Tsunami!

Chapter from Living with earthquakes in the Pacific Northwest

Latest Quaternary paleoseismology and slip rates of the Longriba fault zone, eastern Tibet: Implications for fault behavior and strain partitioning

Although much work has been done on active tectonics of eastern Tibet, little is known about the Longriba fault zone and its role in strain partitioning. Whether its two sub-parallel strands (Longriqu and Maoergai faults) can rupture simultaneously in a large earthquake remains unknown. We conducted trenching combined with the interpretation of satellite imagery, field investigations, topographic surveys, and radiocarbon and Optically Stimulated Luminescence (OSL) dating to reconstruct paleoseismic history, and we used displaced terrace risers to estimate geological slip rates. Our results demonstrate that the Longriba fault zone is predominantly right-lateral with a small southeast-verging thrust component. Four surface-rupturing events occurred on the Longriqu fault at 5080 ± 90, 11,100 ± 380, 13,000 ± 260, and 17,830 ± 530calyr B.P. Together with our previous trenches on the Maoergai fault, we found that the last event probably ruptured both the Longriqu and Maoergai faults. Prior to the last event, the two strands of the Longriba fault zone experienced alternating earthquakes. The fault zone has a high potential for an earthquake larger than Mw 7. The slip rate of the Longriba fault zone decreases from ~ 7.5mm/yr in latest Pleistocene to ~ 2.1mm/yr in the Holocene, probably related to a slowing down of the eastern motion of the Tibetan Plateau. The comparison with slip rates at the Longmen Shan fault zone suggests that the Longriba fault zone has an equally important role in strain partitioning in eastern Tibet. This study is helpful to seismic hazard assessment and an understanding of deformation mechanism in eastern Tibet.Keywords: Slip rate, Paleoseismology, Longmen Shan fault zone, Eastern Tibet, Longriba fault zoneKeywords: Slip rate, Paleoseismology, Longmen Shan fault zone, Eastern Tibet, Longriba fault zon

Dislocation modeling of blind thrusts in the eastern Los Angeles basin, California

The East and West Coyote Hills in the eastern Los Angeles Basin are the surface expression of uplift accompanying blind reverse faulting. Folded Quaternary strata indicate that the hills are growing and that the faults underlying them are active. Detailed subsurface mapping in the East Coyote Oil Field shows that a previously mapped, reverse separation fault is predominantly an inactive, left‐lateral, strike‐slip fault that is not responsible for the uplift of the East Coyote Hills. The fault responsible for folding and uplift of the Coyote Hills does not cut wells in either the East or West Coyote Oil Fields. To characterize the geometry of the blind fault responsible for folding, we employ dislocation modeling. The dip and upper fault tip depths obtained from modeling suggest that the thrust fault beneath the Coyote Hills may be an extension of the Puente Hills blind thrust fault that continues westward beneath the Santa Fe Springs Oil Field. Modeling results suggest that the segment of the thrust fault responsible for folding the Coyote Hills would have accumulated 1500 m of reverse displacement over the last 1.2 Myr, yielding an average slip rate of 1.3 ± 0.5 mm/yr. The Santa Fe Springs segment of the fault has a slip rate of 1.5 ± 0.4 mm/yr for the last 1.2 Myr. The estimated moment magnitude for a reverse displacement earthquake on the Puente Hills blind thrust ranges from 6.6 to 7.2, depending on the length of the rupture. The estimated average recurrence interval for these earthquakes is 1700–3200 years.Keywords: Southern California, Blind thrust faults, Earthquake

Millennial slip rates of the Tazang fault, the eastern termination of Kunlun fault: implications for strain partitioning in eastern Tibet

The way of slip transformation and strain partitioning at the eastern termination of the Kunlun fault system remains unclear, and the question of whether this fault system is an important part for lateral extrusion of Tibetan crust is debatable. The Tazang fault is regarded as the easternmost continuation of the Kunlun fault system, and its late Quaternary activity is unknown. In this paper, we use displaced geomorphic features combined with radiocarbon and optically stimulated luminescence (OSL) dating to determine millennial slip rates along the Tazang fault. Our data yield a 1.4-3.2 mm/yr left-slip rate on the western Tazang fault, similar to that on the Maqu segment of the Kunlun fault. Tectonic geomorphology propose that displacement on the Kunlun fault is probably transferred to the Tazang fault via a pull-apart basin. The eastern Tazang fault has a dominant reverse motion that decreases eastward from ~1.5 mm/yr to 0.2-0.3 mm/yr at the easternmost part. Displaced terraces indicates that the eastern strand of the northern Longriba fault is active in the Holocene and has a ~0.8 mm/yr right-lateral slip rate with a ~0.3 mm/yr reverse component. Millennial slip rates and geodetic results show that the decrease of left-lateral motion along the Tazang fault is mainly transformed into crustal shortening along the nearly N-S-trending Longriba, Minjiang, and Huya faults, probably resulting in uplift of the Min Shan. Our results also indicate that the deformation along the Tazang fault is not transferred to beyond the border of the plateau, and the Kunlun fault is not an important tectonics for Tibetan extrusion.Keywords: Min Shan platform, Eastern Tibet, Slip rate, Displaced terrace riser, Kunlun fault, Tazang faul

