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2017
Sahakian, V, Bormann J, Driscoll N, Harding A, Kent G, Wesnousky S.  2017.  Seismic constraints on the architecture of the Newport-Inglewood/Rose Canyon fault: Implications for the length and magnitude of future earthquake ruptures. Journal of Geophysical Research-Solid Earth. 122:2085-2105.   10.1002/2016jb013467   AbstractWebsite

The Newport-Inglewood/Rose Canyon (NIRC) fault zone is an active strike-slip fault system within the Pacific-North American plate boundary in Southern California, located in close proximity to populated regions of San Diego, Orange, and Los Angeles counties. Prior to this study, the NIRC fault zone's continuity and geometry were not well constrained. Nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture. Four main fault strands are identified offshore, separated by three main stepovers along strike, all of which are 2km or less in width. Empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset. Models of Coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone, which appear to be dependent on the location of rupture initiation and fault geometry at the stepovers. These modeling results show that the southernmost stepover between the La Jolla and Torrey Pines fault strands may act as an inhibitor to throughgoing rupture due to the stepover width and change in fault geometry across the stepover; however, these results still suggest that rupture along the entire fault zone is possible.

2016
Olsen, MJ, Johnstone E, Driscoll N, Kuester F, Ashford SA.  2016.  Fate and transport of seacliff failure sediment in Southern California. Journal of Coastal Research. :185-199.   10.2112/si76-016   AbstractWebsite

Continual erosion and collapse of unstable seacliffs along the economically important coastline of San Diego County, California, threatens existing development and public safety. Frequent time-series mapping of the seacliffs and beaches provides valuable insight into the processes responsible for cliff erosion and into the reworking and transport of the failed material. High-resolution terrestrial laser scan (TLS) data provide quantitative data for analyzing seacliff morphology, capturing patterns over time and across a wide range of spatial scales. Through an ongoing "rapid response" program operational since spring 2007, eleven substantial seacliff failure sites were mapped pre-collapse, immediately post-collapse, and repeatedly after the collapse to constrain processes causing cliff failure and estimate the rate at which failed material is reworked. Comparison of the TLS data with water levels and climate data highlights the contributing mechanisms to the seacliff failures and the rapid reworking of the failed material. Failure sites were categorized based on the frequency of wave contact (i.e., total water level) compared with the beach elevation to assess differences in the rates of sediment reworking. For example, unconsolidated failed material on the beach was reworked quickly by waves at sites where waves reached the failure on a nearly daily basis. Conversely, other failure masses with less wave contact were only reworked during storm events producing larger waves. At sites where the failure material consisted of large boulders, there are feedback mechanisms at play where the failed material protects the cliff toe by stabilizing talus deposits, akin to riprap engineering techniques. Failures due to wave undercutting and notching were observed to migrate laterally at these sites. This lateral progression of failures might explain the long-term linear retreat of the seacliffs in the region, which minimizes the development of embayments and promontories.

Johnstone, E, Raymond J, Olsen MJ, Driscoll N.  2016.  Morphological expressions of coastal cliff erosion processes in San Diego County. Journal of Coastal Research. :174-184.   10.2112/si76-015   AbstractWebsite

High-resolution, Terrestrial Laser Scanning (TLS) data have been acquired seasonally since 2006 to define the style and magnitude of cliff erosion along the southern 20 km of coastline within the Oceanside Littoral Cell (OLC). In particular, twelve sites with cliff collapses were mapped repeatedly to examine how these collapses propagate along the cliffs and to identify feedback mechanisms between the liberated material and subsequent cliff failures. Grain size analyses of the failed material (retention cutoff) were performed to estimate the contribution to the beach sand inventory. Despite a relatively short time series (only six years) on a geologic scale, the high spatial and temporal resolution of the study has provided important insights into the fine details of processes controlling cliff erosion in the OLC. In addition, the seasonal TLS established a quantitative baseline from which future change may be assessed. Both lithological and environmental conditions are known to play a major role in governing the rate and style of cliff erosion; however, other factors such as beach width, elevation, and precipitation also exert control on rates and styles of cliff failures. The findings of this study reveal that cliff erosion is subaerially dominated where the beaches are wider and elevation is higher. Alternatively, erosion is marine dominated where the beaches are narrow and have lower average elevation. A direct relationship exists between beach elevation and undercutting and erosion along the failure edges and thus might provide a mechanism to create the observed linear retreat of the cliffs in the OLC rather than the formation of promontories and embayments. Other morphological expressions on the cliff face, such as honeycomb patterns and sawtooth-style frontage, indicate mechanisms that control predominant styles of erosion in particular locations. This time series documents seasonal and short-term erosional patterns and rates as well as establishes a baseline to understand cliff erosion in response to rapid sea level rise (>3 mm/yr).

