Vol49_2_2006 607 ANNALS OF GEOPHYSICS, VOL. 49, N. 2/3, April/June 2006 Key words ocean bottom seismic observatory – broadband seismology 1. Introduction Two-thirds of the Earth’s surface is covered by oceans, and this represents a significant chal- lenge for the investigation of global scale dy- namic processes in the Earth’s interior, as well as tectonic processes at ocean-continent bound- aries. In particular, the need for long term ocean floor seismic observatories has now been widely recognized, and several national and internation- al efforts are underway to resolve the technolog- ical and logistical issues associated with such de- ployments (e.g., COSOD II, 1987; Purdy and Dziewonski, 1988; Purdy, 1995; Forsyth et al., 1995; Montagner and Lancelot, 1995). Following pioneering efforts in the 1960’s (Sutton et al., 1965), a number of pilot projects have been conducted in the last ten years, coor- dinated internationally by ION (International Ocean Network, Suyehiro et al., 1995), to test technological solutions and demonstrate the feasibility of seafloor seismic observatories ei- ther in boreholes or on the ocean floor (e.g., Suyehiro et al., 1992; Beranzoli et al., 1998). In particular, in 1992, a French experiment involv- ing the manned underwater vessel «Nautile» in- The Monterey Bay broadband ocean bottom seismic observatory Barbara Romanowicz (1), Debra Stakes (2), David Dolenc (1), Douglas Neuhauser (1), Paul McGill (2), Robert Uhrhammer (1) and Tony Ramirez (2) (1) Seismological Laboratory, University of California, Berkeley, CA, U.S.A. (2) Monterey Bay Aquarium Research Institute, Moss Landing, CA, U.S.A. Abstract We report on the installation of a long-term buried ocean-floor broadband seismic station (MOBB) in Monterey Bay, California (U.S.A.), 40 km off-shore, at a water depth of 1000 m. The station was installed in April 2002 using a ship and ROV, in a collaborative effort between the Monterey Bay Aquarium Research Institute (MBARI) and the Berkeley Seismological Laboratory (BSL). The station is located on the western side of the San Gregorio Fault, a major fault in the San Andreas plate boundary fault system. In addition to a 3-component CMG-1T seismometer package, the station comprises a current meter and Differential Pressure Gauge, both sampled at high-enough frequency (1 Hz) to allow the study of relations between background noise on the seis- mometers and ocean waves and currents. The proximity of several land-based broadband seismic stations of the Berkeley Digital Seismic Network allows insightful comparisons of land/ocean background seismic noise at pe- riods relevant to regional and teleseismic studies. The station is currently autonomous. Recording and battery packages are exchanged every 3 months during scheduled one day dives. Ultimately, this station will be linked to shore using continuous telemetry (cable and/or buoy) and will contribute to the earthquake notification sys- tem in Northern California. We present examples of earthquake and noise data recorded during the first 6 months of operation of MOBB. Lessons learned from these and continued recordings will help understand the nature and character of background noise in regional off-shore environments and provide a reference for the installa- tion of future off-shore temporary and permanent broadband seismic stations. Mailing address: Dr. Barbara Romanowicz, Seismolo- gical Laboratory, University of California, 215 McCone Hall #4760, Berkeley, CA 94720-4760, U.S.A.; e-mail: barbara@seismo.berkeley.edu 608 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez stalled two sets of 3-component broadband seismometers in the north-equatorial mid-At- lantic, one directly on the seafloor, and the oth- er, using the IFREMER re-entry vessel NA- DIA, at 300 m depth inside ODP hole 396B (Montagner et al., 1994a,b). The data compari- son between the two systems seemed to indi- cate that the ocean-floor installation was quieter at long periods (Beauduin et al., 1996), howev- er, this remained controversial, as only 10 days of data were acquired in this experiment, and water circulation may have increased the noise in the borehole. Since then, several other buried seafloor installations have been deployed in the deep oceans, some of them making use of aban- doned submarine telecommunications cable (e.g., Kasahara et al., 1998; Butler et al., 2000). During the OSN1 experiment in 1998 (e.g., Collins et al., 2001) 3 broadband systems were installed 225 km southwest of Oahu (Hawaii), at a water depth of 4407 m, one on the seafloor, one buried, and the third one at 248 m below the seafloor, in a borehole drilled by the ODP in 1992 for this purpose (Dziewonski et