Seismic activity in stable continental regions

<div> <p><span>Stable continental regions (SCRs) have low seismicity and large magnitude earthquakes are infrequent and diffuse compared to plate boundary settings. Because of this, seismicity parameters required for seismic hazard analysis (SHA) are difficult to constrain. A method to overcome this challenge involves using an analogue approach to generate seismic hazard inputs in SCRs. Seismic hazard analysis of these regions develops recurrence parameters by drawing upon data from a larger global database than what is typically done for plate boundary regions. This is completed by choosing regions that are considered seismotectonically analogous and then amalgamating data from the regions to generate larger and perceivably more robust seismicity data sets. Historically, this is done by considering all SCRs as analogous and including all of their data into the analysis. </span></p> <p><span>This study refines and updates this approach by assessing whether there is internal variability of seismogenic potential within SCR crust that can be distinguished by comparing properties of the crust to seismicity. We completed this analysis by: (1) compiling a global homogeneous earthquake catalog for earthquakes >= Mw 2 up to July 2020 which includes historical and instrumental events; (2) subdividing global SCR crust into five geological domains that distinguish crustal criteria within SCRs; (3) calculating and comparing the seismic parameters between the different SCRs and sub-domains to better understand the range in values across different SCRs and determine if there is statistically observable variation between sub-domains</span>. <span>Our results provide an initial step towards redefining what crustal characteristics define analog regions for use in seismic hazard studies.</span></p> </div>

Scaling relations between seismic moment and rupture area of earthquakes in stable continental regions

This article describes the development of scaling relations between seismic moment and rupture area of earthquakes in stable continental regions (SCRs). The article reviews the relations developed by Somerville and compares them with relations developed by other investigators. It also compares the scaling relations of SCR earthquakes with those in tectonically active continental regions (TCRs). Three different methods of estimating rupture area, based on aftershocks, slip models, and duration methods were used by Somerville to analyze the relation between seismic moment and rupture area, using earthquake source parameters compiled from published literature. For each category of data, the relations obtained were not significantly different from those obtained by constraining them to be self-similar (scale-invariant), so self-similar relations were adopted. The stress drops corresponding to these scaling relations range from 51 to 86 bars, with an average of 65 bars. This value is comparable with the value of 58 bars obtained by Leonard, and it is recommended that the Leonard scaling relations for SCR earthquakes be used for the NGA East Project. To a first approximation, the results of Somerville and those of Somerville et al. indicate that the rupture areas of SCR earthquakes are about half those of TCR earthquakes, and their stress drops are about 2.8 times higher. Allmann and Shearer find less of a difference, presumably because their intraplate category includes some earthquakes that we would assign to TCR instead of SCR. Their study indicates that the rupture areas of intraplate earthquakes are about two-thirds those of TCR earthquakes, and their stress drops are about 2 times higher.

Testing Ground-Motion Prediction Equations against Moderate Magnitude Earthquake Data Recorded in Korea

ABSTRACT The selection of ground-motion prediction equations (GMPEs) to perform seismic hazard assessments is challenging for stable continental regions that lack a sufficient number of recordings. In this study, we implement various ranking methods to test the efficiencies of a wide range of GMPEs against the recordings from three of the largest magnitude inland earthquakes that occurred in the Korean Peninsula, which belongs to an intraplate region with low seismicity. In this context, we select a total of 14 GMPEs developed for active shallow crustal zones (Next Generation Attenuation-West2 [NGA-West2] project), stable continental regions, and Korea. Three statistical approaches, including the classical residual, log-likelihood (LLH), and Euclidean distance-based ranking (EDR) methods, are used to evaluate the performance of the GMPEs. The residual analyses show that for the very short spectral period (T≤0.1 s), regionally developed GMPEs perform the best, whereas the NGA-West2 GMPEs outperform other equations for short (0.2≤T≤0.5 s) and medium to long periods (T≥0.75 s). The LLH approach is shown to favor a stable continental region GMPE that has the highest standard deviation. The EDR method, which can account for both aleatory uncertainties and model bias, is found to favor the NGA-West2 and Korean GMPEs. NGA-West2 GMPEs show the lowest model bias, whereas the Korean GMPEs exhibit the lowest residual bias. Five GMPEs selected based on the EDR method are recommended for a probabilistic seismic hazard analysis in Korea. For deterministic hazard assessment, using the Korean GMPEs for the very short spectral period and NGA-West2 GMPEs for short and medium to long periods is recommended. Overall, the stable continental region GMPEs are demonstrated to perform poorly when tested against the earthquakes recorded in Korea.

