Investigating Large Earthquake Rupture Kinematics from the Joint Analysis of Seismological, Geodetic and Remote Sensing Data

Citation

Konca, Ali Ozgun (2008) Investigating Large Earthquake Rupture Kinematics from the Joint Analysis of Seismological, Geodetic and Remote Sensing Data. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6B13-4M30. https://resolver.caltech.edu/CaltechETD:etd-05292008-113958

Abstract

This thesis presents detailed studies of 4 large earthquakes. The 2006 Mw 8.6 Nias-Simeulue earthquake and 2007 Sumatra Mw 8.4 and 7.9 earthquake sequence which occurred on the Mentawai Island area of Sunda megathrust are studied using teleseismic, long period, GPS, and field data. Two crustal earthquakes, the 2005 Mw 7.6 Kashmir and the 1999 Mw 7.1 Duzce earthquakes, are studied using satellite image cross-correlation, seismic, GPS and SAR data.

The 2005, Mw 8.6, Nias-Simeulue earthquake was caused by rupture of a portion of the Sunda megathrust offshore northern Sumatra. Based on the excitation of the normal modes and geodetic data, we put relatively tight constrains on the seismic moment and the fault dip, where the dip is determined to be 8o to10o with corresponding moments of 1.24 x 1022 to 1.00 x 1022 Nm, respectively. The geodetic constraints on slip distribution help to eliminate the trade-off between rupture velocity and slip kinematics. Our results indicate a relatively slow average rupture velocity of 1.5 to 2.5 km/s and long average rise time of up to 20 s.

Our study of the 2007 Mentawai Islands earthquakes shows the influence of permanent barriers on the extent of large megathrust ruptures, which can be a cause of some regularity of the seismic behavior, but also that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability arises mostly from the influence of nonpermanent barriers, probably zones with locally lower prestress due to the past earthquakes. The state of stress on that portion of the Sumatra megathrust was not adequate for the development of a single major large rupture at the time of this seismic crisis. However, the slip deficit that has accumulated since the 1833 and 1797 events remains large, and so is the potential for a large megahrust event in the Mentawai area.

We analyzed the rupture process of 1999 Mw 7.1 Duzce earthquake using geodetic and seismic data. Applying subpixel cross-correlation of SPOT images acquired before and after the event, we mapped a continuous fault trace over 55 km; 15 km longer than the field reports. We investigated the spatiotemporal evolution of the earthquake using four-segment fault geometry with constraints on surface offsets based on satellite imagery, incorporated GPS and InSAR data and four strong-motion stations in the vicinity of the rupture. Our joint modeling shows a very stable slip distribution that does not depend on constraints imposed on rupture velocity. We show that no constant rupture velocity can explain the strong-motion data. Due to constraints from fault geometry and geodetic data, the rupture velocity has to vary rapidly. The rupture starts slow, accelerates to supershear speeds toward east and subsequently slows down. Supershear rupture is local and only toward the east of the hypocenter. Teleseismic data are consistent with the joint near-field model when 2 s time shifts are applied to their hand-picked arrivals. This implies that the weak beginning of the earthquake is not observable at teleseismic distances. This appears to be a common problem with teleseismic modeling and leads to more compact models with major slip around the hypocenter than the actual phenomenon. We performed teleseismic inversion models comparing four-segment fault geometry based on satellite imagery to one-segment geometry based on CMT solution. The four-segment model gives better predictions of near-field ground motions.

We analyzed the Mw 7.6 Kashmir earthquake of October 8, 2005, using sub-pixel correlation of ASTER images to measure ground deformation, and modeling SAR imagery data along with seismic waveforms. The surface rupture is continuous over a distance of 75 km. The rupture lasted about 25 s and propagated up-dip and bilaterally by ~2 km/s, with a rise time of 2-5 s. The shallowness and compactness of the rupture, both in time and space, provide an explanation for the intensity of destructions. By comparing the teleseismic models with SAR data, we infer that satellite image correlation puts constraints on teleseismic models, which lead to more coherent models with the geodetic data. This kind of satellite image analysis could be achieved as soon as a postearthquake image is available, and would provide key information for early assessment of damages.

Thesis Files