T23E-01
Transfer Zones in Listric Normal Fault Systems: Insights from Clay Experimental Models
Displacements on adjacent listric normal faults are accommodated within a transfer zone by the development of complex secondary fault systems. These are common features in passive margin settings such as the Gulf of Mexico and Niger Delta. Transfer zones can be classified as two common types (1) Convergent: where two main faults dip towards each other, and (2) Divergent: where faults dip away from each other. Clay experiments have been used to gather insights on the geometry and evolution of secondary faults in these two end member cases. Within each type, three experimental setups have been constructed where the two main faults (a) approach one another, (b) are offset by 90 degrees and (c) overlap each other. The different transfer zones have been created by designing indentations along the frontal edge of both the fixed and moveable base plates. The during extension, the structure develops as a symmetric graben right above the contact of the overlying and underlying base plates and with progressive extension evolves into an asymmetric half-graben. The master faults dip towards the direction of extension and are tied to the overlying fixed footwall. They form by coalescence of a number of synthetic faults. The antithetic faults, on the other hand, constitute a fault zone and are made up of evenly spaced, discrete fault segments that are tied to the underlying moveable base plate. Fault orientations, fault lengths, fault densities and sizes of connected fault clusters varied with type of transfer zone, structural position relative to the offset and total extension. The experimental results show that the antithetic faults in convergent transfer zones and the synthetic faults in divergent transfer zones tend to be consistent in orientation and get connected easily, although higher connectivity is observed in the convergent transfer zones. The synthetic faults in convergent transfer zones and antithetic faults in divergent transfer zones change orientation along strike as they approach the transfer zone, but tend to remain separated from each other. These clay experimental models provide important insights on the kinematic growth of faults, geometry of fault patterns and possible fluid migration pathways within transfer zones.
T23E-02
Multidisciplinary seismotectonics of a low-angle normal fault system (Lunigiana- Garfagnana grabens, Italy)
The Plio-Quaternary Lunigiana and Garfagnana basins (Northern Apennines) are NW-SE grabens originated
in the hanging wall of a ENE-dipping low-angle normal fault system (northern termination of the Etrurian
Fault System of Boncio et al. 2000, Tectonics 19, 1038-1055). The slow stretching rate across the grabens
(less than 2 mm/a), the low-conservative lithology of the outcropping rocks (mostly siliciclastic deposits) and
the rarity of recent syntectonic deposits along the fault traces makes it difficult to recognize and quantify the
late Quaternary activity of the faults. We used a multidisciplinary approach, involving structural survey,
morphotectonic analysis, subsurface data (reflection seismics) and instrumental seismicity.
The grabens are bounded by E- and W-dipping normal fault systems that are synthetic and antithetic splays
of the detachment, respectively. The two systems root on the detachment at increasing depths from W to E
(from 1 to 2.5 s TWT). The outcrop expression of the two systems is similar, but the E-dipping faults show, on
average, lower dip-angles (30° T23E-03 ANALYSIS OF INTERFEROMETRY DATA (PERMANENT SCATTERERS: PS-INSAR) TO IDENTIFY ACTIVE TECTONIC STRUCTURES IN WESTERN ALPS (NW ITALY)
The SAR interferometer technique allows detecting small movements of the ground surface. One of the most
effective is the Permanent Scatters (PS-InSAR) technique proposed by Ferretti et al., 2001. The PS are
points on the ground (buildings, rocks, etc.) that show high long-term phase coherence, as well as strong
and stable backscatter level through time. Since PS do not change their signature with time, they can be
used to estimate the sub-vertical motion of the ground, within the range of millimetre motions/year (fraction of
the 5.6 cm-wavelength). PS velocity values are relative to a chosen reference point that is assumed to be
stable.
Thirty-eight ERS1/2 SAR descending scenes, taken from 1992 to 2000, allow detecting over 2 million PS with
a variable distribution density. This large data set required geostatistical and spatial cluster analysis (Hot
Spot Analysis) that allowed generating maps of iso-kinematic domains to be compared with the present
knowledge of Western Alps tectonic mobility.
The boundaries of the detected iso-kinematic domains can be very narrow (less than one km) or large (tens
of kms), while the area size ranges from tens to some square kms.
The geometry of the boundaries can be very straight or curvilinear. Different geological meanings can be
inferred for these boundaries, depending on their geometry and velocity gradient observed across them. The
boundaries could coincide with known, buried, concealed or inferred active faults, or could represent hinge
areas between different uplifting or subsiding areas. Furthermore, iso-kinematic domains can be bounded by
significant velocity variations induced by landslides or deep-seated gravitational deformations alignment. In
some cases, the boundaries could also be simply determined by the effects of local anomalies in the PS
statistical distribution.