The 2016 Mw 6.1 Petermann Ranges earthquake rupture, Australia: another “one-off” stable continental region earthquake

<p>The 20<sup>th</sup> May 2016 moment magnitude (M<sub>W</sub>) 6.1 Petermann earthquake was the 2<sup>nd</sup> longest single-event historic Australian surface rupture (21 km) and largest M<sub>W</sub> on-shore earthquake in 28 years. Trench logs from two hand-dug trenches show no evidence of penultimate rupture of surface eolian sediments or underlying calcrete. Available dating of eolian dunes 140 to 500 km away from the Petermann fault indicated eolian deposition during either the last glacial maximum (approximately 20 ka) or a period of aridification at approximately 180 - 200 ka. Ten <sup>10</sup>Be cosmogenic nuclide erosion rates of bedrock outcrops at 0 to 50 km from the surface rupture trace are within error of each other between 1.4 to 2.6 mMyr<sup>-1</sup>. These samples have approximate averaging times between 208 to 419 ka. Bedrock erosion rates, trenching results and interpretation of the landscape history suggest the 2016 event is the only surface rupturing earthquake on the Petermann fault in the last 200 to 400 kyrs, and possibly the first ever on this fault. This finding is consistent with a lack of evidence for penultimate rupture for all eleven historic Australian surface rupturing events, as described by either trenching and/or landscape analysis and bedrock erosion rates. These ‘one-off’ events within Precambrian cratonic Australian crust are not consistent with trenching results and geomorphology of paleo-scarps within the Flinders Ranges and Eastern Australia which indicate multiple recurrent fault offset. Variable fault recurrence behaviour highlights that uniform seismic hazard modelling approaches are not applicable across Stable Continental Regions.</p>

The Mw4.9 Le Teil surface-rupturing earthquake in southern France: New insight on seismic hazard assessment in stable continental regions

<p>On November 11th 2019, a Mw 4.9 earthquake shook the Rhone River Valley in southern France, a rather densely populated area with many industrial facilities including several nuclear power plants. The “Le Teil” earthquake was felt as far as Montpellier and Grenoble, 120 km from the epicenter. Seismological data promptly showed that the earthquake corresponded to a reverse faulting event along a NE-SW trending fault with a focus at a very shallow depth (~1 km). In parallel, satellite-based radar observations (InSAR) showed the uplift of the SE compartment (up to 10 centimeters) along a sharp NE-SW trending ~4.5-km-long discontinuity. Field investigations conducted in the following days and weeks in the epicentral area uncovered several evidences of surface ruptures across roads and paths where the InSAR discontinuity is mapped. We also carried out airborne LiDAR surveys to map the rupture below the dense forest cover. Characteristics of surface deformations are fully consistent with InSAR and seismological data, and allow concluding to the reactivation of an Oligocene normal fault segment (i.e. La Rouvière fault) that belongs to the Cévennes fault system, a 120 km long polyphased system bounding the southern rim of the Massif Central. The absence of clear cumulative compressional deformation along the fault rupture, which on the contrary displays inherited extensional deformation (most likely Oligocene in age), suggests that the fault has not moved significantly since millions of years. These observations relaunch the question of seismic hazard assessment in stable continental regions such as continental France and most of Western Europe, where strain rates are very low.</p>

Relative Differences between Nonlinear and Equivalent-Linear 1-D Site Response Analyses

This study investigates the conditions for which one-dimensional (1-D) nonlinear (NL) site response analysis results are distinct from equivalent-linear (EL) results and provides guidance for predicting when differences are large enough to be of practical significance. Relative differences in spectral accelerations and Fourier amplitudes computed from NL and EL analyses are assessed for a range of site conditions and for suites of input motions appropriate for active crustal and stable continental regions. Among several considered parameters, EL/NL differences are most clearly dependent on shear strain index ( I γ), defined as the ratio of input motion peak velocity to time-averaged shear-wave velocity in the top 30 m of the soil profile. For small I γ (generally under 0.03%), EL and NL results are practically identical, whereas at larger strains, differences can be significant for frequencies >0.3 Hz. Frequency-dependent I γ values are recommended for conditions above which NL analyses are preferred to EL.