5.3. Ocean Bottom Seismographic Observation Eiichiro Araki, Nugroho Dwi Hananto, and Kiyoshi Suyehiro

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1 5.3. Ocean Bottom Seismographic Observation Eiichiro Araki, Nugroho Dwi Hananto, and Kiyoshi Suyehiro Seismological background of the study area Large historical earthquakes Both Indonesia and Japan are prone to being hit by devastating earthquakes and tsunamis. Indeed, since 1900, the two countries suffered the world s most deadly tsunamis more than any other countries. Both countries are located in the same tectonic environment where oceanic plates are subducting beneath the island arcs. In this section, we summarize historical earthquakes that occurred along and near the rupture zone of the 2004 December event. Figure from USGS website shows the overall magnitude of the 2004 event. Teleseismically determined aftershocks through Jan 10, 2005 more or less define the rupture zone to extend northward along the Sunda Trench from the epicenter to 14 deg N amounting to about 1300 km in length. Historical large earthquakes which accompanied tsunami disasters along this area is shown in Figure from Yamanaka (2005). Along the Sumatra Island arc, the frequency of interplate subduction earthquakes is 237 events over 36 years with a b-value of 1.0, the largest being the June 4, 2000 event with M 7.9 (Petersen et al., 2004). This past statistics predicts the recurrence time of M8-class on the order of 100 years. Since, very large subduction earthquakes in some cases do not follow the Gutenberg-Richter relations (so was the M7.9), it is difficult to assess the recurrence time by this method with a short historical record. The subduction rate is about 5 cm/year normal to the trench axis. Assuming the interplate earthquakes only account for the trench normal component as supported by the earthquake mechanism solutions, 5-10 m of slip corresponds to years if the plate is fully locked between the earthquakes. Another way of seeing the event is to consider how M8-class might accommodate the plate motion. Since log Mo[Nm] = 1.5 Mw +9.1 (Kanamori, 1977) and Yamanaka (2005) estimate is Mo=3.5x Nm, her average slip estimate is 7m from Mo=µAD (if µ=3x10 10 N/m 2 and A=850x 200 km 2 ). If this 7m slip is to be accommodated by a fault dimension of 200 km x 120 km (length x width), Mw=8.4. This means 7 events equal the 2004 moment release, which, may not be far off from the historical earthquake distribution (Fig ). However, these estimates likely include large errors. It is important to independently determine what the fault dimension is from local seismological observations.

2 Fig Aftershock distribution determined by USGS ( news/2004/). Note the oblique plate convergence motion. The absence of active volcanoes exists between about 6-deg N and 12.5-deg N where Andaman Sea spreading axis seems to align with the Sumatra Fault subparalleling the volcanic front on the Sumatra Island.

3 1941 M M M M M M7.2 Fig Historical earthquakes (from Utsu) since 1800 causing tsunami disasters. Utsu list does not include the 1881 event (Ortiz and Bilham, 2002). Ellipses showing rupture areas are approximate. Contours in red show estimation of slip magnitude obtained by Yamanaka (2005) for the 2004 event (contoured every 1 m over 3m of slip). Red asterisk is the epicenter.

