Microseismicity and Creeping Faults: Hints from Modeling the Hayward Fault

Abstract Creeping segments of strike-slip faults are often characterized by high rates of microseismicity on or near the fault. This microseismicity releases only a small fraction of the slip occurring on the fault and the majority of the accumulating elastic strain is released either through aseismic creep or in rare large events. Distinguishing between creeping or non-creeping patches on faults and determining the resulting accumulated slip deficit is important in assessing the seismic hazard associated with a fault. Unfortunately, surface creep data alone are insufficient to constrain the creep at depth on the fault. Here we analyze the possibility of using microseismicity as a further constraint. An analysis of the accumulation of Coulomb stress associated with the fault creep indicates that the transition from creeping regions to locked patches has the potential to affect the local seismicity pattern. Precise relative relocations of the microseismicity of the Hayward fault [1] [F. Waldhauser, W.L. Ellsworth, Fault structure and mechanics of the Hayward Fault, California, from double-difference earthquake locations, J. Geophys. Res. 107(3), doi:10.1029/2000JB000084, 2002 indicate that a fraction of the events repeat, indicating recurrent ruptures of the same small patch. A comparison of the creeping pattern resulting from a Finite Element deformation Model with this precisely relocated microseismicity indicates that the non-repeating earthquakes mainly occur in the transitional zones from creeping to locked patches, while recurrent (repeating) earthquakes cluster in high creep-rate regions. Building from this observation, we have developed an analysis approach to better define patterns of creep, and thus the slip deficit, on the Hayward fault. Additionally this creep rate and its spatial pattern on the fault vary as a function of time after the system is loaded by earthquakes on the locked patches.

Combination of SAR remote sensing and GIS for monitoring subglacial volcanic activity – recent results from Vatnajökull ice cap (Iceland)

This paper presents latest results from the combined use of SAR (Synthetic Aperture Radar) remote sensing and GIS providing detailed insights into recent volcanic activity under Vatnajökull ice cap (Iceland). Glaciers atop active volcanoes pose a constant potential danger to adjacent inhabited regions and infrastructure. Besides the usual volcanic hazards (lava flows, pyroclastic clouds, tephra falls, etc.), the volcano-ice interaction leads to enormous meltwater torrents (icelandic: jökulhlaup), devastating large areas in the surroundings of the affected glacier. The presented monitoring strategy addresses the three crucial questions: When will an eruption occur, where is the eruption site and which area is endangered by the accompanying jökulhlaup. Therefore, sufficient early-warning and hazard zonation for future subglacial volcanic eruptions becomes possible, as demonstrated for the Bardárbunga volcano under the northern parts of Vatnajökull. Seismic activity revealed unrest at the northern flanks of Bardárbunga caldera at the end of September 2006. The exact location of the corresponding active vent and therefore a potentially eruptive area could be detected by continuous ENVISAT-ASAR monitoring. With this knowledge a precise prediction of peri-glacial regions prone to a devastating outburst flood accompanying a possible future eruption is possible

"Delirium Day": A nationwide point prevalence study of delirium in older hospitalized patients using an easy standardized diagnostic tool

Background: To date, delirium prevalence in adult acute hospital populations has been estimated generally from pooled findings of single-center studies and/or among specific patient populations. Furthermore, the number of participants in these studies has not exceeded a few hundred. To overcome these limitations, we have determined, in a multicenter study, the prevalence of delirium over a single day among a large population of patients admitted to acute and rehabilitation hospital wards in Italy. Methods: This is a point prevalence study (called "Delirium Day") including 1867 older patients (aged 65 years or more) across 108 acute and 12 rehabilitation wards in Italian hospitals. Delirium was assessed on the same day in all patients using the 4AT, a validated and briefly administered tool which does not require training. We also collected data regarding motoric subtypes of delirium, functional and nutritional status, dementia, comorbidity, medications, feeding tubes, peripheral venous and urinary catheters, and physical restraints. Results: The mean sample age was 82.0 \ub1 7.5 years (58 % female). Overall, 429 patients (22.9 %) had delirium. Hypoactive was the commonest subtype (132/344 patients, 38.5 %), followed by mixed, hyperactive, and nonmotoric delirium. The prevalence was highest in Neurology (28.5 %) and Geriatrics (24.7 %), lowest in Rehabilitation (14.0 %), and intermediate in Orthopedic (20.6 %) and Internal Medicine wards (21.4 %). In a multivariable logistic regression, age (odds ratio [OR] 1.03, 95 % confidence interval [CI] 1.01-1.05), Activities of Daily Living dependence (OR 1.19, 95 % CI 1.12-1.27), dementia (OR 3.25, 95 % CI 2.41-4.38), malnutrition (OR 2.01, 95 % CI 1.29-3.14), and use of antipsychotics (OR 2.03, 95 % CI 1.45-2.82), feeding tubes (OR 2.51, 95 % CI 1.11-5.66), peripheral venous catheters (OR 1.41, 95 % CI 1.06-1.87), urinary catheters (OR 1.73, 95 % CI 1.30-2.29), and physical restraints (OR 1.84, 95 % CI 1.40-2.40) were associated with delirium. Admission to Neurology wards was also associated with delirium (OR 2.00, 95 % CI 1.29-3.14), while admission to other settings was not. Conclusions: Delirium occurred in more than one out of five patients in acute and rehabilitation hospital wards. Prevalence was highest in Neurology and lowest in Rehabilitation divisions. The "Delirium Day" project might become a useful method to assess delirium across hospital settings and a benchmarking platform for future surveys

