Magnitude distribution complexity and variation at The Geysers geothermal field

2020 ◽  
Vol 222 (2) ◽  
pp. 893-906
Author(s):  
Konstantinos Leptokaropoulos
Keyword(s):  
Seismic Hazard ◽  
Size Distribution ◽  
Hazard Assessment ◽  
Seismic Hazard Assessment ◽  
B Value ◽  
Geothermal Field ◽  
Earthquake Magnitude ◽  
Fluid Injection ◽  
Magnitude Distribution ◽  
The Geysers

SUMMARY Earthquake magnitude (size) distribution is a major component required for seismic hazard assessment and therefore, the accurate determination of its functional shape and variation is a task of utmost importance. Although often considered as stationary, the magnitude distribution at particular sites may significantly vary over time and space. In this study, the well-known Gutenberg–Richter (GR) law, which is widely assumed to describe earthquake magnitude distribution, is tested for a case study of seismicity induced by fluid injection at The Geysers (CA, USA) geothermal field. Statistical tests are developed and applied in order to characterize the magnitude distribution of a high quality catalogue comprising seismicity directly associated with two injection wells, at the north western part of The Geysers. The events size distribution variation is investigated with respect to spatial, temporal, fluid injection and magnitude cut-off criteria. A thorough spatio-temporal analysis is performed for defining seismicity Clusters demonstrating characteristic magnitude distributions which significantly differ from the ones of the nearby Clusters. The magnitude distributions of the entire seismic population as well as of the individual Clusters are tested for their complexity in terms of exponentiality, multimodal and multibump structure. Then, the Clusters identified are further processed and their characteristics are determined in connection to injection rate fluctuations. The results of the analysis clearly indicate that the entire magnitude distribution is definitely complex and non-exponential, whereas subsequent periods demonstrating significantly diverse magnitude distributions are identified. The regional seismicity population is divided into three major families, for one of which exponentiality of magnitude distribution is clearly rejected, whereas for the other two the GR law b-value is directly proportional to fluid injection. In addition, the b-values of these Families seem to be significantly magnitude dependent, a fact that is of major importance for seismic hazard assessment implementations. To conclude, it is strongly suggested that magnitude exponentiality must be tested before proceeding to any b-value calculations, particularly in anthropogenic seismicity cases where complex and time changeable processes take place.

Seismicity associated with 2018 and 2020 hydraulic stimulations at EGS in Helsinki, Finland, shows limited earthquake interaction: Implication for seismic hazard assessment

2021 ◽  
Author(s):  
Grzegorz Kwiatek ◽  
Maria Leonhardt ◽  
Patricia Martínez-Garzón ◽  
Matti Pentti ◽  
Marco Bohnhoff ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Hazard Assessment ◽  
Energy Input ◽  
Stress Transfer ◽  
Seismic Hazard Assessment ◽  
B Value ◽  
Temporal Distance ◽  
Probabilistic Seismic Hazard Assessment ◽  
Interevent Time ◽  
Spatio Temporal

<p>In this study we investigate the statistical spatio-temporal characteristics induced seismicity associated with two stimulation campaigns performed in 2018 and 2020 in a 6.1 km deep geothermal well near Helsinki, Finland as part of the St1 Deep Heat project. We aim to find out whether the seismic activity is passively responding to injection operations, or whether we observe signatures of significant stress transfer and strong interactions between events. The former suggests stable relaxation of seismic energy proportional to hydraulic energy input, while the latter includes stress transfer as an additional source of stress perturbation, hence implying larger seismic hazard.</p><p>The selected catalogs from 2018 and 2020 stimulation contained in total 60,814 and 4,368 seismic events, respectively, recorded during and after stimulation campaigns and above the local magnitude of M -1.5. The analyzed parameters include magnitude-frequency b-value, correlation integral (c-value), fractal dimension (D-value), interevent time statistics, magnitude correlation, interevent time ratio and generalized spatio-temporal distance between earthquakes. The initial observations suggest significant time-invariance of the magnitude-frequency b-value, and increased D and c-values only at high injection rates, the latter also guiding the rate of seismicity. The seismicity covering the stimulation period neither provide signatures of magnitude correlations, nor temporal clustering or anticlustering. The interevent time statistics are generally characterized with Gamma distribution (close to Poissonian distribution), and the generalized spatio-temporal distance suggest very limited triggering (90% of the catalog was classified as background seismicity). The observable parameters suggest the seismicity passively respond to hydraulic energy input rate with little to no time delay, and the total seismic moment is proportional to total hydraulic energy input. The performed study provides the base for implementation of time-dependent probabilistic seismic hazard assessment for the site.</p>

scholarly journals Revisiting Seismic Hazard Assessment For Peninsular Malaysia Using Deterministic And Probabilistic Approaches

