Cm Of Uplift Research Articles

Optimizing Injection Well Trajectory to Maximize CO2 Storage Security and Minimize Geomechanical Risk

Summary The work demonstrates an optimal well design for a potential geological carbon storage (GCS) project in Kern County, California (USA). Carbon dioxide (CO2) plume shape, size, and pressure response history in the subsurface are outcomes. We created a toolbox (pyCCUS) to standardize the well design optimization process and it is applicable to different carbon storage assets. This toolbox is helpful to maximize storage security and minimize geomechanical risk. The numerical model of the storage formation features two-way coupled transport and geomechanical deformation. It honors a predefined injection scheme with injection rates that ramp up and then decline for a total of 12.3 MtCO2 injection in 18 years. The peak injection rate is greater than 1 MtCO2/yr, whereas the post-injection monitoring period is 100 years. We propose to develop a long, deviated injection well to best address the injectivity and plume migration challenges for this complex, heterogeneous, dipping formation. The chosen well trajectory improves injectivity while minimizing formation pressure buildup. The well design optimization successfully reduces the pressure buildup by 54% over the base design while only increasing the areal extent of the plume by 21%. We quantify the CO2 plume shape and size at the land surface. The plume grows rapidly during injection, but it increases only slightly after shut-in due to slow updip migration driven by buoyancy. The plume becomes stationary within the post-injection monitoring period. The optimal injector design balances the optimization goals of CO2 plume size, pressure increase, and pressure buildup at geological faults. The optimal injection well design is robust under uncertainties from injection schemes and geological model realizations. Rock deformation due to the pressure buildup is also computed. The model estimates 2.1 cm of uplift that occurs during the year of the peak annual injection rate. Land surface uplift strongly correlates with the subsurface pressure response.

Surface Deformation and Seismicity Linked to Fluid Injection in the Raton Basin.

It is suggested that in addition to seismicity deep fluid injection may cause surface uplift and subsidence in oil and gas-producing regions. This study uses the Raton Basin as an example to investigate the hydromechanical processes of surface uplift and subsidence during wastewater injection. The Raton Basin, in southern central Colorado and northern central New Mexico, has experienced wastewater injection related to coalbed methane and gas production starting in 1994. In this study, we estimate the extent and magnitude of total vertical deformation in the Raton Basin from 1994 to 2020 and incremental deformation between the years 2017 to 2020. Results indicate a modeled uplift as much as 15 cm occurring between 1994 and 2020. Between 2017 and 2020, up to 3 cm of uplift occurred, largely near the northwestern injection wells. Most modeled uplift between 1994 and 2020 occurred near the southern wells, where the greatest cumulative volume of wastewater was injected. However, modeled subsidence occurred around the southern and eastern wells between 2017 and 2020, after the rate of injection decreased. Modeling indicates that while the magnitude of modeled uplift corresponds to cumulative injection volume and maximum rate in the long-term, short-term incremental deformation (uplift or subsidence) is controlled by changes in the rate of injection. The number of yearly earthquake events follows periods of rapid modeled uplifting throughout the Basin, suggesting that measurable surface deformation may be caused by the same injection-induced pore pressure perturbations that initiate seismicity.

Using Seafloor Geodesy to Detect Vertical Deformation at the Hikurangi Subduction Zone: Insights From Self‐Calibrating Pressure Sensors and Ocean General Circulation Models

