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Papers by steven wesnousky
10 The largest historical earthquakes along the Himalayan Frontal Thrust reach values of Mw equal... more 10 The largest historical earthquakes along the Himalayan Frontal Thrust reach values of Mw equal to MW 8.711 8.9. A considered view of historical and paleoseismological observations reveals no clear and substantive 12 evidence that any has produced surface rupture along the Himalayan Frontal Thrust. The logical implication then 13 is that the paleoearthquake magnitude detection level for earthquakes on the MHT is on the order of Mw >8.7: 14 Large earthquakes with magnitudes less than this value may not be represented in the geologic record preserved 15 in fault scarps along the Himalayan Frontal Thrust. 16
Faulting in the Inner California Borderlands is complex. In the past, this region has undergone v... more Faulting in the Inner California Borderlands is complex. In the past, this region has undergone various deformational events such as extensional and rotational deformation and variable strike-slip deformation; this has imparted the geomorphology and fault structures observed offshore Southern California. Several hypotheses have been proposed to explain the current fault structures and the hazards they pose to populated coastal regions. The geometry and architecture of these structures can have significant implications for ground motions in the event of a rupture, and therefore impact working models of hazard assessment. Here, focusing on the Newport-Inglewood/Rose Canyon (NI/RC) and Palos Verdes (PV) fault zones, we use new and existing multibeam, CHIRP and Multi-Channel Seismic (MCS) data to describe the geometry of the fault system. We interpret reprocessed (prestack time migration) MCS data collected in 1979, 1986, and 2006 as well as newly acquired high-res MCS datasets collecte...
Geology, 2008
The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the n... more The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the northwest margin of the Indo-Asian collision zone. To address the quantitative contribution of the earthquake to plate convergence, we performed paleoseismological trench excavations at Nisar Camp site near Muzaffarabad across the middle section of the 2005 surface rupture. The fault strands exposed in the trench cut late Holocene fl uvial deposits and record evidence of both the 2005 and a penultimate event, supported by the presence of colluvial deposits and a downdip increase in displacement along the fault strands. The 2005 event produced a net slip of 5.4 m, and the penultimate earthquake exhibits a similar amount of slip. Radiocarbon ages and historical accounts loosely constrain the timing of the penultimate event between 500 and 2200 yr B.P.; however, the exposed section encompasses ~4 k.y. of stratigraphy, suggesting an average interevent interval of ~2 k.y. for the 2005 type events. We thus conclude that the 2005 event did not occur on the plate boundary megathrusts, but on intraplate active faults within the Sub-Himalaya. Consequently, the accumulated elastic strain around the complex northwestern margin of the Indo-Asian collision zone has not been signifi cantly released by the 2005 earthquake.
Bulletin of the Seismological Society of America, 2012
The Wassuk Range fault zone is an 80-km-long, east-dipping, high-angle normal fault that flanks t... more The Wassuk Range fault zone is an 80-km-long, east-dipping, high-angle normal fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from two alluvial fan systems truncated by the fault yield information on the vertical slip rate and Holocene earthquake history along the range front. At the apex of the Rose Creek alluvial fan, radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple earthquakes resulted in 7.0 m of vertical offset along the fault since ∼9400 cal B.P. These data yield a Holocene vertical slip rate of 0:7 0:1 mm=yr. The south trench exposure records at least two faulting events since ∼9400 cal B.P., with the most recent displacement postdating ∼2810 cal B.P. The north trench exposure records an ∼1 m offset between ∼610 cal B.P. and A.D. ∼1850, a 1.3-m minimum offset prior to ∼1460 cal B.P., and one earlier undated earthquake of a similar size. Variations in stratigraphy and limited datable material preclude a unique correlation of paleoevents between the two trenches. Approximately 25 km north, the range-front fault has truncated and uplifted a remnant of the Penrod Canyon fan by > 40 m since the surface was deposited ∼113 ka, based on cosmogenic dating of two large boulders. These data allow an estimate of the minimum late Pleistocene vertical slip rate at > 0:3-0:4 mm=yr for the Wassuk Range fault zone. Online Material: Tables summarizing radiocarbon and cosmogenic analyses, photos of rocks sampled for cosmogenic analyses, and figures summarizing cosmogenic exposure age estimates.
