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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.

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.

Singh, SC, Sinha MC, Harding AJ, Kent GM, Barton PJ, Orcutt JA, White RS, Hobbs RW.  1999.  Preliminary results are in from mid-ocean ridge three-dimensional seismic reflection survey. Eos, Transactions, American Geophysical Union. 80   10.1029/99eo00129   Abstract

The first three-dimensional (3-D) seismic reflection survey of a mid-ocean ridge was shot in 1997 and, while it is still too early for firm interpretations of the data, it can be confirmed that significant crustal melt bodies have been located and one widely accepted model does not seem to apply to the presence of a robust magma supply there. The survey the Anatomy of a Ridge-Axis Discontinuity (ARAD) experiment, was centered over an overlapping spreading center (OSC) system that offsets the ridge axis at 9 degrees 03' north latitude on the East Pacific Rise (EPR) (Figure 1). It was conducted aboard the R/V Maurice Ewing during September and October and included a coincident 3-D crustal seismic tomography experiment.

Singh, SC, Collier JS, Harding AJ, Kent GM, Orcutt JA.  1999.  Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise. Geology. 27:219-222.   10.1130/0091-7613(1999)027<0219:sefahl>2.3.co;2   AbstractWebsite

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50-60-m-thick roof is overlain by a 150-200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

Singh, SC, Harding AJ, Kent GM, Sinha MC, Combier V, Bazin S, Tong CH, Pye JW, Barton PJ, Hobbs RW, White RS, Orcutt JA.  2006.  Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise. Nature. 442:287-290.   10.1038/nature04939   AbstractWebsite

The determination of melt distribution in the crust and the nature of the crust - mantle boundary ( the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres(1-4) but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.

Singh, SC, Kent GM, Collier JS, Harding AJ, Orcutt JA.  1998.  Melt to mush variations in crustal magma properties along the ridge crest at the southern East Pacific Rise. Nature. 394:874-878.   10.1038/29740   AbstractWebsite

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images(1-4) have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250-4,500 m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties(5-7). Here we report that the melt sill along the southern East Pacific Rise, which is about 50 m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14 degrees 00' S is characterized by three 2-4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80-90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

Stephen, RA, Harding AJ.  1983.  Travel Time Analysis of Borehole Seismic Data. Journal of Geophysical Research. 88:8289-8298.   10.1029/JB088iB10p08289   AbstractWebsite

A method is presented for reducing travel time data from multiple offset borehole seismic experiments to velocity-depth structure. The technique, which treats simultaneously data from any number of depths in the borehole in addition to surface data, is based on the linear inversion scheme of Dorman and Jacobson (1981). Given the parameters ζ(p) = T(p) + pX(p) and τ(p) = T(p) − pX(p), the method solves for depth to specified slownesses, assuming linear gradients (in velocity) between slowness values. A practical limitation on the use of surface travel time measurements to resolve velocity-depth structure is the necessity of assuming a surface velocity. This is not necessary for the case of borehole data. For a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel time curve. Thus a direct measure of the uppermost velocity of a section can be obtained from the travel time data of a shallow borehole receiver. Estimation of velocities from the inflection points of deeper receivers improves the resolution of the velocity-depth function, which would be obtained from surface data alone. The technique is applied to data from three borehole seismic experiments in oceanic crust. The experiments were carried out in the western Atlantic (crustal age ∼110 m.y.), the Gulf of California (crustal age ∼1 m.y.) and the Costa Rica Rift Area (crustal age ∼6 m.y.). All three experiments show relatively high upper crustal velocities ( >4.0 km/s), suggesting that layer 2A is not present even in the very young crust. All sites had over 100 m of sediment thickness, and it is postulated that sediment thickness and sediment permeability, not merely age, govern the velocity of the upper crust by accelerating the cementation of fractures and cracks.