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.

The Horizontal Kinematics of the North Island of New Zealand

<p>The advantages and disadvantages of the 'displacement' approach and the 'strain' approach to the analysis of repeated geodetic surveys for crustal deformation are discussed and two methods of geodetic strain analysis are described in detail. Repeated geodetic surveys in the central North Island show i) secular widening of the Taupo Volcanic Zone (TVZ) at 7 mm y-1 without significant transcurrent motion ii) north-south dextral motion at 14 mm y-1 and east-west narrowing at 4 mm y-1 across the northern end of the North Island Shear Belt iii) 3.1 m extension at 135' across a 15 km-wide region north of Lake Taupo, and adjacent zones of compressive rebound all associated with the 1922 Taupo Earthquakes. From the epicentral distribution and horizontal strain pattern a 15 km-square fault dipping 40' and striking parallel to the TVZ is inferred for the 1922 earthquakes. The seismic moment, 1.3 x 10 26 dyne cm, and the stress drop, 134 bars, are abnormally high for the TVZ. Widening of the TVZ is considered to be back-arc spreading. The spreading axis is postulated to extend northeast into the Havre Trough via a north-south dextral transform; and southwest into the Waverley Fault Zone and Waimea Depression via the sinistral reverse Raetihi Transform. Deformation of the North Island is not homogeneous. Fault zones are idealized as line plate boundaries and four plates -Indian, Central, Kermadec and Pacific - are postulated to account for the deformation. The Indian-Pacific macroplate pole is adopted and non-unique positions and rotation rates for the remaining poles are determined from geodetic strain data and the geometry of plate interactions. The Central Plate is moving away from the Indian Plate at the back-arc spreading axis; the Kermadec Plate is moving dextrally with respect to the Central Plate at the North Island Shear Belt which accommodates most of the transcurrent component of motion between the Indian and Pacific plates in the North Island and gives almost pure subduction of the Pacific Plate under the Kermadec Plate at the Hikurangi Margin.</p>

The Horizontal Kinematics of the North Island of New Zealand

<p>The advantages and disadvantages of the 'displacement' approach and the 'strain' approach to the analysis of repeated geodetic surveys for crustal deformation are discussed and two methods of geodetic strain analysis are described in detail. Repeated geodetic surveys in the central North Island show i) secular widening of the Taupo Volcanic Zone (TVZ) at 7 mm y-1 without significant transcurrent motion ii) north-south dextral motion at 14 mm y-1 and east-west narrowing at 4 mm y-1 across the northern end of the North Island Shear Belt iii) 3.1 m extension at 135' across a 15 km-wide region north of Lake Taupo, and adjacent zones of compressive rebound all associated with the 1922 Taupo Earthquakes. From the epicentral distribution and horizontal strain pattern a 15 km-square fault dipping 40' and striking parallel to the TVZ is inferred for the 1922 earthquakes. The seismic moment, 1.3 x 10 26 dyne cm, and the stress drop, 134 bars, are abnormally high for the TVZ. Widening of the TVZ is considered to be back-arc spreading. The spreading axis is postulated to extend northeast into the Havre Trough via a north-south dextral transform; and southwest into the Waverley Fault Zone and Waimea Depression via the sinistral reverse Raetihi Transform. Deformation of the North Island is not homogeneous. Fault zones are idealized as line plate boundaries and four plates -Indian, Central, Kermadec and Pacific - are postulated to account for the deformation. The Indian-Pacific macroplate pole is adopted and non-unique positions and rotation rates for the remaining poles are determined from geodetic strain data and the geometry of plate interactions. The Central Plate is moving away from the Indian Plate at the back-arc spreading axis; the Kermadec Plate is moving dextrally with respect to the Central Plate at the North Island Shear Belt which accommodates most of the transcurrent component of motion between the Indian and Pacific plates in the North Island and gives almost pure subduction of the Pacific Plate under the Kermadec Plate at the Hikurangi Margin.</p>

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.