Bulletin of the Seismological Society of America, 2013
The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and ... more The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and Range that produced a small surface rupture (< 20 cm) during an M L 5.6 earthquake in 1950. We investigate the paleoseismic history of the Fort Sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950. Rupture of the Fort Sage fault ∼5:6 ka resulted in surface displacements of at least 0.8-1.5 m, implying earthquake moment magnitudes (M w) of 6.7-7.1. An older rupture at ∼20:5 ka displaced the ground at least 1.5 m, implying an earthquake of M w 6.8-7.1. A field of precariously balanced rocks (PBRs) is located less than 1 km from the surface-rupture trace of this Holoceneactive normal fault. Ground-motion prediction equations (GMPEs) predict peak ground accelerations (PGAs) of 0:2-0:3g for the 1950 rupture and 0:3-0:5g for the ∼5:6 ka paleoearthquake one kilometer from the fault-surface trace, yet field tests indicate that the Fort Sage PBRs will be toppled by PGAs between 0:1-0:3g. We discuss the paleoseismic history of the Fort Sage fault in the context of the nearby PBRs, GMPEs, and probabilistic seismic hazard maps for extensional regimes. If the Fort Sage PBRs are older than the mid-Holocene rupture on the Fort Sage fault zone, this implies that current GMPEs may overestimate near-fault footwall ground motions at this site.
Bulletin of the Seismological Society of America, 2011
Smith valley is bounded on its western edge by an active normal fault and the Pine Nut mountains.... more Smith valley is bounded on its western edge by an active normal fault and the Pine Nut mountains. Displacement on the fault is responsible for development of one of the westernmost basins of the Basin and Range province of the western United States. Interpretation of an exposure afforded by excavation of a trench across the range-bounding fault places the most recent surface-rupture earthquake after 5176 130 cal B.P. and suggests it was close in time to 3530 82 cal B.P. Utilization of tephrochonologic and cosmogenic analyses and mapping of fault scarps across the Artesia Road fan are the basis for putting forth an initial estimate of the late Pleistocene vertical slip rate of the fault at between 0:125 mm=yr and 0:33 mm=yr.
Bulletin of the Seismological Society of America, 2004
The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 Au... more The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 August 1954. More than 50 field measurements of surface offsets constrain the distribution of slip along the discontinuous and distributed rupture zone that formed during the earthquake sequence. Vertical offsets reach a maximum of ϳ0.8 m with the average vertical offset being ϳ0.2 m. In contrast to original reports, we see evidence for a right-lateral component of slip along portions of the rupture zone, including offset stream channels (0.5-1.0 m), left-stepping en echelon scarps, and a well-preserved, 100-m-long mole track. The right-slip component is consistent with focal plane solutions for the events and recent geodetic results. Previously unmapped surface ruptures now extend the known rupture length of the sequence by 25 km to a total of 70 km. Surface ruptures along the previously unmapped Fourmile Flat fault are subparallel to and form a 10-km left step to the southeast of the Rainbow Mountain fault. Event locations and anecdotal information indicate that the Fourmile Flat ruptures represent minor, primary surface rupture associated with the large 6 July aftershock, triggered ϳ11 hr after the initial 6 July Rainbow Mountain event. The paleoseismic histories of the Rainbow Mountain and Fourmile Flat faults, as recorded in natural and trench exposures, are different although both faults experienced three post ϳ15-ka surface rupturing events, including 1954. Bracketing ages for triultimate events on both faults do not overlap. However, constraints on the penultimate event for the Rainbow Mountain and triultimate event for the Fourmile Flat fault do overlap slightly, allowing the possibility that they may have ruptured close in time as in 1954. The Holocene slip rate for the Fourmile Flat fault (0.40 mm/yr) is similar to the post-latest Pleistocene rate for the Rainbow Mountain fault (0.20-0.46 mm/yr) even though the total length of the Fourmile flat (10 km) is much shorter than the overall length of the Rainbow Mountain rupture zone (ϳ60 km), indicating that even minor faults can be important for assessing regional strain rates and patterns.