AbstractSeafloor pressure sensor data is emerging as a promising approach to resolve vertical displacement of the seafloor in the offshore reaches of subduction zones, particularly in response to slow slip events (SSEs), although such signals are challenging to resolve due to sensor drift and oceanographic signals. Constraining offshore SSE slip distribution is of key importance to understanding earthquake and tsunami hazards posed by subduction zones. We processed seafloor pressure data from January to October 2019 acquired at the Hikurangi subduction zone, offshore New Zealand, to estimate vertical displacement associated with a large SSE that occurred beneath the seafloor array. The experiment included three self‐calibrating sensors designed to remove sensor drift, which, together with ocean general circulation models, were essential to the identification and correction of long‐period ocean variability remaining in the data after applying traditional processing techniques. We estimate that long‐period oceanographic signals that were not synchronous between pressure sensors and reference sites influenced our inferred displacements by 0.3–2.6 cm, suggesting that regionally deployed reference sites alone may not provide sufficient ocean noise correction. After incorporating long‐period ocean variability corrections into the processing, we calculate 1.0–3.3 cm of uplift during the SSE offshore Gisborne at northern Hikurangi, and 1.1–2.7 cm of uplift offshore the Hawke's Bay area at central Hikurangi. Some Hawke Bay displacements detected by pressure sensors near the trench were delayed by 6 weeks compared to the timing of slip onset detected by onshore Global Navigation Satellite System sites, suggesting updip migration of the SSE.

Dynamics of Episodic Magma Injection and Migration at Yellowstone Caldera: Revisiting the 2004–2009 Episode of Caldera Uplift With InSAR and GPS Data

AbstractThe 2004–2009 caldera uplift is the largest instrumentally recorded episode of unrest at Yellowstone caldera. We use GPS and Interferometric Synthetic Aperture Radar (InSAR) time series spanning 2004–2015, with a focus in the aforementioned event to understand the mechanisms of unrest. InSAR data recorded ∼25 and ∼20 cm of uplift at the Sour Creek (SCD) and Mallard Lake (MLD) resurgent domes during 2004–2009, and ∼8 cm of subsidence at the Norris Geyser Basin (NGB) during 2004–2008. The SCD/MLD uplift was followed by subsidence across the caldera floor with a maximum at MLD of ∼1.5–2.5 cm/yr, and no deformation at NGB. The best‐fit source models for the 2004–2009 period are two horizontal sills at depths of ∼8.7 and 10.6 km for the caldera source and NGB, respectively, with volume changes of 0.354 and −0.121 km3, and an overpressure of ∼0.1 MPa. The InSAR and GPS time series record exponentially increasing followed by exponentially decreasing uplift between 2004 and 2009, which is indicative of magma injection into the caldera reservoir, with no need for other mechanisms of unrest. However, magma extraction from NGB to the caldera is unable to explain the subsidence coeval with the caldera uplift. Models of magma injection can also explain other episodes of caldera uplift like that in 2014–2015. Distributed sill opening models show that magma is stored across the caldera source with no clear boundary between MLD and SCD. Since the magma overpressure is orders of magnitude below the tensile strength of the encasing rock, historical episodes of unrest like these are very unlikely to trigger an eruption.

Renewed Posteruptive Uplift Following the 2011–2012 Rhyolitic Eruption of Cordón Caulle (Southern Andes, Chile): Evidence for Transient Episodes of Magma Reservoir Recharge During 2012–2018

AbstractThe VEI 4 rhyolitic eruption of Cordón Caulle volcano in 2011–2012 was immediately followed by ∼0.77 m of exponentially decaying uplift during 2012–2015. In this study, we present evidence of additional transient pulses of inflation with interferometric synthetic aperture radar (InSAR) time series during 2016–2018. We also assess whether the 2012–2015 uplift can be explained by viscoelastic relaxation or not. InSAR detected ∼12 cm of uplift during 2016–2017 and ∼5 cm during 2017–2018. The three inflation episodes have very similar spatial scales and can be modeled with the same inflating sill (z ∼ 6 km), but their time evolution is significantly different. Numerical models of a pressurized reservoir surrounded by a viscoelastic shell do not have a better fit than a magma injection model to the 2012–2015 InSAR time series, indicating that magma injection is the most likely mechanism to explain this uplift signal. The spatial similarities of the three pulses suggest that they are produced by episodic magma injection. These magma injection pulses provide the heat to remobilize the crystal mush beneath the volcano on timescales of a few months, but the end of the uplift is not predicted by existing models. None of these uplift pulses were related to abnormal seismicity, and we speculate that they are mostly aseismic because they caused stresses of lower magnitude than the coeruptive stresses. The meter scale displacement observed at Cordón Caulle between 2007 and 2018 suggests that the volcano undergoes episodic cycles of inflation like those observed in silicic calderas.

