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Seismic Images of of the Mendocino Triple Junction Region

Cruise Report for RV WECOMA 9406A

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Pulling the Rug out From Under California: Seismic Images of the Mendocino Triple Junction Region

Summary

This cruise represents part of a two-year onshore/offshore seismic acquisition program to image the crest and upper mantle of northern Califomia. The primary objective is to place constraints on how the northward migration of the Mendocino triple junction affects the crustal architecture of the Califomia continental margin, which is undergoing a transition from a compressive to a transpressive tectonic regime. A better understanding of the processes associated with triple junction migration in this region will contribute to our understanding of the geological history of western California and of the seismic and volcanic hazards associated with the San Andreas fault system and the Cascadia subduction zone. During the cruise described in this document, we deployed and retrieved ocean bottom seismographs (OBS) operated by the U. S. Geologicai Survey and ocean bottom hydrophones (OBH) operated by the Woods Hole Oceanographic Institution in order to record airgun shots from from a 8385 in3 (137.5 liter) tuned airgun array deployed by the RV Ewing, thus providing large-aperture seismic data for derivation of. crustal velocity and structure. This information complements the multichannel seismic reflection information obtained simultaneously by the RV Ewing. Additional seismographs were deployed on land to record the shots. This field effort builds on an onshore seismic refraction/reflection program conducted during August, 1993, using approximately 600 seismometers and large explosive sources. The need to coordinate the efforts of two ships and a large onshore field crew during the 1994 field experiment placed unusual constraints on the timing of the OBS/OBH operations that were conducted from the RV WECOMA. A secondary objective of the cruise was to obtain high-resolution subsurface images around sites that have been proposed to the Ocean Drilling Program (ODP) as part of a major effort to constrain the paleo-oceanographic effects of the California current through acquisition of a series of long cores recording the sedimentary history of the Califomia continental margin. Logistically, this program complemented the OBS/OBH effort because several proposed drilling sites fall within the boundaries of the OBS/OBH experiment, and time for surveying was available between OBH/OBS deployments. Digital 3.5 and 12 kHz echo-sounding data and digital single channel seismic reflection data using an 80 in3 water gun as a source were acquired on grids surrounding 4 of the proposed drilling sites. Finally, during the transit to and from Newport, we acquired several Acoustic Doppler Current Profiles (ADCP) to map currents offshore Cape Blanco.

Cruise Narrative for RV WECOMA 9406A

date time (local) summary of activity est./actual time needed
6/8/94 0905 Leave Newport for the seaward end of MCS line 2 ~ 36 hours
- 1633 Start ADCP survey, plan A. -
6/9/94 0345 ADCP survey completed. -
- 0809 Start WHOI release tests. 8/9
- 1700 Finish WHOI release tests. -
6/10/94 0600 Arrive at site 1 to begin GROUP 1 deployment (6 0BH, 5 OBS). 21/17.3
- - 2320 Finish deployment at site 11 of GROUP I. Underway to beginning Of GROUP H.
6/11/94 0055 Start SCS survey at CA-7. Because of problems with the water gun and streamer, this is primarily a 3.5 and 12 kHz survey site ?/26
6/12/94 0247 Stop survey at CA-7. -
- 0545 Arrive at site 12 of GROUP H deployment (40BH; 3 OBS). Deployment includes approx 200 nm of transit. 20.5/12.7
- 1828 Finish deployment at site 18 of GROUP II. Stay on site to fix articulated crane. -
- 1945 Crane fixed. Underway to SCS CA-2 survey site. -
- 2326 Deploy SCS for CA-2 survey. ?/24
6/13/94 2357 Finish CA-2 SCS survey. -
6/14/94 0704 Start GROUP I recovery at site 1. 30/31
- 1253 Finish recovery of site 3 in rough seas. Stop recovery ofthe rest of GROUP I to wait for seas to calm down. (this includes 6.5 hrs waiting for calmer seas)
- 1930 Resume recovery of GROUP I at site 10. Sites recovered in the following order: 10, 11, 9, 8, 7, 5, 4, 6. OBS at site 6 was not recovered. -
6/15/94 1400 Abandon search for OBS. Start SCS survey at site CA-7. -
- 2130 Finish SCS survey at CA-7. ?/7.5
6/16/94 0045 Recover OBH at site 12 (first deployment of GROUP II). -
- 0600 Start GROUP III deployment at sites 19-26 (50BH and 3 OBS on the seaward end of line 6 and along line 5) 20/11
- 1715 Finish GROUP III deployment. Head to SCS survey site CA-4 -
- 2027 Arrive at SCS survey site CA-4 and begin survey ?/9.5
6/17/94 0600 Leave CA4 and head to seaward end of line 8 -
- 1030 Start deployment of 20BH and 10BS on line 8 (GROUP 4) 6/4.3
- 1149 Go to SCS site CA1 -
- 1700 SCS profiling at CA1 ?/13
6/18/94 0600 Leave CA 1 and head for north end of line 3 (56 nm) -
- 0953 Pick up 30BH and 30BS from line 3 (82 nm transit). 20/13
6/18/94 2300 Finish pickup of instruments and get underway to OBS site 30 on line 8 -
6/19/94 0934 At OBS station 30 -
- 0959 Underway to SCS site CA-1 -
- 1223 Start SCS survey at CA-1 ?/9.75
- 2212 Finish SCS. Underway to east end of line 6. -
6/20/94 eta 0300 Redeploy 30BH and 20BS line 6 (GROUP 5) 8
- 1100 Finish redeployment. Transit to CA-4 for SCS survey (60 nm) 6
- 1700 Arrive at CA-4. Survey for 15 hrs 15
6/21/94 0800 Transit to OBS site 27 (55 nm) 5.5
- 1330 Start pickup of 4 instruments on line 8.(10.5 hrs) 10.5
6/22/94 0000 Finish pickup of line 8. Start transit to site 23 (68 nm) 7
- 0700 Arrive at site 23. Pick up 23 to 19, heading south. 14.5
6/22/94 2130 Finish line 5 pickup. Spend 12 hours at CA-2 doing SCS. If them are delays earlier in the program, SCS time here will be decreased accordingly. 12
6/23/94 0930 Finish SCS and transit toward west end of line 6.(60 nm) 6
- 1530 Start line 6 pickup of 5 instmments. -
- 2214 Finish line 6 pickup. 12/6.75
- 2300 Deploy SCS system for high-resolution survey of the deformation front between MCS3 and MCS6 -
6/24/94 0248 Abandon SCS survey because of equipment problem. Continue with a 3.5 KHz survey. -
- 1120 Head for Eureka. -
- 1400 Tie up at the Sierra Pacific dock in Eureka. Start packing and unloading ship. -
6/25/94 1220 Leave Eureka. Trehu, Lendl, and WHOI and USGS personnel remain on shore. Lyle and Chambers ride back to Newport. -
6/26/94 1530 Arrive Newport. -
6/27/94 - Finish unloading. -

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Photograph of the research vessel RV Wecoma.
RV Wecoma
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This page last modified on Monday, 24-Nov-2014 13:04:47 EST