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Japanese citizens are still not fully prepared for an occurrence of the Nankai Trough Earthquake. In the Japanese population, it is not an easy task to achieve disaster preparedness, and to address issues such as a lack of initiative and low priority in disaster risk reduction (DRR). In their attempt to address these issues, the authors focused on Japanese elementary school students and their parents by conducting a lighthearted workshop that encouraged them to initiate risk reduction activities against disasters caused by natural hazards, including the Nankai Trough Earthquake. The workshop was a nonschool-based DRR activity, and it had the following characteristics: (1) the contents were comprehensively developed from a collaboration between researchers in science and engineering with a disaster science communicator and media personnel; (2) innovative contents were communicated in a creative manner; (3) incorporation of a “newspaper-making” activity; and (4) an online format that easily enabled those who did not have a close social network in the community to participate in it. The workshop was conducted with 21 families, and they developed original newspapers on DRR. The findings of the completed newspapers, participant observations, and questionnaire survey revealed that the workshop was an effective starting point for citizens to proactively engage in DRR, including the Nankai Trough Earthquake.

SUMMARYEarthquake source parameters play a fundamental role in evaluating seismic hazard. In many countries, the relatively low accuracy of the earthquake source parameters owing to sparse seismic monitoring networks prevents accurate seismic hazard evaluation. Interferometric Synthetic Aperture Radar (InSAR) is a useful tool for accurate fault parameter determination of shallow inland earthquakes. We compared the fault parameters (location and rupture geometry) derived from InSAR with those derived from local and global seismic data to assess the credibility of the earthquake catalogues. We set our targets to Iran and Japan, where seismic networks are relatively sparse and dense, respectively, and analysed a total of 10 shallow inland earthquakes. To compare the fault parameters for each studied earthquake, we first created interferograms using InSAR data from multiple satellites and conducted fault model inversions assuming uniform slip on a rectangular finite fault. For the studied Iranian earthquakes, the epicentres of the United States Geological Survey (USGS) catalogue were more consistent with the InSAR-derived fault model compared to those of the local catalogue of the Iranian Seismological Center (IRSC). For the studied Japanese earthquakes, the epicentres of the Japan Meteorological Agency (JMA) catalogue were more consistent with the InSAR-derived fault model compared to those of the USGS. Assuming that the accuracy of the USGS locations is comparable for the two countries, the JMA catalogue has a higher accuracy than the IRSC catalogue. The difference in the accuracy for the two local catalogues can be explained by the larger azimuthal gaps of the IRSC seismic network. We also showed that the IRSC- and JMA-derived centroids were more consistent with the InSAR-derived fault model compared to those of the Global Centroid Moment Tensor (GCMT) catalogue. The focal mechanism solutions derived from InSAR and seismic CMT solutions were significantly different, highlighting the difficulties in determining the focal mechanism parameters from seismic data, even when the seismic station coverage is dense. As InSAR data can be easily accessed, this study presents a cost-effective method to assess and improve the accuracy of the local earthquake catalogues anywhere in the world.

Liquid bridges in unsaturated soils attach to grain contacts and contribute to strengthening microscopic bonding forces, which leads to macroscopic high strength and stiffness compared with dry or saturated soils. This study investigated microscopic liquid bridge behaviors in monodisperse granular media by combining X-ray micro-computed tomography and image analyses. Evaluating the anisotropy of grain contact orientation by fabric tensor revealed that fabric transitions under a triaxial process for both grain contacts and that with liquid bridges are equivalent, which indicates that the orientation of the liquid bridge contribution does not have unique anisotropy. The liquid bridge ratio (LBR) was defined as the ratio of the number of grain contacts with liquid bridges to the total number of grain contacts, representing the magnitude of the liquid bridge contribution. LBR distributions—the relationship of LBR to degree of saturation obtained by water retention tests for both dense and loose specimens—exhibited different peak positions depending on the void ratio and constant dispersion, which was then modeled using β distribution. A constitutive model on an experimental micromechanics basis for unsaturated soils was proposed by applying the liquid bridge contribution model as a combination of the LBR distribution model and the soil water characteristic curve model to a micromechanics model, in which spherical and monodisperse grains are assumed and the pendular saturation regime is focused. The proposed model qualitatively reproduced the macroscopic suction effect by describing the microscopic liquid bridge contribution.

A major source of errors in interferometric synthetic aperture radar (InSAR), used for mapping ground deformation, is the delay caused by changes in the propagation velocity of radar microwaves in the troposphere. It is common to correct this tropospheric delay noise using numerical weather models because of their widespread global availability. A variety of correction methods and tools exist and selecting the most appropriate one by considering weather models, delay models, and delay calculation algorithms is essential for specific applications. We compared the performance of two tropospheric delay correction methods applied to Advanced Land Observing Satellite-2 (ALOS-2) data acquired over Japan, where the atmospheric field is complex with significant seasonal variation. In particular, we tested 1) a method of delay integration along the slant radar line-of-sight (LOS) path using the mesoscale model (MSM) provided by the Japan Meteorological Agency and 2) the Generic Atmospheric Correction Online Service (GACOS) for InSAR, which estimates delay using the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis products along with an iterative decomposition approach. Results showed that tropospheric delay correction using the slant delay integration approach with MSM, which has a finer temporal and spatial resolution, performed slightly better than GACOS, which used ECMWF data. We further found that the differences in these tropospheric delay models would have limited significance, suggesting that the difference in performance mainly originates from differences in the numerical weather models being used. This study highlights the importance of using the best-available numerical weather model data for tropospheric delay calculations.

