UC Santa Barbara

Currently, the source mechanisms of global large and moderate earthquakes are routinely constrained and reported by multiple agencies using long-period seismic signals. These conventional analyses simplify the rupture process of an earthquake as single point source. On one hand, it becomes not appropriate if this earthquake has a fault dimension comparably larger than the signal's dominant wavelength or includes multiple subevents with different focal mechanisms. On the other hand, these source complexities can yield distinct seismic waves, and therefore might be deciphered through more careful waveform modeling. Such additional information is crucial for the study of earthquake physics as well as post-earthquake hazard evaluation and emergency assistance. The first part of this dissertation introduces a nonlinear inverse algorithm, which approximate an earthquake as multiple double-couple sources (MDC) and constrain them simultaneously using seismic data. The number of double-couples is determined by statistic F-test. This method has been applied to local and global large earthquakes using long-period signals, teleseismic body waves and local broadband strong-motion records. The results demonstrated that more source information could be extracted using this MDC algorithm than the conventional moment tensor inversion algorithms, though the spatiotemporal resolution relies on the station coverage and frequency contents of seismic signals. The implements of high-quality broadband seismograph networks globally and advances in parallel computational capability enable us to conduct the MDC analysis automatically for global large earthquakes and provide more precise priori information for the subsequent finite fault inversions. It then shall be an important contribution to the routine real-time earthquake hazard analysis.

The second part of this thesis focuses on detecting the aftershocks occurring temporally close to the corresponding mainshocks with the MDC analysis and subsequently analyzing their dynamic interactions with the mainshocks using complete Coulomb stress failure criteria. The two examples presented in Chapter 3 and 4 are the 2012 Mw 7.3 Honshu earthquake and 2000 Mw 8.1 New Ireland earthquake. Our results indicated that both events are doublets. The first subevent of the 2012 Honshu earthquake is a Mw 7.3 oblique thrust earthquake beneath the seaside of the Japan trench axis at a depth of ~50 km, and followed ~13 s later by a Mw 7.3 pure normal fault rupture ~50 km to the N260oE, at a depth of 25-30 km beneath the island side of the trench axis, a classic example of the plate bending. The first subevent of the 2000 Mw 8.0 New Ireland earthquake is Mw 8.0 right lateral strike-slip earthquake, which ruptured a fault plane of 140 km and following 90 s later by a Mw 7.4 normal fault earthquake beneath the outer rise region of subducted Solomon sea plate, 263 km to the south of the first subevent. Our subsequent stress calculations indicated that the rupture of the 2011 Mw 9.1 Tohoku earthquake increases the stress levels at the hypocenters of two 2012 Honshu subevents. While the Coulomb stress increment is significantly smaller than the co-seismic stress drop of the first subevent, which at the second subevent is compatible to co-seismic stress drop. At the hypocenter of the second subevent, the rupture of the first subevent produced negative static Coulomb stress. The dynamic Coulomb stress carried by the direct P wave is negative as well, but that associating with the direct S is positive with a peak amplitude of 3.8 MPa.. Although the inverted rupture initiation time of the second subevent is 1.7 s earlier than when the positive stress reaches the maximum, this is within the uncertainty of our kinematic modeling. For the 2000 New Ireland doublet, our calculations indicate that the rupture of first subevent produces either a negative or positive but with negligible amplitude (6x10^-4MPa) static Coulomb failure stress at the hypocenter of second subevent. In contrast, a 0.91 MPa positive dynamic Coulomb stress pulse excited by the stopping phase associating with the rupture of the dominant asperity of the first subevent reaches the hypocenter of the second subevent at 86 s and keeps positive until 90 s. November 16 2000 Mw 8.0 New Ireland earthquake is then a unique example that passing of seismic waves caused a nearly instantaneous triggering of an Mw >7 normal fault earthquake 150 km away. In Chapter 5, we have performed MDC analysis and finite fault joint inversion for 2013 Mw 7.7 Balochistan earthquake using teleseismic body waves and surface waves. Volumetric and Coulomb stress perturbation are then carefully investigated to shed light on revealing the possible dynamic interaction between mud volcano eruption and seismic process of the 2013 Pakistan earthquake. In short, for the all three cases, we find that these subsequent events are correlated with the dynamic stress perturbation, rather than static stress perturbation, excited by the earlier events.