Rocco Malservisi

Earthquake and tsunami forecasts: Relation of slow slip events to subsequent earthquake rupture

Significance Recent destructive megathrust earthquakes and tsunamis in Japan and Sumatra indicate the difficulty of forecasting these events. Geodetic monitoring of the offshore regions of the subduction zones where these events occur has been suggested as a useful tool, but its potential has never been conclusively demonstrated. Here we show that slow slip events, nondestructive events that release energy slowly over weeks or months, are important mechanisms for releasing seismic strain in subduction zones. Better monitoring of these events, especially those offshore, could allow estimates of the size of future earthquakes and their potential for damaging tsunamis. However, the predictive value of slow slip events remains unclear. The 5 September 2012 Mw 7.6 earthquake on the Costa Rica subduction plate boundary followed a 62-y interseismic period. High-precision GPS recorded numerous slow slip events (SSEs) in the decade leading up to the earthquake, both up-dip and down-dip of seismic rupture. Deeper SSEs were larger than shallower ones and, if characteristic of the interseismic period, release most locking down-dip of the earthquake, limiting down-dip rupture and earthquake magnitude. Shallower SSEs were smaller, accounting for some but not all interseismic locking. One SSE occurred several months before the earthquake, but changes in Mohr–Coulomb failure stress were probably too small to trigger the earthquake. Because many SSEs have occurred without subsequent rupture, their individual predictive value is limited, but taken together they released a significant amount of accumulated interseismic strain before the earthquake, effectively defining the area of subsequent seismic rupture (rupture did not occur where slow slip was common). Because earthquake magnitude depends on rupture area, this has important implications for earthquake hazard assessment. Specifically, if this behavior is representative of future earthquake cycles and other subduction zones, it implies that monitoring SSEs, including shallow up-dip events that lie offshore, could lead to accurate forecasts of earthquake magnitude and tsunami potential.

Influence of the earthquake cycle and lithospheric rheology on the dynamics of the Eastern California Shear Zone

The Eastern California Shear Zone is bounded by the high heat flow region of the Basin and Range province and the low heat flow region of the Sierra Nevada block. This difference in thermal state influences the rheology of the lower crust/upper mantle, resulting in a viscosity contrast between the two regions. We analyze the effect of such a contrast on the kinematics and dynamics of the shear zone with numerical models. This viscosity contrast drives asymmetric strain accumulation in the upper crust, producing an asymmetric surface velocity field. An additional consequence of this strain pattern is the potential for asymmetric co-seismic displacement during an earthquake.

Seismic cycle and rheological effects on estimation of present‐day slip rates for the Agua Blanca and San Miguel‐Vallecitos faults, northern Baja California, Mexico

[1] Geodesy can be used to infer long-term fault slip rates, assuming a model for crust and upper mantle rheology. We examine the sensitivity of fault slip rate estimates to assumed rheology for the Agua Blanca and San Miguel-Vallecitos faults in northern Baja California, Mexico, part of the Pacific–North America plate boundary zone. The Agua Blanca fault is seismically quiet, but offset alluvial fans indicate young activity. Current seismicity is confined to the nearby San Miguel-Vallecitos fault, a small offset fault better aligned with plate motion. GPS measurements between 1993 and 1998 suggest that both faults are active, with a combined slip rate of 4–8 mm yr � 1 regardless of rheological model. However, slip rate estimates for the individual faults are sensitive to assumed rheology. Elastic half-space models yield 2–3 mm yr � 1 for the Agua Blanca fault, and somewhat faster rates for the San Miguel-Vallecitos fault, 2–4 mm yr � 1 , with uncertainties of about 1 mm yr � 1 . Models incorporating viscoelastic rheology and seismic cycle effects suggest a faster slip rate for the Agua Blanca fault, 6 ± 1 mm yr � 1 , and a slower rate for the San Miguel-Vallecitos fault, 1 ± 1 mm yr � 1 , in better agreement with geological data, but these rates are sensitive to assumed rheology. Numerical simulations with a finite element model suggest that for similar rheological and friction conditions, slip on the San Miguel-Vallecitos fault should be favored due to better alignment with plate motion. Long-term faulting processes in the larger offset Agua Blanca fault may have lowered slip resistance, allowing accommodation of motion despite misalignment with plate motion. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 1236 Geodesy and Gravity: Rheology of the lithosphere and mantle (8160); 8107 Tectonophysics: Continental neotectonics; KEYWORDS: fault slip rates and the seismic cycle; northern Baja California

Numerical modeling of strike-slip creeping faults and implications for the Hayward fault, California