Hazard Implications of the 2016 Mw 5.0 Cushing, OK Earthquake from a Joint Analysis of Damage and InSAR Data

The Cushing Hub in Oklahoma, one of the largest oil storage facilities in the world, is federally designated as critical national infrastructure. In 2014, the formerly aseismic city of Cushing experienced a Mw 4.0 and 4.3 induced earthquake sequence due to wastewater injection. Since then, an M4+ earthquake sequence has occurred annually (October 2014, September 2015, November 2016). Thus far, damage to critical infrastructure has been minimal; however, a larger earthquake could pose significant risk to the Cushing Hub. In addition to inducing earthquakes, wastewater injection also threatens the Cushing Hub through gradual surface uplift. To characterize the impact of wastewater injection on critical infrastructure, we use Differential Interferometric Synthetic Aperture Radar (DInSAR), a satellite radar technique, to observe ground surface displacement in Cushing before and during the induced Mw 5.0 event. Here, we process interferograms of Single Look Complex (SLC) radar data from the European Space Agency (ESA) Sentinel-1A satellite. The preearthquake interferograms are used to create a time series of cumulative surface displacement, while the coseismic interferograms are used to invert for earthquake source characteristics. The time series of surface displacement reveals 4–5.5 cm of uplift across Cushing over 17 months. The coseismic interferogram inversion suggests that the 2016 Mw 5.0 earthquake is shallower than estimated from seismic inversions alone. This shallower source depth should be taken into account in future hazard assessments for regional infrastructure. In addition, monitoring of surface deformation near wastewater injection wells can be used to characterize the subsurface dynamics and implement measures to mitigate damage to critical installations.

Calibrating coseismic coastal land-level changes during the 2014 Iquique (Mw=8.2) earthquake (northern Chile) with leveling, GPS and intertidal biota

The April 1st 2014 Iquique earthquake (MW 8.1) occurred along the northern Chile margin where the Nazca plate is subducted below the South American continent. The last great megathrust earthquake here, in 1877 of Mw ~8.8 opened a seismic gap, which was only partly closed by the 2014 earthquake. Prior to the earthquake in 2013, and shortly after it we compared data from leveled benchmarks, deployed campaign GPS instruments, continuous GPS stations and estimated sea levels using the upper vertical level of rocky shore benthic organisms including algae, barnacles, and mussels. Land-level changes estimated from mean elevations of benchmarks indicate subsidence along a ~100-km stretch of coast, ranging from 3 to 9 cm at Corazones (18°30’S) to between 30 and 50 cm at Pisagua (19°30’S). About 15 cm of uplift was measured along the southern part of the rupture at Chanabaya (20°50’S). Land-level changes obtained from benchmarks and campaign GPS were similar at most sites (mean difference 3.7±3.2 cm). Higher differences however, were found between benchmarks and continuous GPS (mean difference 8.5±3.6 cm), possibly because sites were not collocated and separated by several kilometers. Subsidence estimated from the upper limits of intertidal fauna at Pisagua ranged between 40 to 60 cm, in general agreement with benchmarks and GPS. At Chanavaya, the magnitude and sense of displacement of the upper marine limit was variable across species, possibly due to species—dependent differences in ecology. Among the studied species, measurements on lithothamnioid calcareous algae most closely matched those made with benchmarks and GPS. When properly calibrated, rocky shore benthic species may be used to accurately measure land-level changes along coasts affected by subduction earthquakes. Our calibration of those methods will improve their accuracy when applied to coasts lacking pre-earthquake data and in estimating deformation during pre–instrumental earthquakes.

