Stable Continental Regions Research Articles

Crack density, saturation rate and porosity at the 2001 Bhuj, India, earthquake hypocenter: a fluid-driven earthquake?

Quantitative estimates of the crack density ( ϵ), saturation rate ( ξ) and porosity ( ψ) parameters from seismic velocities ( V p, V s) and Poisson’s ratio ( σ) in the 2001 Bhuj earthquake area in western India indicate that the 2001 Bhuj earthquake hypocenter is associated with high- ϵ, high- ξ and high- ψ in the depth range of about 23–28 km, extending 15–30 km laterally. These anomalies may be due to a fluid-filled, fractured rock matrix, which might have contributed to trigger the 2001 Bhuj earthquake in the intraplate, stable continental region of the Indian Peninsula. This feature is similar to that of the 1995 Kobe earthquake [D. Zhao et al., Science 274 (1996) 1891–1894]. High- ψ areas are generally consistent with high- ϵ areas, but high- ξ areas have a wider distribution, indicating that micro-cracks exist in more localized areas within the crust, and that permeation of fluids in the hypocenter zone might have occurred extensively through the intergranular and fractured pores due to hidden intersecting fault geometry. Here we suggest the possibility that earthquake occurrence is closely related to in situ material heterogeneities, rather than stress conditions alone.

Seismic tomography beneath stable tectonic regions and the origin and composition of the continental lithospheric mantle

On a large scale, seismic waves travel faster through old continental shields than through Paleozoic platforms. On a smaller scale, the continental lithosphere appears to be composed of blocks with different velocities. Sharp vertical contacts between different blocks of lithosphere are revealed by short-wavelength offsets in P-vertical travel times; some contacts have been preserved for more than one billion years. These observations cannot be explained by temperature differences alone and imply persistent mineralogical differences within the lithosphere. To reconcile global and local seismological observations, a bimodal model is required: the continental lithosphere appears to consist of cores of depleted, refractory lithosphere surrounded by and/or superposed on younger, more fertile lithosphere. The seismic characteristics and mineralogical compositions correlate on a large scale with the age of the lithosphere and reflect episodic growth of the continents. Temperatures in the lithosphere of stable continental regions vary less than commonly assumed and may be close to the geotherms proposed by Jaupart and Mareschal [Lithos 48 (1999) 93–114].

Paleoseismicity of Two Historically Quiescent Faults in Australia: Implications for Fault Behavior in Stable Continental Regions

Paleoseismic studies of two historically aseismic Quaternary faults in Australia confirm that cratonic faults in stable continental regions (SCR) typically have a long-term behavior characterized by episodes of activity separated by quiescent intervals of at least 10,000 and commonly 100,000 years or more. Studies of the approximately 30-km-long Roopena fault in South Australia and the approximately 30-km-long Hyden fault in Western Australia document multiple Quaternary surface-faulting events that are unevenly spaced in time. The episodic clustering of events on cratonic SCR faults may be related to temporal fluctuations of fault-zone fluid pore pressures in a volume of strained crust. The long-term slip rate on cratonic SCR faults is extremely low, so the geomorphic expression of many cratonic SCR faults is subtle, and scarps may be difficult to detect because they are poorly preserved. Both the Roopena and Hyden faults are in areas of limited or no significant seismicity; these and other faults that we have studied indicate that many potentially hazardous SCR faults cannot be recognized solely on the basis of instrumental data or historical earthquakes. Although cratonic SCR faults may appear to be nonhazardous because they have been historically aseismic, those that are favorably oriented for movement in the current stress field can and have produced unexpected damaging earthquakes. Paleoseismic studies of modern and prehistoric SCR faulting events provide the basis for understanding of the long-term behavior of these faults and ultimately contribute to better seismic-hazard assessments.

Differences in attenuation among the stable continental regions

There are systematic differences in the attenuation of damaging earthquake ground motions between different stable continental regions (SCRs). Seismic intensity and weak‐motion data show that the attenuation in seismic waves for eastern North America (ENA) is less than for India, Africa, Australia, and northwest Europe. If ENA ground‐motion attenuation relations are used in seismic hazard models for other SCRs, as is commonly done, then the estimated ground motions and resulting hazard may be too large. If an attenuation model that averages observations from ENA and the other SCRs is used to estimate the magnitudes of large historical earthquakes in ENA, as is the case for recent estimates of M for the 1811–1812 New Madrid, Missouri and the 1886 Charleston, South Carolina events, then the magnitude estimates for these events will be too large, as will be the resulting hazard.

