Plate Kinematics, Tectonics, and Earthquakes
Presiding: C DeMets, University of Wisconsin-Madison; N Gourmelen, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami
G24A-01 15:30h
Toward a Minimum Change Model for Recent Plate Motions: Calibrating Seafloor Spreading Rates for Outward Displacement
We use seafloor spreading distances derived from dense magnetic surveys of young magnetic anomalies flanking seven seafloor spreading centers and the velocities of 398 continuous GPS sites on the plates bordering these spreading centers to study outward displacement, a phenomenon in which seafloor spreading magnetic lineations are displaced outward from their idealized locations as a consequence of extrusive and intrusive emplacement of new magma across a several-km-wide zone centered on the spreading axis and outward sloping reversal boundaries. Linear regressions of age-opening distance series derived from crossings of magnetic reversals 1n-3An.2 (0.78 Ma-6.72 Ma) yield positive Y-intercepts for 42 out of 53 seafloor spreading segments, corresponding to displacement of reversals outward from the seafloor spreading axis. The improvement in the least-squares fit of a model that allows for outward displacement relative to a model in which outward displacement is assumed to be zero is significant at a very high confidence level. Separate inversions of 13 age-distance series derived from magnetic anomaly crossings grouped by plate boundary yields 12 estimates of outward displacement that range from 0.5-3 km and unusually wide outward displacement of 6.1+-0.4 km along the Reykjanes Ridge. Detailed analysis of numerous crossings of Anomaly 1n from the Southeast Indian ridge suggests there is a correlation between axial morphology and the magnitude of outward displacement; however, too few data are available from axial rise segments along other seafloor spreading centers to confirm whether this correlation is characteristic of other seafloor spreading centers. Our results corroborate previous estimates of magnetic polarity transition zone widths derived from near-bottom magnetic measurements, which range from 1-8 km and average 2 km. Statistical tests of our age-distance series indicate that all but two are consistent within errors with a globally averaged value for outward displacement of 1.9+-0.2 km. Seafloor spreading rates corrected for outward displacement agree better with instantaneous rates estimated from GPS-derived plate angular velocity vectors than do uncorrected long-term rates, underscoring the need to correct seafloor spreading rates for outward displacement before attempting to interpret differences between geodetic and geologic estimates of plate motions.
G24A-02 15:45h
Total Motion Across the East African Rift Viewed From the Southwest Indian Ridge
The Nubian plate is known to have been separating from the Somalian plate along the East African Rift since Oligocene time. Recent works have shown that the spreading rates and spreading directions since 11 Ma along the Southwest Indian Ridge (SWIR) record Nubia-Antarctica motion west of the Andrew Bain Fracture Zone complex (ABFZ; between 25E and 35E) and Somalia-Antarctica motion east of it. Nubia-Somalia motion can be determined by differencing Nubia-Antarctica and Somalia-Antarctica motion. To estimate the total motion across the East African Rift, we estimated and differenced Nubia-Antarctica motion and Somalia-Antarctica motion for times that preceded the initiation of Nubia-Somalia motion. We analyze anomalies 24n.3o (53 Ma), 21o (48 Ma), 18o (40 Ma) and 13o (34 Ma). Preliminary results show that the poles of the finite rotations that describe the Nubia-Somalia motions cluster near 30E, 42S. Angles of rotation range from 2.7 to 4.0 degrees. The uncertainty regions are large. The lower estimate predicts a total extension of 245 km at the latitude of the Ethiopian rift (41E, 9N) in a direction N104, perpendicular to the mean trend of the rift. Assuming an age of 34 Ma for the initiation of rifting, the average rate of motion would be 7 mm/a, near the 9 mm/a deduced from present-day geodetic measurements [e.g. synthesis of Fernandes et al., 2004]. Although these results require further analysis, particularly on the causes of the large uncertainties, they represent the first independent estimate of the total extension across the rift. Among other remaining questions are the following: How significant are the differences between these estimates and those for younger chrons (5 or 6 ; respectively 11 and 20 Ma), i.e. is the start of extension datable? Is the region east of the ABFZ part of the Somalian plate or does it form a distinct component plate of Somalia, as postulated by Hartnady (2004)? How has motion between two or more component plates within the African composite plate affected estimates of India-Eurasia motion and of Pacific-North America motion?
G24A-03 16:00h
Co-Seismic and Post-Seismic Crustal Deformation of Sumitra-Andaman Islands Earthquake from GPS Measurements
In view of the devastating Sumitra-Andaman Islands earthquake and tsunami on December, 26th 2004, understating the Indian lithosphere beneath the Andaman Islands has gained great importance. This region is a transition zone between India & Burmese plates possessing high strain rate distribution. To estimate the co-seismic and post-seismic deformation, GPS data from IGS sites IISC, HYDE, LHAS, KUNM, PIMO, NTUS, DGAR, BAKO, COCO, MALE and a network of nine GPS sites in Andaman and Nicobar Islands have been analyzed using GAMIT/GLOBK software. Time series of the baselines and position coordinates indicate significant crustal deformation. The GPS results are discussed in conjunction with the litho-tectonics of the Andaman region.