Principal component analysis of MSBAS DInSAR time series from Campi Flegrei, Italy

Because of its proximity to the city of Naples and with a population of nearly 1 million people within its caldera, Campi Flegrei is one of the highest risk volcanic areas in the world. Since the last major eruption in 1538, the caldera has undergone frequent episodes of ground subsidence and uplift accompanied by seismic activity that has been interpreted as the result of a stationary, deeper source below the caldera that feeds shallower eruptions. However, the location and depth of the deeper source is not well-characterized and its relationship to current activity is poorly understood. Recently, a significant increase in the uplift rate has occurred, resulting in almost 13cm of uplift by 2013 (De Martino et al., 2014; Samsonov et al., 2014b; Di Vito et al., 2016). Here we apply a principal component decomposition to high resolution time series from the region produced by the advanced Multidimensional SBAS DInSAR technique in order to better delineate both the deeper source and the recent shallow activity. We analyzed both a period of substantial subsidence (1993–1999) and a second of significant uplift (2007–2013) and inverted the associated vertical surface displacement for the most likely source models. Results suggest that the underlying dynamics of the caldera changed in the late 1990s, from one in which the primary signal arises from a shallow deflating source above a deeper, expanding source to one dominated by a shallow inflating source. In general, the shallow source lies between 2700 and 3400m below the caldera while the deeper source lies at 7600m or more in depth. The combination of principal component analysis with high resolution MSBAS time series data allows for these new insights and confirms the applicability of both to areas at risk from dynamic natural hazards.

Repeated magmatic intrusions at El Hierro Island following the 2011–2012 submarine eruption

After more than 200years of quiescence, in July 2011 an intense seismic swarm was detected beneath the center of El Hierro Island (Canary Islands), culminating on 10 October 2011 in a submarine eruption, 2km off the southern coast. Although the eruption officially ended on 5 March 2012, magmatic activity continued in the area. From June 2012 to March 2014, six earthquake swarms, indicative of magmatic intrusions, were detected underneath the island. We have studied these post-eruption intrusive events using GPS and InSAR techniques to characterize the ground surface deformation produced by each of these intrusions, and to determine the optimal source parameters (geometry, location, depth, volume change). Source inversions provide insight into the depth of the intrusions (~11–16km) and the volume change associated with each of them (between 0.02 and 0.13km3). During this period, >20cm of uplift was detected in the central-western part of the island, corresponding to approximately 0.32–0.38km3 of magma intruded beneath the volcano. We suggest that these intrusions result from deep magma migrating from the mantle, trapped at the mantle/lower crust discontinuity in the form of sill-like bodies. This study, using joint inversion of GPS and InSAR data in a post-eruption period, provides important insight into the characteristics of the magmatic plumbing system of El Hierro, an oceanic intraplate volcanic island.

The 22 February 2014 Mw 4.1 Bordj-Menaiel Earthquake, Near Boumerdes-Zemmouri, North-Central Algeria

A number of destructive earthquakes have occurred in the region of Algiers, the capital of Algeria. Two well‐known historical earthquakes were recorded in 1365 and 1716 (Harbi et al. , 2004). A more recent one, the 2003 M w 6.9 Boumerdes earthquake (also called the Zemmouri earthquake), took place in the Boumerdes district (Fig. 1), about 50 km east of Algiers (Yelles‐Chaouche et al. , 2003). The Boumerdes earthquake occurred on a blind‐thrust fault offshore from the Algerian coast. The rupture was along a northeast–southwest‐striking fault plane, although the precise fault trace was unknown. During this earthquake, an average value of 55 cm of uplift of the shoreline was observed (Meghraoui et al. , 2004), consistent with the thrust focal mechanism, at least in the high‐slip region on the fault plane (Semmane et al. , 2005). Figure 1. The studied area and location of stations that recorded the earthquake used in this study (closed triangles are broadband stations and open triangles are one‐component short‐period stations). Dots show the seismic epicenters from 1992 to March 2014; the 2003 Boumerdes epicenter is represented by the black star, and the Bordj‐Menaiel earthquake is represented by the white star. The black lines are active faults mapped by Guemache (2010). The inset shows the basement in the region in gray. The locations of the numbered blocks 1 and 2 coincide with the two asperities found in the slip distribution models and also with the uplift zones (two thick lines on the coast). More recently, in the same district (Boumerdes), an earthquake of moment magnitude M w 4.1 ( M d 4.3) occurred on 22 February 2014 at 20:30 local time, 12 km southeast of the epicenter of the 2003 Boumerdes earthquake, and at a depth of 7–11 km. The epicenter was located 5 km northeast of the central part of the town of Bordj‐Menaiel and …

