Gregory A. Lyzenga

The temperature of shock‐compressed water

Temperatures from 3300–5200 K were measured in liquid H2O shocked to 50–80 GPa (500–800 kbar). A six‐channel, time‐resolved optical pyrometer was used to perform the measurements. Good agreement with the data is obtained by calculating the temperature with a volume‐dependent Gruneisen parameter derived from double‐shock data and a heat capacity at constant volume of 8.7 R per mol of H2O.

Multiwavelength optical pyrometer for shock compression experiments

A system for measurement of the spectral radiance of materials shocked to high pressures ( approximately 100 GPa) by impact using a light gas gun is described. Thermal radiation from the sample is sampled at six wavelength bands in the visible spectrum, and each signal is separately detected by solid-state photodiodes, and recorded with a time resolution of approximately 10 ns. Interpretation of the records in terms of temperature of transparent sample materials is discussed. Results of a series of exploratory experiments with metals are also given. Shock temperatures in the range 4000-8000 K have been reliably measured. Spectral radiance and temperatures have been determined with uncertainties of 2%.

GPS observations of fault afterslip and upper crustal deformation following the Northridge earthquake

Global Positioning System (GPS) observations indicate that significant aseismic deformation occurred in the year following the January 17, 1994, Northridge earthquake. The postseismic observations show the same sense of motion as the coseismic. Compared with the coseismic displacements, the far-field postseismic motions (1–2 fault dimensions away) are proportionally larger than those seen in the near field. The postseismic data are best modeled with two faults: one on the rupture plane and one located in the shallow crust. The upper crustal fault may represent an actual fault or may be indicative of viscous relaxation occurring in the upper crust. The inferred afterslip and/or relaxation moment is approximately 2.9×1018 N m or 22% of the mainshock moment release. We expect that the moment release due to the afterslip and relaxation effectively reduce the earthquake hazard locally. It is not clear from this study how the postseismic deformation loads the surrounding faults or alters the state of stress on those faults.

Shortening and thickening of metropolitan Los Angeles measured and inferred by using geodesy

Geodetic observations using the Global Positioning System (GPS) and other techniques record a high rate of north-south shortening in an east-southeast–trending, 5–40-km-wide belt in northern metropolitan Los Angeles, California. Downtown Los Angeles is observed to be converging upon the southern San Gabriel Mountains at 6 mm/yr. Aside from the elastic strain that will be released during earthquakes rupturing the San Andreas and San Jacinto faults, east-west lengthening across northern metropolitan Los Angeles is minor, <2.5 mm/yr. Therefore north-south shortening is accommodated mainly by vertical crustal thickening.

Reconciling rapid strain accumulation with deep seismogenic fault planes in the Ventura Basin, California

Global Positioning System measurements across the east central Ventura basin, Transverse Ranges, southern California, before the nearby 1994 Northridge earthquake show high strain rates. Interpreting this rapid strain accumulation using the usual model of deep slip on a dislocation in a uniform elastic half-space requires slip to extend to within 5 km of the surface. Such shallow slip is difficult to reconcile with the substantial coseismic displacement at depths from 7 to 19 km during the Northridge earthquake. Here we model the displacement and velocity fields throughout the earthquake cycle using a two-dimensional finite element model with a viscoelastic rheology. Displacements are driven by far-field and basal velocity boundary conditions and by imposed periodic earthquakes on the thrust faults bounding the basin. The thrust faults rupture through an elastic upper crust to a depth of 15 km. After a transient stage, during which stresses and strains build up to quasi-equilibrium values, the behavior of the model becomes periodic. The sum of the coseismic displacement divided by the repeat interval, plus the average interseismic velocity, is equal to the geologic velocity. The temporal variation in surface velocity depends mainly on the Elsasser relaxation time (proportional to the product of the Maxwell time of the lower crust and the ratio of the thicknesses of the entire crust and viscoelastic lower crust). We are able to match the observed high strain rate only if we include the observed variations in elastic modulus associated with the deep basin sediments. The model reconciles geologic, geodetic, and seismological observations of deformation. There are trade-offs among the far-field convergence rate, the Elsasser time, the earthquake repeat time, and the time into the earthquake cycle. Acceptable convergence rates range from 8 mm/yr, for a relaxation time of the lower crust of 300 years, to 12 mm/yr, for a 30-year relaxation time.

The relation between the shock‐induced free‐surface velocity and the postshock specific volume of solids

The release of solids from a state of shock compression at a free surface is examined. For isentropic release, the postshock specific volume V′0 is shown to be constrained by V′0? (Ufs−Up)2/P1+V1, where (P1,V1) is the pressure‐volume Hugoniot state of shock compression and Ufs and Up are the free‐surface and shock particle velocities, respectively. When a sudden phase change occurs during the release process, this lower bound is increased, subject to simplifying assumptions about the phase transition.

