Outcrops and well logs as a practicum for calibrating the accuracy of traveltime tomograms

Abstract

We have developed two case studies demonstrating the use of high-resolution seismic tomography and reflection imaging in the field of paleoseismology. The first study, of the Washington fault in southern Utah, USA, evaluated the subsurface deposits in the hanging wall of the normal fault. The second study, of the Mercur fault in the eastern Great Basin of Utah, USA, helped to establish borehole locations for sampling subsurface colluvial deposits buried deeper than those previously trenched along the fault zone. We evaluated the seismic data interpretations by comparison with data obtained by trenching and logging deposits across the Washington fault, and by drill-core sampling and video logging of boreholes penetrating imaged deposits along the Mercur fault. The seismic tomograms provided critical information on colluvial wedges and faults but lacked sufficient detail to resolve individual paleoearthquakes. IntroductionTime-space patterns of earthquake recurrence and magnitude reflect the pulse of the earthquake engine and are the primary parameters required for seismic hazard analysis of faulting (McCalpin, 2009). The pulse period indicates the recurrence interval of large earthquakes, and the pulse strength is the earthquake's magnitude. Knowing these two parameters is important for establishing earthquake risk in an inhabited region.Geologic mapping and regional geophysical surveys provide information on the locations and lengths of faults, but establishing the earthquake history requires excavating and logging unconsolidated deposits that are several hundred thousand years or fewer in age. High-resolution seismic surveys are useful in site characterization, especially prior to excavation. However, the results are commonly limited by resolution and complex stratigraphy adjacent to the faults, where vertically stacked colluvial wedges contain crucial information concerning the number and magnitudes of earthquake events represented in the subsurface deposits (e.g., Stephenson et al., 1993;Morey and Schuster, 1999;McCalpin, 2009).We present two case studies of normal faulting, in which seismic methods image faulted deposits prior to excavation. In both cases, the subsurface deposits were subsequently logged, and the results compared with the seismic interpretations. This "postmortem" cri-

Imaging of subsurface faults using refraction migration with fault flooding

“…The location of the faults in the migration image agree with the P-wave velocity tomogram (Figure 22a), and suggest similar dip angles. The nomal fault indicated at the offset of about 60 meters is also seen in a nearby trench log (Hanafy et al, 2015). In this case an antithetic fault on the left corner of the refraction image is indicated in Figure 22c, which is not visible on the P-wave velocity tomogram due to the limited-offset coverage as shown in the ray-path density diagram (Figure 22b).…”

Hydro/engineering shallow geophysical investigation for sustainable development in Ras Muhammed National Park, Sinai, Egypt

“…A local low velocity anomaly, called colluvial wedge, is shown in Figure 5b, a similar one is also shown on the tomogram of profile S-4 (not shown in this abstract), while the other tomograms S1, S2, and S5 to S-8 don't show a similar anomaly. Such colluvial wedges are usually associated with normal faults (Hanafy et al, 2015;Hanafy, 2012;McCalpin, 1996), to get a better understanding of this anomaly, we recorded another ten parallel 2D seismic profiles between the location of profile S-3 and S-4. We also used 72 shot gather, with 72 receiver per shot gather and 5 meters shot/receiver intervals.…”