Stratovolcano growth by co‐eruptive intrusion: The 2008 eruption of Tungurahua Ecuador

Volcanic edifices are constructed by a combination of erupted material and internal growth. We use the L‐band satellite ALOS to produce InSAR measurements at Tungurahua Volcano, Ecuador. We find a maximum of 17.5 cm of uplift on the upper western flank between December 2007 and March 2008, coincident with an eruption in February 2008. The deformation can be modeled using an ellipsoidal or sill‐like source within the edifice. The models require an elongated aspect ratio with a length of 4–6 km. The intruded volume of 1.2 × 106 m3 is roughly equivalent to the bulk erupted volume of 1.5 × 106 m3 and together the values are roughly equal to the long‐term magma flux. The question remains whether this uplift is permanent, thus contributing to the internal growth of the edifice, or temporary, in which case the magma will be released in a future eruption.

Multiple inflation and deflation events at Kenyan volcanoes, East African Rift

The presence or absence of magma exerts a fundamental control on the distribution of strain in continental rift zones, yet the time scales of magma intrusion remain unconstrained. Using more than a decade of measurements from interferometric synthetic aperture radar (InSAR), we detected geodetic activity at four of the eleven central rift volcanoes in the Kenyan sector of the East African Rift. Subsidence of 2–5 cm occurred at Suswa and Menengai over the period 1997–2000, ~9 cm of uplift was recorded at Longonot in 2004–2006, and ~21 cm of uplift occurred at Paka during 2006–2007. The deformation is episodic, and no deformation was observed at these volcanoes during other time periods. Preceding the infl ation at Paka, we observed transient uplift and subsidence of a second, nearby source, likely associated with fl ow through a complex plumbing system. The best-fi tting source models for each episode include infl ation or defl ation of a horizontal penny-shaped crack at a depth of 2–5 km. The episodic nature of the activity, its lack of correlation with seasons, and the preferred source geometry are all consistent with activity in the volatile-rich cap to a crystal-rich magma chamber beneath each of the four volcanoes. The presence of active magmatic systems beneath more than 40% of the volcanoes, and the 2007–2008 explosive eruptions at nearby Oldoinyo Lengai volcano, have implications for models of continental rifting, caldera volcanoes, geothermal resources, and volcanic and seismic hazard in rift zones.

Geochronology and Geochemistry of the Kuwei Mafic Intrusion, Southern Margin of the Altai Mountains, Northern Xinjiang, Northwest China: Evidence for Distant Effects of the Indo‐Eurasia Collision

