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Recent SCEC Published Research

The following list of recently published papers are based on research sponsored by SCEC. These papers are NOT available from SCEC. Most of the journals containing these papers are available at university libraries, and authors may also have reprints of their papers available by request.

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Recent Research:

(SCEC Contribution numbers are in bold)

230. Tsutsumi, H., R. S. Yeats, and G. J. Huftile, Late Cenozoic Tectonics of the Northern Los Angeles Fault System, California, Geological Society of America Bulletin, 113, pp. 454-468, 2001.

The northern Los Angeles fault system at and south of the southern range front of the Santa Monica Mountains includes potentially seismogenic structures directly beneath major population centers of the Los Angeles metropolitan area. For a better assessment of seismic hazards, we mapped late Cenozoic faults and folds in the northern Los Angeles basin, using an extensive set of oil-well and surface geologic data. The northern Los Angeles fault system developed through an early to late Miocene extensional regime and a Pliocene and Quaternary contractional regime. The Santa Monica, San Vicente, and Las Cienegas faults are early to late Miocene left-oblique normal faults that were later reactivated as reverse faults, suggesting that the rientation of reverse faults is largely controlled by Miocene extensional tectonics rather than by the post-Miocene stress field. Contractional tectonics began in early Pliocene time, with the reactivation of Miocene normal faults and initiation of reverse faults. Many Pliocene structures became inactive by the middle Pleistocene, and younger deformation is taken up by new active structures including the West Beverly Hills Lineament and an active strand of the Santa Monica fault. The West Beverly Hills Lineament is the northernmost segment of the Newport-Inglewood fault zone, which may have propagated northward to the Santa Monica Mountains in the Quaternary. The lineament divides the active Santa Monica-Hollywood fault system into distinct segments and bounds the Hollywood basin on the west. The uplift of the oxygen-isotope substage 5e marine terrace north of the City of Santa Monica suggests an average dip-slip rate <1.3 mm/yr for the Santa Monica Mountains thrust fault underlying and uplifting the Santa Monica Mountains. Crustal shortening across the northern Los Angeles fault system accounts for <1/3 of shortening rates between the San Gabriel Mountains and Palos Verdes Hills based on GPS observations.

470. Hilley, G. E., J R. Arrowsmith, and E. M. Stone, Inferring Segment Strength Contrasts and Boundaries along Low-Friction Faults Using Surface Offset Data, with an Example from the Cholame-Carrizo Segment Boundary along the San Andreas Fault, Southern California, Bulletin of the Seismological Society of America, 91, pp. 427-440, 2001.

Rupture segmentation arises from changes in fault geometry and strength. We use boundary element models of frictionless strike-slip fault segments to quantify how fault geometry and strength change earthquake surface offset distributions. Using these relationships between fault geometry, strength, and surface offsets, we can infer fault strength from the surface offsets in cases where the fault geometry can be independently constrained. This article includes normalized plots of the surface offset distribution expected from rupture along low-friction fault segments with strength contrasts of 1/4, 1/3, 1/2, 1, 2, 3, and 4 for a range of fault segment geometries. These plots may be used with offset data to constrain the strength of two coplanar, adjacent fault segments. This analysis is applied to the Cholame and Carrizo segments of the San Andreas Fault. The available surface offset data suggest that the offset increases where the fault deepens; in addition, the observed offset gradient at the segment boundary requires a 2/3-1/4 strength ratio of the Cholame to the Carrizo segment.

485. Madariaga, R. and K.B. Olsen, Criticality of Rupture Dynamics in Three Dimensions, Pure and Applied Geophysics, 157, pp. 1981-2001, 2001.