The coseismic geodetic signature of the 1999 Hector Mine earthquake

The M = 7.1 Hector Mine earthquake ruptured the Lavic Lake fault near Twentynine Palms, CA at 09:46 UTC October 16, 1999. Because it occurred near the eastern edge of the Southern California Integrated GPS Network (SCIGN), a network of permanent, continuously recording GPS receivers for measuring the crustal deformation field around Los Angeles, CA, it was possible to determine the deformation associated with the earthquake with unprecedented speed and reliability. Thirty-four stations recorded displacements over the 3-sigma level. The displacements measured with GPS can be modeled by a fault 46.2±2.6 km long, 8.2±1.0 km wide, striking 330°, dipping 84° east, with 301±36 cm right lateral strike-slip, and 145±36 cm of east-up dip-slip, yielding a potency of 1.3 km³ and geodetic moment of 3.8 × 1026 dyne-cm. The trace and dip of the model fault is consistent with the observed ground rupture and seismic focal mechanisms.

Influence of anelastic surface layers on postseismic thrust fault deformation

We present the results of a systematic modeling study of postseismic deformation following blind thrust earthquakes. The results include qualitative and quantitative predictions of the surface movement caused by relaxation in viscoelastic near-surface layers. Finite element forward models are used in conjunction with elastic dislocation inversions to characterize the post-seismic deformation. A viscoelastic surface layer overlying a blind thrust fault in an elastic basement shows characteristic signatures of postseismic surface movement. Simple equivalent elastic dislocations located in the hanging wall wedge are found to provide an effective proxy for near-surface postseismic relaxation in two-dimensional numerical simulations. A model survey of a range of fault dip angles and layer geometries shows the time evolution and geometry of the proxy fault to be simply related to fault dip and sediment thickness. The results are of significance in the interpretation of postseismic Global Positioning System (GPS) strain data from the 1994 Northridge, California, earthquake and other similar events in regions characterized by poorly consolidated or otherwise anelastic layers overlying the brittle seismogenic zone.

Stress Patterns in an Interplate Shear Zone: An Effective Anisotropic Model and Implications for the Transverse Ranges, California

Strong lateral variations in geological structure within a transcurrent interplate deformation boundary have a substantial influence upon the way in which ambient stress is related to the relief of regional stress within the boundary zone. Much of the crustal deformational structure in southern California and environs consists of a conjugate wrench fault system. The Quaternary fault system consists of a series of parallel and sub-parallel strike-slip faults that are causally related to the horizontal interplate shearing. A prominent crustal structural inhomogeneity is the Transverse Ranges, where fault orientation is east-west, transverse to the dominant northwesterly trend. We investigate some of the consequences of this transverse inhomogeneity on the overall stress and strain field in the southern California region. The activity of the strike-slip (or wrench) system to the south and north of the Transverse Ranges suggests a mechanical model consisting of weak zones with a relatively strong degree of orientation. An effective anisotropy model is constructed based on: (1) a two-component laminate model consisting of competent unfaulted rock adjacent to incompetent faulted rock; (2) theoretical results for the weakening of a plate due to a doubly periodic array of cracks; and (3) finite element treatment of a checkerboard array of cracks. The fundamental parameter for weakening is = 1 - where is a non-dimensional form of Biot’s slide modulus. In the limit of 1 the crust becomes extremely weak and anisotropic, and as A -> 0 the condition of a strong, isotropic crust is recovered. The components of the stiffness (or compliance) matrix are directly related to the mechanical properties of a finite width fault zone, or to the average fault spacing and asperity density within a particular geological province, or both. An elastic plate model that incorporates the stress-strain channelling caused by multiple, oriented fault systems is constructed. The plate is assumed to be stressed by pure shearing forces maintained at infinity. The ambient field then corresponds to the north-south compressions, east-west extensions tectonic regime that dominates North-American-Pacific interplate shear along the San Andreas fault, California. Embedded within the plate is an elliptical inclusion in which multiple fault stress channelling also occurs. The inclusion thus mimics the misaligned structure of the Transverse Ranges in southern California. The boundary value problem associated with the model is treated both analytically and with finite element computations. The simple model predicts (i) the enhanced seismic energy release associated with the Transverse Ranges; and (ii) the clockwise rigid rotation indicated by a palaeomagnetic studies. The relatively simple nature of the model helps to isolate those features of the southern California tectonic stress regime that might be attributed to the transverse orientation of the Transverse Ranges. Stress channelled into the crosscutting tectonic structure from the ambient interplate field is significant. Contradirectionality alone cannot provide an explanation for the enhanced north-south compressive stress relative to east-west extension.

Rate change observed at JPLM after the Northridge Earthquake

Geodetic time series determined with the Global Positioning System indicate that the geodetic rate of a permanent site in Pasadena, California (JPLM) changed significantly after the 17 January 1994 Northridge California earthquake. Subtracting the pre-quake rate and co-seismic offset leaves 30±4 mm of integrated eastward excess motion observed in the three years following the earthquake. North and vertical components show excess motion of −11 plusmn;3 mm and 25±11 mm respectively. Local surveys to three additional points near JPLM changed by no more than 6 mm E, 3 mm N, and 15 mm V during the two years after the earthquake, ruling out the possibility of a local effect at the JPLM monument. The direction and size of the post-seismic displacements at JPLM are not consistent with additional slip on the fault which ruptured. The most rapid accumulation of excess motion occurs immediately after the earthquake, suggesting a relationship between the two events.