Investigating fault characteristics is necessary for seismic hazard evaluation and can provide useful insights for municipalities to optimize their investments in safer buildings and more resilient infrastructure. The location and the geometry of the faults, especially their relationship with sedimentary layer thickness, and the amount of stress drop are critical components when evaluating strong ground motions caused by earthquakes. Interferometric Synthetic Aperture Radar (InSAR) is a powerful tool for accurately determining fault parameters of shallow inland earthquakes and investigating their characteristics in terms of seismic hazards. In this study, based on our earlier results from InSAR data inversions conducted on five Iranian shallow blind reverse-fault earthquakes, we particularly exploited the resolving power of InSAR to investigate fault characteristics and discussed their implications for seismic hazard evaluation in Iran. For all studied Iranian earthquakes, the InSAR-derived fault was deeper than the sedimentary layer. This may indicate that these earthquakes did not rupture the entire seismogenic layer, likely leaving a significant seismic potential. Moreover, the average stress drop of the Iranian earthquakes was relatively large (13.4 MPa), which might be related to their immature blind reverse faults, leading to a possibility of generating stronger short-period ground motions, which should receive greater consideration in future seismic hazard analyses. As InSAR data can be easily accessed, this study presents a cost-effective method to provide fault parameters that can be used for the evaluation of strong ground motions caused by earthquakes, even in countries where observation networks of strong ground motions are not well developed.

The Mw7.0 2016 Kumamoto earthquake occurred on the previously mapped Futagawa–Hinagu fault causing significant strong ground motions. A ~ 30-km-long dextral surface rupture appeared on the major fault zone and dextral slip was up to 2–3 m. However, the surface ruptures were also broadly and remotely distributed approximately 10 km away from the primary rupture zone. These numerous distributed secondary surface slips with vertical displacement of less than a few tens of centimeters were detected by the interferometric synthetic aperture radar (InSAR) technology in previous studies. Such displacements occurred not only on previously mapped faults but also on unknown traces. Here, we addressed the following fundamental issues: whether the broadly distributed faults were involved in the past major earthquakes in the neighborhood, and how the fault topography of such secondary faults develops, seismically or aseismically. To find clues for understanding these issues, we show the results of field measurements of surface slips and paleoseismic trenching on distributed secondary faults called the Miyaji faults inside the Aso caldera, 10 km away from the eastern end of the primary rupture zone. Field observations revealed small but well-defined dextral slip surface ruptures that were consistent with vertical and dextral offsets derived from InSAR. On the trench walls, the penultimate event with vertical displacements almost similar to the 2016 event was identified. The timing of the penultimate event was around 2 ka, which was consistent with that of the primary fault and archeological information of the caldera. Considering the paleo-slip event and fault models of the Miyaji faults, they were presumed not to be source faults, and slip on these faults have been triggered by large earthquakes along major adjacent active faults. The results provide important insights into the seismic hazard assessment of low-slip-rate active faults and fault topography development due to triggered displacement along secondary faults.

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.

Partially water-saturated granular soils are mixtures of soil grains, pore water, and pore air. This study presents methodologies to obtain local quantities, such as porosity, degree of saturation, and pore volume, for partially water-saturated uniform sand at the pore scale. A sand specimen subjected to different water retention states was scanned using X-ray micro computed tomography. The obtained three-dimensional tomographic images were trinarized, i.e. segmented into soil grain, water, and air phases. The trinarized volumes were divided into Voronoi cells containing individual pores, and the volume and degree of saturation for the pores were calculated. Alternatively, the porosity and degree of saturation were quantified for cubical local subsets arranged regularly in the trinarized volumes. Pore characteristics and local water retention behavior are discussed using the results of these two investigations.

Determining the fault parameters of an earthquake is fundamental for studying the earthquake physics, understanding the seismotectonics of the region, and forecasting future earthquake activities in the surrounding area. Dense crustal deformation data such as Interferometric Synthetic Aperture Radar (InSAR) are useful for fault parameter determination, but determining the dipping direction of a blind fault is often challenging when the size of the earthquake is not large (M

DefVolc is a suite of programs and a web service intended to help the rapid interpretation of InSAR data, acquired on volcanoes at an increased frequency thanks to the various dedicated satellites. Our objective is to help to rapidely inverse volcano displacements, whether these displacements result from fractures (sheet intrusions or faults) or massive magma reservoirs. These sources may have complex geometries, and they may deform simultaneously. Moreover, volcanoes are associated with prominent topographies. This makes the analysis of surface displacements complex. To appropriately analyse the InSAR displacements, we conduct inverse modelling, using 3D elastostatic boundary element models and a neighbourhood optimization algorithm . We simultaneously invert non-linear model parameters (source geometry and location) and linear model parameters (source stress changes), and further assess mean model parameters and confidence intervals. In order to speed up the setting up of inversions, we developed a users friendly graphical interface. In order to accelerate the inversions, they run on clusters. A web server is proposed to registered users in order to run the inversions on University Clermont Auvergne clusters. Because the web server was developped in the framework of the Eurovolc project framework, European volcano observatories are priority users.

