Maximum Depth Of Earthquakes Research Articles

Temperature of the Icelandic crust: Inferred from electrical conductivity, temperature surface gradient, and maximum depth of earthquakes

Two different models of the structure of the Icelandic crust have been presented. One is the thin-crust model with a 10–15 km thick crust beneath the axial rift zones, with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up-domed asthenosphere. The thick-crust model assumes a 40 km thick and relatively cold crust beneath central Iceland. The most important and crucial parameter to distinguish between these different models is the temperature distribution with depth. Three methods are used to estimate the temperature distribution with depth. First, the surface temperature gradient measured in shallow wells drilled outside geothermal areas. Second, the thickness of the seismogenic zone which is associated with a 750 °C isothermal surface. Third, the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 °C at the base of the crust. Combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust. These results are strongly in favour of the thin-crust model. The scattered deep seismic reflectors interpreted as Moho in the thick-crust model could be caused by phase transitions or reflections from melt pockets in the mantle.

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Geological data suggest that, after a period of oceanic lithosphere subduction, about 170 km of continental lithosphere were subducted under the Northern Apennines since about 23 Myear at rates of ca. 1 cm/year, whereas more than 700 km of Mesozoic oceanic lithosphere (Ionian lithosphere) have been subducted under the Calabrian arc at rates of about 3 cm/year. However, a well-developed Wadati-Benioff zone can be only recognised below the Calabrian arc down to depths of 450–500 km, whereas intermediate-depth seismicity reaches maximum depths of about 90 km under the Northern Apennines.We model the thermal state of these two subduction zones and apply temperature-dependent non-linear rheological laws to evaluate the down-dip extent of brittle regions in the two subduction zones. We show that differences in subduction rate and in slab composition (continental vs. oceanic) produce a far deeper (down to 290–380 km) brittle field in the colder Calabrian slab than in the warmer Northern Apenninic slab (70–120 km), thus explaining differences in the maximum depth of seismicity in both regions.Concerning deep earthquakes in the Calabrian subduction zone, models predict that metastable olivine persists down to depths of 430 km. This could possibly explain the maximum depth of earthquakes in the area. The small extent of the metastable olivine wedge produces a small density anomaly that cannot explain the pervasive down-dip compression observed in the Ionian slab.

Subduction-related intermediate-depth and deep seismicity in Italy: insights from thermal and rheological modelling

Geological data suggest that, after a period of oceanic lithosphere subduction, about 170 km of continental lithosphere were subducted under the Northern Apennines since about 23 Myear at rates of ca. 1 cm/year, whereas more than 700 km of Mesozoic oceanic lithosphere (Ionian lithosphere) have been subducted under the Calabrian arc at rates of about 3 cm/year. However, a well-developed Wadati-Benioff zone can be only recognised below the Calabrian arc down to depths of 450–500 km, whereas intermediate-depth seismicity reaches maximum depths of about 90 km under the Northern Apennines. We model the thermal state of these two subduction zones and apply temperature-dependent non-linear rheological laws to evaluate the down-dip extent of brittle regions in the two subduction zones. We show that differences in subduction rate and in slab composition (continental vs. oceanic) produce a far deeper (down to 290–380 km) brittle field in the colder Calabrian slab than in the warmer Northern Apenninic slab (70–120 km), thus explaining differences in the maximum depth of seismicity in both regions. Concerning deep earthquakes in the Calabrian subduction zone, models predict that metastable olivine persists down to depths of 430 km. This could possibly explain the maximum depth of earthquakes in the area. The small extent of the metastable olivine wedge produces a small density anomaly that cannot explain the pervasive down-dip compression observed in the Ionian slab.