al., 1992). Data were collected for 4 months and demon- strated the importance of burying the seismome- ter package below the seafloor to obtain good coupling with the ground and ensure good qual- ity of data at long periods. The OSN1 experi- ment also demonstrated that a borehole installa- tion can be quieter at teleseismic body wave pe- riods than a buried or seafloor deployment, be- cause it avoids signal-generated noise due to re- verberations in the near surface sediment layers. This was confirmed by the long-term Japanese NEREID Observatory deployment, which also documented that a properly cemented ocean- floor borehole in basement rock can be very qui- et at long periods as well (Suyehiro et al., 2002; Araki et al., 2004). Long-term ocean floor observations are also necessary to better constrain regional tectonics, such as on the western margin of North America, where tectonics and seismic activity do not stop at the continental edge. For example, in Califor- nia, the zone with most abundant seismicity is as- sociated with the Mendocino Triple Junction, and is mostly off-shore. Much effort has been ex- pended to deploy networks of seismic stations in the Western US, most recently broadband sta- tions, with multiple goals of monitoring the back- ground seismicity, understanding modes of strain release, documenting seismic hazards and pro- viding constraints on crustal and upper-mantle structure. However, because there are very few off-shore islands in Central and Northern Califor- nia, practically all stations are located on the con- tinent. As a consequence, the study of plate- boundary processes, as afforded by regional seis- mological investigations, is heavily squewed on the continental side of the San Andreas Fault (SAF) system. Off-shore seismicity is poorly constrained, both in location and in mechanisms, as is crustal structure at the continental edge. While consensus seems to have been reached that permanent, borehole installations are best for seafloor deployments of broadband seismome- ters, they are very expensive: the spatial resolu- tion required for regional studies, either off-shore or in the middle of the oceanic plates, may not be achieved for many years to come. It is therefore important to conduct pilot studies to determine how to optimally deploy ocean floor broadband systems, and in particular how to minimize the strong perturbing environmental effects, both through improved installation procedures, and through a posteriori deconvolution of ocean cur- rent, tide, pressure, temperature and other such signals that can be recorded simultaneously. In the summer of 1997, the international MOISE experiment (Monterey Ocean bottom International Seismic Experiment) allowed us to collect 3 months of broadband seismic data from a seafloor system installed 40 km off-shore in Monterey Bay, in a cooperative experiment be- tween MBARI, IPG (Paris France) and UC Berkeley (e.g., Romanowicz et al., 1998; Stakes et al., 1998). During this experiment, the feasi- bility of performing under-water electrical and data cable connections between instruments, us- ing an ROV operated from a ship, were success- fully illustrated for the first time. The MOISE experiment also demonstrated the sensitivity of ocean floor systems to sea currents at long peri- ods and the importance of simultaneous record- ing of current velocity and direction, at a sam- pling rate sufficient for quantitative comparisons with seismic data: the conventional current meter sampling rate used by oceanographers (4 sample points once every 4 min), was too low to correct 609 The Monterey Bay broadband ocean bottom seismic observatory the MOISE seismic data for noise generated by currents, although this is theoretically possible (e.g., Stutzmann et al., 2001). The Monterey Bay Ocean Broad Band Ob- servatory (MOBB, McGill et al., 2002; Uhrham- mer et al., 2002; Romanowicz et al., 2003) was installed in April 2002. It is a direct follow-up of MOISE, and capitalizes on the lessons learned during that pilot experiment. The ultimate goal of this collaborative project between MBARI and the Berkeley Seismological Laboratory (BSL) is to link the MOBB station by continuous teleme- try to the shore, so that MOBB becomes part of the Berkeley Digital Seismic Network (BDSN, Romanowicz et al., 1994). The data can then be contributed to the real-time earthquake monitor- ing system in Northern California (Gee et al., 2003). The opportunity to do so awaits the instal- lation of the MARS cable (Monterey Accelerated Research System; http://www.mbari.org/mars). In the meantime, data are recorded on-site and retrieved every 3 months using MBARI’s ship and ROV, and data analysis is focused on under- standing sources of background noise at long pe- riods in this relatively near-shore environment. We view MOBB as the first step towards extend- ing the on-shore broadband seismic network in Northern California to the seaside of the North- America/Pacific plate boundary, providing better azimuthal coverage for regional earthquake and structure studies. In what follows we describe this observatory and discuss some of the data recorded during the last 18 months. 