Enrique Cabral-Cano

Mexico City subsidence observed with persistent scatterer InSAR

We analyzed 23 satellite SAR (synthetic aperture radar) scenes using Persistent Scatter Interferometry (PSI) to study subsidence in Mexico City associated with groundwater withdrawal. The data were acquired by the Envisat ASAR system between January 2004 and July 2006. The spatial pattern of subsidence and the maximum subsidence rate (300 mm/year) are similar to earlier studies. Comparison to independent GPS data indicates RMS agreement between the two techniques of 6.9 mm/year, about the level expected based on joint data uncertainty. Significant annual variation in the GPS vertical data is not observed, suggesting minimal aquifer recharge during the rainy season, and justifying a simple linear model of phase variation through time for the PSI analysis.

Space Geodetic Imaging of Rapid Ground Subsidence in Mexico City

Since the late 1950s, several areas of Mexico City have undergone accelerated ground subsidence and have developed associated fracturing and faulting. New interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) data indicate that rates of current land subsidence in Mexico City exceed 350 mm/yr. These rates are close to historical maximum levels of the mid-twentieth century, when mitigation efforts were fi rst undertaken to reduce damage to urban infrastructure. The locus of maximum subsidence has shifted from its historical location in the old city center to the east. Correlation of our InSAR results with seismically mapped stratigraphic units suggests that subsidence is primarily controlled by compaction of Quaternary lacustrine clays and silts. We also evaluate spatial gradients in subsidence and suggest that this, rather than subsidence magnitude, is the key factor in risk assessment. Subsidence represents a major geologic risk for Mexico City and imposes serious constraints to any further urban development.

Timing magma ascent at Popocatepetl Volcano, Mexico, 2000–2001

Abstract Magnetic anomalies may be used to constrain magma ascent and are useful as precursors to eruptions especially when correlated with other geophysical and geochemical data. In this paper we present multiparameter data on the magnetics, dome morphology, geochemistry and seismicity associated with the December 2000–January 2001 eruptions, the largest of the recent eruptions at Popocatepetl Volcano. A 6-month data period was studied in order to evaluate the precursors and post-eruption processes. Several cycles of dome construction and destruction occurred from September 2000 through February 2001. In December, large amplitude tremor associated with a higher effusion rate resulted in the formation of a large dome which filled the crater to within about 50 m of the lowest part of the crater rim. Seismic activity in December was marked by many volcanotectonic earthquakes and both high frequency and harmonic tremor. On December 12 and 13, an increase in the tremor amplitude was followed by ash eruptions with 1.7–5-km-high columns. Tremor amplitude increased again on December 15 and oscillated for the next four days. Activity remained high until the end of the month. On January 22, an 18-km-high plume produced ash and pumice fall to the east as well as pyroclastic flows and mudflows which reached 6 km from the crater. The eruption left three concentric explosion pits, partially destroying the December dome. Mixing of a mafic olivine-bearing melt with a more evolved magma triggered the larger eruption on January 22 as can be seen from the higher MgO concentrations in some of the ejecta and the presence of a dark andesitic scoria with lower silica content and a white andesitic pumice with higher silica content. Precursory negative magnetic anomalies up to 5 nT (−3.2 nT, −5 nT, −2.9 nT) were associated with the ascent of the larger batches of magma which preceded the increases in seismicity, before the December 2000–January 22 VEI 3–4 eruptions. No significant increases in seismicity were observed prior to the January 22 eruption, except for a magnitude-2.4 earthquake on the day of the eruption. There was, however, a −0.8-nT magnetic anomaly which lasted from January 10 to 12 and possibly to January 17. Most of the magma ascended in three large batches and several smaller ones from October 22 through December 10. Short-lived crater domes with small volumes were formed at Popocatepetl in September–October and November 2000 and in March 2001. Even the smaller negative anomalies from September to February were associated with the ascent of several batches of magma. They preceded the increases observed in the seismicity and growth of a dome by several days.