Han, L, Hole JA, Stock JM, Fuis GS, Kell A, Driscoll NW, Kent GM, Harding AJ, Rymer MJ, Gonzalez-Fernandez A, Lazaro-Mancilla O.  2016.  Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project. Journal of Geophysical Research-Solid Earth. 121:7469-7489.   10.1002/2016jb013139   AbstractWebsite

A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from similar to 3 to similar to 8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher-grade metamorphosed sediment. The lower crust below similar to 12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 km depth as it does to the south, and a weak reflection suggests Moho at similar to 28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

Smith, KD, Kent GM, von Seggern DP, Driscoll NW, Eisses A.  2016.  Evidence for Moho-lower crustal transition depth diking and rifting of the Sierra Nevada microplate. Geophysical Research Letters. 43:10738-10744.   10.1002/2016gl070283   AbstractWebsite

Lithospheric rifting most often initiates in continental extensional settings where breaking of a plate may or may not progress to sea floor spreading. Generally, the strength of the lithosphere is greater than the tectonic forces required for rupture (i.e., the tectonic force paradox), and it has been proposed that rifting requires basaltic magmatism (e.g., dike emplacement) to reduce the strength and cause failure, except for the case of a thin lithosphere (<30km thick). Here we isolate two very similar and unprecedented observations of Moho-lower crustal transition dike or fluid injection earthquake swarms under southern Sierra Valley (SV: 2011-2012) and North Lake Tahoe (LT: 2003-2004), California. These planar distributions of seismicity can be interpreted to define the end points, and cover similar to 25% of the length, of an implied similar to 56km long structure, each striking N45 degrees W and dipping similar to 50 degrees NE. A single event at similar to 30km depth that locates on the implied dipping feature between the two swarms is further evidence for a single Moho-transition depth structure. We propose that basaltic or fluid emplacement at or near Moho depths weakens the upper mantle lid, facilitating lithospheric rupture of the Sierra Microplate. Similar to the LT sequence, the SV event is also associated with increased upper crustal seismicity. An 27 October 2011, M-w 4.7 earthquake occurred directly above the deep SV sequence at the base of the upper crustal seismogenic zone (similar to 15km depth).

Sahakian, V, Kell A, Harding A, Driscoll N, Kent G.  2016.  Geophysical evidence for a San Andreas subparallel transtensional fault along the northeastern shore of the Salton Sea. Bulletin of the Seismological Society of America. 106:1963-1978.   10.1785/0120150350   AbstractWebsite

The southern San Andreas fault (SSAF) accommodates a significant amount of strain between the Pacific and North American plates; thus, the fault represents a major geohazard to the populated areas of southern California, in particular the larger Los Angeles metropolitan area. Paleoseismic chronology of ruptures along the SSAF segment suggests this fault is near the end of its interseismic period (similar to 180 years), because it has not ruptured in historic times (similar to 320 years). A recent active-source seismic experiment performed in the Salton Sea west of the SSAF provides evidence for extensional deformation along the northeastern shore of the Salton Sea. This study posits that the extensional deformation is due to a previously unmapped fault, here named the Salton trough fault (STF). The seismic reflection data image a divergent sediment package that dips toward the northeast with thicknesses up to at least 2 km. Refraction inversion produces a southwestward-dipping velocity discontinuity that crops out east of the SSAF surface trace, consistent with the existence of a southwest to northeast gradient in lithology. If present, the existence of the STF has scientific and societal relevance. First, the STF appears to control the recent Salton trough architecture north of Bombay Beach. Second, from a seismological hazards perspective, the presence of this structure could alter the current understanding of stress transfer and rupture dynamics in the region, as well as community fault models and ground-motion simulations on the SSAF.

Sievers, KT, Barr RJ, Maloney JM, Driscoll NW, Anderson TW.  2016.  Impact of habitat structure on fish populations in kelp forests at a seascape scale. Marine Ecology Progress Series. 557:51-63.   10.3354/meps11885   AbstractWebsite

Habitat use by a species is a vital component in explaining the dynamics of natural populations. For mobile marine species such as fishes, describing habitat heterogeneity at a seascape scale is essential because it quantifies the spatial extent to which fishes are interacting with their environment. Here, we explored the relationships between habitat metrics and the density and size of kelp forest fishes across a seascape that is naturally fragmented. Multibeam sonar and GIS analysis were employed to create a seascape map that explicitly defined bathymetry and spatial configuration of rocky reefs in southern California (USA). Georeferenced subtidal transects were conducted across this seascape to describe habitat attributes, including the density of macroalgae, and record the number and size of fishes. Multiple regression analyses were conducted to identify which variables of habitat structure were most important in describing numerical density, biomass density, average size, and maximum size for fishes. Responses to different habitat components were dependent on particular species, choice of spatial scale, and the inherent characteristics of the seascape itself. Notably, the relative influence of seascape components was dependent on the configuration of the seascape, where fishes in a more isolated and less connected seascape were more influenced by spatial configuration than fishes in a seascape with greater habitat connectedness. This study demonstrates that explicit habitat maps allow for a more comprehensive understanding of population structure when describing fishes across large spatial scales.

