Rupture History of the Great 2003 Tokochi-Oki, Japan, Earthquake - Evidence of the Influence of Subducted Seamounts on Rupture Dynamics?
The Mw 8.3 September 25, 2003 Tokachi-Oki earthquake ruptured the area of the Japan-Kurile trench that last broke in the great 1952 Tokachi-Oki earthquake. We invert broadband, teleseismic, SH data to find details of the rupture history. Having tested over 150 solutions with differing mechanisms and fault geometries, we find a preferred, robust solution which shows that rupture initiated on a shallowly dipping fault plane (strike 250°, dip 18°, rake 137°) at a depth of 18km. The main rupture proceeded down-dip, approximately northwards from the hypocentre, with a rupture velocity of ∼2 km/s, and ruptured two main asperities in the first 35 seconds of the earthquake. Rupture up-dip of the hypocentre initiated at ∼ 30 s and lasted until ∼ 45 s after earthquake initiation. Eighty-five percent of the total moment occurred by 50 s after eathquake initiation. Maximum slip of ∼18 m is obtained up-dip of the hypocentre with slip of ∼16 m in each of the 2 asperities down-dip of the hypocentre. Maximum slip-rates approaching 2.5 m/s are found in the down-dip asperities. Robustness tests on the solutions obtained were carried out to investigate if it is possible to still fit the data with reduced slip in the region of the asperities. We find that slip in the asperities down-dip of the hypocentre can only be reduced by a small amount (to ∼14 m) without degrading the quality of fits of solution seismograms to data, and are thus robust features of the solution. In contrast, slip up-dip of the hypocentre is less robust and not as well constrained by the data; slip in this region can be reduced to 3-5 m before there is a noticeable degradation in the fits of the data to the solution. The dimensions of the asperities are comparible to those of seamounts clearly visible in the gravity maps of the subducting plate and we propose that the asperities are related to seamounts that have been subducted. The high level of slip seen in the asperities is greater than the slip accumulated since the last great earthquake in the area and suggests that even magnitude 8 earthquakes do not always release all the stresses that have built up in a region.
The 2007 Pisco, Peru, Earthquake - Late Slip Close to the Hypocentre
The 2007, Pisco, earthquake ruptured a region of the Peru trench which has a complex seismological history. Throughout the earthquake record of the area it appears that there are several zones that can break either individually or together producing earthquakes ranging from magnitude ∼8 up to ∼8.5. This study investigates the seismological data surrounding the 2007 earthquake and determines the rupture history of the event. Initially, long-period mantle wave data from 80 stations and 179 channels is examined to evaluate the quality and robustness of the published global CMT solution. We find an optimal mechanism (strike 324°, dip 22°, rake 68°) that is close to the published solution. We then investigate the solution space surrounding this optimal mechanism and find that a wide range of dips are possible while still fitting the data, with dips as shallow as 8-10° having only marginally worse misfit values than the optimal solution. A more shallowly dipping fault plane is consistent with the inferred angle of the subduction interface from other studies. Short period broadband data is also examined with SH waves from 18 stations inverted to find a rupture history. The broadband data strongly favours shallowly dipping mechanisms with an optimum dip of 12°, well within the region of the long-period solution space with low misfit. The preferred solution has a single asperity close to, and to the Southeast of the earthquake hypocentre. This asperity breaks late in the earthquake rupture history, between ∼50 and 70s suggesting that either there were very low rupture velocities of less than 1km/s, or that there was a delay in the breaking of the asperity. Maximum slip and sliprate within the asperity are ∼5 m and ∼0.5 m/s respectively. The general location, size and slip of the asperity is in broad agreement with Geodetic measurements.
Interplay of Strike-slip and Thrust Faulting at the Obliquely Convergent Plate Boundary in Eastern Taiwan
The oblique convergence of the Eurasian and Philippine Sea plates in eastern Taiwan shows distinct strain partitioning between reverse faults and strike-slip faults in the suture zone. We studied three earthquake events occurred at this plate boundary with magnitudes larger than 6 during 2003 to 2006 to reveal the interaction between these two fault systems. We found that the first and the third events in time sequence may occur in the same reverse fault system. By contrast, the second event is clearly accommodated by strike-slip motion along the other fault system according to waveform inversion. Stress analyses show that instead of triggering the third event that shares similar focal mechanism and the same fault system, the first thrust event enhanced failure of the second strike-slip event. Likewise, the stress transfer from the second strike-slip event could have triggered the third thrust event on the reverse fault system. Such alternated slip on the reverse and strike-slip fault systems has also been reported in 1951 earthquake sequence in this suture zone with magnitudes greater than 7.
