Seismic Hazard Assessment for Guayaquil City (Ecuador): Insights from Quaternary Geological Data
by
Kervin Chunga
Dipartimento di Scienze Chimiche e Ambientali, Università dell'Insubria, Via Vallegio 11, 22100, Como, Italia
Coauthors: Carlos León, María Quiñónez (ESCIGEA, Universidad de Guayaquil, Ecuador), Stalín Benítez, Galo Montenegro (ESPOL, Guayaquil, Ecuador)

Guayaquil City, the chief fluvial and maritime Port of Ecuador, is located in the estuarine zone of the lower Guayas River Drainage Basin (South of the Ecuadorian Seaboard). This city has the largest urban residents with 2.156.636 inhabitants (Source: INEC, 2001); and together with Quito, the Capital's Nation city represents the 80% of the Industrial Production. However, this industrialized city is threatened by free from earthquakes. In fact, its closeness with significant seismogenetic structures makes it highly susceptible to tectonic events. The few available seismic hazard studies for the area of Guayaquil have been based essentially on seismological data. These studies have analyzed: (1) damage estimation to residential buildings (i.e., houses, schools, hospitals, etc.) (Argudo et al., 1993); (2) life-loss estimation models to the chief urban areas; and (3) assessment of Guayaquil seismic hazard through an adopted seismogenetic structure capable of generating large earthquakes. For instance, the Radius Project (Radius Project, 1999; http://geohaz.org/radius/LACont.htm) selected as reference structure the Subduction Zone, capable of generating magnitude 8 earthquakes, and located about 200 Km NW of Guayaquil.

In this paper, we discuss the contribution that detailed geological data can provide to the seismic hazard surveys of the area of Guayaquil city. We reviewed in detail the main seismic events that struck the study area and described capable seismogenetic structures through historical seismic data (implement it to the GIS tool); which will be applied to understand the actual situation of Guayaquil and its possible ground environmental effects as a consequence of moderate to strong earthquakes. For this study, we made a GIS database composed of 939 instrumental and historical intensity data (in the range of MM Intensity VI-XI) measured in 122 Ecuadorian seismic events (from 1557 to 2000). Information on these events has been provided by CERESIS Seismic Intensity Catalogue (www.ceresis.org/new/es/index.html), and from Web pages of some governmental entities (www.igp.gob.pe; www.igepn.edu.ec).

The most destructive historical earthquake that struck Guayaquil (with MM IX) occurred on 14th May 1942 (Mw 7.8), causing moderate to high damage in the central urban area (Argudo et al., 1993). The earthquake occurred near the subducting Carnegie Ridge off the coast of Ecuador, where the Nazca Plate subducts beneath the South American Plate. The instrumental epicenter was located up to 240 Km NW from Guayaquil city.

Moreover, other seismogenetic structures have also generated significant earthquakes that struck the area causing moderate to high effects to the Guayaquil suburban and urban sectors. The most representative are:

(A) The 9th July 1653, Guayaquil earthquake: it was located in the surrounding of the current city with MM Intensity of VII. Previous seismic hazard studies have not mentioned this event, perhaps, because of the scarce information available in the historical record.

(B) The 18th August 1980, earthquake (Ms 6.1) whose instrumental epicenter was located up to 28 Km NW from Guayaquil, with MM Intensity measured of VII-VIII. Seismic shaking was felt very strongly in the center and southern part of the city. For their epicentral locations, both the 9th July 1653 and the 18th August 1980 earthquakes could be tectonically linked to the active Colonche fault and to a potential Quaternary uplifting of the Chongon Colonche Ridge.

(C) The 12th Dec 1953, Tumbes earthquake (Northwest Perú and South Ecuador, Ms 7.3), whose instrumental epicenter was located up to 155 Km SW from Guayaquil city, the MM Intensity measured for Guayaquil was of VI-VII (Silgado, 1957). The possible seismogenetic structure which generated this seismic event is still not well defined. Based on our surveys, this earthquake could be linked to the Jubones-Tumbes Fault or to the capable Amistad Fault (see also the other Abstract by Chunga et al. in this vol.).

