Characterization of seismogenic crustal faults in the Gulf of Guayaquil, Ecuador

Abstract

Few moderate-to-strong earthquakes associated with active and capable geological faults have been documented for the southern coastal region of Ecuador. The seismic record of Ecuador initiates with the Guayaquil earthquake in 1787 (Mw 6.5), followed by the 1943 (Mw 6.2), and the most recent one in 1980 (Mw 6.1). The available data is insufficient to evaluate the seismic hazards associated with faults capable of generating seismic events of magnitude Mw≥6.0 in the region. Also, earthquakes of minor magnitudes can be disregarded as they do not induce significant ground coseismic effects. In this context, this study presents a catalog of geological faults, delineating 40 segments of capable and active faults on the sea floor of the Gulf of Guayaquil and inland segments of Guayas, Santa Elena, and El Oro provinces. This methodological approach estimates a variety of seismicity levels ranging between Mw 6.2 and Mw 7.2, with rock peak ground accelerations between 0.24 g and 0.41 g. These values have been obtained from empirical regression equations applied to the length of capable geological faults. The F-40 seismogenic structure located in the accretionary prism, close to the subduction zone, is capable of generating Mw 8.2 earthquakes and potentially causing coseismic ground damage to the city of Guayaquil located ca. 177 km to the NE. Furthermore, local tsunami hazards may affect severely areas that are densely populated, with developing industrial areas, on the coast of the Gulf of Guayaquil. This structural geological analysis provides useful new data for seismic hazard assessment.

Assessing Susceptibility to Soil Liquefaction Using the Standard Penetration Test (SPT)—A Case Study from the City of Portoviejo, Coastal Ecuador

“…Geotechnical, geological, geomorphological, and geophysical data were used to assess the quality of susceptible soil profiles to coseismic deformations, such as phenomenalinduced liquefaction including lateral spreading, sinkholes, sand boils, and ground subsidence [2,8,[59][60][61][62]. The thicknesses of the lithological units of the soil and the seismic shear wave profiles (assessed using the Nakamura method [63]) were analyzed using 23 available boreholes of up to 30 m deep; the number of SPT blows, granulometry, and water content were assessed in the soil, and variation in soil saturation degree, liquid limit (LL), and plas-ticity limit of soil strata samples were analyzed.…”

“…Within areas with high levels of seismicity, where earthquakes occur at intervals of 20 to 70 years and soils are susceptible to coseismic ground deformations, it is absolutely necessary to perform continuous studies in order to understand which urban areas may be susceptible to liquefaction. The poor quality of the soils due to their young age produces unfavorable conditions that correspond to the Holocene [1][2][3][4][5][6][7][8][9][10][11][12]. Historically, older buildings in alluvial valleys filled with Quaternary deposits are the most vulnerable, as was documented in past earthquakes in the province of Manabí, in the central coastal area of Ecuador (Figure 1) [11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”

Caracterización geomecánica de taludes de roca basáltica en la costa sur de Ecuador: implicaciones en su estabilidad

“…La costa sur de Ecuador se caracteriza por dos tipos de amenazas sísmicas. Sismos de subducción lejanos donde las magnitudes alcanzan rangos entre 6,9≤Mw≤8,2; y sismos moderados locales desde fallas geológicas con magnitudes entre 5,9≤Mw≤6,8 (ver catálogo de fallas para la costa sur de Ecuador [20]). Según la NEC-11, el sitio de estudio se encuentra en la zona sísmica de nivel moderado-alto [21],con PGA ≥0,4g en roca (Fig.…”

“…Los lineamientos estructurales fueron cartografiados a partir de un análisis morfométrico, utilizando imágenes de ortofotos y modelos de elevación digital de alta resolución. La información existente del mapa de fallas activas fue consultada desde [20] [27] [28].…”

“…Esta información proporcionará parámetros geotécnicos para macizos rocosos de basalto, así como alternativas de técnicas para la estabilidad de taludes, útiles en la proyección de planificaciones y para la reducción de riesgos por deslizamientos en carreteras. Figura 1.Formaciones cretácicas en la costa sur de Ecuador y su relación con valores PGA-h, según [20] [21]. Sitio de estudio del cerro San José (ver Figura 2), coordenadas UTM, 611435mE y 9786217mN.…”