PIAHSProceedings of the International Association of Hydrological SciencesPIAHSProc. IAHS2199-899XCopernicus GmbHGöttingen, Germany10.5194/piahs-372-243-2015Subsidence monitoring with geotechnical instruments in the Mexicali Valley, Baja California, MexicoGlowackaE.glowacka@cicese.mxSarychikhinaO.Márquez RamírezV. H.RoblesB.NavaF. A.FarfánF.García ArthurM. A.Centro de Investigacion Cientifica y Educacion Superior de Ensenada, Ensenada, MexicoUNAM Campus, Centro de Geociencias, Juriquilla, Querétaro, MexicoInstituto Mexicano de Tecnología de Agua, Jiutepec, Morelos, MexicoE. Glowacka (glowacka@cicese.mx)12November2015372372243248This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://piahs.copernicus.org/articles/372/243/2015/piahs-372-243-2015.htmlThe full text article is available as a PDF file from https://piahs.copernicus.org/articles/372/243/2015/piahs-372-243-2015.pdf
The Mexicali Valley (northwestern Mexico), situated in the southern part of
the San Andreas fault system, is an area with high tectonic deformation,
recent volcanism, and active seismicity. Since 1973, fluid extraction, from
the 1500–3000 m depth range, at the Cerro Prieto Geothermal Field (CPGF),
has influenced deformation in the Mexicali Valley area, accelerating the
subsidence and causing slip along the traces of tectonic faults that limit
the subsidence area. Detailed field mapping done since 1989 (González et
al., 1998; Glowacka et al., 2005; Suárez-Vidal et al., 2008) in the
vicinity of the CPGF shows that many subsidence induced fractures, fissures,
collapse features, small grabens, and fresh scarps are related to the known
tectonic faults. Subsidence and fault rupture are causing damage to
infrastructure, such as roads, railroad tracks, irrigation channels, and
agricultural fields.
Since 1996, geotechnical instruments installed by CICESE (Centro de
Investigación Ciéntifica y de Educación Superior de Ensenada,
B.C.) have operated in the Mexicali Valley, for continuous recording of
deformation phenomena. Instruments are installed over or very close to the
affected faults. To date, the network includes four crackmeters and eight
tiltmeters; all instruments have sampling intervals in the 1 to 20 min range.
Instrumental records typically show continuous creep, episodic slip events
related mainly to the subsidence process, and coseismic slip discontinuities
(Glowacka et al., 1999, 2005, 2010; Sarychikhina et al., 2015).
The area has also been monitored by levelling surveys every few years and,
since the 1990's by studies based on DInSAR data (Carnec and Fabriol, 1999;
Hansen, 2001; Sarychikhina et al., 2011).
In this work we use data from levelling, DInSAR, and geotechnical
instruments records to compare the subsidence caused by anthropogenic
activity and/or seismicity with slip recorded by geotechnical instruments,
in an attempt to obtain more information about the process of fault slip
associated with subsidence.
Introduction
The Mexicali Valley is located, in the southern part of the San Andrés
fault system, within the southern part of the Salton Trough, on the border
between the North America and Pacific tectonic plates. The Valley is
characterized by recent volcanic hydrothermal processes, active tectonics,
and high seismicity. Moreover large earthquakes concentrate along the major,
Imperial and Cerro Prieto faults, while scattered seismicity, (mainly
swarms), and deformation are observed in the Pull-apart Cerro Prieto Basin
(Lomnitz et al., 1970; Nava and Glowacka, 1994; Suárez-Vidal et al.,
2008). Fluid extraction began in the Cerro Prieto Geothermal Field (CPGF) in
1973, and brine injection therein began in 1989; these processes have been
influencing deformation, stress, and seismicity of the area (Majer and
McEvilly, 1981; Glowacka and Nava, 1996; Fabriol and Munguía, 1997;
Glowacka et al., 1999, 2005; Trugman et al., 2014).
Instruments (yellow pins) and the subsidence rate blue isolines
(cm yr-1) for 1997–2006 (modified from Glowacka et al., 2011). Red lines
indicate tectonic faults. The inset shows the geographical location of the
study area.
The subsidence area is limited to the area between the Imperial, Saltillo,
Cerro Prieto and Morelia faults (Fig. 1). This zone, also known as Cerro
Prieto basin, is larger than the extraction zone, suggesting the existence
of a recharging region. Since the sunken area is limited by tectonic faults,
Glowacka et al. (1999, 2005, 2010a), suggest that these faults constitute a
boundary of the subsiding region, due to differential compaction and/or due
to poor permeability in the direction perpendicular to the faults that acts
as a groundwater barrier.
The CPGF has 720 MW production capacity, or 1/3 of electricity production in
Baja California; however, the anthropogenic subsidence caused by deep
extraction of fluids in the CPGF causes damage to roads, railways and
irrigation systems, increasing the natural hazards in this area that is
quite vulnerable because of its tectonic situation.
