Nicaragua/Costa Rica Subduction Zone

Field Reports from R/V Ewing Research Cruise to Costa Rica - June 2000

Report 1:Mon, 05 Jun 2000

After 10 days at sea it is time for a progress report.  We have faced many challenges already during the cruise, but it seems that we are overcoming them and collecting lots of good data.  After the short transit from Puntarenas, Costa Rica to offshore southern Nicaragua we acquired about 24 hours of Hydrosweep swath bathymetry along the Middle America Trench. After this period the 14 OBHs from GEOMAR were ready and we deployed them along a 200 km long profile across the Sandino forearc basin, down the trench slope, and 60 km out onto the subducting Cocos plate.  We have traversed this profile 2 times--once firing airguns at a 60 second interval to reduce noise in the OBH records and once at a 20 second interval for optimum MCS results.  We have now acquired 9 reflection profiles covering almost 1000 km (1200 km if you count each pass on the first profile).   We are less than half way through the cruise so we should come home with a large data set.

We have produced brute stacks for a few of the profiles and the data look promising.  We can clearly see the subducting plate boundary at least 85 km from the trench and probably much farther. We have learned that we need to record 20 seconds of data in order to record this reflection all the way to the coast.  We estimate that we may be able to image the boundary to depths of ~ 40 km.  This means we may be able to follow it through the entire seismogenic zone --from the shallowest levels to the deepest.

At this moment we are recovering OBHs from the first profile.  These data and the land instruments, operated by Jim Dolan, will provide the velocity information to further determine the geometry of the Nicaraguan margin and true depth of the subducting plate.

Looking forward we still plan to acquire ~ 17 more seismic lines, make two more OBH deployments, and acquire 2-3 more days of Hydrosweep swath bathymetry.

All of the group from UTIG (Kevin, Imtiaz, Junru, Tip, and Kirk) seem to be doing well and adapted to life at sea.  The weather has not been a significant factor, with only occasional evening rainstorms.

Regards to all at UTIG,

Kirk

Report 2: Sun, 11 Jun 2000

Since my previous report we have recovered the 14 GEOMAR OBHs, acquired hydrosweep for a day, redeployed the 14 OBHs, shot parts of 4 new seismic profiles, and recovered the OBHs a second time. Of the 14 OBHs in the first deployment, 12 recorded good data and 2 failed to record data.  As yet the cause of failure is unknown.  The records from the remaining OBHs are generally good, with arrivals visible to offsets of up to 120 km on some.   Data from the second deployment will be extracted tomorrow. Completion of the second refraction profile also signaled the end of the land recording program.  Jim Dolan and Stephan Husen (ETHZ) will be working with Wilfried Strauch (INETER) over the next few days to copy data from disk to tape and pack instruments for shipping.

We have continued our preliminary MCS data processing, which includes a brute stack and FK water velocity migration.  So far we are generally happy with the data quality.  Our modified source array seems to have sufficient strength for deep targets, while apparently having a relatively simple source signature and high peak to bubble ratio.  We have now had a chance to examine the first set of closely spaced lines (5 lines at 2 km spacing) and it is clear that there are significant structural and stratigraphic differences along the margin even across 8 km.  We have also noticed some important differences in structure compared to neighboring Costa Rica.  Off the Nicoya Peninsula of Costa Rica landward dipping normal faults are common in the upper slope sedimentary section.  Offshore Nicaragua we have also noticed normal faults in the slope section, but the ones we have observed so far, are predominantly seaward dipping.

Although we have not had any major catastrophes during the cruise we have had some challenges during the last week.  We have had intermittent problems with one of the two tape drives connected to the recording system, which has led to some data loss.  We also have had two episodes of streamer problems, the second of which forced us to recover the streamer and break off a line due to snagged fishing gear.  Fortunately we had already shot the second refraction profile, so we used the opportunity to recover the OBHs earlier than planned.  For the remainder of the cruise we still plan to collect another 12 days of MCS data and make one more OBH deployment/recovery before making the 60+ hour transit to Panama.

The science participants remain in good spirits despite the long hours and no days off.  I think it must help to eat well and watch a lot of silly movies.

Until next time,

Kirk

Report 3: Wed, 21 Jun 2000

This is my third, and final, NicSeis Progress Report

At this time we are shooting the first of two long strike lines. This line is situated along the upper trench slope at water depths of  ~500-1200 m, and it extends 200 km from offshore Santa Elena Peninsula, Costa Rica to off northwest Nicaragua.  We have deployed 13 OBHs along this line and are shooting at 125 m intervals.  We will shoot this line only once for both refraction and reflection.   We will finish the data acquisition Friday afternoon with a parallel strike line across the lower trench slope and recover the OBHs by Saturday afternoon.

To date we have acquired about 2500 km of seismic reflection data and 400 km of OBH data.  Our totals for the cruise should be over 2900 km of MCS and 600 km of OBH data.   The dip line coverage in our survey has extended from northwestern Costa Rica, seaward of the Santa Elena Peninsula, to the northwestern extent of Nicaragua, near the Gulf of Fonseca.  All of the profiles that we have recorded so far show the subduction plate boundary reflection across most, or all, of the sections to estimated depths between 20 and 40 km (usually depending on how far the lines extend landward). Many lines also cross the Sandino forearc basin, and in some areas the sedimentary section extends to as deep as 8 seconds, or to perhaps ~12 km depth.

The science party has retained a positive attitude throughout the cruise, although now we are definitely looking forward to the conclusion of NicSeis data acquisition and our first steps ashore in 32 days. Again, silly movies are playing a constructive role in maintaining our perspective. The highlight for me was the dog attack in "Something about Mary".

Best Regards,

Kirk