Slow Slip History for the MEXICO Subduction Zone: 2005 Through 2011

AbstractTo further our understanding of the seismically hazardous Mexico subduction zone, we estimate the first time-dependent slip distributions and Coulomb failure stress changes for the six major slow slip events (SSEs) that occurred below Mexico between late 2005 and mid-2011. Slip dist ributions are the first to be estimated from all continuous GPS data in central and southern Mexico, which better resolves slow slip in space and time than was previously possible in this region. Below Oaxaca, slip during previously un-modeled SSEs in 2008/9 and 2010/11 extended farther to the west than previous SSEs. This constitutes the first evidence that slow slip accounts for deep slip within a previously noted gap between the Oaxaca and Guerrero SSE source regions. The slip that we estimate for the two SSEs that originated below Guerrero between 2005 and 2011 agrees with slip estimated in previous, mostly static-offset SSE modeling studies; however, we show that both SSEs migrated eastward toward the Oaxaca SSE source region. In accord with previous work, we find that slow slip below Guerrero intrudes up-dip into the potentially seismogenic region, presumably accounting for some of the missing slip within the well-described Guerrero seismic gap. In contrast, slow slip below Oaxaca between 2005 and 2011 occurred mostly down-dip from the seismogenic regions defined by the rupture zones of large thrust earthquakes in 1968 and 1978 and released all of the slip deficit that accumulated in the down-dip region during this period.

The DRASTIC-Sg model: an extension to the DRASTIC approach for mapping groundwater vulnerability in aquifers subject to differential land subsidence, with application to Mexico City