Relative contributions of crustal temperature and composition to controlling the depth of earthquakes in Southern California

Control of the ∼13 km range of the maximum depth of earthquakes in southern California has been attributed previously to lateral variations in either crustal temperature or crustal rock composition. Here I compare heat flow (a proxy for crustal temperature), local maximum depths of earthquakes (d95), and best‐guess crustal rock compositions. Plotting heat flow versus d95 produces distinct trends for each rock composition, with each trend representing an isotherm marking the onset of plastic behavior. Offsets between the trends measure the differences in the plastic behavior temperatures of each rock composition; these offsets are comparable to the d95 variations with heat flow. Thus, crustal temperature and composition contribute about equally to the control of the maximum depths of earthquakes. The observed isotherm slopes imply vertical variations in crustal composition in addition to the lateral variations.

Depth distribution of earthquakes in the Baikal rift system and its implications for the rheology of the lithosphere

AbstractThe correspondence between the predicted brittle–plastic transition within the crust and the maximum depth of earthquakes is examined in the case of the Baikal rift, Siberia. Although little accurate information on depths is available through large- and moderate-size earthquakes, there are frequent indications of foci at 20 km depth and more. We have relocated 632 events recorded at nearby stations that occurred between 1971 and 1997, with depth and epicentral uncertainties less than 5 km, over the eastern and southern parts of the Baikal rift. We have compared these results with other depth distributions obtained in previous studies from background seismicity in the NE rift (1365 events in the Kalar-Chara zone and 704 events in the Muya region). The relative abundance of earthquakes is generally low at depths between 0 and 10 km (7–15 per cent) and high between 15 and 25 km (∼50 per cent). Earthquake activity is still significant between 25 and 30 km (9–15 per cent) and persists between 30 and 40 km (7–13 per cent). Very few earthquakes are below the Moho. We use empirical constitutive laws to obtain the yield-stress limits of several layers made of dominant lithologies and to examine whether the observed distribution of earthquakes at depth (519 events controlled by a close station and located within the extensional domain of the Baikal rift system) can match the predicted crustal strength proportion with depth and the deeper brittle–ductile transition in the crust. A good fit is obtained by using a quartz rheology at 0–10 km depth and a diabase rheology at 10–45 km depth with a moderate temperature field which corresponds to a ∼100 Myr thermal lithosphere. No dioritic composition of the crust is found necessary. In any case, earthquakes occur at deep crustal levels, where the crust is supposed to be ductile, in a way very similar to what is found in the East African Rift System. From these results we conclude that the seismogenic thickness is ∼35–40 km in the Baikal rift system and that the depth distribution of earthquakes is at first order proportional to the strength profile found in a rheologically layered crust dominated by a mafic composition in the ∼10–45 km depth range. An upper mantle core with high strength, however, generally prevents it from reaching stress failure at greater depth.

Seismotectonic analysis of the Tjörnes Fracture Zone, an active transform fault in north Iceland

The Tjörnes Fracture Zone is a transform fault connecting the rift zone of the Kolbeinsey Ridge with that of north Iceland. The main transform motion takes place on the Húsavík‐Flatey Fault, a major 7–9 Myr old right‐lateral fault. In addition to this fault, there are two major seismic lineaments associated with the Tjörnes Fracture Zone; the Grímsey lineament and the Dalvík lineament. These lineaments are marked by concentrations of seismicity with the largest earthquakes reaching magnitude 7. The maximum depth of earthquakes is 10–12 km and increases with distance from the spreading axis. We determined accurate relative locations and focal mechanisms of more than 800 earthquakes in 62 clusters on the principal seismic lineaments. The estimated relative location uncertainty for most of the relocated earthquakes is 2–20 m. The best fitting plane through each cluster is assumed to coincide with the fault plane of the group of earthquakes. For clusters near the Húsavík‐Flatey Fault the fault planes are right‐lateral and strike N122°E–N140°E, similar to the overall strike of the Húsavík‐Flatey Fault. This agrees with the right‐lateral displacement on the fault as well as with field observations of numerous transform‐parallel right‐lateral faults associated with the main fault. By contrast, earthquake clusters on the lineaments of Grímsey and Dalvík define (mostly) left‐lateral planes striking roughly north‐south, i.e., at 40°–90° to the overall trend of these lineaments. Field observations show that left‐lateral, north‐south trending fault planes are also common in the on‐land parts of the Dalvík lineament. The different style of faulting probably represents transform faults at different stages of development.

