The map is interesting, because it is presents the data in 3 ways:
2. With the help of usual points the color of which depends on the magnitude of the earthquake. The values of all attributes of each point you can see by hovering the cursor over it and clicking on it (if while hovering over a point, it is not activated, then move the map a little bit and then again hover the cursor). You need to click on the point of earthquake you are interested in, after that a pop-up window will be opened (See Figure 3). How to create such a color legend you can learn for example from this article: "Customizing legend of the layer with cluster strategy of OpenWebGIS" if you look at Figures 4,5,6 in it.