The Reliance of the Earthquake B-Value on Depth and Focal Mechanism (original) (raw)
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Abstract We map the earthquake frequency – magnitude distribution ( b value ) as a function o depth in the intraplate regions of the earth. About 590 well – located events in the different intraplate regions are selected for this analysis. The earthquake data sets are separated in 10 km zones from 0 to 40 km depth. The b-value of the intraplate regions ranges from 0.72 to 1.20. There is a clear tendency for decrease in b- value in Africa, Asia, North America and Globally, and increase in b-value in Australia and Europe . The low b-values and low seismicity at depth range of 20-30 km may be associated with low degree of heterogeneity and high rheological strength in the Crust. The result shows that the b-value increases below the depth range of 20-30 km in Africa, Asia, North America and Globally and decreases in Australia and Europe. We found that there is high seismicity below the depth range of 20-30 km. Such high seismicity with high b-values may be associated with high heterogen...
Variations in earthquake-size distribution across different stress regimes
Nature, 2005
The earthquake size distribution follows, in most instances, a power law 1,2 , with the slope of this power law, the 'b value', commonly used to describe the relative occurrence of large and small events (a high b value indicates a larger proportion of small earthquakes, and vice versa). Statistically significant variations of b values have been measured in laboratory experiments, mines and various tectonic regimes such as subducting slabs, near magma chambers, along fault zones and in aftershock zones 3 . However, it has remained uncertain whether these differences are due to differing stress regimes, as it was questionable that samples in small volumes (such as in laboratory specimens, mines and the shallow Earth's crust) are representative of earthquakes in general. Given the lack of physical understanding of these differences, the observation that b values approach the constant 1 if large volumes are sampled 4 was interpreted to indicate that b 5 1 is a universal constant for earthquakes in general 5 . Here we show that the b value varies systematically for different styles of faulting. We find that normal faulting events have the highest b values, thrust events the lowest and strike-slip events intermediate values. Given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress.
Tectonophysics, 2014
Interpretation of the b-value of earthquake frequency-magnitude distributions has received considerable attention in recent decades. This paper provides a comprehensive review of previous investigations of spatial and temporal variations in b-value, including their classification and possible causes. Based on least-squares regression of seismicity data compiled from the NEIC, IRIS and ISC catalogs, we find an average value of 1.02 ± 0.03 for the whole Earth and its two hemispheres, consistent with the general view that in seismically active regions the long-term average value is close to unity. Nevertheless, wide-ranging b-variations (0.3 ≤ b ≤ 2.5) have been reported in the literature. This variability has been interpreted to arise from one or more of the following factors: prevailing stress state, crustal heterogeneity, focal depth, pore pressure, geothermal gradient, tectonic setting, petrological/environmental/geophysical characteristics, clustering of events, incomplete catalog data, and/or method of calculation. Excluding the latter, all of these factors appear to be linked, directly or indirectly, with the effective state of stress. Although time-dependent changes in b-value are well documented, conflicting observations reveal either a precursory increase or decrease in b value before major earthquakes. Our compilation of published analyses suggests that statistically significant b-variations occur globally on various timescales, including annual, monthly and perhaps diurnal. Taken together, our review suggests that b-variations are most plausibly linked with changes in effective stress.
Evolution of b-values before large earthquakes of mb ≥6.0 in Andaman region
2015
We have considered six earthquakes of mb ≥6.0, with focal depths within 45 km, in Andaman region during 2000 to 2012 to examine the spatial variations of b-value at epicentre within a one-year period prior to the occurrence of the earthquakes. We have found a correlation between the low b for the one year time interval and the occurrence of large events. The epicentral b-values of six earthquakes are lower than 1.0 and five out of six earthquakes show very low b-value at the epicentre. Our study may indicate that b-value can be employed as a possible precursor for the forecasting of a major earthquake.
Values of b and p: their Variations and Relation to Physical Processes for Earthquakes in Japan
This work reviews some results obtained already for the variations of the seismicity parameters b and p in different seismogenic and tectonic regions in Japan. We bring as well new evidence that the time and space changes in seismicity parameters are correlating well with the crustal structure and/or some parameters of the earthquake process. In the first part of the paper we show that several seismicity precursors (clear b-value changes, quiescence and clustering) occurred about two years before the 1995 Kobe earthquake and they correlate well with other geophysical premonitory phenomena of the major event. The precursory phenomena occurred in a relatively large area, which corresponds probably with the preparation zone of the future event. In the second part, we analyze the b and p value spatial and temporal distribution for the aftershocks of the 2000 Tottori earthquake. The results indicate significant correlations between the spatio-temporal pattern of b and p and the stress distribution after the main shock, as well as the crustal structure. The swarm-like seismic sequences occurred in 1989, 1990 and 1997 showed significant precursory b and p values. In the third part of the paper we analyze the seismicity during the 1998 Hida Mountain earthquake swarm. The double-differencerelocated events are analyzed for their frequency-magnitude distribution and stress changes. While again the b-value is significantly different in south comparing with the north part of the epicentral area, the physical interpretation is difficult and complex. The changes in the Coulomb failure stress (∆CFF) can explain the b-value distribution features, but the crustal structure may be also important. The seismicity distribution and migration, in relation with ∆CFF is also discussed. We refer as well to other world-wide studies.