We study the propagation of seismic ruptures along a fault surface using a 4th order Finite Difference program. When prestress is uniform, rupture propagation is simple but presents some essential differences with the circular shear crack models of Kostrov. The best known is that rupture can only start from a finite initial patch (or asperity). The other is that rupture front becomes elongated in the inplane direction. Finally, if the initial stress is sufficiently high, the rupture front in the in-plane direction becomes super-shear and the rupture front develops a couple of "ears" in the in-plane direction. We show that we can understand these features in terms of single non-dimensional parameter kappa that is roughly the ratio of available strain energy to energy release rate. For low values of kappa rupture does not occur because Griffith's criterion is not satisfied. A bifurcation occurs when kappa is larger than a certain critical value (kappa_c that depends mildly on the geometry of the stress distribution on the fault. For even larger values of kappa rupture jumps to super-shear speeds. We then study carefully spontaneous rupture propagation along a long strike slip fault and along a rectangular asperity. As for the simple uniform fault we observe three regimes: no rupture for subcritical values of kappa, sub-shear speeds for a narrow range of supercritical values of kappa, and super-shear speeds for kappa > 1.3 kappa_c. Thus there seems to be a certain universality in the behavior of seismic ruptures.

493. Hardebeck, J. and E. Hauksson, Stress orientations obtained from earthquake focal mechanisms: What are appropriate uncertainty estimates? Bulletin of the Seismological Society of America, 91, no. 2pp. 250-262, 2001.

Crustal stress orientations provide important information about the mechanics of regional deformation. Numerous methods exist for inverting earthquake focal mechanisms for stress orientation, and the more widely-used methods usually obtain similar results for similar data sets. However, error estimates are highly variable, complicating the interpretation of results. The southern California stress field, for example, contains much statistically significant spatial and temporal variability according to the error estimates of one method (Michael [1984,1987a]), but very little according to those of another (Gephart and Forsyth [1984]). To resolve whether the southern California stress field is generally homogeneous or heterogeneous, it must be determined which of the error estimates best reflects the true inversion uncertainty. To do this, we test both methods on a suite of synthetic focal mechanism data sets containing random errors. The method of Gephart and Forsyth [1984] usually provides more accurate estimates of stress orientation, especially for high-quality data sets, but its confidence regions are in most cases too large. The method of Michael [1984,1987a] is more accurate for very noisy data sets and provides a more appropriate estimate of uncertainty, implying that the stress field in southern California is probably heterogeneous.

515. Louie, J. N., Faster, Better: Shear-Wave Velocity to 100 Meters Depth From Refraction Microtremor Arrays, Bulletin of the Seismological Society of America, 91, no. 2, pp. 347-364, 2001

Current techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use at most construction sites. They require large sources to be effective in noisy urban settings, or specialized independent recorders laid out in an extensive array. This work shows that microtremor noise recordings made on 200-m-long lines of seismic refraction equipment can estimate shear velocity with 20% accuracy, often to 100 m depths. The combination of commonly available equipment, simple recording with no source, a wavefield transformation data processing technique, and an interactive Rayleigh-wave dispersion modeling tool exploits the most effective aspects of the microtremor, spectral analysis of surface wave (SASW), and multichannel analysis of surface wave (MASW) techniques. The slowness-frequency wavefield transformation is particularly effective in allowing accurate picking of Rayleigh-wave phase-velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities, higher-mode Rayleigh waves, body waves, air waves, and incoherent noise. Two locations illustrate the application of this technique in detail: coincident with a large accelerometer microtremor array in Reno, Nevada; and atop a borehole logged for shear velocity in Newhall, Calif. Refraction equipment could duplicate microtremor results above 3 Hz, but could not estimate velocities deeper than 100 m. Refraction microtremor cannot duplicate the detail in the velocity profile yielded by a suspension logger, but can match the average velocity of 10-20 m depth intervals and suggest structure below the 100 m logged depth of the hole. Eight additional examples from southern California and New Zealand demonstrate that the refraction microtremor technique quickly produces good results from a wide range of hard and soft sites.