4 Subduction geometry off Sumatra The area where the 2004 event took place has practically no local geophysical survey data to reliably infer the plate subduction geometry. We note that although the trench axis curves along the axis, so does the volcanic front on the Sumatra Island (Fig ). Kopp et al. (2001) studied the structure offshore southern Sumatra. Since the plate geometry seems to follow the curve and since the free air gravity anomaly shows similar features across the trench axis, we estimate the plate geometry in the area of our study as shown in Fig The estimated plate geometry is an extrapolated and composite one based on the data from Engdahl et al. (1998), Fauzi et al. (1996), and Kopp et al. (2001). This geometry has to be confirmed by this study as well as better defining the zone using focal mechanisms. In particular, it is important to define the updip and downdip end of the seismogenic zone, where the plate boundary slips in unstable condition causing earthquakes. The zone width delimits the maximum size. We will now put this estimate in perspective. The Sumatran subduction zone is where Indo-Australian plate with an age of around 50 Ma subducts and the plate seems to start bending at about 20 km depth more than 100 km away from the trench axis. At the JapanTrench and the Izu Ogasawara Trench, > 100 Ma Pacific Plate subducts and bends at about km depth more than 150 km away from the trench axis. On the other hand, at the Nankai Trough, where much younger Philippine Sea Plate is subducting, the plate bends at about 10 km depth less than 100 km away from the trough axis. These observations suggest that the oceanic plate subducts and increases its subduction angle from a very shallow angle depending on its age, that is older the more rigid. There does not seem to be peculiarities associated with the Sumatra subduction zone except for the partitioning of the plate motion into dip and strike components as mentioned earlier. Note that subduction dip angles are normally referred at deeper depths and not at this shallow depths where plate geometry is not well defined without seafloor observations.

5 5.3 Ocean Bottom Seismographic Ovservation (Araki et al.) Figure Seismicity and isodepth contours from Fauzi et al. (1996). Red contour is additional iso-depth at 50 km depth based on Engdahl et al. (1998). Red dots are active volcanoes.

6 Figure Topmost curve is free-air gravity anomaly from southern Sumatra section where offshore seismic experiment was conducted (Kopp et al., 2001). Second panel is the crustal structure from Kopp et al. (2001). Hypocenter plots from Engdahl et al. (1998) (black dots) and aftershocks of 2004 events (red dots) are partially shown in the third panel (normal to the trench from about 3-degN and 94-degE). Green line approximates the trend of the seismicity excluding shallow events. The bottom panel shows seismicity using Indonesian network data in the section normal from about 1.5-degN and 96.5-degE) from Fauzi et al. (1996). Red curve is the estimated subducting plate boundary following the better estimates of plate boundaries from each portion Seismicity before 26 December 2004 earthquake The epicenters of seismic events before great Sumatra earthquake 26 December 2004 were located mainly in Aceh basin and Simeuleu basin (Fig ). Based on the distribution of the seismicity in that area, from the Aceh basin to the north of Simeulue basin, there is a tendency that the depth of earthquakes becomes shallower. The mechanism of events is dominated by thrust fault mechanism. In the Aceh basin, some earthquakes of normal component were observed. In this region, earthquakes of mechanism due to oblique subduction processes seems to be very rare. (Around the ridges between the Sumatra trench and the Aceh basin several normal mechanism distribution was also observed.) Along the trench focal mechanism was dominated by thrust fault. In general, the depth of the source averaged around 40 km in the Aceh basin and shallower to the trench. The deepest hypocenter in this region exceed 80 km and the shallowest was around 10 km. A less dense seismicity area also observed between Simeuleu and Nias basin Seismicity after 26 December 2004 earthquake The aftershock distribution of the great Sumatra eartquake mainly occur in the western of

7 Aceh basin. There are smaller number of aftershocks observed onland or near Sumatra island. In Simeuleu and Nias basin, the distribution of aftershock is very rare. The depth of hypocenters of the aftershock events, beneath the Aceh basin (A), is approximately between 30 and 60 km. The focal mechanism of aftershock events is dominated by low angle thrust mechanism there. We observe many thrust fault earthquakes offshore W Simelue near the Sumatra trench (B). Several normal fault earthquakes are observed in between (A and B). Figure Regional pre-and-post event seismicity of December 26 Sumatra earthquake. The points without relief show earthquakes from Engdahl et al. (1998) from 1964 to The points with red relief shows earthquakes determined by USGS after the Sumatra earthquake on December 26, 2004 (until mid-february, 2005). Colors of the points represent