Geomorphology, 2012
We present the results of dating glacial landforms in Venezuela using 10 Be terrestrial cosmogeni... more We present the results of dating glacial landforms in Venezuela using 10 Be terrestrial cosmogenic nuclide (TCN) analysis and optical stimulated luminescence (OSL). Boulders on the La Victoria and Los Zerpa moraines of the Sierra Nevada that mark the extent of the local last glacial maximum (LLGM) yield 10 Be TCN surface exposure ages of 16.7 ± 1.4 ka (8 samples). About 25 km to the west in the drainage basin of the Río Mucujún, 10 Be TCN dates for boulders on moraines at La Culata in the Sierra Nevada Norte yield a younger average age of 15.2 ± 0.9 ka (8 samples). The data suggest that glaciation across the Venezuelan Andes during the LLGM was asynchronous. The LLGM in Venezuela may be broadly concurrent with Heinrich Event 1 at~16.8 ka, implying that glaciation here is dominantly temperature driven. A moraine inset into the older laterofrontal moraines of La Culata has an age of 14.1 ± 1.0 ka (5 samples); it may have been deposited by a small Late Glacial readvance. Right-lateral offsets of the La Victoria and Los Zerpa moraines by the Boconó fault are each~100 m. The 10 Be TCN based Boconó fault slip rate is about b~5.5 to 6.5 mm a − 1 , notably less than the total right-lateral slip of 12 ± 2 mm a − 1 of shear documented across the Andes from geodesy. The 10 Be TCN dating of boulders on a faulted alluvial fan along the northwestern range front at Tucanízón yields a late Pleistocene uplift rate of the Andes at between~1.7 ± 0.7 mm a − 1. Glacial outwash has produced valley-fill sequences within the central Andean valley along the trace of the Boconó fault and Río Chama. The valley-fill has been incised to produce the 'meseta', a terrace surface that sits > 100 m above the Río Chama and on which the major city of Mérida is built. Geomorphic observations indicate that the meseta deposits were largely derived from the glaciers of La Culata. The OSL dating suggests that the final aggradation of the valley-fill deposits occurred rapidly over a period of about 5 to 6 ka and that the surface was abandoned and initially incised at~30 ka. The result implies Venezuelan valley fills record phases of aggradation that are likely modulated by climate change on glacial/Milankovitch timescales.
To improve the empirical basis for estimating the likely length of future earthquake ruptures on ... more To improve the empirical basis for estimating the likely length of future earthquake ruptures on mapped active faults, we measure map-scale complexities including fault bends, discontinuous rupture, overlaps, and fault-to-fault rupture from 67 historical ruptures and analyze the measurements for statistical relationships relevant to seismic hazard analysis. We observe that angles of bends at the ends of surface ruptures on strike-slip faults are systematically larger than interior bends (IBs), whereas corresponding interior and ending populations are similar for dip-slip events. The probability of a strike-slip rupture passing a bend decreases systematically with increasing bend angle roughly as PR 3:1 − 0:083 × A, in which PR is the passing ratio and A is the bend angle, with values ranging between 5° and 30°. The regression shows the likelihood of a strike-slip rupture propagating through a bend of 25° is about 50%. The maximum IB angles through which ruptures propagate, and the n...
Bulletin of the Seismological Society of America
Earthquake early warning (EEW) systems can quickly identify the beginning of a significant earthq... more Earthquake early warning (EEW) systems can quickly identify the beginning of a significant earthquake rupture, but the first seconds of seismic data have not been found to predict the final rupture length. We present two approaches for estimating probabilities of rupture length given the rupture initiation from an EEW system. In the first approach, bends and steps on the fault are interpreted as physical mechanisms for rupture arrest. Arrest probability relations are developed from empirical observations and depend on bend angle and step size. Probability of arrest compounds serially with increasing rupture length as bends or steps are encountered. In the second approach, time-independent rates among ruptures from the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), are interpreted to apply to the time-dependent condition in which rupture grows from a known starting point. Length probabilities from a Gutenberg–Richter magnitude–frequency relation provide a referen...
Seismological Research Letters
Geological, historical, and geophysical observations show that the entire Himalayan arc is poised... more Geological, historical, and geophysical observations show that the entire Himalayan arc is poised to produce a sequence of great earthquakes, possibly similar to that which occurred in the twentieth century along the Aleutian subduction zone. The human catastrophe in the densely populated countries astride the arc is likely to be unprecedented when these earthquakes occur.
Journal of Asian Earth Sciences
An exposure created by excavation of a trench across the Himalayan Frontal Thrust provides the ba... more An exposure created by excavation of a trench across the Himalayan Frontal Thrust provides the basis to interpret that a single earthquake produced vertical separation of ∼7 m at Khayarmara, a small community ∼80 km southwest of Kathmandu. The fault trace at Khayarmara is expresssed by a topographic ridge resulting from folding up-dip and toward the surface at the expense of greater fault slip taking place at depth. Structure, stratigraphy, and radiocarbon data are interpreted to indicate displacement occurred after about 1050 CE to 1200 CE. The timing and displacement at Khayarmara are compared to that reported previously at six contiguous sites that extend from 200 km to the west and 250 km to the east of Khayarmara, respectively. The comparison leads us to conclude that the surface rupture at Khayarmara was part of a ≥250 km long synchronous surface rupture earthquake of magnitude approaching if not surpassing Mw 9. We observe in the exposure no record of surface rupture associated with the great 1934 Bihar-Nepal earthquake.
