Christopher M. Dietel

Seismogram offers insight into Oklahoma City bombing

The terrorist bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995, generated seismic waves that were recorded on two permanent seismographs about 7 and 26 km away from the bombing. The seismogram recorded at 26 km shows two low-frequency wave trains, discrete sets of oscillatory signals, that begin about 10 s apart. Public release of this record prompted speculation that each wave train was caused by a different energy source. On May 23, 1995, the U.S. Geological Survey monitored the demolition of the bomb-ravaged Federal Building with portable seismographs (Figure 1). Two wave trains were picked up again. The recordings indicate that the wave trains during both the bombing and demolition represent seismic waves traveling at different velocities. We conclude that the two wave trains recorded during the bombing are consistent with a single impulsive energy source.

On the variability of aftershock ground motions in the San Fernando Valley

We analyze aftershocks of the 1/17/94 Mw6.7 Northridge earthquake recorded at a 3-element small-aperture array within the town of Northridge, above the mainshock rupture plane. Many of the M4-5 aftershocks are observed to have a prolonged shaking duration, up to ~8 seconds, with conspicuous longer period (≈1 s) arrivals in the latter part of the wave train. Recordings of a M4.0 aftershock that occurred at 23:49 GMT on 1/17 show the origin of these waves. A slant-stack cross-correlation method on each of the three components shows that the late arrivals are characterized by low apparent velocities and a back-azimuth that is approximately 10 degrees off that of the direct arrivals. Based on the inferred apparent velocities and consideration of studies in other sedimentary basins, we conclude that these later arrivals consist of surface waves generated within the San Fernando Valley. Similar results are obtained for a M3.4 event recorded across the array. The surface waves are not, however, a ubiquitous feature of the aftershock recordings. We show that other M~4 events recorded at the same site are characterized by simple displacement pulses and durations that are typical for their magnitude, suggesting that 3-dimensional site response may be difficult to predict in cases where the sources are close to a valley or basin and/or the basin structure is complex.