Steven G. Wesnousky

Predicting the endpoints of earthquake ruptures

The active fault traces on which earthquakes occur are generally not continuous, and are commonly composed of segments that are separated by discontinuities that appear as steps in map-view. Stress concentrations resulting from slip at such discontinuities may slow or stop rupture propagation and hence play a controlling role in limiting the length of earthquake rupture. Here I examine the mapped surface rupture traces of 22 historical strike-slip earthquakes with rupture lengths ranging between 10 and 420 km. I show that about two-thirds of the endpoints of strike-slip earthquake ruptures are associated with fault steps or the termini of active fault traces, and that there exists a limiting dimension of fault step (3–4 km) above which earthquake ruptures do not propagate and below which rupture propagation ceases only about 40 per cent of the time. The results are of practical importance to seismic hazard analysis where effort is spent attempting to place limits on the probable length of future earthquakes on mapped active faults. Physical insight to the dynamics of the earthquake rupture process is further gained with the observation that the limiting dimension appears to be largely independent of the earthquake rupture length. It follows that the magnitude of stress changes and the volume affected by those stress changes at the driving edge of laterally propagating ruptures are largely similar and invariable during the rupture process regardless of the distance an event has propagated or will propagate.

Displacement and Geometrical Characteristics of Earthquake Surface Ruptures: Issues and Implications for Seismic-Hazard Analysis and the Process of Earthquake Rupture

There now exist about three dozen historical earthquakes for which in- vestigators have constructed maps of earthquake rupture traces accompanied by de- scriptions of the coseismic slip observed along the fault strike. The maps and slip distributions are compiled here to place observational bounds on aspects of seismic- hazard analysis and fault mechanics. Analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults. The observations also give support to the ideas that there exists a process zone or volume of about 3-4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults, and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes. To this may be added the observation that the form of earthquake surface-slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike. Online Material: Maps of surface ruptures, digitized values and curve fits to surface-slip distributions, and notes and references for Tables 1 and 2.

Uplift and convergence along the Himalayan Frontal Thrust of India

Along the Himalayan thrust front in northwestern India, terrace deposits exposed 20 to 30 m above modern stream level are interpreted to have been uplifted by displacement on the underlying Himalayan Frontal Thrust. A radiocarbon age limits the age of the terrace to ≤1665±215 calendar BC (≤3663±215 radiocarbon years before present), yielding a vertical uplift rate of ≥6.9±1.8 mm/yr. In combination with published studies constraining the dip of the Himalayan Frontal Thrust fault to about 30° in the study area, the observed uplift rate equates to horizontal shortening across the Himalayan Frontal Thrust of ≥11.9±3.1 mm/yr and the slip rate of the Himalayan Frontal Thrust of ≥13.8±3.6 mm/yr. This is similar to previously reported rate estimates along the Himalayan arc based on displacement of older Plio-Miocene age rocks, or the much shorter records of geodesy and historical seismicity. The similarity is consistent with the idea that convergence across the Himalayan front has occurred at a relatively steady rate through time. The seismic expression of this deformation includes several great (M∼8) historical earthquakes which, due to lack of surface rupture during those events, have been attributed to their occurrence on blind thrusts. Yet, the occurrence of a possible fault scarp in the field area indicates that past earthquakes have been sufficiently large to rupture to the surface and produce coseismic scarps. These observations suggest a potential for earthquakes along the Himalayan Frontal Thrust larger than those observed historically.

Shoreline processes and the age of the Lake Lahontan highstand in the Jessup embayment, Nevada

The well-developed shoreline record of pluvial Lake Lahontan in the Jessup embayment, Nevada, is used to refine the history of late Pleistocene lake-level fluctuations and to assess controls on shoreline development and distribution. Controls on the strength and type of shorelines developed include local slope, the amount and characteristics of sediment available for transport, the availability of accommodation space, and length of time the lake level resides at a particular shoreline elevation. At the Sehoo highstand and during the early part of the regression, strong storm winds and waves from the south-southeast set up a clockwise net shore-drift pattern near the head of the embayment. Although significant differences in local slope, geometry of the shoreline, and wave energy existed in the embayment, crestal heights of constructional shoreline features formed at the highstand vary <2.6 m in elevation and hence provide a relatively precise marker of the highstand elevation. Radiocarbon dating of a camel bone preserved in high shoreline deposits indicates that the lake reached its highest elevation of 1338.5 m in the embayment and receded from that elevation immediately prior to 13 070 ± 60 yr B.P. Similar and slightly older radiocarbon ages on gastropod shells preserved in barrier deposits at 1327 m (13 280 ± 110 yr B.P.) and 1331 m (13 110 ± 110 yr B.P.) suggest that the final rise to the highstand was very rapid and that the lake maintained its highest stand for a very brief period of time, perhaps only for years or decades. The brevity of the highstand is reasonable in light of the recent formation of similar barrier features in modern Pyramid Lake, which formed in less than seven months due to a rapid increase in lake level.

Paleoseismological evidence of surface faulting along the northeastern Himalayan front, India: Timing, size, and spatial extent of great earthquakes

The similar to 2500 km long Himalayan arc has experienced three large to great earthquakes of M-w 7.8 to 8.4 during the past century, but none produced surface rupture. Paleoseismic studies have been conducted during the last decade to begin understanding the timing, size, rupture extent, return period, and mechanics of the faulting associated with the occurrence of large surface rupturing earthquakes along the similar to 2500 km long Himalayan Frontal Thrust (HFT) system of India and Nepal. The previous studies have been limited to about nine sites along the western two-thirds of the HFT extending through northwest India and along the southern border of Nepal. We present here the results of paleoseismic investigations at three additional sites further to the northeast along the HFT within the Indian states of West Bengal and Assam. The three sites reside between the meizoseismal areas of the 1934 Bihar-Nepal and 1950 Assam earthquakes. The two westernmost of the sites, near the village of Chalsa and near the Nameri Tiger Preserve, show that offsets during the last surface rupture event were at minimum of about 14 m and 12 m, respectively. Limits on the ages of surface rupture at Chalsa (site A) and Nameri (site B), though broad, allow the possibility that the two sites record the same great historical rupture reported in Nepal around A.D. 1100. The correlation between the two sites is supported by the observation that the large displacements as recorded at Chalsa and Nameri would most likely be associated with rupture lengths of hundreds of kilometers or more and are on the same order as reported for a surface rupture earthquake reported in Nepal around A.D. 1100. Assuming the offsets observed at Chalsa and Nameri occurred synchronously with reported offsets in Nepal, the rupture length of the event would approach 700 to 800 km. The easternmost site is located within Harmutty Tea Estate (site C) at the edges of the 1950 Assam earthquake meizoseismal area. Here the most recent event offset is relatively much smaller (<2.5 m), and radiocarbon dating shows it to have occurred after A.D. 1100 (after about A.D. 1270). The location of the site near the edge of the meizoseismal region of the 1950 Assam earthquake and the relatively lesser offset allows speculation that the displacement records the 1950 M-w 8.4 Assam earthquake. Scatter in radiocarbon ages on detrital charcoal has not resulted in a firm bracket on the timing of events observed in the trenches. Nonetheless, the observations collected here, when taken together, suggest that the largest of thrust earthquakes along the Himalayan arc have rupture lengths and displacements of similar scale to the largest that have occurred historically along the worlds subduction zones.