2. Location, instrument packages and deployment The MOBB station is located at a water depth of 1000 m, 40 km off-shore in Monterey Bay, in an area called «Smooth ridge» on the western side of the San Gregorio Fault, and closer to it Fig. 1. Location of the MOBB and MOISE stations in Monterey Bay, California, against seafloor and land to- pography. Fault lines are from the California Division of Mines and Geology database. MOBB is located at a water depth of 1000 m. 610 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez than was MOISE (fig. 1). The planned MARS ca- ble route is down the center of Smooth ridge and with a termination near the MOBB site. The San Gregorio Fault (SGF) splays from the SAF at the Golden Gate and extends south past the San Francisco Peninsula and Santa Cruz mountains, mostly off-shore. It is the principal active fault west of the SAF in central coastal California, yet it remains the largest known fault whose seismogenic potential is not well charac- terized in this region. However, Begnaud and Stakes (2000) and Begnaud et al. (2000) used a temporary off-shore seismic network to demon- strate the unusually high seismicity levels of the Northern SGF, dominated by compressional mechanisms as well as an east-dipping focal plane (Simila et al., 1998). The SGF is thought to be capable of M > 6 earthquakes, making the MOBB site particularly interesting from the tec- tonic and seismic hazards point of view. Using refined crustal velocities based on the results of Begnaud et al. (2000), Simila et al. (1998) relo- cated the 1926 M > 6 doublet to show that the first event occurred on the Northern SGF followed by the second on the adjacent Monterey Bay fault zone. The coseismic geodetic slip on the SAF dur- ing the 1906 earthquake and the Late Holocene geologic slip rate on the San Francisco peninsu- la and southward are about 50-70% of their values north of San Francisco (Thatcher et al., 1997; Schwartz et al., 1998). Review by the Working Group on Northern California Earth- quake Potential (1996) suggests that the slip rate on the SAF in the Santa Cruz mountains is about 14 mm/yr, which is 58% of the slip rate north of San Francisco, so the rest of the slip must be ac- commodated by other faults, both on-land and off-shore. This slip gradient reflects partitioning of the plate boundary slip onto the San Gregorio, Sargent and other faults south of the Golden Gate. Because of the limited on-shore extent, few detailed geologic studies have been conduct- ed to evaluate the style and rate of late Quater- nary deformation along this complex fault zone. The ocean-bottom MOBB station currently comprises a three-component seismometer pack- age, a current meter, a Differential Pressure Gauge Fig. 2a,b. a) Photo of a Guralp CMG-1TD seismometer in the Byerly Vault (BKS). Shown are the various cir- cuit boards on the sides and the top of the sensor package. Three of the nine batteries used by the leveling sys- tem are on the left front and the system clock is on the circuit board on the right. The seismometers are in the µ metal shielded container mounted on leveling gimbals in the center. b) Photo of the titanium pressure vessel con- taining the CMG-1TD and resting on a ~ 1 cm thick bed of kiln dried fine sand on the concrete pier at BKS, for the purpose of noise comparisons (Uhrhammer et al., in prep.). a b 611 The Monterey Bay broadband ocean bottom seismic observatory (DPG) and a recording and battery package. The data logger, battery, and DPG are contained in a modular frame which is removed and replaced when the ROV services the system. This config- uration permits hardware and software upgrades to take place as required. For instance, the DPG (Cox et al., 1984) was not present in the initial deployments, but was added later during a data retrieval dive in September, 2002. The seismic package contains a low-power (2.2 W), three-component CMG-1T broadband seismometer system, built by Guralp Inc., with a 24-bit digitizer, a leveling system, and a preci- sion clock (fig. 2a). The seismometer package is mounted in a cylindrical titanium pressure ves- sel 54 cm in height and 41 cm in diameter (fig. 2b), custom built by the MBARI team and out- fitted for