Characteristics of Lg attenuation in the Tibetan Plateau

Lg, a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide, is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties. In stable continental regions, Lg propagates with a group velocity of about 3.5 km/s and can often be observed at distances up to 4000 km. To better understand the absence of high‐frequency Lg arrivals for paths traversing the northern boundary of the Tibetan Plateau, we investigate spatial variations of broadband (0.15–5.0 Hz) energy in the Lg group velocity window (3.6–3.0 km/s) using regional waveforms recorded at the Chinese Digital Seismic Network station WMQ. Vertical component seismograms are analyzed for 90 events with magnitudes of 4.4 ≤ mb ≤ 6.4 that occurred between 1987 and 1999 in the Tibetan Plateau and around its margins. The Lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1–2 Hz, nearly identical to those for comparable size events at similar distances outside the plateau. High‐frequency (>1 Hz) Lg energy recorded at WMQ decreases rapidly as a function of source distance into the plateau. A path length of 300–400 km within the northern Plateau suffices to eliminate 2–5 Hz Lg energy. For events in southern Tibet with paths crossing the central portion of the Tibetan Plateau, almost total Lg extinction occurs, even for energy in the low‐frequency band of 0.2–1 Hz. Corresponding apparent corner frequencies of the “Lg” amplitude spectra range between 0.2 and 0.4 Hz. The corner frequency shift is found to vary systematically with path length across the plateau. Linear regressions demonstrate that the shift in apparent corner frequency of Lg amplitude spectra is negatively correlated with features of the Tibetan Plateau, such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds. The systematic variations in the amplitude and frequency content of energy in the Lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in Lg extinction for paths traversing central and northern Tibet, superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau. Spectral ratios of many event pairs along great circle paths give estimates of frequency‐dependent Lg attenuation for paths crossing western, central, and eastern sectors of Tibet. The region of strong Sn attenuation in northern central Tibet also has strong Lg attenuation with QLg for 1 Hz on the order of Q0 = 80–90, while in southern central Tibet, Q0 increases to about 316, and in eastern and western Tibet, Q0 is on the order of 120–200 for paths traversing the entire plateau. The strong Lg attenuation in northern central Tibet is responsible for the so‐called Lg blockage and may be associated with partial melting in the crustal low‐velocity layer in northern Tibet. Our QLg values for Tibet are significantly lower than most earlier estimates, primarily as a result of not excluding blocked observations along with allowing for lateral variations within Tibet.

Stable Continental Region Earthquakes in South China

— This paper reviews some remarkable characteristics of earthquakes in a Stable Continental Region (SCR) of the South China Block (SCB). The kernel of the SCB is the Yangtze platform solidified in late Proterozoic time, with continental growth to the southeast by a series of fold belts in Paleozoic time. The facts that the deviatoric stress is low, the orientations of the major tectonic features in the SCB are substantially normal to the maximum horizontal principal stress, and a relatively uniform crust, seem to be the major reasons for lack of significant seismicity in most regions of the SCB. Earthquakes in this region are mainly associated with three seismic zones: (1) the Southeast China Coast seismic zone related to Guangdong-Fujian coastal folding belt (associated with Eurasia-Philippine Sea plate collision); (2) the Southern Yellow Sea seismic zone associated with continental shelf rifts and basins; and (3) the Downstream Yangtze River seismic zone spatially coinciding with Tertiary rifts and basin development. All three seismic zones are close to one or two major economic and population centers in the SCB so that they pose significant seismic hazards. Earthquake focal mechanisms in the SCB are consistent with strike-slip to normal faulting stress regimes. Because of the global and national economic significance of the SCB and its dense population, the seismic hazard of the region is of outstanding importance. Comparing the SCB with another less developed region, a pending earthquake with the same size and tectonic setting would cause substantially more severe social and economic losses in the SCB. This paper also compiles an inventory of historic moderate to great earthquakes in the SCB; most of the data are not widely available in English literature.

Characteristics of Deformation and Past Seismicity Associated with the 1819 Kutch Earthquake, Northwestern India

The 1819 earthquake in Kutch, northwestern India, is one of the most significant events to have occurred in a plate-interior setting. Despite being the second largest among the stable continental region (SCR) earthquakes, this event has not been analyzed within the context of present-day understanding of earthquake seismology. Coseismic changes related to this earthquake include massive ground deformation in a wide low-lying tidal-flat area. Although detailed historic accounts of this earthquake exist, many questions regarding the mode of deformation and the seismic history of the region remain unresolved. We explored the region nearly 180 years after the earthquake, and the information gathered adds to our understanding of this event and provides a fresh perspective on this unique intraplate seismogenic zone. A 90-km-long tract of elevated land with a peak height of 4.3 m is the most visible surface expression of this earthquake. We surveyed and analyzed the morphological features of this scarp and also carried out exploratory trenching in this region. The scarp morphology is suggestive of a growing fold related to a buried north-dipping thrust rather than a discrete fault that could have resulted from a surface rupture. The extensive liquefaction field associated with the earthquake offered an ideal setting to explore the paleoearthquake history. Age data of liquefaction features suggest that a previous event of comparable size must have occurred 800–1000 years ago. Seismic activity appears to be related to the reactivation of an ancient rift in a stress regime that is dominated by nearly north–south compression.

Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.

Stable continental regions, free from tectonic activity, are generally found only within ancient cratons-the centres of continents which formed in the Archaean era, 4.0-2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium-osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.

Detection of widespread fluids in the Tibetan crust by magnetotelluric studies.

Magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north-to-south width of the Tibetan plateau. The integrated conductivity (conductance) of the Tibetan crust ranges from 3000 to greater than 20,000 siemens. In contrast, stable continental regions typically exhibit conductances from 20 to 1000 siemens, averaging 100 siemens. Such pervasively high conductance suggests that partial melt and/or aqueous fluids are widespread within the Tibetan crust. In southern Tibet, the high-conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust. In northern Tibet, the conductive layer is at 30 to 40 kilometers and is due to partial melting. Zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow.

Heat flow scaling for mantle convection below a conducting lid: Resolving seemingly inconsistent modeling results regarding continental heat flow

Numerical models of mantle convection below a conducting lid, meant to mimic a continent, have been used 1) to argue that heat flow variations in stable continental regions result principally from lithospheric thickness variations, and 2) to argue that the relationship between lithospheric thickness and continental heat flow is too weak to allow for this. We reconcile these results using a theoretical heat flow scaling which shows that the relationship between heat flow and lid thickness can take on two end‐member forms. As the ratio of lid thickness to convecting layer depth increases, the effects of lid thickness variations on heat flow move from weak to strong. Modeling studies that concluded that lithospheric thickness variations could lead to significant continental heat flow variations assumed a convecting layer depth appropriate to upper mantle convection and, thus, maximized the relative thickness of the continental lithosphere, i.e., the conducting lid. Studies that concluded that the relationship is weak assumed whole mantle convection which placed them on the flatter end of the heat flow versus relative lid thickness curve.

Triggered Earthquakes and the 1811-1812 New Madrid, Central United States, Earthquake Sequence

The 1811–1812 New Madrid, central United States, earthquake sequence included at least three events with magnitudes estimated at well above M 7.0. I discuss evidence that the sequence also produced at least three substantial triggered events well outside the New Madrid Seismic Zone, most likely in the vicinity of Cincinnati, Ohio. The largest of these events is estimated to have a magnitude in the low to mid M 5 range. Events of this size are large enough to cause damage, especially in regions with low levels of preparedness. Remotely triggered earthquakes have been observed in tectonically active regions in recent years, but not previously in stable continental regions. The results of this study suggest, however, that potentially damaging triggered earthquakes may be common following large mainshocks in stable continental regions. Thus, in areas of low seismic activity such as central/eastern North America, the hazard associated with localized source zones might be more far reaching than previously recognized. The results also provide additional evidence that intraplate crust is critically stressed, such that small stress changes are especially effective at triggering earthquakes.

The deadliest stable continental region earthquake occurred near Bhuj on 26 January 2001

A large destructive earthquake occurred on 26 January 2001 in the region of Kutch, Gujarat, in Western India, with magnitude Mw 7.7. The earthquake caused very heavy damage and a large number of casualties with more than 20,000 deaths. A preliminary study of ground deformation, damage pattern and aftershock distribution is presented.

Seismic hazard in regions of present day low seismic activity: uncertainties in the paleoseismic investigations along the Bree Fault Scarp (Roer Graben, Belgium)

