Crustal Cracks in Areas of Active Deformation: Correlation of GPS and Seismic Anisotropy

<p>Seismic anisotropy in the upper crust can be observed from shear-wave split- ting. It is closely related to crack distribution, orientation and density via the orientation of fast polarisation and the delay time between the two perpendic- ular components of the original shear-wave. Since stress variations can a ect crustal cracks, they should change shear-wave splitting. Another observable result of variations in stress can be deformation measured by GPS (Global Positioning System). Although there are as yet few publications linking these di erent methods, some have suggested an alignment of fast direction with maximum horizontal compressive stress and maximum horizontal compressive strain. We examine whether we can observe this or a di erent relation of seis- mic anisotropy to strain and stress changes in three di erent settings. We performed shear-wave splitting analyses of local earthquakes and baseline and strain calculations around Taupo caldera (New Zealand), Aso caldera (Japan), and around an area on the Raukumara peninsula (New Zealand) associated with slow slip on the Hikurangi subduction interface. Both anisotropy and deformation vary with time in all three regions, but the time variations do not strongly correlate with each other. We suggest that a strong deformation signal observed at Taupo caldera might have a regional, non-volcanic source, and small variations in shear-wave splitting cannot be linked to variations in GPS time series or baselines. At Aso caldera strong deformation seems to be directly related to volcanic activity. Seismic anisotropy also shows a change, but at a slightly di erent time than the GPS signal. On the Raukumara peninsula, the strong deformation associated with slow slip does not show up as a variation in seismic anisotropy, although variations in shear-wave splitting do exist in this area. Overall, we observe an alignment of fast direction with either maximum horizontal compressive strain or stress or both for a subset of time periods and stations. In addition, there is a contribution of structure to the observed anisotropy. We conclude that deformation and seismic anisotropy cannot always be linked in a straightforward way. Instead, shear-wave splitting may be connected to smaller scale processes than can be detected by the current densities of the GPS networks.</p>

Crustal Cracks in Areas of Active Deformation: Correlation of GPS and Seismic Anisotropy

<p>Seismic anisotropy in the upper crust can be observed from shear-wave split- ting. It is closely related to crack distribution, orientation and density via the orientation of fast polarisation and the delay time between the two perpendic- ular components of the original shear-wave. Since stress variations can a ect crustal cracks, they should change shear-wave splitting. Another observable result of variations in stress can be deformation measured by GPS (Global Positioning System). Although there are as yet few publications linking these di erent methods, some have suggested an alignment of fast direction with maximum horizontal compressive stress and maximum horizontal compressive strain. We examine whether we can observe this or a di erent relation of seis- mic anisotropy to strain and stress changes in three di erent settings. We performed shear-wave splitting analyses of local earthquakes and baseline and strain calculations around Taupo caldera (New Zealand), Aso caldera (Japan), and around an area on the Raukumara peninsula (New Zealand) associated with slow slip on the Hikurangi subduction interface. Both anisotropy and deformation vary with time in all three regions, but the time variations do not strongly correlate with each other. We suggest that a strong deformation signal observed at Taupo caldera might have a regional, non-volcanic source, and small variations in shear-wave splitting cannot be linked to variations in GPS time series or baselines. At Aso caldera strong deformation seems to be directly related to volcanic activity. Seismic anisotropy also shows a change, but at a slightly di erent time than the GPS signal. On the Raukumara peninsula, the strong deformation associated with slow slip does not show up as a variation in seismic anisotropy, although variations in shear-wave splitting do exist in this area. Overall, we observe an alignment of fast direction with either maximum horizontal compressive strain or stress or both for a subset of time periods and stations. In addition, there is a contribution of structure to the observed anisotropy. We conclude that deformation and seismic anisotropy cannot always be linked in a straightforward way. Instead, shear-wave splitting may be connected to smaller scale processes than can be detected by the current densities of the GPS networks.</p>

Comprehensive Investigation of Capabilities of the Left-Looking InSAR Observations in Coseismic Surface Deformation Mapping and Faulting Model Estimation Using Multi-Pass Measurements: An Example of the 2016 Kumamoto, Japan Earthquake