2018 ◽  
Author(s):  
Daniel Weijie Loi ◽  
Mavinakere Eshwaraiah Raghunandan ◽  
Varghese Swamy
Keyword(s):  
Seismic Hazard ◽  
Hazard Assessment ◽  
Seismic Hazard Assessment ◽  
B Value ◽  
Peninsular Malaysia ◽  
Point Sources ◽  
Potential Hazard ◽  
Ground Acceleration ◽  
Proximity Potential ◽  
Source Models

Abstract. Seismic hazard assessments – both deterministic and probabilistic, for Peninsular Malaysia have been carried out using peak ground acceleration (PGA) data recorded between 2004 and 2016 by the Malaysian Meteorological Department – using triaxial accelerometers placed at 19 seismic stations within the peninsula and monitored. Seismicity source modelling for the deterministic seismic hazard assessment (DSHA) used historical point sources whereas in the probabilistic (PSHA) approach, line and areal sources were used. The earthquake sources comprised the Sumatran Subduction Zone (SSZ), Sumatran Fault Zone (SFZ), and local intraplate (LI) faults. Gutenberg–Richter law b-value for the various zones identified within the SSZ ranged between 0.56 and 1.06 (mean = 0.83) and that for the zones within SFZ, between 0.53 and 1.13 (mean = 0.84). Suitable ground motion prediction equations (GMPEs) for Peninsular Malaysia along with other pertinent information were used for constructing a logic tree for PSHA of the region. The DSHA critical-worst scenario suggests PGAs of 0.07–0.80 ms−2, whilst the PSHA suggests mean PGAs of 0.06–0.42 ms−2 and 0.12–0.70 ms−2 at 10 % and 2 % probability of exceedance in 50 years, respectively. Both DSHA and PSHA, despite using different source models and methodologies, conclude that the central-western cities of Peninsular Malaysia located between 2° N and 4° N are most susceptible to high PGAs due to neighbouring active Sumatran sources SFZ and SSZ. Surprisingly, the relatively less active SFZ source with low magnitude seismicity appeared as the major contributor, due to its close proximity. Potential hazard due to SSZ mega-earthquakes should not be dismissed, however. Finally, DSHA performed using the limited intraplate seismic data from the Bukit Tinggi (LI) fault at a reasonable Mw 5.0 predicted a PGA of ~ 0.40 ms−2 at Kuala Lumpur.

Crustal strain and seismic hazard of the NE Tibetan Plateau

2021 ◽  
Author(s):  
Qi Ou ◽  
Simon Daout ◽  
Chris Rollins ◽  
Jonathan Weiss ◽  
Barry Parsons
Keyword(s):  
Tibetan Plateau ◽  
Seismic Hazard ◽  
Strain Rate ◽  
Hazard Assessment ◽  
Release Rate ◽  
Seismic Hazard Assessment ◽  
B Value ◽  
Earthquake Catalogues ◽  
Seismic Moment Release ◽  
Ne Tibetan Plateau