Lithologic Control of the Depth of Earthquakes in Southern California

The depth distribution of southern California earthquakes indicates that areas underlain by schist basement rocks have a shallower (4 to 10 kilometers) maximum depth of earthquakes than do areas with other types of basement rocks. The predominant minerals in the schists become plastic at lower temperatures, and thus at shallower depths, than the minerals in the other basement rocks. The lateral variations in lithology will control the depth extent (and thus the magnitudes) of potential future earthquakes; these depths can be determined from the depth of the current background seismicity.

The Hengill geothermal area, Iceland: Variation of temperature gradients deduced from the maximum depth of seismogenesis

Given a uniform lithology and strain rate and a full seismic data set, the maximum depth of earthquakes may be viewed to a first order as an isotherm. These conditions are approached at the Hengill geothermal area S. Iceland, a dominantly basaltic area. The likely strain rate calculated from thermal and tectonic considerations is 10 −15 s −1, and temperature measurements from four drill sites within the area indicate average, near-surface geothermal gradients of up to 150 °C km −1 throughout the upper 2 km. The temperature at which seismic failure ceases for the strain rates likely at the Hengill geothermal area is determined by analogy with oceanic crust, and is about 650 ± 50 °C. The topographies of the top and bottom of the seismogenic layer were mapped using 617 earthquakes located highly accurately by performing a simultaneous inversion for three-dimensional structure and hypocentral parameters. The thickness of the seismogenic layer is roughly constant and about 3 km. A shallow, aseismic, low-velocity volume within the spreading plate boundary that crosses the area occurs above the top of the seismogenic layer and is interpreted as an isolated body of partial melt. The base of the seismogenic layer has a maximum depth of about 6.5 km beneath the spreading axis and deepens to about 7 km beneath a transform zone in the south of the area. Beneath the high-temperature part of the geothermal area, the maximum depth of earthquakes may be as shallow as 4 km. The geothermal gradient below drilling depths in various parts of the area ranges from 84 ± 9 °Ckm −1 within the low-temperature geothermal area of the transform zone to 138 ± 15 °Ckm −1 below the centre of the high-temperature geothermal area. Shallow maximum depths of earthquakes and therefore high average geothermal gradients tend to correlate with the intensity of the geothermal area and not with the location of the currently active spreading axis.