These different boundary-types were recognized in several sectors of Western Alps and adjoining alluvial
basins (Gran Paradiso, Argentera, Torino Hill and Monferrato tectonic units) where normal, thrust and strike-
slip faulting are presently active with moderate/low seismicity, and as complex local responses to the ongoing
Europe-Adria convergence. The PS data revealed differential uplift or subsidence of large areas, probably
controlled by km-scale gentle flexures, precisely located individual faults, not precisely located fault systems,
gravitational instability or local human effects.
References: Ferretti A., Prati C., Rocca F. (2001) Permanent scatterers in SAR interferometry. Geoscience
and Remote Sensing, IEEE Trans., 39, 1: 8-20
T23E-04 Accelerating Uplift Rate and Non-uniform Inheritance: Cosmogenic Be10 Depth Profiles from the Montecito Anticline, Mendoza, Argentina
The Andean orogenic front between 31° S and 33° S marks the transition between west-vergent thick-
skinned faulting in the Sierras Pampeanas, and east vergent thin-skinned faulting in the Precordillera. This
area has experienced several devastating earthquakes in the last century, and geodetic studies indicate that
this area has a long-term shortening rate of ~5mm/yr. One of the growing anticlines in this region that
partially accommodates this shortening is the Montecito anticline, a fault propagation fold above a blind thrust
fault. Uplifted and deformed fluvial terraces formed along the axis of this anticline were dated using Be10
cosmogenic depth profiles. In 3 of the 5 dated terraces the horizon sampled directly above the strath had a
higher nuclide concentration than the other samples in the profile, which suggests that the assumption of
constant inheritance with depth is violated. One possible explanation for the cosmogenic profile irregularity is
that catastrophic flooding events mobilize sediment from storage locations which are not normally tapped,
such as hillslopes or alluvial fan surfaces. This sediment, which has an abnormally high inheritance value due
to its increased time in storage, is deposited on the strath of the newly formed terrace, thus leading to higher
Be10 values at the base of the profile. After discarding the abnormally high bottom value in each profile the
terrace ages are found to be ~6.5 ka, ~4.1 ka, ~1.9 ka. Given their height above the river the
uplift rate is 0.63-0.68 mm/yr for the period between 6.5 ka and 1.9 ka. However, surprising preliminary data
indicates that the uplift rate of the Montecito Anticline has increased 4-fold since 1.9 ka. This could reflect
stochastic variations in slip rate or interactions with other regional faults, and suggests a greater seismic
threat in this earthquake prone region.
T23E-05 Geomorphic Evidence for Multiple Large Post-glacial Earthquakes on the Western Seattle Fault
An apparently-warped late-glacial outwash surface west of Bremerton suggests at least 23 meters of post-16
ka differential uplift across the western end of the Seattle fault. If the 7-9 meter vertical offset during the 900
AD earthquake farther east is typical of large Seattle fault events, deformation of such magnitude indicates 3
large earthquakes on this segment of the Seattle fault in the last 16,000 years.
Geomorphic mapping from lidar topography (6 ft DEM from 1 pulse/m2 data aquired in leaf-off
conditions, 2000 and 2001, survey contracted by the Puget Sound Lidar Consortium, data and DEM
available at http://pugetsoundlidar.ess.washington.edu) outlines extensive relict alluvial flats. These flats were
formed by meltwater that flowed south from the decaying Puget Lobe of the Cordilleran Ice Sheet during its
last retreat about 16,000 years ago. In the Wildcat Lake 7.5-minute quadrangle, west of Bremerton, one of
these flats extends south from upper Big Beef Creek, past William Symington Lake, and into the headwaters
of the Tahuya River. This flat slopes southwards except for the part that extends from Symington Lake to the
Big Beef-Tahuya divide, which slopes gently north. Projection of alluvial-flat elevations onto a north-south
cross-section and correction for 0.1% up-to-the north tilting by post-glacial isostatic rebound closely defines
a smooth surface with a 23 meter elevation difference between the low at Symington Lake and the high at the
Big Beef-Tahuya divide 5 km to the SW. Inclusion of the (unknown) paleo-gradient of the outwash stream
would increase the amount of offset. No surface scarps are evident in this region, which suggests that young
surface deformation records folding above a buried fault.
The primary uncertainty in this analysis is the inference that the Big Beef-Symington-Tahuya flat was formed
by a continuous south-flowing outwash stream. If the flat included a paleo-divide that separated N- from S-
flowing streams, the observed elevation differences could be primary. Presence of a wetland and pond near
the possible paleo-divide, lack of evident up-stream steepening near the possible paleo-divide, and our
(limited) understanding of the history of Puget Lobe retreat all argue for a continuous south-flowing outwash
stream.
