Stress dissipation and seismic potential in the central seismic gap of the north-west Himalaya
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Abstract In the Netherlands, over 190 gas fields of varying size have been exploited, and 15% of these have shown seismicity. The prime cause for seismicity due to gas depletion is stress changes caused by pressure depletion and by differential compaction. The observed onset of induced seismicity due to gas depletion in the Netherlands occurs after a considerable pressure drop in the gas fields. Geomechanical studies show that both the delay in the onset of induced seismicity and the nonlinear increase in seismic moment observed for the induced seismicity in the Groningen field can be explained by a model of pressure depletion, if the faults causing the induced seismicity are not critically stressed at the onset of depletion. Our model shows concave patterns of log moment with time for individual faults. This suggests that the growth of future seismicity could well be more limited than would be inferred from extrapolation of the observed trend between production or compaction and seismicity. The geomechanical models predict that seismic moment increase should slow down significantly immediately after a production decrease, independently of the decay rate of the compaction model. These findings are in agreement with the observed reduced seismicity rates in the central area of the Groningen field immediately after production decrease on 17 January 2014. The geomechanical model findings therefore support scope for mitigating induced seismicity by adjusting rates of production and associated pressure change. These simplified models cannot serve as comprehensive models for predicting induced seismicity in any particular field. To this end, a more detailed field-specific study, taking into account the full complexity of reservoir geometry, depletion history and mechanical properties, is required.
Seismic moment
Stress field
Natural gas field
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Seismic moment
Focal mechanism
Scaling law
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The spatial distribution of seismicity is often used as one of the indicators of zones where future large earthquakes are likely to occur. This is particularly true for intraplate regions such as the central and eastern United States, where geology is markedly enigmatic for delineating seismically active areas. Although using past seismicity for this purpose may be intuitively appealing, it is only scientifically justified if the tendency for past seismicity to delineate potential locations of future large earthquakes is well-established as a real, measurable, physical phenomenon as opposed to an untested conceptual model. This paper attempts to cast this problem in the form of scientifically testable hypotheses and to test those hypotheses. Ideally, thousands (or even millions) of years of data would be necessary to solve this problem. Lacking such a long-term record of seismicity, I make the "logical leap" of using data from other regions as a proxy for repeated samples of seismicity in intraplate regions. Three decades of global data from the National Earthquake Information Center are used to explore how the tendency for past seismicity to delineate locations of future large earthquakes varies for regions with different tectonic environments. This exploration helps to elucidate this phenomenon for intraplate environments. Applying the results of this exercise to the central and eastern United States, I estimate that future earthquakes in the central and eastern United States (including large and damaging earthquakes) have ∼86% probability of occurring within 36 km of past earthquakes, and ∼60% probability of occurring within 14 km of past earthquakes.
Earthquake prediction
Proxy (statistics)
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Epicenter
Asperity (geotechnical engineering)
Seismic moment
Seismogram
Moment magnitude scale
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Seismic hazard analyses are associated with large uncertainties when historical data are insufficient to define secular rates of seismicity. Such uncertainties may be decreased with geological data in areas where seismicity is shallow and produced by Quaternary faulting. To illustrate, we examine intraplate Japan. Large intraplate earthquakes in Japan characteristically produce surface ruptures along mappable Quaternary faults and show a systematic relation between seismic moment, M_0 and rupture length I (log M_0 = 23.5 + 1.94 × log I). It is observed that, within the bounds placed by geologically assessed slip rates, the mean regional moment release rate M_0 resulting from slip on mapped Quaternary faults is in accord with estimates of M_0 determined with the 400-yr record of seismicity. Recent work also shows that when the repeat time T of earthquakes on Quaternary faults in southwest Japan is assumed to equal M_0/M_0^g (where M_0 is estimated for rupture extended over the entire fault length and M_0^g is the geologically assessed moment release rate of each fault), the moment frequency distribution of earthquakes predicted from the geologic record is virtually identical to that seen with the 400-yr record of seismicity. These observations indicate that the geologic record of Quaternary fault offsets contains sufficient information to predict both the spatial and size distribution of intraplate earthquakes in Japan. A contour map of the average recurrence time of ground shaking of JMA intensity ≧V is thus computed using an empirical relation between seismic moment and the areal distribution of seismic intensity and assuming that the repeat time T of earthquakes on each Quaternary fault equals M_0/M_0^g. The map demonstrates how Quaternary fault data may be used to assess long-term seismic hazard in areas of active faulting where historical records of seismicity are relatively short or absent. Another shortcoming of conventional seismic hazard analysis is that hazard is not considered a function of the time since each fault in a region last ruptured. A simple procedure is used to demonstrate how the time-dependent nature of the earthquake cycle affects the evaluation of seismic hazard. The distribution of seismic shaking characteristic of large interplate earthquakes offshore of Japan is estimated from published isoseismal maps. The observed average repeat times of ruptures along specific segments of the plate boundaries then provide the basis to make probabilistic estimates of the next expected time of seismic shaking due to plate boundary earthquakes. When data are too few to document the average repeat times of rupture, the estimates of probability are calculated with data relating to the relative coseismic slip during past earthquakes and the rate of interseismic strain accumulation, interpreted within the framework of the time predictable model of earthquake occurrence. Results are displayed as maps of instantaneous seismic hazard: the probability that seismic shaking will occur conditional to knowledge of where in time each fault in a region presently resides with respect to the earthquake cycle.