523. Eisner, L. and R. W. Clayton, A reciprocity method for multiple source simulations, Bulletin of the Seismological Society of America, 91, no. 3, pp. 553-560, 2001.

Reciprocity is applied to the situation where simulations are needed for a number of source locations, but relatively few receiver positions. By invoking source-receiver reciprocity, the number of simulations can be generally reduced to three times the number of receiver positions. The procedure is demonstrated for a heterogeneous medium with both single force and double couple sources. The numerical tests using a finite-difference implementation show that the reciprocal simulations can be performed with the same level of accuracy as the forward calculations.

534. R. Madariaga, S. Peyrat, and K.B. Olsen, Rupture Dynamics in 3-D: a review, Annali di Geofisica, in Problems in Geophysics for the New Millenium, a collection of papers in honour of Adam Dziewonski, E.Boschi, G. Ekstrom and A. Morelli, editors, pp. 89-110, 2001.

We review some recent results on the propagation of seismic ruptures along a planar fault surface subject to a friction law that contains a finite length scale and therefore has a well defined fracture energy flow. For this study we use the new fourth-order finite-difference method developed by Olsen, Archuleta and Madariaga. We look first at the rupture of an unbounded fault plane starting from a single circular asperity. Rupture propagation is simple but presents substantial differences with the self-similar shear crack model of Kostrov: the most important being that ruptures do not have circular symmetry because rupture resistance can never be uniform around the edge of the rupture front.We study the non-linear parameterization of this problem and show that there is a simple non-dimensional parameter kappa that controls the overall properties of rupture propagation. This number generalizes previous studies in 2D and 3D by Andrews, Das and Aki, Day, Burridge and many others. We demonstrate for several models that the rupture process has a bifurcation point at a critical value (kappa=kappa_c), so that for values of kappa less than critical rupture does not grow, while for values barely above critical ruptures grow indefinitely at sub-Rayleigh or sub-shear speeds. For values of kappa larger than 1.5 kappa_c an additional bifurcation occurs: rupture in the in-plane direction becomes super-shear and the rupture front develops a couple of "ears" in the in-plane direction. Finally we study a realistic stress distribution derived from the inversion of the accelerogramsof the Landers earthquake. This earthquake started from a critical patch that was probably a few km in radius and then rupture evolved under the control of stress and strength heterogeneities. We find that rupture in Landers occurred for a value of kappa that was barely above critical, which is the reason rupture was sub-shear on the average. In the presence of stress or strengthheterogeneity, rupture propagation becomes very complex and it propagates only in those regions where preexisting stress is high over relatively broad zones. Thus, rupture is a sort of ercolation process controlled by the local ratio of available energy to energy release rate.

539. Madariaga, M. and K.B. Olsen, Earthquake Dynamics, International Handbook of Earthquake and Engineering Seismology, Lee, Kanamori, and Jennings, editors, IASPEI, Chapter 7, 2001.

The propagation of seismic ruptures along a fault subject to an initial stress distribution and a set of frictional parameters has been studied extensively over the years. When prestress is uniform, rupture propagation is relatively simple with rupture speed in the in-plane direction being faster than that in the antiplane one. Spontaneous rupture fronts are therefore elongated in the in-plane direction. For uniform stress fields, most of the seismic observables (seismic moment, corner frequency, etc) scale with the fault length. Numerical models with a single length scale show that in the initial stages of rupture and near the rupture front stress and velocity fields scale with the slip weakening distance, a characteristic length scale of the friction law. When stress is heterogeneous, as seems to always be the case for earthquakes, rupture propagation is much more complicated. The rupture is then controlled by local length scales determined by the initial stress distribution as well as the local rupture resistance. Thus, the occurrence of phenomena such as rupture pulses with short rise times, rupture arrest, stopping phases,super-shear rupture velocities and spatial variation of slip arecontrolled by the heterogeneous stress field on the fault.State-of-the-art dynamic rupture models with realistic stress distributions suggest that large earthquakes are typically characterized by a complex rupture path with a spatially strong variation of rupture energy release. Such complex rupture behavior can be modeled numerically by methods such as boundary integral elements and finite differences for historic earthquakes with the friction parameters and stress distributions constrained bykinematic analyses and strong motion data.

