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Hadley, DM, Helmberger DV, Orcutt JA.  1982.  Peak Acceleration Scaling Studies. Bulletin of the Seismological Society of America. 72:959-979. AbstractWebsite

An acceleration time history can be decomposed into a series of operations that transfers energy from each point on the fault to the recording stationACC(t) = S * R * E * Qwhere S is the source time function, R represents rupture over a finite fault, E is the elastic propagation through the earth, and Q is the path attenuation, assumed to be linear. If these operators were exactly known, a deterministic approach to predicting strong ground motions would be straightforward. For the current study, E was computed from a velocity model that incorporates a stiff sedimentary layer over a southern California crust. A range of realistic rupture velocities have been obtained by other investigators and is incorporated into the simulation. Assumptions of the path averaged attenuation, Q, can be tested by comparing with observational data, as a function of distance, the parameters peak acceleration, and computed ML. This provides a check on both the high frequency (~ 5 Hz) and long-period (∼ 1 sec) behavior of E* Q. An average curstal shear wave Qβ of 300 is found to be compatible with observational data (ML = 4.5 to 5.0). Assumptions of S can be avoided by using real sources derived from accelerograms recorded at small epicentral distances (epicentral distance/source depth < 1). Using these operators, accelerograms have been simulated for strike-slip faulting for four magnitudes: 4.5; 5.5; 6.5; and 7.0. The shapes of the derived average peak ground acceleration (PGA) versus distance curves are well described by the simple equation PGA α [R + C(M)]−1.75, where R is the closest distance to the fault surface and C(4.5) = 6, C(5.5) = 12, C(6.5) = 22, and C(7.0) = 36 km.

Spudich, P, Orcutt J.  1980.  Petrology and Porosity of an Oceanic Crustal Site - Results from Wave Form Modeling of Seismic Refraction Data. Journal of Geophysical Research. 85:1409-1433.   10.1029/JB085iB03p01409   AbstractWebsite

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.

Jacobson, RS, Adair R, Orcutt J.  1984.  Preliminary Seismic Refraction Results Using a Borehole Seismometer in Deep-Sea Drilling Project Hole-395A. Initial Reports of the Deep Sea Drilling Project. 78:783-792.   10.2973/dsdp.proc.78b.113.1984   AbstractWebsite

Three seismic refraction lines shot to the Marine Seismic System, a vertical-component borehole seismometer in DSDP Hole 395A at 609 m sub-bottom depth, have been analyzed. Despite inconsistencies between various velocity determinations in the upper crust, the velocity of the uppermost 600 m appears to be high, about 4.5 km/s. Between600 m and 1.8 km sub-bottom, the crust yields an apparent velocity of 4.6 km/s. This value is somewhat lower than that determined near the base of the hole using acoustic logging tools (5.0-5.5 km/s), and is also lower than the average velocity of core samples from the same depth (5.7 km/s). The lower crust is unusually thin, only 1.8 to 2.4 km thick, andhas compressional-wave velocities of 6.8 to 7.3 km/s. Mantle velocities range from 7.8 to 8.2 km/s. There is little or no velocity gradient in the uppermost kilometers of the crust, but our results indicate widespread lateral inhomogeneities, supporting the observations based on refraction profiles shot for the original site survey.

Adair, RG, Orcutt JA, Jordan TH.  1987.  Preliminary-Analysis of Ocean-Bottom and Subbottom Microseismic Noise During the Ngendei Experiment. Initial Reports of the Deep Sea Drilling Project. 91:357-375.   10.2973/dsdp.proc.91.108.1987   AbstractWebsite

Simultaneous measurements of ambient microseismic noise at and below the seafloor are compared over the band 0.2-7.0 Hz using data collected during the Ngendei Seismic Experiment. Borehole data were collected with a triaxial set of seismometers which rested undamped at the bottom of DSDP Hole 595B, 124 m sub-bottom, 54 m within basementrock. Ocean-bottom data were collected with six 4-component ocean-bottom seismographs (OBSs) deployed at distances ranging from 0.5 to 30 km from the borehole. Noise spectra of displacement power density at both borehole and ocean-bottom sites typically displayed the microseism peak between 0.15 and 0.25 Hz. Vertical-component spectra fell off from this peak at 60-80 dB/decade with various peaks superposed. The peaks suggest that the noise consisted of seismic waves trapped in the seafloor. Vertical ocean-bottom and borehole noise levels are nearly identical at the microseism peak, on the order of I0^8 nm^2/Hz, but OBS values exceed Marine Seismic System (MSS) values by 10 dB or more at frequencies between 0.5 and 7 Hz. In the borehole, horizontal noise levels were essentially the same as the vertical-component levels. At the ocean bottom, horizontal noise levels exceed vertical levels between 0.4 and 7 Hz, but havecomparable values at the microseism peak. Ocean-bottom pressure measurements of ambient noise are comparable to other published measurements; Ngendei values are on the order of I0^4 Pa^2/Hz at the microseism peak. Leg 91 noise measurements contrast with those from the prototype MSS deployment during DSDP Leg 78B, when levels at a proximate seafloor site exceeded borehole levels by 10-30 dB over the entire band of valid comparisons (0.16-2.2 Hz). Between 0.3 and 2.2 Hz, Leg 78B borehole levels were greater than those of Leg 91 by 10-20 dB. Leg 78B spectra did not display any obvious peaks other than the microseism peak, where high vertical-component coherence was observed between pairs of OBSs separated by 0.7 km. At the quietest land sites, vertical-component noise levels at the microseism peak are 10-30 dB lower than those measured in the borehole during Leg 91. At higher frequencies, the noise levels measured in the borehole still exceed those of the quietest land sites, but are lower than at average land sites.

Shaw, P, Orcutt J.  1984.  Propagation of Pl and Implications for the Structure of Tibet. Journal of Geophysical Research. 89:3135-3152.   10.1029/JB089iB05p03135   AbstractWebsite

PL is a long-period (20 s or more) wave train beginning just after the P arrival in seismograms and continuing until the S arrival or the Rayleigh wave. The wave train can be observed at epicentral distances of about 5°–20° with instruments sensitive to these low frequencies. PL propagates as a partially trapped P-SV wave in the crust; S wave energy is lost to the mantle during propagation making PL a “leaky mode.” We study PL propagation for a variety of earth models using the synthetic seismogram algorithm wave number integration and find that the vertical travel time in the crust is the most important parameter controlling PL's oscillation period. This period can vary by more than a factor of two between oceanic and continental paths. P-SV leaky mode propagation includes many different modes; the low-frequency motion termed “PL” is only the first, or fundamental mode, in this family. A second, higher-frequency mode roughly equal in amplitude to the fundamental appears for models without a net positive velocity gradient in the crust. We use these results to match an observed PL wave train whose propagation path consisted almost entirely of the Tibetan Plateau. By considering first and second PL mode behavior we find that the Tibetan crust is about 85 km thick, with an uncertainty of about 20 km, and possesses a significant (≥0.01 s−1) positive velocity gradient. The presence of a large, low-velocity zone in the lower Tibetan crust thus seems unlikely. Much higher-frequency PL modes also appear in the Green's functions for layer over half space models and appear to be responsible for the high-frequency Pg phase, making PL and Pg different members of the same type of leaky mode propagation.

Sereno, TJ, Orcutt J.  1986.  The propagation of Pn. Ocean seismo-acoustics : low-frequency underwater acoustics. ( Akal T, Berkson JM, Eds.)., New York: Published in cooperation with NATO Scientific Affairs Division [by] Plenum Press Abstract
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