The Kuwei mafic intrusion, consisting of hornblende gabbro, gabbro, gabbro norite, and olivine norite, lies in the southern Altai Mountains, northern Xinjiang. A combined field, geochronological, and geochemical study of the Kuwei intrusion is reported here. This study provides the first reliable SHRIMP U‐Pb zircon dating results for the intrusion, and these yielded an age of $$47\pm 1$$ Ma, which is the first documented report of Eocene magmatism in the region. The chondrite‐normalized rare earth element patterns for the Eocene intrusions are flat, and most of the incompatible elements are comparably depleted. Thus, geochemical data suggest that the Kuwei mafic intrusion was produced by partial melting of asthenospheric mantle that was slightly contaminated by lithospheric material. We interpret the 47‐Ma magmatism to result from asthenospheric mantle upwelling following the progressive India‐Eurasian collision. Although the Kuwei intrusion is laterally beyond the limit of Eocene deformation normally attributed to the India‐Asia collision, the timing of magmatism in the intrusion suggests that lateral extension may have initially affected a wider region than the area later thickened by convergence in the Tibetan Plateau. The Kuwei intrusion and other plutons likely related to it may have been emplaced into dilational jogs in fault systems activated by the India‐Asia collision. The emplacement depth is estimated to be ∼6 km, based on geobarometric determinations. Erosion was imperceptible before 25 Ma but has worn away an average of 0.024 cm of uplift every year since 25 Ma. The 6 km of exhumation since the late Oligocene is also attributed to far‐field effects of the India‐Asia collision.

Seismicity associated with the 2004–2006 renewed ground uplift at Campi Flegrei Caldera, Italy

Following the significant ground uplift (∼1.8 m) of the 1982–1984 bradyseismic crisis, the recent history of Campi Flegrei volcanic complex (Italy) has been dominated by a subsidence phase. Recent geodetic data demonstrate that the subsidence has terminated, and that positive ground deformation renewed in November 2004, at a low but accelerating rate leading to about 4 cm of uplift by the end of October 2006. As in previous episodes, ground uplift has been accompanied by swarms of micro-earthquakes ( M ≤ 1.4) in three distinct episodes: October 2005, October 2006 and December 2006. Hypocenters of these earthquakes are mainly located beneath the Solfatara Volcano at depths ranging between 0.5 and 4 km. Inversion of S-wave spectra indicates source radius and stress drop on the order of 30–60 m and 10 4–9 × 10 5 Pa, respectively. Fault plane solutions indicate predominantly normal mechanisms. Accompanying the October 2006 swarm, we detected intense long-period (LP) activity for about 1 week. These signals consist of weak, monochromatic oscillations whose spectra exhibit a main peak at frequency ∼0.8 Hz. This peak is common to all the stations of the network, and not present in the noise spectra, suggesting that it is a source effect. About 75% of the detected LPs cluster into three groups of mutually similar events. Adjustment of waveforms using cross-correlation allows for precise alignment and stacking, which enhances signal onsets and permits accurate absolute arrival picks, and thus better absolute as well as relative locations. Locations associated with the three different clusters are very similar, and appear to delineate the SE rim of the Solfatara Volcano at a depth of about 500 m. The most likely source process for the LP events involves the resonance of a fluid-filled, buried cavity. Quality factors of the resonator cluster in a narrow interval around 4, which is consistent with the vibration of a buried cavity filled with a water-vapour mixture at poor gas-volume fractions. We propose a conceptual model to interpret the temporal and spatial patterns of the observed seismicity.

The 2005 eruption of Sierra Negra volcano, Galápagos, Ecuador

Sierra Negra volcano began erupting on 22 October 2005, after a repose of 26 years. A plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly followed by a ~2-km-long curtain of lava fountains. The eruptive fissure opened inside the north rim of the caldera, on the opposite side of the caldera from an active fault system that experienced an mb 4.6 earthquake and ~84 cm of uplift on 16 April 2005. The main products of the eruption were an `a`a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank. The `a`a flow grew in an unusual way. Once it had established most of its aerial extent, the interior of the flow was fed via a perched lava pond, causing inflation of the `a`a. This pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow, many of which were subsequently overridden by `a`a, as the crust slowly spread from the center of the pond and tumbled over the pahoehoe. The curtain of lava fountains coalesced with time, and by day 4, only one vent was erupting. The effusion rate slowed from day 7 until the eruption’s end two days later on 30 October. Although the caldera floor had inflated by ~5 m since 1992, and the rate of inflation had accelerated since 2003, there was no transient deformation in the hours or days before the eruption. During the 8 days of the eruption, GPS and InSAR data show that the caldera floor deflated ~5 m, and the volcano contracted horizontally ~6 m. The total eruptive volume is estimated as being ~150×106 m3. The opening-phase tephra is more evolved than the eruptive products that followed. The compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2.1 km beneath the caldera floor.

