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Stefan Wiemer - One of the best experts on this subject based on the ideXlab platform.
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A Stochastic Forecast of California Earthquakes Based on Fault Slip and Smoothed Seismicity
Bulletin of the Seismological Society of America, 2013Co-Authors: Stefan Hiemer, David D. Jackson, Qi Wang, Yan Y. Kagan, Jochen Woessner, J. D. Zechar, Stefan WiemerAbstract:We present a testable stochastic earthquake source model for intermedi- ate- to long-term forecasts. The model is based on fundamental observations: the Frequency-Magnitude Distribution, slip rates on major faults, long-term strain rates, and source parameter values of instrumentally recorded and historic earthquakes. The basic building blocks of the model are two pairs of probability density maps. The first pair consists of smoothed seismicity and weighted focal mechanisms based on observed earthquakes. The second pair corresponds to mapped faults and their slip rates and consists of smoothed moment-rate and weighted focal mechanisms based on fault geometry. We construct from the model a "stochastic event set," that is to say, a large set of simulated earthquakes that are relevant for seismic hazard calculations and earthquake forecast development. Their complete descriptions are determined in the following order: magnitude, epicenter, moment tensor, length, displacement, and down-dip width. Our approach assures by construction that the simulated magnitudes are consistent with the observed Frequency-Magnitude Distribution. We employ a magnitude-dependent weighting procedure that tends to place the largest simulated earthquakes near major faults with consistent focal mechanisms. Nevertheless, our stochastic model allows for surprises such as large off-fault earthquakes, events that comply with the observation that several recent destructive earthquakes occurred on previously unknown fault structures. We apply our model to California to illustrate its features.
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influence of pore pressure on the event size Distribution of induced earthquakes
Geophysical Research Letters, 2012Co-Authors: C E Bachmann, Stefan Wiemer, Bettina P Goertzallmann, Jochen WoessnerAbstract:[1] During an Enhanced Geothermal System (EGS) experiment, fluid is injected at high pressure into crystalline rock, to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted. The fracturing of the basement caused by these high pore-pressures is associated with microseismicity. However, the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates, and thus pore-pressure, is unknown. Here we show how pore-pressure can be linked to the seismic frequency–magnitude Distribution, described by its slope, theb-value. We evaluate the dataset of an EGS in Basel, Switzerland and compare the observed event-size Distribution with the outcome of a minimalistic model of pore-pressure evolution that relates event-sizes to the differential stressσD. We observe that the decrease of b-values with increasing distance of the injection point is likely caused by a decrease in pore-pressure. This leads to an increase of the probability of a large magnitude event with distance and time.
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Mapping spatial variability of the Frequency-Magnitude Distribution of earthquakes
Advances in Geophysics Volume 45, 2002Co-Authors: Stefan Wiemer, Max WyssAbstract:Publisher Summary This chapter summarizes the new facts surrounding the spatial variability of the b -value—the slope of the Frequency-Magnitude Distribution (FMD) describing the relative size Distribution of events—to discuss the new hypotheses that grew from the observations, to report how far the tests of these hypotheses have advanced, and to describe the methods used. In addition, the concepts such as temporal variations, fractal dimension, and the correct estimate of b and its uncertainty are discussed. Spatially mapping b -values has proven to be a rich source of information about the seismotectonics of a region. The ample, high-quality earthquake catalogs collected primarily over the past several years and the availability of increased computing power have enabled researchers to investigate spatial variations in b with an unprecedented level of detail. The discovery of strong differences in b is simply a reflection of the heterogeneity of the Earth that emerges on all scales, once suitable datasets become available. The first step in extracting information from the heterogeneity in the b -value is to determine with what other parameters these variations correlate.
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A systematic test of the hypothesis that the b value varies with depth in California
Geophysical Research Letters, 2001Co-Authors: Matt Gerstenberger, Stefan Wiemer, Domenico GiardiniAbstract:We spatially map the shallow to deep b value ratio in the crust in California. Previous studies of the frequency magnitude Distribution, as a function of depth, for selected crustal regions indicated that b decreases from b > 1.1 in the 0–5 km depth range to b < 0.8 in the depth range 7–15 km. Our detailed mapping confirms that this pattern can be established at the 99% significance level for about 32% of the entire seismically active crust. About 2% of the crust displays the opposite b-gradient. One such area is the San Francisco Bay Area. We speculate that differences in stress levels are the main factor controlling the depth dependency of b. These results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations.
