Wayne Thatcher

Tectonic contraction across Los Angeles after removal of groundwater pumping effects.

After the 1987 Whittier Narrows and 1994 Northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan Los Angeles, a network of 250 continuously recording global positioning system (GPS) stations was deployed to monitor displacements associated with deep slip on both blind and surface faults. Here we augment this GPS data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike-slip faulting. After removing these non-tectonic signals, we are left with 4.4 mm yr-1 of uniaxial contraction across the Los Angeles basin, oriented N 36° E (perpendicular to the major strike-slip faults in the area). This indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast-trending strike–slip faults. We have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults. In the 40-km-long Santa Ana basin, groundwater withdrawal and re-injection produces 12 mm yr-1 of long-term subsidence, accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally.

Coseismic slip resolution along a plate boundary megathrust: The Nankai Trough, southwest Japan

Geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 Tonankai (Mw=8.1) and 1946 Nankaido (Mw=8.3) earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust. Data used include 798 angle changes from triangulation surveys, 328 leveling section differences, and 5 coseismic tidal gage offsets. Many of the nominally coseismic triangulation data span ∼50 years, nearly half the earthquake cycle, and correction for interseismic deformation using post-1950 observations is applied. Microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous, multisegment fault model. Because the onshore geodetic data have very limited resolving power for offshore fault segments, offshore coseismic slip was constrained by Satakes [1993] estimation based on tsunami data. The majority of the coseismic slip occurs between 15 and 25 km depth. Although resolution declines toward the trench axis, it is sufficiently good to define two distinct high-slip regions, one off southeastern Shikoku Island (11 m maximum) and the other offshore of Kii Peninsula (3 m maximum). The slip magnitude off southeastern Shikoku, coupled with the plate convergence rate, would imply an recurrence interval of about 270 years, much longer than the average repeat time of ∼120 years for historical great earthquakes on the Nankai Trough. However, the maximum coseismic slip is sensitive to the assumed fault geometry, and slippage on trough-parallel splay faults could significantly decrease the maximum slip to about 6 m.

Seismic Moments of the Larger Earthquakes of the Southern California Region

The seismic moment (M_0) of an earthquake is a more consistent and more physical measure of source strength than magnitude (M) or strain release (see pdf for formula), and this measure of source strength is determined for 47 of the larger earthquakes occurring in the Southern California region since 1857. Most of the seismic moments are obtained by conventional seismological means, but a relationship between M_0 and the areal distribution of Intensity VI (A_(VI)) is developed and scaled to estimate M_0 when intensity data are available but instrumental data are not. This relationship is log M_0 = 1.97 log A_(VI) − 2.55. For the region as a whole, earthquakes at the threshold of M_0 ≥ 10^(25), ≥10^(26), and ≥10^(27) dyne-cm have occurred once every 3, 8, and 25 yr, respectively. The spatial occurrence of the five largest earthquakes (M_0 ≥ 1 × 10^(27) dyne-cm) is not limited to a particular geologic province, mode of tectonic accommodation, or geographic locality. It is unlikely that this data set can reliably predict long-term spatial and temporal patterns of the M_0 ≥ 10^(25) dyne-cm seismicity of the Southern California region.

Resolution of fault slip along the 470‐km‐long rupture of the great 1906 San Francisco earthquake and its implications

Data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade-off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture. Because of varying network aperture and station density, slip resolution is very uneven. Although slip is determined within uncertainties of ±1.0 m along 60% of the fault, constraints are poor on the remaining, mostly offshore portions of the rupture. Slip decreases from maxima of 8.6 and 7.5 m at Shelter Cove and Tomales Bay to 4.5 m near Mount Tamalpais and 2.7 m at Loma Prieta. The geodetically derived slip distribution is in poor agreement with estimates based on analysis of S wave seismograms, probably because these waves register only 20–30% of the total seismic moment obtained from longer-period surface waves. Consideration of a range of fault geometries for 1906 slip near Loma Prieta indicates right-lateral motions lie between 2.3 and 3.1 m. These values are considerably greater than the 1.5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based. This factor, along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts, casts doubt on some claims of success in predicting the 1989 M=6.9 Loma Prieta earthquake.

Microplate versus continuum descriptions of active tectonic deformation

Whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi-continuous flow has important implications for lithospheric dynamics, fault mechanics, and earthquake hazard assessment. Actively deforming regions of the western United States, central Asia, Japan, and New Zealand show features that argue for both styles of movement, but new observations are necessary to determine which is most appropriate and at what scale the description applies. Geologic, geodetic, seismic, and paleomagnetic measurements tend to sample complementary aspects of the deformation field, so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases. Provided the total relative motion across each region is known and the distribution of active faults is well mapped, determination of fault slip rates can provide potentially decisive constraints. Reconnaissance geological studies supply useful slip rate estimates, but precise values depend upon detailed intensive investigation of individual sites. Geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information, but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long-term deformation pattern. Earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs. Paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years. Which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere, especially the strength and thickness of the elastic crust relative to the ductile lithosphere, and dynamical models can provide contrasting forecasts of observable features with testable consequences.

