A Comparison of Site Response Techniques Using Weak‐Motion Earthquakes and Microtremors

The applicability of the microtremor spectral ratio method is examined by comparing microtremor and weak‐motion earthquake site responses at seven permanent strong‐motion sites in Victoria, British Columbia. For each site, a weak‐motion earthquake standard spectral ratio (bedrock reference), the average horizontal‐to‐vertical spectral ratio of up to five weak‐motion earthquakes, and the average microtremor (Nakamura method) spectral ratio are compared. The geologic setting of Victoria is ideal for site response studies with a near‐surface high impedance contrast between thin geologic layers of Victoria clay (about 11 m maximum in this study) and Pleistocene till or bedrock. Regardless of excitation source (weak‐motion earthquakes or microtremors) and spectral ratio method, similar peak amplitudes and fundamental frequencies were found. Thicker material (>10 m ) sites displayed higher peak amplitudes (up to six times amplification) at frequencies of 2–5 Hz compared to sites with a thin lens of material (<3 m ) over bedrock that showed peak amplitudes at frequencies of >8 Hz .