Far-field aftershocks of the 1906 earthquake

This is the publisher's version, also available electronically from "http://bssa.geoscienceworld.org".During the 24 hr following the great San Francisco, California, earthquake of 18 April 1906, separate seismic events were felt at Paisley, Oregon; Phoenix, Arizona; Los Angeles, California; and Brawley, California (MMIX). Using probability theory, we show that the occurrence of felt earthquakes in each of these widespread locations on the same day would constitute a rare event. Rates of felt-earthquake occurrences over a 9-yr period from 1897 to 1906 were determined for the four different regions that experienced earthquakes within 24 hr after the 1906 event. We modeled the likelihood of occurrence of these aftershocks in the spirit of the “ball-in-the-box” probability problem, and the results indicated a very high probability that the aftershock zone of the great earthquake extended at least 500 km beyond the extent of ground breakage, implying a disturbance of the stress field over an area at least two to three times longer than the fault break itself

Kansas refraction profiles

Historically, refraction surveys have been conducted in hopes of mapping distinct layers within the earth. Refraction is a useful tool provided its limitations and the assumption that layers increase in seismic velocity with increasing depth are kept in mind. A traditional reversed-refraction profile was conducted along a 500-km (300- mi)-long east-west line extending from Concordia, Kansas, to Agate, Colorado. Analysis of the data showed an average crustal velocity of 6.1 kmlsec (3.7 milsec) and an average upper-mantle P phase velocity of 8.29 kmlsec (4.97 milsec) with a Moho depth calculated to be 36 km (23 mi) on the eastern end and 46 km (29 mi) on the western end. Some evidence suggests velocities as high as 7.2 kmlsec (4.3 milsec) in the crust at various locations along the survey line. The strong east-west regional gravity gradient of -0.275 mgalb supports the seismically drawn conclusion of a thinning of crust in north-central Kansas. In order to supplement the data from this refraction survey, we took advantage of the Kansas earthquake seismograph network. A crustal study using earthquakes as energy sources and a regional earthquake network as seismometer locations resulted in a crustal-velocity model that will improve determination of local earthquake locations. A large anomalous body in the upper mantle/lower crust, assumed to be related to the Precambrian-aged Midcontinent Geophysical Anomaly (MGA), resulted in early Pwave arrivals from refracted energy from the Moho recorded at Concordia, Salina, Tuttle Creek, and Milford. An omnidirectional positive P residual zone near El Dorado may be related to the Wichita geomagnetic low. Some evidence suggests the presence of a lower velocity material on the western and eastern flanks of the MGA, possibly representing the Rice Formation. A P velocity of 8.25 krn/sec±0.1k m/sec (4.95 mi/sec+0.09m i/sec) with the crust thinning from west to east and an apparent thinning from the north and from the south was determined from the 16 regional earthquakes studied. Crustal thickness from central Kansas through western Missouri seems to be relatively consistent

Avoiding pitfalls in shallow seismic reflection surveys

This is the publisher's version, also available electronically from "http://library.seg.org".Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air‐coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common‐midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near‐surface reflection data correctly often requires more than a simple scaling‐down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section therefore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing

Shallow structure from a seismic reflection profile across the Borah Peak, Idaho, fault scarp

This is the publisher's version, also available electronically from “http://onlinelibrary.wiley.com”.A short 12-fold CDP seismic-reflection survey was performed along the road to Doublespring Pass across the fault scarp formed by the October 28, 1983, magnitude-7.3, Idaho earthquake. This high-resolution reflection survey was conducted to determine the feasibility of using reflection seismology to delineate shallow structures in a fault zone. Field-recording parameters were designed to optimize seismic reflections in the 30-150 msec range corresponding to 10-100 m in depth. A modified 30-06 hunting rifle was used as the energy source. Single 100-Hz geophones at 1.5-m group intervals in conjunction with 220-Hz low-cut recording filters (24 dB/octave) provided dominant frequencies above 150 Hz on field records. As would be expected from geologic considerations, the processed data suggest the existence of faulting in the subsurface. Strong events between 30 and 80 msec on the upthrown side of the scarp are of distinctly different character and frequency than those on the downdropped side at similar times. This indicates different geologic units are present at approximately the same reflection time on opposite sides of the fault zone. The northeastern edge of the scarp may not represent the true subsurface boundary of the upthrown block. Projection to the surface of the northeasternmost edge of the seismically determined subsurface graben is 10-15 m farther northeast than expected from surface faulting. High-frequency energy present within the subsurface expression of the graben is primarily noise and is related to the deformed and incoherent nature of materials within the graben

Shallow faults mapped with seismic reflections: Lost River Fault, Idaho

This is the publisher's version, also available electronically from "http://onlinelibrary.wiley.com".A high-resolution seismic-reflection survey, conducted at the intersection of Arentson Gulch road and the western splay of the Lost River fault scarp in central Idaho, defines a bedrock surface about 80 m deep which is segmented by several faults forming graben structures. Six meters of total fault displacement can be interpreted on the bedrock reflector while only 1 to 2 m of displacement can be observed on a shallower refracting interface and the surface fault scarp. This relatively small displacement suggests the western splay has either been active only recently or extremely infrequently since deposition of the bedrock, or that strike-slip motion may be present. A westward deflection of the major activity along the Lost River fault was probably responsible for the gap in 1983 surface faulting between the Warm Spring and Thousand Springs segments. The inconsistency in total bed displacement based on reflection, refraction, and fault-scarp evidence suggests tectonic activity on the western splay spans more than just a single episode

