Authors: Laura Gulia, Stefan Wiemer, and Max Wyss
Abstract: Man-made contaminations and heterogeneity of reporting are present in all earthquake catalogs. Often they are quite strong and introduce errors in statistical analyses of the seismicity. We discuss three types of artifacts in this chapter: The presence of reported events, which are not earthquakes, but explosions; heterogeneity of resolution of small events as a function of space and time; and inadvertent changes of the magnitude scale. These problems must be identified, mapped, and excluded from the catalog before any meaningful statistical analysis can be performed. Explosions can be identified by comparing the rate of day-time to night-time events because quarries and road construction operate only during the day and often at specific hours.
Spatial heterogeneity of reporting small events comes about because many stations record small earthquakes that occur near the center of a seismograph network, but only relatively large ones can be located outside the network, for example offshore. To deal with this problem, the minimum magnitude of complete reporting, Mc, has to be mapped. Based on the map of Mc, one needs to define the area and the corresponding Mc, the choice of which leads to a homogeneous catalog. There are two approaches to the strategy for selecting an Mc and its corresponding area of validity: If one wishes to work with the maximum number of earthquake per area for statistical power of resolution, one needs to eliminate from consideration areas of inferior reporting and use a small Mc(inside), appropriate for the inside of a network. However, if one wishes to include areas outside of the network, such as offshore areas, then one has to cull the catalog by deleting all small events from the core of the network and accept only earthquakes with magnitude larger than Mc(outside). In this case, one pays with loss of statistical power for the advantage of covering a larger area.
As a function of time, changes in hardware, software, and reporting procedure bring about two types of changes in the catalog. (1) As a function of time the reporting of small earthquakes improves because seismograph stations are added or detection procedures are improved. (2) The magnitude scale is inadvertently changed due to changes in hardware, software, or analysis routine. The first problem is dealt with by calculating the mean Mc as a function of time in the area chosen for analysis. This will usually identify steps of Mc downward (better resolution with time) at fairly discrete times. Once these steps are identified, one is faced with choosing a homogeneous catalog that covers a long period, but with a relatively large Mc(long time). This way one gains coverage of time, but pays with loss of statistical power because small events, which are completely reported during recent times, have to be eliminated. On the other hand, if one wishes to work with a small Mc(recent), then one must exclude the older parts of the catalog in which Mc(old) is high.
To define the magnitude scale in a local or regional area in such a way that it corresponds to an international standard is not trivial, nor is it trivial to keep the scale constant as a function of time, when hardware, software, and reporting procedures keep changing. Resulting changes are more prominent in societies characterized by high intellectual mobility, and may not be found in totalitarian societies, where observatory procedures are adhered to with military precision. There are two types of changes: simple magnitude shifts and stretches (or compressions) of the scale. Here, we show how to identify changes of the magnitude scale and how to correct for them, such that the catalog approaches better homogeneity, a necessity for statistical analysis.
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