Mexico City relies on groundwater for most of its domestic supply. Over the years, intensive pumping has led to significant drawdowns and, subsequently, to severe land subsidence. Tensile cracks have also developed or reactivated as a result. All such processes cause damage to urban infrastructure, increasing the risk of spills and favoring contaminant propagation into the aquifer. The effects of ground deformation are frequently ignored in groundwater vulnerability studies, but can be relevant in subsiding cities. This report presents an extension to the DRASTIC methodology, named DRASTIC-Sg, which focuses on evaluating groundwater vulnerability in urban aquifers affected by differential subsidence. A subsidence parameter is developed to represent the ground deformation gradient (Sg), and then used to depict areas where damage risk to urban infrastructure is higher due to fracture propagation. Space-geodetic SqueeSAR data and global positioning system (GPS) validation were used to evaluate subsidence rates and gradients, integrating hydrogeological and geomechanical variables into a GIS environment. Results show that classic DRASTIC approaches may underestimate groundwater vulnerability in settings such as the one at hand. Hence, it is concluded that the Sg parameter is a welcome contribution to develop reliable vulnerability assessments in subsiding basins.RésuméLa ville de Mexico est dépendante de l’eau souterraine pour l’essentiel de son usage domestique. Au fil des années, les pompages intensifs ont conduit à des abaissements importants du niveau piézométrique et par conséquence à d’important affaissement de terrain. Des fissures de tension se sont développées ou ont été réactivées. Tous ces processus causent des dommages sur les infrastructures urbaines augmentant le risque de fuites favorisant la propagation de contaminants dans l’aquifère. Les effets des déformations du sol sont souvent ignorés dans les études de vulnérabilité des eaux souterraines, mais peuvent être pertinents pour les villes soumises à des affaissements de terrain. Cet article présente un développement de la méthodologie DRASTIC, nommée DRASTIC-Sg, ciblant l’évaluation de la vulnérabilité des eaux souterraines pour des aquifères en milieu urbain impactés par un affaissement différentiel des terrains. Un paramètre affaissement est développé pour représenter le gradient de déformation du sol, et ensuite utilisé pour décrire les zones où les risques de dommage des infrastructures urbaines sont plus élevés dus à la propagation des fractures. Les données spatiales et de géodésie (SqueeSAR) et les données de validation du système global de positionnement (GPS) ont été utilisées pour évaluer les vitesses d’affaissement des terrains et les gradients, intégrant les variables hydrogéologiques et géomécaniques dans un système d’information géographique. Les résultats montrent que les approches classiques DRASTIC sous-estiment la vulnérabilité des eaux souterraines dans des environnements tels que celui de la ville de Mexico. Par conséquence, on conclut que l’ajout du paramètre Sg contribue à élaborer des évaluations fiables de la vulnérabilité dans des bassins subsidents.ResumenLa ciudad de México depende del agua subterránea para la mayoría de sus abastecimientos domésticos. A través de los años, el bombeo intensivo ha llevado a depresiones significativas y, subsecuentemente, a una severa subsidencia del terreno. Como resultado también se han desarrollado o reactivado fracturas de tensión. Tales fenómenos han causado daños a la infraestructura urbana, incrementando el riesgo de vertidos y favoreciendo la propagación de contaminantes en el acuífero. Los efectos de la deformación del terreno son frecuentemente ignorados en los estudios de vulnerabilidad del agua subterránea, pero pueden ser relevantes en las ciudades subsidentes. Este trabajo presenta una extensión de la metodología DRASTIC, denominada DRASTIC-Sg, que se enfoca para evaluar la vulnerabilidad del agua subterránea en acuíferos afectados por subsidencia diferencial. Se desarrolla un parámetro de subsidencia para representar el gradiente de deformación del terreno (Sg), y luego se usa para delinear áreas donde el riesgo al daño para la infraestructura urbana es mayor debido a la propagación de las fracturas. Se usaron datos de espacio geodético SqueeSAR y la validación por el sistema de posicionamiento global (GPS) para evaluar las tasas y gradientes de subsidencia, integrando variables hidrogeológicas y geomecánicas en un ambiente GIS. Los resultados muestran que el enfoque DRASTIC clásico puede subestimar la vulnerabilidad del agua subterránea en configuraciones como la que nos ocupa. Por lo tanto, se concluye que el parámetro Sg es una contribución bienvenida para desarrollar evaluaciones confiables de vulnerabilidad en cuencas subsidentes.摘要墨西哥城大部分家庭用水依赖地下水。多年以来,强烈开采地下水 导致地下水水位大幅下降低,并由此造成严重的地面沉降。还使拉伸缝进 一步发育或使原来的拉伸缝再度激活。所有这些过程对城市基础设施造成 损坏,增加泄漏物和污染物进入含水层的风险。地面变形的影响在地下水 脆弱性研究中经常受到忽略,但在沉降的城市中有重要意义。本文介绍了 DRASTIC 方法的延伸,即DRASTIC-Sg。这个方法聚焦于评价受不同沉 降影响的城市含水层中地下水的脆弱性。找到了代表地面变形梯度(Sg)的 沉降参数,用来描述由于断裂蔓延造成对城市基础设施损害风险较高的地 区。太空大地测量SqueeSAR 资料和全球定验证用来评价沉降速度和梯 度,把水文地质变量和岩土力学变量整合在地理信息系统环境中。结果显 示,传统的DRASTIC 方法可能低估所研究的环境中地下水的脆弱性。因 此,结论就是,在进行沉降盆地可靠的脆弱性评价中,Sg 参数是一个积 极贡献。ResumoA Cidade do México depende de águas subterrâneas para a maior parte do seu abastecimento doméstico. Ao longo dos anos, bombeamentos intensivos levaram a significativos decréscimos do nível freático e, subsequentemente, a severa subsidência de terrenos. Como resultado, foram desenvolvidas ou reativadas fendas de tração. Todos estes processos causam danos nas infraestruturas urbanas, aumentando o risco de derramamentos e favorecendo a propagação da contaminação no aquífero. Os efeitos da deformação dos terrenos são frequentemente ignorados nos estudos de vulnerabilidade das águas subterrâneas, mas podem ser significativos em cidades sob subsidência. Este artigo apresenta uma extensão à metodologia DRASTIC, denominada DRASTIC-Sg, que se foca na avaliação da vulnerabilidade da água subterrânea em aquíferos urbanos afetados por subsidência diferencial. É desenvolvido um parâmetro de subsidência para representar o gradiente de deformação do solo (Sg), sendo depois usado para descrever áreas onde o risco de danos às infraestruturas urbanas é maior devido à propagação de fraturas. Foi usada validação com dados espaço-geodésicos SqueeSAR e sistemas de posicionamento global (GPS) para avaliar as taxas de subsidência e os gradientes, integrando variáveis hidrogeológicas e geomecânicas num ambiente SIG. Os resultados mostram que a abordagem DRASTIC clássica pode subestimar a vulnerabilidade da água subterrânea em ambientes como este. Conclui-se portanto que o parâmetro Sg é uma contribuição bem-vinda para o desenvolvimento de avaliações de vulnerabilidade em bacias em subsidência.

