The electronic supplement presents a database of intensities compiled for Colombia and Western Venezuela. The data was extracted from isoseismal maps and processed/referenced documents. A total of 68 events occurred between 1766 and 2004 is included. 283 pairs, intensity-equivalent radii, were processed to be used as the main source for modeling probability distributions.
The first step was to access the USGS/NEIC database (see Data and Resources section), delimiting a rectangular area between coordinates 12°N, 62°W and 17°S, 81.5°W, which widely encompasses the area of Venezuela, Colombia, Ecuador and Peru. The available USGS/NEIC databases which offered historical information about the earthquakes in this region were then selected, namely “seismicity of the Andean Region”, “SISRA”, “catalog (1471–1981)” and “data sets of significant worldwide earthquakes (2150 B.C–1994 A.D)”.
More than 12,000 events were displayed, a great percentage of them lacking basic details like depth, magnitude, exact date and occurrence time. The search was expanded to secondary information sources, such as journals, proceedings, technical reports and consulting studies.
In the specific case of Colombia, seismic instrumentation was firstly installed in the 1920’s by Jesuit scientists. Previous accounts of earthquakes go back to the pre–Hispanic time, mixed into the mythology of the native inhabitants. Such is the case of the Guayupe Indians, dwellers of the eastern plains of Colombia, who believed that the god Chibchacun had been condemned by god Bochica to carry the earth on his shoulders, and when ever he changed it from side or laid it down, an earthquake occurred (Dimaté et al., 2005).
Spanish conquistadors arrived at Bogotá region in 1538, but the first written description of an earthquake in the zone dates back to 1566. Probably the most relevant contributions to the description of historical earthquakes in the colonial period come from reports and narrations of missionaries and refer especially to damages on churches (Dimaté, et al., 2005).
It is worth noting the efforts made by Father Ramírez (1948, 1953, 1975a, 1975b, 2004), who pioneered the compilation of historic seismicity in Colombia and laid the corner stone of seismological research in Colombia. His work was continued and complemented among others by the works of Sarria (1985), Espinosa (1992), Gómez (1997), Salcedo (1994), Salcedo and Gómez (2000), Briceño (2004), and Dimaté et al. (2005).
Ramírez (1975a) based his compilation of historic earthquakes mainly on ancient reports written by priests, historians, traveling scientists and soldiers in the colonial and in republican times. He complemented his compilation with data from international seismic catalogs published at that time in seismic bulletins and scientific journals, on the scarce seismic records registered with the first incipient Colombian seismological network established by the Jesuits in 1923, and on surveys among eye–witnesses after an earthquake took place (after 1943).
For earthquakes in Venezuela, Ecuador and Peru, the works by MasVall (1950), Fiedler (1961), Villacis et al. (1994), Grases and Rodríguez (2001), Rodríguez and Audemard (2001), Ccallo (2002), Escobar and Rengifo (2003), Saragoni et al. (2004), were taken into account, particularly the isoseismal maps drawn or referenced by these authors.
A considerable number of the published isoseismal maps were processed by Sarria (1985). His catalog is based on the historic compilation made by Ramírez (1975a). The historic events related in the mentioned compilation were incorporated into a corrected and extended database, in which the earthquakes were geographically located and correlated with major geologic fault systems. In 1982 the seismic catalog for Colombia was concluded, as part of the SISRA project sponsored by the Regional Center of Seismology for South America (CERESIS), based on information from 4,784 earthquakes, from which 999 events were historic and 3,782 instrumental.
Sarria (1985) mentions that the reliability of the compiled earthquakes in the SISRA catalog is not as good as one would have wished, because nearly 2,000 events were recorded in 5 or fewer seismological stations. It is worth mentioning that at that time, Colombia had only 7 seismological stations, and 10 accelerometers. In the frame of the SISRA project, information about the effects of 132 historic earthquakes was gathered, resorting to local newspapers, historic chronicles and in some cases to interviews with witnesses.