underwater connection. The compo- nent design of the instruments permit the sensor, datalogger, and current meter to be carried to the seafloor separately, then tested and connected in situ. This component design permits us to up- date software, change batteries and replace in- struments without disturbing the sensor pack- age. The system has been designed to permit a GPS time mark, applied during an ROV visit, to establish the offset and drift rate of the Guralp clock. These errors are recorded and the timing of the seismic data is corrected in post-process- ing. The clock is not adjusted in situ to prevent abrupt jumps in the time marking of the data. Establishing these corrections on the seafloor and after the system has reached thermal equi- librium is a critically important feature for long- term autonomous deployments. Because of the extreme sensitivity of the seismometer, air movement within the pressure vessel must be minimized. In order to achieve this, after extensive testing at BSL, the top of the pressure vessel was thermally isolated with two inches of insulating foam and reflective Mylar. The sides were then insulated with mul- tiple layers of reflective Mylar space blanket, and the vessel was filled with argon gas (fig. 2b). The low thermal conductivity of argon al- lows better thermal insulation. This resulted in significant noise reduction on the 3 compo- nents, in the 10-100 s period range (fig. 3a,b). Fig. 3a,b. Comparison of typical background noise PSD levels observed by the CMG-1TD and the co-sited STS-1’s in the BKS vault, a) before and b) after it had been installed in the titanium pressure vessel, appropri- ately insulated and purged with argon gas. The large dashed line and the solid lines are the Z component and hor- izontal component PSD’s, respectively, and the small dashed lines are the STS-1 PSD’s. Note that the CMG- 1TD PSD levels are within ∼ 5 dB of the STS-1 PSD levels at long periods after insulation (Uhrhammer et al., in prep.). a b 612 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez Near-bottom water currents are measured by a Falmouth Scientific 2D-ACM acoustic current meter. It is held by a small standalone fixture and measures the current speed and di- rection about one meter above the seafloor. The recording system is a GEOSense LP1 data log- ger with custom software designed to acquire and record digital data from the Guralp sensor and from the current meter over RS-232 serial interfaces, as well as analog data from the DPG. The seismic data are sampled at 20 Hz and the current meter and DPG are sampled at 1 Hz. Data are stored on a 6-GB, 2.5-inch disk drive. All the electronics, including the seismometer, current meter, and DPG, are powered by a sin- gle 10 kWh lithium battery. All installations were done using the MBARI ship Point Lobos and the ROV Ventana. Prior to the instrumentation deployment, the MBARI team manufactured and deployed a 1181 kg gal- vanized steel trawl-resistant bottom mount to house the recording and power systems, and in- stalled a 53 cm diameter by 61 cm deep cylindri- cal PVC caisson to house the seismometer pres- sure vessel. The bottom mount for the recording system was placed about 11 m away from the caisson to allow the future exchange of the recording and battery package without disturbing the seismometer. The seismometer package was tested extensively at BSL, then brought to MBARI where its internal clock drift was cali- brated against GPS time in an environmental test chamber at seafloor temperature. The actual deployment occurred over 3 days (04/09-11/2002). On the first dive, the seismome- ter package was lowered into the PVC caisson (fig. 4a), and its connection cable brought to the site of the recording unit. On the second dive, the Fig. 4a-c. MOBB installation snapshots. a) The seismometer package is being lowered into the hole bounded by the PVC pipe, held by the arm of the ROV Ventana. b) The recording and battery package is being installed inside the trawl-resistant mount. c) The ROV arm (at front) is connecting the current meter cable to the recording system. The connector of the seismometer package on the right is already in place. a b c 613 The Monterey Bay broadband ocean bottom seismic observatory recording package was emplaced in its trawl-re- sistant mount (fig. 4b), and connected to the seis- mometer package (fig. 4c). Tiny (0.8 mm diame- ter) glass beads were poured into the caisson un- til the seismometer was completely covered, to further isolate it from water circulation, as dictat- ed by lessons learned from previous experiments (e.g., Sutton et al., 1992; Duennebier and Sutton, 1995). The seismometer package is now buried at least 