Time-Independent Forecast Model For Large Crustal Earthquakes in Southwest Japan Using GNSS Data

In this study, we developed a regional likelihood model for crustal earthquakes using geodetic strain rate data from southwest Japan. First, smoothed strain rate distributions were estimated from continuous GNSS measurements. Second, we removed the elastic strain rate attributed to interplate coupling on the subducting plate boundary, including the observed strain rate, under the assumption that it is not attributed to permanent loading on crustal faults. We then converted the geodetic strain rates to seismic moment rates and calculated the 30-year probability for M ≥ 6 earthquakes in 0.2 × 0.2° cells, using a truncated Gutenberg–Richter law and time-independent Poisson process. Likelihood models developed using different conversion equations, seismogenic thicknesses, and rigidities were validated using the epicenters and moment distribution of historical earthquakes. The average seismic moment rate of crustal earthquakes recorded during 1583–2020 was only 13–20 % of the seismic moment rate converted from the geodetic data, which suggests that the observed geodetic strain rate includes considerable inelastic strain. Therefore, we introduced an empirical coefficient to calibrate the moment rate converted from geodetic data with the moment rate of the earthquakes. Several statistical scores and the Molchan diagram showed that all models could predict real earthquakes better than the reference model, in which earthquakes occur uniformly in space. Models using principal horizontal strain rates exhibited better predictive skill than those using the maximum horizontal shear strain rate. There was no significant difference in the predictive skill between uniform and variable distributions for seismogenic thickness and rigidity. The preferred models suggested high 30-year-probability in the Niigata–Kobe Tectonic Zone and central Kyushu, exceeding 1% in more than half of the analyzed region. Model predictive skill was also verified by a prospective test using earthquakes recorded during 2010–2020. This study suggests that the proposed forecast model based on geodetic data can improve the regional likelihood model for crustal earthquakes in Japan in combination with other forecast models based on active faults and seismicity.

Statistical Correlation of Seismicity and Geodetic Strain Rate in the Chinese Mainland

The correlation between seismicity and crustal deformation is investigated by introducing the earthquake catalogs from 1900 to 2019 and the Global Positioning System (GPS) strain rate during 1999–2007 in the Chinese mainland. The primary results show that (1) about 76.9% of earthquakes with M>6.5 since 1900 and 74.7% of events with M>5.5 between 1970 and 2019, occur in 30% of the region with high-strain rate; (2) the statistics of earthquakes greater than M 5.0 since 1990 show when temporally approaching to the time period of GPS observation, more earthquakes concentrated in the region with high-strain rate. In details, in a 9 yr statistics window, the ratio of cumulative seismicity that occurred in 30% of the high-strain rate regions from 2008 to 2017 is about 16.1%–20.7% lower than those in the previous two periods. Meanwhile, in a 4 yr statistics window, the ratios of cumulative seismicity that occurred in 30% of the high-strain rate regions in the periods of 2011–2015 and 2015–2019 are lower than other five periods by about 9.6%–31.2%. These periodic statistics show that high-strain rate matches well with the historical earthquakes (M≥5.0) than the future earthquake events. In general, the 9 yr GPS observations correspond well to the historical seismicity in the Chinese mainland, but the predicted effect for the future moderate earthquakes is not as good as that of the retrospective test.

On the relationship between strain rate and seismicity in the India-Asia collision zone: Implications for probabilistic seismic hazard

