Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low V p, high V p/V s in the transparent frontal prism
© Nakamura et al.; licensee Springer. 2014
Received: 10 March 2014
Accepted: 13 September 2014
Published: 22 September 2014
Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration (PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (or chaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the PSDM analysis shows low velocity in the frontal prism and velocity reversal across the backstop interface. The PSDM velocity model around the drill site is similar to the P wave velocity model calculated from the ocean bottom seismograph (OBS) data and agrees with the P wave velocities measured from the core experiments. The average V p/V s in the hanging wall sediments around the drill site, as derived from OBS data, is significantly larger than that obtained from core sample measurements.
KeywordsJapan Trench Seismic image P and S wave velocities JFAST
The 2011 Tohoku earthquake occurred along the Japan Trench subduction zone east off the northern Honshu island, Japan. The earthquake rupture likely propagated along the plate boundary up to the trench axis. (e.g., Ide et al. 2011; Kodaira et al. 2012). Rapid response drilling by the Integrated Ocean Drilling Program (IODP) Expedition 343 (Japan Trench Fast Drilling Project (JFAST)) successfully drilled through the inferred plate boundary fault approximately 6 km landward of the trench axis. The expedition obtained logging data and core samples, and installed and recovered temperature sensors (Chester et al. 2012). The drilled plate boundary fault is very thin (<5 m) and localized in a pelagic clay layer approximately 15 m above the basal chert layer (Chester et al. 2013). Borehole breakout orientations indicate the stress state of the hanging wall is in normal fault regime and a complete stress drop occurred during the earthquake (Lin et al. 2013). The temperature observatory recorded a residual co-seismic frictional heat anomaly at the plate boundary (Fulton et al. 2013) and estimated a low friction coefficient (<0.1) that is consistent with values from high-speed friction experiments on the fault core sample (Ujiie et al. 2013). All of these results support the hypothesis that co-seismic slip extended to the very shallow portion of the plate boundary. However, it is still difficult to fully understand the in situ physical properties of the hanging wall of the plate boundary fault because of the limited logging data (gamma ray and resistivity) and limited cored intervals obtained during the drilling expedition. Local scale seismic images were previously obtained in a high-resolution seismic survey around the drill site (Chester et al. 2012, Nakamura et al. 2013), but a regional scale seismic section crossing the drill site had not been obtained. A regional scale seismic data is necessary to determine the velocity structure of the wedge and directly connect the drill site information to the background regional scale geological structure. To understand the structure and the physical properties of hanging wall sediments, a post-drilling multi-channel seismic (MCS) survey was conducted around the JFAST drill site. We processed the MCS data up to the pre-stack depth migration (PSDM) to produce an accurate depth seismic image and the P wave velocity structure. We also modeled travel-times of converted S wave arrivals observed at the ocean bottom seismographs (OBSs) to obtain the shear-wave velocity information around the drill site, which cannot be retrieved using the MCS data. We compare the seismic-derived velocity information with drilling results and controlled pressure V p and V s core measurements and discuss the similarities and differences between them.
Seismic survey KR13-01
MCS data processing
The MCS data were first processed following a conventional procedure that includes minimum phase conversion, predictive deconvolution, common midpoint (CMP) sorting, velocity analysis, normal moveout, stacking, and post-stack time migration (PoSTM). Noise reduction by f-x projection (Soubaras 1995) and f-x prediction filtering (Canales 1984) was applied to reduce the swell noise and seismic waves by local earthquakes. Surface-related multiple elimination and parabolic radon transform were applied to suppress multiples. Pre-stack time migration (PSTM) analysis was applied to produce a fine-tuned time-domain seismic image. This PSTM section was used to establish the structural formation model for further PSDM analysis.
The MCS data of the line JFD1 were processed until PSDM to produce the depth seismic section and the corresponding P wave velocity model. A ‘layer-cake’ approach was used for PSDM velocity analysis where we first determined the velocity in the shallowest formation and then sequentially moved down to deeper formations. The velocity model v(z) in each formation has the following form: v(z) = v0 + g (z - z0), where z is the depth, v0 is the P wave velocity at the top of the formation, z0 is the top depth of the formation, and g is the velocity gradient. A depth section was created by PSDM with a defined layer-cake velocity model, and horizon semblance was examined to evaluate the velocity model. We determined v0 and g to achieve the best improved horizon semblance over the target horizon by trial and error. Once a satisfactory layer-cake model was achieved, the velocity model was updated by a horizon-based residual velocity analysis which evaluates the moveout of migrated common reflection point (CRP) gathers using horizon semblance. The velocity was modified to shift the semblance peak to zero residual. The velocity model was finally updated by grid-based travel-time tomography to accommodate the local scale velocity variation. Clear but local reflective events (inter-layer segments) were picked for the input to the tomography, and grid-based velocity model was updated to flatten the CRP gathers at picked events. The velocity model cannot be resolved below the top of the oceanic crust due to the length of the streamer cable, so we use the deeper velocities determined in previous studies of the Japan Trench (Tsuru et al. 2000, Tsuru et al. 2002, Miura et al. 2005). Automatic gain control was used to enhance the deeper reflections in the gathers because we prioritized obtaining the best depth image and corresponding velocity model over amplitude preservation in this study.