Preliminary forecast model of crustal earthquakes in southwest Japan based on GNSS data

&lt;p&gt;In Japan, the Headquarters for Earthquake Research Promotion has developed a nationwide probabilistic earthquake model called &amp;#8220;National Seismic Hazard Maps for Japan&amp;#8221; since the destructive 1995 Kobe earthquake. This model covers both subduction and crustal earthquakes based on a history of past large earthquakes from seismological, archaeological, and geological data. The model for crustal earthquakes relies on geological and geomorphological data of active faults and never use geodetic data, whereas contemporary deformation of the Japanese Islands has been observed by a dense GNSS network. Here, we attempt to develop a preliminary forecast model of shallow crustal earthquakes using GNSS velocity data.&lt;/p&gt;&lt;p&gt;We follow the procedure of Shen et al.(2007) to calculate the forecast model. The GNSS velocities at continuous GNSS stations from April 2005 to December 2009 are used for the model in southwest Japan. Elastic deformation due to interplate coupling along the Nankai Trough is removed using the block model of Nishimura et al. (2018). Strain rate field is calculated at a grid point of 0.2&amp;#186; x 0.2&amp;#186; by a method of Shen et al (1994). The strain rates are converted to geodetic moment rates by a formula proposed in Savage and Simpson (1997). The thickness of a seismogenic layer, rigidity, b value of the Gutenberg-Richter law, and magnitude of the maximum earthquake are assumed to be 12 km, 30 GPa, 0.9, and 7.5, respectively. They are uniform in the modeled region. Previous studies (e.g., Shen-Tu et al., 1994) revealed that geodetic strain rates were much larger than seismological ones in southwest Japan because geodetic strain includes both elastic and inelastic strain. Elastic strain rates presumably equal to seismological ones on a long-term average. We compared seismic moment rates released by shallow historical earthquakes since AD1586 with the geodetic moment rates. Their ratio is 0.24 and 0.16 in the Chubu, Kinki, and Chugoku region and the whole southwest Japan. This difference is probably attributed to the distribution of historical documents and may also reflect the regionality of the ratio between elastic and inelastic strain. Applying 0.16 for calculating elastic rates and the stationary Poisson process of the earthquake occurrence, a probability of M&amp;#8805;6 earthquakes for 30 years ranges from 5.1 % to 0.2 % in each 0.2&amp;#186; x 0.2&amp;#186; grid of southwest Japan. We verify this probability model by using shallow (Depth&amp;#8804; 20 km) M&amp;#8805;5 earthquakes occurred in 2010-2019, which is a period after the used GNSS data. The number of earthquakes was 36, which is roughly concordant to the predicted number of the model (3.04 per year). About 58 % of the earthquakes occurred with 25 % of the area with the highest strain rates, which suggests many crustal earthquakes occur in high strain-rate regions. The verification suggests the preliminary forecast model has the predictive power reasonably.&lt;/p&gt;

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

&lt;p&gt;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&lt;sup&gt;4&lt;/sup&gt;&amp;#8211;10&lt;sup&gt;6&lt;/sup&gt; 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.&lt;/p&gt;

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.

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.

GPS Constraints on the Active Deformation in Tunisia: Implications on the Geodynamics of the Western Mediterranean

&lt;p&gt;The plate boundary in the western Mediterranean includes the Tunisian Atlas Mountains. We study the active deformation of this area using GPS data collected from 2014 to 2018. WNW to NNW trending velocities express the crustal motion and geodetic strain field from the Sahara platform to the Tell Atlas, consistent with the African plate convergence. To the south, the velocities indicate a nearly WNW-ESE trending right-lateral motion of the Sahara fault-related fold belt with respect to the Sahara Platform. Further north and northeast, the significant decrease in velocities between the Eastern Platform and Central &amp;#8211; Tell Atlas marks the NNW trending shortening deformation associated with local ENE &amp;#8211; WSW extension visible in the Quaternary grabens. The velocity field and strain distribution associated with the active E-W trending right-lateral faulting and NE-SW fault-related folds sustain the existence of three main tectonic blocks and related transpression tectonics. The velocity field and pattern of active deformation in Tunisia document the oblique plate convergence of Africa towards Eurasia.&amp;#160;&lt;/p&gt;