From 132 earthquakes, only 19 events presented sufficient information in terms of quantity and quality that allowed the drawing of isoseismal maps. Salcedo and Gómez (2000) complemented this database, adding up to 34 isoseismal maps. The criterion for selecting events and their corresponding maps was to select those earthquakes with magnitude equal or greater than 5.0, with one exception, or events where the highest observed intensity was greater than 5. Magnitudes were taken from the CERESIS database, which is one of the most reliable for the Colombian region.
The intensity scale used was the Modified Mercalli (MMI), and the epicentral intensity (I0) was defined as the isoseismal with the highest value (Imax) as in Gómez and Salcedo (2002). Recent earthquakes have resulted in high quality isoseismal maps based on surveys conducted among people living not only in the areas worst–hit by the events, but also on more distant regions where the events were barely felt. The database presented in this article compiles information from 68 earthquakes, which were selected because either their isoseismal maps or the processed information (areas inside isoseismals and their equivalent radii) were available.
Despite the fact that Colombia and Western Venezuela are earthquake–prone regions with an ample range of recorded damaging effects, no formal intensity scale has been adapted for considering the geologic, structural, and cultural conditions of the countries (Salcedo and Gómez, 2000). Therefore, each researcher chooses his/her preferred scale to evaluate and draw isoseismal maps, according to his/her experience and best judgment.
The great majority of collected maps for this study were originally drawn using the MMI scale. About 13% of the original data represented other intensity scales like R–F (Rossi–Forel), MCS (Mercalli–Cancani–Sieberg), MSK (Medvedev, Sponheuer, Karnik), and EMS–92 (European macroseismic scale). In many cases, it was observed that the researchers took those maps with their original scales and converted them into the Mercalli Modified scale. This procedure can be easily implemented when the differences between the degrees of the intensity scales (the original scale and the desired one) for the analyzed earthquakes are not significantly important. However, when there is an isoseismal map whose contours can be grouped or split to obtain an equivalent isoseismal (for example when a isoseismal X from a map drawn in the R–F scale has to be converted into a MMI–scale isoseismal map), researchers need access to the primary information from which the isoseismals were derived (interviews, newspaper articles, surveys, etc). In this way, it would be possible to re–evaluate in a more accurate way the reported effects of the earthquake in the light of the desired scale. By doing so, the isoseismal contour could be redrawn, following the detailed description given for each particular degree in the scale of interest.
Unfortunately, for this project the access to the primary sources of information was impossible.
The information compiled for the 68 events from the USGS/NEIC database was compared, corrected and complemented (when needed) with the collected information from the aforementioned documents. Special care was taken with the depth of the earthquakes. Epicentral depth is one of the most important criteria to group the earthquakes (shallow, intermediate, deep), in order to derive attenuation relations for different seismological sources. The list with the 68 chosen earthquakes and their date, location, depth, magnitude, scale of magnitude, maximum intensity, scale of intensity and number of isoseismals is presented in Table S1.
The 68 earthquakes compiled in this database are described in terms of their intensity level, the equivalent isoseismal radius R, the epicentral intensity I0 and the difference between I and I0 (see Table S2.)
The map of the area of study with the location of the 68 earthquakes listed in Table S1 is presented in Figure S1.
A total of 45 isoseismal maps were scanned and digitized in a CAD system. Some examples of the collected isoseismal maps that were drawn for some of the most important events in the last century are shown in Figure S2a for the event of Tumaco 31/01/1906 (Espinosa, 1992); Figure S2b for the event of Pasto 14/07/1947 (Ramírez, 1948); and Figure S2c for the event of Quindío 25/01/1999 (Briceño, 2004).
The USGS/NEIC database was consulted in http://neic.usgs.gov/neis (last accessed on January 15, 2005).
The map of isoseismals was digitalized using AutoCAD-2008. The dwg file can be freely downloaded.
Download: Isoseismals-Maps.dwg. AutoCAD file [266 KB] containing the digitalized isoseismals for the events collected in the database. The numbering of the earthquakes presented in Table S1 coincides with the names of the layers in the AutoCAD file.
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