International audience; Delineating seismic and aseismic slip on faults allows the exploration of the complex relationship between these different modes of slip. Further, quantifying them helps in the assessment of seismogenic potential. We present a distributed slip model of the rate of aseismic slip along the Leyte island section of the Philippine Fault and make comparisons with the extent of seismic slip from the latest significant earthquakes in July 2017. We derived both coseismic and aseismic slip distributions from kinematic inversions of synthetic aperture radar interferometric (InSAR) observations using a probabilistic (Bayesian) framework. Velocity maps from stacking and time series analysis of ALOS interferograms spanning 4 years (2007–2011) show a step change in ground deformation right on the fault. Inverting for slip at depth reveals along‐strike variations of aseismic slip rate over ∼100 km. Aseismic slip on the surface reaches rates of more than 3 cm/yr, equivalent to the long‐term slip rate (3.3 ± 0.2 cm/yr). Over the same period, a 20‐km segment in Tongonan appears to be locked. This segment ruptured in Mw 6.5 and Mw 5.8 earthquakes on 6 and 10 July 2017, respectively, as constrained by Sentinel‐1 and ALOS‐2 InSAR data. Seismic slip appears to be restricted within the Tongonan segment, with up to 152 ± 21 cm of left‐lateral displacement. The slip budget and complementarity between the extents of interseismic and coseismic slip suggest that a seismogenic asperity exists in Tongonan. The presence of active hydrothermal systems and rate‐strengthening materials provide physical conditions that can promote aseismic slip.

Active faults commonly repeat cycles of sudden rupture and subsequent silence of hundreds to tens of thousands of years, but some parts of mature faults exhibit continuous creep accompanied by many small earthquakes. Discovery and detailed examination of creeping faults on land have been in a rapid progress with the advent of space-borne synthetic aperture radar interferometry. In this study, we measured the spatial variation of the creep rate along the Philippine fault on Leyte Island using ALOS/PALSAR data acquired between October 2006 and January 2011. Prominent creep of$$33\pm {11}$$33±11 mm/year was estimated in northern and central parts of the island except for a locked portion around latitude 11.08–11.20$$^\circ$$∘N. We compared the creep rate distribution along the fault with the slip distribution of the 2017$$M_w$$Mw6.5 Ormoc earthquake which occurred in northern Leyte, estimated from the displacements mapped by ALOS-2/PALSAR-2 interferometric data. The estimated slip of the 2017 earthquake amounted up to 2.5 m and to moment magnitude of 6.49, with the dominant rupture area coinciding with the locked portion identified from the interseismic coupling analysis. Teleseismic waveforms of the 2017 earthquake and another event that occurred in 1947 ($$M_s$$Ms6.9) exhibit close resemblance, indicating two ruptures of rather similar locations and magnitudes with a time interval of 70 years.

The February 1993 dike intrusion in the East Rift Zone (ERZ) of Kīlauea Volcano, Hawai'i, was recognized from tilt and seismic data, but ground-based geodetic data were too sparse to constrain the characteristics of the intrusion. Analysis of Interferometric Synthetic Aperture Radar (InSAR) from the Japan Aerospace Exploration Agency (JAXA) JERS-1 satellite reveals a maximum of ~30 cm of line-of-sight (LOS) displacement occurring near Makaopuhi Crater in the middle ERZ of Kīlauea. We model this deformation signal as a subvertical dike using a 3D-Mixed Boundary Element Method (3D-MBEM) paired with a nonlinear inversion algorithm to find the best-fit model. The best-fit dike is located just to the west of Makaopuhi Crater striking N50°W, extends to within 100 m of the surface, is ~1.3 km in length by ~4.2 km in width along strike, and has a total volume of ~7.4 × 106 m3. In addition, a post-intrusion interferogram from JERS-1 spanning 1993–1997 was analyzed. Guided by previous results, our model for the 4-year period consists of opening of the deep rift zones by about 0.5 m at 3–8.5 km depth beneath the Southwest Rift Zone (SWRZ), ERZ and the summit. A sub-horizontal detachment fault is connected to the seaward side of the vertical dike-like source to mimic the decollement known to exist beneath the volcano. We classify the 1993 dike intrusion as a passive intrusion similar to those that occurred in 1997 and 1999. Passive intrusions lack precursory inflation at Kīlauea's summit, and the likely triggering mechanism is persistent deep rift opening combined with seaward motion of the south flank along the basal decollement. Passive intrusions make forecasting and hazard assessment difficult since they are not preceded by inflation nor by large increases in seismicity.

