Creep events at Oakland Zoo July 2004
Two episodes of accelerated creep have been observed at Oakland Zoo. The creepmeter is buried beneath the park grasslands and due to the difficulty of exhumation has been serviced twice only in 11 years. Stiction (friction resulting in stick-slip behavior) is manifest as artificial steps in the data with amplitudes of 0.1-0.45 mm. True slip on the fault is an inferred displacement that leads each step with minimal overshoot. This was corrected in March 2006.
28 Sept 2002 A slow acceleration occurred between late 2002 and early 2003 preceeded by microearthquakes on the Hayward fault to the NW. Although this appears to have an amplitude of 7.5 mm it partly corresponds to a seasonal trend at this time of the year. Probably steady slip is incremented by only 2-3 mm.
19 July 2004 5.5 mm in three weeks. This event occurred after apparently no slip on the fault for more than 6 months. A thermoelastic signal with amplitude 0.5 mm is the cause of the apparent left lateral trend prior to the creep event. The creep event outlined by the box (enlarged below), was preceeded and followed by microseismicity as explained below the figure. The data have been calibrated, and converted to dextral slip (*1/cos30°), and can be downloaded as numeric listings.
Download calibrated dextral slip data for the creep event 10 minute samples: Excel or Text
Download calibrated dextral slip data 1 January to 4 August 2004. Excel or Text The data were converted to 1 day samples with a cubic spline interpolation scheme (IGO Pro)
Bob Uhrhammer reports the following events recorded at Berkeley since July 1st at HFN stations SMCB (10.4 km SSW of COZC) and BRIB
(18.7 km N of COZC). 12 small local events:
2004.183.213923 *
2004.185.192843
2004.187.005619 *
2004.194.011609 *
2004.194.072356
2004.194.133403 *
2004.196.135109
2004.196.183641
2004.211.104314 *
2004.212.142825 *
2004.212.154235 *
2004.216.225128
The 7 events marked with a "*" are also in the redi.event.log and their redi determined event parameters are:
-----COZC---- delta azim
2004.183,21:39:19.7500 37.8557 -122.0132 4.88 1.99 16.81 45.6
2004.187,00:56:17.2200 37.8633 -122.2625 5.14 1.10 16.01 321.9
2004.194,01:16:06.9600 37.7533 -122.1833 6.62 1.51 2.95 277.1
2004.194,13:34:00.5400 37.9895 -122.0895 12.36 1.88 27.16 11.3
2004.211,10:43:11.0300 37.8693 -122.2772 13.51 1.08 17.34 319.9
2004.212,14:28:21.9200 37.9263 -122.3000 11.63 2.06 23.62 326.1
2004.212,15:42:29.7800 38.0122 -122.1575 12.81 1.60 29.16 358.7
The raw BRIB and SMCB waveforms are shown in the attached plot. The creep event began circa 2004.200. Thus 8 local earthquakes
were detected in the 17 days preceding the COZC creep event onset and 4 local earthquakes were detected and in the 17 days following the
onset and 3 of these events were located NW of COZC by redi. Only one of these events (2004.194.0116), however, is located within 3 km of
COZC while the rest are 16-30 km distant.