(D) The 27th July 1971, earthquake (Ms 7.5), whose instrumental epicenter was located up to 290 Km SE from Guayaquil, is the most distant event that produced significant effects in Guayaquil, with MM Intensity of VII. This event could be linked to the Macuma reverse Fault (Prov. Morona Santiago).

(E) The 4th February 1797, Riobamba earthquake (epicentral MM: XI): is considered as the strongest historical earthquake recorded in Ecuador (www.igepn.edu.ec/sismologia/sismicidad/historica/efectos.htm). The instrumental epicenter was located up to 150 Km NE from Guayaquil city. The MM Intensity in Guayaquil has not been measured neither studied in detail (e.g. Argudo et al., 1993). This event has been attributed to the capable Pallatanga strike slip Fault (this fault crosses the western Andean Cordillera and joins the Gulf of Guayaquil), which is characterized by slip rates ranging from 1 to 5 mm/yr (USGS, 2003; http://pubs.usgs.gov/of/2003/ofr-03-289/). This large seismic event should be taken in account for future detailed paleoseismological studies, inasmuch as, its closeness to the Continental Megashear (composed by a right-lateral fault system about 2200 Km long, extended from the Gulf of Guayaquil, on the Pacific coast until the surrounding of Caracas, on the Caribbean coast) makes it an excellent test site.

All these historical facts emphasize the importance of understanding the main ground natural effects caused by earthquakes, such as: natural subsidence or acceleration of anthropogenic subsidence (at present day, this last phenomenon is observed in the southwest sector of Guayaquil), ground cracking, liquefaction or settlement of soils, massive rockfalls and landslides and other effects common to earthquakes that have occurred in similar geologic settings. In fact, the lithologic characteristics of the soil on which Guayaquil lies is very uneven. For instance: (1) the commercial and urban center rest on unconsolidated Holocene alluvial clay deposits interbedded with silty and clayey sand sediments; (2) the southwestern and southern part of the city, where it is concentred the less developed urban area, directly lies on filling in marshland; (3) in the northern part of the city: a) the residential areas rest on consolidated stratified silt siliceous deposits, belonging to the Guayaquil Formation (Danian age, Cretaceous-Tertiary contact) and as well upon volcano-sedimentary successions appertain to the Cayo Formation (Late Cretaceous age); b) the marginal urban areas lie on unstable foothills; and c) in the Kennedy Norte residential zone, large buildings lie on soft-sediments. Indeed, these lithologic characteristics would create suitable conditions for amplification of ground motions.

Therefore, for assessing the impact of seismically-induced ground effects in natural environment, we recommend to apply the new INQUA Intensity Scale which could be a powerful tool for the engineering community, allowing to properly taking into account the geological setting and the geotechnical aspects in the seismic hazard surveys.

References

Argudo, J., Bravo, F., Yela, R. (1993). Metodología para la reducción de la vulnerabilidad sísmica de escuelas y bibliotecas en Guayaquil. La Educación (115) II.

Egred, J. (1968). Breve historia sísmica de la Republica del Ecuador. Observatorio Astronómico de Quito, Vol.4, pp.147-190.

Michetti, A.M.; Esposito, E.; Gürpinar, A.; Mohammadioun, B.; Mohammadioun, J.; Porfido, E.; Rogozhin, E.; Serva, L.; Tatevossian, R., Vittori, E., Audemard, F., Comerci, V., Marco, S. McCalpin, J., Mörner, N.A. (2004). The Scale INQUA: An innovative approach for assessing earthquake intensities based on seismically-induced ground effects in natural environment. Special paper. Memorie - Descrittive della Carta Geologica d'Italia, Vol. LXVII, E. Vittori and V. Comerci eds.. APAT-Dip. Difesa del Suolo/ Servizio Geologico d'Italia. Roma.

Silgado, F.E. (1957). El movimiento sísmico del 12 de diciembre de 1953. Boletín de la Sociedad Geológica del Perú, vol.32, Part 2, pp.225-238.

Date received: July 21, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-47.