William C. Hammond

Uplift and seismicity driven by groundwater depletion in central California

Groundwater use in California’s San Joaquin Valley exceeds replenishment of the aquifer, leading to substantial diminution of this resource and rapid subsidence of the valley floor. The volume of groundwater lost over the past century and a half also represents a substantial reduction in mass and a large-scale unburdening of the lithosphere, with significant but unexplored potential impacts on crustal deformation and seismicity. Here we use vertical global positioning system measurements to show that a broad zone of rock uplift of up to 1–3 mm per year surrounds the southern San Joaquin Valley. The observed uplift matches well with predicted flexure from a simple elastic model of current rates of water-storage loss, most of which is caused by groundwater depletion. The height of the adjacent central Coast Ranges and the Sierra Nevada is strongly seasonal and peaks during the dry late summer and autumn, out of phase with uplift of the valley floor during wetter months. Our results suggest that long-term and late-summer flexural uplift of the Coast Ranges reduce the effective normal stress resolved on the San Andreas Fault. This process brings the fault closer to failure, thereby providing a viable mechanism for observed seasonality in microseismicity at Parkfield and potentially affecting long-term seismicity rates for fault systems adjacent to the valley. We also infer that the observed contemporary uplift of the southern Sierra Nevada previously attributed to tectonic or mantle-derived forces is partly a consequence of human-caused groundwater depletion.

Upper mantle seismic wave attenuation: Effects of realistic partial melt distribution

Frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated. We consider the contribution of the melt squirt mechanism, through which pressure differences between disk-shaped inclusions are equalized by melt passing through connecting tubes. The pressure differences arise as a result of shear strain compressing disk-shaped pores differently on the basis of disk orientation with respect to the applied shear. The frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores. The pressure equalization process is modeled by a system of first-order linear differential equations, whose eigenvalues are the characteristic frequencies for melt squirt relaxation. It is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity, as well as distribution of melt inside pores and conduits. Use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation. Some conceivable melt distributions, however, would result in detectable attenuation in the seismic band. In all cases investigated, attenuation increases with frequency, indicating that melt squirt is not responsible for global upper mantle Q observations.

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.

Asymmetric mantle dynamics in the MELT region of the East Pacific Rise

Abstract The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.

Contemporary uplift of the Sierra Nevada, western United States, from GPS and InSAR measurements

Modern space geodesy has recently enabled the direct observation of slow geological processes that move and shape Earth’s surface, including plate tectonics and crustal strain accumulation that leads to earthquakes. More elusive has been the direct observation of active mountain growth, because geodetic measurements have larger uncertainties in the vertical direction, while mountain growth is typically very slow. For the Sierra Nevada of California and Nevada, western United States, the history of elevation is complex, exhibiting features of both ancient (40–60 Ma) and relatively young (<3 Ma) elevation. Here we exploit the complementary strengths of high-precision three-component point positions from the GPS and blanket coverage line-of-sight measurements from interferometric synthetic aperture radar (InSAR) to show that contemporary vertical motion of the Sierra Nevada is between 1 and 2 mm/yr. The motion is upward with respect to Earth’s center of mass and with respect to a relatively stable eastern Nevada, indicating generation of relief and uplift against gravity. Uplift is distributed along the entire length of the range, between latitude 35°N and 40°N, and is not focused near localized, seismically imaged mantle downwellings. These results indicate that the modern episode of Sierra Nevada uplift is still active and could have generated the entire modern range in <3 m.y.

MIDAS robust trend estimator for accurate GPS station velocities without step detection

Abstract Automatic estimation of velocities from GPS coordinate time series is becoming required to cope with the exponentially increasing flood of available data, but problems detectable to the human eye are often overlooked. This motivates us to find an automatic and accurate estimator of trend that is resistant to common problems such as step discontinuities, outliers, seasonality, skewness, and heteroscedasticity. Developed here, Median Interannual Difference Adjusted for Skewness (MIDAS) is a variant of the Theil‐Sen median trend estimator, for which the ordinary version is the median of slopes vij = (xj–xi)/(tj–ti) computed between all data pairs i > j. For normally distributed data, Theil‐Sen and least squares trend estimates are statistically identical, but unlike least squares, Theil‐Sen is resistant to undetected data problems. To mitigate both seasonality and step discontinuities, MIDAS selects data pairs separated by 1 year. This condition is relaxed for time series with gaps so that all data are used. Slopes from data pairs spanning a step function produce one‐sided outliers that can bias the median. To reduce bias, MIDAS removes outliers and recomputes the median. MIDAS also computes a robust and realistic estimate of trend uncertainty. Statistical tests using GPS data in the rigid North American plate interior show ±0.23 mm/yr root‐mean‐square (RMS) accuracy in horizontal velocity. In blind tests using synthetic data, MIDAS velocities have an RMS accuracy of ±0.33 mm/yr horizontal, ±1.1 mm/yr up, with a 5th percentile range smaller than all 20 automatic estimators tested. Considering its general nature, MIDAS has the potential for broader application in the geosciences.

