Merging plot of massive earthquake catalogs using the minimum number of active records

Abstract

Generating and editing vector graphics of earthquake catalogs are time consuming when the volume of catalogs is huge; however, most symbols of earthquakes are actually covered by others thus are not visible. We need to condense the catalogs to a limited number by disregarding those invisible symbols to simplify the process. In this paper, we only select the superficial portrait of the whole data volume by ignoring all invisible records. In numerical implementation, we mark one record as active if at least one of its pixels in the virtual bitmap is visible, after judging the superposition relationship between this record and all previous active records; otherwise, we mark this record as inactive and remove it from the catalogs. This approach does not degrade the quality of the final vector graphics, since it can completely separate active records from inactive ones. It can greatly suppress the final size of vector graphics file and significantly reduce the time consumption of drawing, editing, and displaying vector graphics, since the number of active records was condensed to a minimum. In an example of 410,642 earthquake catalogs, only 2577 records (~0.63% the whole catalog) were finally selected as the candidates to be plotted. The run time of our code for selecting these active records spends only ~14 s. The resulted file is only ~2.2 megabyte in size, which is much smaller than that of >100 megabyte when plotting the whole catalog. The run time of plotting active records is about one minute, greatly reduced from >10 h when plotting the whole catalog.