Bulletin of the Seismological Society of America
We analyze a set of 76 mapped surface ruptures for relationships between geometrical discontinuit... more We analyze a set of 76 mapped surface ruptures for relationships between geometrical discontinuities in fault traces and earthquake rupture extent. The combined set includes 46 strike-slip, 16 normal, and 14 reverse mechanism events. The survey shows ∼90% of ruptures have at least one end at a mappable discontinuity, either a fault end or a step of 1 km or greater. Dip-slip ruptures cross larger steps than strike-slip earthquakes, with maxima of ∼12 versus ∼5 km, respectively. Large steps inside strike-slip ruptures are rare; only 8% (5 of 62) are ≥ 4 km. A geometric probability distribution model of steps as "challenges" to rupture propagation predicts that steps of 1 km or greater will be effective in stopping rupture about 46% of the time. The rate is similar for dip-slip earthquakes, but, within this set, steps are relatively more effective in stopping reverse ruptures and less effective in stopping normal ruptures. By comparing steps at rupture terminations to the set of steps broken in rupture, we can estimate the importance of step size for stopping rupture. We define the passing ratio for a given step size as the fraction of steps broken divided by the corresponding fraction that stop rupture. A linear model for steps from 1 to 6 km in strike-slip ruptures leads to the passing ratio 1:89-0:31× step width. Steps of ∼3 km are equally likely to be broken or to terminate rupture, and steps ≥ 6 km should almost always stop rupture. A similar comparison suggests that extensional steps are somewhat more effective than compressional steps in stopping ruptures. We also compiled the incidence of gaps of 1 km and longer in surface ruptures. Gaps occur in ∼43% of ruptures and occur more frequently in dip-slip than strike-slip ruptures.
Geophysical Research Letters
An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement ... more An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement on a fault plane dipping~22°has produced vertical separation across a scarp equal to 5.5 m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate that the displacement is the result of a single earthquake of 11.3 ± 3.5 m of dip-slip displacement that occurred 1146-1256 A.D. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 to >800 km and moment magnitudes M w of 8.6 to >9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800 years since the 1146-1256 A.D. earthquake to produce another earthquake displacement of similar size. Plain Language Summary The densely populated country of Nepal sits above the Himalayan Frontal Thrust fault. It is repeated displacements on this fault that are responsible for the uplift of the Himalaya mountains and considered capable of producing great earthquakes. Here we excavate a trench across the fault to show a great earthquake occurred 1146-1256 AD in eastern Nepal. It has been a sufficiently long time since then that stresses have accumulated to a level capable of producing another such great earthquake.
El Riesgo Sismico Prevencion Y Seguro 2001 Pags 53 68, 2001
Seismological Research Letters, 2016
At about 10 p.m. on the first day of October 1915, some 200 km west of Reno, Nevada, the U.S.A.'s... more At about 10 p.m. on the first day of October 1915, some 200 km west of Reno, Nevada, the U.S.A.'s Biggest Little City, and 100 km or so north of our nation's Loneliest Highway, the sky was cold and moonless. And in this particular location of the arid and sparsely settled Basin and Range, as is generally the case, absolutely nothing happened. The next day was different though. It was then, only nine years after the great 1906 California earthquake, that Nevada's contributions to earthquake science began. After a couple of quite strong earthquakes in the late afternoon, things had apparently quieted down sufficiently for the residents of the little mining community of Kennedy, adjacent to a little valley called Pleasant, to consider safely retiring for the night. It was not to be. At about 10:50 p.m., a yet more violent shaking occurred.
The San Andreas fault system and strike-slip faults of the Walker Lane accommodate most Pacific-N... more The San Andreas fault system and strike-slip faults of the Walker Lane accommodate most Pacific-North American plate motion. The style of faulting differs in the two regions. Whereas the San Andreas system is characterized by a relatively continuous and anastomosing set of faults, northwest trending zones of strike-slip faults within the Walker Lane are discontinuous and truncated by northeast trending cross-faults. The differences in style may be coupled to the observations that strike-slip faulting in the Walker Lane is transtensional, and that the cumulative amount of strike-slip in the Walker Lane (<60-80km) is less than that accumulated by the San Andreas. The Excelsior Trend includes a set of east to northeast-striking faults and is immediately between faults of the Central and Southern Walker lane, which are defined by zones of northwest-striking right-lateral and normal faults. Mapping within the Excelsior Trend shows that transfer of slip between the Central and Southern...