10 cm beneath the seafloor. On the third dive, the ROV immobilized the cable between the seismometer and recording package with steel «wickets» inserted into the sediment. It then con- nected the seismometer to the recording system, leveled and recentered the seismometer, and ver- ified that everything was operational. Finally, the current meter was installed and connected to the recording system. On April 22nd, 2002, the ROV returned to the MOBB site to check the functioning of the seis- mometer and recording system. Some slight set- tling of the seismometer pressure vessel had oc- curred, and so the seismometer was recentered electronically. Over 3 Mb of data were then down- loaded from the recording system over a period of about two and a half hours, using a 9600 bps Fig. 5. Mass position data, for portions of the time period 11/04/2002-07/01/2003, showing the progressive set- tling of the seismometers. At periods longer than the free period of the seismic sensor, the mass position is is proportional to tilt on the horizontal components, and to perturbations in gravity on the vertical component. The mass position has therefore been converted to acceleration (vertical component, MMZ) and tilt (horizontal com- ponents (MMN, MME). The large steps on the horizontal components are associated with: 1) installation (day 100); 2) re-centering (day 112, day 263). There is a smaller step on day 134, associated with a local Mw 4.95 earthquake. The vertical component data have been detrended by subtracting a running 36 h average (±18 h) from each 1/2 h duration smoothed data sample, to bring out the clearly visible tide signal. 614 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez serial data connection through the ROV. These data included the recordings of two regional earthquakes in California and two teleseismic events that occurred in Guerrero, Mexico and in Northern Chile. The site was revisited two months later, on June 27th, to check the functioning of the sys- tem and replace the data recording and battery module, in the first of a series of such dives planned for the next 3 years. The following functions were performed: 1) Disconnected the current meter and seis- mometer from old datalogger. 2) Removed old datalogger frame with da- talogger and batteries from the trawl-resistant mount. 3) Installed new datalogger frame in the trawl-resistant mount. 4) Connected the current meter to the new datalogger. 5) Connected the ROV to the new datalog- ger and verified that the datalogger was opera- tional. 6) Connected the seismometer to the new datalogger, and monitored its reboot. 7) Centered the seismometer. 8) Re-centered the seismometer. 9) Verified that the Guralp was receiving the GPS clock signals from the ROV (NMEA time messages and pulse per second), and recorded the clock offset. During this dive, the Guralp clock was not resynchronized to GPS time. 10) Brought the old datalogger frame with datalogger and batteries back to the ship. Figure 5 shows the evolution of the tilt signal obtained from the seismometer mass position channels (MMZ, MME, MMN), for the first 7 months of deployment. These data indicate that the seismometer package has been experiencing an exponentially decaying tilt in a south-south- westerly direction, which is also the down slope direction (e.g., fig. 1). The large step on day 112 (22/04/2002) was caused by re-centering, when the instrument was checked 12 days after instal- lation. The small step on day 134 (14/05/2002) is coincident with the occurrence of a Mw 4.96 earthquake which occurred 55 km N59W of MOBB on the San Andreas Fault near the town of Gilroy. The instrument was recentered again on day 178 (27/06/2002). The slow drift rate of the horizontal component mass positions in the latter half of the plot (day 263 onward) indicates that the OBS pressure vessel stabilized in the ocean floor sediments after about two and a half months after deployment. On the MMZ component, the semidiurnal gravitational tide is visible, riding on the tilt signal. As with the horizontal components, the largest signals are associated with rapid changes in the second derivative of the tilt, caused by recentering or by significant ground shaking (day 134). During the first two months of recording, many regional and teleseismic events were recorded, as described later. The site has been re- visited regularly every three months since. Dur- ing each visit, the datalogger and battery pack- ages are changed, the seismometers are recen- tered, and the clock re-synchronized to GPS time. Due to multiple datalogger problems (hard- ware and software) encountered in the first half of 2003, the best data available so far span the time period April-December 2002, as illustrated below. 