Interseismic deformation above the Sunda Megathrust recorded in coral microatolls of the Mentawai islands, West Sumatra

The geomorphology and internal stratigraphy of modern coral microatolls show that all the outer arc Mentawai islands of West Sumatra have been subsiding over the past several decades. These same islands rose as much as 3 m during the giant megathrust earthquakes of 1797 and 1833, and the current subsidence probably reflects strain accumulation that will lead to future large earthquakes. Average subsidence rates over the past half century vary from 2 to 14 mm yr^(−1) and increase southwestward, toward the subduction trench. The pattern is consistent with rates of subsidence measured by a sparse network of continuously recording Global Positioning System (cGPS) stations and with locking of a 400-km-long section of the underlying subduction megathrust, between about 1°S and 4°S. This record of subsidence and tilting, extending nearly a century into the past, implies that the region is advancing toward the occurrence of another giant earthquake. However, evidence of episodic rather than steady subsidence reflects a behavior that is more complex than simple elastic strain accumulation and relief. Most prominent of these episodes is an extensive emergence/subsidence couplet in about 1962, which may be the result of rapid, aseismic slip on the megathrust, between the islands and the trench. Lower subsidence rates recorded by the corals since about 1985 may reflect failure on many small patches within the locked section of the megathrust

Interseismic coupling and seismic potential along the Central Andes subduction zone

We use about two decades of geodetic measurements to characterize interseismic strain build up along the Central Andes subduction zone from Lima, Peru, to Antofagasta, Chile. These measurements are modeled assuming a 3-plate model (Nazca, Andean sliver and South America Craton) and spatially varying interseismic coupling (ISC) on the Nazca megathrust interface. We also determine slip models of the 1996 M_w = 7.7 Nazca, the 2001 M_w = 8.4 Arequipa, the 2007 M_w = 8.0 Pisco and the M_w = 7.7 Tocopilla earthquakes. We find that the data require a highly heterogeneous ISC pattern and that, overall, areas with large seismic slip coincide with areas which remain locked in the interseismic period (with high ISC). Offshore Lima where the ISC is high, a M_w∼8.6–8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four M_w∼8.0 earthquakes in 1940, 1966, 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a M_w > 8.5 earthquake. The region where the Nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period (low ISC) and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years. In southern Peru, ISC is relatively high and the deficit of moment accumulated since the M_w∼8.8 earthquake of 1868 is equivalent to a magnitude M_w∼8.4 earthquake. Two asperities separated by a subtle aseismic creeping patch are revealed there. This aseismic patch may arrest some rupture as happened during the 2001 Arequipa earthquake, but the larger earthquakes of 1604 and 1868 were able to rupture through it. In northern Chile, ISC is very high and the rupture of the 2007 Tocopilla earthquake has released only 4% of the elastic strain that has accumulated since 1877. The deficit of moment which has accumulated there is equivalent to a magnitude M_w∼8.7 earthquake. This study thus provides elements to assess the location, size and magnitude of future large megathurst earthquakes in the Central Andes subduction zone. Caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic. Also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench, except offshore Lima where sea bottom geodetic measurements have been collected suggesting strong coupling

Gradients from GOCE reveal gravity changes before Pisagua Mw = 8.2 and Iquique Mw = 7.7 large megathrust earthquakes

Considerable improvements in the measurement of the Earth gravity field from GOCE satellite mission have provided global gravity field models with homogeneous coverage, high precision and good spatial resolution. In particular, the vertical gravity gradient (Tzz), in comparison to the classic Bouguer anomaly, defines more accurately superficial mass heterogeneities. Moreover, the correction of these satellitederived data from the effect of Earth topographic masses by means of new techniques taking into account the Earth curvature, improves results in regional analyses. In a recent work we found a correlation between Tzz and slip distribution for the 2010 Maule Mw= 8.8 earthquake. In the present work, we derive the vertical gravity gradient from the last GOCE only model, corrected by the topographic effect and also by the sediments on depocenters of the offshore region at the PerueChile margin, in order to study a spatial relationship between different lobes of the gravity derived signal and the seismic sources of large megathrust earthquakes. In particular, we analyze this relation for the slip models of the 1996 Mw = 7.7 Nazca, 2001 Mw = 8.4 Arequipa, 2007 Mw = 8.0 Pisco events and for the slip models of the 2014 Mw = 8.2 Pisagua and Mw = 7.7 Iquique earthquakes from Schurr et al. (2014), including the previously analyzed 2010 Mw = 8.8 Maule event. Then we find a good correlation between vertical gravity gradients and main rupture zones, correlation that becomes even stronger as the event magnitude increases. Besides this, a gravity fall in the gravity gradient was noticed over the area of the main slip patches at least for the two years before 2014 Mw = 8.2 Pisagua and Mw = 7.7 Iquique earthquakes. Additionally, we found temporal variations of the gravity field after 2010 Mw = 8.8 Maule event, related to the main patches of the slip distribution, and coseismic deformation. Therefore, we analyzed vertical gravity gradient field variations as an indirect measure of the variable seismic coupling finding a potential relationship between Tzz and the seismic b-value. These relationships exemplify the strong potential of the satellite only derived models as a predictive tool to determine potential seismic energy released in a subduction segment, determining the potential size of a potential rupture zone, and in particular internal slip distribution that allows inferring coseismic displacement field at surface

