Colin B. Amos

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.

Channel width response to differential uplift

[1] The role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious. Here high-resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift. For five Late Quaternary alluvial paleochannels crossing small folds along the active Ostler fault zone of southern New Zealand, nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift. When the magnitude of differential uplift is sufficiently small, narrowing alone permits these channels to remain antecedent. In the context of a unit stream power model for fluvial erosion, observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift, continued incision requires concomitant changes in channel gradient. Thus when crossing small growing folds, alluvial rivers simply narrow their channels, whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening.

Surface slip during large Owens Valley earthquakes

Data sets and expanded results contributing to this study are available in the supporting information. The EarthScope Southern and Eastern California Lidar Project (available online at http://opentopo.sdsc.edu) involved data acquisition and processing for the Plate Boundary Observatory (PBO) by NCALM (http://www.ncalm.org). UNAVCO operates the PBO for EarthScope (http://www.earthscope.org), supported by the National Science Foundation (EAR-0350028 and EAR-0732947). Funding for this study was provided by the Southern California Earthquake Center (SCEC) (Project 12140), the Geological Society of America Graduate Student Research fund, the Community Foundation of San Bernardino county, and the Western Washington University Geology Department. We thank G. Seitz, M. Price, and K. Morgan for assistance in the field, and S. Bacon, J. Arrowsmith, R. Weldon, K. Scharer, J. Unruh, C. Madden-Madugo, and D. Haddad for helpful discussions. Constructive reviews by D. Schwartz, R. Briggs, E. Schermer, D. Clark, and one anonymous reviewer substantially improved the paper. We also thank the staff at the UC White Mountain Research Center for facilitating this work. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Late Quaternary slip rate on the Kern Canyon fault at Soda Spring, Tulare County, California

The Kern Canyon fault represents a major tectonic and physiographic boundary in the southern Sierra Nevada of east-central California. Previous investigations of the Kern Canyon fault underscore its importance as a Late Cretaceous and Neogene shear zone in the tectonic development of the southern Sierra Nevada. Study of the late Quaternary history of activity, however, has been confounded by the remote nature of the Kern Canyon fault and deep along-strike exhumation within the northern Kern River drainage, driven by focused fluvial and glacial erosion. Recent acquisition of airborne lidar (light detection and ranging) topography along the ∼140 km length of the Kern Canyon fault provides a comprehensive view of the active surface trace. High-resolution, lidar-derived digital elevation models (DEMs) for the northern Kern Canyon fault enable identification of previously unrecognized offsets of late Quaternary moraines near Soda Spring (36.345°N, 118.408°W). Predominately north-striking fault scarps developed on the Soda Spring moraines display west-side-up displacement and lack a significant sense of strike-slip separation, consistent with detailed mapping and trenching along the entire Kern Canyon fault. Scarp-normal topographic profiling derived from the lidar DEMs suggests normal displacement of at least 2.8 +0.6/–0.5 m of the Tioga terminal moraine crest. Cosmogenic 10Be exposure dating of Tioga moraine boulders yields a tight age cluster centered around 18.1 ± 0.5 ka ( n = 6), indicating a minimum normal-sense fault slip rate of ∼0.1–0.2 mm/yr over this period. Taken together, these results provide the first clear documentation of late Quaternary activity on the Kern Canyon fault and highlight its role in accommodating internal deformation of the southern Sierra Nevada.

Map of the late Quaternary active Kern Canyon and Breckenridge faults, southern Sierra Nevada, California

Surface traces of the Quaternary active Kern Canyon and Breckenridge faults were mapped via aerial reconnaissance, analysis of light detection and ranging (LiDAR) elevation data, review and interpretation of aerial photography, field reconnaissance, and detailed field mapping. This effort specifically targeted evidence of late Quaternary surface deformation and, combined with separate paleoseismic investigations, identified and characterized the North Kern Canyon, South Kern Canyon, and Lake Isabella sections of the Kern Canyon fault and the Breckenridge fault. The mapping presented here provides definitive evidence for previously unrecognized Holocene and late Pleistocene east-down displacement along the Kern Canyon and Breckenridge faults. Our results indicate that much of the Kern Canyon fault has undergone Quaternary reactivation to accommodate internal deformation of the otherwise rigid Sierra Nevada block. This deformation reflects ongoing, seismogenic crustal thinning in the southern Sierra Nevada, and highlights the effects of localized tectonic forces operating in this part of the Sierra Nevada.

Differential uplift and incision of the Yakima River terraces, central Washington State

The fault-related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. Geodesy implies ~2 mm/yr of NNE directed shortening across the folds, but until now the distribution and rates of Quaternary deformation among individual structures has been unclear. South of Ellensburg, Washington, the Yakima River cuts a ~600 m deep canyon across several Yakima folds, preserving gravel-mantled strath terraces that record progressive bedrock incision and related rock uplift. Here we integrate cosmogenic isochron burial dating of the strath terrace gravels with lidar analysis and field mapping to quantify rates of Quaternary differential incision and rock uplift across two folds transected by the Yakima River: Manastash and Umtanum Ridge. Isochron burial ages from in situ produced 26Al and 10Be at seven sites across the folds date episodes of strath terrace formation over the past ~2.9 Ma. Average bedrock incision rates across the Manastash (~88 m/Myr) and Umtanum Ridge (~46 m/Myr) anticlines are roughly 4 to 8 times higher than rates in the intervening syncline (~14 m/Myr) and outside the canyon (~10 m/Myr). These contrasting rates demonstrate differential bedrock incision driven by ongoing Quaternary rock uplift across the folds at rates corresponding to ~0.13 and ~0.06 mm/yr shortening across postulated master faults dipping 30 ± 10°S beneath the Manastash and Umtanum Ridge anticlines, respectively. The reported Quaternary shortening across the anticlines accounts for ~10% of the ~2 mm/yr geodetic budget, suggesting that other Yakima structures actively accommodate the remaining contemporary deformation.

Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California

New geochronologic and geomorphic constraints on the Little Lake fault in the Eastern California shear zone reveal steady, modest rates of dextral slip during and since the mid-to-late Pleistocene. We focus on a suite of offset fluvial landforms in the Pleistocene Owens River channel that formed in response to periodic interaction with nearby basalt flows, thereby recording displacement over multiple time intervals. Overlap between 40 Ar/ 39 Ar ages for the youngest intracanyon basalt flow and 10 Be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the Little Lake channel at ca. 64 ka. Older basalt flows flanking the upper and lower canyon margins indicate localization of the Owens River in its current position between 212 ± 14 and 197 ± 11 ka. Coupled with terrestrial light detection and ranging (lidar) and digital topographic measurements of dextral offset, the revised Little Lake chronology indicates average dextral slip rates of at least ∼0.6–0.7 mm/yr and 4 to 10 5 yr. Despite previous geodetic observations of relatively rapid interseismic strain along the Little Lake fault, we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles. Instead, our results indicate that accelerated fault loading may be transient over much shorter periods (∼10 1 yr) and perhaps indicative of time-dependent seismic hazard associated with Eastern California shear zone faults.

Palaeoseismic constraints on Holocene surface ruptures along the Ostler Fault, southern New Zealand

Palaeoseismic trenching along the central Ostler fault zone reveals the nature and timing of past surface-rupturing earthquakes. A 26 m long trench excavated into a last-glacial (26.5 ka) outwash surface cut by the Ruataniwha strand of the North Central Ostler fault reveals evidence for at least two metre-scale surface displacements in the last c. 8 ka. Detailed logging of colluvial wedge and alluvial stratigraphy, combined with optically stimulated luminescence dating of loess within colluvial packages, provides a maximum bound on the most recent earthquake (MRE) of 2.3–4.5 ka. The MRE resulted in a surface displacement of at least 1.8 m, consistent with an estimated moment magnitude (M) 6.9–7.1 earthquake based on the total 60 km length of the Ostler fault zone. The penultimate event occurred sometime before 4.1–8.4 ka and resulted in a comparable or larger surface displacement. Similar ages from samples collected in previous trenches on the Ostler fault suggest that surface ruptures may persist across kilometre-scale stepovers defined by the active surface trace of the fault. At the trench scale, comparison between dip slip calculated from topographic fault-scarp profiling (7.8 m) and the total offset of exposed outwash gravels (6.0 m) suggests that surface folding immediately adjacent to the fault scarp accommodates roughly 25% of the total slip. Given a previously reported recurrence interval of c. 2–5 ka for the Ostler fault, the presence of only 2–3 palaeoearthquakes on the 26.5 ka outwash surface indicates that additional events likely occurred on nearby fault scarps in an overall complex zone of surface faulting.

Refining the Southern Extent of the 1872 Owens Valley Earthquake Rupture through Paleoseismic Investigations in the Haiwee Area, Southeastern California

Recent upward revision of the 1872 Owens Valley earthquake from Mw 7.4-7.5 to 7.7-7.9 implies either additional unrecognized rupture length or anoma- louslystronggroundmotionsassociatedwiththisevent.Weinvestigatethefirstpossibility through paleoseismic trenching south of the mapped surface rupture in the Haiwee area, where historical accounts suggest significant surface deformation following the earth- quake. Trenching focused on a prominent north-striking scarp, herein termed the Sage Flat fault, expressed in Pleistocene alluvial fans east of Haiwee Reservoir. Surficial map- ping and ground-based Light Detection and Ranging (lidar) surveying suggest that this faultaccommodateseast-downnormalmotion,andpossiblyacomparableamountofdex- tralslip.Trenchingandluminescencedatingbracketsthetimingofthemostrecentsurface- rupturing earthquake between ∼25:7 and 30.1 ka, and provides evidence for an earlier event predating this time. In combination with scarp profiling, these dates also suggest am aximum rate of normal, dip-slip fault motion up to∼0:1 mm=yr over this period. Although we discovered no evidence for recent surface rupture on the Sage Flat fault, a series of subvertical fractures and fissures cut across young trench stratigraphy, consis- tentwithsecondarydeformationassociatedwithseismicshaking.Assuch,wesuggestthat possible ground disturbance in the Haiwee area during the 1872 event primarily reflected ground shaking or liquefaction-related deformation rather than triggered slip. In addition, we infer a structural and kinematic connection between the Owens Valley fault and oblique-dextral faults north of Lower Cactus Flat in the northwestern Coso Range, rather thanawest-stepintonorthernorwesternRoseValley.Considerationofthesestructuresin the total extent of the Owens Valley fault suggests a length of 140 km, of which at least 113 km ruptured during the 1872 event. Online Material: Procedural details and expanded results from the OSL sample analyses, as well as high-resolution paleoseismic trench logs.