Switzer, RD, Parnell EP, Leichter JL, Driscoll NW.  2016.  The effects of tectonic deformation and sediment allocation on shelf habitats and megabenthic distribution and diversity in southern California. Estuarine, Coastal and Shelf Science. 169:25-37.   10.1016/j.ecss.2015.11.020   Abstract

Landscape and seascape structures are typically complex and manifest as patch mosaics within characteristic biomes, bordering one another in gradual or abrupt ecotones. The underlying patch structure in coastal shelf ecosystems is driven by the interaction of tectonic, sedimentary, and sea level dynamic processes. Animals and plants occupy and interact within these mosaics. Terrestrial landscape ecological studies have shown that patch structure is important for ecological processes such as foraging, connectivity, predation, and species dynamics. The importance of patch structure for marine systems is less clear because far fewer pattern-process studies have been conducted in these systems. For many coastal shelf systems, there is a paucity of information on how species occupy shelf seascapes, particularly for seascapes imbued with complex patch structure and ecotones that are common globally due to tectonic activity. Here, we present the results of a study conducted along a myriameter-scale gradient of bottom and sub-bottom geological forcing altered by tectonic deformation, sea level transgression and sediment allocation. The resulting seascape is dominated by unconsolidated sediments throughout, but also exhibits increasing density and size of outcropping patches along a habitat patch gradient forced by the erosion of a sea level transgressive surface that has been deformed and tilted by tectonic forcing. A combination of sub-bottom profiling, multibeam bathymetry, and ROV surveys of the habitats and the demersal megafauna occupying the habitats indicate (1) significant beta diversity along this gradient, (2) biological diversity does not scale with habitat diversity, and (3) species occupy the patches disproportionately (non-linearly) with regard to the proportional availability of their preferred habitats. These results indicate that shelf habitat patch structure modulates species specific processes and interactions with other species. Further studies are needed to examine experimentally the mechanics of how patch structure modulates ecological processes in shelf systems. Our results also provide further support for including multiple spatial scales of patch structure for the application of remote habitat sensing as a surrogate for biological community structure.

Noble, PJ, Ball IG, Zimmerman SH, Maloney J, Smith SB, Kent G, Adams KD, Karlin RE, Driscoll N.  2016.  Holocene paleoclimate history of Fallen Leaf Lake, CA., from geochemistry and sedimentology of well-dated sediment cores. Quaternary Science Reviews. 131, Part A:193-210.   10.1016/j.quascirev.2015.10.037   Abstract

Millennial-scale shifts in aridity patterns have been documented during the Holocene in the western United States, yet the precise timing, severity, and regional extent of these shifts prompts further study. We present lake sediment core data from Fallen Leaf Lake, a subalpine system at the southern end of the Lake Tahoe basin for which 80% of the contemporary inflow is derived from snowpack delivered by Pacific frontal storm systems. A high quality age model has been constructed using 14C ages on plant macrofossils, 210Pb, and the Tsoyowata tephra datum (7.74–7.95 cal kyr BP). One core captures the transition from the Late Tioga-younger Dryas glaciolacustrine package to laminated opaline clay at 11.48 cal kyr BP. Early Holocene sedimentation rates are relatively high (∼1.9 mm/year) and cooler winter temperatures are inferred by the presence of pebbles interpreted to be transported out into the lake via shore ice. There is a geochemically distinct interval from ∼4.71 to 3.65 cal kyr BP that is interpreted as a late Holocene neopluvial, characterized by depleted δ13C and lower C:N that point to reduced runoff of terrigenous organic matter, increased winter precipitation, and increased algal productivity. The largest Holocene signal in the cores occurs at the end of the neopluvial, at 3.65 cal kyr BP, and marks a shift into a climate state with variable precipitation, yet is overall more arid than the neopluvial. This new climate state persists for ∼3 ka, until the Little Ice Age. Low sedimentation rates (0.5 mm/year), the homogeneous opaline sediment, and steadily increasing contributions of terrestrial vs. algal organic matter in these cores suggest that the lowstand state of Fallen Leaf Lake may have been the norm from 3.65 to 0.55 cal kyr BP, punctuated by short term high precipitation years or multi-year intervals capable of rapid short duration lake level rise. Fallen Leaf Lake is strongly influenced by changes in winter precipitation and temperature, manifested largely by the geochemical proxies, and highlights unique advantages of subalpine lakes in regional paleoclimate reconstructions.

2015
Klotsko, S, Driscoll N, Kent G, Brothers D.  2015.  Continental shelf morphology and stratigraphy offshore San Onofre, California: The interplay between rates of eustatic change and sediment supply. Marine Geology. 369:116-126.   10.1016/j.margeo.2015.08.003   AbstractWebsite

New high-resolution CHIRP seismic data acquired offshore San Onofre, southern California reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply. Throughout the majority of the study region, a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from similar to 72 to 53 m below present sea level. These erosional features appear to have formed between Melt Water Pulse 1A and Melt Water Pulse 1B, when the rate of sea-level rise was lower. There are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the Holocene transgressive surface in the seismic data. Unit I, the deepest unit, is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform. Unit I thins seaward by downlap and pinches out landward against the shoreline cutoff. Unit II is a mid-shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap. The youngest, Unit III, is interpreted to represent modern sediment deposition. Faults in the study area do not appear to offset the transgressive surface. The Newport Inglewood/Rose Canyon fault system is active in other regions to the south (e.g., La Jolla) where it offsets the transgressive surface and creates seafloor relief. Several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area. Nevertheless, our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression. The Cristianitos fault zone also causes a shoaling of the transgressive surface. This may be from resistant antecedent topography due to an early phase of compression on the fault. The Cristianitos fault zone was previously defined as a down-to-the-north normal fault, but the folding and faulting architecture imaged in the CHIRP data are more consistent with a strike-slip fault with a down-to-the-northwest dip-slip component. A third area of shoaling is observed off of San Mateo and San Onofre creeks. This shoaling has a constructional component and could be a relict delta or beach structure. (C) 2015 Elsevier B.V. All rights reserved.

