High-resolution surface-seismic imaging techniques for NEHRP soil profile classifications and earthquake hazard assessments in urban areas

The 17 January 1994 magnitude 6.7 Northridge earthquake caused widespread damage throughout the San Fernando Valley. There were also well-defined zones of significantly higher levels of building damage in the valley, such as in Sherman Oaks and in the vicinity of the California State Northridge campus. Previous aftershock site-response studies indicate that areas with extensive building damage are associated with high levels of ground motion amplification. We acquired high-resolution seismic-refraction data on the ground surface in some of these areas to help explain the earthquake damage patterns and the variation in ground motion. We used these data to determine the compressionaland shear-wave velocities (Vp and Vs) at 20 aftershock recording sites to 30-m depth. Two other sites are located in Sherman Oaks next to 90-m deep USGS boreholes with downhole Vp and Vs data and show that we imaged very similar seismic-velocity structures in the upper 40 m. We acquired two additional profiles in North Hollywood, an area lacking site response data but containing anomalous damage patterns. In Sherman Oaks, we correlate the areas of high building damage and ground motion south of the Los Angeles River with a thick sequence of low Vs (~ 10 m thick at < 185 m/s) and a 10-m deep water table. These deposits are distinct both in seismic velocity and grain size from the deposits located north of the Los Angeles River in Sherman Oaks. Overall, high site response appears to be associated with low Vs in the near surface. Eleven sites can be classified as soil profile type D using the 1994 NEHRP guidelines (average Vs ( VS3o) of 180 to 360 m/s in the upper 30 m). Nine of the sites in this soil type had aftershock site amplification at least a factor of 3 higher than a reference site located on rock (Hartzell and others, 1996); the remaining 2 sites in this group had ground motions amplified by a factor of 2.3 and 2.8. We classified seven sites as NEHRP soil type C ( VS3o: 360 to 760 m/s), and five of these sites had site amplification factors above 2.0. One of the type C sites was considered to be a_rock site during the aftershock site response study. Two rock sites were classified as soil type B ( VS3o: 760 to 1500 m/s), and their site amplification factors were 0.7 and 1.5. INTRODUCTION The 17 January 1994 magnitude 6.7 Northridge earthquake (Hauksson and others, 1994) ranks as one of the most damaging in history with total economic losses near 20 billion dollars (U.S. Geol. Survey and Southern California Earthquake Center, 1994). The earthquake caused widespread damage throughout the San Fernando Valley (SFV), but there were also well-defined zones of significantly higher levels of building damage in the valley. Heavily damaged areas included Sherman Oaks and several buildings on the California State Northridge campus. Previous studies show that decreasing mean shear-wave velocity in the near surface generally correlates with an increase in the average amplification of earthquake ground motion (Borcherdt, 1970; Borcherdt and Gibbs, 1976; Fumal, 1978). These earlier studies used downhole techniques to determine Pand Swave seismic-velocity profiles in the upper 30 m of the ground surface. Following the guidelines of earlier studies, but changing the approach slightly, we used seismic-refraction methods on the ground surface to determine the near-surface compressionaland shear-wave velocities. In this study, we (1) report on the Vp and Vs determined for each site, (2) compare our results from two sites with nearby borehole data, (3) describe the correlation of near-surface geology with Vp and Vs , and (4) assess the correlation of site response from Hartzell and others (1996) with our assignment of a 1994 National Earthquake Hazard Reduction Program (NEHRP) (Building Seismic Safety Council, 1994) Vs soil profile type to 20 sites (table 1). Table 1. NEHRP Soil Profile Type Classifications Soil Profile Type Average Shear-Wave Velocity to 30-m depth ( Vs30) A Vs30 > 1500 m/s, hard rock B 760 m/s < Vs3o< 1500 m/s, rock C 360 m/s < VS3o < 760 m/s, very dense soil and soft rock D 180 m/s < Vs30 < 360 m/s, stiff soil E VS3o<180m/s Nine high-resolution Pand 5-wave refraction profiles were acquired at or near the portable seismograph stations established for the U.S.G.S. -Golden Northridge aftershock study (fig. 1; table 2). These sites are in addition to the eleven sites described in Williams and others, (1996) which are also shown in Figure 1. A generalized surficial geology map from Tinsley and Fumal (1985) is also shown in figure 1. In addition, we determined Vp and Vs at four other sites in the SFV: two sites (SOW and SOP) were selected adjacent to recently-drilled holes in Sherman Oaks to compare our results with Vp and Vs data derived from the boreholes (Gibbs and others, 1996). Additionally, two sites (NOHO and BAK) were located in North Hollywood, an area lacking aftershock site response data, to compare an area containing heavier building damage against an area of lighter damage 1.5 km away.