The history of subsidence at the CPGF area has been well documented.
Geodetic studies in the Mexicali Valley began in the 1960's. Leveling
surveys have been done in the area of the CPGF and the Mexicali Valley since
1977 (Velasco, 1963; Lira and Arellano, 1997; Glowacka et al., 1999,
2012). Subsidence in the CPGF has also been measured via DInSAR
(Differential Synthetic Aperture Radar Interferometry) by Carnec and Fabriol (1999)
and Hanssen (2001) using ERS1/2 images acquired during 1993–1997
and 1995–1997, respectively, and interpreted as an anthropogenic effect of
fluid extraction.
Sarychikhina et al. (2011) applied the DInSAR technique using C-band
ENVISAR ASAR data acquired between 2003 and 2006, to determine the extent
and amount of land subsidence in the Mexicali Valley near the CPGF. Recently
Sarychikhina et al. (2015) applied DInSAR technique, together with leveling
results and geotechnical instruments data and modeling, to estimate seismic
and aseismic deformation in Mexicali Valley for the period 2006–2009. The
current subsidence rate, evaluated from DInSAR data is of the order of
12 cm yr-1 in the production area and 18 cm yr-1 for the recharging zone
(Sarychikhina et al., 2015; Sarychikhina and Glowacka, 2015a).
Geotechnical instruments monitoring
To study the spatial and temporal distribution of crustal deformation in the
Mexicali Valley, CICESE installed a network of geotechnical instruments,
starting in 1996. Since then, the REDECVAM (Red de Deformaciones de la
Corteza en el Valle de Mexicali) network has included four creepmeters (wide
range extensometer) and nine tiltmeters installed over different periods in
different places; all instruments have sampling intervals in the 1 to
20 min range (Nava and Glowacka, 1999; Glowacka et al., 2002, 2010b, c;
Sarychikhina et al., 2015). Creepmeters (Geokon model 4420) were installed
mainly on vertical planes perpendicular to faults, in order to record
vertical displacement. Biaxial tiltmeters (Applied Geomechanics, models 711,
712, and 722) were installed in shallow vaults or wells, close to faults.
Data from a creepmeter and a tiltmeter installed on the Saltillo fault
(Fig. 1) show fault vertical displacement rate at ∼ 5.3 cm yr-1
until 2003 and ∼ 7.3 cm yr-1 since then. The
distance-time relationship between changes in extraction at the CPGF and the
displacement rate change found at the Saltillo fault suggests that the fault
is probably affected by extraction through diffusive transmission of pore
pressure changes, with a characteristic hydraulic diffusivity (Glowacka et
al., 2010a).
Tilt observations. X and Y are the East-West and North–South
components of tilt, respectively, and R is the resultant tilt magnitude in
the azimuth direction, Ang, for 1999–2003 (a) and 2003–2012 (b). X, Y and R are
referred to the left axes, while azimuth Ang is referred to the right axes.
Azimuth is also shown by the direction in which brown arrowheads point, with
North straight up. Extension ES-V and magnitude R for 1999–2003 (c) and
2008–2012 (d). The vertical arrow indicates the M= 7.2 hector Mine
earthquake in (c), and M= 5.8 2009 earthquake in (d). The dotted-line indicates
slip event in August 2008.
Vertical displacement at the Saltillo fault consists of continuous creep and
episodic slip events, sometimes concentrated in suites (Nava and Glowacka,
1999). Episodic slips have 1 to 3 cm magnitudes and 1–3 days duration,
separated by months of monotone creep, and release about 50 % of total
slip. The episodic fault slip in the Saltillo fault appears mainly as
slip-predictable, normal, aseismic slip (e.g. Glowacka et al., 2001,
2010a). Using reservoir model, geological structure, and subsidence data,
Glowacka et al. (2010a), speculatively proposed the depth range of slip
events to be from 1 to 2.5 km.
Some of the slip events are triggered by distant earthquakes. This includes,
for example, Hector Mine Earthquake (of magnitude 7.2 and at a distance
260 km away 15 on 1999), as described by Glowacka et al. (2002), and, probably, by
the Canal de las Ballenas earthquake (of magnitude 6.9 and a distance
∼ 400 km away 2009), as described by Glowacka et al. (2015).
These studies suggest that distant earthquakes can influence fault slip
related to the subsidence. Both, sporadic and triggered slip events confirm
slip predictable fault behavior.
Detailed analysis of tiltmeter and creepmeter data done by Sarychikhina et
al. (2015), identified which part of subsidence observed by DInSAR in the
Mexicali valley during 2006–2009 was caused by local seismicity. Glowacka et
al. (2015) estimated that during the 2006–2009 period of relatively high
seismicity, the anthropogenic subsidence was of the order of 80 % of the
total subsidence, which is close to estimates done by Glowacka et al. (2005),
Camacho Ibarra (2006), and Sarychikhina et al. (2015).
