Relative moment tensors and deep Yakutat seismicity

• Published in: Geophysical Journal International
• Main Authors: Plourde, Alexandre P, Bostock, Michael G
• Other Authors: Seismological Facilities for the Advancement of Geoscience and EarthScope, National Science Foundation, Natural Sciences and Engineering Research Council of Canada
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2019
• Subjects: Geochemistry and Petrology; Geophysics; Yakutat; Alaska
• Online Access: http://dx.doi.org/10.1093/gji/ggz375; http://academic.oup.com/gji/advance-article-pdf/doi/10.1093/gji/ggz375/29158429/ggz375.pdf; http://academic.oup.com/gji/article-pdf/219/2/1447/29807102/ggz375.pdf

SUMMARY We introduce a new relative moment tensor (MT) inversion method for clusters of nearby earthquakes. The method extends previous work by introducing constraints from S-waves that do not require modal decomposition and by employing principal component analysis to produce robust estimates of excitation. At each receiver, P and S waves from each event are independently aligned and decomposed into principal components. P-wave constraints on MTs are obtained from a ratio of coefficients corresponding to the first principal component, equivalent to a relative amplitude. For S waves we produce constraints on MTs involving three events, where one event is described as a linear combination of the other two, and coefficients are derived from the first two principal components. Nonlinear optimization is applied to efficiently find best-fitting tensile-earthquake and double-couple solutions for relative MT systems. Using synthetic data, we demonstrate the effectiveness of the P and S constraints both individually and in combination. We then apply the relative MT inversion to a set of 16 earthquakes from southern Alaska, at ∼125 km depth within the subducted Yakutat terrane. Most events are compatible with a stress tensor dominated by downdip tension, however, we observe several pairs of earthquakes with nearly antiparallel slip implying that the stress regime is heterogeneous and/or faults are extremely weak. The location of these events near the abrupt downdip termination of seismicity and the low-velocity zone suggest that they are caused by weakening via grain-size and volume reduction associated with eclogitization of the lower crustal gabbro layer.