P and converted S waves observed in refraction stations FF1, FF2, and FF4 of the 1959 Fanfare cruise of the Scripps Institution of Oceanography are analyzed by synthetic seismogram modeling of the data using the reflectivity algorithm and by inversion of the P and S wave travel time data to obtain extremal bounds on the P and S velocity (vp and vs) profiles. While the FF1 data are inadequate for detailed analysis, the FF2 and FF4 data yield vp profiles displaying rapidly increasing velocity with depth in layer 2, a small velocity discontinuity between layers 2 and 3, gently increasing velocity with depth in layer 3, and a 1‐km‐thick Moho transition. The vs profiles for FF2 and FF4 show rapidly increasing velocity with depth in layer 2, fairly uniform velocities in the top of layer 3, a slight low‐velocity zone extending through most of layer 3, and a 1‐km‐thick Moho transition. Using theories of seismic wave velocities in cracked media and a laboratory velocity measurement made on a basalt sample from this site, a porosity of 18% is inferred for the top of the igneous crust at this site. A further reduction of porosity to 2% can explain the observed velocity gradients only to a depth of 0.6 km into the igneous crust. In the 0.8‐ to 1.5‐km depth interval, Poisson's ratio appears to drop below 0.27 to a minimum of 0.24, which may indicate a zone of trondhjemites or other quartz‐rich rocks at this depth or which may be related to state of fluid saturation of the rocks. Within layer 3, observed vp and vs agree well with laboratory velocity measurements of ophiolite samples from the western U.S. and from the Bay of Islands, Newfoundland. The observed velocities suggest the disappearance of hornblende and the appearance of augite and olivine with increasing depth in layer 3. There is no evidence for more than 30% serpentine anywhere within the crust or upper mantle at this site, except possibly within unresolvably thin zones or pods. Evidence is also given which suggests that velocities and velocity gradients in the shallow crust may be partly controlled by differential pressure (externally applied pressure minus pore fluid pressure) and its spatial gradients and that laboratory velocity measurements made on water‐saturated basalt samples at zero differential pressure are more representative of in situ velocities in the shallow crust than lab measurements made at which are usually employed as in situ conditions, namely, elevated externally applied pressure and zero pore fluid pressure. The factors affecting the efficiency of shear wave conversion at the sea floor are investigated, and the important role of basement vp and especially vs are shown. Since basement vs is very sensitive to fracture geometry, the high lateral variability of shear wave conversion may be related to variability in the extent and character of basement porosity. A useful explosive source function for marine synthetic modeling is presented, and a nomenclature for marine seismic phases is suggested.

[ 1 ] We estimate fracture energy on extended faults for several recent earthquakes by retrieving dynamic traction evolution at each point on the fault plane from slip history imaged by inverting ground motion waveforms. We define the breakdown work ( W b )a s the excess of work over some minimum traction level achieved during slip. W b is equivalent to "seismological" fracture energy ( G )i np revious investigations. Our numerical approach uses slip velocity as ab oundary condition on the fault. We employ a three-dimensional finite difference algorithm to compute the dynamic traction evolution in the time domain during the earthquake rupture. We estimate W b by calculating the scalar product between dynamic traction and slip velocity vectors. This approach does not require specifying ac onstitutive law and assuming dynamic traction to be collinear with slip velocity.I ft hese vectors are not collinear,t he inferred breakdown work depends on the initial traction level. We show that breakdown work depends on the square of slip. The spatial distribution of breakdown work in as ingle earthquake is strongly correlated with the slip distribution. Breakdown work density and its integral over the fault, breakdown energy,scale with seismic moment according to ap ower law (with exponent 0.59 and 1.18, respectively). Our estimates of breakdown work range between 4 10 5 and 2 10 7 J/m 2 for earthquakes having moment magnitudes between 5.6 and 7.2. We also compare our inferred values with geologic surface energies. This comparison might suggest that breakdown work for large earthquakes goes primarily into heat production.Citation: Tinti, E., P. Spudich, and M. Cocco (2005), Earthquake fracture energy inferred from kinematic rupture models on extended faults,

The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0–7.9; updating NGA ground-motion prediction equations (GMPEs) for the “average” horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.

We present a technique to infer the rupture history of an earthquake from near‐source records of ground motion. Unlike most previous studies, each point on the fault is assumed to slip only once, when the rupture front passes, with a spatially variable slip intensity. In this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time. We perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model. The inverse problem for the model perturbation is solved using a tomographic back projection technique. Smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible. Asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters. In test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity, provided the station coverage is adequate. We apply the technique to recordings of the April 24, 1984, Morgan Hill earthquake. The results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter. In particular, rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter. The rupture front surrounded the region, which subsequently failed with a component of rupture propagation back toward the hypocenter. Similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities. This segment of the fault ruptured with a large slip amplitude, releasing 12% of the total seismic moment from 4% of the total area of the aftershock zone. The surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault. However, the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right. In either case, our rupture model suggests that this segment of the fault represents an asperity, which initially resisted rupture but eventually ruptured massively. We estimate the shear fracture energy for this earthquake to be 2×106 J/m2.

Within the last decade a new picture of the oceanic crust has emerged from advances in seismic experimental design, instrumentation, and analysis techniques. In this new picture, layer 2 is a region in which velocity increases rapidly with depth. While there is evidence of finer structure within layer 2, the exact nature of this structure is still poorly resolved. Layer 3 is much more homogeneous vertically than layer 2 and appears to have gentle vertical velocity gradients and occasional low-velocity zones in v•, and vs. Although it has been observed at several sites, the widespread existence of a high-velocity basal crustal layer is in doubt. The thickness of the crust-mantle transition has been observed to vary between 0 and 2 km from site to site, and even at fiat lying, uncomplicated sites, seismic evidence for lateral heterogeneities on a scale of a few kilometers can be found. This seismic picture of the crust is in good agreement with the seismic velocities of rocks from ophiolite complexes and is consistent with the theoretically expected behavior of seismic velocities in porous, water-saturated rocks at elevated pressures. A review of velocity results obtained from use of synthetic seismogram modeling techniques is given, and the types of synthetic techniques suitable for marine work are described. anic crust and comment on both its current limitations and its advantages over the classical thick layered view of the crust. As useful companions to this paper we suggest the works of Kennett [1977] and Lewis [1978].Our main points in this paper are as follows: 1. Although the travel time data from marine refraction experiments can generally be well explained by oceanic crustal velocity models consisting of a small number of homogeneous layers, this cannot be construed to imply the necessary existence of homogeneous layers, velocity interfaces, or reflectors within the crust. Crustal velocity models in which velocity varies smoothly with depth in most depth regions generally explain seismic wave amplitude variations better than the classical thick homogeneously layered models.3. In oceanic 'layer 2,' seismic velocities increase quite rapidly with depth (velocity gradients are -1-2 s-'). Finer structure has been observed within layer 2, but it appears to be highly variable laterally. The layer 2-layer 3 transition may be a velocity discontinuity at some sites and a broad gradient zone at other sites. The finer structure of layer 2 is only • Now at U.S. Geological Survey, marginally within the ability of explosion seismology to resolve. Oceanic 'layer 3' has more gentle vertical seismic velocity gradients (---0.1 s-') and possible low-velocity zones inboth compressional (P) and shear (S) wave velocity. While a high-velocity (vp = 7.2-7.7 km/s) basal crustal layer may exist at some sites, there is little clear evidence that such a layer is widespread throughout the oceans. 5. The width of the crust-mantle transition is quite variable._6. Velocity gradients within the crust are more naturally interpreted petrol...