3. Examples of data and preliminary analysis Figure 6 shows the location of MOBB with respect to the nearby BDSN stations. Notably, we will be discussing comparisons between recordings on the ocean floor (MOBB), in the noisy Farallon Island environment (FARB) and on the continent (SAO, JRSC). Figure 7a-c shows power density spectra for two different time periods, comparing back- ground noise at MOBB and three land stations of the BDSN network. Day 143 (23/05/2002) is a «quiet» day, as assessed from the ocean wave da- ta recorded on the NOAA buoy in Monterey Bay, whereas day 350 (16/12/2002) is a «stormy» day (spectral ocean wave density is an order of mag- nitude higher at around 30 s). Increased noise level for periods between 20 and 500 s, due to ocean currents and infragravity waves, is ob- served at MOBB on all 3 components on the stormy day, but only on the vertical component on the quiet day. The spectral width of the noisy long-period band is larger on the stormy day, and, interestingly, it also corresponds to a band of increased noise at the Island site FARB, where it 615 The Monterey Bay broadband ocean bottom seismic observatory Fig. 6. Location of MOBB (and MOIS) with respect to nearby broadband stations of the Berkeley Digital Seismic Network (BDSN). FARB is located on the Farallon Islands. MOBB is located just west of the San Gregorio Fault. Fig. 7a. Comparison of noise recorded at MOBB and 2 other stations of the BDSN network, on two days in 2002 when no significant earthquake signals were recorded: a «quiet day» (143), and a «stormy» day (350), as assessed by the mean wave height recordings at a nearby NOAA buoy, located in Monterey Bay. The USGS high- and low-noise models for land stations are shown in black. Increased noise level for periods between 20 and 500 s, due to ocean cur- rents and infragravity waves, is observed at MOBB, as well as at the island station FARB. The noise level at MOBB between 10 and 20 s is comparable to the land station YBH, one of the quietest stations of the BDSN. See fig. 6 for FARB and SAO locations. Station YBH is 560 km north of MOBB. Here it is shown the vertical component. 616 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez is largest on the East component. This most like- ly indicates loading of shallow water around the small Farallon Island by gravity waves, similarly to what is observed in Hawaii (Stephen et al., 2003). We are currently investigating the source of this noise in more detail. In particular, it should be correlated with the DPG data, if it is in- deed due to gravity waves generated by breaking waves at the coastline (Webb et al., 1991). The bell shape of this noise peak is in agreement with theoretical calculations by Araki et al. (2004). On the other hand, the noise level at MOBB between 30 and 100 s on a quiet day is comparable to the noise level at the island station FARB on a stormy day. The «low noise notch» (Webb, 1998) is very narrow at MOBB (10-30 s), and contains the single-frequency micro-seismic peak (12 s), Fig. 7b,c. Comparison of noise recorded at MOBB and 2 other stations of the BDSN network, on two days in 2002 when no significant earthquake signals were recorded: a «quiet day» (143), and a «stormy» day (350), as as- sessed by the mean wave height recordings at a nearby NOAA buoy, located in Monterey Bay. The USGS high- and low-noise models for land stations are shown in black. Increased noise level for periods between 20 and 500 s, due to ocean currents and infragravity waves, is observed at MOBB, as well as at the island station FARB. The noise level at MOBB between 10 and 20 s is comparable to the land station YBH, one of the quietest stations of the BDSN. See fig. 6 for FARB and SAO locations. Station YBH is 560 km north of MOBB. b) North component; c) east component. b c but the level of noise is comparable to the land station YBH, one of the quietest BDSN stations. The corresponding «double-frequency» micro- seismic peak around 6 s is visible at all stations most of the time, but on day 350, it is hidden by higher amplitudes between 2 and 4 s at MOBB. On day 143, two additional narrow-band micro- seismic peaks are clearly resolved at MOBB (around 2.5 and 4 s). These could be related to a combination of local sea-state and distant storms (e.g., Bro-mirski and Duennebier, 2002). They are clearly distinct in frequency from those ob- served in the open sea (i.e., Stephen et al., 2003). Noise levels at frequencies higher than 2 Hz and lower than 200 s are comparable to those ob- served at some of the land stations. FARB (island site) is sometimes noisier at long periods, but the 617 The