Bulletin of The Seismological Society of America, 2011
10 The largest historical earthquakes along the Himalayan Frontal Thrust reach values of Mw equal... more 10 The largest historical earthquakes along the Himalayan Frontal Thrust reach values of Mw equal to MW 8.711 8.9. A considered view of historical and paleoseismological observations reveals no clear and substantive 12 evidence that any has produced surface rupture along the Himalayan Frontal Thrust. The logical implication then 13 is that the paleoearthquake magnitude detection level for earthquakes on the MHT is on the order of Mw >8.7: 14 Large earthquakes with magnitudes less than this value may not be represented in the geologic record preserved 15 in fault scarps along the Himalayan Frontal Thrust. 16
Faulting in the Inner California Borderlands is complex. In the past, this region has undergone v... more Faulting in the Inner California Borderlands is complex. In the past, this region has undergone various deformational events such as extensional and rotational deformation and variable strike-slip deformation; this has imparted the geomorphology and fault structures observed offshore Southern California. Several hypotheses have been proposed to explain the current fault structures and the hazards they pose to populated coastal regions. The geometry and architecture of these structures can have significant implications for ground motions in the event of a rupture, and therefore impact working models of hazard assessment. Here, focusing on the Newport-Inglewood/Rose Canyon (NI/RC) and Palos Verdes (PV) fault zones, we use new and existing multibeam, CHIRP and Multi-Channel Seismic (MCS) data to describe the geometry of the fault system. We interpret reprocessed (prestack time migration) MCS data collected in 1979, 1986, and 2006 as well as newly acquired high-res MCS datasets collecte...
Geology, 2008
The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the n... more The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the northwest margin of the Indo-Asian collision zone. To address the quantitative contribution of the earthquake to plate convergence, we performed paleoseismological trench excavations at Nisar Camp site near Muzaffarabad across the middle section of the 2005 surface rupture. The fault strands exposed in the trench cut late Holocene fl uvial deposits and record evidence of both the 2005 and a penultimate event, supported by the presence of colluvial deposits and a downdip increase in displacement along the fault strands. The 2005 event produced a net slip of 5.4 m, and the penultimate earthquake exhibits a similar amount of slip. Radiocarbon ages and historical accounts loosely constrain the timing of the penultimate event between 500 and 2200 yr B.P.; however, the exposed section encompasses ~4 k.y. of stratigraphy, suggesting an average interevent interval of ~2 k.y. for the 2005 type events. We thus conclude that the 2005 event did not occur on the plate boundary megathrusts, but on intraplate active faults within the Sub-Himalaya. Consequently, the accumulated elastic strain around the complex northwestern margin of the Indo-Asian collision zone has not been signifi cantly released by the 2005 earthquake.
Bulletin of the Seismological Society of America, 2012
The Wassuk Range fault zone is an 80-km-long, east-dipping, high-angle normal fault that flanks t... more The Wassuk Range fault zone is an 80-km-long, east-dipping, high-angle normal fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from two alluvial fan systems truncated by the fault yield information on the vertical slip rate and Holocene earthquake history along the range front. At the apex of the Rose Creek alluvial fan, radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple earthquakes resulted in 7.0 m of vertical offset along the fault since ∼9400 cal B.P. These data yield a Holocene vertical slip rate of 0:7 0:1 mm=yr. The south trench exposure records at least two faulting events since ∼9400 cal B.P., with the most recent displacement postdating ∼2810 cal B.P. The north trench exposure records an ∼1 m offset between ∼610 cal B.P. and A.D. ∼1850, a 1.3-m minimum offset prior to ∼1460 cal B.P., and one earlier undated earthquake of a similar size. Variations in stratigraphy and limited datable material preclude a unique correlation of paleoevents between the two trenches. Approximately 25 km north, the range-front fault has truncated and uplifted a remnant of the Penrod Canyon fan by > 40 m since the surface was deposited ∼113 ka, based on cosmogenic dating of two large boulders. These data allow an estimate of the minimum late Pleistocene vertical slip rate at > 0:3-0:4 mm=yr for the Wassuk Range fault zone. Online Material: Tables summarizing radiocarbon and cosmogenic analyses, photos of rocks sampled for cosmogenic analyses, and figures summarizing cosmogenic exposure age estimates.
