Seismically Induced Soil Liquefaction and Geological Conditions in the City of Jama due to the M7.8 Pedernales Earthquake in 2016, NW Ecuador

Abstract

Seismically induced soil liquefaction has been documented after the M7.8, 2016 Pedernales earthquake. In the city of Jama, the acceleration recorded by soil amplification yielded 1.05 g with an intensity of VIII to IXESI-07. The current study combines geological, geophysical, and geotechnical data in order to establish a geological characterization of the subsoil of the city of Jama in the Manabi province of Ecuador. Then, the liquefaction potential index (LPI) has been evaluated considering the PGA-rock values calculated from deterministic methods applied to nearby geological faults, as well as the soil acceleration records for the city of Jama since the Pedernales megathrust earthquake. The importance of conducting geotechnical evaluations of particular colluvial, alluvial, and floodplain deposits, for which the liquefaction probability profiles have been additionally obtained, may serve as a useful tool for edifices foundations or earthquake resistant designs. Finally, the site response analysis is presented using a linear equivalent analysis, where previously seismic records compatible with the target spectrum have been selected. Hereby, the results of ground surface effects have been compared with the spectra of the Ecuadorian Regulation of Construction (NEC) in the context of local seismic amplification.

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

“…In seismic areas with moderate to high magnitude levels, the soil liquefaction is among the main causes of structural damage in saturated loose silt and/or sand sediments of Holocene age during earthquakes [11,81,82]. In many cases, urban areas susceptible to soil liquefaction have seen rapid unplanned population growth; however, there are few regulations from the competent agencies related to territorial planning that involve types of soils prone to seismic environmental effects [2,[83][84][85], i.e., they are not considered in municipal cadastral systems.…”

“…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].…”

“…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]. Some specialists have evaluated the surface coseismic effects related to liquefaction, exposing their criteria in a variety of studies [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]…”

“…In the epicentral area, the earthquake environmental effects were represented in cartography by the isosists of maximum macroseismic intensities using the Environmental Seismic Intensity, the ESI 2007 scale [2]; the maximum intensity (IX ESI-07 intensity) was recorded in the Jama-Pedernales epicentral area. The urban area of Portoviejo, which is located 150 km away, was assigned a VII-VIII ESI-07 intensity [11,14], and registered the greatest damage to buildings. The poor geotechnical quality of the soils was exacerbated by the total breakdown of 2678 collapsed dwellings [51].…”

“…Other historical effects of soil liquefaction were documented in 1942 (Mw 7.9), in the same seismogenetic segment of the South American subduction zone, on the central coast of Ecuador [13][14][15]18,21,25,27]. During the recent thrust of the Pedernales earthquake, coseismic geological effects were documented in an area of around 18,000 km 2 , which included various soil liquefaction features in recent sedimentary deposits, such as alluvial and alluvial-colluvial sequences, where high frequencies are related to Quaternary climatic oscillations [11,14,21,22,25,50,51]. In the seismic event that occurred on 16 April 2016, the peak ground acceleration (PGA) of the soil was 1.40 g for the city of Pedernales, while in the city of Portoviejo it was approximately 0.38 g [52].…”