8 depths of the earthquakes as shown by the scale inside the map. Beach balls are Harvard CMT solutions (red ones for after the event and black ones for before the event). Black lines from some of beach ball connect to their hypocenter. Gray lines show projection plane for those cross section seismicity shown in Fig Figure 5-3-6: Cross section view of seismicity shown in Fig Black points: seismicity before the December 26th event by Engdahl et al. (1998). Red points: seismicity of the aftershocks determined by USGS. Each projection plane is shown in Fig as gray lines whose ends correspond to the edges of the plate (-1.5 and 1.5 degrees from the projection center). Projection center latitude and longitude is shown at upper left corner of each plate, and unit of horizontal axis is geographical degree Section description Fig shows cross-sectional view of pre-seismic and post-seismic events for the Sumatra

9 earthquake on December 26, Section #1 (Uppermost section) Aftershock events occur from the trench side and distributed with low angle pattern to landward side. Aftershocks distributes mainly between offset 0.0 to 0.5 degrees with depth around 20 to 40 km. No event observed before shocks in trench side. Before the mainshock, events are distributed with low angle toward land, and a group of shallow ones beneath the Sumatra Island. Section #2 This section corresponds to the location of our OBS network. Aftershocks events in the trench side appears more dense than in section #1. The low angle pattern of aftershocks appears with main distribution on offset 0.0 to 0.5 degree. Again, before the main shock no event has been observed in the trench side. Before the main shock, subduction earthquakes start to occur from -0.5 degree with low angle distribution to landward. The same group of shallow earthquakes observed in the section #1 can be seen here. Section #3 Aftershocks event occur from trench side direction to landward direction. Main distribution of this event is in between 0.0 and 0.5 degree. Very rare before shock event occur in trench side. Low angle subducting slab was observed from the distribution of events before the main shock. For the earthquakes after the main shock, the depth is not well determined. Section #4 Several aftershock events occur in rather dense in the trenchward and lower activity in landward side. Before shock, distribution was dense in the between -0.5 to 0.5 degree. A gap of events was observed in -1.0 to -0.5 degree but several events occur in trench as shallow events. Section #5 (Lowermost section) Aftersock events are very rare, distributed in the trench and about -0.5 offset with a gap in between. One aftershock more than 80 km depth (offset ~ +0.2). Before shock, events (?) concentrated mainly between offset -0.5 to +0.5 degree with depth varies between 30 km to 60 km Objective of OBS observation and the network geometry Seismic observation using ocean bottom seismographs (OBS) was carried out offshore northern part of the Sumatra Island. We laid an OBS network to determine detail of after shock seismicity after the great Sumatran earthquake on December 26, The purposes of

10 this OBS observation are to determine precise location of aftershocks around the area of the main shock. The image of precise aftershocks as well as those mechanisms should help us to delineate the geometry of subducting oceanic plate that caused the great Sumatran earthquake, and assess possibilities for large aftershocks by recovering activity of subsurface faults. The OBS network consisted of two long-term OBS (LT-OBS) and 17 short-term OBS (we call one-month observation as "short-term"). Ten out of 17 OBSs are from JAMSTEC and others including LT-OBSs from ERI, the Univ. of Tokyo. The 17 OBS were deployed with variable spacing between the stations from 15km to 30km as shown by Fig The JAMSTEC OBSs were deployed in the northern part of the network covering the Aceh basin and in the outer high in the south of the Aceh basin. The ERI OBSs were deployed in the southern part of the network in the outer high. The area of the SP-OBS network covers the area of seaward transition about 100km to the Sumatra trench from the one of the largest slip area in the main shock determined by Yamanaka (2005). The location of the network also corresponds to the area of large seafloor deformation inferred from Tsunami waveform inversion by Hirata (2005). The most landward edge of the network is situated at the high aftershock activity area found from the aftershock seismicity determined by USGS using the global seismic network (Fig ). The centroid locations of Harvard CMT solutions for these aftershocks shift about 20~30km seaward from those hypocenters determined by USGS. The OBS network observation should help confirm precise location of these aftershocks. To the southwest of the network, the number of known aftershocks becomes significantly small, although there are some of which mechanisms are not well known. This transition may define the dipping end of the slipped area in the main shock, or less possibly remaining locked plate boundary. In both cases, the precise geometry of aftershock distribution is a key data to re-analyze Tsunami and teleseismic data to reveal the mechanism of the Sumatra earthquake in December 26, The along arc coverage of the network is approximately 50km, restricted from the number of the OBSs available and the spacing required for precise determination of depths of earthquakes. The distances between stations are made shorter to the Sumatra trench. The intention to set such a variable station-to-station distances is to better resolve depths of aftershocks in the subducting slab, and yet to have wider network coverage with a limited number of OBSs. As a rule of thumb, the distances are set to be similar to the depth of the subducting plate boundary. The depths of the subducting slab is not known very well, but from globally determined hypocenters from 1964 to 2004 (Engdahl et al., 1998), the depths seem to span from approximately 10km at the south-western end of the network to approximately 50km in the north-eastern end of the network (Fig 5-3-7). The OBSs are aligned as three parallel arrays in 55 degrees of north, parallel to the dipping direction of the Wadati-Benioff zone there. Two LT-OBS, combined with existing land seismographic station in Banda-Aceh, cover the area of the whole short-term OBS network and around. From our interests of

