No abstract

Relative crustal motions along active faults generate earthquakes, and repeated earthquake cycles build mountain ranges over millions of years. However, the long-term summation of elastic, earthquake-related deformation cannot produce the deformation recorded within the rock record. Here, we provide an explanation for this discrepancy by showing that increases in strain facilitated by plastic deformation of Earth’s crust during the earthquake cycle, in conjunction with isostatic deflection and erosion, transform relative fault motions that produce individual earthquakes to geologic deformations. We focus our study on the data-rich Santa Cruz Mountains, CA, USA and compare predicted and observed quantities for rock uplift, apatite (U-Th)/He thermochronology, topographic relief, 10 Be-based erosion rates, and interseismic surface velocities. This approach reconciles these disparate records of mountain-building processes, allowing us to explicitly bridge decadal measures of deformation with that produced by millions of years of plate motion.

The 1989, Mw = 6.9 Loma Prieta earthquake resulted in tens of lives lost and cost California almost 3% of its gross domestic product. Despite widespread damage, the earthquake did not clearly rupture the surface, challenging the identification and characterization of these hidden hazards. Here, we show that they can be illuminated by inverting fluvial topography for slip‐and moment accrual‐rates—fundamental components in earthquake hazard assessments—along relief‐generating geologic faults. We applied this technique to thrust faults bounding the mountains along the western side of Silicon Valley in the San Francisco Bay Area, and discovered that these structures may be capable of generating a Mw = 6.9 earthquake every 250–300 years based on moment accrual rates. This method may be deployed broadly to evaluate seismic hazard in developing regions with limited geological and geophysical information.

Transform faults have anomalously low rates of seismicity, but it’s not clear whether this reflects persistent earthquake-generating fault patches surrounded by creep, or the presence of creep and earthquakes at different times along the same patch. We use new, autonomous underwater vehicle high-resolution seafloor mapping to image the morphology of and offsets along transform fault segments in the Gulf of California, offshore Mexico. Fault zone structure imaged in this study shows evidence for the initiation and cessation of activity along individual fault splays over geologic time. A series of six identically offset depositional fans evidence 21–23 m of slip along the main transform fault, which could not have been produced by a single earthquake given the length of the transform. Rather, the lack of smaller-magnitude offsets indicates synchronous deposition and an absence of multiple slope failure–inducing earthquakes, which is consistent with the idea that creep and/or small-magnitude events occur asynchronously with large earthquakes in the slip history of a given transform fault segment.

Valleys produced by glaciers are morphologically distinct from those created by rivers, but these differences are surprisingly difficult to automatically detect using digital topographic data. This contribution tests whether glacial and fluvial valleys can be discriminated using scaling between watershed area and valley width. We present a method for estimating valley width at each point in digital topographic data by determining the cross-valley scale at which normalized principal curvature is minimized. We assess how well this approach measures valley width in synthetic valleys using artificial valley cross sections with random topographic noise. These width estimates are validated against manual measurements in glacial and fluvial valleys. Finally, we assess the sensitivity of these quantities to the pixel dimension of the input data to determine the detection limits of the method for moderate (20 m) and coarse-resolution (>30 m) digital elevation models (DEMs). We find that valley widths are underpredicted by a factor of 2.5 to 4; nonetheless, they are well correlated with both synthetically prescribed and empirically measured widths and so can be used as reliable measures of valley width when scaled to account for this underprediction. We find clear distinctions between valley width-catchment area relationships in glacial and nonglacial valleys, indicating that this methodology might be deployed globally to characterize the distribution and extent of glacial landscapes across Earth. In addition, this technique could be used to determine anomalous downstream width changes related to processes such as valley aggradation. Plain Language Summary Valleys formed by glaciers and rivers have distinct appearances, with river valleys having a distinct V-shaped cross section and glacial valleys a U shape. We study whether these differences can be automatically detected in elevation data by comparing the width of each type of valley at similar watershed sizes. We estimate valley width by fitting the shape of the valley with a mathematical equation that forms a curve. We test how accurately this method calculates valley widths compared to those measured by hand on the same data. While the mathematical fitting tends to estimate the valleys as less wide than measurements made by hand, it does so in a predictable way. By applying a correction for this bias, we can automatically classify valley size in landscapes and recognize areas where different landscape-forming processes may be occurring.