Aseismic slip events are proving to be extremely frequent at subduction zones worldwide, and they... more Aseismic slip events are proving to be extremely frequent at subduction zones worldwide, and they come in a wide variety of sizes and durations. One of the largest not immediately following an earthquake affected southern Alaska from 1998 through 2002. In the summer of 1998, sites across a broad region of southern Alaska began to move in a southerly direction, relative to their pre-event motions. North of Anchorage, sites that had been moving to the NNW, in the direction of relative plate motion, instead began moving to the SSW. The observed change in velocity was as much as 25 mm/yr. The southward motion decreased with time, until in 2002 the sites began to move northward again. As of this writing, the event has not totally decayed to zero, but it has very nearly done so. The total horizontal surface displacements caused by this event are up to ~100 mm. We have investigated this event using a variety of 3D modeling techniques. The event affected a part of the plate interface that lies well downdip of the 1964 coseismic rupture, with the updip end of the creep event being 70-100 km downdip of the downdip end of the coseismic rupture. Because the dip angle of the megathrust is exceptionally shallow here, the updip end of the creep event lies at only 35-40 km. From GPS data recorded prior to the event, we infer that this region of the plate interface was creeping at approximately the average rate of plate motion (and produced no surface deformation); during the first two years of the event this rate doubled to approximately twice the rate of plate motion. Over 1998-2001, the temporal behavior of the slip event is consistent with a simple afterslip model, in which the displacement is proportional to log(t-t*), with t* being the time of the event. Such a response could result from a fault obeying rate and state dependent friction undergoing a sudden increase in shear stress. The puzzle raised by this sort of model is that the slip event did not follow any seismic event - it appears to have begun and ended aseismically. A new model of subduction zone dynamics is required that can explain the occurrence of events like this.
Annual Global Positioning System (GPS) campaign surveys since 1995 in the area of the 1964 Great ... more Annual Global Positioning System (GPS) campaign surveys since 1995 in the area of the 1964 Great Alaska earthquake (Mw 9.2) show evidence for a large aseismic slip event beginning in late 1997 or early 1998 and apparently ending in 2001. Prior to 1998, velocities of sites in Anchorage and the area to the north were oriented toward the NNW, consistent with strain associated with a locked subduction interface to the south. Between fall 1997 and summer 1998, velocities of GPS sites in an area at least 150 km by 100 km in size changed by as much as 25 mm/year. North of Anchorage, the change in site velocities was large enough that sites changed direction, from NNW-directed motion to SSE-directed motion. One permanent site in the area, installed in late 1998, shows a clear time-dependent signal, moving rapidly to the south shortly after installation and then slowing down over the next two years. A preliminary evaluation of data from summer 2001 suggests that the anomalous southward motion has ended or nearly so. These observations are consistent with the sudden activation (taking less than several months) of some process that causes southward motion of the sites, and a slow decay of that process over a span of 3-4 years. We can explain this change in velocities by a model of increased creep on a large section of the plate interface downdip of the 1964 rupture zone. In this model, slip on the interface increased from roughly the rate of plate motion to roughly double that, decaying back to roughly the rate of plate motion after 3+ years. This event is different from the recent creep event observed in Cascadia, as it extends well downdip of the seismogenic zone and appears to have affected a very large area simultaeously rather than propagating along strike. The westward extent of the zone of anomalous creep is uncertain due to a lack of data prior to 1997, but this zone is inferred to be at least 100 km by 100 km, lying at a depth of 35 km or more. The change in velocities was accompanied by a significant change in the rate of microseismicity in two volumes within the subducting Pacific plate, which lie on the edges of the inferred creeping zone. We infer that these changes, in one case a reduction in the rate of seismicity and the other an increase, had the same root cause as the creep event.
Physics of the Earth and Planetary Interiors, 2002
Global Positioning System (GPS), triangulation and leveling data are used to derive models for th... more Global Positioning System (GPS), triangulation and leveling data are used to derive models for the present day (last ∼5 years) and the 30-year average postseismic deformation on the Kenai Peninsula, Alaska following the 1964 Alaska earthquake. The two datasets are inverted using a three-dimensional elastic dislocation model to estimate the magnitude and spatial distribution of slip on the North America–Pacific plate interface, allowing us to examine the time dependence of the processes controlling postseismic deformation. We determine that the 30-year average postseismic slip rate beneath the western Kenai Peninsula was about twice as large as the present day slip rate. The observations suggest a time-decaying process, but are not consistent with a single exponentially decaying relaxation process initiated immediately after the 1964 earthquake. We conclude that the postseismic deformation observed on the western Kenai Peninsula cannot be explained in terms of any single time-decaying process. Either multiple processes acting on different timescales or significant spatial propagation of the postseismic deformation, or both, must occur. In the latter case, postseismic deformation would not begin everywhere at the same time and the rate of spatial propagation would affect the timescale inferred for the postseismic processes.
2012 IEEE International Geoscience and Remote Sensing Symposium, 2012
ABSTRACT The COSMOS network will eventually consist of several hundred sensors throughout the Uni... more ABSTRACT The COSMOS network will eventually consist of several hundred sensors throughout the United States that report kilometer-scale soil water content via measurement of the intensity of neutrons immediately above Earth's surface. We show that COSMOS sensors must be corrected for the effects of growing vegetation. Once this phenomenon is completely understood the COSMOS network could be a useful source of information for the validation of both soil moisture and vegetation products obtained from current and future microwave remote sensing satellites.