Is the Local Seismicity Along the North Shore of Lake Erie (Southwestern Ontario) Related to the oil/gas Production and Water Injection?
Over the past decades a number of small earthquakes have been recorded in Leamington - Ridgetown area along the north shore of Lake Erie (southwestern Ontario). A new seismic cluster is forming in this area, away from the already known clusters in Ontario. The new seismic area lays across the seismic area south of Lake Erie (along the Pennsylvania- Ohio border), known for some moderate induced events related to the oil production there. Another cluster related to the oil/gas production- in the region of Gobles, north of Lake Erie-has been documented and studied by Mereu et al. (1986). The induced seismicity is usually related not to the oil/gas production itself but to the water injection accompanying this production. The water injection is used in southern Ontario in Leamington - Ridgetown area to increase the oil/gas recovery from the existing reservoirs. The relationship between the new forming cluster and the ongoing oil/gas production north of Lake Erie is studied here. The parameters of the earthquakes in the area (hypocenter location, magnitudes, seismic moment, stress drop, and focal mechanism and/or seismic moment tensors for some events) are calculated using the POLARIS and Canadian National Seismic Network (CNSN) data. A temporary seismic network, consisting of four high-frequency three-component stations, has been installed in the fall of 2008 to record data from possible smaller events, not recorded by the permanent stations. The lithology, structural geology, and hydrology of the site are critical for determining if the water injection can induce seismic events. This type of data as well as data about the local tectonics (the existing faults) have been collected and analyzed. The main goal of this work was to find if any spatial or temporal correlations between the seismicity pattern and oil production/water injection exist. The preliminary results of the study suggest a correlation between the seismic activity and the oil/gas production. The study provides also additional information about the tectonic regime in southern Ontario and on throws some light on the hypothesis for induced seismicity due to the oil/gas production north of Lake Erie.
3D Ground Motion in the Georgia Basin Region of SW British Columbia for Intra-slab Earthquake Scenarios
We investigate long-period (> 2 s) ground motions in the Georgia basin region of SW British Columbia (BC) for intra-slab earthquake scenarios using 3D finite-difference simulations of viscoelastic wave propagation. The Georgia basin is a site of concentrated deep (25-80 km) Juan de Fuca plate seismicity in a region with over 2 million inhabitants and vital economic facilities. Earthquake waves are altered by 3D basin structure due to the generation of surface waves and S-wave focusing at the basin edges. To validate our simulation, synthetic surface waveforms are compared with 32 strong- and weak-motion recordings of the 2001 Mw 6.8 Nisqually earthquake spanning from Puget Sound, Washington, to southern BC. To investigate intra-slab earthquake scenarios we initiate the Nisqually-model source in six different locations beneath the NW-SE trending Georgia basin. The largest ground motions always occur NW of the source location, so the peak ground velocity pattern alters dramatically with source location. In all cases, ground motion is amplified at the edges of the basin due to S-wave focusing, as well as along a NE-SW velocity contrast that runs beneath the city of Vancouver. In greater Vancouver, the largest simulated ground motions (100 cm/s) occur for sources located beneath the SE portion of the basin.
Improvement of Algorithm for Detecting Land-surface Deformations Associated with Earthquakes from a Satellite-borne Microwave Radiometer Data
Interferograms formed from satelliteborne synthetic-aperture radar (SAR) data enable us to detect slight land- surface deformations related to volcanic eruptions and earthquakes. However, since the time lag between two scenes acquired by the SAR used to form interferograms becomes longer than the recurrent period of the satellite carrying it, we cannot determine when land-surface deformations occurred with high time resolution. Therefore, we have investigated another approach to detecting land-surface deformations with higher time resolution from satelliteborne sensor data. Laboratory experiments recently confirmed that microwave energy is emitted when rocks are fractured. Land-surface deformations are likely to be accompanied by rock failures. Therefore, if rocks are crushed by land-surface deformations, microwave energy generated by rock failures should be detectable by a satelliteborne microwave radiometer. Based on this concept, we first developed an algorithm to evaluate microwave energy generated by rock failures on the land surface using only the data of the Advanced Microwave Scanning Radiometer for Earth-Observation System (AMSR-E). We then corrected the data of AMSR-E using the profiles of air temperature and water vapor density retrieved from the data of the Advanced Microwave Sounding Unit (AMSU) and the Atmospheric InfraRed Sounder (AIRS) in order to more accurately evaluate microwave emission from the ground. We can assimilate the data of AMSR-E, AMSU and AIRS with small time error since they are carried by the same Aqua satellite and observed on the same area simultaneously. We presents the improvement process and the verification result of our algorithm.