Lithosphere, asthenosphere, and perisphere

“Lithosphere” is a mechanical concept implying strength and relative permanence. Unfortunately, the term has also been applied to the surface thermal boundary layer (TBL) and a shallow enriched geochemical reservoir, features having nothing to do with strength. The “strong” lithosphere is about one half the thickness of the TBL. The bottom of the elastic plate, the maximum depths of midplate and fracture zone earthquakes, and the top of the low‐velocity zone all occur at depths corresponding to the 550°–750°C temperature range. This may also correspond to the base of the coherent plate. Earthquakes in subducted slabs are bounded by isotherms in this temperature interval. Mantle hotter than ∼650°C cannot support long‐term stresses and does not qualify as lithosphere. There is very little ancient lithosphere (mantle colder than 650 ± 100°C for long periods of time), and this is not a suitable reservoir for continental flood basalts (CFB). A chemical characteristic, that is, “enriched,” has been attributed to old lithosphere to distinguish it from the “depleted” upper mantle. The continental lithosphere (CL) is often treated as a viable reservoir for CFB or, when delaminated, for ocean island basalts and enriched mid‐ocean ridge basalts (MORB). The CFB reservoir is more likely to be a hot, weak sublithospheric layer which may include the lower part of the TBL. The term “perisphere” has been introduced to accommodate the need for a term for a global, shallow, enriched reservoir or boundary layer. It is physically isolated from the depleted mantle (usually called the “convecting mantle,” “asthenosphere” or “upper mantle”) not by its strength but by its weakness and buoyancy; it is chemically isolated by its location relative to subduction recycling. It has the chemical characteristics often attributed to continental lithosphere (or plume heads), but it is a permanent part of the sublithospheric shallow mantle and is constantly refreshed by recycling. It is an open system and can also be called the “active layer” or “mixing zone.” It is proposed that the depleted reservoir (MORB source or depleted mantle) is below and protected from contamination (chemical isolation) by the filtering action of this boundary layer. Depleted MORB is most prominent at fast spreading ridges, which induce or localize deep, broad upwellings. Melts from enriched mantle (EM) are most evident at new or slowly rifting regions, infant subduction zones, new backarc basins, slab windows, and mid‐plate environments away from spreading induced upwelling. EM is therefore probably shallow. It is not known if volatiles and large‐ion lithophiles can recycle much deeper than ∼200 km, or into the lower mantle, as is implied by some plume theories. The base of the (strong) lithosphere and plate may correspond to a phase change. If so, the correspondence among the brittle‐ductile boundary (the maximum depth of earthquakes), the top of the low‐velocity zone, and the elastic plate thickness can be understood. Candidate phase changes include dehydration and clinoenstatite to orthoenstatite since these occur near 600°C. Another rheological boundary may set in near the solidus, but silicates lose most of their strength at absolute temperatures about one half the dry solidus temperature. The region of the subcontinental mantle between ∼600°C and the solidus may provide some of the material in continental magmas, but this cannot be considered part of the continental plate. The “continental lithosphere” reservoir of petrologists is actually a weak enriched layer that may spread across the top of the convecting mantle. This is the perisphere. Its existence makes it possible to understand CFB and ocean island chemistry and kinematics without postulating plumes from the lower mantle, plume heads, fossil plume heads, or delaminated CL. The upper mantle is inhomogeneous in chemistry.

Crustal structure of the offshore southern Santa Maria Basin and transverse ranges, southern California, from deep seismic reflection data

Seismic reflection data from the 1986 EDGE/Pacific Gas and Electric experiment together with refraction and gravity data are used to construct an integrated north‐south geophysical cross section along the offshore Santa Maria Basin and across the offshore extension of the Transverse Ranges. Thrust faults offsetting acoustic basement and folding Miocene sediments root in a midcrustal detachment at 12–13 km depth. The detachment coincides with an increase in reflectivity seen on the seismic reflection section and the maximum depth of earthquakes. A high‐velocity oceanic crustal layer at the base of the crust (17–22 km) and Moho (≈ 25 km) is offset by a low‐angle thrust fault with an apparent dip ≈ 10° to the north offshore Point Arguello. Gravity models indicate crustal thickening and a change in the depth to Moho from 25 to 30 km in this region. However, the contribution of elastic flexure from loading of the Transverse Ranges is considered to be minor in this part of the borderland. The cross section constrains Neogene shortening to less than 3% based on faulting and folding of the sedimentary section. Shortening of the upper crust is balanced by a combination of ductile flow and subduction of the oceanic crust beneath the midcrustal detachment.

A relation among geology, tectonics, and velocity structure, western to central Nevada Basin and Range