It has been inferred that the Philippine fault on Leyte Island has been creeping. On 6 July 2017, an Mw 6.5 earthquake struck northern Leyte, suggesting that not all parts along the fault have been creeping. We conducted InSAR time-series analyses using ALOS/PALSAR data (2006-2011) to estimate the spatial variation of the fault creep rate, and fault slip modeling of the 2017 earthquake using the displacements obtained from ALOS-2/PALSAR-2 interferometry. We found that the section ruptured by the 2017 earthquake had been locked (lack of fault creep) while the neighboring sections, in northernmost and central areas of the island, have been creeping with rates of up to 30 mm/year. We speculate from the creep analysis that the size of the 2017 earthquake would be the maximum-class earthquake along the Philippine fault in the northern half of Leyte. Considering the similarity in size and location, we infer that the 1947 earthquake also ruptured the same locked portion as the 2017 earthquake. Our findings demonstrate the usefulness of InSAR time-series analysis to identify future earthquake-prone sections along active faults.

Studies using spaceborne interferometric synthetic aperture radar (InSAR) analysis showed that two megathrust earthquakes – the 2011Mw9.0 Tohoku-oki earthquake in Japan and the 2010Mw8.8Maule earthquake in Chile – triggered unprecedented subsidence in multiple volcanoes. There are strong similarities in the characteristics of the surface deformation in Japan and Chile: (1) Maximum subsidence is about 15 cm. (2) Areas of subsidence are elliptically elongated in a north-south direction perpendicular to the principal axis of the extensional stress change. (3) Most of this subsidence is coseismic. These similarities imply that volcanic subsidence triggered by the megathrust earthquakes is a ubiquitous phenomenon. Nonetheless, the mechanism of subsidence is yet to be investigated. Two main hypotheses have been proposed thus far: 1) The localized deformation of hot and weak plutonic bodies. 2) Water release from large hydrothermal reservoirs beneath the volcanoes.

We estimated recent surface displacements around Bangkok by means of persistent scatterer interferometry with ALOS/PALSAR images acquired from November 2007 to December 2010. Land subsidence due to excessive groundwater pumping has been reported in this region. However, we detected ground surface uplift around the mega-city, along with seasonal surface displacement, with high spatial resolution. We then discriminated long-term natural rebound and seasonal displacement by fitting exponential and sinusoidal functions to displacement time-series, and mapped their spatial distributions. This mapping allowed us to infer that the second and third shallowest aquifers are laterally continuous, whereas the shallowest aquifer has lateral discontinuities. The temporal decay rate of the long-term rebound might reflect spatial changes of the Chao Phraya River watershed or the magnitude of the preceding groundwater extraction. We demonstrated that our method of decomposing the displacement time series into different spatial and temporal patterns is useful for understanding aquifer connectivity and the elastic response pattern in an aquifer system.

We used two tracks of ALOS PALSAR images to investigate the focal mechanism and slip distribution of the 2011 March 24, M W 6.8 Burma strike-slip earthquake. Three different SAR techniques, namely conventional interferometry, SAR pixel offsets (SPO) and multiple-aperture InSAR (MAI), were employed to obtain the coseismic surface deformation fields along the ∼30 km length of the fault rupture. Along-track measurements from SPO and MAI techniques show a high correlation, and were subsequently used to precisely determine the location and extent of the surface fault trace. The best-fitting fault model geometry derived from an iterative inversion technique suggests that the rupture occurred on a near-vertical sinistral strike-slip fault west of the Nam Ma fault with a strike of 70°. A maximum slip of 4.2 m occurs at a depth of 2.5 km, with significant slip constrained only to the upper 10 km of the crust.

The 2011 Tohoku earthquake caused high levels of crustal deformation in Japan. Analysis of satellite radar and GPS data show that the earthquake caused nearby volcanic regions to subside instantaneously, creating elliptical depressions that are parallel to the direction of quake-induced crustal extension.

After the 11 March 2011 M w 9.0 Tohoku earthquake, Japan, a swarm of shallow normal‐faulting earthquakes was triggered in a localized region beneath the border between Fukushima and Ibaraki prefectures in eastern Honshu. Here we examine the coseismic displacement field and a fault model of the largest event of the swarm, the 11 April 2011 M w 6.6 Iwaki earthquake. Radar interferometry applied to data of the Japanese ALOS satellite reveals complex ruptures associated with the Iwaki earthquake. In particular, the interferogram clearly shows multiple surface ruptures along each of the subparallel Yunodake and Itozawa faults. The slip distributions on these faults, as estimated by inverting the interferogram, suggest that the faults dip 60°–70° westward and that slip on the Itozawa fault extended to a splay fault at depths shallower than about 5 km. Our results indicate that the highland area west of the Itozawa fault was downthrown by up to 2.4 m during the Iwaki earthquake. Online Material: Original and de‐trended interferograms over a wider area.

[1] The ongoing eruption of the Lusi mud volcano in East Java, Indonesia offers the unprecedented opportunity to study a large eruption from its beginning to its eventual end. We use new observations of ground deformation obtained from multitemporal interferometric analysis of L-band synthetic aperture radar data to show that Lusi will stop erupting much sooner than previously anticipated. Using principal component analysis, we find that the rate of ground deformation, and by implication, pressure in the mud source region, has been decaying exponentially with an e-folding time scale of 2.1 ± 0.4 years. We anticipate that discharge will decrease to 10% of the present rate in 5 years.