Source area and rupture parameters of the 31 December 1881 Mw = 7.9 Car Nicobar earthquake estimated from tsunamis recorded in the Bay of Bengal

On the morning of 31 December 1881 a submarine earthquake beneath the Andaman Islands generated a tsunami with a maximum crest height of 0.8 m that was recorded by eight tide gauges surrounding the Bay of Bengal. Since the earthquake occurred 8 years before the construction of the world's first teleseismic recording seismometer, little has been known about its rupture parameters or location. Waveform and amplitude modeling of the tsunami indicate that it was generated by a Mw = 7.9 ± 0.1 rupture on the India/Andaman plate boundary resulting in 10–60 cm of uplift of the island of Car Nicobar. The rupture consisted of two segments: the northern 40‐km‐long segment is separated from the southern 150‐km‐long segment by a 100‐km region corresponding to the westward projection of the West Andaman spreading center. The main rupture occurred between 8.5°N and 10°N with a total area of 150 km × 60 km dipping 20°E with a mean slip of 2.7 m. The recurrence time for 1881‐type events is estimated to be 114–200 years on the basis of inferred GPS convergence rates and inferred plate closure vectors, although slip partitioning in the region may extend this estimate by as much as 30%.

Preeruptive inflation and surface interferometric coherence characteristics revealed by satellite radar interferometry at Makushin Volcano, Alaska: 1993–2000

Pilot reports in January 1995 and geologic field observations from the summer of 1996 indicate that a relatively small explosive eruption of Makushin, one of the more frequently active volcanoes in the Aleutian arc of Alaska, occurred on 30 January 1995. Several independent radar interferograms that each span the time period from October 1993 to September 1995 show evidence of ∼7 cm of uplift centered on the volcano's east flank, which we interpret as preeruptive inflation of a ∼7‐km‐deep magma source (ΔV = 0.022 km3). Subsequent interferograms for 1995–2000, a period that included no reported eruptive activity, show no evidence of additional ground deformation. Interferometric coherence at C band is found to persist for 3 years or more on lava flow and other rocky surfaces covered with short grass and sparsely distributed tall grass and for at least 1 year on most pyroclastic deposits. On lava flow and rocky surfaces with dense tall grass and on alluvium, coherence lasts for a few months. Snow and ice surfaces lose coherence within a few days. This extended timeframe of coherence over a variety of surface materials makes C band radar interferometry an effective tool for studying volcano deformation in Alaska and other similar high‐latitude regions.

Synthetic aperture radar interferometry of Okmok volcano, Alaska: Radar observations

ERS‐1/ERS‐2 synthetic aperture radar interferometry was used to study the 1997 eruption of Okmok volcano in Alaska. First, we derived an accurate digital elevation model (DEM) using a tandem ERS‐1/ERS‐2 image pair and the preexisting DEM. Second, by studying changes in interferometric coherence we found that the newly erupted lava lost radar coherence for 5–17 months after the eruption. This suggests changes in the surface backscattering characteristics and was probably related to cooling and compaction processes. Third, the atmospheric delay anomalies in the deformation interferograms were quantitatively assessed. Atmospheric delay anomalies in some of the interferograms were significant and consistently smaller than one to two fringes in magnitude. For this reason, repeat observations are important to confidently interpret small geophysical signals related to volcanic activities. Finally, using two‐pass differential interferometry, we analyzed the preemptive inflation, coeruptive deflation, and posteruptive inflation and confirmed the observations using independent image pairs. We observed more than 140 cm of subsidence associated with the 1997 eruption. This subsidence occurred between 16 months before the eruption and 5 months after the eruption, was preceded by ∼18 cm of uplift between 1992 and 1995 centered in the same location, and was followed by ∼10 cm of uplift between September 1997 and 1998. The best fitting model suggests the magma reservoir resided at 2.7 km depth beneath the center of the caldera, which was ∼5 km from the eruptive vent. We estimated the volume of the erupted material to be 0.055 km3 and the average thickness of the erupted lava to be ∼7.4 m.