Abstract The seismic potential of creeping faults such as the Hayward fault (San Francisco Bay Area, CA) depends on the rate at which moment (slip deficit) accumulates on the fault plane. Thus, it is important to evaluate how the creep rate observed at the surface is related to the slip on the fault plane. The surface creep rate (SCR) depends on the geometry of locked and free portions of the fault and on the interaction between the fault zone and the surrounding lithosphere. Using a viscoelastic finite element model, we investigate how fault zone geometries and physical characteristics such as frictionless or locked patches affect the observed surface creep when the system is driven by far field plate motions. These results have been applied to creep observations of the Hayward fault. This analysis differs from most previous fault creeping models in that the fault in our model is loaded by a distributed viscous flow induced by far field velocity boundary conditions instead of imposed slip beneath the major faults of the region. The far field velocity boundary conditions simulate the relative motion of the stable Pacific plate respect to the Rigid Sierra Nevada block, leaving the rheology, fault geometry, and mechanics (locked or free to creep patches), to determinate the patterns of fault creep. Our model results show that the fault geometry (e.g. length and depth of creeping) and the local rheology influence the surface creep rate (SCR) and the slip on the fault plane. In particular, we show that the viscoelastic layer beneath the elastic seismogenic zone plays a fundamental role in loading the fault. Additionally, the coupling with the surrounding lithosphere results in a smooth transition from regions free to creep to locked patches.

Multiscale postseismic behavior on a megathrust: The 2012 Nicoya earthquake, Costa Rica

The Nicoya Peninsula in northwest Costa Rica overlies a section of the subduction megathrust along the Middle America Trench. On 5 September 2012, a moment magnitude 7.6 megathrust earthquake occurred beneath a dense network of continuous GPS and seismic stations. Many of the GPS stations recorded the event at high rate, 1 Hz or better. We analyze the temporal and spatial evolution of surface deformation after the earthquake. Our results show that the main rupture was followed by significant afterslip within the first 3 h following the main event. The behavior of the surface displacement can be represented by relaxation processes with three characteristic times: 7, 70, and more than 400 days. We assume that the long relaxation time corresponds to viscoelastic relaxation and the intermediate relaxation time corresponds to afterslip on the main fault. The short relaxation time may represent a combination of rapid afterslip, poroelastic adjustment in the upper crust, or other processes. During the first few months that followed the earthquake, afterslip likely released a significant amount of slip deficit still present following the coseismic rupture, in particular updip of the rupture. Afterslip seems to be bounded updip by regions affected by slow slip events prior to the earthquake, suggesting that the two processes are influenced by different frictional properties.

Dynamic uplift in a transpressional regime: numerical model of the subduction area of Fiordland, New Zealand

Abstract Bending of the downgoing plate in subduction zone typically leads to an offshore peripheral bulge. This leads to dynamic uplift generated by the elastic bending of the subducted slab, and is generally enough to support the topography of the bulge in a non-isostatic manner and produce a positive gravity anomaly. The Southwest region of the South Island of New Zealand, Fiordland, is characterized by high elevation and a large positive Bouguer gravity anomaly. This combination of high topography with high Bouguer gravity argues against isostatic equilibrium and suggests an additional support mechanism. Earthquakes as deep as 150 km, a deformed Benioff zone and inferences from plate reconstructions all support a tectonic model where the eastern margin of the Australian plate is subducting beneath Fiordland and is sharply bent. This bending of the Australian plate provides the needed non-isostatic support for Fiordland topography and generates the observed gravity anomaly. Although the peripheral bulge in subduction zones is generally localized offshore, the positive gravity anomaly (Bouguer and free air) in Fiordland is onshore, close to the shoreline, and generally corresponds spatially with high elevations. Here we propose a mechanism that allows the subducted sliver of slab to be decoupled from the main Australian plate and strongly bent beneath Fiordland. We test this scenario with a finite-element model. The model allows us to study the flexural response of a subducting elastic slab bent by lateral compression into a shape similar to the one inferred from seismicity. We test how different plate geometries and plate boundary forces influence the flexural dynamic support of Fiordland topography, providing important constraints on the local plate dynamics. The model results show that for a tectonically reasonable combination of plate geometries and boundary forces, the deformation of the lithosphere produces the observed topography and gravity signature. In particular we find that the bending of the subducted Australian plate can supply the needed uplift and support for the topography of Fiordland. However, a weak area west of but nearby the Fiordland shoreline, perhaps a fault or tear, is needed to decouple the subducted sliver, confine the bulge, and localize the uplift within Fiordland.