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Spatial variability of seismicity parameters in aftershock zones
Journal of Geophysical Research: Solid Earth, 1999Co-Authors: Stefan Wiemer, Kei KatsumataAbstract:The spatial variability of the b value of the Frequency-Magnitude relationship and the decay rate of aftershocks as described by the p value of the modified Omori law is investigated. By using dense spatial grids we map out the Distribution of b and p values within the Landers, Northridge, Morgan Hill, and Kobe aftershock sequences. Considerable spatial variability is found, with b values of independent subvolumes ranging from 0.6 to 1.4, and p values ranging from 0.6 to 1.8. These systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km. The spatial Distribution of these two parameters is compared with the slip Distribution during the mainshock, suggesting that the areas of largest slip release correlate with high b value regions. We hypothesize that the frictional heat created during the event may influence the p value Distribution within an aftershock zone, while applied shear stress, crack density and pore pressure govern the Frequency-Magnitude Distribution. By investigating the Frequency-Magnitude Distribution separately for preseismic and postseismic periods for the Morgan Hill mainshock, we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value, which decays to premainshock values within a year. Surrounding areas of the aftershock zone show an approximately constant b value with time. Because the aftershock hazard after a mainshock depends strongly on both the b and p value, we propose that aftershock hazard assessment can be improved by taking into account the spatial Distribution of the parameters.
Max Wyss - One of the best experts on this subject based on the ideXlab platform.
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Spatial Variations in the Frequency-Magnitude Distribution of Earthquakes at Mount Pinatubo Volcano
Bulletin of the Seismological Society of America, 2004Co-Authors: John J. Sánchez, John A. Power, Stephen R Mcnutt, Max WyssAbstract:The Frequency-Magnitude Distribution of earthquakes measured by the b -value is mapped in two and three dimensions at Mount Pinatubo, Philippines, to a depth of 14 km below the summit. We analyzed 1406 well-located earthquakes with magnitudes M D ≥0.73, recorded from late June through August 1991, using the maximum likelihood method. We found that b -values are higher than normal ( b = 1.0) and range between b = 1.0 and b = 1.8. The computed b -values are lower in the areas adjacent to and west-southwest of the vent, whereas two prominent regions of anomalously high b -values ( b ∼ 1.7) are resolved, one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth. The statistical differences between selected regions of low and high b -values are established at the 99% confidence level. The high b -value anomalies are spatially well correlated with low-velocity anomalies derived from earlier P -wave travel-time tomography studies. Our dataset was not suitable for analyzing changes in b -values as a function of time. We infer that the high b -value anomalies around Mount Pinatubo are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma bodies. Manuscript received 16 December 2002.
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Mapping spatial variability of the Frequency-Magnitude Distribution of earthquakes
Advances in Geophysics Volume 45, 2002Co-Authors: Stefan Wiemer, Max WyssAbstract:Publisher Summary This chapter summarizes the new facts surrounding the spatial variability of the b -value—the slope of the Frequency-Magnitude Distribution (FMD) describing the relative size Distribution of events—to discuss the new hypotheses that grew from the observations, to report how far the tests of these hypotheses have advanced, and to describe the methods used. In addition, the concepts such as temporal variations, fractal dimension, and the correct estimate of b and its uncertainty are discussed. Spatially mapping b -values has proven to be a rich source of information about the seismotectonics of a region. The ample, high-quality earthquake catalogs collected primarily over the past several years and the availability of increased computing power have enabled researchers to investigate spatial variations in b with an unprecedented level of detail. The discovery of strong differences in b is simply a reflection of the heterogeneity of the Earth that emerges on all scales, once suitable datasets become available. The first step in extracting information from the heterogeneity in the b -value is to determine with what other parameters these variations correlate.
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Spatial variations in the frequency‐magnitude Distribution of earthquakes at Soufriere Hills Volcano, Montserrat, West Indies
Geophysical Research Letters, 1998Co-Authors: John A. Power, Max Wyss, Joan L. LatchmanAbstract:The Frequency-Magnitude Distribution of earthquakes measured by the b-value is determined as a function of space beneath Soufriere Hills Volcano, Montserrat, from data recorded between August 1, 1995 and March 31, 1996. A volume of anomalously high b-values (b > 3.0) with a 1.5 km radius is imaged at depths of 0 and 1.5 km beneath English's Crater and Chance's Peak. This high b-value anomaly extends southwest to Gage's Soufriere. At depths greater than 2.5 km volumes of comparatively low b-values (b∼1) are found beneath St. George's Hill, Windy Hill, and below 2.5 km depth and to the south of English's Crater. We speculate the depth of high b-value anomalies under volcanoes may be a function of silica content, modified by some additional factors, with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths.