The Metropolis and Evangelical Life: Coherence and Fragmentation in the ‘Lost City of London’

This article examines the interplay of different processes of cultural and subjective fragmentation experienced by conservative evangelical Anglicans, based on an ethnographic study of a congregation in central London. The author focuses on the evangelistic speaking practices of members of this church to explore how individuals negotiate contradictory norms of interaction as they move through different city spaces, and considers their response to tensions created by the demands of their workplace and their religious lives. Drawing on Georg Simmel’s ‘The Metropolis and Mental Life’, the author argues that their faith provides a sense of coherence and unity that responds to experiences of cultural fragmentation characteristic of everyday life in the city, while simultaneously leading to a specific consciousness of moral fragmentation that is inherent to conservative evangelicalism

Oblique strike-slip faulting of the central Cascadia submarine forearc

At least nine WNW trending left-lateral strike-slip faults have been mapped on the Oregon-Washington continental margin using sidescan sonar, seismic reflection, and bathymetric data, augmented by submersible observations. The faults range in length from 33 to 115 km and cross much of the continental slop. Five faults offset both the Juan de Fuca plate and North American plates and cross the plate boundary with little or no offset by the frontal thrust. Left-lateral separation of channels, folds, and Holocene sediments indicate active slip during the Holocene and late Pleistocene. Offset of surficial features ranges from 120 to 900 m, and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2.2 to 5.5 km of total slip. Near their western tips, fault ages range from 300 ka to 650 ka, yielding late Pleistocene-Holocene slip rates of 5.5 ± 2 to 8.5 ± 2 mm/yr. The geometry and slip direction of these faults implies clockwise rotation of fault-bounded blocks about vertical axes within the Cascadia forearc. Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the overlying forearc. The basement-involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate-coupling forces are probably insufficient to rupture the oceanic lithosphere. The set of sinistral faults is consistent with a model of regional deformation of the submarine forearc (defined to include the deforming slab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.Copyrighted by American Geophysical Union

The occupation of a box as a toy model for the seismic cycle of a fault

We illustrate how a simple statistical model can describe the quasiperiodic occurrence of large earthquakes. The model idealizes the loading of elastic energy in a seismic fault by the stochastic filling of a box. The emptying of the box after it is full is analogous to the generation of a large earthquake in which the fault relaxes after having been loaded to its failure threshold. The duration of the filling process is analogous to the seismic cycle, the time interval between two successive large earthquakes in a particular fault. The simplicity of the model enables us to derive the statistical distribution of its seismic cycle. We use this distribution to fit the series of earthquakes with magnitude around 6 that occurred at the Parkfield segment of the San Andreas fault in California. Using this fit, we estimate the probability of the next large earthquake at Parkfield and devise a simple forecasting strategy.Comment: Final version of the published paper, with an erratum and an unpublished appendix with some proof

Listric normal faulting on the Cascadia continental margin

Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E-W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low-angle (0.1°-2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction-related compression

Tectonic controls for transverse drainage and timing of the Xin-Ding paleolake breach in the upper reach of the Hutuo River, North China

The upper reach of the Hutuo River flows along the Xin-Ding basin and cuts a transverse drainage through Xizhou Mountain and Taihang Range into the North China Plain. Previous studies showed that the Xin-Ding basin was occupied by a lake during the Early-Middle Pleistocene. However, the timing of the paleolake breach and the mechanism for the creation of the transverse drainage are unknown. We constructed the fluvial terrace sequence in the upper reach of the Hutuo drainage combined with thermoluminescence (TL) and optically stimulated luminescence (OSL) dating, as well as the timescale of the overlying loess-paleosol units. Our results reveal that (1) five terraces (T5-T1) are developed along the upper reach of the Hutuo River, amongst which terraces T4-T1 were formed synchronously at ~600 ka, ~120-130 ka, ~21-26 ka and ~6-7 ka, respectively; (2) the creation of the transverse drainage and breach of the Xin-Ding paleolake occurred between ~600 ka and ~130 ka; (3) the mechanism for the creation of the transverse drainage is via river piracy of paleostreams on both sides of the drainage divide. Localized differential uplift and associated tilting of the Xizhou Mountain block during the Middle Pleistocene result in the formation of the transverse drainage and breach of the Xin-Ding paleolake.Keywords: Hutuo River, River piracy, Tectonic uplift, Loess-paleosol sequence, Transverse drainage, Xin-Ding paleolake, Fluvial terrac

New trends in active faulting studies for seismic hazard assessment

Vulnerability to earthquakes increases steadily as urbanization and development expand in areas that are prone to the effects of significant earthquakes. As virtually all of the largest earthquakes of the past decade demonstrated, the development of large cities in high seismicity areas is often based on an insufficient knowledge or distorted perception of the local seismic hazard, a condition often worsened by the construction of seismically unsafe buildings and infrastructures