[work in progress] Dave Harris. Tern Hauk

Transcription

1 UCRLJC PREPRNT Can T Phases Be Used to Map Blockage? [work in progress] Dave Harris Tern Hauk Presented at the 1995 Monitoring Technologies Conference Westfields nternational Conference Center Chantilly, Virginia, May 15-18,1995 May 1995 n Thisisa prcphtofapapcrintcndcd forpublicationina journalorprocccdings. Since changes may be made bcforc publication, this preprint is made available with the understanding that it will not bc cited or reproduced without the permission of thc author. ' DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof. nor any of their employees, makes any warranty. express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement. recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. MASTER

2 DSCLAMER Portions of this document may be illegible in electronic image products. mages are produced from the best available original document.

3 Can T Phases Be Used to Map B oc kag e? [work in progress] Dave Harris and Terri Hauk Lawrence Livermore National Laboratory 1995 ARPA Monitoring Technologies Conference May 16,1995 *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livemore National Laboratory under contract No. W-745-Eng-48.

4 ntroduction The placement of stations in a CTBT hydroacoustic monitoring network is controlled, in large part, by the presence of bathymetric features or land masses that block propagation. n the absence of blocking features, propagation is very efficient in the SOFAR channel, allowing surveillance over large basins with hydrophone networks that are sparse compared to seismic networks. Blockage can be estimated from theoretical calculations of acoustic attenuation. However, some measurement of blockage is desirable, since not all parameters required for attenuation calculations are known everywhere with complete certainty. n addition, acoustic modeling that correctly accounts for bottom interactions and diffractions around complex bathymetric structures is still a subject of active research. While calibration of attenuation with controlled sources is best, it is also prohibitively expensive [Munk et al., This fact motivated us to look for sources of opportunity. The T phases generated by undersea earthquakes are known to be sensitive to interruptions of the SOFAR channel [Walker, McCreery, and Hiyoshi; Earthquakes along ridges may illuminate regions of interest to define blockage areas. Figure 1 shows a collection of earthquake epicenters along the East Pacific Ridge and the Pacific-Antarctic Ridge. Great circle paths extending from these source locations to the San Nicolas s. SOSUS arrays are color coded to illustrate which events produced observable T phases (red)

5 9 and which did not (black) at the receivers, based on visual interpretation of filtered beam data. Some of the observations are shown in Figure 2, which displays beam data around the expected T phase arrival times at the receiver. No T phases are observed for events with great circle paths traversing the Tasman Sea or the Campbell Plateau and the Chatham Rise, suggesting strong attenuation in these regions. These observations are consistent with the failure of the Heard sland transmissions to be detected through the Tasman Sea [Forbes, The pattern of T phase attenuation may be useful to assess the actual coverage of a CTBT network, once the network has been in operation for a few years. However, both attenuation along the path (bathymetric shadowing and other propagation effects) and coupling at the source affect signal strength at the receiver. A technique to assess coverage, even crudely, must account for the uncertainty in coupling (which is large, Johnson and Northrop [1966] ).

6 Distinguishing Coupling and Path Effects To resolve the ambiguity between path and source effects, a second hydroacoustic station near the source to determine coupling would be ideal. Lacking such data for our study area, we propose to quantify T phase coupling for shallow ridge earthquakes with an empirical probabilistic model. We assume that the coupling mechanisms for such events are similar on ridges worldwide and will result in similar distributions of signal strength in the absence of path attenuation. We will use the following model for signal strength (energy in decibels) measured at the receiver: 1OlogE = a mb loga + 21Ogc + 21gP + p * mb : USGS reported earthquake magnitude A: event - receiver distance, in correction for cylindrical spreading and intrinsic seawater attenuation 21g C : source amplitude coupling factor (db) 2logP : path amplitude factor (a), which accounts for blockage a and p: model parameters to be estimated We propose to treat the coupling factor as a random variable, and to estimate its distribution along with the model scaling parameters a and p, using a population of events with free

7 acoustic paths (zero path attenuation factor, 2logP) to the receivers near San Nicolas sland. seismic magnitude (mb) 21gc Coupling calibration from events with presumed free acoustic paths We will use the resulting calibration to estimate the attenuation factor (and the reliability of the estimate) along paths suspected to be blocked or partially blocked.