552. Li, Y.G., and J. E. Vidale, Healing of the shallow fault zone from 1994 to 1998 after the 1992 M7.5 Landers, California, earthquake, Geophysical Research Letters, 28, no. 15, pp. 2999-3002, 2001.

We conducted seismic surveys at the Johnson Valley fault in 1994,1996, 1997, and 1998. We found that the shear velocity of the fault zone rock increased by ~1.2% between 1994 and 1996, and increased further by ~0.7% between 1996 and 1998. This trend indicates the Landers rupture zone has been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 1992 earthquake. The observedfault-zone strength recovery is consistent with a decrease of ~0.03 in the apparent crack density within the fault zone. The ratio of decrease in travel time for P to S waves changed from 0.75 in the earlier two years to 0.65 in the later two years between 1994 and 1998, suggesting that cracks near the fault zone are partially fluid-filled and have became more fluid saturated with time.

555. Gottschaemmer, E. and K.B. Olsen, Accuracy of the Explicit Planar Free-Surface Boundary Condition Implemented in a Fourth-Order Staggered-Grid Velocity-Stress Finite-Difference Scheme, Bulletin of the Seismological Society of America, 91, pp. 617-623, 2001.

We compute the accuracy of two implementations of the explicit planar free-surface boundary condition for 3-D fourth-order velocity-stress staggered-grid finite-differences, 1/2 grid apart vertically, in a uniform halfspace. Due to the staggered grid, theclosest distance between the free surface and some wavefield components for both implementations is 1/2 grid spacing. Overall, the differences in accuracy of the two implementations are small.When compared to a reflectivity solution computed at the staggered positions closest to the surface, the total misfit for all three components of the wavefield is generally found to be larger for the free surface co-located with the normal stresses, compared tothat for the free-surface co-located with the xz and yz stresses. However, this trend is reversed when compared to the reflectivity solution exactly at the free surface (the misfit encountered in staggered-grid modeling). When the wavefield is averaged across the free surface, thereby centering the staggered wavefield exactly on the free surface, the free surface condition co-located with the xz and yz stresses generates the smallest total misfit for increasing epicentral distance. For an epicentral distance/hypocentral depth of 10 the total misfit of this condition is about 15% smaller than that for the condition co-located with the normal stresses, mainly controlled by the misfit on the Rayleigh wave.

572. Oglesby, D. D., and S. M. Day, The Effect of Fault Geometry on the 1999 Chi-Chi (Taiwan) Earthquake, Geophysical Research Letters, 28, pp. 1831-1834, 2001.

The September 20, 1999 M7.6 Chi-Chi (Taiwan) earthquake produced enough near-source seismic data to verify many theoretical predictions of the effects of fault geometry on the physics of the earthquake process. These effects include increased motion on the hanging wall (peaked at the fault trace), a transition from thrust to significant left-lateral slip as one proceeds northward on the fault, and a mismatch between the near-field and far-field estimates of faulting style, energy, and apparent stress. Through rigorous 3-D dynamic models of this earthquake, all of these features can be seen to be robust consequences of the three-dimensional, asymmetric fault geometry and its angle with the free surface of the earth. The results of this study imply that for dipping faults that intersect the earth's surface, many important features of earthquakes are controlled by the fault geometry, and in principle might be predicted ahead of time.

573. Wdowinski,S., Y. Sudman, and Y. Bock, Geodetic detection of active faults in southern California, Geophysical Research Letters, 28, pp. 2321-2324, 2001.