Bulletin of the Seismological Society of America, 2013
The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and ... more The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and Range that produced a small surface rupture (< 20 cm) during an M L 5.6 earthquake in 1950. We investigate the paleoseismic history of the Fort Sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950. Rupture of the Fort Sage fault ∼5:6 ka resulted in surface displacements of at least 0.8-1.5 m, implying earthquake moment magnitudes (M w) of 6.7-7.1. An older rupture at ∼20:5 ka displaced the ground at least 1.5 m, implying an earthquake of M w 6.8-7.1. A field of precariously balanced rocks (PBRs) is located less than 1 km from the surface-rupture trace of this Holoceneactive normal fault. Ground-motion prediction equations (GMPEs) predict peak ground accelerations (PGAs) of 0:2-0:3g for the 1950 rupture and 0:3-0:5g for the ∼5:6 ka paleoearthquake one kilometer from the fault-surface trace, yet field tests indicate that the Fort Sage PBRs will be toppled by PGAs between 0:1-0:3g. We discuss the paleoseismic history of the Fort Sage fault in the context of the nearby PBRs, GMPEs, and probabilistic seismic hazard maps for extensional regimes. If the Fort Sage PBRs are older than the mid-Holocene rupture on the Fort Sage fault zone, this implies that current GMPEs may overestimate near-fault footwall ground motions at this site.
Bulletin of the Seismological Society of America, 2011
Smith valley is bounded on its western edge by an active normal fault and the Pine Nut mountains.... more Smith valley is bounded on its western edge by an active normal fault and the Pine Nut mountains. Displacement on the fault is responsible for development of one of the westernmost basins of the Basin and Range province of the western United States. Interpretation of an exposure afforded by excavation of a trench across the range-bounding fault places the most recent surface-rupture earthquake after 5176 130 cal B.P. and suggests it was close in time to 3530 82 cal B.P. Utilization of tephrochonologic and cosmogenic analyses and mapping of fault scarps across the Artesia Road fan are the basis for putting forth an initial estimate of the late Pleistocene vertical slip rate of the fault at between 0:125 mm=yr and 0:33 mm=yr.
Bulletin of the Seismological Society of America, 2004
The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 Au... more The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 August 1954. More than 50 field measurements of surface offsets constrain the distribution of slip along the discontinuous and distributed rupture zone that formed during the earthquake sequence. Vertical offsets reach a maximum of ϳ0.8 m with the average vertical offset being ϳ0.2 m. In contrast to original reports, we see evidence for a right-lateral component of slip along portions of the rupture zone, including offset stream channels (0.5-1.0 m), left-stepping en echelon scarps, and a well-preserved, 100-m-long mole track. The right-slip component is consistent with focal plane solutions for the events and recent geodetic results. Previously unmapped surface ruptures now extend the known rupture length of the sequence by 25 km to a total of 70 km. Surface ruptures along the previously unmapped Fourmile Flat fault are subparallel to and form a 10-km left step to the southeast of the Rainbow Mountain fault. Event locations and anecdotal information indicate that the Fourmile Flat ruptures represent minor, primary surface rupture associated with the large 6 July aftershock, triggered ϳ11 hr after the initial 6 July Rainbow Mountain event. The paleoseismic histories of the Rainbow Mountain and Fourmile Flat faults, as recorded in natural and trench exposures, are different although both faults experienced three post ϳ15-ka surface rupturing events, including 1954. Bracketing ages for triultimate events on both faults do not overlap. However, constraints on the penultimate event for the Rainbow Mountain and triultimate event for the Fourmile Flat fault do overlap slightly, allowing the possibility that they may have ruptured close in time as in 1954. The Holocene slip rate for the Fourmile Flat fault (0.40 mm/yr) is similar to the post-latest Pleistocene rate for the Rainbow Mountain fault (0.20-0.46 mm/yr) even though the total length of the Fourmile flat (10 km) is much shorter than the overall length of the Rainbow Mountain rupture zone (ϳ60 km), indicating that even minor faults can be important for assessing regional strain rates and patterns.