The goal of this paper is to find if there are differences, in the signal
characteristics observed on the creepmeter and/or tiltmeter records, between
continuous creep, sporadic slip, triggered slip and coseismic slip recorded
on the Saltillo fault.
Results
In the following we will analyze records from ES-V creepmeter and ES-I
biaxial tiltmeter installed very close to each other on the Saltillo fault.
A biaxial tiltmeter (ES-I) was installed in 1998, very close to the
creepmeter and recorded ground tilt until 2003 (Fig. 2a), and after a small
reconstruction, again since 2008 (Fig. 2b). E–W (X) and N–S (Y)
inclination recorded on ES-I tiltmeter are presented on Fig. 2a (1999–2003) and
Fig. 2b (2008–2012).
For the tilt we calculate the resultant magnitude, R, and the azimuth,
Ang, as
R=X2+Y21/2,Ang=arctan(ΔX/ΔY),
where X and Y are the measured tilts in the N–S and E-W directions,
respectively; ΔX=Xcosθ+Ysinθ, and
ΔY=Ycosθ-Xsinθ, and θ is the correction for the azimuth
orientation of the Y axis from North. θ=-10∘ for ES-I.
The tilt magnitude, R, and the ES-V vertical extension recorded by the
creepmeter, are shown in Fig. 2c (1999–2003) and Fig. 2d (2008–2012).
Tilt observations vs. time for episodic slip (a), triggered
slip (b) and coseismic slip (c). R is the resultant tilt magnitude in the azimuth
direction Ang. R is referred to the left axes, while azimuth Ang is referred to
the right axes. Extension ES-V is shown for the period when the creepmeter
was working. The vertical arrow indicates the M= 7.2 1999 hector Mine
earthquake (b) and the M= 5.8, 2009 earthquake (c).
Figure 3 shows R and Ang for the August 2008 slip episode (Fig. 3a), the slip
event triggered by the M= 7.2 Hector Mine earthquake (Fig. 3b) and by
the local M= 5.8, 30 December 2009, earthquake (Fig. 3c). While episodic
slip events and triggered slip events show very similar behavior of R and
Ang, the slip related to the 5.8 local earthquake has different behavior.
Discussion
Figure 2c and d show R and creepmeter extension ES-V observed between
1999–2003 and 2008–2012, respectively. The similarity between R and ES-V
seen on both Fig. 2 suggests that tilt increases when vertical
displacement increases on the fault, as observed during continuous creep and
slip events.
There is not such an evident tendency for azimuthal behavior (Fig. 2a and b);
the azimuth oscillated during 1999–2000, slightly increased
during 2001–2003, and diminished with time during 2008–2012, except during
December 2009–April 2010, which will be discussed later.
However, a significant azimuth change can be seen between Fig. 2a and b.
Tilt tends to be more north-oriented for the 2008–2012 period.
This phenomenon can be related to the subsidence amplitude increase in the
recharge area observed for 1993–2009 by Sarychikhina and Glowacka (2015b) (this book).
Three kinds of deformation are shown in Fig. 3. The typical slip event
shown on Fig. 3a is characterized by an R increase, related to the
extension (subsidence) increase and azimuth decrease, with about one day
duration. Triggered by the M= 7.2 HME, the slip event shown on Fig. 3b is
characterized by abrupt R increase, extension (subsidence) increase, and
azimuth decrease. This is followed by the slow deformation with the same
orientation, similar to the regular slip event, but with smaller magnitude,
expected since the slip predictable character of deformation suggested by
Glowacka et al. (2002). The deformation shown on Fig. 3c was caused by a
local M= 5.8 earthquake and about 1 h of deformation was recorded after
the earthquake before the instrument went out of range. Within the time
precision of the tiltmeter (one sample every 4 min) the immediate R
increase followed by postseismic R increase can be observed. However, the
azimuth shows immediate increase and postseismic decrease.
Although creepmeters were not working during the M= 5.8, 2009 earthquake,
Sarychikhina et al. (2015), using data from InSAR and leveling, showed there
was subsidence (about 20 cm) and slip associated with this earthquake. They
suggested that the observed deformation could be caused by triggered
aseismic slip on the Saltillo fault or by earthquake-triggered soft
sediments subsidence.
Conclusions
Our results show that the tilt magnitude is proportional to the extension
recorded by a creemeter, which may eventually allow to substitute results
when one instrument is missing.
Since magnitude and azimuth behaviors are very similar for episodic and
triggered slip we suggest that they occur along the same fault segment and
the same depth.
However, the different tilt azimuthal behavior for local earthquakes
suggests that the slip on the fault has a different behavior, possibly
including a horizontal component or/and that the slip was deeper than for
episodic slip. More dense instrumentation with a higher frequency of
measurements could be useful to confirm this suggestion.
Acknowledgements
This research was sponsored in part by CONACYT, project 105907 and CICESE
internal funds.
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