Aseismic slip events are proving to be extremely frequent at subduction zones worldwide, and they... more Aseismic slip events are proving to be extremely frequent at subduction zones worldwide, and they come in a wide variety of sizes and durations. One of the largest not immediately following an earthquake affected southern Alaska from 1998 through 2002. In the summer of 1998, sites across a broad region of southern Alaska began to move in a southerly direction, relative to their pre-event motions. North of Anchorage, sites that had been moving to the NNW, in the direction of relative plate motion, instead began moving to the SSW. The observed change in velocity was as much as 25 mm/yr. The southward motion decreased with time, until in 2002 the sites began to move northward again. As of this writing, the event has not totally decayed to zero, but it has very nearly done so. The total horizontal surface displacements caused by this event are up to ~100 mm. We have investigated this event using a variety of 3D modeling techniques. The event affected a part of the plate interface that lies well downdip of the 1964 coseismic rupture, with the updip end of the creep event being 70-100 km downdip of the downdip end of the coseismic rupture. Because the dip angle of the megathrust is exceptionally shallow here, the updip end of the creep event lies at only 35-40 km. From GPS data recorded prior to the event, we infer that this region of the plate interface was creeping at approximately the average rate of plate motion (and produced no surface deformation); during the first two years of the event this rate doubled to approximately twice the rate of plate motion. Over 1998-2001, the temporal behavior of the slip event is consistent with a simple afterslip model, in which the displacement is proportional to log(t-t*), with t* being the time of the event. Such a response could result from a fault obeying rate and state dependent friction undergoing a sudden increase in shear stress. The puzzle raised by this sort of model is that the slip event did not follow any seismic event - it appears to have begun and ended aseismically. A new model of subduction zone dynamics is required that can explain the occurrence of events like this.
Annual Global Positioning System (GPS) campaign surveys since 1995 in the area of the 1964 Great ... more Annual Global Positioning System (GPS) campaign surveys since 1995 in the area of the 1964 Great Alaska earthquake (Mw 9.2) show evidence for a large aseismic slip event beginning in late 1997 or early 1998 and apparently ending in 2001. Prior to 1998, velocities of sites in Anchorage and the area to the north were oriented toward the NNW, consistent with strain associated with a locked subduction interface to the south. Between fall 1997 and summer 1998, velocities of GPS sites in an area at least 150 km by 100 km in size changed by as much as 25 mm/year. North of Anchorage, the change in site velocities was large enough that sites changed direction, from NNW-directed motion to SSE-directed motion. One permanent site in the area, installed in late 1998, shows a clear time-dependent signal, moving rapidly to the south shortly after installation and then slowing down over the next two years. A preliminary evaluation of data from summer 2001 suggests that the anomalous southward motion has ended or nearly so. These observations are consistent with the sudden activation (taking less than several months) of some process that causes southward motion of the sites, and a slow decay of that process over a span of 3-4 years. We can explain this change in velocities by a model of increased creep on a large section of the plate interface downdip of the 1964 rupture zone. In this model, slip on the interface increased from roughly the rate of plate motion to roughly double that, decaying back to roughly the rate of plate motion after 3+ years. This event is different from the recent creep event observed in Cascadia, as it extends well downdip of the seismogenic zone and appears to have affected a very large area simultaeously rather than propagating along strike. The westward extent of the zone of anomalous creep is uncertain due to a lack of data prior to 1997, but this zone is inferred to be at least 100 km by 100 km, lying at a depth of 35 km or more. The change in velocities was accompanied by a significant change in the rate of microseismicity in two volumes within the subducting Pacific plate, which lie on the edges of the inferred creeping zone. We infer that these changes, in one case a reduction in the rate of seismicity and the other an increase, had the same root cause as the creep event.
Physics of the Earth and Planetary Interiors, 2002
Global Positioning System (GPS), triangulation and leveling data are used to derive models for th... more Global Positioning System (GPS), triangulation and leveling data are used to derive models for the present day (last ∼5 years) and the 30-year average postseismic deformation on the Kenai Peninsula, Alaska following the 1964 Alaska earthquake. The two datasets are inverted using a three-dimensional elastic dislocation model to estimate the magnitude and spatial distribution of slip on the North America–Pacific plate interface, allowing us to examine the time dependence of the processes controlling postseismic deformation. We determine that the 30-year average postseismic slip rate beneath the western Kenai Peninsula was about twice as large as the present day slip rate. The observations suggest a time-decaying process, but are not consistent with a single exponentially decaying relaxation process initiated immediately after the 1964 earthquake. We conclude that the postseismic deformation observed on the western Kenai Peninsula cannot be explained in terms of any single time-decaying process. Either multiple processes acting on different timescales or significant spatial propagation of the postseismic deformation, or both, must occur. In the latter case, postseismic deformation would not begin everywhere at the same time and the rate of spatial propagation would affect the timescale inferred for the postseismic processes.
2012 IEEE International Geoscience and Remote Sensing Symposium, 2012
ABSTRACT The COSMOS network will eventually consist of several hundred sensors throughout the Uni... more ABSTRACT The COSMOS network will eventually consist of several hundred sensors throughout the United States that report kilometer-scale soil water content via measurement of the intensity of neutrons immediately above Earth's surface. We show that COSMOS sensors must be corrected for the effects of growing vegetation. Once this phenomenon is completely understood the COSMOS network could be a useful source of information for the validation of both soil moisture and vegetation products obtained from current and future microwave remote sensing satellites.
Uploads
Papers