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Moment magnitude scale
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Abstract The evolution of fluid injection‐induced seismicity, generally characterized through the number of events or their seismic moment, depends on, among other factors, the injected fluid volume. Migration of seismicity is observed during those sequences and might be caused by a range of mechanisms: fluid pressure diffusion, fluid‐induced aseismic slip propagating along a stimulated fault, interactions between earthquakes. Recent theoretical and observational developments underline the important effect on seismicity migration of structural parameters, like fault criticality, or injection parameters, like flow rate or pressurization rate. Here, we analyze two well‐studied injection‐induced seismic sequences at the Soultz‐sous‐Fôret and Basel geothermal sites, and find that the evolution of the seismicity front distance primarily depends on the injected fluid volume. Based on a fracture mechanics model, we develop new equations relating seismicity migration to injected fluid volume and frictional and structural properties of the fault. We find that the propagation of a fluid‐induced aseismic slip front along the stimulated fault, triggering seismicity, explains well the observations made on the two sequences. This model allows us to constrain parameters describing the seismicity front evolution and explains the diversity of migration patterns observed in injection‐induced and natural earthquake swarms.
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Despite that earthquakes in stable continental regions (SCR) often cause more damage than interplate seismicity, they remain poorly understood. This is mainly because of the lower rate of intraplate seismicity and because of its different behaviour compared to the better-known seismicity at the plate boundary. Understand the characteristics of the intraplate seismicity is a challenge for the seismic risk studies. We study and characterise an SCR (NW Iberian Peninsula), which not only registers moderate instrumental intraplate seismicity, but also important historic seismicity and paleoseismic activity. To tackle some of the difficulties posed by intraplate seismicity, we analyse a wide and multidisciplinary data set (e.g., geological structures, seismicity, focal mechanisms, and geophysical data). Seismicity in this region is not associated with an old rift, but with inherited faults widely distributed throughout the region with a great variety of orientations. The reactivation kinematics of these faults are coherent with the current regional stresses. Instrumental seismicity is not associated with the large active faults nor with crustal limits. Seismicity is mainly clustered in swarms and sequences. Although seismic swarms present lower magnitudes, they are the most common. Based on swarms' characteristics (high b-values, upward spatiotemporal migration), reported mantellic CO2 in some thermal springs, and the reactivation of inherited steeply-dipping faults, we propose the migration of deep fluids through steeply-dipping fractured areas as the cause of the intraplate seismicity. These processes could increase the pore pressure and decrease the stresses necessary for the fault rupture in a fault-valve behaviour. In general, in intraplate context, the important control in the seismicity of the inherited fault systems favourable oriented under the current stress tensor is observed, and also the need for mechanisms that can decrease the effective stress for the fault ruptures. Mechanisms as hydrothermal fluids in arterial faults with fault-valve processes has been identified as an effective driver of intraplate seismicity, playing an important role in stability of tectonic faults. The large number and variety of these faults, that share the low strain rates in intraplate polyorogenic context, may explain the different characteristics of these intraplate regions compared with the interplate regions, as the "unanticipated" behaviour, variety of kinematics, the long quiescence periods without seismicity associated and erosion obliterating their morphotectonic expression.
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Seismic moment
Scaling law
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