<p>Seismic hazard assessment for the NE Tibetan Plateau is of paramount importance because of the growing population density and the accelerated communication and trade activities along the rejuvenated Ancient Silk Road, following the Belt and Road Initiative, and the opening of the high speed railways. Previous-generation seismic hazard assessments were largely based on earthquake catalogues which are shorter than typical earthquake cycles and are temporally and spatially incomplete. This is exacerbated by the fact that magnitudes of many historical Chinese earthquakes are overestimated. In this study, we present new earthquake rate estimates for the NE Tibetan Plateau derived from both an InSAR strain rate map and a re-estimated magnitude of the 1920 Haiyuan Earthquake. First, we obtain a ~100 m resolution strain rate map from five years of Sentinel-1 InSAR data covering an area of 439254 km2 which shows strain concentrated along the Haiyuan and East Kunlun Faults and distributed across the Qilian thrusts and the West Qingling Fault. Second, the magnitude of the Haiyuan Earthquake has been re-estimated to Mw 7.9 ± 0.2 using both historical seismograms and offset measurements. Taking the total moment release rate given by the strain rate map and the magnitude of the 1920 Haiyuan Earthquake as the largest magnitude in the Gutenberg-Richter relationship, we generate rate-balancing frequency-magnitude models with different b values and percentages of seismic moment release. Comparing our models against four earthquake catalogues covering different periods and magnitude ranges suggests the following: (1) With a b value of 1 and 75% seismic moment release, the calculated relationship fits well the International Seismological Centre - Global Earthquakes Catalogue (ISC-GEM, 97 years) catalogue in the range Mw>6.5, but overestimates all other catalogues not containing the Haiyuan Earthquake; (2) keeping a b value of 1 and in order to fit the Global Centroid Moment Tensor Catalogue (GCMT, 34 years), the China Earthquake Networks Center Catalogue (CENC, 12 years) and the China Historical Strong Earthquakes Catalogue (CHSEC, 411 years), a low seismic release rate of 30% would be required; the resultant relationship also fits the ISC-GEM catalogue excluding the Haiyuan Earthquake and its aftershocks; (3) to fit all of the catalogues, it is necessary to reduce the b value to 0.7, in which case only 25% aseismic moment release would be required, giving confidence that Mw 7.9 ± 0.2 is likely the largest magnitude required to balance the tectonic strain in the NE Tibetan Plateau. This study highlights the dominating strain release by, and the effect on the b value of, the largest earthquake and demonstrates the advantage of combining tectonic strain and earthquake catalogues for seismic hazard assessment.</p>

scholarly journals Revisiting seismic hazard assessment for Peninsular Malaysia using deterministic and probabilistic approaches

2018 ◽  
Vol 18 (9) ◽  
pp. 2387-2408 ◽  
Author(s):  
Daniel Weijie Loi ◽  
Mavinakere Eshwaraiah Raghunandan ◽  
Varghese Swamy
Keyword(s):  
Seismic Hazard ◽  
Hazard Assessment ◽  
Seismic Hazard Assessment ◽  
B Value ◽  
Peninsular Malaysia ◽  
Point Sources ◽  
Ground Acceleration ◽  
Earthquake Sources ◽  
Source Models ◽  
Pertinent Information

Abstract. Seismic hazard assessments, both deterministic and probabilistic, for Peninsular Malaysia have been carried out using peak ground acceleration (PGA) data recorded between 2004 and 2016 by the Malaysian Meteorological Department using triaxial accelerometers placed at 19 seismic stations on the peninsula. Seismicity source modelling for the deterministic seismic hazard assessment (DSHA) used historical point sources whereas in the probabilistic (PSHA) approach, line and areal sources were used. The earthquake sources comprised the Sumatran subduction zone (SSZ), Sumatran fault zone (SFZ) and local intraplate (LI) faults. Gutenberg–Richter law b value for the various zones identified within the SSZ ranged between 0.56 and 1.06 (mean=0.82) and for the zones within the SFZ, between 0.57 and 1.03 (mean=0.89). Suitable ground motion prediction equations (GMPEs) for Peninsular Malaysia along with other pertinent information were used for constructing a logic tree for PSHA of the region. The DSHA “critical-worst” scenario suggests PGAs of 0.07–0.80 ms−2 (0.7–8.2 percent g), whilst the PSHA suggests mean PGAs of 0.11–0.55 ms−2 (0.5–5.4 percent g) and 0.20–1.02 ms−2 (1.9–10.1 percent g) at 10 % and 2 % probability of exceedance in 50 years, respectively. DSHA and PSHA, despite using different source models and methodologies, both conclude that the central-western cities of Peninsular Malaysia, located between 2 and 4∘ N, are most susceptible to high PGAs, due to neighbouring active Sumatran sources, SFZ and SSZ. Of the two Sumatran sources, surprisingly, the relatively less active SFZ source with low magnitude seismicity appeared as the major contributor due to its proximity. However, potential hazards due to SSZ mega-earthquakes should not be dismissed. Finally, DSHA performed using the limited LI seismic data from the Bukit Tinggi fault at a reasonable moment magnitude (Mw) value of 5.0 predicted a PGA of ∼0.40 ms−2 at Kuala Lumpur.