A new analysis of velocities of geodetic markers straddling the San Andreas Fault System in southern California reveals that interseismic deformation is localized along a dozen sub-parallel narrow belts of high shear strain rate that correlate well with active geologic fault segments and concentrated zones of microseismicity. The highest shear strain rate (0.95 mstrain/year) is observed along the creeping Parkfield segment of the San Andreas Fault. High shear strain rates (0.3-0.6 mstrain/year) are also observed northward and southward of the big bend, whereas the big bend itself is characterized by a diffuse low magnitude shear strain rate (< 0.3 mstrain/year). Dilatational deformation is diffuse and of relatively low magnitude (< 0.2 mstrain/year), with the highest contraction rates reflecting the ongoing contraction within the Ventura and Los Angeles basins. Because no prior assumptions were made regarding the geology, tectonics, or seismicity of the region, our analysis demonstrates that geodetic observations alone can be used to detect active fault segments.

576. Yeats, R., B. Swanson, J. Dolan, T. Hopps, D. Kunitomi, and S. Bachman, Geology and Tectonics of the East Ventura Basin, Geology and Tectonics of the East Ventura Basin and San Fernando Valley, Wright, T.L, and R.S. Yeats, editors, San Joaquin Geological Society, Bakersfield, CA, 77, pp. 203-224, 2001.

This is a field-trip guide accompanying Field Trip 7, Geology and Tectonics of the east Ventura Basin, conducted on Sunday, April 8, 2001 as part of the Cordilleran Section, Geological Society of America meeting at Universal City, California. The field trip focuses on regional geology, engineering geology, petroleum geology, and active fault mapping. Active faults visited included the San Gabriel fault and the San Cayetano fault.

577. Yeats, R. S., Neogene Tectonics of the East Ventura and San Fernando Basins, California: An Overview, Geology and Tectonics of the East Ventura Basin and San Fernando Valley, Wright, T.L, and R.S. Yeats, editors, San Joaquin Geological Society, Bakersfield, CA, 77, pp. 9-36, 2001.

The east Ventura Basin and San Fernando Valley have had a long, complex history, including extension and volcanism in the Miocene, initial contraction in the Miocene, and contraction at higher rates in the Quaternary along exposed and blind faults. Most faults are reverse except for the San Gabriel fault, which is a combination of reverse and right-slip fault. A table of potentially-active faults in the two basins with estimated slip rates is included. The paper accompanies field trips to the east Ventura basin and San Fernando Valley and a theme session, San Fernando Valley Geology and Tectonics as part of the Cordilleran Section, Geological Society of America meeting April 9-11, 2001.

593. Freed, A.M. and J. Lin, Delayed triggering of the 1999 Hector Mine earthquake by viscoelastic stress transfer, Nature, 411, pp. 180-183, 2001.

Stress changes in the crust due to an earthquake can hasten the failure of a neighboring fault, inducing earthquake sequences in some cases. The 1999 Mw = 7.1 Hector Mine, California, earthquake occurred only 20 km from and 7 years after the 1992 Mw = 7.3 Landers quake, suggesting a potential triggering relationship between these two events. Uncertainties on the Landers quake slip distribution and rock friction properties have prevented a consensus on the sign and magnitude of the stress changes at the Hector Mine hypocenter caused by the Landers quake, with estimates varying from -1.4 to +0.5 bars. More importantly, the coseismic stress changes alone cannot satisfactorily explain the 7 year delay between the two events. Therefore, the close relationship of the two events argues for postseismic stress triggering mechanisms. Here we present the results of a 3-D viscoelastic model that simulates stress transfer from the ductile lower crust and/or upper mantle to the brittle upper crust in the 7 years following the Landers quake. Using viscoelastic parameters that can reproduce the observed post-Landers horizontal surface deformation, our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1-2 bars at the Hector Mine hypocenter during this time period, contributing to the eventual occurrence of the 1999 Hector Mine earthquake. These results attest to the importance of considering viscoelastic processes in our assessment of seismic hazard.

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