OBS data analysis
where d t is the time lag between refracted P wave and the converted PPS wave, and tc and tsf are the two-way travel time of the conversion point and the seafloor shown on the time-domain seismic section.
P and S wave velocity measurements of the JFAST core sample
List of the core samples used in this study
Unit 3 wedge sediments
Unit 3 wedge sediments
Unit 3 wedge sediments
Unit 3 wedge sediments
Unit 3 wedge sediments
Unit 3 wedge sediments
Line JFD1 PSDM section
Velocity uncertainties in the PSDM analysis
Line JFS12 PoSTM section
Velocities measured from core samples
In situ velocities, V p/V s, and Poisson's ratio estimated from core experiments
V p (km/s)
V s (km/s)
V p/ V s
Seismic structure and velocity in the frontal prism
The frontal prism is located at the seaward edge of the hanging wall, and it is thought to have been horizontally displaced more than 50 m during the Tohoku earthquake (Fujiwara et al. 2011, Ito et al. 2011). Our PSDM image (Figures 3a and 4) shows a relatively transparent frontal prism, although several landward dipping discontinuous reflections can be distinguished (Figure 4). Some of these reflections extend from the plate boundary to the seafloor and have been previously interpreted as reverse faults (e.g., Tsuji et al. 2013); however, the lack of stratigraphy in the frontal prism makes it difficult to determine the sense of slip. The seismic velocity within the frontal prism is approximately 2.0 km/s in average and 2.5 km/s at maximum (Figure 3c). The velocity model is comparable with the model in Tsuru et al. (2000) obtained in the Japan Trench approximately 50 km north of our survey area, but the model in this study has slower velocity in the landward deeper part within the frontal prism. The difference is probably a consequence of the lower velocity gradient value in the velocity model adopted in this study. Park et al. (2010) obtained a detailed seismic image and velocity volume from a 3D reflection seismic data in the Nankai subduction margin off the southwest Japan. Their velocity model shows that the P wave velocity in the hanging wall accretionary prism is approximately 2.0 km/s at the trough axis, but it increases to approximately 3.0 km/s at 10 km landward of the trough axis. They reported the presence of a low velocity zone (LVZ) within the outer wedge of the accretionary prism, 10 to 25 km landward of the trough axis; however, the velocity is 2.7 to 3.2 km/s in the LVZ.
P wave velocities around the site C0019
We obtained V p around the drill site C0019 from two different seismic data analyses: one is from the PSDM analysis using the MCS data and the other is the 1D velocity derived from tau-p/tau-sum analysis on the OBS data (Figure 8). The PSDM velocity has lower velocity (1.6 km/s) at the top of the sediment layer and higher velocity (2.3 km/s) at the bottom, with larger gradient than the OBS velocity. Small scale velocity change within the frontal prism is not well resolved from the PSDM analysis due to the lack of internal reflections (Figures 4 and 5). However, the averaged value of the PSDM velocity in the frontal prism formation is approximately 2.0 km/s, which is comparable with that obtained from the OBS data analysis (Figure 8). The comparison of the seismic data with the drilling results during the Expedition 343 indicated that averaged interval velocity of the interval between the seafloor and the strong reflection near the top of the chert layer is approximately 1.9 km/s which is also comparable to the OBS velocity and the averaged PSDM velocity (Figure 8). Estimated in situ V p values are 2.00 to 2.23 km/s for cores 4R, 6R, 7R, and 8R, which is also consistent with the velocity obtained from the seismic data analysis (Figure 8). For deeper cores, 15R and 16R, in situ V p is slightly higher, ranging between 2.35 to 2.48 km/s. These values match with the PSDM velocities (Figure 8); however, the PSDM velocity curve in this depth range (7,700 to 7,800 mbsl) could be affected by the vertical smoothing during the velocity model building. The higher velocity values measured from core samples may represent the faster velocity layer at the bottom of the prism, which is below the nominal resolution of the seismic system at this depth level.