Universal law for waiting internal time in seismicity and its implication to earthquake network

In their paper (Europhys. Lett., 71 (2005) 1036), Carbone, Sorriso-Valvo, Harabaglia and Guerra showed that "unified scaling law" for conventional waiting times of earthquakes claimed by Bak et al. (Phys. Rev. Lett., 88 (2002) 178501) is actually not universal. Here, instead of the conventional time, the concept of the internal time termed the event time is considered for seismicity. It is shown that, in contrast to the conventional waiting time, the waiting event time obeys a power law. This implies the existence of temporal long-range correlations in terms of the event time with no sharp decay of the crossover type. The discovered power-law waiting event-time distribution turns out to be universal in the sense that it takes the same form for seismicities in California, Japan and Iran. In particular, the parameters contained in the distribution take the common values in all these geographical regions. An implication of this result to the procedure of constructing earthquake networks is discussed.Comment: 21 pages, 5 figure

Seismic-reflection surveys at sinkholes in central Kansas

Salt-dissolution sinkholes have developed at many localities in Kansas during the past 25 years. Most of the sinkholes subside gradually over a period of years, although catastrophic collapse has occurred in some cases. We have performed high-resolution seismic-reflection surveys across more than a half dozen of these sinkholes. It is possible to discern considerable geologic detail at depths of 50 to 1,000 m (160-3,300 ft) within the sinkholes by seismic-reflection methods. At one site astride I-70, we obtained acoustic images of grabens within the sinkhole that showed approximately 40 to 50 m (130-160 ft) of vertical downdrop at a depth of 400 m (1,300 ft) in an area where surface displacement was less than 5 m (16 ft). At another site we detected two paleosinkholes adjacent to a presently active sink. The paleosinks are filled with alluvial material probably of Pleistocene age; one of them shows indications of two different geologic ages of active sinking. While many of the new sinkholes that have formed appear to be related to oil-field brine disposal or salt-solution mining activities, the detection of the paleosinks by seismic-reflection methods reconfirms the natural occurrence of some salt-dissolution sinkholes in Kansas prior to the encroachment of civilization

Seismic-reflection surveys at sinkholes in central Kansas

Salt-dissolution sinkholes have developed at many localities in Kansas during the past 25 years. Most of the sinkholes subside gradually over a period of years, although catastrophic collapse has occurred in some cases. We have performed high-resolution seismic-reflection surveys across more than a half dozen of these sinkholes. It is possible to discern considerable geologic detail at depths of 50 to 1,000 m (160-3,300 ft) within the sinkholes by seismic-reflection methods. At one site astride I-70, we obtained acoustic images of grabens within the sinkhole that showed approximately 40 to 50 m (130-160 ft) of vertical downdrop at a depth of 400 m (1,300 ft) in an area where surface displacement was less than 5 m (16 ft). At another site we detected two paleosinkholes adjacent to a presently active sink. The paleosinks are filled with alluvial material probably of Pleistocene age; one of them shows indications of two different geologic ages of active sinking. While many of the new sinkholes that have formed appear to be related to oil-field brine disposal or salt-solution mining activities, the detection of the paleosinks by seismic-reflection methods reconfirms the natural occurrence of some salt-dissolution sinkholes in Kansas prior to the encroachment of civilization

Ultra-shallow seismic imaging of the top of the saturated zone

This is the published version. Copyright 2010 by the American Geophysical Union. All Rights Reserved.We collected ultra-shallow seismic-reflection data to image the near-surface stratigraphy of a Kansas River point bar. We were successful in identifying a discontinuous clay layer and the top of the saturated zone at depths of 0.95 and 1.4 m. Seismic walkaway data collected using various .22-caliber ammunition show that decreased source energy is necessary to generate higher frequencies and prevent clipping of critical near-offset traces needed to identify ultra-shallow reflections. The seismic reflections exhibited average normal moveout velocities of 180–195 m/s with dominant frequencies of 200–450 Hz. Coincident subsurface features were also imaged using 200-MHz ground-penetrating radar. This study presents the shallowest seismic reflection from the top of the saturated zone reported in the literature to date and further demonstrates the potential of using seismic-reflection methods for ultra-shallow imaging of the subsurface as a stand-alone tool or in conjunction with other high-resolution geophysical techniques

Shallow seismic AVO variations related to partial water saturation during a pumping test

This is the published version. Copyright 2007 by the American Geophysical Union. All Rights Reserved.High-resolution shallow seismic reflection experiments were conducted during and after a pumping test of an agricultural irrigation well to image the cone of depression. Although variations in the reflection time from the top of the saturated zone were not observed, amplitude-versus-offset (AVO) analysis revealed changes in reflection amplitude responses that correlate temporally and spatially to expected changes to the partially saturated zone induced by the pumping and recovery of the aquifer. The AVO responses exhibit dependence on aquifer drawdown and recovery cycles and the distance from the pumping well. We propose that near-surface soil heterogeneity and relatively rapid changes in the water table elevation during irrigation cycles caused a thickening of the partially saturated zone above the water table, which resulted in detectable changes in seismic reflection amplitudes. This study offers insights about the response of shallow seismic reflections to changes in subsurface water saturation and the potential application of seismic techniques to hydrogeophysical problems