Day, S, Llanes P, Silver E, Hoffmann G, Ward S, Driscoll N.  2015.  Submarine landslide deposits of the historical lateral collapse of Ritter Island, Papua New Guinea. Marine and Petroleum Geology. 67:419-438.   10.1016/j.marpetgeo.2015.05.017   AbstractWebsite

The March 13th 1888 collapse of Ritter Island in Papua New Guinea is the largest known sector collapse of an island volcano in historical times. One single event removed most of the island and its western submarine flank, and produced a landslide deposit that extends at least 70 km from the headwall of the collapse scar. We have mapped and described the deposits of the debris avalanche left by the collapse using full-coverage multibeam bathymetry, side-scan sonar backscatter intensity mapping, chirp seismic-reflection profiles, TowCam photographs of the seafloor and samples from a single dredge. Applying concepts originally developed on the 1980 Mount St. Helens collapse landslide deposits, we find that the Ritter landslide deposits show three distinct morphological facies: large block debris avalanche, matrix-rich debris avalanche and distal debris flow facies. Restoring the island's land and submarine topography we obtained a volume of 4.2 km(3) for the initial collapse, about 75% of which is now forming the large block facies at distances less than 12 km from the collapse scar. The matrix-rich facies volume is unknown, but large scale erosion of the marine sediment substrate yielded a minimum total volume of 6.4 km(3) in the distal debris flow and/or turbidite deposits, highlighting the efficiency of substrate erosion during the later history of the landslide movement. Although studying submarine landslide deposits we can never have the same confidence that subaerial observations provide, our analysis shows that well-exposed submarine landslide deposits can be interpreted in a similar way to subaerial volcano collapse deposits, and that they can in turn be used to interpret older, incompletely exposed submarine landslide deposits. Studying the deposits from a fades perspective provides the basis for reconstructing the kinematics of a collapse event landslide; understanding the mechanisms involved in its movement and deposition; and so providing key inputs to tsunami models. (C) 2015 Elsevier Ltd. All rights reserved.

Magnusdottir, S, Brandsdottir B, Driscoll N, Detrick R.  2015.  Postglacial tectonic activity within the Skjalfandadjup Basin, Tjornes Fracture Zone, offshore Northern Iceland, based on high resolution seismic stratigraphy. Marine Geology. 367:159-170.   10.1016/j.margeo.2015.06.004   AbstractWebsite

Multibeam bathymetric and high resolution seismic reflection data (Chirp) have been used to illuminate the structural framework and tectonic evolution of the 300 km(2) Skjalfandadjup Basin, one of three large rift basins within the Tjornes Fracture Zone, a complex transform zone offshore Northern Iceland. The Skjalfandadjup Basin together with the Nafir seamounts, form the nascent Nafir-Skjalfandadjup volcanic system of the oblique Grimsey Volcanic Zone. The Skjalfandadjup Basin is made up of normal faults with maximum displacement of 50-60 m and a dominant strike of N10-20 degrees W. Correlation of Chirp data with tephrochronology from sediment core MD99-2275 provided constraints on tectonic movements along individual faults from Late-glacial time throughout Holocene. Maximum vertical displacement is observed on three listric normal faults marking the eastern boundary of the main basin. Postglacial tectonic activity commenced as early as 14-15 kyr BP, followed by three major rifting episodes prior to 10 kyr BP and three smaller rifting episodes during the last 4.2 kyr. The 10-12 kyr period of enhanced tectonic activity was accompanied by eruptions within the Tjornes Fracture Zone. Isostatic rebound following rapid deglaciation caused rifting with significant vertical displacement on normal faults offshore N-Iceland. (C) 2015 Elsevier B.V. All rights reserved.

Eisses, AK, Kell A, Kent GM, Driscoll NW, Baskin RL, Smith KD, Karlin RE, Louie JN, Pullammanappallil SK.  2015.  New constraints on fault architecture, slip rates, and strain partitioning beneath Pyramid Lake, Nevada. Geosphere. 11:683-704.   10.1130/ges00821.1   AbstractWebsite

A seismic compressed high-intensity radar pulse (CHIRP) survey of Pyramid Lake, Nevada, defines fault architecture and distribution within a key sector of the northern Walker Lane belt. More than 500 line-kilometers of high-resolution (decimeter) subsurface imagery, together with dated piston and gravity cores, were used to produce the first comprehensive fault map and attendant slip rates beneath the lake. A reversal of fault polarity is observed beneath Pyramid Lake, where down-to-the-east slip on the dextral Pyramid Lake fault to the south switches to down-to-the-west displacement on the Lake Range fault to the north. Extensional deformation within the northern two thirds of the basin is bounded by the Lake Range fault, which exhibits varying degrees of asymmetric tilting and stratal divergence due to along-strike segmentation. This structural configuration likely results from a combination of changes in slip rate along strike and the splaying of fault segments onshore. The potential splaying of fault segments onshore tends to shift the focus of extension away from the lake. The combination of normal-and oblique-slip faults in the northern basin gives Pyramid Lake its distinctive "fanning open to the north" geometry. The oblique-slip faults in the northwestern region of the lake are short and discontinuous in nature, possibly representing a nascent shear zone. In contrast, the Lake Range fault is long and well defined. Vertical slip rates measured across the Lake Range and other faults provide new estimates on extension across the Pyramid Lake basin. A minimum vertical slip rate of similar to 1.0 mm/yr is estimated along the Lake Range fault. When combined with fault length, slip rates yield a potential earthquake magnitude range between M6.4 and M7.0. Little to no offset on the Lake Range fault is observed in the sediment rapidly emplaced at the end of Tioga glaciation (12.5-9.5 ka). In contrast, since 9.5 ka, CHIRP imagery provides evidence for three or four major earthquakes, assuming a characteristic offset of 2.5 m per event. Regionally, our CHIRP investigation helps to reveal how strain is partitioned along the boundary between the northeastern edge of the Walker Lane and the northwest Basin and Range Province proper.

