Graduate Thesis Or Dissertation
 

The 1988 Lancang-Gengma, China, earthquake sequence : teleseismic body wave, surface wave and strong ground motion studies

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  • On November 6, 1988, two strong earthquakes (Mw: 7.0 and 6.8) separated by about 13 minutes occurred in Yunnan Province, China. The aftershocks located by Kunming Telemetered Seismic Network form a lineament approximately 120 km long and 20 km wide with the long dimension oriented approximately N30°W. The epicenter of the first event lies about 30 km from the southern terminus of the aftershock zone while the epicenter of the second event is 60 km further to the northwest. Field investigations indicate that the surface fault ruptures associated with the first and second shock and a variety of ground deformations. We analyze teleseismic data recorded by the GDSN network to determine the rupture process of these two mainshocks (referred to as Ml and M2) and the two largest aftershocks (referred to as Al and A2). Inversion of long-period body waves gives the following centroid source parameters for Ml: strike 154°±4°, dip 86°±1°, slip 181°±1°, centroid depth shallower than 15 km (least-misfit centroid depth 12 km), and seismic moment 4.5-4.9 X 10²⁶ dyn cm (least-misfit seismic moment 4.6 X 10²⁶ dyn cm). The source time function, further constrained by broadband seismograms, indicates that the source duration for this event is 12 seconds. Due to signal interference with Ml, body wave inversion techniques cannot be applied to M2. The Rayleigh waves provide a better look at this event. In order to identify the energy contributions from the two events, group velocity analysis was performed on the surface wave trains. The energy from the individual events was then isolated based on their dispersion patterns. The amplitude spectra in the period range of 100 to 66 s were inverted for the source parameters. The inversion constrains the strike of M2 precisely (155°±3°), however, dip and slip angles were not well resolved by the inversion. Similar Rayleigh wave amplitude spectra and radiation patterns of Ml and M2, however, suggest that they had very similar mechanisms and centroid depths. On the average, the amplitude spectra of M2 are smaller than those of Ml by a factor of 2.2, indicating the seismic moment of M2 is 2.1 X 10²⁶ dyn cm. The two largest aftershocks, Al (Mw 6.1) and A2 (Mw 5.3), which occurred at the southern terminus of the aftershock zone, were analyzed by modeling teleseismic and strong ground motion data. Teleseismic body wave inversion gives source orientation of Al: strike 165°±2.5°, dip 90°±1.5°, slip 178°±0.5°, centroid depth shallower than 12 km (least-misfit centroid depth 7 km from broadband waveform inversion), and seismic moment 1.5-1.6 X 10²⁵ dyn cm. The inversion of A2 gives the source orientation and centroid depth very similar to those of Al. The seismic moment for this event is 1.3-1.6 X 10²⁴ dyn cm. Modeling of strong ground motion seismograms adds more constraints on centroid depths and source time functions of Al and A2. To minimize the effect of scattering caused by upper crustal heterogeneity, we confined our analysis to frequencies lower than 1 Hz. A crustal model, with a low velocity sedimentary layer, was found that predicts common features of observed strong ground motion seismograms for both events. Derived source orientation is consistent with that found from teleseismic body wave inversion. The centroid depths of Al and A2 were constrained to be between 4 and 12 km. A source duration of 7 s and 2 s was obtained for Al and A2, respectively. Derived rupture parameters of Ml and M2, aftershock distribution, field investigations, geological information and concepts of geometrical barriers and fault asperities, indicate that the preexisting fault intersections played the key role in rupture terminations and initiations. The 12 s source duration of Ml and about 60 km long zone of ground deformation along the strike suggest that Ml rupture was bilateral. The rupture initiated near a fault intersection and propagated to NNW and SSE along the strike. The SSE propagating rupture was terminated by a preexisting fault which intersects the ruptured fault 30 km to the south. The aftershock Al and A2 as well as a dense group of small aftershocks were associated with the termination of the SSE segment. The NNW propagating rupture was also terminated by a NE striking preexisting fault on which several of the largest aftershocks appear to have occurred. This NE striking fault right-laterally offsets the fault on which Ml and M2 occurred forming a geometrical barrier for the rupture. M2 presumably nucleated near this barrier and unilaterally ruptured about 25 km toward NNW where it was terminated by a well documented preexisting fault.
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