G14A-01 INVITED 16:00h
`Geologic time series' of earth surface deformation
The debate of whether the earth has evolved gradually or by catastrophic change has dominated the geological sciences for many centuries. On a human timescale, the earth appears to be changing slowly except for a few sudden events (singularities) such as earthquakes, floods, or landslides. While these singularities dramatically affect the loss of life or the destruction of habitat locally, they have little effect on the global population growth rate or evolution of the earth's surface. It is also unclear to what degree such events leave their traces in the geologic record. Yet, the earth's surface is changing! For example, rocks that equilibrated at depths of > 30 km below the surface are exposed at high elevations in mountains belts indicating vertical motion (uplift) of tens of kilometers; and rocks that acquired a signature of the earth's magnetic field are found up to hundreds of kilometers from their origin indicating significant horizontal transport along great faults. Whether such long-term motion occurs at the rate indicated by the recurrence interval of singular events, or whether singularities also operate at a higher-order scale ("mega-singularities") are open questions. Attempts to address these questions require time series significantly longer than several recurrence intervals of singularities. For example, for surface rupturing earthquakes (Magnitude > 7) with recurrence intervals ranging from tens to tens of thousands of years, observation periods on the order of thousands of years to a million years would be needed. However, few if any of the presently available measurement methods provide both the necessary resolution and "recording duration." While paleoseismic methods have the appropriate spatial and temporal resolution, data collection along most faults has been limited to the last one or two earthquakes. Geologic and geomorphic measurements may record long-term changes in fault slip, but only provide rates averaged over many recurrence intervals. Space-geodetic measurements provide high-resolution time series of contemporary surface deformation, but most systems have been recording for less than ten years (GPS data), and the spatial coverage is only beginning to improve (e.g., InSAR data). Assuming that we are not willing to wait hundreds of years of global geodetic recording, what would be our most realistic options for collecting appropriate data in the near future?
G14A-02 16:15h
A 3-D Semi-analytic Viscoelastic Model of the San Andreas Fault System: A 1000-year Perspective of the Earthquake Cycle
Combining historical earthquake data, coastal tide gauge measurements, and continuous vertical and horizontal geodetic velocities, we simulate one thousand years of the earthquake cycle for the entire San Andreas Fault System. We employ a model based on a new semi-analytic solution that provides the displacement and stress caused by time-dependent dislocations embedded in an elastic layer overlying a Maxwell viscoelastic half-space. The problem is solved analytically in both the vertical dimension and the time dimension, while the solution in the two horizontal dimensions is developed in the Fourier transform domain to exploit the computational advantages offered by the convolution theorem. Hundreds of dislocations imbedded in a 2048 x 2048 km sized grid are used to represent the San Andreas Fault System from the Gulf of California to the Mendocino Triple Junction. Major historical earthquakes (from 1812-present, M$_{w}$ $>$ 6.0) are used in conjunction with published recurrence intervals to produce the time-dependent velocity and stress tensor spanning the past 1000 years. The model simulates interseismic stress accumulation on the upper locked portion of subfaults and adjacent crust, repeated earthquakes on prescribed fault segments, and the viscoelastic response of the asthenosphere following major ruptures. Continuous geodetic observations ($\sigma$ $<$ 1.5 mm/yr) from the Scripps Orbit and Permanent Array Center (SOPAC) and USGS networks are used to constrain model parameters of elastic plate thickness ({\it H}), half-space viscosity (${\eta$), Poisson's ratio (${\nu$), and apparent locking depth. We identify best fitting models with rms $<$ 2.5 mm/yr for {\it H} $>$ 60 km, ${\eta$ = 1-5 x10$^{19}$ Pa s, ${\nu$ = 0.35-0.45, and locking depths that are approximately 1/4 less deep than those required to fit an equivalent elastic half-space model. Using these model parameters, we calculate present-day Coulomb stress and observe large amounts of stress focused along the Carrizo and Mojave regions of the San Andreas, as expected from the absence of major earthquakes along these fault segments over the past 150 years. These results, along with corresponding time-dependent deformation results, have been assembled to form animations of the San Andreas Fault System that capture temporal variations in the plate-boundary velocity vector and stress tensor spanning the past 1000 years of the earthquake cycle.