Maloney, JM, Grupe BM, Pasulka AL, Dawson KS, Case DH, Frieder CA, Levin LA, Driscoll NW.  2015.  Transpressional segment boundaries in strike-slip fault systems offshore southern California: Implications for fluid expulsion and cold seep habitats. Geophysical Research Letters. 42:4080-4088.   10.1002/2015gl063778   AbstractWebsite

The importance of tectonics and fluid flow in controlling cold seep habitats has long been appreciated at convergent margins but remains poorly understood in strike-slip systems. Here we present geophysical, geochemical, and biological data from an active methane seep offshore from Del Mar, California, in the inner California borderlands (ICB). The location of this seep appears controlled by localized transpression associated with a step in the San Diego Trough fault zone and provides an opportunity to examine the interplay between fluid expulsion and restraining step overs along strike-slip fault systems. These segment boundaries may have important controls on seep locations in the ICB and other margins characterized by strike-slip faulting (e.g., Greece, Sea of Marmara, and Caribbean). The strike-slip fault systems offshore southern California appear to have a limited distribution of seep sites compared to a wider distribution at convergent plate boundaries, which may influence seep habitat diversity and connectivity.

2014
Umhoefer, PJ, Maloney SJ, Buchanan B, Arrowsmith JR, Martinez-Gutierrez G, Kent G, Driscoll N, Harding A, Kaufman D, Rittenour T.  2014.  Late Quaternary faulting history of the Carrizal and related faults, La Paz region, Baja California Sur, Mexico. Geosphere. 10:476-504.   10.1130/ges00924.1   AbstractWebsite

The southwest margin of the Gulf of California has an array of active normal faults despite this being an oblique-divergent plate boundary with spreading centers that localized deformation along the plate boundary 2-3 million years ago. The Carrizal and Centenario faults form the western border fault of the Gulf of California marginal fault system within and south of La Paz Bay, and similar to 20-30 km west of the capital city of La Paz, Baja California Sur, Mexico. Geologic and geomorphic mapping, optically stimulated luminescence (OSL) geochronology, and paleoseismic investigations onshore, compressed high-intensity radar pulse (CHIRP) profiling offshore, and analysis of uplifted marine terraces in the footwall of the offshore Carrizal fault provide some of the first numerical and geometrical constraints on late Pleistocene-Holocene faulting along the Carrizal fault. The onshore Carrizal fault has ruptured with up to similar to 1-2 m of vertical displacement per event, likely producing similar to M 6.3-6.9 earthquakes, and at least two to three surface rupturing earthquakes have occurred since 22 ka. Onshore paleoseismic excavations and uplifted marine terraces on the western side of La Paz Bay both suggest offset rates of 0.1-0.2 mm/yr, with a footwall uplift rate of 0.13 mm/yr since 128 ka, and an approximately constant rate since marine oxygen-isotope stage (MIS) 11 terraces (420 ka). A CHIRP survey identified underwater fault scarps with heights ranging from 21 to 86 m on the Carrizal fault in La Paz Bay and from 3 to 5 m along the Centenario fault. The offshore Carrizal fault lies 8-10 km east of the western edge of La Paz Bay, forming a right step from the onshore Carrizal fault. The offshore Carrizal fault is the oldest fault of the fault system, and the fault likely grew in the latest Miocene to Pliocene in a complex way to the south toward the onshore Centenario and Carrizal faults. When the Alarcon spreading center started its modern rates at 2.4 Ma, the Carrizal fault likely slowed to the 0.1-0.2 mm/yr rates of the late Quaternary determined in this study.

Dong, SP, Ucarkus G, Wesnousky SG, Maloney J, Kent G, Driscoll N, Baskin R.  2014.  Strike-slip faulting along the Wassuk Range of the northern Walker Lane, Nevada. Geosphere. 10:40-48.   10.1130/ges00912.1   AbstractWebsite

A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). Recessional shorelines of pluvial Lake Lahontan that reached its highstand ca. 15,475 +/- 720 cal. yr B.P. are displaced similar to 14 m and yield a right-lateral slip-rate estimate approaching 1 mm/yr. The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is similar to 15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical dessication of the lake caused by upstream water diversion and consumption. High-resolution seismic CHIRP (compressed high intensity radar pulse) profiles acquired in Walker Lake reveal similar to 20 k.y. of stratigraphy that is tilted westward similar to 20-30 m to the Wassuk Range front, consistent with similar to 1.0-1.5 mm/yr (20-30 m/20 k.y.) of vertical displacement on the main range-bounding normal fault. Direct evidence of the northwest-trending right-lateral strike-slip fault is not observed, although a set of folds and faults trending N35 degrees E, conjugate to the trend of the strike-slip fault observed to the north, is superimposed on the west-dipping strata. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. Regardless of their genesis, the faults and folds appear to have been inactive during the past similar to 10.5 k.y. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strike-slip faults that occurs in regions of oblique extension such as the Walker Lane.