Earthquake hazard assessment in stable continental regions, such as northern Europe, has traditionally been evaluated on the basis of the instrumentally and historically recorded seismicity, which indicates relatively low hazard levels. Reliability of such estimates is a matter of debate as the long-term potential of large earthquakes usually cannot be determined based on short observational periods generally less than a few hundred years. A significant improvement to this lack of knowledge can be achieved by extending the past observations into the geological time scale. Paleoseismic investigations can provide valuable information to bridge this gap, where the potential for large earthquakes can be quantified both in magnitude and recurrence period, based on the observation of prehistoric earthquakes (paleoearthquakes) in the geological record (particularly in the last 20,000 years). However, using these records in seismic hazard analysis requires systematic treatment of uncertainties. Usually uncertainties are inherent to the interpretation of geological record, which leads, in the end, to the identification of paleoearthquakes. Field observations used in the analysis may satisfy several alternative interpretations. Such interpretations become useless when alternative solutions exist but not documented in detail, and especially when the relative reliability of the favored interpretation with respect to the alternative interpretations is not known. The recently introduced method using logic-tree formalism, which is based on qualitative description of the uncertainties related to the paleoseismic data and especially in its interpretation, is applied in the paleoseismic investigations performed on the Bree Fault Scarp, along the Feldbiss Fault (Roer Graben, Belgium). The cumulative uncertainties associated with the different stages of the study are computed as the combination of the preferred alternative branches in the logic-tree presentation. The final uncertainty and its relative importance in seismic hazard analysis is expressed as the paleoseismic quality factor (PQF), which indicate 0.76. This value can directly be used in seismic hazard analysis.

Long-term seismicity in regions of present day low seismic activity: the example of western Europe

In western Europe, the knowledge of long-term seismicity is based on reliable historical seismicity and covers a time period of less than 700 years. Despite the fact that the seismic activity is considered as low in the region extending from the Lower Rhine Embayment to England, historical information collected recently suggests the occurrence of three earthquakes with magnitude around 6.0 or greater. These events are a source of information for the engineer or the scientist involved in mitigation against large earthquakes. We provide information relevant to this aspect for the Belgian earthquake of September 18, 1692. The severity of the damage described in original sources indicates that its epicentral intensity could be IX (EMS-98 scale) and that the area with intensity VII and greater than VII has at least a mean radius of 45 km. Following relationships between average macroseismic radii and magnitude for earthquakes in stable continental regions, its magnitude M s is estimated as between 6.0 and 6.5. To extend in time our knowledge of the seismic activity, we conducted paleoseismic investigations in the Roer Graben to address the question of the possible occurrence of large earthquakes with coseismic surface ruptures. Our study along the Feldbiss fault (the western border of the graben) demonstrates its recent activity and provides numerous lines of evidence of Holocene and Late Pleistocene large earthquakes. It suggests that along the 10 km long Bree fault scarp, the return period for earthquakes with magnitude from 6.2 to 6.7 ranges from 10,000 to 20,000 years during the last 50,000 years. Considering as possible the occurrence of similar earthquakes along all the Quaternary faults in the Lower Rhine Embayment, a large earthquake could occur there each 500–1000 years. These results are important in two ways. (i) The evidence that large earthquakes occur in western Europe in the very recent past which is not only attested by historical sources, but also suggested by paleoseismic investigations in the Roer Graben. (ii) The existence of a scientific basis to better evaluate the long-term seismicity in this part of Europe (maximal magnitude and return period) in the framework of seismic hazard assessment.

Geology in the 1996 USGS Seismic-hazard Maps, Central and Eastern United States

The current (1996) national probabilistic seismic-hazard maps utilize information about geologic structure and tectonics of the central and eastern U.S. to compensate for uncertainty that arises from the short seismicity record. Geology was incorporated into the maps mainly as seven source zones that are delineated in three distinct ways. The North American stable continental region is divided into two large zones, the sparsely seismic Precambrian craton and the more active Phanerozoic rim. Five other source zones are much smallerqthe Wabash Valley source zone is within the craton, whereas the Reelfoot Rift, eastern Tennessee, Charleston, and Charlevoix source zones are in the Phanerozoic rim of the continent. We document these zones and explain and justify their use. The seven zones provide a foundation from which we suggest a criterion for including more geology in future maps.

Anatomy of surface rupture zones of two stable continental region earthquakes, 1967 Koyna and 1993 Latur, India

Soil‐helium surveys in the surface rupture zones of the 1993 Latur (Mw 6.2) and the 1967 Koyna (Mw 6.3) stable continental region (SCR) earthquake sites in Peninsular India showed anomalies defining surface traces of the causative faults. Propagating from the Archaean crystalline basement through the Deccan basalt cover, the seismic fault produced a scarp by uplift along a thrust in the Killari area of the Latur earthquake, whereas the Koyna earthquake was associated with a strike‐slip fault which expressed itself as an en echelon fissure zone. Core drilling has confirmed that the fault at Killari extends downward from the surface rupture zone with an approximate dip of 50° towards SSW. The level differences of flow contacts obtained by drilling in the hanging wall and foot wall sides of the fault, do not unequivocally establish the amount of displacement, but suggest that it might be anything from 1 m to 6 m. If the higher figure of 6 m is accepted, it would indicate reactivation of an old fault. Drilling established a WNW dip of the Koyna fault, resolving a long‐standing debate.