The relative motion between the Caribbean and North American plates is accommodated by several active faults around Hispaniola Island1, 2, 3. The Enriquillo–Plantain Garden fault in southern Haiti is one of these structures2, 4. Strain equivalent to a magnitude 7.2 earthquake is estimated to have accumulated along this fault since its last significant activity4. The Haiti earthquake of 12 January 2010 was initially reported to have occurred along this fault5, 6, but more recent studies proposed slips on previously unrecognized, neighbouring faults5, 7, 8. Here we use interferometric synthetic aperture radar data to show that surface deformation caused by the earthquake does not correspond to the present topography. Alluvial fan deltas were uplifted on the north side of the Enriquillo–Plantain Garden fault, whereas mountains located on the south side of the fault subsided, implying that faults other than the Enriquillo–Plantain Garden fault were responsible for the deformation. To determine fault structure, we fit the satellite surface deformation data to a fault model. We show that slip occurred on a fault dipping northward at 42°, with large thrust components. The maximum displacement on the fault was about 4 m at 10–20 km depth, offshore from the Tiburon peninsula. We confirm that the earthquake ruptured a blind thrust fault and show that the fault could not be identified from large-scale present-day topography.

We analyzed ALOS/PALSAR images along eight paths acquired before and after the mainshock of an earthquake in Wenchuan, in the Sichuan province of China, May 12, 2008. We found entire coseismic deformation features and detected many fringes in an approximately 100-km wide region surrounding the Longmen Shan fault zone. We found 6–7 broad elliptic fringes north of the epicenter, which indicated around 70–80-cm increase in range (distance between the ground and satellite). This pattern corresponded to subsidence or eastward shift of ground, which is often seen above the lower margin of a low-angled thrust. We also counted 8–9 fringes, i.e., 90–100-cm range decrease, between the fault zone and Chengdu. Chengdu is located roughly 50 km away from the Pengguan fault, which is the southernmost trace of the Longmen Shan fault zone. As this area is located on the footwall side, the pattern suggested a westward shift of the earth’s surface. However, we could not obtain high correlations due to large displacements in the about 240 km long belt along the surface faults. Several concentric fringes were found along the Beichuan fault, which is the central trace of the Longmen Shan fault zone. This implied that the rupture process of this earthquake was complicated. We also performed pixel matching to estimate offsets in the region of low correlation. Positive and negative range offsets larger than 2 m were found on the north and south sides of the Beichuan fault, respectively. We observed a belt of negative range and azimuth offsets between the Beichuan and Pengguan faults. We calculated an interferogram using a multi-segment fault model with uniform slip along the Beichuan and Pengguan faults and estimated the slip distribution. Based on previous seismological studies, we assumed that the southwestern part dipped toward the northwest by 33 degrees, whereas the central and northeastern parts had a much steeper dip angle. We found two regions of slips larger than 4 m in the southwest and central parts of the Beichuan fault. The Pengguan segments may have much smaller slips (around 1 m). The southwestern segments of the Beichuan fault and the entire Pengguan fault had dominant thrust components, whereas dextral slip prevailed in the central and northeastern segments of the Beichuan fault. The estimated geodetic moment was 5.87 × 1020Nm (Mw = 7.78).

A mud volcano LUSI initiated its eruption on 29 May 2006, adjacent to a hydrocarbon exploration well in East Java. Ground subsidence in the vicinity of the LUSI eruptive vent was well recorded by a Synthetic Aperture Radar (SAR) PALSAR onboard the Japanese ALOS satellite. We apply an Interferometric SAR (InSAR) technique on ten PALSAR data scenes, acquired between 19 May 2006 and 21 May 2007, in order to obtain continuous maps of ground displacements around LUSI. Although the displacements in the area closest to the eruptive vent (spatial extension of about 1.5 km) are not detectable because of the erupted mud, all the processed interferograms indicate subsidence in an ellipsoidal area of approximately 4 km (north–south) × 3 km (east–west), centered at the main eruptive vent. In particular, interferograms spanning the first four months until 4 Oct. 2006 and the subsequent 46 days between 4 Oct. 2006 and 19 Nov. 2006 show at least about 70 cm and 80 cm of displacements away from the satellite, respectively. Possible causes of the subsidence, i.e., 1) loading effect of the erupted mud, 2) creation of a cylindrical mud conduit, and 3) pressure decrease and depletion of materials at depth, are investigated. The effects of the first two causes are found to be insufficient to explain the total amount of subsidence observed in the first six months. The third possibility is quantitatively examined using a boundary element approach by modeling the source of deformation as a deflating oblate spheroid. The spheroid is estimated to lie at depths of a few hundred to a thousand meters. The estimated depths are significantly shallower than determined from analyses of erupted mud samples; the difference is explained by presence of significant amount of inelastic deformation including compaction and downward transfer of material.

