Local Crustal Imaging Using High-Magnitude Earthquakes

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Global phases are seismic waves that travel through the earth's core before emerging at the surface. Traditionally, global phases are used to obtain subsurface information up to 1 Hz. A recent study, however, found that signal present in the coda of strong, distant earthquakes contains higher frequencies. Quantitative analysis of the coda of earthquakes, of at least magnitude 6, shows that frequencies of, on average, 3 Hz can be found, with the most promising results coming from P- and PKIKP-phases. By autocorrelating the coda of these phases an estimate of the zero-offset reflection response below a seismic station can be retrieved.
The method allows to successfully delineate basins in both Argentina and the United States. Even higher frequencies, up to 8 Hz, are used at specific stations, that have been active for an extended period of time (~ 8 years). Again the method succeeds in retrieving structural information below these stations. The results of both studies are confirmed by existing literature as well as Ambient Noise Seismic Interferometry (ANSI) studies. ANSI is a method, which retrieves the reflection response below a station by applying seismic interferometry to ambient noise.
Advantages are that the method is computationally cheap and fast and that it is a passive measurement thus requiring low-effort. A disadvantage, however, is the unpredictability of earthquakes, which means that the exact duration the stations have to be active is unknown. Instead, the station will have to record over a prolonged amount of time, during which suitable earthquakes should occur. Research into the upper crustal structure, in particular, may benefit from the method. For example, the depth of a sedimentary basin can be discovered and imaged using this method. This gives important constraints, e.g. for geothermal prospecting.