Geothermal investigations in the 1993 Latur earthquake area, Deccan Volcanic Province, India

Heat flow measurements have been made in four boreholes in the epicentral area of the Latur earthquake ( M w 6.2, 1993) in the southeastern part of the Cretaceous–Eocene Deccan Volcanic Province (DVP), central India. Three of these, located 4 to 8 km from the surface rupture zone near Killari, were about 180 m deep and yielded heat flow values in the range 33 to 40 mW m −2. A hole was specially drilled in the surface rupture zone and penetrating the entire 338 m Deccan basalt cover and a further 271 m into the Archaean granite-gneiss basement. Temperature gradients over both the basaltic as well as the granite-gneiss sections, which have a large thermal conductivity contrast, yielded a consistent heat flow value of 43 mW m −2. Earlier measurements at Koyna, the site of another stable continental region (SCR) earthquake ( M w 6.3, 1967) in the western part of the DVP, and at another locality, Lonar, in the central part of the DVP had yielded heat flow values of 41 mW m −2 and 47 mW m −2, respectively. Thus the southern part of the DVP is characterised by a low heat flow regime, similar to that over the Archaean Dharwar province immediately to the south. The 617 m deep hole at Killari provided an opportunity for carrying out measurements of radioactive heat production on core samples of the 271 m long basement section. Thirty-nine core samples of the migmatitic gneisses and the granites encountered in the 271 m long basement section of KLR-1 were analysed for U, Th and K, and resulted in estimates of heat production of 0.5 and 2.6 μW m −3, respectively, for the two types. A crustal heat production model is envisaged, with the thicknesses of the gneisses and the granites constrained from available pressure estimates in the Dharwar craton, and also considering a 17 km `granitic' upper crust and a 20 km `granulitic' lower crust based on deep seismic sounding data. Using this crustal heat production model and the heat flow value of 43 mW m −2, the estimated crustal temperatures imply a deep (>30 km) brittle–ductile transition. Even though heat flow–crustal temperature considerations allow for deeper, crustal earthquake foci, most of the M w>6 events of the last three decades in the Australian and Indian SCRs show shallow foci (<10 km). This calls for seeking possible mechanisms for frictional failure to take place in the top part of a crust which is largely brittle.

Seismogenesis in the stable continental interiors: an appraisal based on two examples from India

Characteristics of two recent moderate earthquakes from India (1993 Killari, M w 6.1 and 1997 Jabalpur, M w 5.8) are presented in this paper to illustrate the influence of geologic and tectonic setting on the nature of seismicity in stable continental regions (SCRs). The Killari event, which occurred in the middle of a cratonic region of low background seismicity, long recurrence interval and poorly developed neotectonic features, may be qualified as a common type of SCR event. This earthquake is also characterized by a shallow focal depth and a long sequence of aftershocks. In contrast, the Jabalpur earthquake shows a spatial association with a well-defined tectonic structure with significant background seismicity. A remarkable feature of this earthquake is its deep focus, not commonly observed in SCR seismicity. This event was associated with fewer aftershocks unlike most other SCR earthquakes. Analogous events are found in other shield regions as well. Based on their general characteristics, a broad classification of moderate SCR earthquakes is attempted here. Two broad groups have been identified in this study: (1) a set of deep focus earthquakes that are related to intracratonic paleorifts, and (2) shallow-focus earthquakes associated with discrete faults in the less deformed Precambrian terrains. A synthesis of earthquake data from different geologic environments will improve our understanding of the seismogenesis in the shield regions.

Rupture histories of two stable continental region earthquakes of India

Rupture histories of two stable continental region earthquakes of India

Stable continental regions are more vulnerable to earthquakes than once thought

Seismic events at shield areas throughout the world suggest that the stable continental regions (SCR) are much more vulnerable to earthquakes than was once thought. Earthquakes have struck SCRs at a number of locations, including the New Madrid Zone, United States; Tennant Creek, Australia; Ungava, Canada; and Kachchh, Koyna, Latur, and Jabalpur, India. In several developing countries, such as India, the problems caused by SCR earthquakes have become very serious because of high population density and the proliferation of structures not built to withstand earthquake damage.A recent Chapman Conference on SCR earthquakes attracted 90 researchers from 12 countries. Eighty papers were presented. Globally, the stable continental region earthquakes account for about 0.5% of total seismic energy released. The largest SCR earthquakes occurred in the New Madrid zone from 1811 to 1812, when three earthquakes of Mw 7.8‐8.1 occurred within just 53 days.