Research Article| September 01, 1992 A relation among geology, tectonics, and velocity structure, western to central Nevada Basin and Range R. D. CATCHINGS R. D. CATCHINGS 1U.S. Geological Survey, 345 Middlefield Road, M.S. 977, Menlo Park, California 94587 Search for other works by this author on: GSW Google Scholar Author and Article Information R. D. CATCHINGS 1U.S. Geological Survey, 345 Middlefield Road, M.S. 977, Menlo Park, California 94587 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (9): 1178–1192. https://doi.org/10.1130/0016-7606(1992)104<1178:ARAGTA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. D. CATCHINGS; A relation among geology, tectonics, and velocity structure, western to central Nevada Basin and Range. GSA Bulletin 1992;; 104 (9): 1178–1192. doi: https://doi.org/10.1130/0016-7606(1992)104<1178:ARAGTA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In the northwestern to central Nevada Basin and Range, there are correlations between velocity and specific geologic structures of the crust. Mapped range-bounding faults at the surface can be traced to appreciable (10 km) depths based on velocity variations and are consistent with subsurface projections of the faults based on seismic reflection images. The limiting depth of the faults, as indicated by the velocity variations, corresponds to the maximum depth of earthquakes along the seismic profile. Correlations between velocity and the surface geology show that in the upper crust the pre-Cenozoic rocks are underlain by high-velocity (6.0 km/s) rocks, whereas the Tertiary ranges are underlain by lower-velocity (4.0-5.7 km/s) rocks to depths as great as 10 km. Although the Tertiary rocks differ in composition from the Mesozoic rocks, the lower-velocity Tertiary rocks may also be attributed to rock masses which are broken (4.0-5.7 km/s), and the higher-velocity Mesozoic rocks (6.0 km/s) may be attributed to largely unbroken rock masses. The regional seismicity pattern is consistent with this interpretation, as earthquakes are largely confined within or near the base of the low-velocity rocks. These low-velocity, highly fractured rocks are laterally distributed in discrete zones, suggesting that extension is not uniformly distributed but occurs in discrete, highly extended zones. Beneath these highly extended zones, the lower-crustal layers show structural evidence of extension, and velocity measurements suggest that the lowermost crust has been magmatically underplated. The superposition of Tertiary volcanic rocks, highly fractured upper crust, and lower-crustal magmatic underplating suggests that the Tertiary volcanic rocks originated from lower-crustal magmas that migrated to the surface via the highly extended zones. The velocity structure of one of the highly extended zones and the Lahonton Basin resembles that of many continental rifts. The velocity structure beneath central Nevada, however, is much more like normal continental crust. On the basis of isotopic studies, it is concluded that the transition between highly extended crust and more normal crust occurs in the area inferred to be the edge of the North American craton. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Thermal‐mechanical controls on seismicity depth distributions in the San Andreas Fault Zone

Earthquake depth distributions in continental crust can be related to factors which determine the depth of the transition from brittle to ductile failure. These factors include temperature, strain rate and lithology. The San Andreas Fault Zone of Northern and Central California is an ideal place to test the relative importance of these factors since there is abundant earthquake data and our understanding of the tectonic interaction of the Pacific and North American plates provides constraints for several parameters. In the San Andreas Fault Zone we have found an approximately 3 km increase from Northern to Central California of the median depth of events and the depth above which 90% of events occur. In addition, depth distributions of earthquakes vary systematically from Northern to Central California. Both the maximum depth of earthquakes and the depth of the peak of seismic activity are shallower in Northern California. Utilizing Critical Strength Envelopes we have found that, unlike the oceanic case, this variation is not thermally controlled. Rather, a combination of low strain rate and low stress level cause the seismicity in Northern California to be shallow relative to Central California.

A Brief Update on the Focal Depths of Intracontinental Earthquakes and their Correlations with Heat Flow and Tectonic Age

