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The neotectonics of southern Alaska (USA) are characterized by a several hundred kilometers--wide zone of dextral transpressional that spans the Alaska Range. The Denali fault system is the largest active strike-slip fault system in interior Alaska, and it produced a Mw 7.9 earthquake in 2002. To evaluate the late Quaternary slip rate on the Denali fault system, we collected samples for cosmogenic surface exposure dating from surfaces offset by the fault system. This study includes data from 107 samples at 19 sites, including 7 sites we previously reported, as well as an estimated slip rate at another site. We utilize the interpreted surface ages to provide estimated slip rates. These new slip rate data confirm that the highest late Quaternary slip rate is ~13 mm/yr on the central Denali fault near its intersection with the eastern Denali and the Totschunda faults, with decreasing slip rate both to the east and west. The slip rate decreases westward along the central and western parts of the Denali fault system to 5 mm/yr over a length of ~575 km. An additional site on the eastern Denali fault near Kluane Lake, Yukon, implies a slip rate of ~2 mm/yr, based on geological considerations. The Totschunda fault has a maximum slip rate of ~9 mm/yr. The Denali fault system is transpressional and there are active thrust faults on both the north and south sides of it. We explore four geometric models for southern Alaska tectonics to explain the slip rates along the Denali fault system and the active fault geometries: rotation, indentation, extrusion, and a combination of the three. We conclude that all three end-member models have strengths and shortcomings, and a combination of rotation, indentation, and extrusion best explains the slip rate observations. [ABSTRACT FROM AUTHOR]

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 ~3-4 k.y. for large earthquakes on the Fallen Leaf Lake segment of the WTDPF, and the time since the most recent event (~4.5 k.y. ago) exceeds this recurrence time. In addition, Chirp data beneath Cascade Lake image strands of the WTDPF offsetting the lake fl oor as much as ~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. [ABSTRACT FROM AUTHOR]

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 14C 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. [ABSTRACT FROM AUTHOR]

Examines earthquake recurrence and rupture dynamics of the Himalayan Frontal Thrust in India. Displacement of two large surface rupture earthquakes; Reason of the uplift of the abandoned terraces of the Markanda River; Effects of the collision of India into Eurasia on the thrust.

Paleoseismologic trenches excavated across the North Anatolian fault near the village of Çukurçimen in north-central Turkey yield a complete record of surface ruptures for the past 2500 yr. The trenches provide mutually consistent evidence for the timing of the five most recent surface ruptures at the site, as well as at least two older events. These are interpreted as: (1) the historic 1939 Mw 7.9 earthquake; (2) the historic 1254 A.D. earthquake; (3) the historic 1045 A.D. earthquake; (4) an earthquake that probably occurred late in the interval between 250 and 540 A.D., possibly the historic 499 A.D. earthquake; and (5) an earthquake that occurred sometime between 770 and 50 B.C. (and probably between 230 and 50 B.C.). One additional earthquake occurred sometime between 1450 and 800 B.C., and at least one other surface rupture occurred between 2880 and 200 B.C., but the stratigraphic section at the site was not completely exposed for sediments older than 2500 yr. As a result, it is unclear whether the oldest event horizons represent a single earthquake, or multiple events. Our findings, when coupled with other published results and the historical record, enable us to construct a space-time history of earthquakes along the North Anatolian fault. The most striking aspects of this analysis are: (1) the rarity of earthquakes at any given place along the fault, suggesting that the fault typically ruptures in large, infrequent events; (2) earthquake occurrence is relatively temporally regular, with inter-event times that range from 200 to <900 yr, and vary by a factor of only 3-4; and (3) almost the entire length of the North Anatolian fault ruptured in short-lived clusters of activity in the seventeenth to eighteenth centuries and in the tenth to twelfth centuries, similar to the twentieth-century sequence. The magnitude and sequence of the individual ruptures, however, have varied considerably between clusters. Moreover, the paleoseismologic and historical data indicate that the North Anatolian fault does not always rupture in sequences similar to the twentieth-century cluster. These results suggest that the relatively regular recurrence of rare, large-magnitude earthquakes may be the expected mode of earthquake occurrence on mature strike-slip faults in settings where such faults are structurally isolated from other major seismic sources. [ABSTRACT FROM AUTHOR]