Summary The increasing density of geodetic measurements makes it possible to map surface strain rate in many zones of active tectonics with unprecedented spatial resolution. Here we show that the strain tensor rate calculated from GPS in the India-Asia collision zone represents well the strain released in earthquakes. This means that geodetic data in the India-Asia collision zone region can be extrapolated back in time to estimate strain buildup on active faults, or the kinematics of continental deformation. We infer that the geodetic strain rates can be assumed stationary through time on the timescale needed to build up the elastic strain released by larger earthquakes, and that they can be used to estimate the probability of triggering earthquakes. We show that the background seismicity rate correlates with the geodetic strain rate. A good fit is obtained assuming a linear relationship ($\dot{N} = \lambda \ \cdot \dot{\epsilon }$ where $\dot{N}$ is the density of the rate of Mw ≥ 4 earthquakes, $\dot{\epsilon }$ is strain rate and λ = 2.5 ± 0.1 × 10−3 m−2), as would be expected from a standard Coulomb failure model. However, the fit is significantly better for a non-linear relationship ($\dot{N} = \gamma _1 \cdot \dot{\epsilon }^{\gamma _2}$ with γ1 = 2.5 ± 0.6 m−2 and γ2 = 1.42 ± 0.15). The b-value of the Gutenberg-Richter law, which characterize the magnitude-frequency distribution, is found to be insensitive to the strain rate. In the case of a linear correlation between seismicity and strain rate, the maximum magnitude earthquake, derived from the moment conservation principle, is expected to be independent of the strain rate. By contrast, the non-linear case implies that the maximum magnitude earthquake would be larger in zones of low strain rate. We show that within areas of constant strain rate, earthquakes above Mw4 follow a Poisson distribution in time and and are uniformly distributed in space. These findings provide a framework to estimate the probability of occurrence and magnitude of earthquakes as a function of the geodetic strain rate. We describe how the seismicity models derived from this approach can be used as an input for probabilistic seismic hazard analysis. This method is easy to automatically update, and can be applied in a consistent manner to any continental zone of active tectonics with sufficient geodetic coverage.

Glacial Isostatic Adjustment as a process of deformation but not seismicity in Western Alps: Coupling geodetical strain rate and numerical modeling

<p>In the last decade, geodetic data has become fundamental in studies of active faults, seismicity and seismic hazard. In particular, GNSS strain rates and velocities are used to constrain fault-slip rates and seismicity parameters, on the premise that these short-term (ca. 10 yr) measurements are representative of long-term (10<sup>4</sup>–10<sup>6</sup> yr) fault activity. The Western Alps are a good example of such development in a very-low-strain region with a high-density ongoing seismic activity. There, the first-order agreement between GNSS strain rates and earthquake deformation patterns suggest that a large part of the geodetic deformation observed in the area is seismic. This correlation also suggests that geodetic strain rates can provide constraints on seismicity and seismic hazard. With a numerical modeling approach, we point out the similarities between strain rates predicted for Glacial Isostatic Adjustment (GIA) from the Last Glacial Maximum and the geodetic strain rate field, suggesting that a large part of the GNSS signal is related to GIA. However, we show that the apparent compatibility between geodetic strain rates and seismicity hides a strain rate - stress paradox. In fact, stress perturbations due to GIA are not compatible with observed seismicity, and even tend to inhibit fault activity (as observed from focal mechanisms). Thus, the Western Alps present a typical example of a tectonic system where a transient deformation process precludes, or at least strongly complexifies, the use of geodetic strain rates in seismicity and seismic hazard analyses.</p>

Crustal deformation rates in Kashmir valley and adjoining regions from continuous GPS measurements from 2008 to 2019

We present GPS velocities in Kashmir valley and adjoining regions from continuous Global Positioning System (cGPS) network during 2008 to 2019. Results indicate total arc normal shortening rates of ~ 14 mm/year across this transect of Himalaya that is comparable to the rates of ~ 10 to 20 mm/year reported else-where in the 2500 km Himalaya Arc. For the first time in Himalayas, arc-parallel extension rate of ~ 7 mm/year was recorded in the Kashmir valley, pointing to oblique deformation. Inverse modeling of the contemporary deformation rates in Kashmir valley indicate oblique slip of ~ 16 mm/year along the decollement with locking depth of ~ 15 km and width of ~ 145 km. This result is consistent with the recorded micro-seismicity and low velocity layer at a depth of 12 to 16 km beneath the Kashmir valley obtained from collocated broadband seismic network. Geodetic strain rates are consistent with the dislocation model and micro-seismic activity, with high strain accumulation (~ 7e−08 maximum compression) to the north of Kashmir valley and south of Zanskar ranges. Assuming the stored energy was fully released during 1555 earthquake, high geodetic strain rate since then and observed micro-seismicity point to probable future large earthquakes of Mw ~ 7.7 in Kashmir seismic gap.

