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On April 25, 1992, a magnitude 7.1 earthquake occurred in the Cape Mendocino area.
Two additional earthquakes, magnitudes 6.6 and 6.7 occurred the next morning. The
first earthquake was located six miles north of Petrolia, California, in a sparsely
populated part of southwestern Humboldt County. Five small communities were located
within a 50 mile radius of these events: Honeydew, Petrolia, Rio Dell, Scotia, and
Ferndale. This slide set illustrates the effects and damage of a moderately large
earthquake and moderate aftershocks on this sparsely settled area.Statistics - The
first earthquake was located six miles The main earthquake came immediately after
a Saturday morning parade at Ferndale. Horses reportedly became spooked during the
parade (before the earthquake), but riders attributed it to flags fluttering in the
wind along the parade route.Seismicity and Geology - The Cape Mendocino region of
California's north coast is one of the most seismically active regions in the conterminous
United States. This seismic zone coincides with the Mendocino Triple Junction, a tectonically
unstable area where three tectonic plates join together.The earthquake of April 25
is exceptional for several reasons. It generated a tsunami that was recorded along
the West Coast and measurable in the Hawaiian Islands. It also produced distinct uplift
(maximum uplift = 1 m) along a 25-km-long section of the coastline. The April 25 event
was the largest historic earthquake in this region with an epicenter clearly located
on land, and the only damaging event that may have resulted from thrust faulting along
the Cascadia subduction zone.Earth Effects - A broad zone of ground fractures, coastal
uplift, and elevation changes are the only surface changes to indicate the location
and tilt of the fault plane. Strong ground shaking during the earthquakes triggered
numerous landslides in steep mountainous areas. A landslide at Scotia Bluffs (south
of Scotia) temporarily interrupted traffic on the North Coast Railroad. Tension cracks,
due to soil compaction and downhill slumping, restricted traffic on the Mattole Road
between Honeydew, Petrolia, and Ferndale. Liquefaction took place in modern deposits
of the Mattole River between Petrolia and Honeydew, and in the flood-plain deposits
of the Eel and Salt Rivers. Soils liquefy when ground water near the surface is forced
between the grains of sand during an earthquake. The sandy soil behaves like a very
thick liquid. Structures then settle or tip in the liquefied soil, or are ripped apart
as the ground spreads laterally or flows. Sand blows result from the eruption of liquefied
sediment from the subsurface on to the ground. The eruption of liquefied sediment
is facilitated by fissures in the ground, which are also produced by strong shaking
during an earthquake.Damage - Collectively, these shocks caused significant damage
to older structures in the sparsely populated, mountainous, epicentral region. The
earthquake caused 98 injuries and moderate damage in Humboldt County. Losses topped
$66 million. Single- and double-story, wood-frame houses and commercial buildings
were the most common type of structures in the epicentral region, where lumber is
the principal industry. Many foundations were the pier and post type, providing little
resistance to lateral ground shaking. The earthquakes jarred many older homes off
their foundations.
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