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

The July 27, 1976 Tangshan, China earthquake—A complex sequence of intraplate events

The Tangshan earthquake (M_s = 7.7), of July 27, 1976 and its principal aftershock (M_s = 7.2), which occurred 15 hr following the main event, resulted in the loss of life of over 650,000 persons in northeast China. This is the second greatest earthquake disaster in recorded history, following the 1556 Shensi Province, Chinese earthquake in which at least 830,000 persons lost their lives. Detailed analyses of the teleseismic surface waves and body waves are made for the Tangshan event. The major conclusions are: (1) The Tangshan earthquake sequence is a complex one, including strike-slip, thrust, and normal-fault events. (2) The main shock, as determined from surface waves, occurred on a near vertical right-lateral strike-slip fault, striking N40°E. (3) A seismic moment of 1.8 × 10^(27) dyne-cm is obtained. From the extent of the aftershock zone and relative location of the main shock epicenter, symmetric (1:1) bilateral faulting with a total length of 140 km may be inferred. If a fault width of 15 km is assumed, the average offset is estimated to be 2.7 meters with an average stress drop of about 30 bars. (4) The main shock was initiated by an event with a relatively slow onset and a seismic moment of 4 × 10^(26) dyne-cm. The preferred fault-plane solution, determined from surface-wave analyses, indicates a strike 220°, dip 80°, and rake −175°. (5) Two thrust events follow the strike-slip event by 11 and 19 sec, respectively. They are located south to southwest of the initial event and have a total moment of 8 × 10^(25) dyne-cm. This sequence is followed by several more events. (6) The principal aftershock was a normal-fault double event with the fault planes unconstrained by the P-wave first motions. Surface waves provide additional constraints to the mechanism to yield an oblique slip solution with strike N120°E, dip 45°SW, and rake −30°. A total moment of 8 × 10^(26) dyne-cm is obtained. (7) The triggering of lesser thrust and normal faults by a large strike-slip event in the Tangshan sequence has important consequences in the assessment of earthquake hazard in other complex strike-slip systems like the San Andreas

Surface-wave constraints on the August 1, 1975, Oroville earthquake

Observations of Love and Rayleigh waves on WWSSN and Canadian Network seismograms have been used to place constraints upon the source parameters of the August 1, 1975, Oroville earthquake. The 20-sec surface-wave magnitude is 5.6. The surface-wave radiation pattern is consistent with the fault geometry determined by the body-wave study of Langston and Butler (1976). The seismic moment of this event was determined to be 1.9 × 10^(25) dyne-cm by both time-domain and long-period (T ≥ 50 sec) spectral amplitude determinations. This moment value is significantly greater than that determined by short-period studies. This difference, together with the low seismic efficiency of this earthquake, indicates that the character of the source is intrinsically different at long periods from those aspects which dominate the shorter-period spectrum

SMART Cables for Observing the Global Ocean: Science and Implementation

The ocean is key to understanding societal threats including climate change, sea level rise, ocean warming, tsunamis, and earthquakes. Because the ocean is difficult and costly to monitor, we lack fundamental data needed to adequately model, understand, and address these threats. One solution is to integrate sensors into future undersea telecommunications cables. This is the mission of the SMART subsea cables initiative (Science Monitoring And Reliable Telecommunications). SMART sensors would “piggyback” on the power and communications infrastructure of a million kilometers of undersea fiber optic cable and thousands of repeaters, creating the potential for seafloor-based global ocean observing at a modest incremental cost. Initial sensors would measure temperature, pressure, and seismic acceleration. The resulting data would address two critical scientific and societal issues: the long-term need for sustained climate-quality data from the under-sampled ocean (e.g., deep ocean temperature, sea level, and circulation), and the near-term need for improvements to global tsunami warning networks. A Joint Task Force (JTF) led by three UN agencies (ITU/WMO/UNESCO-IOC) is working to bring this initiative to fruition. This paper explores the ocean science and early warning improvements available from SMART cable data, and the societal, technological, and financial elements of realizing such a global network. Simulations show that deep ocean temperature and pressure measurements can improve estimates of ocean circulation and heat content, and cable-based pressure and seismic-acceleration sensors can improve tsunami warning times and earthquake parameters. The technology of integrating these sensors into fiber optic cables is discussed, addressing sea and land-based elements plus delivery of real-time open data products to end users. The science and business case for SMART cables is evaluated. SMART cables have been endorsed by major ocean science organizations, and JTF is working with cable suppliers and sponsors, multilateral development banks and end users to incorporate SMART capabilities into future cable projects. By investing now, we can build up a global ocean network of long-lived SMART cable sensors, creating a transformative addition to the Global Ocean Observing System

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