Geomorphology, 2012
We present the results of dating glacial landforms in Venezuela using 10 Be terrestrial cosmogeni... more We present the results of dating glacial landforms in Venezuela using 10 Be terrestrial cosmogenic nuclide (TCN) analysis and optical stimulated luminescence (OSL). Boulders on the La Victoria and Los Zerpa moraines of the Sierra Nevada that mark the extent of the local last glacial maximum (LLGM) yield 10 Be TCN surface exposure ages of 16.7 ± 1.4 ka (8 samples). About 25 km to the west in the drainage basin of the Río Mucujún, 10 Be TCN dates for boulders on moraines at La Culata in the Sierra Nevada Norte yield a younger average age of 15.2 ± 0.9 ka (8 samples). The data suggest that glaciation across the Venezuelan Andes during the LLGM was asynchronous. The LLGM in Venezuela may be broadly concurrent with Heinrich Event 1 at~16.8 ka, implying that glaciation here is dominantly temperature driven. A moraine inset into the older laterofrontal moraines of La Culata has an age of 14.1 ± 1.0 ka (5 samples); it may have been deposited by a small Late Glacial readvance. Right-lateral offsets of the La Victoria and Los Zerpa moraines by the Boconó fault are each~100 m. The 10 Be TCN based Boconó fault slip rate is about b~5.5 to 6.5 mm a − 1 , notably less than the total right-lateral slip of 12 ± 2 mm a − 1 of shear documented across the Andes from geodesy. The 10 Be TCN dating of boulders on a faulted alluvial fan along the northwestern range front at Tucanízón yields a late Pleistocene uplift rate of the Andes at between~1.7 ± 0.7 mm a − 1. Glacial outwash has produced valley-fill sequences within the central Andean valley along the trace of the Boconó fault and Río Chama. The valley-fill has been incised to produce the 'meseta', a terrace surface that sits > 100 m above the Río Chama and on which the major city of Mérida is built. Geomorphic observations indicate that the meseta deposits were largely derived from the glaciers of La Culata. The OSL dating suggests that the final aggradation of the valley-fill deposits occurred rapidly over a period of about 5 to 6 ka and that the surface was abandoned and initially incised at~30 ka. The result implies Venezuelan valley fills record phases of aggradation that are likely modulated by climate change on glacial/Milankovitch timescales.
To improve the empirical basis for estimating the likely length of future earthquake ruptures on ... more To improve the empirical basis for estimating the likely length of future earthquake ruptures on mapped active faults, we measure map-scale complexities including fault bends, discontinuous rupture, overlaps, and fault-to-fault rupture from 67 historical ruptures and analyze the measurements for statistical relationships relevant to seismic hazard analysis. We observe that angles of bends at the ends of surface ruptures on strike-slip faults are systematically larger than interior bends (IBs), whereas corresponding interior and ending populations are similar for dip-slip events. The probability of a strike-slip rupture passing a bend decreases systematically with increasing bend angle roughly as PR 3:1 − 0:083 × A, in which PR is the passing ratio and A is the bend angle, with values ranging between 5° and 30°. The regression shows the likelihood of a strike-slip rupture propagating through a bend of 25° is about 50%. The maximum IB angles through which ruptures propagate, and the n...
Bulletin of the Seismological Society of America
Earthquake early warning (EEW) systems can quickly identify the beginning of a significant earthq... more Earthquake early warning (EEW) systems can quickly identify the beginning of a significant earthquake rupture, but the first seconds of seismic data have not been found to predict the final rupture length. We present two approaches for estimating probabilities of rupture length given the rupture initiation from an EEW system. In the first approach, bends and steps on the fault are interpreted as physical mechanisms for rupture arrest. Arrest probability relations are developed from empirical observations and depend on bend angle and step size. Probability of arrest compounds serially with increasing rupture length as bends or steps are encountered. In the second approach, time-independent rates among ruptures from the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), are interpreted to apply to the time-dependent condition in which rupture grows from a known starting point. Length probabilities from a Gutenberg–Richter magnitude–frequency relation provide a referen...
Seismological Research Letters
Geological, historical, and geophysical observations show that the entire Himalayan arc is poised... more Geological, historical, and geophysical observations show that the entire Himalayan arc is poised to produce a sequence of great earthquakes, possibly similar to that which occurred in the twentieth century along the Aleutian subduction zone. The human catastrophe in the densely populated countries astride the arc is likely to be unprecedented when these earthquakes occur.
Journal of Asian Earth Sciences
An exposure created by excavation of a trench across the Himalayan Frontal Thrust provides the ba... more An exposure created by excavation of a trench across the Himalayan Frontal Thrust provides the basis to interpret that a single earthquake produced vertical separation of ∼7 m at Khayarmara, a small community ∼80 km southwest of Kathmandu. The fault trace at Khayarmara is expresssed by a topographic ridge resulting from folding up-dip and toward the surface at the expense of greater fault slip taking place at depth. Structure, stratigraphy, and radiocarbon data are interpreted to indicate displacement occurred after about 1050 CE to 1200 CE. The timing and displacement at Khayarmara are compared to that reported previously at six contiguous sites that extend from 200 km to the west and 250 km to the east of Khayarmara, respectively. The comparison leads us to conclude that the surface rupture at Khayarmara was part of a ≥250 km long synchronous surface rupture earthquake of magnitude approaching if not surpassing Mw 9. We observe in the exposure no record of surface rupture associated with the great 1934 Bihar-Nepal earthquake.