The stresses at transducer contacts were estimated from accurate particle velocity measurements by using a laser Doppler vibrometer (LDV). We then evaluated the performance of LDV for ultrasonic waveform measurements in physical model experiments that are employed for modeling seismic observations. For such experiments, the characteristics of the source and detector should be exactly known. Disc-shaped compression and shear-mode piezoelectric transducers were attached on a flat surface of a steel semicircular column, and ultrasonic waves were excited by single-shot sine waves with 0.25, 0.5, and 1 MHz frequencies. Radial and transverse components were measured by LDV at a distance of 150 mm from the source. The maximum amplitudes of waves with respect to radiation angle give a radiation pattern of a transducer. Each observed radiation pattern was fitted to the calculated radiation pattern by assuming a harmonically oscillating stress distributed uniformly on a flat circular area. The observed radiation patterns show fairly good agreement with the calculated radiation patterns for both radial and transverse components when the source frequencies are 0.25 and 0.5 MHz. Because the best-fit stress values were independently estimated from the radial and the transverse radiation patterns, the 2 stress values should be equal for each source and frequency. The discrepancy between the estimated radial and transverse stress values becomes larger as the source frequency increases. Provided that coincidence of the 2 stress values indicates the validity of waveform measurements, the results suggest that LDV is applicable for measuring the 3-D particle-velocity at frequencies up to 0.5 MHz.

Piton de La Fournaise is in a period of intense volcanic activity since 1998. To constrain the magma dynamics responsible for this activity, we combined GPS ground deformation monitoring interpreted through numerical modelling and geochemistry. Two cycles of continuous volcano inflation are evidenced for the May 2004–December 2005 period, with a rest from March to October 2005. These inflations are consistent with two cycles of compatible major element enrichment in the emitted lavas. Numerical models indicate that the pressurization of a single magma reservoir may be responsible for the observed pre-eruptive inflations of the volcano. The reservoir, located at 2300 m depth, has a radius of ∼ 500 m. At the beginning of each cycle, dykes propagate from the roof of the reservoir and yield eruptions of differentiated basalt near the summit. At the end of the cycle, dykes propagate from the eastern sidewall of the reservoir and yield distal eruptions of primitive magmas away from the summit. The volumes of magma emitted during the primitive eruptions seem too large to explain the surface deformations and therefore suggest some refill of the reservoir by deeper magmas. Our results may be used to predict the location and lava volume of future eruptions at Piton de La Fournaise volcano, depending on the timing of these eruptions within a cycle of volcanic activity.

The 2007 Noto Hanto earthquake (M w 6.6) is the first major earthquake in inland Japan since the launch of the Japanese ALOS satellite. Interferometric synthetic aperture radar successfully mapped the coseismic displacements from both ascending and descending orbits in and around the earthquake epicentral region. The line-of-sight displacements toward the satellite from the ascending and descending orbits amount to about 50 and 20 cm, respectively. The interferograms as well as the GPS data of the GEONET network are inverted in order to determine the slip distribution on the source fault. The dip and strike angles of the preferred model are 48° and N51°E, respectively, and its fault slip area reaches the seafloor. The dip angle is not well constrained in our tested range between 40° and 60°. The dominant slip area is located just above the hypocenter, at depths between 4 and 10 km. Many aftershocks seem to have occurred on and around the area of dominant slip.

To understand the scattering characteristics of seismic shear waves in the Earth, a laboratory physical model experiment in ultrasonic frequency range was performed. By analyzing waveform envelopes and particle motions of shear waves in media with different correlation distance and fluctuation intensity, we studied the strength of scattered wave excitation and the distortion of shear-wave polarization as a function of ka (wavenumber times the correlation distance). We used gabbro and granite as media having different small-scale random heterogeneities. The granite we used contained preferred-oriented thin microcracks. The correlation distance a and the standard deviation of the fractional fluctuation of the shear-wave velocity ϵ were estimated by fitting exponential-type autocorrelation functions. The estimated values were a = 0.84 mm and ϵ = 8.1% for the gabbro, and a = 0.39 mm and ϵ = 17.0% for the granite. Waveform measurements were performed in the frequency range 0.25-1 MHz, which roughly corresponds to ka as 0.25-1. Envelope broadening appeared when ka exceeds 0.4 for the granite and 1.4 for the gabbro. Shear-wave polarizations were distorted by scattering when ka exceeds 0.2 for the granite and 0.7 for the gabbro. The results on envelope broadening suggest the importance of large-angle scattering due to small-scale random heterogeneities and cracks in the envelope broadening, in addition to diffraction effects due to large-scale heterogeneities. The effect of aligned microcracks on scattering was also examined. It was found that scattering was prominent when shear wave propagates perpendicular to the aligned crack plane. Manuscript received 1 March 2002.