On the importance of path for phase unwrapping in synthetic aperture radar interferometry

Phase unwrapping is a key procedure in interferometric synthetic aperture radar studies, translating ambiguous phase observations to topography, and surface deformation estimates. Some unwrapping algorithms are conducted along specific paths based on different selection criteria. In this study, we analyze six unwrapping paths: line scan, maximum coherence, phase derivative variance, phase derivative variance with branch-cut, second-derivative reliability, and the Fisher distance. The latter is a new path algorithm based on Fisher information theory, which combines the phase derivative with the expected variance to get a more robust path, potentially performing better than others in the case of low image quality. In order to compare only the performance of the paths, the same unwrapping function (phase derivative integral) is used. Results indicate that the Fisher distance algorithm gives better results in most cases.

Volcanomagnetic signals during the recent Popocatépetl (México) eruptions and their relation to eruptive activity

Abstract An interdisciplinary approach correlating magnetic anomalies with composition of the ejecta in each eruption, as well as with seismicity, was used to study the effect of magmatic activity on the local magnetic record at Popocatepetl Volcano located 65 km southeast of Mexico City. Eruptions began on December, 1994, and have continued with dome growth and ash emissions since then. The Tlamacas (TLA) geomagnetic total field monitoring station, located 5 km away from Popocatepetl’s crater, was installed in December, 1997, in order to detect magnetic anomalies induced by this activity. Spatial correlation and weighted difference methods were applied to detect temporal geomagnetic anomalies using TLA’s record and the Teoloyucan Magnetic Observatory as a reference station. Weighted differences were applied to cancel the effects of non-vulcanogenic external field variations. Magnetic anomalies over a 2-year time span were classified into four types correlating them with geochemical, seismic and visual monitoring of the volcanic activity. Magnetic anomalies are believed to be caused by magma injection and gas pressure build-up, which is sensitive to vent morphology and clearing during eruption, although some anomalies appear to be thermally related, changes in the stress field are very important. Most magnetic anomalies are short time signals that reverse to baseline level. Decreasing anomalies (−0.5 to −6.8 nT) precede eruptions by 1–8 days. The presence of a mafic magmatic component was determined by mineral examination and silica and magnesium analyses on the ejecta from the 1997–1999 eruptions. Whole rock analyses ranged from dacitic (65% SiO 2 ) to andesitic (57% SiO 2 ) with 2–6.6% MgO. The higher MgO, lower silica samples contain forsteritic olivine (Fo90). SiO 2 does not increase and MgO does not increase with time, suggesting ascent of small magma pulses which are consistent with the magnetic data.

Lateral Variations of Interplate Coupling along the Mexican Subduction Interface: Relationships with Long-Term Morphology and Fault Zone Mechanical Properties

Although patterns of interseismic strain accumulation above subduction zones are now routinely characterised using geodetic measurements, their physical origin, persistency through time, and relationships to seismic hazard and long-term deformation are still debated. Here, we use GPS and morphological observations from southern Mexico to explore potential mechanical links between variations in inter-SSE (in between slow slip events) coupling along the Mexico subduction zone and the long-term topography of the coastal regions from Guerrero to Oaxaca. Inter-SSE coupling solutions for two different geometries of the subduction interface are derived from an inversion of continuous GPS time series corrected from slow slip events. They reveal strong along-strike variations in the shallow coupling (i.e. at depths down to 25 km), with high-coupling zones (coupling >0.7) alternating with low-coupling zones (coupling <0.3). Coupling below the continent is typically strong (>0.7) and transitions to uncoupled, steady slip at a relatively uniform

New perspective on the transition from flat to steeper subduction in Oaxaca, Mexico, based on seismicity, nonvolcanic tremor, and slow slip

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