11 offshore extension of small number of seismicity, one of the OBS was placed close to the Sumatra trench. We placed the OBS at shallower than 3000m depth on the landward slope of the trench, because we wanted to keep a chance to recover it by the ROV Hyper Dolphin to minimize a possibility losing the OBS data.

12 Figure The network of Ocean Bottom Seismographs and existing land seismographic stations (by open triangles with name). The points without relief show earthquakes from Engdahl et al. (1998). The points with red relief shows earthquakes determined by USGS after the Sumatra earthquake on December 26, 2004 (until mid-february, 2005). Colors of the points represent depths of the earthquakes as shown by the scale inside the map. SCS lines that pass above the OBSs are plotted by black lines with its starting point indicated by the line numbers.

13 5.3.6 Onland seismographic station in Sumatra Island Indonesian Meteorological and Geophysical Agency (BMG) in coordination with JISNET (Japan Indonesia Seismographic Network by NIED, Japan) has established several seismographic stations in Sumatra Island. The nearest seismographic station to the epicenter of the Sumatra great earthquake on 26 December 2004 is Banda Aceh (BSI), a broadband type seismographic station. This station located at lat 5 o and lon 95 o on the northern tip of Banda Aceh. At first, we are in doubt about the activity of this station after the earthquake and tsunami event. A message have been sent to BMG to ensure of the activity of this station and returned with positive response of the status of this station. The geographical situation of this station given in Fig and the position of another seismographic station around Sumatra Island listed in Table Seismographic net in Indonesia as of 豪州 ( 計画中 ) JISNET seismometer(cmg3t)*23 OHP seis. (STS1)+VSAT *2 CTBT Aux obs.(sts2 + VSAT)*5 IRIS obs.of USA Figure Seismograph network in Indonesia.

14 Table Seismographic station around Sumatra island. No. Station Type Code Island Latitude Longitude deg min sec deg min sec 1 Banda Aceh Broadband BSI Sumatra Tuntungan Short Per TSI Sumatra Parapat Short Per PSI Sumatra Gunung Sitoli Short Per GSI Sumatra Padang Panjang Short Per PPI Sumatra Deployment We conducted a series of deployment operation from February 18th to February 19th as Table except for the LT-OBSs. The LT-OBSs are deployed on February 24th and March 4th. The universal time is used for all OBS records. The recording period of the SP-OBS is set for JAMSTEC OBSs from 2/21 17h UT, and for ERI OBSs from 2/20 05h UT until 3/26 00h UT (after recovery). The deployment position, time, transponder code, and beacon call sign are listed in the Table The settings of the OBS recorders are summarized in the Table Figure Long-term OBS being deployed from R/V Natsushima at LT-1.