G24A-04 INVITED 16:15h
Use of Geological Data to Investigate the Rheology, Geometry, and Temporal Evolution of Faulting in the New Madrid Seismic Zone
In this study we use paleoseismic observations to investigate possible earthquake generation models in the New Madrid Seismic Zone (NMSZ). Observations from the NMSZ include mean recurrence times of ~500 years, finite fault length, and ~100 m of total offset. Faulting is inferred to have begun in the Holocene. To be successful, understanding of the proposed models at a number of time-scales and rheological conditions is extremely important. If the NMSZ is modeled as a relaxing weak zone at depth, the time-scale of that relaxation process must match observations. The duration of the modeled relaxation process is highly dependent on the rheology of the weak zone. We have investigated a number of rheologies including Maxwell viscoelastic, standard linear solid, and power-law rheologies. Acceptable rheologies must satisfy the duration of the total relaxation process, as stated above. Postseimic effects, which act at a shorter time-scale are also observed in the model. During the earthquake, stresses are transferred back the underlying weak zone. This effectively recycles strain and prolongs relaxation of the proposed weak zone. Though relatively short-lived, postseismic effects clearly play an important part in the overall behavior of the system. There are also geometric effects that influence the relaxation time of the proposed weak zone. If the weak zone is wide, then stress is transferred upward to areas far from the actual faults. This prolongs the relaxation process as these stresses must work their way to the actual fault as relaxation continues. For narrow weak zones this effect is less important. Finally, there are time-scales that are much longer than the time-scales inferred from observations in the NMSZ. These are the processes that act to initially localize stress within the NMSZ. We investigate whether or not these stresses are localized or generated in the far-field. Further, we investigate the possible geometrical evolution of faulting using plastic models of strain accumulation through time. These models incorporate the source(s) of stress which drive seismicity in the NMSZ.
G24A-05 16:30h
Comparing geological and geodetic deformation on megathrust
The `seismic cycle' concept assumes that earthquakes rupture fault portions that remain locked over the period between recurring earthquakes. Each event would release some the stresses accumulated in the interseismic period. This kinematic description offers a convenient and quite successful framework to integrate geodetic and geological observations. In this presentation we will overview geodetic, geological and geophysical observations made in the context of intracontinental thrust faults in the Himalaya and Taiwan and discuss their implications regarding faults rheology and the mechanics of the seismic cycle. We will in particular discuss the role of temperature, fluids, and stress transfer in governing strain and seismicity over the seismic cycle. We will show in particular that background seismicity is relatively well explained from stress build up in the interseismic period and that reloading of the upper brittle crust, due to postseismic afterslip and viscous relaxation is a viable mechanism to explain jointly geodetic data and the decay rate of aftershocks following large thrust event, such as for example the Chichi earthquake. The model is also used to assess eventual non-stationary of strain in the interseismic period. Depending on the viscosity and thickness of the viscous shear zone at depth, stress transfers during the seismic cycle may indeed lead to significant variations of interseismic strain that could be detected from comparing geodetic rates and geological rates.
G24A-06 16:45h
Constraining the Kinematics of the A.D. 900 Seattle Fault Earthquake With Geomechanical Modeling and LIDAR Surface Elevation Data
Within the Puget Sound region of Washington State, several lines of geological evidence suggest that the largest upper-crustal earthquake within the past 2500 years occurred on the Seattle fault system in A.D. 900. Constraining the rupture characteristics of this event is of singular importance in evaluating the upper-bound seismic hazard and tsunami threat posed by upper-crustal (non-subduction) earthquakes to the Puget Lowland region. It is only possible to model the fault geometry, slip distribution, and moment magnitude of this earthquake with a data set of the surface elevation changes caused by this event. Due to the historic age of this earthquake, we use elevations of an uplifted marine terrace, digitally extracted from LIDAR images, as a novel source of coseismic surface deformation data for this event. Ideal for this forested region, LIDAR images, acquired via airborne laser swath mapping (ALSM), offer a drastic improvement over earlier topographic mapping techniques due to its improved resolution and its ability to measure the ground surface beneath dense vegetative cover. The LIDAR images reveal a single uplifted terrace, dated to 1000 cal yr B.P. near Restoration Point, that is morphologically continuous along the southern shoreline of Bainbridge Island and is visible at comparable elevations within a 25 km by 12 km region encompassing coastlines of West Seattle, Bremerton, East Bremerton, Port Orchard, and Waterman Point. Considering sea level changes since A.D. 900, the maximum uplift magnitudes of shoreline inner edges approach nine meters and are located at the southernmost coastline of Bainbridge Island and the northern tip of Waterman Point, while tilt magnitudes are modest - approaching 0.1 degrees. Although the terrace is locally offset and tilted near the Toe Jam Hill and Waterman north-dipping, reverse fault scarps, the regional uplift pattern is a doubly-plunging antiform with steepened north limb, consistent with its location directly above the hanging wall of the frontal thrust of the south-dipping Seattle fault zone. Preliminary forward modeling results show that the earthquake magnitude is greater than M7. The greatest slip of the A.D. 900 event was most likely confined to the frontal thrust of the Seattle fault system and was centered beneath Puget Sound betweem Restoration Point and Alki Point and localized in the upper ten kilometers of the crust. Slip along the Toe Jam Hill and Waterman backthrusts merely modify this larger uplift pattern and would not have been sufficient to generate the observed regional distribution of terrace uplift.