8 Choice of Events for Calibration and Mapping Figure 3 shows the locations of the seismic events which have occurred since we began recording at San Nicolas s. in April, We intend to divide these events into two populations, a calibration population selected from events in the southeast Pacific (red) and an attenuation mapping population (black). We currently are searching the events indicated in red for those with free acoustic paths to the San Nicolas s. arrays. An example of the procedure we use is shown in Figure 4. The figure (4a) shows a ray path between the event epicenter and the receiver, which accounts for earth flattening and horizontal refraction due to lateral changes in sound velocity (refracted geodesic). The bathymetric profile and sound velocity profile (4b) are examined for features that might lead to blockage. f none are found, the event T phase (4c) is included in the subset of events used to construct the empirical coupling relation. Other factors may complicate the calibration process. The T phase may exhibit a radiation pattern. We intend to examine azimuthal dependence, with observations of some of the events at both San Nicolas s. and Wake s. stations. Additional factors that may effect coupling (earthquake focal depth, depth of the SOFAR axis, water column depth) may be estimated from USGS locations. These estimates will be uncertain due to the inaccuracy of seismic locations in the oceans.

9 Conclusion Our initial examination of T phase amplitudes (Figure 1) suggests that T phases can be used to map blockage or other strong path attenuation. We are undertaking a quantitative study to verify or reject this conclusion. The principal difficulty to be surmounted is the ambiguity between source coupling and path attenuation. We are attempting to quantify coupling with a probabilistic model, which would permit us to estimate attenuation and to quantify the reliability of the estimate.

10 References Forbes, A. (1994), The Tasman blockage - An acoustic sink for the Heard sland feasibility test? JASA, 96(4), Johnson, R. and J. Northrup (1966), A comparison of earthquake magnitude with T-phase strength, BSSA, 56(1), Munk, W., R. Spindel, A. Baggeroer, T. Birdsall (1994), The Heard sland Feasibility Test, JASA, 96(4), Walker, D., C. McCreery, and Y. Hiyoshi (1992), T-phase spectra, seismic moments, and tsunamigenesis, BSSA, 82(3),

11 Figure la. Great circle paths in red indicate observation of a T-phase at SN; black indicates no T-phase was observed. Green stars mark earthquakes used in plotting the section in Figure 2.

12 h Bathymetry map of the southern Pacific Ocean 25 5 Lati tude Longitude Figure lb. Shallow bathymetric features in the Tasman Sea and east of New Zealand may contribute to acoustic blockage of the energy from the earthquakes marked in Figures la and 2. The prominent ridge structures are sites of earthquakes recorded by SN hydrophones. dkpth (m).

13 ACOLS~~C blockage expected for these particular eqs 1 35 n.;3 E u wl Q) W Q) E Magnification (x2) to show observed T-phase..rl b c E F 5 t rl Backazimuth from San Nicolas sland (( legrees east of north) 16 Figure 2. 'Record section' of data segments around the expected T-phase arrival times for the earthquakes indicated by green stars in Figure 1.

14 ' Figure 3. Map of selected earthquake locations that have occurred while we have been collecting data at San Nicolas s. Hydroacoustic recording sites at SN and Wake s. are marked in green. Red indicates earthquakes from which we will select a 'calibration' subset of events with free acoustic paths.

15 Figure 4a. We are collecting events along the East Pacific Ridge and the Pacific- Antarctic Ridge to calibrate the relation between earthquake magnitude and T phase energy. The calibration events must have a free acoustic path to San Nicolas s. Latitude :; -5 - f:' J ' ; Map of Bathymetry in the East Pac ific Longitude [credit: ATOC software, Shaun Leach] :pth 1)

16 Map of Sound Speed and Bathymetry along Refracted Geodesic Path.,,..i!?..., sound velocity 15 (mkec) Vertical Exaggeration 2: range from source (km) [credit: ATOC software, Shaun Leach] Figure 4b. This figure shows the bathymetry (water depth) and sound velocity along the path shown in figure 4a. This path appears to be free of features that would block acoustic propagation.

17 Figure 4c. Beamformed T phase from earthquake 9351, mb 5.1, on the Pacific-Antarctic Ridge (Figure 4a) recorded at San Nicolas s. Energy from this earthquake has presumably experienced little to no attenuation along its path to SN. Distance to SN 95 km 6 seconds