http://topex.ucsd.edu/body_force
G14A-03 16:30h
Comparing Geodetic and Geologic Data from for the Creeping Segment of the San Andreas Fault, California
We present the results of geodetic and geologic measurements from the central creeping segment of the San Andreas fault. The 175-km creeping segment of the San Andreas fault, stretching from San Juan Bautista to Cholame, is characterized by aseismic slip and shallow microseismicity. The central, 55-km long segment has high, uniform creep rates. New differential GPS measurements, based on 30-year reoccupations of alignment arrays, yield a maximum right-lateral slip rate of 25 $\pm$ 1 mm/yr for the San Andreas fault. This slip rate is significantly slower than both an earlier slip rate estimate of 30 mm/yr and recent geodetic estimates of 39 $\pm$ 2 mm/yr of motion predicted between the Sierra Nevada - Great Valley block and the Pacific plate. New continuous GPS measurements between pairs of sites that flank the creeping segment at respective inter-site distances of 1 km and 70 km give relative fault-parallel slip rates of 28 $\pm$ 2 and 30 $\pm$ 2 mm/yr respectively for 17 months of observation. These observations indicate that right-lateral deformation rates increase with distance from the fault. Possible explanations for the cross-fault gradient observed in the geodetic data, and the 14 mm/yr deficit observed between the slip rate on the San Andreas fault and the predicted plate motion rate, are elastic strain accumulation along the creeping segment or significant distributed deformation on off-fault structures. Preliminary evidence from the Monterey Formation adjacent to the central creeping segment supports the latter model. Folds located away from the fault show evidence for hinge-parallel extension, consistent with fold rotation during progressive deformation in the borderlands. This type of deformation cannot be explained by a simple elastic strain accumulation model for the San Andreas fault or solely by strain/stress partitioning. Rather, the hinge-parallel extension of the folds indicates that some of the slip deficit is accommodated by permanent wrench deformation in the borderlands.
G14A-04 16:45h
Crustal Deformation Along the Northern San Andreas Fault System From Geodetic and Geologic Data
The San Andreas fault system north of the San Francisco Bay area is composed of three sub-parallel right-lateral faults: the San Andreas, Rodgers Creek-Ma'acama, and Green Valley-Bartlett Springs. The San Andreas has been essentially aseismic since it last ruptured in 1906, and no major historical earthquakes have occurred on the more seismically active Ma'acama and Bartlett Springs faults, although the slip deficit on the Ma'acama fault may now be large enough to generate a magnitude 7 earthquake. Since 2002, we have been collecting GPS measurements at about 80 monuments that form roughly 10-station profiles across the northern San Andreas fault system from Pt. Reyes to Cape Mendocino. Most of the monuments were last observed in 1993 or 1995, so the new observations significantly improve estimates of their relative motion and models of average interseismic strain accumulation, including possible spatial variations along the fault system. We use angular velocity-backslip block modeling to determine a self-consistent northern California deformation field and rates of strain accumulation along the northern San Andreas fault system. Preliminary results from our modeling, which includes 2 blocks within the San Andreas fault system, as well as a Sierran-Great Valley block, and the Pacific and North America plates, show agreement between observed and predicted velocities at less than 2 mm/yr. Fault-parallel deformation across the entire San Andreas fault system is 38 mm/yr, but deep slip rates on the sub-parallel faults are poorly constrained due to significant correlations between the deep slip rates and locking depths, which we fully characterize using Monte Carlo techniques. We use Bayesian techniques to combine the GPS observations with constraints derived from other seismic, geodetic, and paleoseismic observations, such as locking depths, surface creep rates, and inferred geologic slip rates. These additional constraints significantly improve the estimates of the slip rates and locking depths on the faults, allowing better assessment of seismic hazards along the northern San Andreas fault system.
G14A-05 INVITED 17:00h
Long-Range and Long-Term Fault Interactions in Southern California
Paleoseismological data reveal four clusters of large earthquakes in the Los Angeles region during the past 12,000 years. The historic period is part of an ongoing, >=1,000-year-long lull between clusters. These Los Angeles-region clusters have occurred during the lulls between similar clusters observed on the eastern California shear zone (ECSZ) in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the San Andreas system suppress activity on faults in the ECSZ, and vice versa, can explain the switching of activity between the two fault networks. Interestingly, available geologic and geodetic rate data suggest that the interseismic loading rates of faults with relatively slow recent seismic slip rates (e. g., SAF Big Bend, Garlock, and LA-region faults) are slower than the long-term slip rates of those faults. Conversely, the geodetically determined loading rate of the ECSZ in the Mojave, currently in the midst of a seismic cluster, appears to be faster than the long-term rate suggested by available paleoseismological data. These observations suggest that when the upper crust is seismically most active the lower crust beneath the major faults is also deforming more rapidly, whereas in regions where the upper crust is less seismically active the loading rate of these faults is also relatively slow. Thus, either rapid ductile slip in the lower crust (and upper mantle?) beneath faults loads the upper crust, resulting in a seismic cluster, or rapid seismic displacement along upper crustal faults increase the ductile loading rate of the downward extensions of the faults. At present, we cannot distinguish the relative importance of these competing hypotheses. Similar behavior at other plate boundaries suggests that alternating clusters of large earthquakes may be the expected mode of seismicity when two fault systems accommodate the same plate boundary motion and slip on one system suppresses slip on the other.