2013
Coyan, MM, Arrowsmith JR, Umhoefer P, Coyan J, Kent G, Driscoll N, Gutierrez GM.  2013.  Geometry and Quaternary slip behavior of the San Juan de los Planes and Saltito fault zones, Baja California Sur, Mexico: Characterization of rift-margin normal faults. Geosphere. 9:426-443.   10.1130/ges00806.1   AbstractWebsite

An array of north-striking, left-stepping, active normal faults cuts the southwest margin of the Gulf of California and across the southern tip of the Baja California peninsula. This is the gulf margin fault system of the oblique-divergent plate boundary within the Gulf of California. Detailed geologic and geomorphic mapping along the onshore San Juan de los Planes and Saltito fault zones allowed us to delineate geometric sections and to infer the tectonic history of the fault zones. To achieve a more complete understanding of these individual normal faults within a larger array, we mapped faults to similar to 10 km offshore using seismic CHIRP (compressed high-intensity radar pulse) profiling. Both onshore faults slip at a low rate and have a low total offset. Along the San Juan de los Planes fault zone, which is entirely onshore, the young, scarp-forming fault reactivated older faults to rupture a broad, low-relief pediment surface with thin Quaternary cover, reflecting a two-stage slip history along this fault zone. The offshore study suggests a northward continuation of the onshore Saltito fault, and a complex fault array north of the La Gata fault on the east side of the San Juan de los Planes basin extending northward to the west Cerralvo fault. Our results suggest relatively low rates of active faulting of <1 mm/yr across the San Juan de los Planes system of faults compared to high rates on the active gulf-axis system, and relatively higher rates on earlier Neogene gulf margin faults in other areas along the southwest Gulf of California margin.

Maloney, JM, Noble PJ, Driscoll NW, Kent GM, Smith SB, Schmauder GC, Babcock JM, Baskin RL, Karlin R, Kell AM, Seitz GG, Zimmerman S, Kleppe JA.  2013.  Paleoseismic history of the Fallen Leaf segment of the West Tahoe-Dollar Point fault reconstructed from slide deposits in the Lake Tahoe Basin, California-Nevada. Geosphere. 9:1065-1090.   10.1130/ges00877.1   AbstractWebsite

The West Tahoe-Dollar Point fault (WTDPF) extends along the western margin of the Lake Tahoe Basin (northern Sierra Nevada, western United States) and is characterized as its most hazardous fault. Fallen Leaf Lake, Cascade Lake, and Emerald Bay are three subbasins of the Lake Tahoe Basin, located south of Lake Tahoe, and provide an opportunity to image primary earthquake deformation along the WTDPF and associated landslide deposits. Here we present results from high-resolution seismic Chirp (compressed high intensity radar pulse) surveys in Fallen Leaf Lake and Cascade Lake, multibeam bathymetry coverage of Fallen Leaf Lake, onshore Lidar (light detection and ranging) data for the southern Lake Tahoe Basin, and radiocarbon dates from piston cores in Fallen Leaf Lake and Emerald Bay. Slide deposits imaged beneath Fallen Leaf Lake appear to be synchronous with slides in Lake Tahoe, Emerald Bay, and Cascade Lake. The temporal correlation of slides between multiple basins suggests triggering by earthquakes on the WTDPF system. If this correlation is correct, we postulate a recurrence interval of similar to 3-4 k.y. for large earthquakes on the Fallen Leaf Lake segment of the WTDPF, and the time since the most recent event (similar to 4.5 k.y. ago) exceeds this recurrence time. In addition, Chirp data beneath Cascade Lake image strands of the WTDPF offsetting the lake floor as much as similar to 7.5 m. The Cascade Lake data combined with onshore Lidar allow us to map the WTDPF continuously between Fallen Leaf Lake and Cascade Lake. This improved mapping of the WTDPF reveals the fault geometry and architecture south of Lake Tahoe and improves the geohazard assessment of the region.

Smith, SB, Karlin RE, Kent GM, Seitz GG, Driscoll NW.  2013.  Holocene subaqueous paleoseismology of Lake Tahoe. Geological Society of America Bulletin. 125:691-708.   10.1130/b30629.1   AbstractWebsite