15 5.3.8 Airgun shot for OBS As well as natural earthquakes, the OBS arrays recorded airgun signal from R/V Natsushima during the recording period. Airgun was shot for single channel seismic survey for most of the seismic lines, so the shooting interval and airgun volume was not very suitable for OBS recording. Three seismic lines (#9, #18, and #19) were shot above the OBS arrays as shown by Figure (OBS network). Airgun shot strategy for the line #18, #19 was optimized for OBS recording. For line #18 and #19, airgun volume was increased to 250 (generator) (injector) cuin at 130 atm, and these survey lines were designed to pass right above each OBS along the seismic line. The geographical coordinates start time, and end time of these seismic lines are summarized in Table in SCS section of this cruise report. It is necessary to record precise time of airgun shot to use with OBS record. Timing of shot triggering generated by SCS recording system was measured by the master clock, which was controlled by GPS, in 1ms accuracy. The measured time was recorded in shot files as listed in Table (SCS line). There is delay between the shot triggering and actual airgun shot. We measured the delay from the signal from hydrophone attached to the airgun using an oscilloscope. For SCS Line #18 and #19, shot delay was 208ms after triggering. For SCS Line #8~#17, shot delay was 120ms after triggering OBS recovery All short-term OBS were recovered in the second leg of the cruise from the morning of March 11th to the afternoon of March 13th. The order of OBS recovery was 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 16, 14, 11, 12, 13, and 17. On recovery, R/V Natsushima came above the OBS location and send acoustic command to release anchor weight of the OBS. The acoustic command was sent from the equipment in the control room behind the bridge. It took approximately 13 minutes after command until anchor weight is release from OBS. After release, we keep track of OBS ascending by acoustic positioning system (SSBL) for JAMSTEC OBS until we find the OBS in the surface. For ERI OBS, we continue ranging OBS by acoustic transponder to get idea about OBS position until surfacing. After surfacing, all the OBS were recovered during the daytime using a rubber-boat launched from R/V Natsushima. In the rubber boat, two men picked up the OBS using wood bar and the other one steered the boat. The OBS was then transferred to Natsushima by a crane on the starboard side of the vessel. After recovery, OBS was disassembled of acoustic transponder, release parts, radio beacon, strobe light, and hydrophone (if it has). These parts were washed by fresh water and cleaned. Also, OBS clock was compared with GPS as soon as it was recovered to estimate as accurate timing in the seafloor as we could. We recovered data from all JAMSTEC OBSs onboard. Glass sphere of JAMSTEC OBS, which stores data recorder and batteries, was opened after one-day waiting after recovery. This is to ensure equal temperature in and outside the glass sphere to prevent

16 moisture in the air making dew in the electronics board. We checked condition of the recorder and stopped the recorder. The data was stored in a HDD package. We take out the HDD pack and connected to PC to backup all the recorded data. The data recovery work was done in the second laboratory. Data recovery of ERI-OBS is planned after transportation of OBSs to ERI office in mid-april Figure OBS being transferred to R/V Natsushima after recovery by rubber boat.

17 Figure Ocean Bottom Seismometer being disassembled just after recovery. Left: Shimizu from NME Right: Fujiwara from JAMSTEC Preliminary analysis: determination of hypocenters from OBS data We recovered data from JAMSTEC OBS onboard. We confirmed that data was normally recorded except for OBS01. OBS01 had no record in the hard disc drive, which may be due to malfunction of recorder by some reason. The record from OBS was plotted as shown in Appendix-C.1. These figures represent monitor record for one day in a single page. There are many earthquakes detected from the smallest as less than of magnitude 1 to the largest as of USGS magnitude 6.7 near Simeulue Island on February 26th. We detected seismic events that were seen at more then three OBSs from all available OBS data onboard (OBS02-OBS10). The number of detected events was more than 2800 for period from February 21st until March 11th. There are also airgun shots clearly seen in the plots, which look like spikes that occur in the same time intervals for an hour or two. Time-distance diagram of airgun shot record was plotted as shown in the Figure Appendix-C.2 for OBS10 for example. Similar records were obtained from other OBSs. We examined the detected seismic events from February 21st to February 24th and determined their hypocenter location using the method by Matsuura-Hirata (1980). The result of location is shown as Fig and Fig The seismic velocity model used in this preliminary determination of earthquake is similar to that in offshore Sanriku near Japan Trench. The velocity model can be significantly different from that of Aceh basin, although we expect to improve the velocity model from analysis of airgun record in the future analysis.