Abstract This paper summarizes the characteristics of focal depths of intracontinental earthquakes based on a compilation of reliably determined depths reported in the past five years. Since the study of Chen and Molnar (1983), several additional moderate-sized sub-crustal events (mb ≥ 5) have occurred in southern Tibet, the Tien Shan, and the Karakorum where such events are known to exist. Furthermore, three events have been identified near the Moho beneath the Brazilian shield, the coldest part of the Baltic shield, and the passive margin of Newfoundland. Since these events took place in stable parts of the continent, they must be an integrated part of the deformation of the intracontinental lithosphere at depth, not that of subducted material. Therefore there seems to be little doubt that earthquakes near the Moho is an incessant element of intracontinental seismicity. Due to the lack of detailed information on the crustal and mantle structures in the source regions, many of the events near the Moho seem to be located in the uppermost mantle but some might have occurred in the lowermost crust. In any case, the events near the Moho in most regions seem to be separated from the familiar seismicity in the upper part of the crust, forming a pattern of two seismogenic zones straddling an aseismic region in the lower crust. With data from South America and the Baltic shield, the maximum depth of earthquakes in cratons can be correlated with tectonic age within a single continent, and with low heat flow of the source region in a given tectonic province, respectively. These observations reinforce the view point that to a first approximation, the distribution of focal depth serves as an indicator of temperature or mechanical strength of the lithosphere, a result consistent with extrapolation of data from experimental rock mechanics. The aseismic zone in the lower crust is interpreted as a relatively weak region of the lithosphere sandwiched between two layers of more competent material where sufficient strain can accumulate to produce earthquakes.

Structural transect of the western Transverse Ranges, California: Implications for lithospheric kinematics and seismic risk evaluation

A retrodeformable cross section that integrates surface and subsurface data across the western Transverse Ranges, California, illustrates an actively developing fold and thrust belt that began forming at 2-3 Ma. High-level thrusts are interpreted to root in a mid-crustal detachment at 12-15 km depth, which coincides with the maximum depth of earthquakes. The cross section documents 53 km of convergence above the mid-crustal detachment; dividing this by the time since onset of deformation yields convergence rates of 17.6-26.5 mm/yr. The high-density lithospheric anomaly beneath the Transverse Ranges is related to subduction of lower crust and lithosphere below the mid-crustal detachment to balance the shallow crustal shortening. Thrust ramps coincide with zones of high seismicity in the Transverse Ranges; this suggests that ramp regions have the highest potential for compressive earthquake events: e.g., the recent Whittier Narrows earthquake of October 1, 1987 (M/sub l/ = 5.9), occurred along the eastern part of a ramp zone.

Localized polymorphic phase transformations in high‐pressure faults and applications to the physical mechanism of deep earthquakes

Earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes. Such mechanisms, in unmodified form, are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300–700 km. Several alternative mechanisms have been proposed. Here, I discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me. These shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics: (1) A single sharp shear zone forms in the maximum shear stress orientation. (2) Confining pressure does not suppress the instability and, under some conditions, raising confining pressure Pc lowers the differential stress σƒ at which it occurs. (3) Stress drops can occur repeatedly after reloading at about the same differential stress. (4) The mean stress vs. temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice II and diopside + talc, respectively. Although I have not as yet confirmed the existence of these high pressure phases in the shear zones, transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur. Reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non‐hydrostatically near transition boundaries. Moreover, by analogy, the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double‐couple first‐motion distribution. Finally, if this is the earthquake source mechanism in deeply subducted lithosphere, then earthquakes should shut off when all of the polymorphic phase changes have occurred. This may explain the maximum depth of earthquakes at about 680 km.

Subducted lithosphere, hotspots, and the geoid

The earth's largest positive geoid height anomalies are associated with subduction zones and hotspots. Although the correlation with subduction has been noted for many years, the correlation with hotspots is fully evident only when the subduction-related geoid highs are removed from the observed field. Using the assumption that subducted lithospheric slabs are uncompensated and are thermally re-equilibrated with the asthenosphere at the maximum depth of earthquakes, the expected geoid anomaly over subduction zones is calculated. This field provides a satis-factory fit to the observed circum-Pacific high. Subtraction of this predicted anomaly leaves a residual field which is correlated, at greater than the 99% confidence level, with the distribution of hotspots. Broad residual geoid highs occur over the central Pacific and the Africa/eastern Atlantic regions, the same areas where the hotspots are concentrated. The mass anomalies associated with hotspots and subducted slabs apparently control the location of the earth's spin axis.