We used accelerograms from foreshocks recorded by the closest strong-motion stations to the epicentral location of the 2010 El Mayor-Cucapah earthquake (Mw7.2) to estimate S-wave near-source attenuation and to investigate the spatial–temporal evolution of the spectral decay parameter kappa (κ). We found that κ estimated from the foreshocks has significantly higher values compared with those estimated using the mainshock recordings. Since κ is inversely proportional to the quality factor Q and this may vary depending on the state of stress and the presence of fluids, this observation indicates that Q was higher in the epicentral region during the mainshock rupture process, probably due to a higher concentration of stress. We calculated the average regional S-wave attenuation <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$\widetilde{\kappa }(r)$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mi>κ</mi> <mo stretchy="false">~</mo> </mover> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> before and during the mainshock using a nonparametric approach, and we also found higher attenuation before the occurrence of the main event, suggesting a possible role of fluid flow in the rupture process of the main rupture. Before and during the mainshock <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$\widetilde{\kappa }(r)$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mi>κ</mi> <mo stretchy="false">~</mo> </mover> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> increased with increasing hypocenter distance, but at short distances (r < 17 km) <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$\widetilde{\kappa }(r)$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mi>κ</mi> <mo stretchy="false">~</mo> </mover> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> increased faster before the main shock. However, during the main rupture <InlineEquation ID="IEq4"> <EquationSource Format="TEX">$\widetilde{\kappa }(r)$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mover accent="true"> <mi>κ</mi> <mo stretchy="false">~</mo> </mover> <mrow> <mo stretchy="false">(</mo> <mi>r</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> increased faster than during the foreshock sequence for r > 17 km, suggesting that the tectonic stress probably decreased beyond that distance. 17 days before the mainshock the near-source attenuation (<InlineEquation ID="IEq5"> <EquationSource Format="TEX">${\kappa }_{s}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>κ</mi> <mi>s</mi> </msub> </math></EquationSource> </InlineEquation>) was very low in the ruptured area, <InlineEquation ID="IEq6"> <EquationSource Format="TEX">${\kappa }_{s}=0.0374$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>κ</mi> <mi>s</mi> </msub> <mo>=</mo> <mn>0.0374</mn> </mrow> </math></EquationSource> </InlineEquation> s, increasing during the next 30 h to <InlineEquation ID="IEq7"> <EquationSource Format="TEX">${\kappa }_{s}=$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>κ</mi> <mi>s</mi> </msub> <mo>=</mo> </mrow> </math></EquationSource> </InlineEquation> 0.0490 s, then <InlineEquation ID="IEq8"> <EquationSource Format="TEX">${\kappa }_{s}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>κ</mi> <mi>s</mi> </msub> </math></EquationSource> </InlineEquation> started to decrease to the value of <InlineEquation ID="IEq9"> <EquationSource Format="TEX">${\kappa }_{s}=0.0024$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>κ</mi> <mi>s</mi> </msub> <mo>=</mo> <mn>0.0024</mn> </mrow> </math></EquationSource> </InlineEquation> s during the occurrence of the main event. We interpret this decrease in <InlineEquation ID="IEq10"> <EquationSource Format="TEX">${\kappa }_{s}$</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>κ</mi> <mi>s</mi> </msub> </math></EquationSource> </InlineEquation> as due to increasing Q resulting from an important increase of tectonic stress before the occurrence of the Mw7.2 earthquake. We conclude that κ, in combination with other geophysical parameters, it is useful to understand the preparatory phases of the earthquake rupture process.

Near-source attenuation of foreshocks preceding the 2010 El Mayor-Cucapah, Baja California, Mexico earthquake (Mw7.2)

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发表日期：

2025年11月10日

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Raúl R. Castro,
Carlos E. Reinoza,
Favio Cruz-Hernández

摘要

We used accelerograms from foreshocks recorded by the closest strong-motion stations to the epicentral location of the 2010 El Mayor-Cucapah earthquake (Mw7.2) to estimate S-wave near-source attenuation and to investigate the spatial–temporal evolution of the spectral decay parameter kappa (κ). We found that κ estimated from the foreshocks has significantly higher values compared with those estimated using the mainshock recordings. Since κ is inversely proportional to the quality factor Q and this may vary depending on the state of stress and the presence of fluids, this observation indicates that Q was higher in the epicentral region during the mainshock rupture process, probably due to a higher concentration of stress. We calculated the average regional S-wave attenuation $\widetilde{\kappa }(r)$ $\tilde{κ} (r)$ before and during the mainshock using a nonparametric approach, and we also found higher attenuation before the occurrence of the main event, suggesting a possible role of fluid flow in the rupture process of the main rupture. Before and during the mainshock $\widetilde{\kappa }(r)$ $\tilde{κ} (r)$ increased with increasing hypocenter distance, but at short distances (r < 17 km) $\widetilde{\kappa }(r)$ $\tilde{κ} (r)$ increased faster before the main shock. However, during the main rupture $\widetilde{\kappa }(r)$ $\tilde{κ} (r)$ increased faster than during the foreshock sequence for r > 17 km, suggesting that the tectonic stress probably decreased beyond that distance. 17 days before the mainshock the near-source attenuation ( ${\kappa }_{s}$ $κ_{s}$ ) was very low in the ruptured area, ${\kappa }_{s}=0.0374$ $κ_{s} = 0.0374$ s, increasing during the next 30 h to ${\kappa }_{s}=$ $κ_{s} =$ 0.0490 s, then ${\kappa }_{s}$ $κ_{s}$ started to decrease to the value of ${\kappa }_{s}=0.0024$ $κ_{s} = 0.0024$ s during the occurrence of the main event. We interpret this decrease in ${\kappa }_{s}$ $κ_{s}$ as due to increasing Q resulting from an important increase of tectonic stress before the occurrence of the Mw7.2 earthquake. We conclude that κ, in combination with other geophysical parameters, it is useful to understand the preparatory phases of the earthquake rupture process.