Bulletin of the Seismological Society of America
We analyze a set of 76 mapped surface ruptures for relationships between geometrical discontinuit... more We analyze a set of 76 mapped surface ruptures for relationships between geometrical discontinuities in fault traces and earthquake rupture extent. The combined set includes 46 strike-slip, 16 normal, and 14 reverse mechanism events. The survey shows ∼90% of ruptures have at least one end at a mappable discontinuity, either a fault end or a step of 1 km or greater. Dip-slip ruptures cross larger steps than strike-slip earthquakes, with maxima of ∼12 versus ∼5 km, respectively. Large steps inside strike-slip ruptures are rare; only 8% (5 of 62) are ≥ 4 km. A geometric probability distribution model of steps as "challenges" to rupture propagation predicts that steps of 1 km or greater will be effective in stopping rupture about 46% of the time. The rate is similar for dip-slip earthquakes, but, within this set, steps are relatively more effective in stopping reverse ruptures and less effective in stopping normal ruptures. By comparing steps at rupture terminations to the set of steps broken in rupture, we can estimate the importance of step size for stopping rupture. We define the passing ratio for a given step size as the fraction of steps broken divided by the corresponding fraction that stop rupture. A linear model for steps from 1 to 6 km in strike-slip ruptures leads to the passing ratio 1:89-0:31× step width. Steps of ∼3 km are equally likely to be broken or to terminate rupture, and steps ≥ 6 km should almost always stop rupture. A similar comparison suggests that extensional steps are somewhat more effective than compressional steps in stopping ruptures. We also compiled the incidence of gaps of 1 km and longer in surface ruptures. Gaps occur in ∼43% of ruptures and occur more frequently in dip-slip than strike-slip ruptures.
Geophysical Research Letters
An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement ... more An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement on a fault plane dipping~22°has produced vertical separation across a scarp equal to 5.5 m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate that the displacement is the result of a single earthquake of 11.3 ± 3.5 m of dip-slip displacement that occurred 1146-1256 A.D. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 to >800 km and moment magnitudes M w of 8.6 to >9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800 years since the 1146-1256 A.D. earthquake to produce another earthquake displacement of similar size. Plain Language Summary The densely populated country of Nepal sits above the Himalayan Frontal Thrust fault. It is repeated displacements on this fault that are responsible for the uplift of the Himalaya mountains and considered capable of producing great earthquakes. Here we excavate a trench across the fault to show a great earthquake occurred 1146-1256 AD in eastern Nepal. It has been a sufficiently long time since then that stresses have accumulated to a level capable of producing another such great earthquake.
El Riesgo Sismico Prevencion Y Seguro 2001 Pags 53 68, 2001
Seismological Research Letters, 2016
At about 10 p.m. on the first day of October 1915, some 200 km west of Reno, Nevada, the U.S.A.'s... more At about 10 p.m. on the first day of October 1915, some 200 km west of Reno, Nevada, the U.S.A.'s Biggest Little City, and 100 km or so north of our nation's Loneliest Highway, the sky was cold and moonless. And in this particular location of the arid and sparsely settled Basin and Range, as is generally the case, absolutely nothing happened. The next day was different though. It was then, only nine years after the great 1906 California earthquake, that Nevada's contributions to earthquake science began. After a couple of quite strong earthquakes in the late afternoon, things had apparently quieted down sufficiently for the residents of the little mining community of Kennedy, adjacent to a little valley called Pleasant, to consider safely retiring for the night. It was not to be. At about 10:50 p.m., a yet more violent shaking occurred.
The San Andreas fault system and strike-slip faults of the Walker Lane accommodate most Pacific-N... more The San Andreas fault system and strike-slip faults of the Walker Lane accommodate most Pacific-North American plate motion. The style of faulting differs in the two regions. Whereas the San Andreas system is characterized by a relatively continuous and anastomosing set of faults, northwest trending zones of strike-slip faults within the Walker Lane are discontinuous and truncated by northeast trending cross-faults. The differences in style may be coupled to the observations that strike-slip faulting in the Walker Lane is transtensional, and that the cumulative amount of strike-slip in the Walker Lane (<60-80km) is less than that accumulated by the San Andreas. The Excelsior Trend includes a set of east to northeast-striking faults and is immediately between faults of the Central and Southern Walker lane, which are defined by zones of northwest-striking right-lateral and normal faults. Mapping within the Excelsior Trend shows that transfer of slip between the Central and Southern...
Bulletin of The Seismological Society of America, 2011