Continuous-cyclic variations in the b-value of the earthquake frequency-magnitude distribution
Earthquake Science, 2013
Seismicity of the Earth (M C 4.5) was compiled from NEIC, IRIS and ISC catalogues and used to compute b-value based on various time windows. It is found that continuous cyclic b-variations occur on both long and short time scales, the latter being of much higher value and sometimes in excess of 0.7 of the absolute b-value. These variations occur not only yearly or monthly, but also daily. Before the occurrence of large earthquakes, b-values start increasing with variable gradients that are affected by foreshocks. In some cases, the gradient is reduced to zero or to a negative value a few days before the earthquake occurrence. In general, calculated b-values attain maxima 1 day before large earthquakes and minima soon after their occurrence. Both linear regression and maximum likelihood methods give correlatable, but variable results. It is found that an expanding time window technique from a fixed starting point is more effective in the study of b-variations. The calculated b-variations for the whole Earth, its hemispheres, quadrants and the epicentral regions of some large earthquakes are of both local and regional character, which may indicate that in such cases, the geodynamic processes acting within a certain region have a much regional effect within the Earth. The b-variations have long been known to vary with a number of local and regional factors including tectonic stresses. The results reported here indicate that geotectonic stress remains the most significant factor that controls b-variations. It is found that for earthquakes with M w C 7, an increase of about 0.20 in the b-value implies a stress increase that will result in an earthquake with a magnitude one unit higher.
Geophysical Research Letters, 2013
1] We explore the idea that the relative size distribution of earthquakes, quantified using the so-called b-value, is negatively correlated with differential stress. Because the maximum possible differential stress increases linearly in the brittle upper crust, we expect to find a decrease of b with depth. We test this expectation for seven continental areas around the world, each of which is described by a regional earthquake catalog. We find a monotonic decrease in b-value between 5 and 15 km depth. The decrease stops near the brittle-ductile transition. We specifically focus on the high-quality catalogs of earthquakes in California to perform a sensitivity test with respect to depth uncertainty; we also estimate the probability-depth gradient for the occurrence of a target magnitude event and study the behavior of b with depth in near-and off-fault zones. We also translate the observed b-depth gradients into b-differential stress gradients. Our findings suggest that b-values are negatively correlated with differential stress and hus have the potential to act as stress meters in the Earth's crust. Citation: Spada, M., T. Tormann, S. Wiemer, and B.
Frequency-Magnitude Distribution of Earthquakes
Earthquakes - Forecast, Prognosis and Earthquake Resistant Construction
Frequency magnitude distribution of all types of earthquakes has received considerable attention in the last few decades. Their linear logarithmic relationship remains the most accepted. The a and b constants of this equation, their values and variations have been studied in detail. It is largely agreed that for the seismicity of the whole Earth, its hemispheres, quadrants and large epicentral regions are a=10.0 and b=1.0. The b-variations have long been investigated and reported to occur in different forms and values. Long-and short-term b-variations occur in a continuous cyclic manner and may exceed ± (0.6-0.7) of its absolute value. These are observed to occur not only yearly or monthly but also daily. The b-value always attains maxima and minima before and after the occurrence of all large earthquakes. Many days before the occurrence of large earthquakes, b-values start increasing at variable gradients that are affected by foreshocks. It attains a maximum value shortly before each large earthquake and a minimum on its occurrence. Many factors have been proposed to explain the b-variations including prevailing stress, crustal heterogeneity, focal depth, pore pressure, geothermal gradient, tectonic setting, and other factors. Considering the b-variations of the whole Earth, its hemispheres and quadrants and considering that most proposed factors are directly or indirectly related to the stress, it is concluded that this remains the major factor. It is found that for large earthquakes with Mw ≥ 7, an increase of about 0.20 in the b-value implies a stress increase that will result in an earthquake with a magnitude one unit higher.