G14A-06 INVITED 17:15h
From Isolated Sites to Complete Ruptures; the Next Hurdle for Paleoseismology on the Southern San Andreas Fault
At least fifteen sites on the southern San Andreas fault provide some information on the timing or displacement of recent earthquakes. While the data are often sufficient to determine mean recurrence intervals, the time interval since the last earthquake, and crude estimates of magnitude, the wide range of ages, recurrence intervals and displacements seen from site to site currently preclude any simple correlation of site specific information into compelling ruptures. During the past 1300-1400 years at least twelve ruptures, averaging 200-300 km in length, are required to explain the data. These twelve events could be similar-length, quasi-periodic earthquakes rupturing the northern 2/3 or the southern half of the fault south of Parkfield with substantial overlap. In this scenario 1857 was a typical northern rupture and the 1812 event was anomalously short. Another 12-rupture scenario includes five complete ruptures of the 530 km Southern San Andreas, six approximately 100 km infilling events in the Mojave to San Bernardino region, like 1812, and the 1857 event that was an anomalously short rupture that should have completed the fault. As more ruptures are added to explain the data, an increasingly random distribution of recurrence intervals and lengths with highly variable overlap becomes possible. The existing age data favor more, smaller ruptures because of the imperfect overlap of ages from multiple sites. The displacement data generally favor larger ruptures spanning sites despite the relatively poor overlap of ages, perhaps suggesting problems with existing age control. Advances in determining the probability of rupture length given displacement, the creative use of sites that provide only event count or total displacement in an interval of time spanning multiple earthquakes, better C-14 dating accounting for the multiple sources of carbon on paleoseismic samples, and investment of time and effort into a few keystone sites with robust results promise to greatly clarify this picture in the next 3-5 years.
G14A-07 17:30h
Slip Distribution and Temporal Slip Rate Variability on the San Andreas and San Jacinto Fault Zones, Southern California
The San Andreas plate boundary fault system through Southern California ($34\deg$-$37\deg$ N) is composed of a complex set of strike-slip faults, with the San Andreas (SAF) and San Jacinto (SJF) Faults Zones playing a major role in partitioning deformation between the Pacific and North American plates. The total present-day (REVEL-1) Pacific-North American plate motion rate is $\sim$50 mm/yr in Southern California, while the GPS velocity differences observed across the major faults in this region range from 35 to 40 mm/yr, accommodating only $\sim$70% of the total plate motion. GPS data in the northern section of the San Andreas show a relatively abrupt change in velocity across the SAF, implying rigid block behavior (or at least a relatively narrow elastic strain accumulation region). Moving southward, the zone of elastic strain accumulation becomes broader, until velocities are generally increasing fairly smoothly across the faults in the southernmost section. Thus, the amount of strain being accommodated on the SAF and SJF appears to be increasing to the south. Possible explanations for this increase include a broadening of the plate boundary deformation zone, changes in locking depth of the faults, or long-term elastic strain accumulation related to fault growth. Along fault strike (N-S) in the southernmost region, the geodetic velocity differences across the SAF/SJF plate boundary increase from $\sim$10 to $\sim$30 mm/yr, relative to a stable Pacific plate. These are significant variations that, if maintained over longer time periods, would require the accommodation of a substantial amount of strain. A comparison of geodetic velocities with published geologic slip rates determined over longer time scales reveals discrepancies in geologically determined fault slip rates (based on offset geologic features), from a few mm/yr to greater than 30 mm/yr over the past million years, even for the most rapidly moving plate-boundary faults. Along the northern part of the southern SAF, for example, geologic slip rates over the last 5 to 30 ka range from 20 to 35 mm/yr, while the geodetic rates are much slower at 4 to 7 mm/yr.