Great
Earthquakes In Diverse Places
California: More than 70 earthquakes have shaken the Pacific
coastal State since late Thursday
Posted By Mark Dunphy On July 16, 2011 @ 1:50 PM
San Francisco
Earthquakes are a common occurrence in California but the past few
days have seen a notable rise in the number of tremors recorded in
the US State.
More than 70 earthquakes [2] have shaken the Pacific coastal State
since late Thursday, the latest being a magnitude 3.3 tremor in
the San Francisco Bay area on Saturday morning.
The shallow quake struck at a depth of just 6.4 km (4.0 miles) at
03:51 AM local time (11:51 A.M. GMT). According to the United
States Geological Survey (USGS [3]) the quake epicentre was
located 2 km (2 miles) ESE (115°) from Berkeley, 4 km (3 miles) NE
(47°) from Emeryville, 5 km (3 miles) NNW (340°) from Piedmont,
and 8 km (5 miles) NNW (345°) from Oakland.
Over the past 48 hours earth tremors have been felt throughout the
state, including the Greater Los Angeles area, San Diego and the
San Francisco Bay area. The majority of the earthquakes ranged
between 2-4 magnitude. Other areas to experience earth tremors
include Los Banos, Holtville, Parkfield, Pinnacles, Johnville,
Valle Vista, La Jolla, Anderson Springs, Cloverdale, Cobb,
Hawthorns, Anza, The Geysers, Mettler, San Carlos, Hemet and
Keene.
Meanwhile, a magnitude 4.0 earthquake struck near Baja
California’s (Mexico) border with Arizona (USA) early on Friday.
The quake was located 36 km (22 miles) S (180°) from Guadalupe
Victoria, Baja California; 63 km (39 miles) SSW (210°) from San
Luis Roo Colorado, Sonora, Mexico; 67 km (41 miles) SSW (209°)
from San Luis, Arizona; and 190 km (118 miles) ESE (109°) from
Tijuana, Baja California.
The shallow earth tremor struck at 11.37 a.m. GMT (3.37 a.m.
local time) at a depth of approximately 7 km (4.3 miles), reports
the USGS. The quake was followed by a 3.6 magnitude tremor in the
early hours of Saturday morning.
Baja California also is located along the San Andreas Fault.
SEISMIC ACTIVITY IN CALIFORNIA
Earthquakes in California [4] are common occurrences as the state
is located on the San Andreas Fault, which traverses across
California and forms the tectonic boundary between the Pacific and
the North American Plate. There are thousands of small earthquakes
per year, most of them are so small that they are not felt.
However, scores of earthquakes in the 2-4 magnitude range have
been recorded in the past few days.
California has hundreds of active faults located throughout the
state that are capable of producing large earthquakes. The most
active fault is the San Jacinto Fault in Southern California,
which has produced large events on a regular basis throughout
recent history. The Mendocino Triple Junction located offshore of
Northern California is also very active, producing several
earthquakes above magnitude 7 throughout history.
EARTHQUAKE SWARMS
Earthquake swarms [5] are events where a local area experiences
sequences of many earthquakes striking in a relatively short
period of time. The length of time used to define the swarm itself
varies, but the United States Geological Survey (USGS [6]) points
out that an event may be on the order of days, weeks, or months.
They are differentiated from earthquakes succeeded by a series of
aftershocks by the observation that no single earthquake in the
sequence is obviously the main shock. Earthquake swarms also are
one of the events typically preceding eruptions of volcanoes.