18 Also, these results are not reviewed at all, and are only meant for preliminary onboard analysis. Therefore, the following description is subject to systematic errors especially for their depth. The aftershocks derived from the preliminary analysis infer that a large number of earthquakes occur beneath Aceh basin at depth greater than 20km. The aftershocks are aligned as a dipping plane at similar angle as of previous seismicity by Engdahl et al. Therefore, it is inferred that these aftershocks occur at plate boundary interface or in the subducting plate, not in the crust of upper side of the plate boundary. On the other hand, there are not many earthquakes but none beneath the outer arc high. This coincides with the location of large slip inferred by Yagi (2005). We expect locating these small number of earthquake will help us to define plate boundary that seemed to slip during the last Sumatra event on December 26, 2004.

19 Figure seismicity from February 21 17h UT to February 24th 00h UT from OBS network. Hypocenters are shown by colored squares with different colors with depth. The color scale is shown in the bottom left. Location of OBS and land station is shown by triangles with name. In this analysis, only data from OBS02-OBS10 were used. Figure Cross sectional view of the same hypocenters shown in Fig. by blue circles. The locations of OBS are projected by inverse triangles as well as seafloor bathymetry shown by solid line. Projection is along SCS line #18 centered at 'N 'E. Unit of horizontal axis is degrees.

20 References E.R. Engdahl, Van der Hilst, R.D., and Buland, R.P., 1998, Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seism. Soc. Amer., v. 88, pp

21 Table 5-3-1: OBS deployment data Site No. OBS No. Casting time(utc) Cast Water depth transponder S/N code freq[mhz] callsign /2/18 00: N E 2005/2/18 0: N E JX1053 9E JS : N E 2: N E JX1020 7B JS : N E 4: N E JX1057 1C JS : N E 6: N E JX1036 3F JS : N E 8: N E JX1030 1F JS : N E 10: N E JX1037 4D JS : N E 12: N E JX1013 5A JS : N E 14: N E JX1004 2A JS : N E 16: N E JX1099 6F JS : N E 17: N E JX1023 8B JS1118 Landing Vessel position at casting Landing time OBS depth at OBS landing position Lat Long (UTC) landing[m] Lat Long beacon Site OBS Vessel position at casting Landing time S/R at OBS Vessel position at OBS landing transponder beacon Casting time(utc) Water depth No. No. (UTC) landing[m] Lat Long Lat Long S/N code freq[mhz] callsign 11 ERI-2G 2005/2/18 19: N E 2005/2/18 19: N E MX D JS ERI-2E 21: N E 21: N E MX B JS ERI-2B 22: N E 22: N E MX C JS ERI-2D 2005/2/19 00: N E 2005/2/19 0: N E MX C JS ERI-2F 02: N E 2: N E MX D LS ERI-2A 03: N E 4: N E MX A JS ERI-2C 05: N E 6: N E MX A JS122 OBS landing position Site OBS Vessel position at casting Water depth for transponder beacon Casting time(utc) Water depth Landing time(utc) (three points calibration) No. No. calibration Lat Long Lat Long S/N code freq[mhz] callsign LT /2/24 03: N E SI JS125 LT /3/4 03: N E N E SI JS1284 1) Before deployment, the following check was made: Transponder ( call and response, terminal resistence, terminal voltage, S/N, code), Radio beacon (Signal, callsign), Flusher function, and taking out clamp screws of sinker. 2) Slant range mesurement for LT-1 site was made, but determination of the OBS landing position by the mesured data has problems, possibly due to insufficient offset from the vessel to the OBS deployment l