Global deformation from the great 2004 Sumatra-Andaman Earthquake observed by GPS: Implications for rupture process and global reference frame

Static coseismic offsets > 1 mm are observed up to 7800 km away from the great Sumatra-Andaman earthquake of 26 Dec. 2004 using global GPS network data. We investigate the rupture process based on far-field continuous GPS data. To reduce error in the coseismic offset estimates due to post-seismic deformation in the days following the main shock, we simultaneously fit a model of co- and postseismic offsets for nearby stations SAMP (500 km) and NTUS (900 km). The 3-month cumulative postseismic displacement for station SAMP amounts to 20% of the coseismic displacement, and can be well modeled by velocity-strengthening afterslip. We find that coseismic slip on the northern rupture segment is ∼3 m, which is consistent with seismic estimates. Our best estimate of the moment magnitude is Mw = 9.13 if we take into account the expected increase of the shear modulus with depth (for uniform μ = 30 GPa, the moment-magnitude would only be 8.97). Our geodetic results, and thus our inferred rupture model, are different from a similar study using far-field data of Banerjee et al. (2005). These differences highlight the challenge in earthquake studies on a global scale in terms of the sensitivity of far-field offset estimates to the analysis strategy and reference frame treatment. Our predicted coseismic offsets from this event are at least 1 mm across almost the entire globe. This warrants a reconsideration of how to maintain the global terrestrial reference frame affected by earthquakes of Mw > 9.0.

Seismogeodesy of the 2014 Mw6.1 napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike‐slip fault

Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 Mw6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

Geodetic constraints on areal changes in the Pacific–North America plate boundary zone: What controls Basin and Range extension?

Using ∼1500 geodetic velocities we model the present-day spatial patterns of areal changes inside the Pacific–North America plate boundary zone. From this model we show that between the central Gulf of California and the Queen Charlotte Islands there is no significant net change in surface area. This zero net areal-change result allows us to relate regions of areal growth to areas of equivalent contraction elsewhere within the plate boundary zone. We find that areal growth of the Basin and Range province (BRP) and its eastern margin (∼5.2 ± 0.1 × 10 3 m 2 /yr) is balanced by areal reduction near northwestern California between 38°N and 42°N. The San Andreas fault system south of 38°N and the plate boundary zone north of ∼42°N (including the Juan de Fuca and Gorda Ridge systems) each have no significant net areal change. Our results suggest a kinematic relationship between extension in the BRP and contraction near the northern California Coast Ranges and Klamath Mountains. From these observations we propose that, although BRP extension may be caused by internal forces, the southernmost Cascadia subduction zone provides a “window of escape” that acts as a stress guide to BRP extension as well as northwestward Sierra Nevada motion. Such a dynamic model is consistent with independent findings that (1) the least principal horizontal stress orientations in the BRP are toward northern California, (2) extension directions in the BRP have changed orientation to track the northward migration of the Mendocino triple junction, and (3) the southernmost Cascadia subduction zone is a relatively weak plate boundary.

Evidence for an active shear zone in southern Nevada linking the Wasatch fault to the Eastern California shear zone

Previous studies have shown that ~5% of the Pacifi c‐North America relative plate motion is accommodated in the eastern part of the Great Basin (western United States). Near the Wasatch fault zone and other nearby faults, deformation is currently concentrated within a narrow zone of extension coincident with the eastern margin of the northern Basin and Range. Farther south, the pattern of active deformation implied by faulting and seismicity is more enigmatic. To assess how present-day strain is accommodated farther south and how this relates to the regional kinematics, we analyze data from continuous global positioning system (GPS) stations and model the strain rate tensor fi eld using the horizontal GPS velocities and earthquake focal mechanisms. The results indicate an ~100-km-wide zone of ~3.3 mm/yr extension at 40.5°N that broadens southward from the Wasatch fault zone to a width of >400 km at 36°N. This broadening involves at least one zone of localized extension in northwestern Arizona that encroaches into the southwestern plateau, and an eastnortheast‐trending sinistral shear zone (the Pahranagat shear zone) through southern Nevada. This shear zone may accommodate as much as 1.8 mm/yr, and is a key feature that enables westward transfer of extension, thereby providing a kinematic connection between the Wasatch fault zone and the Eastern California shear zone.