Monterey Bay broadband ocean bottom seismic observatory Fig. 8. Comparison of instrument responses of the CMG-1T at MOBB and the CMG-3T at FARB. Fig. 9. Spectrum of current speed [units are (mm/s)/ Hz] based on a 78-day period of current me- ter data. The four dominant group of peaks coincide precisely with the frequencies of the diurnal, semi- diurnal, 8 h and 6 h components of the gravitational tides. Fig. 10. Distribution of current velocity data as a function of azimuth for the 78-day period shown in fig. 9. The contour label units are fractions of the av- erage density distribution of the current velocity. The two dominant maxima (centered at 60° and 240°, i.e. orthogonal to the continental shelf) are associated with the semi-diurnal tidal currents. The third directional peak is roughly parallel to the coastline and appears to be related to the dominant ocean circulation. seismometer there is a Guralp CMG-3 with a shorter period high-pass corner (100 s) than MOBB (360 s) (fig. 8). Some of the longer peri- od background noise is clearly correlated with currents that are associated with tidal flows, as was documented from MOISE (Romanowicz et al., 1998; Stutzmann et al., 2001). Figure 9 shows a spectrum of the ocean current speed at MOBB computed using a 78 day time series, il- lustrating the dominant effects of tides on the bottom currents. This is also illustrated in fig. 10, which shows the corresponding distribution of current direction and velocity as a function of az- imuth. With the current meter and DPG data sampled at 1 s, it will now be possible to decon- volve the tide and current related noise from the MOBB data. Unfortunately, the DPG dataset is still too small (recording system problems since 01/01/2003) to assess the effectiveness of such a deconvolution. 618 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez Figures 11-13 illustrate observations that have been recorded at MOBB during the two months period 10/04/2002-28/06/2002, and which were retrieved during the first datalogger exchange, on 29/06/2002. Figure 11 shows a component-by-com- ponent comparison of the recording of the 26/04/2002 Mw 7.1 teleseism in the Mariana Is- lands at MOBB and 3 nearby BDSN stations (see location on fig. 6). The comparison shows consistency between the recordings of MOBB and nearby stations. On the horizontal compo- nents, there appears to be some signal-generat- ed noise following the S-waves and the Love wave, which is associated with ringing in the shallow mud layers as we will show below. In fig. 12, such ringing is clearly demonstrated for a deep earthquake (17/11/2002, depth = 459 km, Mw = 7.3). Only the vertical component P- wave portion of the seismograms is displayed, filtered in two pass-bands. Signal-generated noise is very apparent in the P-waves in the 0.03-0.3 Hz pass-band, where the P-wave at MOBB displays a 3 min long coda. The ringing is narrow-band and disappears at frequencies lower than 0.1 Hz. Such observations should be helpful in understanding the triggering of sub- marine landslides in strong motion events, and may be relevant for ocean floor structures such as oil platforms and pipelines. On the other Fig. 11. Comparison of vertical, N and E component records of the 26/04/2002 Mw 7.1 Mariana earthquake (depth = 85.7 km, distance = 85.2°, azimuth = 283° from MOBB), at MOBB and 3 stations of the BDSN. The records have been band-pass filtered between 10-100 s. Fig. 12. Comparison of vertical component records at stations FARB, JRSC, MOBB and SAO for the deep Kurile Island earthquake of 17/11/2002 (Mw = 7.3; depth = 459 km; distance to MOBB = 65°). The data are shown in two pass-bands: 0.03-0.1 Hz and 0.03-0.3 Hz to emphasize the narrow-band character of the ringing in the MOBB P-wave data. Clearly visible in the lower frequency band are the P, pP and sP arrivals. 11 12 619 The Monterey Bay broadband ocean bottom seismic observatory seismic events and could be similarly removed. On the vertical component, the water reflection of the P-wave is clearly seen 1.3 s after the P- wave. In spite of these strong site effects, these data can be used in moment tensor studies, as il- lustrated in fig. 15, which shows the results of a moment tensor inversion using a time domain whole waveform methodology (Dreger and Ro- manowicz, 1994). A robust solution is obtained using data from 5 stations, including data from MOBB. The waveform fit at MOBB is out- standing with a 92.8% variance reduction. The addition of MOBB to the existing BDSN sta- tions in this particular case provides an addi- tional SH lobe but, since 3 SH lobes are already sampled by other BDSN stations, this particular strike-slip mechanism is well constrained