222Radon Concentration Measurements biased to Cerro Prieto Fault for Verify its Continuity to the Northwest of the Mexicali Valley.
The need to know the exact location in the field of the fault traces in Mexicali has been an important affair due that the topography in this valley is almost flat and fault traces are hidden by plow zone, for this reason, the southern and northern ends of the San Jacinto and Cerro Prieto fault zones, respectively, are not well defined beneath the thick sequence of late Holocene Lake Cahuilla deposits. The purpose of this study was to verify if Cerro Prieto fault is the continuation to the southeast of the San Jacinto Fault proposed by Hogan in 2002 who based his analysis on pre-agriculture geomorphy, relocation and analysis of regional microseismicity, and trench exposures from a paleoseismic site in Laguna Xochimilco, Mexicali. In this study, four radon (222Rn) profiles were carried out in the Mexicali Valley, first, to the SW-NE of Cerro Prieto Volcano, second, to the W-E along the highway Libramiento San Luis Río Colorado-Tecate, third, to the W-E of Laguna Xochimilco and fourth, to the W-E of the Colonia Progreso. The Radon results allow us to identify in the Cerro Prieto profile four regions where the values exceed 100 picocuries per liter (pCi/L), these regions can be associated to fault traces, one of them associated to the Cerro Prieto Fault (200 pCi/L) and other related with Michoacán de Ocampo Fault (450 pCi/L). The profile Libramiento San Luis Río Colorado-Tecate, show three regions above 100 pCi/L, two of them related to the same faults. In spite of the results of the Laguna Xochimilco, site used by Hogan (2002), the profile permit us observe three regions above the 100 pCi/L, but we can associate only one of the regions above this level to the Michoacán de Ocampo Fault, but none region to the Cerro Prieto Fault. Finally in spite of the Colonia Progreso is the shortest profile with only five stations, it shows one region with a value of 270 pCi/L that we can correlate with the Cerro Prieto Fault. The results of this study allow us to think in the possibility that the Michoacán de Ocampo Fault is the Continuation to the South of the San Jacinto Fault, not the Cerro Prieto Fault.
Estimation of seismic energy and its correlation with focal mechanisms. Case of study: Venezuelan seismic events.
We investigate the correlation between focal mechanisms and seismic energy radiated in a sample of 180 seismic events occurred in Venezuela during the period 1957-2008. In order to study the apparent stress of the sample, we apply the method develop by Pérez-Campos and Beroza . We classify as well as characterize the sets of strike-slip, normal and reverse events through the parametric bootstrap resampling. In this way we make an attempt to use this technique to improve the analysis and better comprehension of the Venezuelan seismicity.
Envelope of coda waves for a double-couple source due to non-linear elasticity.
We have shown recently (GJI 2008) that for an explosive source the envelope of coda waves can be understood by non-linear elasticity. This non-linearity is observed in laboratory experiments which show that the non-linear behavior of rocks exists close to the rupture condition. Since the study of non-linearity with an explosive source has shown a clear correlation between the theoretical model and experimental results, the next necessary step is to extend the theoretical work to other sources. Here we develop the theory of the envelope of coda waves for a general moment tensor source and discuss its implications. In particular, we show that for a double-couple source the non-linear behavior also affects the radiation pattern which is calculated for PP, PS, SP and SS waves. Finally, we investigate the fit between this model and the coda of some earthquakes, such as the Tarapaca, Chile earthquake and its replicas.
Coincident Geophysical and Petrological Evidence for a Metasomatic Boundary Associated With Subduction in the Slave Cratonic Lithosphere
Archean cratons have remained stable for billions of years, and therefore provide a unique window into the tectonic processes that took place during the time when they were formed. In this study, we present a new receiver function (RF) image of the lithospheric mantle beneath the Slave craton. We analyze P-to-S (Ps) converted waves from a dataset consisting of 62 events recorded at a linear array of 23 broadband seismic stations of POLARIS and MIT, spanning ~400 km across the Slave craton. Our RF image shows a consistent positive velocity gradient across the array at depths from 34-41 km, indicating the location of Moho. Furthermore, we observe a pronounced southward- dipping negative gradient at ~103-134 km depths beneath central Slave. This low velocity layer is spatially coincident with an electrical conductive anomaly derived from previous magnetotelluric experiments, and with the upper boundary of the petrologically-constrained ultra- depleted region. One possible explanation for this geophysical/petrological boundary is that it represents a compositional interface marked by alteration minerals that cause the low seismic velocities (e.g., phlogopite) and inter-granular graphite films that cause the conductive anomaly. We speculate that this front may have originated through metasomatism associated with a subduction event that played an important role in the assembly of the Slave craton.
Continental Topography Related to Intraplate Deformation and Lithosphere Structural Inheritance - the Eurekan Orogen of Ellesmere Island: Model Testing with Seismology
Ellesmere Island, in Canada's Arctic, comprises a series of ~SW-NE trending tectonic provinces, the crustal structure and geological expression of which represent a combination of interplate, accretionary orogenesis in the Palaeozoic (Caledonian equivalent and Ellesmerian orogenies) and intraplate orogenesis in the Tertiary (Eurekan Orogeny). The present-day topography of Ellesmere Island is closely related to the crustal architecture of these tectonic provinces, which includes the adjacent polar continental margin. The first-order crustal structure of the area has been deduced from the regional gravity field: the high topography of northwest Ellesmere Island is isostatically compensated by a thick crust; the Hazen Trough (Hazen Foldbelt) running most of the length of central Ellesmere Island is underlain by a shallow Moho; and the central Ellesmere fold- and-thrust belt loads (Greenland-Laurentian) cratonic basement that flexes to the northwest beneath it. The first-order geological and crustal structure can be explained by a model, supported by preliminary analogue modelling results, that depends on lithosphere-scale structures imposed during Palaeozoic orogenesis being reactivated during Eurekan (Tertiary) intraplate shortening ("mega-basin inversion"). In order to test this model - and to complement the scarce seismological data that is available in the area (only two receiver function estimates of Moho depth) - a passive seismology campaign is planned for 2009-2012. The objectives are to collect sufficient high quality seismological data to allow first-order (i) receiver function analyses for crustal structure (Moho depth and other first-order discontinuities) at key (additional) locations (comprising the structurally diverse elements of the Eurekan Orogen), (ii) shear-wave splitting analyses to determine the presence, geometry, and character of lithosphere anisotropy predicted by the tectonic model in, and (iii) surface wave studies to determine lithosphere thickness and for joint inversion studies in this frontier region. The author explicitly acknowledges past and future input and collaboration from and with colleagues in the UK (SEIS-UK), Canada (GSC Halifax and Calgary), Denmark (University of Aarhus and GEUS, Copenhagen), and the Netherlands (VU University Amsterdam).
Travel-time tomography of the Abitibi-Grenville region, eastern Canada
Seismic studies of the Canadian Shield have indicated certain structural anomalies within the cratonic lithosphere. A low-velocity anomaly has been imaged near the Ontario-Québec border, in the Abitibi-Grenville province, but its 3D geometry was poorly-defined due to a lack of seismograph station coverage on the Québec side of the border. With the help of the 5 new seismograph stations installed in western Québec in 2007, 26 others belonging to the POLARIS project and the Canadian National Seismograph Network (CNSN), and a data set of travel time picks from the ABI-96 teleseismic experiment (Rondenay et al., 2000), we analyse the P-wave velocity structure of the lithosphere in order to better understand the complexity of the region and the interaction of the lithosphere with thermal anomalies in the underlying mantle. Several analysis steps have been carried out. We first measured the relative arrival times of teleseismic P waves across the array, using the cross-correlation method of VanDecar & Crosson (1990). We present the results of an analysis of azimuthal variations of these arrival times for representative stations across the array. We have also calculated maps of relative arrival time residuals across the array for earthquakes coming from different back- azimuths, in order to examine systematic patterns of travel-time anomalies resulting from mantle heterogeneity. Finally, we have inverted the travel time data to estimate a preliminary model of the 3D P-wave velocity structure beneath the region, using a standard tomographic inversion technique.
Reflection Seismics for Ore Exploration: A Case Study From Vihanti, Finland
Applicability of the reflection seismic soundings for ore exploration is tested in the HIRE-project of the Geological Survey of Finland (GTK). Seismic data were acquired in 15 mining camps and exploration targets in Finland. One of the targets is the Vihanti Zn mining area in western Finland. Area is mainly composed of the Paleoproterozoic intrusive rocks and lower and upper Svecofennian supracrustal rocks. In Vihanti volcanic rocks and volcaniclastic sediments form layers of intermediate to felsic volcanites, tuffites and calc-silicate rocks. The multi-phased deformation history varies among crustal blocks delimited by the faults and shear zones. The mined Vihanti VHMS deposit (total production 28 Mt massive sulphide ore) is located in an anticline. The lateral extend of Vihanti-type rocks sequence is estimated to be 60 km. The HIRE Vihanti survey consisted of 8 Vibroseis (total of 90 km) and 4 explosion (total of 30 km) survey lines. Preliminary processing using a 6.25 m CMP interval revealed prominent reflectors associated with the Lampinsaari formation, hosting the mined ore bodies in Vihanti. These reflectors can be traced southward to another former drill target. Known SE-trending faults within the survey area were also clearly visible in seismic sections. The 2D reflection seismic images suggest that potential host rocks of sulphide deposits may be much more common in the area than previously anticipated. Conventional processing of the Vihanti high resolution reflection seismic data illuminated the subsurface structures of the area, but in order to create more reliable depth and attribute estimation for reflectors, further processing was found necessary. Previous studies have shown that special attention should be paid for the static corrections in the hard rock areas, where variation of the thickness and seismic velocity in overburden cause substantial time delays to seismic signal. Velocity analysis is also challenging in hard-rock surveys, and preliminary processing of the Vihanti seismic data produced erroneously high interval velocities.
The Junction of Hellenic and Cyprus Arcs: a Detailed Study of the Morphology and Neogene Tectonic Evolution of the Anaximander Mountains
The Anaximander Mountains are enigmatic highs located at the complex corner that links the Cyprus and Hellenic Arcs in the eastern Mediterranean. They are made up of several different highs: Anaximander (sensu stricto), Anaxagoras and Anaximenes. Previous work had shown that rock samples from the Anaximander Mountain have affinity with rocks exposed on land nearby in southern Turkey. This had been explained by rifting of the Mountain away from Turkey. In contrast to that, our interpretation of around 1750 km of high-resolution multi-channel seismic reflection data acquired in 2001 showed that Anaximander Mountain is part of a broadly south-verging Miocene thrust system associated with relative southward motion of the Tauride Mountains in southern Turkey. Post-Miocene motion also involves thrusting but is accompanied by transpression and rotation. The 3-dimensional nature of the geology makes mapping of the linkage of structures difficult, so we collected an additional 500 km of multi-channel seismic reflection data acquired in 2007, extending our 2001 survey further southwards into the Mediterranean Ridge. These new profiles are shot in a grid oblique to that obtained in 2001, such that the new profile intersections provide a basis for better correlation of the earlier data. We are testing our earlier interpretation through processing and interpretation of these new profiles. Here, we present examples of the new profiles and give first indications of how our earlier interpretation is broadly corroborated by the new data, but with minor adjustments. Anaximenes Mountain is imaged to the south of our previously- mapped area and is characterized as a large south-verging thrust lifting pre-Messinian strata by up to 2 km in a 12-km wide pop-up structure. Internally, Anaximenes is dissected by several splays from the bounding thrusts.
The Junction of Hellenic and Cyprus Arcs: the Bey Daglari Lineament, Offshore Termination of the Antalya Basin
The Antalya Basin is one of a series of basins that sweep along the Cyprus Arc in the forearc region between the (formerly) volcanic Tauride Mountains on Turkey in the north and the subduction zone and associated suture between the African plate and the Aegean-Anatolian microplate in the eastern Mediterranean, south of Cyprus. Miocene contraction occurs widely on southwest verging thrusts. Pliocene-Quaternary structures vary from extension/transtension in the northeast, adjacent to the Turkish coastline, to transpression in the southwest, farther offshore. All these structures are truncated at the northwest end of the Antalya Basin by a broad zone of NNE-SSW-trending transverse structure that appears to represent a prolongation of the extreme easterly transform end of the Hellenic arc. Our mapping suggests that this broad zone links the Hellenic Arc with the Isparta Angle in southern Turkey, which we suggest is an earlier location of the junction of Hellenic and Cyprus Arcs: the junction migrated to the southwest over time, as the Hellenic Arc rolled back. The Turkish coastline turns from parallel to the Antalya Basin structures in the east to a N-S orientation, cutting across the trend of the Antalya Basin. The Antalya Complex and the Bey Dağları Mountains provide a spectacular backdrop to this edge of the offshore basin. Somewhere offshore lies the structural termination of the Antalya Basin. In 2001, we acquired around 400 km of high-resolution multi-channel seismic reflection data across the western end of the Antalya Basin to explore the nature of the termination, which we call the Bey Dağları lineament. We present a selection of the seismic profiles with interpretation of the nature and Neogene history of the lineament. Landward of the N-S-trending coastline, ophiolites of the Antalya Complex are exposed in a series of westerly-verging thrust slivers that extend to the carbonate sequences of the Bey Dağları Mountains. Our seismic data indicate that N-S trending west- and east-verging thrusts define a transpressional continental margin. The shelf is underlain by a prominent angular unconformity between overlying shallow-dipping Pliocene-Quaternary sediments and underlying, easterly- dipping ?Miocene sediments.
Crustal seismic experiment along the KCRT-2008 profile in the Korean peninsula
In order to investigate the velocity structure along the KCRT-2008 profile in the southern part of the Korean peninsula, seismic refraction data were obtained along a 299-km NW-SE line in November of 2008. The profile was set across the peninsular at a large angle to the dominant trend of major tectonic boundaries to minimize three-dimensional effects such as out-of-plane refractions and misleading apparent dips. Seismic waves were generated by detonating 250-1500 kg explosives in eight drill holes at depths of 50-100 m. The shots were detonated at roughly 7- to 15-minute intervals. The seismic signals were detected by 4.5 Hz geophones and recorded by portable seismometers at nominal intervals of 500 m. The ground motions of each velocity sensor were digitized at a 250-Hz sample rate. The refraction data are being analyzed based on the travel-time tomography method. A preliminary velocity tomogram was derived from first arrival times using a series expansion method of traveltime inversion. The raypaths indicate existence of several mid-crust boundaries and the Moho discontinuity. The boundary between crust and mantle gets shallower toward the Yellow Sea and the East Sea. Relatively low velocities are apparent in the Cretaceous Gyeongsang sedimentary basin near the east coast.
Anisotropy studies in the Korean peninsular using seismic signals from large explosions
Seismic signals from the four and eight large explosions in 2004 and 2008, respectively, were recorded with a sample rate of 100 Hz by seismographs of the Korea Meteorological Administration (KMA) network. Among the seismograms recorded from 32 seismometers and 75 accelerometers in 2004, 16 records with epicentral distances less than 150 km and high signal-to-noise ratios were chosen to analyze velocity anisotropy of the Pg phase. These data were reformatted, interpolated with a spline function, resampled at a 120 Hz sample rate, and superimposed on the profile data (KCRT-2004) to facilitate both reliable picking and easy comparison. For each shot location, the traveltime differences between the fixed stations on the KMA network and the interpolated arrival times at the same offset from the profile data were then examined. With the exception of anomalously fast velocities along the Chugaryeong fault zone in the central part of the Korean peninsula, the analysis of crustal velocity anisotropy using the Pg phase generated by our shots indicates overall isotropy in the southern half of the peninsula. To convince the result and reveal further anisotropic features, we are analyzing the seismograms recorded at the KMA's stations during the crustal seismic experiment in 2008.
A Study of Gas Hydrates With Ocean-Bottom-Seismometer Data on the East Coast of Canada
A number of BSR locations have been identified along the Scotian Slope from geophysical evidence, but as yet none have been confirmed to contain gas hydrate through direct sampling. Studies near the Mohican Channel (200 km offshore Halifax) show the BSR is around 450 ms below sea floor with a possible underlying low- velocity zone (indicative of free gas in the sediments). Models with hydrate as part of the sediment frame give hydrate concentrations of 2--6% and free gas concentrations of less than 1% (LeBlanc et al., 2007). In a joint project between the Geological Survey of Canada and the Dalhousie University, 19 ocean-bottom- seismometers (OBS) were deployed in 2006 to study the geophysical structure of the Mohican Channel BSR. In this area, a clear BSR beneath the channel and in its levee disappears in a direction away from the channel and parallel to slope. This observation may be related to fining of sediments distal to the channel levee, but there are no direct samples in this zone to confirm this hypothesis. Fining of sediment may restrict vertical fluid flow by reducing permeability, as well as reducing pore spaces available for hydrate formation. Wide-angle reflection and refraction data are used to ascertain the thickness of the sub-BSR low-velocity zone as a function of position on the seismic transect away from the Mohican Channel, and to use these thicknesses to calculate the variation in upward fluid flow along slope. Preliminary results show refractions with apparent velocities of 1850 to 1900 m/s for a depth range of 350 to 600 mbsf. However, initial traveltime inversions using refractions and wide-angle reflections indicate no significant low-velocity zone below the BSR. Furthermore, there appears to be no strong lateral velocity contrast between regions with and without BSR observations. Further use of S-wave arrivals from the geophone components may provide additional constraints on hydrate and gas distribution and help to characterize fault patterns.
Complex Seismic Signatures of the Athabasca Basin Subsurface
Precursor of PKKPbc: an Evidence for Core Rigidity Zone?
As the boundary between solid mantle and liquid outer-core, Core Mantle Boundary (CMB) is one of the most important discontinuities in the deep interior of the Earth, controlling exchange of heat, angular moment and material. Usually, the mantle side of the CMB was regarded as a complex zone, as inferred from the observation of ultra low velocity zone (ULVZ) and etc, while the core side was regarded as less complicated. Recent studies showed that ScP phases in some regions can be modeled with a thin rigidity zone at the top of the core (Rost and Revenaugh, 2001), which called core rigidity zone (CRZ), which can also explain SPdKS waveform complexities. However, mantle side core phases such as ScP may be affected by complexities from the bottom of mantle. To get some evidence for CRZ from core side reflection phase from CMB, We observed PKKPbc occurred from events in Fiji-Tonga region recorded at US networks, and compared the waveforms with Hilbert transform of direct P as PKKP is a maximum arrival phase. The main phase of them fitted well, but there's a significant pulse arriving ~1s earlier in PKKPbc waveform. This precursor indicates the existence of CRZ at the bounce point region. Then we performed waveform modeling with Reflectivity Method (Kennett and Engdahl, 1991), and got a best-fit thickness of 1.0km when the Vp,Vs and density of CRZ accord with Rost's velocity model. Although this precursor can be also explained by a liquid layer at the uppermost outer core, the thickness of the layer must be greater than 5km. Helffrich et al has limited the thickness of the layer to less than 3km in a recent research (2004). Moreover, such distinct precursor if from liquid layer must have occurred from a discontinuity with large contrast, which may result in strong following phases of PcP at short distances where PcP direct phase is weak. Thus the anomaly layer is most likely a rigidity zone. Additionally, we analyzed S and ScS waveforms on models with CRZ or ULVZ, and found that the ScS behavior caused by 1km-thick CRZ can be also explained with 10km-thick ULVZ model with -30% anomalies in S velocity. However, ULVZ model should result in clear SdS phases between S and ScS arrivals. Anyway, ScS observations should be helpful in distinguishing CRZ and ULVZ.
Monte Carlo Simulations Of Elastic Wave Energy In Random Media With Multiple Scales
Radiation transport theory (RTT) describes the propagation of wave energy in scattering media that means
especially in media with small scale heterogeneities. For this we look at squared seismogram envelopes
which are proportional to wave energy. RTT is one of the most powerful tools to picture the multiple scattering
regime of waves and to obtain informations about small scale heterogeneities. Basic validity assumptions of
RTT are: fluctuations of wave velocities are weak, waves are scattered incoherently and correlation length is of
the same order of magnitude as the wavelength.
One of the simplest models for small scale heterogeneities is a medium with random fluctuations around a constant background velocity, that are characterized by an autocorrelation function (ACF), a characteristic scale called the correlation length and fluctuation strength. However, results from borehole velocity logs show, that there is a need for more than one scale to correctly characterise small scale heterogeneities of the earth medium.
Here we present Monte Carlo simulations of RTT in random media with more than one scale. To obtain such a model we superpose several ACF's, with different correlation lengths. The numerical simulations show, how wave energy propagate through a random medium with multiple scales. We compare our results with Monte Carlo simulations in a single scale random medium. This comparison shows especially in P-coda clearly visible differences between a single scale and a multiple scale random medium.
Waveform Tomography Applied to Long-Streamer MCS Data From the Nova-Scotia Slope: Challenges and Applications
Standard methods of seismic imaging treat the derivation of lower wave-number velocity models using refraction tomography separately from higher resolution images produced by MCS reflection techniques. Recent advances using synthetic datasets, however, suggest that waveform tomography may now permit the joint inversion of both types of seismic arrivals to produce high-resolution velocity images. An accurate starting velocity model is crucial for such a highly non-linear inversion to succeed. Phase and amplitude information of mid-offset refracted arrivals are particularly sensitive to velocity variation. The method, therefore, requires wide- angle datasets with long shot-receiver offsets. Modern MCS data, collected with streamers from 6 to 12 km long, can fulfill the required offset criteria for application of 2D waveform tomography. Accurate high-resolution velocity images of the shallower subsurface can be determined because the MCS data are characterized by high density of shots and receivers. These data can then be combined with larger offset data from ocean bottom receivers to allow extension of the velocity images to greater depth at improved accuracy. We analyzed 2D MCS data acquired in 2003 on the Nova Scotia continental slope (water depth 1600 m) by GX Technology (now ION), using a 9-km-long streamer with a shot interval of 50 m and receiver spacing of 25 m. The data show a refracted phase arriving ahead of the seafloor reflection in the 7.5-9 km offset range and some additional later arrivals from deeper refraction events. We test both standard NMO and prestack depth migration velocity models as initial input to 2D waveform tomography but find that in our case neither is sufficiently accurate for the inversion to converge to global minima. The picked refracted arrivals are accurately forward-modeled only with a starting velocity derived from traveltime tomography. This starting velocity field is then updated and refined using 2D waveform tomography in frequency domain. We also plan to combine and compare the GX Technology MCS dataset to ocean bottom data recorded independently along the same profile. A validation of the velocity model will be possible by comparison to in-situ velocity measurements from the 3-km deep Torbrook well which is crossed by the seismic profiles.
Imaging with multiply scattered waves
If singly scattered seismic waves illuminate the entirety of a subsurface structure of interest, standard methods can be applied to image it. This is rarely the case, however, and due to a combination of restricted data acquisition geometry and imperfect background velocity models, it is not generally possible to illuminate all structures with only singly scattered waves. We present an approach to ameliorate this problem by including multiply scattered waves in the imaging process to illuminate structures not sensed by singly scattered waves. Examples from oil exploration will be shown, along with discussion of the relationship between this method and interferometry.
Analysis of the Pre-stack Split-Step Migration Operator Using Ritz Values
The Born approximation for the acoustic wave-field is often used as a basis for developing algorithms in seismic imaging (migration). The approximation is linear, and, as such, can be written as a matrix-vector multiplication (Am=d). In the seismic imaging problem, d is seismic data (the recorded wave-field), and we aim to find the seismic reflectivity m (a representation of earth structure and properties) so that Am=d is satisfied. This is the often studied inverse problem of seismic migration, where given A and d, we solve for m. This can be done in a least-squares sense, so that the equation of interest is, AHAm = AHd. Hence, the solution m is largely dependent on the properties of AHA. The imaging Jacobian J provides an approximation to AHA, so that J-1AHA is, in a broad sense, better behaved then AHA. We attempt to quantify this last statement by providing an analysis of AHA and J-1AHA using their Ritz values, and for the particular case where A is built using a pre-stack split-step migration algorithm. Typically, one might try to analyze the behaviour of these matrices using their eigenvalue spectra. The difficulty in the analysis of AHA and J-1AHA lie in their size. For example, a subset of the relatively small Marmousi data set makes AHA a complex valued matrix with, roughly, dimensions of 45 million by 45 million (requiring, in single-precision, about 16 Peta-bytes of computer memory). In short, the size of the matrix makes its eigenvalues difficult to compute. Instead, we compute the leading principal minors of similar tridiagonal matrices, Bk=Vk-1AHAVk and Ck = Uk-1 J-1 AHAUk. These can be constructed using, for example, the Lanczos decomposition. Up to some value of k it is feasible to compute the eigenvalues of Bk and Ck which, in turn, are the Ritz values of, respectively, AHA and J-1 AHA, and may allow us to make quantitative statements about their behaviours.
The Effect of Surface Overburden on 2D Seismic Response
This study aims to investigate the effects of overburden layers in seismic surveys. Here, we look at the effect of low velocity, low density overburden on seismic imaging. Overburden layers continue to be a problem for recording and analyzing information in seismic surveys. Placing shots and receivers in or near an overburden layer can mask responses from deep subsurface structures and cause elastic wave scattering. To investigate this problem, a finite difference elastic wave modeling study was conducted to evaluate the effects of overburden layer when using 3-component surface or borehole receivers. In this study, models were used with a reflective, angled lens having the velocity and density parameters of a sulfide orebody. The depth of the overburden layer in the first model is uniform and the second layer varies sinusoidally. The parameters used for the overburden are 2.0 g/cm3 density, 600 m/s S-wave, and 2000 m/s P-wave; in contrast, the background parameters are 2.73 g/cm3 density, 3550 m/s S-wave, and 6140 m/s P-wave. The study looks at responses from the lens in models with and without the overburden layer. The relatively slow P-wave and S-wave velocity of the overburden material impacts the travel time and the shape of the wave. As expected with borehole receivers, only the first few traces are corrupted by highly dispersed surface waves while deeper receivers show clear reflections from the sulfide lens. The location of the shot also affects the seismic response depending on whether it originates inside the overburden or below.