Heterogeneous coupling of the Sumatran megathrust constrained by geodetic and paleogeodetic measurements

Geodetic and paleogeodetic measurements of interseismic strain above the Sumatran portion of the Sunda subduction zone reveal a heterogeneous pattern of coupling. Annual banding in corals provides vertical rates of deformation spanning the last half of the 20th century, and repeated GPS surveys between 1991 and 2001 and continuous measurements at GPS stations operated since 2002 provide horizontal velocities. Near the equator, the megathrust is locked over a narrow width of only a few tens of kilometers. In contrast, the locked fault zone is up to about 175 km wide in areas where great interplate earthquakes have occurred in the past. Formal inversion of the data reveals that these strongly coupled patches are roughly coincident with asperities that ruptured during these events. The correlation is most spectacular for rupture of the M_w 8.7 Nias-Simeulue earthquake of 2005, which released half of the moment deficit that had accumulated since its previous rupture in 1861, suggesting that this earthquake was overdue. Beneath the Mentawai islands, strong coupling is observed within the overlapping rupture areas of the great earthquakes of 1797 and 1833. The accumulated slip deficit since these events is slowly reaching the amount of slip that occurred during the 1833 earthquake but already exceeds the slip that occurred during the 1797 earthquake. Thus, rerupture of part of the Mentawai patch in September 2007 was not a surprise. In contrast, coupling is low below the Batu islands near the equator and around Enggano island at about 5°S, where only moderate earthquakes (M_w < 8.0) have occurred in the past two centuries. The correlation of large seismic asperities with patches that are locked during the interseismic period suggests that they are persistent features. This interpretation is reinforced by the fact that the large locked patches and great ruptures occur beneath persistent geomorphologic features, the largest outer arc islands. Depth- and convergence-rate-dependent temperature might influence the pattern of coupling, through its effect on the rheology of the plate interface, but other influences are required to account for the observed along-strike heterogeneity of coupling. In particular, subduction of the Investigator Fracture Zone could be the cause for the low coupling near the equator

Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls

Large uplifts and tilts occurred on the Sumatran outer arc islands between 0.5° and 3.3°S during great historical earthquakes in 1797 and 1833, as judged from relative sea level changes recorded by annually banded coral heads. Coral data for these two earthquakes are most complete along a 160-km length of the Mentawai islands between 3.2° and 2°S. Uplift there was as great as 0.8 m in 1797 and 2.8 m in 1833. Uplift in 1797 extended 370 km, between 3.2° and 0.5°S. The pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes (M_w) in the range 8.5 to 8.7. The region of uplift in 1833 ranges from 2° to at least 3.2°S and, judging from historical reports of shaking and tsunamis, perhaps as far as 5°S. The patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north, between 0.5° and 3°N, during the giant Nias-Simeulue megathrust earthquake of 2005; the outer arc islands rose as much as 3 m and tilted toward the mainland. Elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8.6 to 8.9. Sparse accounts at Padang, along the mainland west coast at latitude 1°S, imply tsunami runups of at least 5 m in 1797 and 3–4 m in 1833. Tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports. The tsunami modeling further indicates that the Indian Ocean tsunamis of both 1797 and 1833, unlike that of 2004, were directed mainly south of the Indian subcontinent. Between about 0.7° and 2.1°S, the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes. The interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833

The Arabidopsis thaliana K+-Uptake Permease 5 (AtKUP5) Contains a Functional Cytosolic Adenylate Cyclase Essential for K+ Transport

Potassium (K+) is the most abundant cation in plants, and its uptake and transport are key to growth, development and responses to the environment. Here, we report that Arabidopsis thaliana K+ uptake permease 5 (AtKUP5) contains an adenylate cyclase (AC) catalytic center embedded in its N-terminal cytosolic domain. The purified recombinant AC domain generates cAMP in vitro; and when expressed in Escherichia coli, increases cAMP levels in vivo. Both the AC domain and full length AtKUP5 rescue an AC-deficient E. coli mutant, cyaA, and together these data provide evidence that AtKUP5 functions as an AC. Furthermore, full length AtKUP5 complements the Saccharomyces cerevisiae K+ transport impaired mutant, trk1 trk2, demonstrating its function as a K+ transporter. Surprisingly, a point mutation in the AC center that impairs AC activity, also abolishes complementation of trk1 trk2, suggesting that a functional catalytic AC domain is essential for K+ uptake. AtKUP5-mediated K+ uptake is not affected by cAMP, the catalytic product of the AC, but, interestingly, causes cytosolic cAMP accumulation. These findings are consistent with a role for AtKUP5 as K+ flux sensor, where the flux-dependent cAMP increases modulate downstream components essential for K+ homeostasis, such as cyclic nucleotide gated channels