22 Table OBS recorder configration Site No. OBS No. Rec ID Number of disks HDD ID OBS depth at landing [m] OBS landing position Recording (UTC) befor deployment time offset after recovery time Lat Long start time end time True time OBS time delay +[sec] True time OBS time offset N E 2005/2/21 17:00: /3/11 02: /2/17 23:50: /3/11 03:26: N E 2005/2/21 17:00: /3/11 04: /2/18 01:33: /3/11 05:21: :22: N E 2005/2/21 17:00: /3/11 06: /2/18 03:28: :28: /3/11 07:35: :35: N E 2005/2/21 17:00: /3/11 08: /2/18 05:42: :42: /3/11 09:58: :58: N E 2005/2/21 17:00: /3/11 11: /2/18 07:41: :41: /3/11 12:14: :14: N E 2005/2/21 17:00: /3/11 22: /2/18 09:51: :51: /3/12 00:04: :04: N E 2005/2/21 17:00: /3/12 01: /2/18 11:37: :38: /3/12 02:12: :12: N E 2005/2/21 17:00: /3/12 03: /2/18 13:37: :37: /3/12 04:29: :29: N E 2005/2/21 17:00: /3/12 03: /2/18 15:36: :36: /3/12 06:36: :36: N E 2005/2/21 17:00: /3/12 07: /2/18 16:49: :49: /3/12 08:23: :23: Number S/R at OBS Vessel position Time calibration (UTC) Site OBS Rec HDD Recording (UTC) of landing at OBS landing befor deployment time after recovery time No. No. ID disks ID [m] Lat Long start time end time True time OBS time offset True time OBS time offset 11 ERI-2G 065N N E 2005/2/20 5:00: /3/13 00: /2/18 18:53: :53: /3/13 01:29: :30: ERI-2E 015N N E 2005/2/20 5:06: /3/13 02: /2/18 20:43: :43: /3/13 03:16: :16: ERI-2B 042N N E 2005/2/20 5:05: /3/13 03: /2/18 21:58: :58: /3/13 04:55: :56: ERI-2D 047N N E 2005/2/20 5:01: /3/12 23: /2/18 23:48: :48: /3/13 23:49: :49: ERI-2F 074N N E 2005/2/20 5:02: /3/12 09: /2/19 01:42: :42: /3/12 10:06: :06: ERI-2A 048N N E 2005/2/20 5:03: /3/12 11: /2/19 03:31: :31: /3/12 12:06: :07: ERI-2C 038N N E 2005/2/20 5:04: /3/13 05: /2/19 05:05: :05: /3/13 06:24: :25: Number depth OBS landing position Time calibration (UTC) Site OBS Rec HDD Recording (UTC) of at casting (three points calibration) befor deployment time after recovery time No. No. ID disks ID [m] Lat Long start time end time True time OBS time offset True time OBS time offset LT , CF5 2+CF N E 2005/2/24 5:00: /7/30 5:00: /2/24 02:21: :21: LT N E 2005/3/4 5:00: /7/31 0:00: /3/4 01:41: :42: Remarks 1)'Recording end time' : time when OBS accepted recovery command. 1) Site01, OBS08 : No data was recorded in HDD. 2) Site04, OBS64 : Seafloor material was soft(muddy). Problem? 3) Site 12, ERI-2E : HDD ID#1 (Toshiba 33R22197S) had problem in spinning up. The recorder was set up to start from HDD ID #2. 4) Site 12, ERI-2E : Connector for the recorder power is bypassed for some conductors. 5) Accelorometer of Site LT-2 starts recording from 2005/2/17 03:00:00 Time calibration (UTC)