SUMMARY Using laboratory-scale physical models, we studied seismic waveform fluctuation in 3-D heterogeneous media, and evaluated the effects of short-wavelength random heterogeneity on seismic waves. We made two series of experiments: (1) waves are excited by the same source signal and propagate through three kinds of granitic rocks with different scales of heterogeneity, and (2) waves are excited by different frequencies but propagating through the same rock. A compression-mode piezoelectric transducer was attached to one side of the block, and the waveforms were measured at a 10 mm radius circular array with a 2 ° spacing, located on the other side. 180 waveforms are obtained at equal source–receiver distances with an equal wave radiation from the source. The source signals are one- or two-cycle sine-wave pulse with 0.5 MHz for different heterogeneities, and 0.25, 0.5, 1 and 2 MHz for the same heterogeneity. Waveforms were recorded by using a system equipped with a laser Doppler vibrometer as a sensor of elastic waves. The rock heterogeneities were investigated as the 1-D velocity fluctuations obtained from the 2-D microstructure images of the rock. By applying the exponential autocorrelation function, heterogeneities in the three rocks are characterized by similar fluctuation intensities (7.9–9.3 per cent) but different scale lengths ranging from 0.22 to 0.92 mm. We calculated the spatial correlation of waveforms, the correlation between an averaged waveform and observed waveforms, the energy partition with respect to lapse time, and the statistical distributions of the waveform parameters: correlation coefficients, traveltimes of P wave, and the log values of the P-phase energy. Correlation of waveforms, energy partition and statistical parameters of waveforms are investigated as a function of the normalize scale parameter ka: the product of wave number and the characteristic scale length of heterogeneity. We found that waveform parameters change the trends at ka∼0.2–0.3. When ka exceeds 0.2–0.3, scattered waves become strong and waveforms become more complicated. This may indicate a transition from an equivalent homogeneous medium to a scattering medium for seismic waves propagating through random heterogeneous medium.

SUMMARY The structure of the Earth is represented by a wide spectrum of small- and large-scale structures. However, tomographic imaging techniques based on ray theory are often applied inappropriately in models with a characteristic length of heterogeneity smaller than the wavelength and width of the Fresnel zone. In other words, the conditions for ray theory are not satisfied in such models. It is therefore necessary to apply the diffraction theory of waves in tomographic reconstruction techniques in order to retrieve images of the Earth with a more general theory for wave propagation than ray theory. Physically speaking, scattering theory takes the finite-frequency effect of waves into account. We performed a test of ray theory and scattering theory in an ultrasonic wave experiment and in a numerical finite-difference experiment using random media with correlation lengths smaller than the width of the Fresnel zone. We used a stochastic approach to compute the mean squared value of time-shift variations calculated from ray theory and diffraction theory. The theoretical results were compared with the experimental values obtained in the laboratory experiment using rock samples with different length-scales of heterogeneity and from numerical experiments on wave propagation in quasi-random media. We observed that ray theory systematically overestimates the mean squared value of time-shift variations, while the observed statistical values from the laboratory experiments are well predicted by scattering theory. This means that tomographic imaging techniques based on ray theory suffer from a loss of resolution when the reconstructing models have a characteristic length of heterogeneity smaller than the width of the Fresnel zone.

In order to study the effect of heterogeneity on the arrival time and energy fluctuations of seismic waves, we made laboratory-scale physical model experiments by observing elastic waves propagating through crystalline rock samples having different scales of heterogeneity. A laser Doppler vibrometer was used to measure the waveforms. About 100 waveforms were obtained over a square grid with a spacing of 1 mm in linear profiles for Westerly and Oshima granite samples, which correspond to small and large scales of heterogeneity, respectively. The same measurement was done for steel block, which is a homogeneous medium. Characteristic scale length of heterogeneity of Westerly and Oshima granites are found to be 0.22 mm and 0.46 mm, respectively; the values were obtained from spatial autocorrelation function of velocity fluctuation based on microstructure photo image. The arrival time of P wave was determined by searching the best-fit point that divides the waveform into two different autoregressive (AR) models based on Akaike information criterion (AIC). This method automatically yields the arrival time of P waves, and the fluctuation of arrival time can be studied as a statistical distribution. We also calculated logarithmic values of the wave energy and studied their fluctuation. The observed fluctuations of the arrival time and the log of wave energy show nonsymmetric distributions. Both the variances of arrival time and log-energy distributions increase with increasing scale length of heterogeneity. This indicates a positive correlation between the variances of the fluctuation and the scale length of heterogeneity. The study also reveals that the correlation coefficient between fluctuations of arrival time and energy of seismic wave increases with increasing length of heterogeneity.

The Tohoku earthquake on March 11, 2011, caused a remarkably large deformation on the island of Honshu, Japan. By analyzing ALOS/PALSAR data, a range increase of up to 3.6 m at the tip of the Oshika Peninsula, the closest point to the epicenter, was detected from ascending orbits. Combining ascending and descending interferograms, this peninsula was confirmed to have subsided and shifted eastward. This deformation may have been caused by a huge reverse slip on the plate interface near the trench axis.

