The Subdue structural discovery system is being used as the Data Mining tool to study the "Orizaba Fault " located in Mexico, as part of a research project of the geologist Dr. Burke Burkart. We analyze the information of the Earthquake Database
the weakest point of the fault. This is more complex than itappears, because the fault is not continuous. It isinterrupted in some locations, changes direction and isprobably connected to other faults. This means that theearthquakes might take place in a location out of the fault,but still as a consequence of this fault.
Figure 5: Area of Study of the Orizaba Fault.
This study started with the identification of the areawith more possibilities of being affected by the fault. Westarted by selecting two rectangles. The first hascoordinates 94.5W Longitude through 101.0W Longitudeand 17.0N Latitude through 18.0N Latitude. The secondhas coordinates 94.0W Longitude through 98.0WLongitude and 18.0N Latitude through 19.0N Latitude. Thearea includes parts of the states of Guerrero, Oaxaca,Puebla and Veracruz. We can see this area in figure 5.
We ran Subdue over the graph representation of theearthquakes in these two rectangles. Subdue helped us tofind not only a subarea with a high concentration ofearthquakes, but also some of the area’s characteristics.The most representative substructures found with Subdueare shown in figure 6. In Substructure 1 we can see that anearthquake Event is related to another earthquake Eventwith a Near_in_distance edge. We also see that one of theearthquake Events is linked to a node representing theregion number “59” and to another node representing theCatalog “PDE.” What this substructure is telling us is thatregion number 59, which is located in the state ofGuerrero, is the one with more earthquake activity (it hasmore occurrences than other regions), in this case with 556earthquakes. Finally the substructure tells us that there is adistance relation between some of the earthquakes,identified by the Near_in_distance edges, that means thatthere is a distance of less than 75 km. between the events.Dr. Burkart identified this area as very active. However,the cause of these earthquakes is not related to the fault instudy, at least this is not yet clear. Substructure 2 links twosubstructure 1s with a “Near_in_distance” edge. It alsolinks one of the Substructure 1s to a vertex describing thedepth of one of its events (Substructure 1 contains two
events as can be seen in figure 6) at 33 km. Thissubstructure tells us about a common depth among some ofthe earthquakes in the area of study.
Substructure 1, 278 instances.
Substructure 2, 138 instances.Figure 6: Substructures Found in the Whole Area of Sudy.
Next, we decided to divide the area and study the sub-areas. We divided the rectangles in small pieces of one halfof a degree in both longitude and latitude. For example,one of those rectangles has coordinates 101.0W to 100.5Wof Longitude and 17.0N to 17.5N of Latitude. We dividedthe total area into 44 sub-areas. After we divided the areaof study, we got all the available information aboutearthquakes in each sub-area from the EarthquakeDatabase described before (the database containsearthquake information from 1973 up to the present date).Table 1 shows how many earthquakes we found in each ofthese sub-areas.
Table 1: Sub-Areas of Study for the Orizaba Fault.
Once we collected the information about earthquakesregistered in each sub-area, we were ready to study itscharacteristics (e.g. common depth and intensity perregion). Here Subdue takes part in this research again. Wetook the information of each sub-area where more than tenearthquakes were registered and converted it into the graphrepresentation used by Subdue. Then we ran Subdue tofind out the characteristics of the earthquakes in that sub-area. As an example lets take sub-area 26 of table 1 labeledwith the name of “Ver1.” Figure 7 shows the first twosubstructures found by Subdue in that sub-area.Substructure 1 in the figure shows that the events happenedin the region number “61,” which corresponds to theselected sub-area. We can also see that the events’information was taken from the “PDE” catalog. Insubstructure 2 we find a pattern of some of the events at a