T23E-06 Structural and Paleoseismologic Characterization of Shallow, Subsurface Folding Above Segmented Blind-Thrust Systems Beneath Los Angeles, California
Over the past nine years we have been developing methodologies to understand the structural evolution and
paleoearthquake history of blind thrust faults and their associated folds. To comprehend these issues our
studies focused on two major blind thrust systems beneath metropolitan Los Angeles; the Puente Hills thrust
fault (PHT) and the Compton fault. We acquired data from multiple study sites in order to analyze shallow
subsurface folding for individual thrust ramps, as well as segmented thrust systems. Our research
demonstrated that both the Puente Hills and Compton blind thrust faults have generated multiple, large-
magnitude (Mw >7) earthquakes during the past 14 ka.
In order to resolve how folds grow in response to slip on the underlying thrust ramps, we utilized a
multidisciplinary approach, combining the acquisition of high-resolution seismic reflection profiles and
borehole excavations across the overlying growth folds of major blind thrust faults. High-resolution seismic
reflection profiles were acquired across the updip projection of the active axial fold surfaces located from
petroleum industry seismic reflection profiles. These shallow seismic data allowed us to observe folding at
multiple depths and identify discrete buried fold scarps at study sites above individual thrust ramps of both
the PHT and Compton fault.
The results of this research provided insights into the detailed kinematics of earthquake-by-earthquake fold
growth above the underlying blind thrust ramps. At all our study sites, folds are well expressed as classic
fault-bend folds growing predominantly by kink-band migration, generally as predicted by folding theory. In
all examples, however, the borehole data show that the folded strata within the kink bands acquired their dips
incrementally, suggesting that fold kinematics involves components of both kink-band migration and limb
rotation. Based on discrete element models of this folding process, we suggest that these fold kinematics
result from the finite width of the axial surfaces and fold hinge, as well as the governing deformation
mechanism of loosely consolidated near-surface sediments. Through this analysis of the geometry and
evolution of young, shallow growth structures we deciphered the paleoearthquake history of the PHT and
Compton fault for seismic hazard analysis.
T23E-07 Incorporating and reporting uncertainties in fault slip rates
Quantitative slip rates are essential to understanding crustal deformation processes and assessing seismic
hazard. The two fundamental ingredients for computing slip rates--age and displacement of the fault of
interest--contain uncertainty, and therefore slip rates are inherently uncertain. In this presentation, we
emphasize a rigorous, probabilistic approach to computing and reporting fault slip rates. We have developed
a software implementation of this approach that accommodates arbitrary age and displacement uncertainty
models; we provide illustrations of our method using recent observations from the Death Valley-Fish Lake
Valley fault zone.
T23E-08 Fault Slip Rates in the Western U.S. From a Joint Fit to Geologic Offsets, GPS Velocities, and Stress Directions
I merge the SCEC, WGCEP, PBO, & WSM community datasets in neotectonic deformation models for the
western US. In California I use: (1) fault traces, dips, and slip senses from WGCEP Fault Models 2.1 or 2.2;
(2) fault offset rates and uncertainties obtained by Bird [2007, Geosphere, 3(6)] from offsets in the
USGS Paleosites Database; (3) a 2006 California joint GPS solution for interseismic benchmark velocities by
Shen, King, Wang, and Agnew; and (4) stress-direction indicators from World Stress Map. In other
western United States I use: (5) my collection of fault traces and offset rates as documented in Bird
[2007]; and (6) selected GPS velocities from PBO. All are fit by weighted least-squares in kinematic F-E
program NeoKinema. As described previously, this program (a) interpolates stress directions to determine
their uncertainties, (b) attempts to minimize off-fault strain-rates and align them with stress, and (c) iteratively
corrects geodetic velocities from short-term to long-term using local dislocation-in-halfspace corrections. All
datasets can be fit at a common level of 1.8 standard errors (RMS or N2 norm). If "acceptable" fit is defined
as N2 < 2 for all datasets, there is a range of acceptable models, defining a range of long-term fault slip
rates and (anelastic) continuum strain-rates. In preferred model GCN2008060, the mean long-term slip rates
for trains of the San Andreas fault are (SE to NW): Coachella 15 mm/a, San Gorgonio Pass-Garnet Hill 6,
San Bernardino South 12, San Bernardino North 19, Mojave South 16, Mojave North 17, Big Bend 15,
Carrizo 25, Cholame 26, Parkfield 31, Creeping 29, Santa Cruz Mt. 23, Peninsula 18, North Coast 16, and
Offshore 9 mm/a. Up to Cajon Pass, these all agree with 2007 WGCEP [2008], but my Mojave N and S
and Big Bend rates are much slower, my Carrizo and Cholame rates are marginally slower, and my North
Coast and Offshore rates are much slower. These differences are due to greater amounts of permanent
(anelastic) straining off the mapped fault traces in NeoKinema, relative to the elastic-microplate models of 2007 WGCEP [2008]. I have not been able to lower the RMS continuum strain rate in these models below
5×10-16 /s (=1.6%/Ma). Such distributed straining results from gaps and geometric
incompatibilities in the fault network and from geologic/geodetic discrepancies. This straining probably also
occurs on faults (which are not part of WGCEP Fault Models), and it probably also produces earthquakes.
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