Strain rate patterns from dense GPS networks

The knowledge of the crustal strain rate tensor provides a description of geodynamic processes such as fault strain accumulation, which is an important parameter for seismic hazard assessment, as well as anthropogenic deformation. In the past two decades, the number of observations and the accuracy of satellite based geodetic measurements like GPS greatly increased, providing measured values of displacements and velocities of points. Here we present a method to obtain the full continuous strain rate tensor from dense GPS networks. The tensorial analysis provides different aspects of deformation, such as the maximum shear strain rate, including its direction, and the dilatation strain rate. These parameters are suitable to characterize the mechanism of the current deformation. Using the velocity fields provided by SCEC and UNAVCO, we were able to localize major active faults in Southern California and to characterize them in terms of faulting mechanism. We also show that the large seismic events that occurred recently in the study region highly contaminate the measured velocity field that appears to be strongly affected by transient postseismic deformation. Finally, we applied this method to coseismic displacement data of two earthquakes in Iceland, showing that the strain fields derived by these data provide important information on the location and the focal mechanism of the ruptures

Numerical modeling of strike-slip creeping faults and implications for the Hayward fault, California, Tectonophysics 361

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.

Influence of the Earthquake Cycle and Lithospheric Rheology on the Dynamics of the Eastern California Shear Ζone

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

Fault Creep and Microseismicity on the Hayward Fault, California: Implications for Asperity Size

The Hayward fault is documented to undergo significant creep, with some patches accommodating 50% or more of the long‐term fault displacement. In spite of this, the fault has also experienced moderate to large earthquakes. By comparing the patterns of microseismicity observed on the fault with models of fault zone creep, we can investigate the long‐term displacement/deformation history of the fault in terms of the relative roles of aseismic creep, fault slip accommodated through microseismicity, and strain accumulation (slip deficit). We find that microseismicity on the Hayward fault produces a negligible percentage of the seismic moment dissipated on the fault. Combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault. For small asperities associated with repeating earthquakes on the Hayward fault, the rupture areas of these asperities range from 20 to 60 m2

Microseismicity and Creeping Faults: Hints from Modeling the Hayward Fault, California (USA)

Creeping segments of strike-slip faults are often characterized by high rates of microseismicity on or near the fault. This microseismicity releases only a small fraction of the slip occurring on the fault and the majority of the accumulating elastic strain is released either through aseismic creep or in rare large events. Distinguishing between creeping or non-creeping patches on faults and determining the resulting accumulated slip deficit is important in assessing the seismic hazard associated with a fault. Unfortunately, surface creep data alone are insufficient to constrain the creep at depth on the fault. Here we analyze the possibility of using microseismicity as a further constraint. An analysis of the accumulation of Coulomb stress associated with the fault creep indicates that the transition from creeping regions to locked patches has the potential to affect the local seismicity pattern. Precise relative relocations of the microseismicity of the Hayward fault [1] [F. Waldhauser, W.L. Ellsworth, Fault structure and mechanics of the Hayward Fault, California, from double-difference earthquake locations, J. Geophys. Res. 107(3), doi:10.1029/2000JB000084, 2002.] indicate that a fraction of the events repeat, indicating recurrent ruptures of the same small patch. A comparison of the creeping pattern resulting from a Finite Element deformation Model with this precisely relocated microseismicity indicates that the non-repeating earthquakes mainly occur in the transitional zones from creeping to locked patches, while recurrent (repeating) earthquakes cluster in high creep-rate regions. Building from this observation, we have developed an analysis approach to better define patterns of creep, and thus the slip deficit, on the Hayward fault. Additionally this creep rate and its spatial pattern on the fault vary as a function of time after the system is loaded by earthquakes on the locked patches