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temporal and three dimensional spatial analyses of the frequency magnitude Distribution near long valley caldera california
Geophysical Journal International, 1998Co-Authors: Stefan Wiemer, Stephen R Mcnutt, Max WyssAbstract:SUMMARY The 3-D Distribution of the b value of the frequency‐magnitude Distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N=150 earthquakes and centred at nodes of a grid separated by 0.3 km. Significant variations in the b value are detected, with b ranging from b#0.6 to b#2.0. High b-value volumes are located near the resurgent dome, and at depths below 5 km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial Distribution of temporal variations of the frequency‐magnitude Distribution by introducing diVerential b-value maps. b values increased from b#0.8 to b#1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average Distribution of cracks at 5‐10 km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not suYcient) condition near a magmatic body, and therefore spatial b-value mapping can be used to aid in the identification of active magma bodies.
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Spatial variations in the Frequency-Magnitude Distribution of earthquakes at Soufriere Hills Volcano, Montserrat, West Indies
1998Co-Authors: West Indies, Max Wyss, John A. Power, Joan L. LatchmanAbstract:The Frequency-Magnitude Distribution of earthquakes measured by the b-value is determined as a function of space beneath Soufriere Hills Volcano, Montserrat, from data recorded between August 1, 1995 and March 31, 1996. A volume of anomalously high b-values (b > 3.0) with a 1.5 km radius is imaged at depths of 0 and 1.5 km beneath English's Crater and Chance's Peak. This high b-value anomaly extends southwest to Gage's Soufriere. At depths greater than 2.5 km volumes of comparatively low b-values (b∼1) are found beneath St. George's Hill, Windy Hill, and below 2.5 km depth and to the south of English's Crater. We speculate the depth of high b-value anomalies under volcanoes may be a function of silica content, modified by some additional factors, with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths.
Stephen R Mcnutt - One of the best experts on this subject based on the ideXlab platform.
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Spatial Variations in the Frequency-Magnitude Distribution of Earthquakes at Mount Pinatubo Volcano
Bulletin of the Seismological Society of America, 2004Co-Authors: John J. Sánchez, John A. Power, Stephen R Mcnutt, Max WyssAbstract:The Frequency-Magnitude Distribution of earthquakes measured by the b -value is mapped in two and three dimensions at Mount Pinatubo, Philippines, to a depth of 14 km below the summit. We analyzed 1406 well-located earthquakes with magnitudes M D ≥0.73, recorded from late June through August 1991, using the maximum likelihood method. We found that b -values are higher than normal ( b = 1.0) and range between b = 1.0 and b = 1.8. The computed b -values are lower in the areas adjacent to and west-southwest of the vent, whereas two prominent regions of anomalously high b -values ( b ∼ 1.7) are resolved, one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth. The statistical differences between selected regions of low and high b -values are established at the 99% confidence level. The high b -value anomalies are spatially well correlated with low-velocity anomalies derived from earlier P -wave travel-time tomography studies. Our dataset was not suitable for analyzing changes in b -values as a function of time. We infer that the high b -value anomalies around Mount Pinatubo are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma bodies. Manuscript received 16 December 2002.
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temporal and three dimensional spatial analyses of the frequency magnitude Distribution near long valley caldera california
Geophysical Journal International, 1998Co-Authors: Stefan Wiemer, Stephen R Mcnutt, Max WyssAbstract:SUMMARY The 3-D Distribution of the b value of the frequency‐magnitude Distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N=150 earthquakes and centred at nodes of a grid separated by 0.3 km. Significant variations in the b value are detected, with b ranging from b#0.6 to b#2.0. High b-value volumes are located near the resurgent dome, and at depths below 5 km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial Distribution of temporal variations of the frequency‐magnitude Distribution by introducing diVerential b-value maps. b values increased from b#0.8 to b#1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average Distribution of cracks at 5‐10 km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not suYcient) condition near a magmatic body, and therefore spatial b-value mapping can be used to aid in the identification of active magma bodies.