High average V p/V s in the frontal prism
According to our analysis, the average V p/V s ratio in the prism around the site C0019 (or OBS JF1) is 4.4 + 0.7/-0.5 or larger. This is equivalent to a Poisson's ratio > 0.46 and V s < 0.48 km/s. V p and V s under the saturated and confined pressure situation (Figure 10, Table 2) give much lower V p/V s 2 to 2.5.
We have compared our results with existing empirical V p-V s relationships. We have used mudrock line for clastic silicate rock (Castagna et al. 1985), which is often used to evaluate the V p and V s in marine sediments. Following the mudrock line V p = 1.16 V s + 1.36, the V p/V s can take significantly large value if the V p is low. The V p/V s is estimated at 4.08 assuming V p = 1.9 km/s (5% reduction from 2.0 km/s), which is located at the lowest possible range from our observation if we consider all uncertainties on the velocity analysis to reduce V p/V s. We note that the core measurements were limited to the section deeper than 600 mbsf (Chester et al. 2012), and that the velocity measurements can only be carried on the samples with enough stiffness. It is likely that the shallower frontal prism, which was not sampled during the Expedition 343, and/or the intervals with poor recovery that did not provide adequate samples for measurement, could have larger V p/V s increasing the average V p/V s value in the hanging wall sediments. Large V p/V s values have been reported in the sediments on the incoming plates seaward of the trench and trench fill sediments. The V p/V s of the incoming sediments on the Pacific plate in the Kuril Trench was estimated to be approximately 8 from the PPS-converted wave observed by the OBSs (Fujie et al. 2013). Peacock et al. (2010) reported high Poisson's ratio (>0.45) in the trench fill sediments in the Nankai Trough, which corresponds to the V p/V s values larger than 3.3. Similar high V p/V s ratio (approximately 3) was reported for the incoming sediments in the Nankai Trough (Tsuji et al. 2011). On the other hand, Tsuji et al. (2011) reported that the V p/V s value is approximately 2 at the toe of the hanging wall accretionary prism landward of the trough axis in the Nankai Trough, which is approximately half of our observation in the Japan Trench. The Poisson's ratio of sediments is affected by the consolidation, i.e., unconsolidated sediments show high Poisson's ratio (e.g., Prasad 2002). Unconsolidated sediments in the shallowest portion of the prism could contribute to increase the average V p/V s and Poisson's ratio. Tsuji et al. (2011) pointed out that the sand-rich turbidite could reduce the V p/V s (and Poisson's ratio). In the Japan Trench, at least at the site C0019, sand-rich sediment is not observed in core samples and not suggested by gamma-ray log data. The lack of the sand-rich sediments might be one of the possible reasons to cause the high Poisson's ratio in the Japan Trench prism toe.
A newly conducted seismic experiment around the JFAST drill site provides a regional-scale depth seismic image and the corresponding velocity models. The obtained PSDM profile exhibits seismically transparent or chaotic frontal prism with several semi-continuous landward dipping reflections beneath the toe of the Japan Trench landward slope. The P wave velocity model derived from the PSDM analysis shows that frontal prism has the low average interval velocity approximately 2.0 km/s. The backstop interface is clearly imaged, and there is a velocity inversion from the Cretaceous sequences to the underlying frontal prism. The velocity values around the drill site derived from the PSDM analysis are reasonably comparable with the 1D P wave velocity model estimated from OBS data. The controlled effective pressure core P wave velocity is also similar to the velocities from the seismic data analysis. The averaged V p/V s calculated from the analysis of seismic data is approximately 4.4 in the hanging wall sediments around the drill site from OBS data, which is considerably larger than that calculated from core measurements. To reconcile these two apparently contradictory observations, it is necessary that the shallower part of the hanging wall sediments, which were not retrieved from core measurements, should have very high V p/V s values. The OBS-derived V p/V s in the hanging wall sediments is also significantly higher than that in the toe of other subduction zones.
We thank Dr. Tetsuro Tsuru, Dr. Masataka Kinoshita, and an anonymous reviewer for their valuable comments and suggestions which helped to improve this manuscript. We are grateful to the captains and crews of the research vessel Kairei for their help during the KR13-01 and KR13-08 cruises. We thank Makoto Ito of Nippon Marine Enterprises and his colleagues for their technical assistance during the KR13-01 cruise. We acknowledge Tsutomu Hayashi and Susumu Abe at JGI Inc. for their support in the PSDM analysis. The core sample used in this study was obtained by the Integrated Ocean Drilling Program. We also thank the captain and crews of the drilling vessel Chikyu during the Expedition 343. The operational and scientific support teams in the Center for Deep Earth Exploration, JAMSTEC are also acknowledged for their support. YN is also supported by a Grant-in-Aid for Scientific Research on Innovative Areas Number 21107002. Some figures were made with Generic Mapping Tools software.
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