Shear Wave Splitting parameters of the Los Humeros Geothermal Field, Puebla, Mexico.
A study of anisotropic characteristics based on shear wave splitting parameters (fast shear wave polarization and delay time) was carried out from microearthquakes recorded from a network of fifteen seismic stations temporarily installed in Los Humeros Volcanic Complex, Puebla, Mexico from 2011 to 2016. This analysis determined the effects on seismic activity caused by fluid injection, as well as possible variations of the tectonic stress regime by changes in both parameter, where most of the fast shear wave polarization results correlate with the orientation of the main geological structures that tend mainly NW – SE and follow the inferred Malpais, Antigua, Iman and Nueva faults with some secondary polarizations E – W and WNW - ESE related to minor local faults (employing the values of azimuths) in the tectonic ambient and effects generated by variations in pore pressure of fluid injection. On the other hand, the annual tendency for both parameters shows a significant change from September 2014 to June 2016, based on statistical tests that indicates that possible changes in the stress level are due to an increase in fluid injection. Graphs of the annual averages from 2014 to 2016, also indicate a change in the stress level (decrement), similar to our results of fast shear-wave polarizations, also confirmed by a t-test. The delays results have allowed characterizing the main routes of permeability in the geothermal field, indicating the Los Humeros Fault, and the inferred Antigua and Malpaís faults as the main routes from internal heat sources. The database, consisting of the results of both parameters (fast shear wave polarization and delay time), allows interpreting them in a general analysis for the period studied, by years, months or by areas, since all have been referred to a seismic station, which also allows a local analysis. Likewise, raw data related to the hypocentral parameters of the seismic events employed are also available, allowing to know more about the characteristics of the phenomenon. For example, the raw data of Figure 4 are represented by columns of years, latitudes, longitudes, depths, and magnitudes, which can show the locations of the seismic events registered from 2011 to 2016; the filtered and analyzed data of Figure 6 are represented by columns with the number of azimuths (left) and the value of azimuth (right) of each seismic station, which can be represented by rose diagrams for the studied period 2011 –2016; the filtered and analyzed data of Figure 8 are represented by a columns of time (left), and values of the azimuth parameter (right), which can be depicted by graphs of individual values and annual averages of the azimuth parameter; or the analyzed data in Table 6 are represented by a column with the values of the delay time parameter of each station, which allow to calculate the mean delay time and the standard deviations.