Magnitude distribution complexity and variation at The Geysers geothermal field

Abstract

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 Properties of the Chain Transform Fault Inferred Using Data From the PI‐LAB Experiment

“…The completeness magnitude of the GCMT catalog for the study area is determined as a function of time. The Anderson Darling Test (Leptokaropoulos, 2020; Marsaglia & Marsaglia, 2004; Text S2 in Supporting Information S1) is performed in various time windows and establishes that M C = 5.4 for 1976–2002 and M C = 5.0 starting in 2004. This changing point is consistent with the work of Ekström et al.…”

Broad fault zones enable deep fluid transport and limit earthquake magnitudes

“…The exponentiality of magnitude distribution is investigated by the Anderson-Darling Test 46,47 and further established by goodness of t test 48,49 , applying the Maximum Likelihood Estimation (MLE) of b-value 50 . Both techniques suggest that the magnitude distribution can be su ciently modelled by an exponential distribution for M L ≥2.3.…”

Tidal Triggering of Microseismicity at the Equatorial Mid‐Atlantic Ridge, Inferred From the PI‐LAB Experiment

“…This part of analysis requires a complete catalog, since it involves seismicity rates. The Anderson‐Darling (Leptokaropoulos, 2020; Marsaglia & Marsaglia, 2004) test (AD‐test) is applied for different magnitude cut‐off ( M cut ) values, for verifying whether the magnitudes are drawn from an exponential distribution. The completeness magnitude is selected equal to the magnitude above which the exponentiality hypothesis is not rejected by the AD‐test at 0.05 significance.…”

“…These studies have focused on intermediate (Juan de Fuca) and fast (East Pacific Rise) spreading ridges and less attention has been paid to slow spreading ridges such as the MAR. Although there are some microseismicity studies for various MAR segments (e.g., Grevemeyer et al, 2013;Horning et al, 2018;Parnell-Turner et al, 2017, 2020, temporal variations in seismicity, such as tidal effects and triggering mechanisms have not been fully investigated along the MAR. Tidal pressure was shown to modulate high temperature hydrothermal discharge at the Lucky Strike deep sea vent field at 37°N, although seismicity was only briefly discussed (Barreyre et al, 2014).…”