We here present an example of the 2016 Kumamoto earthquake with its coseismic surface deformation mapped by the ALOS-2 satellite both in the right- and left-looking observation modes. It provides the opportunity to reveal the coseismic surface deformation and to explore the performance of the unusual left-looking data in faulting model inversion. Firstly, three tracks (ascending and descending right-looking and descending left-looking) of ALOS PALSAR-2 images are used to extract the surface deformation fields. It suggests that the displacements measured by the descending left-looking InSAR coincide well with the ascending right-looking track observations. Then, the location and strike angle of the fault are determined from the SAR pixel offset-tracking technique. A complicated four-segment fault geometry is inferred for explaining the coseismic faulting of the Kumamoto earthquake due to the interpretation of derived deformation fields. Quantitative comparisons between models constrained by the right-looking only data and by joint right- and left-looking data suggest that left-looking InSAR could provide comparable constraints for geodetic modelling to right-looking InSAR. Furthermore, the slip model suggests that the series of events are dominated by the dextral strike-slip with some normal fault motions. The fault rupture initiates on the Hinagu fault segment and propagates from southwest to northeast along the Hinagu fault, then transforms to Futagawa fault with a slip maximum of 4.96 m, and finally ends up at ~7 km NW of the Aso caldera, with a rupture length of ~55 km. The talent of left-looking InSAR in surface deformation detection and coseismic faulting inversion indicates that left-looking InSAR can be effectively utilized in the investigation of the geologic hazards in the future, same as right-looking InSAR.

Characteristics of secondary-ruptured faults in the Aso Caldera triggered by the 2016 Mw 7.0 Kumamoto earthquake

The 16 April 2016 Mw 7.0 Kumamoto earthquake caused prominent fault displacements and crustal deformation, not only around the main rupture faults but also around numerous secondary-ruptured faults. The physics and characteristics of such secondary faulting have not yet been studied in detail. We investigated a set of two secondary faults that appeared at the timing of the Mw 7.0 quake in the Aso Caldera by mainly using synthetic aperture radar interferometry and fault slip modeling. The two faults were found to be associated with surface displacement offsets of several centimeters or more, in the oblique sense of right-lateral and vertical motion. Fault slip inversions found that the slip was dominantly in normal sense with smaller contribution from the right-lateral component. The deeper limit of the slips was estimated to be around 1.3 km, which may coincide with the boundary between the superficial sediment layer and the basement rock. The shallowness of the slip and the difference in the dip angles of the main secondary fault and the Mw 7.0 seismogenic fault suggest separation of the two fault systems, although the fault strike and sense of motions were similar. The amount of slip on the two secondary faults was larger than that expected from the scaling law derived from seismogenic faults, which may indicate the difference in the physics of seismogenic and secondary faultings.

Postseismic deformation following the 2016 Kumamoto earthquake detected by ALOS-2/PALSAR-2

I have been conducting a study of postseismic deformation following the 2016 Kumamoto earthquake using ALOS-2/PALSAR-2 acquired till 2018. I apply ionospheric correction to interferograms of ALOS-2/PALSAR-2. L-band SAR gives us high coherence enough to reveal surface deformation even in vegetated or mountainous area for pairs of images acquired more than 2 years. Postseismic deformation following the Kumamoto earthquake exceeds 10 cm during 2 years at some spots in and around Kumamoto city and Aso caldera. Westward motion of ~ 6 cm/year was dominant on the southeast side of the Hinagu fault, while westward shift was detected on both sides of the Futagawa fault. The area of latter deformation seems to have correlation with distribution of pyroclastic flow deposits. Significant uplift was found around the eastern Futagawa fault and on the southwestern frank of Aso caldera, whose rate reaches 4 cm/year. There are sharp changes across several coseismic surface ruptures such as Futagawa, Hinagu, and Idenokuchi faults. Rapid subsidence between Futagawa and Idenokuchi faults also found. It is confirmed that local subsidence continued along the Suizenji fault, which newly appeared during the mainshock in Kumamoto City. Subsidence with westward shift of up to 4 cm/year was also found in Aso caldera. Time constant of postseismic decay ranges from 1 month to 600 days at selected points, but that postseismic deformation during the first epochs or two is dominant at point in the Kumamoto Plain. This result suggests multiple source of deformation. Westward motion around the Hinagu fault may be explained with right lateral afterslip on the shallow part of this fault. Subsidence along the Suizenji fault can be attributed to normal faulting on dipping westward. Deformation around the Hinagu and Idenokuchi faults cannot be explained with right lateral afterslip of Futagawa fault, which requires other sources. Deformation in northern part of Aso caldera might be the result of right lateral afterslip on a possible buried fault.

