Shear-wave travel times from SS

The seismic shear phase SS is considered as a tool in the reconnaissance of the Earth. The Hibert transform is empirically verified as a reasonable mimic of the distortion incurred at the internal caustic in the propagation of SS. Travel times are obtained by a waveform correlation technique for 26 well-recorded SHSH waves from the 1968 Borrego Mountain earthquake. Significant variation is found in the travel-time residuals for paths reflected under the Canadian shield. A correlation of the variation with tectonic sub-province is suggested. The data are sufficiently precise to indicate lateral heterogeneity of several percent in the upper mantle velocities within the Canadian shield

Teleseismic short-period amplitudes: Source and receiver variations

Short-period P-wave amplitude data from nuclear explosions in the Soviet Union recorded by WWSSN stations in the United States are presented. Thirty-four events in five test sites are analyzed. The consistency and similarity of the initial P waveforms allow a stable amplitude measure. A well-defined amplitude pattern is obtained for each source region. The test sites at northern and southern Novaya Zemlya show a relative amplitude trend of a factor of 3 across the United States in their respective amplitude patterns. This is in contrast to two sites at Semipalatinsk which are in good relative agreement. A pattern of lateral variation of amplitude in the United States is obtained for a northern azimuth of approach. Stations situated on sediments are corrected for amplification effects. In contrast to previous studies, stations in the Western United States do not have systematically lower amplitudes than Eastern United States stations. Lowest amplitudes are found in Golden, Colorado (GOL) and Albuquerque, New Mexico (ALQ), a factor of 4 lower than high amplitude stations. Preliminary amplitude data are presented from earthquakes in the Kuriles and South America. Events are chosen for consistency of waveforms across the United States to minimize earthquake source and directivity effects. These earthquake data indicate that amplitude variations in the United States are azimuthally dependent

Long-period ground motion from a great earthquake

Direct body waves and fundamental surface waves are calculated for a credible, hypothetical great earthquake on the San Andreas Fault. The prototype event assumed is the Fort Tejon earthquake of January 9, 1857. Amplitudes and durations of long-period ground motion (T > 1 sec) are found for a receiver in downtown Los Angeles. Calculations are carried out for various epicenters, dislocation profiles, and time functions. Ground motion from Love radiation is found to be most important, with peak-to-peak amplitudes up to 14 cm and durations up to 5 min. This duration is a factor of 3 longer than has been assumed by previous design earthquakes whose estimates have been based upon acceleration criteria. Although the present result reveals several important features of long-period ground motion resulting from a great earthquake, more details of rupture propagation need to be known before a more definitive prediction can be made. The present result should be considered tentative

Focal mechanism of the August 1, 1975 Oroville earthquake

Long-period teleseismic P and S waves from the WWSS and Canadian networks are modeled to determine the focal parameters for the main shock in the Oroville earthquake series. Using the techniques of P first motions, wave-form synthesis, and phase identification, the focal parameters are determined as follows: dip 65°; rake −70°; strike 180°; depth 5.5 ± 1.5km; moment 5.7 ± 2.0 × 10^(24) dyne-cm; and a symmetric triangular time function 3 sec in duration. This is a north-south striking, westward dipping, normal fault with a small component of left-lateral motion. The time separation between the small foreshock and mainshock appears to be 6.5 sec at teleseismic distances, rather than 8.1 sec as observed at short distances

Scientific Rationale and Requirements for a Global Seismic Network on Mars

Following a brief overview of the mission concepts for a Mars Global Network Mission as of the time of the workshop, we present the principal scientific objectives to be achieved by a Mars seismic network. We review the lessons for extraterrestrial seismology gained from experience to date on the Moon and on Mars. An important unknown on Mars is the expected rate of seismicity, but theoretical expectations and extrapolation from lunar experience both support the view that seismicity rates, wave propagation characteristics, and signal-to-noise ratios are favorable to the collection of a scientifically rich dataset during the multiyear operation of a global seismic experiment. We discuss how particular types of seismic waves will provide the most useful information to address each of the scientific objectives, and this discussion provides the basis for a strategy for station siting. Finally, we define the necessary technical requirements for the seismic stations

The Great Sumatra-Andaman Earthquake of 26 December 2004

The two largest earthquakes of the past 40 years ruptured a 1600-kilometer-long portion of the fault boundary between the Indo-Australian and southeastern Eurasian plates on 26 December 2004 [seismic moment magnitude (M_w) = 9.1 to 9.3] and 28 March 2005 (M_w = 8.6). The first event generated a tsunami that caused more than 283,000 deaths. Fault slip of up to 15 meters occurred near Banda Aceh, Sumatra, but to the north, along the Nicobar and Andaman Islands, rapid slip was much smaller. Tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer