Seismic Depth Research Articles

An inverse method to derive fault slip and geometry from seismically observed vertical stratigraphic displacements using elastic dislocation theory

This contribution describes the development of an inverse method to extract fault slip and fault geometry from subsurface horizons derived from seismic reflection datasets. The inversion process uses non-linear minimisation together with a forward model using elastic dislocation (ED) theory. The forward model calculates subsurface vertical displacements from fault parameters such as fault slip, dip, length and width for planar, listric or seismically observed fault geometries. Vertical subsurface displacements are measured at selected stratigraphic horizons from seismic depth sections, and act as observations for the inversion process. The non-linear inversion uses a standard procedure (Powell's method) to minimise an objective function (misfit parameter). The method has been tested using synthetic data generated by theoretical forward models. The inverse method can successfully recover slip distributions and non-planar, including listric, fault geometries. The method has also been applied to stratigraphic horizons derived from seismic reflection data. Fault slip and fault geometry derived from the inverse method are consistent with direct seismic interpretations, and are reproducible and geologically realistic. This inverse method provides a useful complement to conventional subjective seismic interpretations of fault slip and geometry. ED theory together with the retrieved fault parameters from the application of inverse theory may be used to determine strains for fracture prediction.

Isolated deep earthquakes beneath the North Island of New Zealand

Seismicity shallows towards the south along the Tonga-Kermadec-Hikurangi margin, deep and intermediate seismicity being absent altogether in the South Island of New Zealand. Beneath the Taranaki region of the North Island the maximum depth of the main seismicity is 250 km, but very rare events occur directly below at 600 km. These could be associated with a detached slab or a vertical, aseismic continuation of the subducted Pacific Plate. Six small events that occurred in the 1990s were recorded extensively by digital instruments of the New Zealand National Network (NZNN) and temporary deployments. We relocate these events by a joint hypocentre determination (JHD) method and find their focal mechanisms using first motions and relative amplitudes of P and S arrivals. The earthquakes relocate to a remarkably uniform depth of 603 +/- 3 kmrelative error (+/- 10 km absolute error) in a line 30- km long orientated 40 NE, roughly parallel to the strike of the intermediate- depth seismicity. The only consistent component of the focal mechanisms is the tension axis: all lie close to horizontal and tend to align with the line of hypocentres. We interpret this deep seismic zone as a detached sliver of plate lying horizontally with the same orientation as the main subducted plate above. Volume change caused by a phase change controlled by the pressure at 600 km and temperature in the sliver produces a pattern of strain that places the sliver under tension along its length

Geodynamics of the deep sedimentary basin of the Caspian Sea region: paragenetic correlation of seismicity and mud volcanism

In this paper we analyze seismic regime and earthquake depth distribution and correlation of seismicity and mud volcanism in the Azerbaijan and the Caspian Sea region. For the present region we have calculated accurate source locations, seismic activity, earthquake repetition and released earthquake energy parameters. It is shown that the active tectonic processes in the region are concentrated within the thick sedimentary cover that we consider as a general source of contemporary stress and a main structural element responsible for the origin of regional earthquakes. The correlation of seismicity and mud volcanism is of paragenetic character.

Ghosts of Mars

What is the most exciting geology news event in the 21st century? Recent earthquakes? Satellite-based interferometric radar images of crustal movements? Seismic depth slices of the earth that reveal ancient meandering river channels at depths of 10 km or more? Tomographic images of mantle plumes that underly hot spots? None of the above are even close, in my opinion. Paradoxically, the most exciting geology events in the 21st century are the spaceborne explorations of extraterrestrial planets, moons, and comets.

Focal mechanisms of shallow and intermediate depth earthquakes along the Hellenic Arc

The active tectonics of the southern Aegean Sea, a region of intense shallow and intermediate depth seismicity, is studied based on well-constrained focal mechanisms. We use the focal mechanisms of 28 earthquakes (with moment magnitudes M>4.9) of the period 1977–2002 whose source parameters we determined from inversion of body waves and the focal mechanisms of 85 other earthquakes to re-examine the style of faulting in the shallow and deeper parts of the subducting slab. The deepest event that we studied has depth of ∼160 km. The combined data set of focal mechanisms, indicate the following, starting from south to north: (a) NW–SE compression with thrusts along the outer central Mediterranean Rise; (b) reverse and low angle thrust faulting at depths up to 40 km with P axis having a constant direction which is almost perpendicular to the strike of the arc, in the western and central parts and parallel to the arc in the eastern part; (c) normal faulting with E–W trending T-axes showing along—arc extension at depths up to 40 km; and (d) strike–slip faulting with P-axes parallel to the strike of the arc for events deeper than 40 km. Of special interest is the area between the island of Zante and the western coasts of Peloponnese where shallow and deeper events are of strike–slip faulting. The eastern part of the Hellenic arc exhibits a different behaviour than the western and central part. In the eastern part P-axes for both shallow and deeper events have the same NE–SW parallel to the arc trend. This is not the case in the western and central part of the arc where there is a clear discrimination between shallow and deeper events. In the eastern part the angle of the subducting plate is steeper than the other parts and the T - axes are well aligned along dip of the subducting plate. If for the deeper than 40 km events the ∼E–W trending planes are assumed as the fault planes, then the western parts of the arc (Kythira, Milos islands) are connected to dextral strike–slip motions while the eastern parts (near Rodos Island) are connected to sinistral strike–slip motions.

Thermal Constraints on Earthquake Depths in California

The high-quality databases for California seismicity (from the Southern and Northern California Earthquake Centers) and an extensive compilation of thermal measurements in California are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone. The base of this zone, below which only a small percentage of crustal earthquakes occur, is termed the “cutout depth,” and it is at or near the brittle-ductile transition in the crust. Based on laboratory deformation studies, this transition should be temperature, strain rate, lithology, and stress-state dependent. In this study, qualitative comparisons between the heat flow in California and earthquake hypocentral distributions show first that, as expected, earthquake cutout depths are inversely related to heat flow. Second, the epicentral distributions tend to parallel thermal transitions. This correlation is probably related to stress concentrations in these locations. An ancillary observation is that the cooler western Mojave block in southern California appears to be behaving similarly to the Tibetan indenter block in Asia, as faults tend to go around it into areas of higher heat flow where the seismogenic zone is thinner and the crust may be weaker, and it is pushed toward (and under) the southern cold Sierra Nevada block. Third, to quantitatively compare the data sets, temperatures for the seismicity cutout depth in California were calculated using the steady-state, 1D heat conduction equation, with the variables based on published values for heat flow, heat generation, and thermal conductivity. The analysis was restricted to profiles along which the heat flow and earthquake databases were constrained, which allowed the error in the temperature calculation to be determined using Monte Carlo simulations. The results show that two distinct ranges of temperatures (dependent upon location) describe the cutout depth ( D 99%) of seismicity for California: 450 ± 50°C and 260 ± 40°C. The 450 ± 50°C cutout depth temperature is most widespread geographically, occurs within many provinces, and is higher than the frequently referenced temperature of 300 ± 50°C for the brittle-ductile transition. The lower temperature (260°C) seems to be restricted to provinces where the heat flow is near or below 50 mW m-2, such as the Mojave block and Sierra Nevada. The differences in these cutout temperatures suggest that additional factors, such as strain rate, stress regime, and/or lithology, may contribute to the seismicity cutout depth. Fourth, along a profile with significant seismic activity and a 450°C cutout temperature, the envelope for the maximum energy released by earthquakes falls at or below the 300°C isotherm. Detailed characterization of the heat flow and earthquake synergy in this manner furthers the understanding of the earthquake process and can aid in the estimation of the maximum depth of rupture for great earthquakes, particularly in areas of low seismicity, thus reducing uncertainties in hazard calculations. Manuscript received 26 February 2003.

Integrated prestack depth migration of vertical seismic profile and surface seismic data from the Rocky Mountain Foothills of southern Alberta, Canada

We have developed an anisotropic prestack depth migration code that can migrate either vertical seismic profile (VSP) or surface seismic data. We use this migration code in a new method for integrated VSP and surface seismic depth imaging. Instead of splicing the VSP image into the section derived from surface seismic data, we use the same migration algorithm and a single velocity model to migrate both data sets to a common output grid. We then scale and sum the two images to yield one integrated depth‐migrated section. After testing this method on synthetic surface seismic and VSP data, we applied it to field data from a 2D surface seismic line and a multioffset VSP from the Rocky Mountain Foothills of southern Alberta, Canada. Our results show that the resulting integrated image exhibits significant improvement over that obtained from (a) the migration of either data set alone or (b) the conventional splicing approach. The integrated image uses the broader frequency bandwidth of the VSP data to provide higher vertical resolution than the migration of the surface seismic data. The integrated image also shows enhanced structural detail, since no part of the surface seismic section is eliminated, and good event continuity through the use of a single migration–velocity model, obtained by an integrated interpretation of borehole and surface seismic data. This enhanced migrated image enabled us to perform a more robust interpretation with good well ties.

A possible model for large preseismic slip on a deeper extension of a seismic rupture plane

I develop a numerical model for preseismic slip and episodic aseismic slip events (slow earthquakes) on a deeper extension of a seismogenic plate interface, using a rate- and state-dependent friction law. The deep preseismic slip and episodic slow slip were indicated by models based on geodetic observations, though they cannot be explained by common rate- and state-dependent laws. I investigate whether this incongruity can be removed by including use of additional known constraints on the frictional property. The steady-state friction on a model fault is taken to be velocity weakening at low velocities and velocity strengthening at high velocities. The transition velocity from velocity-weakening friction to velocity-strengthening is assumed to be the seismic slip velocity at shallow seismic depths and decreases with depth at deeper parts. Simulation results for a 2D model of interplate earthquakes at a subduction zone are as follows: (1) During an interseismic period between successive large earthquakes at shallow seismogenic depths, some episodic aseismic slip events with durations of a few years occur on the plate interface just below the seismogenic depths. (2) Preseismic slip occurs at nearly the same location where the episodic aseismic slip events occur. These may explain some characteristics of observed preseismic slip of the 1944 Tonankai earthquake ( M=7.9) along the Nankai trough, central Japan, and episodic aseismic slip events in the same area.

Seismicity and tectonics of the South Shetland Islands and Bransfield Strait from a regional broadband seismograph deployment

We investigate the tectonics of the South Shetland Trench and Bransfield Strait by performing a detailed study of local seismicity. During 1997–1999 we deployed seven land seismometers and 14 ocean bottom seismometers in the South Shetland Island region. The data we obtained indicate a high level of local seismicity (mb 2–5), and we accurately located ∼150 earthquakes. Many of the earthquakes occur at locations and depths indicative of ongoing subduction in the South Shetland trench. A focal mechanism for the largest event in the forearc indicates shallow angle thrusting. The maximum depth of seismicity is ∼65 km, but the majority of the events are shallower than 30 km. These seismic results are consistent with recent magnetic, GPS, and multichannel seismic reflection data that suggest continued subduction at a very slow rate. The South Shetland trench thus represents an extreme end‐member of hot subduction resulting from slow convergence of young lithosphere, and the absence of intermediate depth earthquakes is consistent with thermal assimilation of the slab at shallow depths. We have located many earthquakes associated with volcanism and rifting in Bransfield Strait. A swarm of events near a submarine volcano suggests current magmatic activity. A normal faulting focal mechanism in the northeastern part of the strait gives evidence of extension. Earthquakes associated with rifting in the northeastern portion of the strait are clustered along well‐established rifts, but the seismicity is much more diffuse to the southwest. This observation is consistent with other evidence that extension has propagated from northeast to southwest.

A workshop on enhanced seismic imaging

One of the hottest topics in seismic depth imaging these days is gravity modeling. What? That's right, most major oil companies and many smaller ones routinely integrate rigorous gravity models with their seismic depth migration workflow. If you think about it, it makes sense. Gravity data have always been effective at predicting the distribution of mass in the subsurface. Although once thought limited in resolution, gravity has gained new respect in the last five years due to careful modeling and thoughtful integration into the workflow of seismic depth migration.

A workshop on enhanced seismic imaging

One of the hottest topics in seismic depth imaging these days is gravity modeling. What? That's right, most major oil companies and many smaller ones routinely integrate rigorous gravity models with their seismic depth migration workflow. If you think about it, it makes sense. Gravity data have always been effective at predicting the distribution of mass in the subsurface. Although once thought limited in resolution, gravity has gained new respect in the last five years due to careful modeling and thoughtful integration into the workflow of seismic depth migration.

Lower-crustal strength under the Dead Sea basin from local earthquake data and rheological modeling

We studied the local seismicity of the Dead Sea basin for the period 1984–1997. Sixty percent of well-constrained microearthquakes ( M L≤3.2) nucleated at depths of 20–32 km and more than 40% occurred below the depth of peak seismicity situated at 20 km. With the Moho at 32 km, the upper mantle appeared to be aseismic during the 14-year data period. A relocation procedure involving the simultaneous use of three regional velocity models reveals that the distribution of focal depths in the Dead Sea basin is stable. Lower-crustal seismicity is not an artifact created by strong lateral velocity variations or data-related problems. An upper bound depth uncertainty of ±5 km is estimated below 20 km, but for most earthquakes depth mislocations should not exceed ±2 km. A lithospheric strength profile has been calculated. Based on a surface heat flow of 40 mW m −2 and a quartz-depleted lower crust, a narrow brittle to ductile transition might occur in the crust around 380°C at a depth of 31 km. For the upper mantle, the brittle to ductile transition occurs in the model at 490°C and at 44 km depth. The absence of micro-seismicity in the upper mantle remains difficult to explain.

Rheological implications of the strong seismic reflector in the Kakkonda geothermal field, Japan

We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.

Crustal architecture and tectonic evolution of the Gulf of Cadiz (SW Iberian margin) at the convergence of the Eurasian and African plates

The Gulf of Cadiz, located at the southwestern Iberian margin, is characterized by widespread seismicity, compressional and strike‐slip fault plane solutions and by a large, elongated positive free‐air gravity anomaly, the Gulf of Cadiz Gravity High (GCGH). Multichannel seismic profiles across and along GCGH, together with bathymetric and gravity data, allow us to study in detail the tectonic architecture and crustal structure of the Gulf of Cadiz. The upper shelf and slope of the Gulf of Cadiz includes the main structural domains of the Betic fold and thrust belt. In the middle part of the Gulf, the Paleozoic basement crops out on the shallow Guadalquivir Bank and is associated with the largest signature of the GCGH, whereas toward the outer part of the Gulf, the basement deepens progressively. A large NW‐SE normal fault and conjugate NE‐SW faults define a prominent basement high associated with the GCGH. Modeling of the GCGH suggests localized crustal thinning of 10 km along the central part of the Gulf of Cadiz, probably generated during the Mesozoic rifting episode between the Iberian and African plates. Concentric wedges of fold and thrust belts and large allochthonous masses were emplaced in the Gulf of Cadiz during the Neogene compressional phase. The final emplacement of these units becomes progressively young from the SE (pre‐early Langhian) toward the foreland in the NW (late Tortonian). Seafloor surface ruptures, pockmarks, and submarine landslides provide evidence of active faulting in the Gulf of Cadiz. To accommodate the present‐day convergence between the African and Eurasian plates, previously extensional faults have probably been reactivated and inverted at depth, as suggested by the intermediate depth seismicity.

IMPLEMENTING SEISMIC MOHO DEPTHS IN GEOID COMPUTATION

The paper studies the effect of implementing seismically estimated Moho depths in gravimetric geoid computation, instead of assuming an isostatic hypothesis to create the compensating masses, within the well-known remove-restore technique of geoid computation. Three different techniques were applied when using the seismic Moho depths in the geoid computation. In the first technique, a constant density contrast between the lower crust and the upper mantle for all topographic elements has been assumed. In the second technique, variable density contrast computed as a function of the seismic Moho depth has been assumed. In the third technique, variable density contrast computed as a function of the topographic height has been assumed. A comparison with a geoid computed using the commonly used Airy-Heiskanen isostatic model has been made. For all cases, a gravimetric geoid has been computed for a test area by applying Stokes' integral in the frequency domain using the ID-FFT technique. All geoids are scaled (fitted) to the geoid derived from GPS and levelling. A broad comparison between all geoids is carried out. The results show that using the seismic Moho depths with variable density contrast between the lower crust and upper mantle computed as a function of the seismic Moho depth improves the accuracy of the computed geoid.

RETRACTED: Is the plastic flow uniformly distributed below the seismogenic region?

We compared the cutoff depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in intraplate Japan with the brittle–ductile transition depth of the widely accepted strength profile model of the crust. We successfully determined the temperature profile from borehole measurements, since almost the same geothermal gradients were observed at two boreholes located about 4 km apart from each other, and the thermal conductivity and heat production were also measured by taking numerous core samples. We found that the cutoff depth was much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 3×10−15 s−1. This small strain rate implies, however, that plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10−13 s−1, the cutoff depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region, even in the intraplate region.

THE MUTINEER COMPLEX AND EXETER OIL DISCOVERIES: MUTINY IN THE DAMPIER

The oil discoveries at Norfolk–1 and Exeter–1 in the Northern Dampier Sub-basin (permit WA-191-P) have realised major commercial potential in an area with a prolonged exploration history. This paper presents the results of the drilling campaign which was undertaken in 2002 comprising two successful exploration discovery and five appraisal wells. The Norfolk–1 (28 m gross oil column), Norfolk–2 (9 m oil column), Exeter–1 (23 m oil column), Mutineer–3 (8 m oil column) and finally Exeter–2 (12 m oil column) confirmed a significant commercial oil volume within the Jurassic Angel Sandstones.The recent exploration of this area has improved understanding of the geology through the integration of technology with geoscientific understanding. Highs have been revealed from seismic time data using advanced 3D seismic techniques and sophisticated depth conversion processes.The time structural high of the Mutineer area was first tested by Bounty–1 in 1983 which is now mapped outside the southern limit of closure. Pitcairn–1 was drilled in 1997 discovering three thin oil columns and was followed by a near crestal well at Mutineer–1B drilled 2.6 km northwest of Pitcairn–1 in 1998, discovering an 8m column. The key issue was the understanding of the velocity gradient and depth conversion over the Mutineer Complex which revealed the true structural picture.This paper summarises results of the exploration and appraisal wells drilled and describes the evolution of the structural/stratigraphic understanding of the area, covering critical components hindering the oil field’s early detection. The first component is a significant seismic velocity gradient which causes true structural closure to be significantly offset from the time closure. The second component is the reservoir pressures within the oil reservoir and older sandstone intervals within the Angel and Legendre Sandstones show differences due to hydrodynamic cells and/or depletion resulting from production from the adjacent NWS Venture oil fields. The final component is the oil is primarily reservoired in the top Angel Sandstone, belonging to the J40 sequence and is sealed by a thin shale from the underlying mainly water bearing sandstones (J35/J30 and Legendre Sandstones).The combined reserves for the Mutineer Complex and Exeter Oil Fields reservoired in these laterally continuous turbidites are estimated to be 70–160 MMBBL recoverable.

The West Firsby Oilfield, Development Licence 003, Lincolnshire

Abstract The West Firsby Oilfield exemplifies the classic east Midlands oil play, sourced by early Namurian pro-deltaic shales. Stacked fluvio-deltaic sandstone reservoirs of latest Namurian to early Westphalian 'A' age are structured into a Variscan inversion anticline in the hanging wall of the northeastern boundary fault of the Dinantian-Namurian Gainsborough Trough. The Enterprise West Firsby-1 discovery well, drilled on a 'Top Westphalian 'A'' seismic depth closure, encountered oil-bearing sandstones at three levels. These are thought to represent the earliest Westphalian 'A' Sub-Alton/Crawshaw distributary channel and mouthbar sandstones (Reservoir Zone [RZ] 1), the stacked fluvial channel sands of the Namurian Rough Rock (RZ2) and an underlying high quality mouth-bar sandstone (RZ3). In the hanging wall setting, these units can be correlated over an area far larger than the West Firsby Field. On acquiring operatorship of the Development Licence (DL 003) in 1991, Tullow Exploration put the WF-1 well into production from RZ2 and 3 disposing of produced water in the earlier-drilled and unsuccessful WF-2 step-out. Between 1992 and 1996, three new production wells (WF-4, -5 and -6) were drilled from the same surface site. The wells principally produce from RZ2 and involve high-angle/horizontal sections in these sandstones, approaching 170 m in length in WF-6. Strong aquifer support, coupled with the injection of produced water into RZ2 of WF-2, promotes early natural flow from such wells at rates up to 800BOPD, and when this declines jet-pumping can maintain production at rates of several hundred BOPD/well. As of 1st January 1999, the West Firsby Oilfield had produced 0.97 MMBBL of an estimated 13 MMBBL OIIP, maintaining an average annual rate during 1998 of 345 BOPD from four wells. Remaining recoverable reserves are estimated at around 1.03 MMBBL.

秋田‐新潟海域油・ガス探鉱への地震探査の寄与―１９６０年代後期からの２０年を振り返って―

Until the digital processing of seismic data was introduced to Japan in 1968, Japanese geologists had no means but to depend on the seismic depth profiles which displayed very short lines showing inclination of reflection planes picked up by seismologists. There was only a small discovery of the Tuchizaki-oki field those days. After the air-gun and digital processing became available in 1968, offshore exploration activities expanded and led to the discovery of the Aga-oki field in 1972. From the viewpoint of seismology, this discovery was based on stack sections and “hand-made depth sections” which displayed continuous curves indicating prominent reflections. Abnormal reflection patterns such as bright spots, flat spots and pull down began to attract geologists' attention in the mid-1970s, and various processing menus in addition to depth profiles were provided for geologists. In 1981, certain abnormal patterns were found in seismic record sections obtained in 1973, an exploratory well, Iwafune-oki SIM-1 was drilled at the center ofthe distribution area of such anomalies and became the discovery well of the Iwafune-oki field.Based on these experience, I emphasize the importance of stack sections in conjunction with wide and deep geological knowledge and thought.-Oil and gas fields are present in geologists' brains-

Simulation of subduction zone seismicity by dehydration of serpentine.

We measured acoustic emission energy during antigorite dehydration in a multianvil press from 1.5 to 8.5 gigapascals and 300 degrees to 900 degrees C. There was a strong acoustic emission signal on dehydration, and analysis of recovered samples revealed brittle deformation features associated with high pore-fluid pressures. These results demonstrate that intermediate depth (50 to 200 kilometers) seismicity can be generated by dehydration reactions in the subducting slab.