Slip Partitioning along Major Convergent Plate Boundaries

Along plate boundaries characterized by oblique convergence, earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors. Consequently, relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back-arc regions. The deformation behind the trench may take the form of strike-slip motion, back-arc extension, or some combination of both. We observe from our analysis of the Harvard Moment Tensor Catalog that convergent arcs characterized by back-arc spreading, specifically the Marianas and New Hebrides, are characterized by a large degree of slip partitioning. However, the observed rates, directions, and location of back-arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs, implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate. In the case of the Sumatran arc, where partitioning is accommodated by strike-slip faulting in the overriding plate, the degree of partitioning is similar to that observed along the Marianas, but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors. In the case of the Alaskan-Aleutian arc, where back-arc spreading is also absent, the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence. If partitioning in the Alaskan-Aleutian arc is accommodated by strike-slip faulting within the upper plate, the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either (1) the ratio of the depth extent of strike-slip faults behind the trenchZs to the subduction thrustZt increases westward along the arc, (2) the dip of the subduction thrust increases westward along the arc, or (3) the strength of the subduction thrust decreases westward along the arc.

Isostatic rebound, active faulting, and potential geomorphic effects in the Lake Lahontan basin, Nevada and California

The high shoreline of the late Pleistocene (Sehoo) lake in the Lahontan basin is used as a passive strain marker to delineate the magnitude and character of regional deformation since 13 ka. The elevations of 170 high shoreline sites document that the once horizontal (equipotential) shoreline, which traverses almost 4° of latitude and 3° of longitude, is now deflected vertically about 22 m. Most of the deformation is attributed to isostatic rebound, but a small down-to-the-north regional tilting also appears to contribute to the overall deformation pattern. Active faults locally offset the high shoreline, but cannot explain the regional upwarping attributed to isostatic rebound since 13 ka. Preliminary models of the rebound yield an upper mantle viscosity of 10 18 Pa s that implies a Maxwell relaxation time of about 300 yr. The rapid Earth response, coupled with the rapid fall in lake level at the end of Pleistocene time, may have acted to divert some of the major rivers flowing into the basin from one terminal subbasin to another. The regional deformation caused by the rebound may also have acted to control the present location of Honey Lake. These shoreline data, therefore, support the potential for a link be

Scaling of Fault Parameters for Continental Strike-Slip Earthquakes

The long-standing conflict between the predictions of elastic dislocation models and the observation that average coseismic slip increases with rupture length is resolved with application of a simple displacement-depth function and the assumption that the base of the seismogenic zone does not result from the onset of viscous re- laxation but rather a transition to stable sliding in a medium that remains stressed at or close to failure. The resulting model maintains the idea of self-similarity for earth- quakes across the entire spectrum of earthquake sizes.

Viscosity structure of the crust and upper mantle in western Nevada from isostatic rebound patterns of the late Pleistocene Lake Lahontan high shoreline

Received 13 July 2005; revised 26 October 2006; accepted 31 January 2007; published 8 June 2007. [1] Large lakes can both produce and record significant crustal deformation. We present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake Lahontan, in western Nevada, using a layered Maxwell viscoelastic model. The inferred viscosity structure depends on loading history. We use three variants of a well-documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure. A simple two-layer model, with an elastic plate over an inviscid half-space, fits the observed elevation pattern quite well, with a residual variance of 32% of the data variance. Using multilayered, finite viscosity models, the residual variance is reduced to 20% of the data variance, which is very near to the noise level. In the higher-resolution models, the viscosity is below 10 18 Pa s over the depth range from 80 to 160 km. The minimum viscosity is very similar to the value that has been seen in the eastern Great Basin, from similar analyses of Lake Bonneville shorelines, but the lowviscosity zone is thinner beneath Bonneville. Making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations. Additionally, we find that small variations in proposed loading models can result in presumably spurious density inversions, and suggest that this modeling approach provides a test for loading histories.

Oblique slip, slip partitioning, spatial and temporal changes in the regional stress field, and the relative strength of active faults in the Basin and Range, western United States

When viewed with stress transformation laws and an idealized physical model, observations of oblique slip and slip partitioning in the Basin and Range (western United States) are interpreted to show that (1) separate regions with the same net extension direction are not necessarily characterized by the same regional stress field, (2) fault systems exhibiting partitioning where one of the faults is near vertical generally do not require temporal changes in the stress field to explain the disparate slip vectors on the adjacent faults, and (3) the relative strengths of active fault zones may vary by more than an order of magnitude.