Temporal evolution of continental lithospheric strength in actively deforming regions

It has been agreed for nearly a century that a strong, loadbearing outer layer of earth is required to support mountain ranges, transmit stresses to deform active regions, and store elastic strain to generate earthquakes. However, the depth and extent of this strong layer remain controversial. Here we use a variety of observations to infer the distribution of lithospheric strength in the active western United States from seismic to steady-state time scales. We use evidence from post-seismic transient and earthquake cycle deformation, reservoir loading, glacio-isostatic adjustment, and lithosphere isostatic adjustment to large surface and subsurface loads. The nearly perfectly elastic behavior of Earth’s crust and mantle at the time scale of seismic wave propagation evolves to that of a strong, elastic crust and weak, ductile upper mantle lithosphere at both earthquake cycle (EC, ~10 0 to 103 yr) and glacio-isostatic adjustment (GIA, ~103 to 104 yr) time scales. Topography and gravity field correlations indicate that lithosphere isostatic adjustment (LIA) on ~106–107 yr time scales occurs with most lithospheric stress supported by an upper crust overlying a much weaker ductile substrate. These comparisons suggest that the upper mantle lithosphere is weaker than the crust at all time scales longer than seismic. In contrast, the lower crust has a chameleon-like behavior, strong at EC and GIA time scales and weak for LIA and steady-state deformation processes. The lower crust might even take on a third identity in regions of rapid crustal extension or continental collision, where anomalously high temperatures may lead to large-scale ductile flow in a lower crustal layer that is locally weaker than the upper mantle. Modeling of lithospheric processes in active regions thus cannot use a one-size-fits-all prescription of rheological layering (relation between applied stress and deformation as a function of depth) but must be tailored to the time scale and tectonic setting of the process being investigated.

GPS constraints on the kinematics of continental deformation

Recent GPS observations from the western United States, New Zealand, central Greece, and Japan indicate that present-day continental deformation is typically focused in narrow deforming zones whose extent is much smaller than the intervening largely inactive regions. However, these narrow zones are heterogeneously distributed, reflecting the inherent heterogeneity of continental lithospheric strength and internal buoyancy. Plate driving and resisting forces stress plate boundary zones and plate interiors and drive deformation. These forces change continuously and discontinuously, leading to continental deformation that typically evolves and migrates with time. Magmatic and tectonic processes alter lithospheric rheology and internal buoyancy and also contribute to the time-varying character of continental deformation.

Fault orientations in extensional and conjugate strike-slip environments and their implications

Seismically active conjugate strike-slip faults in California and Japan typically have mutually orthogonal right- and left-lateral fault planes. Normal- fault dips at earthquake nucleation depths are concentrated between 40° and 50°. The observed orientations and their strong clustering are surprising, because conventional faulting theory suggests fault initiation with conjugate 60° and 120° intersecting planes and 60° normal-fault dip or fault reactivation with a broad range of permitted orientations. The observations place new constraints on the mechanics of fault initiation, rotation, and evolutionary development. We speculate that the data could be explained by fault rotation into the observed orientations and deactivation for greater rotation or by formation of localized shear zones beneath the brittle-ductile transition in Earth9s crust. Initiation as weak frictional faults seems unlikely.

Crustal deformation across the Sierra Nevada, northern Walker Lane, Basin and Range transition, western United States measured with GPS, 2000-2004

[1] Global Positioning System (GPS) data collected in campaigns in 2000 and 2004 were processed and interpreted with other GPS data in the western Basin and Range province to provide new constraints on the rate, style, and pattern of deformation of the central and northern Walker Lane (WL), which lies near the western boundary of the Basin and Range. Across the central WL, near 38N latitude, the velocities with respect to North America increase westward by � 10 mm/yr inducing dextral shear. Farther north between 40 and 41N latitude, a western zone of � 7 mm/yr relative motion undergoes dextral shear, and an eastern zone of � 3 mm/yr relative motion undergoes extension and shear. These data show that the northern WL is essentially a dextral shear zone experiencing minor net dilatation (eD = 2.6 ± 0.8 nstrain/yr). Near most Holocene normal faults, dilatation inferred from the velocity field is not greater than the uncertainties. However, near the central Nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts (eD = 23.0 ± 3.9 nstrain/yr), some of which is attributable to transient postseismic deformation following large historic earthquakes. A block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the Honey Lake, Warm Springs, east Pyramid fault system, and Mohawk Valley faults is � 7 mm/yr. The WL is a zone whose width and dilatation rate increase northwestward, consistent with counterclockwise rotation of the Sierra Nevada microplate and transfer of deformation into the Pacific Northwest.

Seismic Slip Distribution along the San Jacinto Fault Zone, Southern California, and Its Implications

The amount and distribution of seismic slip along 240 km of the San Jacinto fault zone between Cajon Pass and Superstition Mountain has been obtained from determinations of seismic moment and estimates of source dimension for each of the nine moderate earthquakes (6 < M < 7) which have occurred there since 1890. There are two significant gaps in seismic slip, one between Cajon Pass and Riverside, the other from Anza to Coyote Mountain. Each is about 40 km long and both are characterized by complex fault zones and a currently high level of minor seismicity (M < 5). No aseismic fault creep has been identified on either segment. These gaps may mark the sites of the next moderate earthquakes (M = 6 → 7) to occur along the San Jacinto fault zone. The two remaining sections of the fault, Riverside and Anza, and Coyote Mountain to Superstition Mountain, may have been ruptured along their entire lengths, in 1890–1923 and 1942–1968, respectively.