A three-dimensional surface velocity field for the Mississippi Delta: Implications for coastal restoration and flood potential

Accurate estimates of the current rate of subsidence in the Mississippi Delta (southern United States) provide a context for planning of wetland restoration and predictions of storm surge flooding. We present a comprehensive three-dimensional surface velocity field for the Mississippi Delta based on a network of 36 high-precision continuous GPS stations. We show that while the majority of the delta is relatively stable, the southern portion continues to experience high rates of subsidence (5–6 mm yr –1 ). Our data are consistent with long-term tide gauge records at Grand Isle, Louisiana, and several stations in Florida. The current rate of relative sea-level rise (combined effect of land subsidence and sea-level rise) along parts of the coastal delta is ~8–9 mm yr –1 . Most tide gauge stations have recorded sea-level-rise acceleration after A.D. 1970. These data have implications for land reclamation and wetland restoration in the region; parts of the delta may not be viable in the long term.

Thermal-rheological controls on deformation within oceanic transforms

Abstract Transform faults that offset mid-ocean ridge (MOR) segments accommodate plate motion through deformations that involve complex thermal and mechanical feedbacks involving both brittle and temperature-dependent ductile rheologies. Through the implementation of a 3D coupled thermal-mechanical modelling approach, we have developed a more detailed picture of the geometry of plate boundary deformation and its dependence on plate velocity and the age offset of MOR transforms. The modelling results show that cooling of near-ridge lithosphere (lateral heat transfer) has significant effects in the ductile mantle lithosphere for both the location and style of deformation. The region where strain is accommodated in the subjacent mantle lithosphere is systematically offset from the position of the overlying linear transform fault in the brittle crust. This offset causes the boundary to be oblique to plate motions along much of the transform’s length, producing extension in regions of significant obliquity modifying the location of the surface fault segments. An implication of this complex plate-boundary geometry is that in the near-ridge region, the older (cooler) lithosphere will extend beneath the ridge tip, restricting the upwelling of mantle to the MOR. The melt to generate the oceanic crust adjacent to the transform must migrate laterally from its offset source, resulting in a reduced volume and thinner crust. This near-ridge plate boundary structure also matches the pattern of core-complex extension observed at inside corners of many slow-spreading ridges. The oblique extensional structure may also explain magmatism that is observed along ‘leaky’ transforms, which could ultimately result in the generation of new ridge segments that effectively ‘split’ large transforms.

Space geodetic observation of the deformation cycle across the Ballenas Transform, Gulf of California

The Gulf of California, Mexico, accommodates ~90% of North America-Pacific plate relative motion. While most of this motion occurs on marine transform faults and spreading centers, several fault segments in the central Gulf come close to peninsular Baja California. Here we present Global Positioning System and interferometric synthetic aperture radar data near the Ballenas transform fault, separating the peninsula from Angel de la Guarda Island. We observe interseismic motion between June 2004 and May 2009 and displacements associated with the 3 August 2009 Mw 6.9 earthquake. From the interseismic data we estimate a locking depth of 9–12.5 km and a slip rate of 44.9–48.1 mm/yr, indicating that faults east of Angel de la Guarda deform at negligible rates and that the Ballenas Transform accommodates virtually all of the relative motion between the North American plate and the Baja California microplate. Our preferred model for coseismic slip on a finite rectangular fault plane suggests 1.3 m of strike-slip displacement along a vertical rupture plane that is 60 km long and extends from the surface to a depth of 13 km in the eastern Ballenas Channel, striking parallel to Baja California-North America relative plate motion. These estimates agree with the seismic moment tensor and the location of the major foreshock and aftershocks and are compatible with the fault location identified from high-resolution bathymetric mapping. The geodetic moment is 33% higher than the seismic moment in part because some afterslip and viscous flow in the first month after the earthquake are included in the geodetic estimate. Coulomb stress changes for adjacent faults in the Gulf are consistent with the location of smaller aftershocks following the 2009 main shock and suggest potential triggering of the 12 April 2012 Mw 6.9 Guaymas earthquake.

Strain release at the trench during shallow slow slip: The example of Nicoya Peninsula, Costa Rica

The near-trench behavior of subduction megathrust faults is critical for understanding earthquake hazard and tsunami generation. The shallow subduction interface is typically located in unconsolidated sediments that are considered too weak to accumulate elastic strain. However, the spectrum of shallow fault slip behavior is still elusive, due in large part to the lack of near-field observations. Here we combine measurements from seafloor pressure sensors near the trench and an onshore GPS network in a time-dependent inversion to image the initiation and migration of a well-documented slow slip event (SSE) in 2007 at the Nicoya Peninsula, Costa Rica. Our results show that the shallow SSE initiated on the shallow subduction interface at a depth of ~15 km, where pore fluid pressure is inferred to be high, and propagated all the way to the trench. The migrating event may have triggered a second subevent that occurred one month later. Our results document release of elastic strain at the shallow part of the subduction megathrust, and suggest prior accumulation of elastic strain. In conjunction with near-trench shallow slow slip recently reported for the Hikurangi subduction zone, and trench breaching ruptures revealed in some large earthquakes, our results suggest that near-trench strain accumulation and release at the shallower portions of the subduction interface is more common than previously thought.