|
by A. Shakal, R. Darragh, M. Huang,
T. Cao, R. Sherburne, R. Sydnor, P. Malhotra,
C. Cramer, J. Wampole, P. Fung and C. Petersen
Click on the link below for the full text:
Introduction
Some of the highest accelerations ever recorded were obtained at stations of
the California Strong Motion Instrumentation Program (CSMIP) during the Ms 6.9
April 25 earthquake near Petrolia. The Cape Mendocino station recorded a peak
acceleration of near 2 g, the largest acceleration ever recorded in
California. One of the highest accelerations ever recorded on a structure,
1.23 g, occurred on the deck of a freeway overpass near Rio Dell.
The accelerograms in this report were recorded during three earthquakes with
magnitude over 6 that occurred in an 18-hour time span between 11:06 PDT on
April 25 and 04:18 PDT on April 26. The largest event, the first, had a
magnitude of 6.9 Ms. The epicenters of these three earthquakes were within
about 25 km of each other.
The preliminary earthquake locations and magnitudes estimated by the USGS and
UC Berkeley are:
| Earthquake
|
Date
|
Time
|
N. Lat.
|
W. Long.
|
Depth
|
Ms
|
ML
|
| Mainshock
|
4/25/92
|
11:06:04 PDT
|
40.37
|
124.31
|
15 km
|
6.9
|
6.4
|
| Aftershock No. 1
|
4/26/92
|
00:41:40 PDT
|
40.44
|
124.58
|
18 km
|
6.2
|
6.2
|
| Aftershock No. 2
|
4/26/92
|
04:18:26 PDT
|
40.40
|
124.56
|
21 km
|
6.5
|
6.4
|
The April 25th event is the largest earthquake to occur in California since
the 7.1 Ms Loma Prieta earthquake of 1989. It caused significant damage in
Ferndale. The second large event, at 00:41 PDT on April 26, caused greater
damage in Rio Dell, Fortuna, and Scotia, according to press reports. The USGS
source mechanism indicates that the mainshock had pure thrust motion on a
fault striking N45°W (approximately parallel to the San Andreas fault) and
dipping 30° to the east. The epicenters of the first and second large
aftershocks are northwest of the mainshock. The source mechanism of these
events indicates strike-slip faulting on a fault striking northeast.
|