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Temporal and Three-Dimensional Spatial of the Frequency-Magnitude (FMD) Distribution Near Long Valley Caldera, California
Geophysical Journal International, 1998Co-Authors: Stefan Wiemer, Stephen R Mcnutt, Max WyssAbstract:SUMMARY The 3-D Distribution of the b value of the frequency‐magnitude Distribution is analysed in the seismically active parts of the crust near Long Valley Caldera, California. The seismicity is sampled in spherical volumes, containing N=150 earthquakes and centred at nodes of a grid separated by 0.3 km. Significant variations in the b value are detected, with b ranging from b#0.6 to b#2.0. High b-value volumes are located near the resurgent dome, and at depths below 5 km at Mammoth Mountain. b values are found to be much lower south of the Long Valley Caldera. We interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath Mammoth Mountain in 1989, and that anomalous crust, either highly fractured or containing unusually high pore pressure, such as is the case in the vicinity of active magma bodies, exists north of the seismically active area beneath the resurgent dome at all depths. We also investigate the spatial Distribution of temporal variations of the frequency‐magnitude Distribution by introducing diVerential b-value maps. b values increased from b#0.8 to b#1.5 underneath Mammoth Mountain at the onset of the 1989 earthquake swarm and remained high thereafter. This suggests that an intrusion permanently altered the average Distribution of cracks at 5‐10 km depth, or that the pore pressure permanently increased. We propose that high b values are a necessary (but not suYcient) condition near a magmatic body, and therefore spatial b-value mapping can be used to aid in the identification of active magma bodies.
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Variations in the Frequency-Magnitude Distribution with Depth in Two Volcanic Areas: Mount St. Helens, Washington, and Mt. Spurr, Alaska
Geophysical Research Letters, 1997Co-Authors: Stefan Wiemer, Stephen R McnuttAbstract:The Frequency-Magnitude Distribution of earthquakes, characterized using the b-value, is examined as a function of space beneath Mount St. Helens (1988–1996), and Mt. Spurr (1991–1995). At Mount St. Helens, two volumes of anomalously high b (b > 1.3) can be observed at depths of 2.6–3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b-value (b ≥ 1.3) at a depth of about 2.3–4.5 km below the crater floor of the active vent Crater Peak. We propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b-value at shallow depths. Alternatively, interaction of magma with groundwater may have increased pore pressure and lowered the effective stress. The deeper anomaly at Mount St. Helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber. Our results indicate that detailed mapping of the Frequency-Magnitude Distribution can be used as a tool to trace vesiculation and locate active magma chambers.
B. Bodri - One of the best experts on this subject based on the ideXlab platform.
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A FRACTAL MODEL FOR SEISMICITY AT IZU-TOKAI REGION, CENTRAL JAPAN
Fractals, 1993Co-Authors: B. BodriAbstract:Fractal approach has been applied to investigate regional seismicity at the Izu peninsula—Tokai area, Central Japan. The Frequency-Magnitude Distribution of earthquakes, Distribution of epicenters, origin times of earthquakes, the fracture fault system in the region have been considered, and the fractal dimensions corresponding to them were calculated. A good correspondence in the fractal dimension values was found. The Frequency-Magnitude Distribution in the area shows a fractal dimension of 1.28, whilst D=1.15±0.18 is representative of the geometry of the Distribution of earthquake epicenters. The fractal dimension of faults for the Izu peninsula is found to be 1.16±0.04, and in the whole Izu-Tokai region, values 1.1
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A FRACTAL MODEL FOR SEISMICITY AT IZU-TOKAI REGION, CENTRAL JAPAN
Fractals, 1993Co-Authors: B. BodriAbstract:Fractal approach has been applied to investigate regional seismicity at the Izu peninsula—Tokai area, Central Japan. The Frequency-Magnitude Distribution of earthquakes, Distribution of epicenters, origin times of earthquakes, the fracture fault system in the region have been considered, and the fractal dimensions corresponding to them were calculated. A good correspondence in the fractal dimension values was found. The Frequency-Magnitude Distribution in the area shows a fractal dimension of 1.28, whilst D=1.15±0.18 is representative of the geometry of the Distribution of earthquake epicenters. The fractal dimension of faults for the Izu peninsula is found to be 1.16±0.04, and in the whole Izu-Tokai region, values 1.1<D<1.3 are characteristic. The temporal Distribution of earthquakes yields a fractal dimension of 0.51±0.03, which indicates a relatively weak clustering of events in time. Independent autocorrelation analysis also shows that the earthquakes in the area of study occur to a large extent statistically independent. The general conclusion is that crustal deformation in the Izu-Tokai region occurs on a scale-invariant matrix faults. The behavior of the system is controlled by a single parameter, the fractal of dimension.