in any case. However, this example serves to show that the MOBB data are well calibrated and have potential for providing valuable con- straints in moment tensor studies of events of hand, this type of noise may be unavoidable in a shallow buried installation. We are currently evaluating ways to eliminate this signal-gener- ated noise by post-processing. One possibility is to design an «observational» transfer-func- tion, using data from near-by land stations that do not show the ringing. This is illustrated in fig. 13, where we show a comparison of origi- nal P-wave train at MOBB (blue) and JRSC (green) and «cleaned» MOBB data (red) after removal of the corresponding transfer function. We are working on combining this type of pro- cessing with direct modelling of the ringing ef- fect, by computing theoretical transfer func- tions based on simple sediment layer models (Uhrhammer et al., 2003; Dolenc et al., 2006) that can be obtained from local studies (e.g., Begnaud et al., 2000). Figure 14 shows the records, deconvolved to ground velocity, for a Mw 3.63 regional event which occurred on 23/04/2002 on the SAF at a distance of 53.4 km from MOBB. The very large S-wave pulse on the horizontal compo- nents as well as the subsequent ringing are like- ly due to site response, as observed for the tele- Fig. 13. Raw vertical component data (P-wave and depth phases) observed on the vertical component at stations MOBB (blue) and JRSC (green) for the 17/11/2002 deep Kurile Island earthquake. Clearly seen is the ringing due to the soft sediment layer in Monterey Bay. The red trace shows the MOBB data after removal of the transfer function constructed us- ing JRSC data. Fig. 14. Deconvolved ground velocity records at MOBB of the 23/04/2002 M 3.63 San Andreas Fault event (latitute = 36.866; longitude = −121.61; depth = = 9 km). 620 Barbara Romanowicz, Debra Stakes, David Dolenc, Douglas Neuhauser, Paul McGill, Robert Uhrhammer and Tony Ramirez Fig. 15. Results of moment tensor inversions for the M 3.63 regional event shown in fig. 14. Top: inversion us- ing 4 stations of the BDSN and MOBB (BDM, BKS, CMB are bandpass filtered between 0.02 and 0.05 Hz; MHC and MOBB, between 0.05 and 0.10 Hz). Bottom: results of inversion using only MOBB, showing the good fits of the single station solution to the other BDSN waveform data. 621 The Monterey Bay broadband ocean bottom seismic observatory other types, such as reverse fault events in the Coast Ranges or strike slip events on faults closer to the shore or off-shore. To further demonstrate the consistency of the MOBB data, we also show the results of a single station moment tensor inversion using only MOBB, and the comparison of the corre- sponding synthetic predictions with the actual data at the four other BDSN stations. The single station solution results in a nearly identical fo- cal mechanism, but a slightly larger CLVD component and scalar moment, which is not un- like other single station inversions. 4. Discussion and future work Data collected at the MOBB site can be used for several purposes. They provide com- plementary constraints for regional crustal and upper mantle structure to land-based broad band stations, as well as for the study of earth- quakes along the San Andreas fault system. Currently, they are noisier than records from land-based stations, so that only relatively large events can be successfully analyzed. As we have seen, there are two sources of in- creased noise: signal-generated noise due to re- verberation in the sediment pile, and back- ground noise generated by currents as well as local and distant waves in the ocean. The first type of noise can be dealt with by designing appropriate deconvolution filters. In particular, the vicinity of high quality broadband land sta- tions of the BDSN provides a helpful refer- ence. The second type of noise is complex and time variable, however, the combination of MOBB and BDSN seismic data, current meter and DPG data, sampled at sufficiently high rates, as well as local and regional buoy data, provides a promising dataset to try to reduce the MOBB background noise – at least in the infragravity wave band. Ultimately, the level of success that we may reach in this endeavor will provide a reference for what can be expected of data from shallow-buried broadband ocean bottom systems. Finally, the collection of these different types of data and their geographical distribution also provides an opportunity to further study the generation of such infragravity waves, at least in the particular setting of a relatively shal- low ocean bay, as was pioneered by Bromirski and Duennebier (2002). 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