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.

本研究基于2011年至2016年在墨西哥普埃布拉州洛斯乌梅罗斯火山复合体（Los Humeros Volcanic Complex）临时布设的15个地震台站网络记录的微地震数据，以横波分裂参数（快横波偏振（fast shear wave polarization）与延迟时间（delay time））为核心开展各向异性特征分析。本次分析旨在明确流体注入对地震活动的影响，以及两类参数变化所引发的构造应力场潜在变异；研究发现，多数快横波偏振结果与主要地质构造的走向具有相关性，这些构造整体呈NW-SE向，且与推断的马尔派斯、安提瓜、伊曼及Nueva断层相契合，同时存在少量E-W及WNW-ESE向的次级偏振方向，对应局部小型断层（基于方位角数值），该现象受构造环境及流体注入孔隙压力变化产生的效应影响。另一方面，基于统计检验结果，两类参数的年度变化趋势显示，2014年9月至2016年6月期间存在显著变化，表明应力水平的潜在变化源于流体注入量的增加。2014年至2016年的年度平均值图表同样显示应力水平出现下降趋势，这与快横波偏振的分析结果一致，且通过t检验（t-test）得到了验证。延迟时间结果可用于刻画地热田的主要渗流通道，研究指出洛斯乌梅罗斯断层以及推断的安提瓜和马尔派斯断层是内部热源的主要渗流通道。本数据集包含两类参数（快横波偏振与延迟时间）的计算结果，可按年份、月份或区域开展通用分析，所有数据均关联至对应地震台站，支持局部尺度的精细化解读。此外，研究所用地震事件的震源参数（hypocentral parameters）原始数据同样公开可用，可用于进一步解析该现象的相关特征。例如，图4的原始数据以年、纬度、经度、深度及震级列为列项，可用于展示2011年至2016年记录的地震事件分布；图6的滤波分析数据以台站方位角计数（左列）与方位角数值（右列）为列项，可通过玫瑰图（rose diagrams）呈现2011-2016年研究期内的相关特征；图8的滤波分析数据以时间（左列）与方位角参数数值（右列）为列项，可绘制方位角参数的单值与年度平均值图表；而表6的分析数据则以各台站的延迟时间参数值为列项，可用于计算平均延迟时间及标准差（standard deviations）。