[1] At basaltic volcanoes, magma is transported to the surface through dikes (magma-filled fractures), but the evolution of these dikes as eruptions proceed is rarely documented. In March 1998, after five and a half years of quiescence, Piton de la Fournaise volcano (Reunion Island) entered into a new eruptive phase characterized by intense eruptive activity. Coeruptive displacements recorded by interferometric synthetic aperture radar (InSAR) for the first five eruptions of the cycle are analyzed using 3-D boundary element models combined with a Monte Carlo inversion method. We show that the eruptions are associated with the emplacement of lateral dikes rooted at depths of less than about 1000 m, except for the first March 1998 event where an additional deeper source is required. The dikes are located above preeruptive seismic swarms. This is consistent with nearly isotropic stress caused by repeated dike intrusions and low confining pressure enhanced by the presence of pores in the shallowest 1000 m of the edifice. The volumes of the modeled dikes represent 17% of the volume of emitted lava, showing that exogenous growth plays a major role in building the volcano. By taking into account the preeruptive seismicity and tilt data together with the results of InSAR data modeling, we find that dikes first propagate vertically from a source region below sea level before being injected laterally at shallow depth. This behavior is consistent with the presence of levels of neutral buoyancy at shallow depth in the edifice.

Subsurface structures contain small‐scale random heterogeneities which generate scattered waves. Considerable fluctuations appear in seismic waveform when heterogeneity scales are comparable or a little smaller than the dominant seismic wavelengths. Waveform fluctuation by scattering can be examined in laboratory experiments by using random heterogeneous materials as scale models. A laser Doppler vibrometer is used to accurately record waveforms propagating through a model specimen. By taking advantages of laboratory experiments, one can reveal some quantitative relationships between wave fluctuations and the intensity or the characteristic scale length of random heterogeneity. Variations in travel times, fluctuations of amplitude, phase, and particle‐motion, as well as envelope formation are examined with respect to the statistical properties of random heterogeneities. The variations can be characterized in terms of the scale‐invariant values, ka and kL, the wavelength‐normalized values for the characteristic scale length of heterogeneity and the wave travel distance, respectively. On the basis of experimental results, we obtain the boundary between equivalent homogeneous media approach and scattering random media approach. The two different approaches come from two different properties of the same medium in wave propagation problems, depending on the values of ka and kL. The boundary is critical in seismic imaging techniques, because strong scattered waves degrade the seismic signals used for imaging and deteriorate the image quality.

International audience; Dike intrusions often cause complex ground displacements that are not sufficiently explained by simple analytical models. We develop a method to find complex and realistic dike geometries and overpressures from interferometric synthetic aperture radar (InSAR) data. This method is based on a combination of a boundary element method with realistic topography and a neighborhood algorithm inversion. Dike model geometry is roughly a quadrangle with its top reaching the ground. The inversion has two stages: search and appraisal. The appraisal stage involves calculations of model marginal probability density functions using misfit values calculated during the search stage. The misfit function takes into account the variance and correlation of data noise. Synthetic tests show that a model is successfully retrieved within predicted narrow confidence intervals. We apply the method on InSAR data of the February 2000 flank eruption at Piton de la Fournaise and get a trapezoid dike dipping seaward (61.0°-67.3°) with its bottom passing 800-1000 m beneath the summit. A model with a basal slip plane does not better explain observed asymmetric displacements, and thus this asymmetry is solely attributed to the dipping dike. The dike lies above a narrow band of preeruption seismicity, suggesting that lateral magma propagation occurred. Neglecting topography results in poor modeling at depth and in overestimations of overpressure (or opening), height (both about 30%), and volume (about 20%).

[1] Three independent ASAR interferograms spanning the August 2003 Piton de la Fournaise eruption, reveal a 3 by 3 km asymmetric pattern of range changes centred on the Dolomieu crater northern flank. It corresponds to 30 cm of displacement towards the satellite east of the eruptive fissures and 7 cm away from the satellite west of the fissures. Displacements are caused by dyke emplacement below fissures. We model the deformation using a 3D mixed boundary element method for elastic media. This consists of a dyke defined by six geometric parameters and an overpressure gradient. A neighbourhood algorithm was applied to explore this 7 dimensional parameter space. The best-fit model is a 57° eastward dipping dyke with a base lying around 1520 m a.s.l. The model provides new evidence of the dyke intrusion – related seaward displacements of the volcano eastern flank.

The termination of the Japanese Earth Resources Satellite (JERS 1) in October 1998 and the subsequent delay of the launch of its successor, the Advanced Land Observing Satellite (ALOS), caused a troublesome interruption in the availability of L-band synthetic aperture radar (SAR) data to the several research institutions and government agencies in Japan that had been studying surface deformation by SAR techniques since the Japan Aerospace Exploration Agency (JAXA, formerly the National Space Development Agency of Japan) launched JERS 1 in 1992. SAR is an all-weather type of sensor that is suitable for the derivation of disaster mitigation information, and—as shown, for example, by Massonnet and Feigl [1998]—interferometric SAR (InSAR) techniques can be used to derive precise ground surface deformation maps associated with earthquakes, volcanism, landslides, and slow tectonic motions, as well as with oil pumping and water withdrawal, with fine spatial accuracy. The electromagnetic similarity (coherence) between two SAR images acquired over the same area before and after an event—for instance, an earthquake—is measured, and where appropriately high coherence is achieved, the change in the distance between SAR and the ground target can be derived with fine (centimeter) precision. Both airborne and spaceborne SAR systems are commonly used, although the latter often are preferred due to their long-term monitoring capacity and global coverage.