Gravity-flow deposits recovered in a suite of sediment cores in Lake Tahoe were examined to determine if the event deposits were triggered by strong shaking from earthquakes on active faults within and in close proximity to the Lake Tahoe Basin. The acoustic character and distribution of individual lacustrine deposits as well as potential source regions were constrained by high-resolution seismic Chirp reflection and multibeam bathymetric data. Between 14 and 17 Holocene event deposits have been identified in Lake Tahoe, and examination of their source areas suggests they originated from different initiation points along the steep margin, with some being synchronous around the basin, as opposed to flood-related deposits. Lithologic characteristics, magnetic susceptibility, carbon and nitrogen isotopic signatures, opal content, and C-14 dating indicate that these event deposits are reworked lacustrine material. Radiocarbon dates indicate that the emplacement of these event deposit sediments correlates well with the late Holocene paleoseismic earthquake record developed for the Tahoe Basin. When taken alone, the causality of these events may appear ambiguous, but when the evidence is examined comprehensively, it suggests that strong shaking may likely have been the primary trigger for many of the event deposits observed in the lake throughout the Holocene. For example, four event deposits are assigned to Tahoe Basin faults. The most recent earthquakes occurred on the Incline Village fault (between 630 and 120 cal. yr B.P.); the southern segment of the West Tahoe fault (between 4510 and 4070 cal. yr B.P.); on the central and northern segments of the West Tahoe fault (5600-5330 cal. yr B.P.); and on the West Tahoe fault (between 7890 and 7190 cal. yr B.P.). The oldest of the four associated Tahoe Basin events coincides with the beginning of an extended period when Lake Tahoe was likely not spilling or spilling intermittently, and this suggests that active faulting and footwall uplift cut off the outlet at this time, exaggerating drought conditions downstream. Likewise, the event between 5600 and 5330 cal. yr B.P. on the West Tahoe fault may have exaggerated a smaller drought reflected downstream in Pyramid Lake. This event may also be the most recent event (MRE) on the largest segment of the West Tahoe fault. If correct, the period since the last rupture is approximately twice the estimated average recurrence interval for the Rubicon segment of the West Tahoe fault. A more complete Holocene record of strong shaking greatly extends the paleoseismic record in the region and indicates a combined recurrence interval of between 750 and 800 yr for all faults in the region.

Denny, JF, Schwab WC, Baldwin WE, Barnhardt WA, Gayes PT, Morton RA, Warner JC, Driscoll NW, Voulgaris G.  2013.  Holocene sediment distribution on the inner continental shelf of northeastern South Carolina: Implications for the regional sediment budget and long-term shoreline response. Continental Shelf Research. 56:56-70.   10.1016/j.csr.2013.02.004   AbstractWebsite

High-resolution geophysical and sediment sampling surveys were conducted offshore of the Grand Strand, South Carolina to define the shallow geologic framework of the inner shelf. Results are used to identify and map Holocene sediment deposits, infer sediment transport pathways, and discuss implications for the regional coastal sediment budget. The thickest deposits of Holocene sediment observed on the inner shelf form shoal complexes composed of moderately sorted fine sand, which are primarily located offshore of modern tidal inlets. These shoal deposits contain similar to 67 M m(3) of sediment, approximately 96% of Holocene sediment stored on the inner shelf. Due to the lack of any significant modern fluvial input of sand to the region, the Holocene deposits are likely derived from reworking of relict Pleistocene and older inner-shelf deposits during the Holocene marine transgression. The Holocene sediments are concentrated in the southern part of the study area, due to a combination of ancestral drainage patterns, a regional shift in sediment supply from the northeast to the southwest in the late Pleistocene, and proximity to modern inlet systems. Where sediment is limited, only small, low relief ridges have formed and Pleistocene and older deposits are exposed on the seafloor. The low-relief ridges are likely the result of a thin, mobile veneer of sediment being transported across an irregular, erosional surface formed during the last transgression. Sediment textural trends and seafloor morphology indicate a long-term net transport of sediment to the southwest. This is supported by oceanographic studies that suggest the long-term sediment transport direction is controlled by the frequency and intensity of storms that pass through the region, where low pressure systems yield net along-shore flow to the southwest and a weak onshore component. Current sediment budget estimates for the Grand Strand yield a deficit for the region. Volume calculations of Holocene deposits on the inner shelf suggest that there is sufficient sediment to balance the sediment budget and provide a source of sediment to the shoreline. Although the processes controlling cross-shelf sediment transport are not fully understood, in sediment-limited environments such as the Grand Strand, erosion of the inner shelf likely contributes significant sediment to the beach system. Published by Elsevier Ltd.

2012
Brothers, D, Harding A, Gonzalez-Fernandez A, Holbrook WS, Kent G, Driscoll N, Fletcher J, Lizarralde D, Umhoefer P, Axen G.  2012.  Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California. Geophysical Research Letters. 39   10.1029/2011gl050828   AbstractWebsite

Subduction of the Farallon plate beneath northwestern Mexico stalled by similar to 12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (similar to 24.5 degrees N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached similar to 40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting similar to 8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes. Citation: Brothers, D., A. Harding, A. Gonzalez-Fernandez, W. S. Holbrook, G. Kent, N. Driscoll, J. Fletcher, D. Lizarralde, P. Umhoefer, and G. Axen (2012), Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California, Geophys. Res. Lett., 39, L09307, doi:10.1029/2011GL050828.

Hogarth, LJ, Driscoll NW, Babcock JM, Orange DL.  2012.  Transgressive deposits along the actively deforming Eel River Margin, Northern California. Marine Geology. 303:99-114.   10.1016/j.margeo.2012.02.005   AbstractWebsite

