Our research group is confident that there are scientific errors, and errors of omission, in the ... more Our research group is confident that there are scientific errors, and errors of omission, in the Thomas et al. presentation. The attached Comments were submitted to PEPI to let the community understand that the reported magnetic precursor of the 1989 Loma Prieta earthquake is a subject of continuing discussion and uncertainty, and not as Thomas et al. (2009) would have the community believe a disproved hypothesis.
We have undertaken a series of controlled field experiments to develop seismoelectric experimenta... more We have undertaken a series of controlled field experiments to develop seismoelectric experimental methods for near-surface applications and to improve our understanding of seismoelectric phenomena. In a set of off-line geometry surveys source separated from the receiver line , we place seismic sources and electrode array receivers on opposite sides of a man-made target two sand-filled trenches to record separately two previously documented seismoelectric modes: 1 the electromagnetic interface response signal created at the target and 2 the coseismic electric fields located within a compressional seismic wave. With the seismic source point in the center of a linear electrode array, we identify the previously undocumented seismoelectric direct field, and the Lorentz field of the metal hammer plate moving in the earth’s magnetic field. We place the seismic source in the center of a circular array of electrodes radial and circumferential orientations to analyze the source-related direc...
The Himalaya orogenic belt produces frequent large earthquakes that affect population centers alo... more The Himalaya orogenic belt produces frequent large earthquakes that affect population centers along a length of over 2500 km. The 2015 Gorkha, Nepal earthquake (Mw 7.8) ruptured the Main Himalayan Thrust (MHT) and allows direct measurements of the behavior of the continental collision zone. We study the MHT using seismic waveforms recorded by local stations that completely cover the aftershock zone. The MHT exhibits clear lateral variation along geologic strike, with the Lesser Himalayan ramp having moderate dip on the MHT beneath the mainshock area and a flatter and deeper MHT beneath the eastern end of the aftershock zone. East of the aftershock zone, seismic wave speed increases at MHT depths, perhaps due to subduction of an Indian basement ridge. A similar magnitude wave speed change occurs at the western end of the aftershock zone. These gross morphological structures of the MHT controlled the rupture length of the Gorkha earthquake.
ABSTRACT Stanford-USGS and UC Berkeley have been maintaining five ultra-low frequency electromagn... more ABSTRACT Stanford-USGS and UC Berkeley have been maintaining five ultra-low frequency electromagnetic (ULFEM) stations along the San Andreas Fault system over a multi-year period. The standard site is equipped with two sets of orthogonal 100-meter electrodes, as well as a set of 3 orthogonal magnetometers. These sites are intended to monitor and record ULFEM signals associated with seismic activity, if any such signals exist, so that we can better understand the mechanisms associated with such fluctuations. In order to recognize anomalous ULFEM fluctuations, we require a thorough understanding of long-term instrument behavior, and of instrument response to local anthropogenic phenomena even in the absence of seismicity-related ULFEM signals. To this end, we are examining the behavior of the apparent electrical resistivity over time. The apparent resistivity at any given site should remain constant over time in the absence of changes in instrumentation or subsurface conditions. Temporal changes in apparent resistivity could reflect processes associated with earthquake preparation or aftershocks, such as dilatancy or porosity changes. However, temporal changes could also be due to seasonal changes in groundwater saturation, cultural interference, or instrument instability. It is important for us to determine which, if any, of these may be affecting our data so that their effects are not inappropriately attributed to geological or geophysical changes. We present evidence for a multi-year trend of increasing apparent resistivity at our JRSC site that we believe is related to well-known problems of electrode deterioration. We also note shorter-term variability (time-scales of days to months) that we cannot yet explain. All ULFEM data acquired through this NSF-Earthscope project are available in near real-time from http://ulfem-data.stanford.edu
Our research group is confident that there are scientific errors, and errors of omission, in the ... more Our research group is confident that there are scientific errors, and errors of omission, in the Thomas et al. presentation. The attached Comments were submitted to PEPI to let the community understand that the reported magnetic precursor of the 1989 Loma Prieta earthquake is a subject of continuing discussion and uncertainty, and not as Thomas et al. (2009) would have the community believe a disproved hypothesis.
We have undertaken a series of controlled field experiments to develop seismoelectric experimenta... more We have undertaken a series of controlled field experiments to develop seismoelectric experimental methods for near-surface applications and to improve our understanding of seismoelectric phenomena. In a set of off-line geometry surveys source separated from the receiver line , we place seismic sources and electrode array receivers on opposite sides of a man-made target two sand-filled trenches to record separately two previously documented seismoelectric modes: 1 the electromagnetic interface response signal created at the target and 2 the coseismic electric fields located within a compressional seismic wave. With the seismic source point in the center of a linear electrode array, we identify the previously undocumented seismoelectric direct field, and the Lorentz field of the metal hammer plate moving in the earth’s magnetic field. We place the seismic source in the center of a circular array of electrodes radial and circumferential orientations to analyze the source-related direc...
The Himalaya orogenic belt produces frequent large earthquakes that affect population centers alo... more The Himalaya orogenic belt produces frequent large earthquakes that affect population centers along a length of over 2500 km. The 2015 Gorkha, Nepal earthquake (Mw 7.8) ruptured the Main Himalayan Thrust (MHT) and allows direct measurements of the behavior of the continental collision zone. We study the MHT using seismic waveforms recorded by local stations that completely cover the aftershock zone. The MHT exhibits clear lateral variation along geologic strike, with the Lesser Himalayan ramp having moderate dip on the MHT beneath the mainshock area and a flatter and deeper MHT beneath the eastern end of the aftershock zone. East of the aftershock zone, seismic wave speed increases at MHT depths, perhaps due to subduction of an Indian basement ridge. A similar magnitude wave speed change occurs at the western end of the aftershock zone. These gross morphological structures of the MHT controlled the rupture length of the Gorkha earthquake.
ABSTRACT Stanford-USGS and UC Berkeley have been maintaining five ultra-low frequency electromagn... more ABSTRACT Stanford-USGS and UC Berkeley have been maintaining five ultra-low frequency electromagnetic (ULFEM) stations along the San Andreas Fault system over a multi-year period. The standard site is equipped with two sets of orthogonal 100-meter electrodes, as well as a set of 3 orthogonal magnetometers. These sites are intended to monitor and record ULFEM signals associated with seismic activity, if any such signals exist, so that we can better understand the mechanisms associated with such fluctuations. In order to recognize anomalous ULFEM fluctuations, we require a thorough understanding of long-term instrument behavior, and of instrument response to local anthropogenic phenomena even in the absence of seismicity-related ULFEM signals. To this end, we are examining the behavior of the apparent electrical resistivity over time. The apparent resistivity at any given site should remain constant over time in the absence of changes in instrumentation or subsurface conditions. Temporal changes in apparent resistivity could reflect processes associated with earthquake preparation or aftershocks, such as dilatancy or porosity changes. However, temporal changes could also be due to seasonal changes in groundwater saturation, cultural interference, or instrument instability. It is important for us to determine which, if any, of these may be affecting our data so that their effects are not inappropriately attributed to geological or geophysical changes. We present evidence for a multi-year trend of increasing apparent resistivity at our JRSC site that we believe is related to well-known problems of electrode deterioration. We also note shorter-term variability (time-scales of days to months) that we cannot yet explain. All ULFEM data acquired through this NSF-Earthscope project are available in near real-time from http://ulfem-data.stanford.edu
Uploads