Postseismic Deformation Following the 2016 Kumamoto Earthquake Detected by ALOS-2/PALSAR-2

I have been conducting a study of postseismic deformation following the 2016 Kumamoto earthquake using ALOS-2/PALSAR-2 acquired till 2018. I apply ionospheric correction to interferograms of ALOS-2/PALSAR-2. L-band SAR gives us high coherence enough to reveal surface deformation even in vegetated or mountainous area for pairs of images acquired more than 2 years. Postseismic deformation following the Kumamoto earthquake exceeds 10 cm during two years at some spots in and around Kumamoto city and Aso caldera. Westward motion of ~6 cm/yr was dominant on the southeast side of the Hinagu fault, while westward shift was detected on both side of the Futagawa fault. The area of latter deformation seems to have correlation with distribution of pyroclastic flow deposits. Significant uplift was found around the eastern Futagawa fault and on the southwestern frank of Aso caldera, whose rate reaches 4 cm/yr. There are sharp changes across several coseismic surface ruptures such as Futagawa, Hinagu, and Idenokuchi faults. Rapid subsidence between Futagawa and Idenokuchi faults also found. It is confirmed that local subsidence continued along the Suizenji fault, which newly appeared during the mainshock in Kumamoto City. Subsidence with westward shift of up to 4 cm/yr was also found in Aso caldera.Time constant of postseismic decay ranges from 1 month to 600 days at selected points, but that postseismic deformation during the first epochs or two are dominant at point in the Kumamoto Plain. This result suggests multiple source of deformation. Westward motion around the Hinagu fault may be explained with right lateral afterslip on the shallow part of this fault. Subsidence along the Suizenji fault can be attributed to normal faulting on dipping westward. Deformation around the Hinagu and Idenokuchi faults cannot be explained with right-lateral afterslip of Futagawa fault, which requires other sources. Deformation in northern part of Aso caldera might be the result of right lateral afterslip on a possible buried fault.

Land Cover Classification by Integrating NDVI Time Series and GIS Data to Evaluate Water Circulation in Aso Caldera, Japan

Grasslands in Aso caldera, Japan, are a type of land cover that is integral for biodiversity, tourist attractions, agriculture, and groundwater recharge. However, the area of grasslands has been decreasing in recent years as a result of natural disasters and changes in social conditions surrounding agriculture. The question of whether the decrease in spring water discharge in Aso caldera is related to the decrease in grasslands remains unanswered. To clarify this relationship, a water circulation model that considers land covers with different hydrological features is needed. In this study, by integrating Normalized Difference Vegetation Index (NDVI) time series and Geographic Information System (GIS) data, we generated land cover maps from the past (in 1981 and 1991) to the present (in 2015 and 2016), before and after the 2016 Kumamoto earthquake, and then for the future (in the 2040s); these maps formed the dataset for building a water circulation model. The results show that the area of grasslands, which are reported to have a higher groundwater recharge rate than that of forests, in 2016 had decreased to 68% of the area in 1981 as a result of afforestation and transformation into forests, as well as landslides induced by the earthquake. The area of grasslands is predicted to further drop to 60% by the 2040s. On the other hand, the area of forests (conifers and hardwoods) in 2016 had increased by 119% relative to that in 1981 because of the transformation of grasslands into forests, although these areas decreased as a result of landslides due to the 2016 Kumamoto earthquake. Quantification of groundwater recharge from grasslands and forests using the land cover maps generated for 1981, 1996, 2015, and 2016 shows that the annual increase in precipitation in these years significantly affected groundwater recharge; these effects were greater than those associated with the type of land cover. Thus, the groundwater recharge increased, despite the decrease in grasslands. However, when constant precipitation was assumed, the groundwater recharge presented a decreasing trend, indicating the importance of maintaining and conserving grasslands from the viewpoint of groundwater conservation.