New Insights into Long-Term Aseismic Deformation and Regional Strain Rates from GNSS Data Inversion: The Case of the Pollino and Castrovillari Faults

We present a novel inverse method for discriminating regional deformation and long-term fault creep by inversion of GNSS velocities observed at the spatial scale of intraplate faults by exploiting the different spatial signatures of these two mechanisms. In doing so our method provides a refined estimate of the upper bound of the strain accumulation process. As case study, we apply this method to a six year GNSS campaign (2003–2008) set up in the southern portion of the Pollino Range over the Castrovillari and Pollino faults. We show that regional deformation alone cannot explain the observed deformation pattern and implies high geodetic strain rate, with a WSW-ENE extension of 86±41×10−9/yr. Allowing for the possibility of fault creep, the modelling of GNSS velocities is consistent with their uncertainties and they are mainly explained by a shallow creep over the Pollino fault, with a normal/strike-slip mechanism up to 5 mm/yr. The regional strain rate decrease by about 70 percent and is characterized by WNW-ESE extension of 24±28×10−9/yr. The large uncertainties affecting our estimate of regional strain rate do not allow infering whether the tectonic regime of the area is extensional or strike-slip, although the latter is slightly more likely.

A probabilistic seismic hazard model for Mainland China

We construct a probabilistic seismic hazard model for mainland China by integrating historical earthquakes, active faults, and geodetic strain rates. We delineate large seismic source zones based on geologic and seismotectonic characteristics. For each source zone, a tapered Gutenberg–Richter (TGR) distribution is used to model the total seismic activity rates. The TGR a- and b-values are calculated using a new earthquake catalog, while corner magnitudes are constrained using the seismic moment rate inferred from a geodetic strain rate model. For hazard calculations, the total TGR distribution is split into two parts, with smaller ( MW < 6.5) earthquakes being distributed within the zone using a smoothed seismicity method, and larger earthquakes put both onto active faults, based on fault slip rates and dimensions, and into the zone as background seismicity. We select ground motion models by performing residual analysis using ground motion recordings. Site amplifications are considered based on a site condition map developed using geology as a proxy. The resulting seismic hazard is consistent with the fifth-generation national seismic hazard model for most major cities.

Surface and deep deformation of the great Alpine region from GNSS and seismic anisotropy measurements

&lt;p&gt;The comparison of crustal and mantle shear directions can provide insights into the extent of crustal-mantle coupling and the dynamics guiding surface movements and active tectonics in continental deformation zones. Here we present a first attempt of comparing surface deformation from GNSS and deep deformation from seismic anisotropy observations for the Great Alpine Area, mainly through France, Switzerland, Italy, Germany and Slovenia. The developments of the European GNSS infrastructure, integrating public and private GNSS networks, allow now to precisely determining crustal deformation over the Alps. We present a new 3D surface velocity field obtained from a recent re-analysis of 22 years of GPS data obtained from &gt;800 continuous GNSS stations operating across the Alps and its surroundings. Unlike the crust, the orientation of the strain field within the mantle cannot be directly measured and must be inferred from either mantle earthquakes or seismic observations, such as seismic anisotropy observations. We compiled a new map of SKS directions merging data collected during several experiments and available from different databases, deriving a new continuous mantle deformation pattern over the Great Alpine Region. Geodetically determined displacements of the Earth&amp;#8217;s surface reflect the response to different processes acting at different spatial scales. In the comparison between crustal and mantle deformation we accounted for the intrinsic multi-scale characteristics of geodetic deformation measurements, estimating the geodetic strain-rate field using a multi-scale spherical wavelet-based method, where the velocity value at a given point of the Earth&amp;#8217;s surface is obtained as a superposition of values obtained at different spatial scales. From the geodetic strain-rate tensors we computed the two planes of shear (or no-length-changes) directions, which are compared with the directions of SKS splitting over the study region.&lt;/p&gt;

Author Correction: A Geodetic Strain Rate Model for the East African Rift System

An amendment to this paper has been published and can be accessed via a link at the top of the paper.