Berkeley Local Earthquake

At 10:35 UT on May 5, 2005, a $M_{L}$ 3.4 earthquake occurred 4.6 km S$29^o$E of CMSB at a depth of 5 km. The CMSB recording of this event demonstrates the signal-to-noise ratio and signal bandwidth that can be achieved using modern low-noise borehole emplanted accelerometers of an NHFN station and its capacity for providing high-resolution registration of a local earthquake. The absolute ground accelerations observed at CMSB are shown in Figure 5.4. Given the observed $M_{L}$ 3.4 magnitude and corresponding peak ground accelertion of 26,650 counts or 2.44 gal for this event and the effective acceleration clipping level of 196 gals for the CMSB hardware, the upper limit on the magnitude clipping level is estimated to be $M_{L}$ 5.3 (3.4 + log10(196/2.44)) for a local earthquake.

Figure 5.4: Plot showing the absolute ground accelerations recorded at CMSB, the 167 m deep borehole station at the Cal Memorial Stadium, for a $M_{L}$ 3.4 earthquake at a hypocentral distance of 6.8 km. The horizontal traces have been rotated to radial and transverse and the observed peak ground ground acceleration is 2.44 gals (or 26,650 counts on the CMSB data logger. The sampling rate is 500 Hz and the bandwidth, limited by the FIR anti-aliasing filters in the data logger, is 200 Hz.

The corresponding vertical component (CL1) acceleration spectra shown in Figure 5.5 demonstrates the signal-to-noise ratio and signal bandwidth that can be achieved using modern low-noise borehole emplanted accelerometers. The signal bandwidth is 200 Hz and the SNR is above 50 dB in the 1-50 Hz band and the SNR is above 0 dB up to 170 Hz.

Figure 5.5: Plot showing the vertical component absolute ground acceleration spectra recorded at CMSB for a $M_{L}$ 3.4 local earthquake (solid line) and the corresponding pre-event noise spectra dashed line).