TheML5.3 Épagny (French Alps) earthquake of 1996 July 15: a long-awaited event on the Vuache Fault

The ML 5.3 Épagny earthquake that occurred on 1996 July 15 in the vicinity of Annecy (French Alps) was the strongest event to shake southeastern France in the last 34 years. Moderate to serious damage in the Annecy area is consistent with MSK intensities of VII–VIII. This earthquake occurred on the Vuache Fault, a geologically well-known, morphologically clear, NW-SE-trending strike-slip fault that links the southern Jura Mountains with the northern Subalpine chains. The hypocentre was located in Mesozoic limestones at shallow depths (1–3 km). The focal mechanism indicates left-lateral strike-slip motion on a N136°E-striking plane dipping 70° to the NE. Abundant field evidence was gathered in the days following the main shock. Several hundred aftershocks were recorded thanks to the rapid installation of a 16-station seismic network. All aftershocks occurred along the southernmost segment of the Vuache Fault, defining a 5-km-long, 3.5-km-deep, N130°E-striking rupture zone dipping 73° to the NE. The fault plane solutions of 60 aftershocks were found to be consistent with left-lateral slip on NW-SE-striking planes. At the SE tip of the aftershock zone we found ground cracks parallel to the fault close to the Annecy-Meythet airport runway; at the NW tip, near Bromines, we observed left-lateral displacement of concrete walls in a building. We also noticed flow changes in two springs close to that locality. Geodetic levelling across the fault revealed about 1 cm of uplift for the region north of the fault. The recording of aftershocks with a six-station accelerometric network showed that lacustrine deposits locally amplified the ground motion up to eight times, which explains how this moderate-magnitude shock could cause such heavy damage. Historical records draw attention to the central segment of the Vuache Fault, which has been locked for at least 200 years. Situated NW of the 1996 aftershock zone, between the Mandallaz and Vuache mountains, this segment forms a 12-km-long potential seismic gap where other M5 events or one single M6 event might occur.

Shallow and peripheral volcanic sources of inflation revealed by modeling two‐color geodimeter and leveling data from Long Valley Caldera, California, 1988–1992

We refined the model for inflation of the Long Valley caldera near Mammoth Lakes, California, by combining both geodetic measurements of baseline length and elevation changes. Baseline length changes measured using a two‐color geodimeter with submillimeter precision revealed that the resurgent dome started to reinflate in late 1989. Measurements between late 1989 and mid‐1992 revealed nearly 13 cm of extension across the resurgent dome. Geodetic leveling surveys with approximately 2‐mm precision made in late 1988 and in mid‐1992 revealed a maximum of about 8 cm of uplift of the resurgent dome. Two ellipsoidal sources satisfy both the leveling and two‐color measurements, whereas spherical point sources could not. The model's primary inflation source is located 5.5 km beneath the resurgent dome with the two horizontal axes being nearly equal in size and the vertical axis being 4 times the length of the horizontal axes. A second ellipsoidal source was added to improve the fit to the two‐color measurements. This secondary source is located at a depth between 10 and 20 km beneath the south moat of the caldera and has the geometry of an elongated ellipsoid or pipe that dips down to the northeast. In addition, the leveling data suggest dike intrusion beneath Mammoth Mountain during the 1988–1992 interval, which is likely associated with an intense swarm of small earthquakes during the summer of 1989 at that location. Our analysis shows the dike intrusion to be the shallowest of the three sources with a depth range of 1–3 km below the surface to the top of the intrusion.