New high-resolution CHIRP seismic data acquired along the Eel River margin, northern California, reveal that stratal architecture and sediment thickness of the Holocene transgressive deposits are, in large part, controlled by tectonic deformation and sediment supply. A thick (>20 m) transgressive deposit is observed across the Eel margin, a forearc basin that is undergoing active folding perpendicular to the coastline at rates of mm/yr. The transgressive deposits on the Eel margin exhibit marked variations in thickness alongshore; being thickest in the Eel River Syncline and thinnest over the Eureka Anticline. The divergent character of the infill in the syncline suggests that deposition is syntectonic. Fault displacement and structural relief observed along the transgressive surface are consistent with deformation rates measured onshore. The transgressive surface is offset similar to 0.5 m across the Eureka Anticline suggesting deformation has been active since similar to 10 ka. Two distinct acoustic units have been identified within the transgressive systems tract: a basal deposit that infills relief on the transgressive surface and an upper onlapping unit. The basal deposit infills lows along the outer shelf with a maximum thickness of 10 m and appears to be controlled by the early sea-level rise (21-7 ka) of the last deglaciation. It is separated from the overlying acoustically well-laminated unit by a pronounced surface of onlap. Moving shoreward along the inner shelf (<60 m water depth) the transgressive sequence thins and becomes acoustically transparent, which suggests that the finer-grained material is bypassing the inner shelf and being sequestered on the middle to outer shelf. It is here, on the inner shelf where tectonically induced accommodation exhibits the greatest control on sediment thickness. Thus, tectonics played a greater role when sea-level rise slowed after 7 ka to rates comparable to or slower than tectonic rates (similar to 3 mm/yr). On the middle to outer shelf offshore of the Eel River, there is evidence for progradation and highstand deposition. (C) 2012 Elsevier B.V. All rights reserved.

2011
Kleppe, JA, Brothers DS, Kent GM, Biondi F, Jensen S, Driscoll NW.  2011.  Duration and severity of Medieval drought in the Lake Tahoe Basin. Quaternary Science Reviews. 30:3269-3279.   10.1016/j.quascirev.2011.08.015   AbstractWebsite

Droughts in the western U.S. in the past 200 years are small compared to several megadroughts that occurred during Medieval times. We reconstruct duration and magnitude of extreme droughts in the northern Sierra Nevada from hydroclimatic conditions in Fallen Leaf Lake, California. Stands of submerged trees rooted in situ below the lake surface were imaged with sidescan sonar and radiocarbon analysis yields an age estimate of similar to 1250 AD. Tree-ring records and submerged paleoshoreline geomorphology suggest a Medieval low-stand of Fallen Leaf Lake lasted more than 220 years. Over eighty more trees were found lying on the lake floor at various elevations above the paleoshoreline. Water-balance calculations suggest annual precipitation was less than 60% normal from late 10th century to early 13th century AD. Hence, the lake's shoreline dropped 40-60 m below its modern elevation. Stands of pre-Medieval trees in this lake and in Lake Tahoe suggest the region experienced severe drought at least every 650-1150 years during the mid- and late-Holocene. These observations quantify paleo-precipitation and recurrence of prolonged drought in the northern Sierra Nevada. Published by Elsevier Ltd.

Brothers, D, Kilb D, Luttrell K, Driscoll N, Kent G.  2011.  Loading of the San Andreas fault by flood-induced rupture of faults beneath the Salton Sea. Nature Geoscience. 4:486-492.   10.1038/ngeo1184   AbstractWebsite

The southern San Andreas fault has not experienced a large earthquake for approximately 300 years, yet the previous five earthquakes occurred at similar to 180-year intervals. Large strike-slip faults are often segmented by lateral stepover zones. Movement on smaller faults within a stepover zone could perturb the main fault segments and potentially trigger a large earthquake. The southern San Andreas fault terminates in an extensional stepover zone beneath the Salton Sea-a lake that has experienced periodic flooding and desiccation since the late Holocene. Here we reconstruct the magnitude and timing of fault activity beneath the Salton Sea over several earthquake cycles. We observe coincident timing between flooding events, stepover fault displacement and ruptures on the San Andreas fault. Using Coulomb stress models, we show that the combined effect of lake loading, stepover fault movement and increased pore pressure could increase stress on the southern San Andreas fault to levels sufficient to induce failure. We conclude that rupture of the stepover faults, caused by periodic flooding of the palaeo-Salton Sea and by tectonic forcing, had the potential to trigger earthquake rupture on the southern San Andreas fault. Extensional stepover zones are highly susceptible to rapid stress loading and thus the Salton Sea may be a nucleation point for large ruptures on the southern San Andreas fault.

Olsen, MJ, Johnstone E, Kuester F, Driscoll N, Ashford SA.  2011.  New Automated Point-Cloud Alignment for Ground-Based Light Detection and Ranging Data of Long Coastal Sections. Journal of Surveying Engineering-Asce. 137:14-25.   10.1061/(asce)su.1943-5428.0000030   AbstractWebsite

This paper presents new techniques with corresponding algorithms to automate three-dimensional point-cloud georeferencing for large-scale data sets collected in dynamic environments where typical controls cannot be efficiently employed. Beam distortion occurs at the scan window edges of long-range scans on near-linear surfaces from oblique laser reflections. Coregistration of adjacent scans relies on these overlapping edges, so alignment errors quickly propagate through the data set unless constraints (origin and leveling information) are incorporated throughout the alignment process. This new methodology implements these constraints with a multineighbor least-squares approach to simultaneously improve alignment accuracy between adjacent scans in a survey and between time-series surveys, which need to be aligned separately for quantitative change analysis. A 1.4-km test survey was aligned without the aforementioned constraints using global alignment techniques, and the modified scan origins showed poor agreement (up to 8 m) with measured real-time kinematic global positioning system values. Further, the effectiveness of the constrained multineighbor alignments to minimize error propagation was evidenced by a lower average, range, and standard deviation of RMS values compared with various single neighbor techniques.