Kiyoshi Ito - One of the best experts on this subject based on the ideXlab platform.
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The 1998 Hida Mountain, Central Honshu, Japan, earthquake swarm: Double-difference event relocation, frequency–magnitude Distribution and Coulomb stress changes
Tectonophysics, 2005Co-Authors: Bogdan Enescu, Kiyoshi ItoAbstract:Abstract By using the double-difference relocation technique, we have determined the fine structure of seismicity during the 1998 Hida Mountain earthquake swarm. The Distribution of seismic activity defines two main directions (N–S and E–W) that probably correspond to the regional stress pattern. The detailed structure of seismicity reveals intense spatio-temporal clustering and earthquake lineations. Each cluster of events contains a mainshock and subsequent aftershock activity that decays according to the Omori law. The seismicity and the b -value temporal and spatial patterns reflect the evolution of the static stress changes during the earthquake swarm. About 80% of the swarm's best-relocated events occur in regions of increased ΔCFF. The smaller value of b found in the northern part of the swarm region and a larger b -value observed to the south, for the same period of time, could be well explained by the static stress changes caused by the larger events of the sequence. We argue that the state of stress in the crust is the main factor that controls the variation of b -value.
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Spatial analysis of the Frequency-Magnitude Distribution and decay rate of aftershock activity of the 2000 Western Tottori earthquake
Earth Planets and Space, 2002Co-Authors: Bogdan Enescu, Kiyoshi ItoAbstract:The b-value of the Frequency-Magnitude Distribution and the parameters in the modified Omori law, describing the decay rate of aftershock activity, are investigated for more than 4000 aftershocks identified in the first four months after the Western Tottori earthquake (October 6, 2000). We used the JMA data catalog, containing aftershocks with magnitude larger than or equal to 2.0. The studied area is first divided into three areas: one region (A) corresponding to the main aftershock area and other two (B and C) corresponding to seismic activity probably triggered by the stress change caused by the main shock. For region A, the magnitude of completeness (Mc) decreases with time, from the largest value of 3.2 in the first two hours of the sequence, to 2.0, about four days after the main shock. Taking the threshold magnitude as 3.2, we estimated the b-value for the whole region A to be about 1.3 and p-value around 1. However, highly significant variations in both b and p values are found when analyzing their spatial Distribution in region A. The seismic activity in the regions B and C started about 2.5 days after the main shock. The b-value for region B (Mc = 2) is 1.05. The decay rate of earthquake activity in Region B is well modeled by the modified Omori law and the p-value is found to be relatively low (0.83). The number of events in region C is too small for a meaningful study. The physical interpretation of the spatial variation of the parameters is not straight forward. However, the variation of b-value can be related to the stress Distribution after the main shock, as well as the history of previous ruptures. Thus, the relatively low stress in the regions that have already experienced rupture is probably responsible for the larger value of b found in these areas. Regions with relatively low b-value, on the other hand, are probably regions under higher applied shear stress after the main shock. Alternatively, one can hypothesize that the areas that experienced slip are more fractured, favoring higher b-values. The larger p-values correlate well with the regions that experienced larger slip during the main shock, while small p-values are found generally in regions that have not ruptured recently. The variation of p-value can be related with the frictional heating produced during rupture. The crustal structure may explain some local features of b and p value spatial Distribution. In order to verify our hypothesis we also analyzed the seismic activity that occurred before the Tottori earthquake, starting in 1978, using the data of DPRI, Kyoto University. It seems that the previous seismic activity associated with some moderate events in 1989, 1990 and 1997 had an influence on the following seismicity in the area—in particular on the spatial Distribution of b and p values observed for the aftershocks of the Tottori earthquake. The aftershocks of the 1997 M5.5 earthquake have a larger p-value than previous aftershock sequences, while the b-value has a clear increase following the M5.5 event.