The Effects of Aleatory Variabilities of Seismicity on Probabilistic Seismic Hazard Assessment

2020 ◽  
Vol 110 (3) ◽  
pp. 1162-1171
Author(s):  
Hongliu Ran
Keyword(s):  
Seismic Hazard ◽  
Hazard Assessment ◽  
Seismic Hazard Assessment ◽  
Source Zone ◽  
Occurrence Rate ◽  
Probabilistic Seismic Hazard Assessment ◽  
Probabilistic Seismic Hazard ◽  
Ground Acceleration ◽  
Magnitude Distribution ◽  
Aleatory Variability

ABSTRACT Aleatory variability is the natural randomness in a process and can affect probabilistic seismic hazard assessment (PSHA). In this study, considering a simple case of a square areal source zone, I employ Monte Carlo methods to estimate aleatory uncertainties due to random variations in temporal, spatial, and magnitude distribution of seismicity within the zone for PSHA. The results show that (1) uncertainty from aleatory variability in PSHA is significant for areas with low-seismic activity, (2) the ratio of the 85th to 15th percentiles of peak ground acceleration (PGA) decreases as the occurrence rate increases, and (3) accounting for random variations in seismic parameters changes the estimated PGA by more than 10%. My analysis applies to the case in which there are fewer than 10 earthquakes over 50 yr, the site is located outside of the areal source, and b≥1.0. This situation should be considered in PSHA due to the cutoff effect of the magnitude lower limit. In addition, the sensitivity analysis shows that random variations in earthquake magnitude distribution are the largest contributor to aleatory uncertainty in most cases.

Regionally Optimized Background Earthquake Rates from ETAS (ROBERE) for Probabilistic Seismic Hazard Assessment

2020 ◽  
Vol 110 (3) ◽  
pp. 1172-1190 ◽  
Author(s):  
Andrea L. Llenos ◽  
Andrew J. Michael
Keyword(s):  
Seismic Hazard ◽  
San Francisco ◽  
Hazard Assessment ◽  
Seismic Hazard Assessment ◽  
B Value ◽  
Tectonic Activity ◽  
Probabilistic Seismic Hazard Assessment ◽  
Probabilistic Seismic Hazard ◽  
Background Rate ◽  
Aftershock Sequences

ABSTRACT We use an epidemic-type aftershock sequence (ETAS) based approach to develop a regionally optimized background earthquake rates from ETAS (ROBERE) method for probabilistic seismic hazard assessment. ROBERE fits parameters to the full seismicity catalog for a region with maximum-likelihood estimation, including uncertainty. It then averages the earthquake rates over a suite of catalogs from which foreshocks and aftershocks have been removed using stochastic declustering while maintaining the same Gaussian smoothing currently used for the U.S. Geological Survey National Seismic Hazard Model (NSHM). The NSHM currently determines these rates by smoothing a single catalog from which foreshocks and aftershocks have been removed using the method of Gardner and Knopoff (1974; hereafter, GK74). The parameters used in GK74 were determined from subjectively identified aftershock sequences, unlike ROBERE, in which both background rate and aftershock triggering parameters are objectively fitted. A major difference between the impacts of the two methods is GK74 significantly reduces the b-value, a critical value for seismic hazard analysis, whereas ROBERE maintains the original b-value from the full catalog. We apply these methods to the induced seismicity in Oklahoma and Kansas and tectonic activity in the San Francisco Bay Region. Using GK74 gives lower overall earthquake rates but estimates higher hazard due to the reduction in the b-value. ROBERE provides higher earthquake rates, at the magnitude of completeness, but lower hazard because it does not alter the b-value. We test two other declustering methods that produce results closer to ROBERE but do not use objectively fit parameters, include uncertainty, and may not work as well in other areas. We suggest adopting ROBERE for the NSHM so that our hazard estimates are based on an objective analysis, including uncertainty, and do not depend strongly on potentially biased b-values, which was never the goal of the existing methodology.

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