthquake
distribution in terms of A) Date, B) Magnitude, and C) Depth.
Conclusions
Figure 5. Morphotectonic of the Tobosi area and the Transtension
Tobosi structure composed by three faults traces and each of them
displays a left-lateral strike-slip displacement with a normal
component.
The name of these faults are: Tobosi, Tablon, and Alumbre.
Figure 6. Focal mechanisms for the Tobosi swarm. Quality-A
(best) focal mechanisms are shown as thicker black lines and
Quality-B as thinner grey lines.
This earthquake swarm occurred on a portion of the CCRDB where faults are mainly of strike-slip type
with right lateral displacement along faults striking northwest-southeast and left lateral displacement
along faults striking northeast-southwest.
The Tobosi Transtension structure is located between two major left-lateral fault systems and composed
of three faults, named Tobosi, Tablon, and Alumbre. Each displays geomorphological features that suggest
left-lateral strike-slip displacements with a normal component (Figure 7).
A strong trend on the polarity observations for focal mechanisms was found for most earthquakes analyzed.
High-quality focal mechanism solutions display mostly strike-slip faults. These solutions correspond to a
left-lateral strike-slip displacement on a nodal plane striking 162-172º and dipping 78-89º, and to a
right-lateral strike-slip displacement on a nodal plane striking 7-12º and dipping 71-80º.
Figure 7. Earthquake locations (small gray circles) and Quality-A
focal mechanisms in the neotectonic context of the Tobosi area.
The main cluster of epicenters occurs to the west of the town of Tobosi is aligned ENE-WSW.
The epicenters are also located at the northeastern portion of the Tobosi Fault where it has ENE-WSW
strike. The left-lateral strike-slip solutions for the high-quality focal mechanisms seem to agree very
well with the tectonic geomorphology observations for the Tobosi Fault in this region. Based on these
observations, we proposed that the Tobosi Fault was the source of the 2011-2012 earthquake swarm.
References
AMBRASEYS, N.N., & ADAMS, R.D., 2001: The Seismicity of Central America. A
Descriptive Catalogue 1898–1995: London, Imperial College Press, 309 p.
FERNÁNDEZ, M. & MONTERO, W., 2002: Fallamiento y sismicidad del área entre
Cartago y San José, valle Central de Costa Rica.- Rev. Geol. Amér. Central,
26: 25-37.
OTTEMÖLLER, L., VOSS, P. & HAVSKOV, J., 2011: SEISAN: the Earthquake Analysis
Software for Windows, Solaris, LINUX, and MACOSX, version 9.0.1.- 361 p.
Univ. of Bergen, Bergen.
MONTERO, W., 2001: Neotectónica de la región Central de Costa Rica: Frontera oeste
de la microplaca de Panamá.- Rev. Geol. Amér. Central, 24: 29-56.
MONTERO, W., ROJAS, W., LINKIMER, L., 2013: Neotectónica de las fallas Ochomogo
y Capellades y su relación con el sistema de falla Aguacaliente, falda sur macizo
Irazú-Turrialba, Costa Rica. – Rev Geol. Amér. Central, 48: 119-139.
SNOKE, J. A., MUNSEY, J. W., TEAGUE, A. G. & BOLLINGER, G. A., 1984: A program for focal
mechanism determination by combined use of polarity and Sv-P amplitude ratio
data. En: Earthquake Notes, 55: 15-20.