Our Trembling Ea rt hby Paul Remeika Educational Bulletin #1 0-2 A publication of the Desert Protective Council www.dpcinc.org“Earthquakes Shake Valley.” “Quakes Jar Wide Area.” “Quake Biggest Known in Desert.” “Twin Quakes Jolt Area.” “Atershocks Keep Desert on Edge.” “5.6 earthquake rattles region.” “Desert Quakes Shake up the World oGeology.” “Earthquake centered on active, dangerous ault.” “7.2 quake rocks region.” On a scale worthy oGenesis, a zig-zag pattern oen echelonstrike-slip aults such as the Cerro Prieto, Imperial, and San Andreas tears the land horizontally in a northwest right-lateral sense, osetting in opposite directions the northernmost sea- oor spreading centers that have migrated up the GuloCali- ornia. Like the seams on a baseball they defne pull-apart basins along a plate boundary as Alta and Baja Caliornia rit and rat obliquely away rom mainland Mexico, opening up a new seawayflled by the Sea oCortez. In doing so, new crust is generated beneath these basins at Cerro Prieto, Brawley Seismic Zone, and at the Salton Buttes. Other dynamic aults, such as the brutally-powerul San Ja- cinto, Elsinor e, Laguna Sa lada, and Sierra Juarez, stretch and t hin the crust through dip-slip (vertical) motion with subordinate strike-slipping. They control elongated northwest-trending desert mountain ranges like the ault-bounded Sierra Cucapá, with its asymmetric sedimentary basin (Laguna Salada), both canted into the depths othe Salton Basin incompletely buried in a vast arena osand and sea. These aults have uplited the Peninsular Ranges and widened the valley, making it one othe lowest and driest places on the North American continent. As a daily consequence oaulting, or indigestion , eart hquakes are dangerous natural hazards that annual ly assault the troubled belly othe Salton Basin. This restlessness has t riggered at least 34 earthquakes greater than M6.0 in the basin since 1850. That’s an average oone such strong earthquake every 4.6 years. Since 1954 the area has experienced 50 shakers measuring M5.0 or greater, and thousands olesser events. It is one othe most geologicallyintense places on Earth. Up until April 4, 2010, it had been 23 years since the last large temblor occurred within the basin— the M6.6 Superstition Hills Earthquake on November 24, 1987. Dancing With Demons The Salton Basin is indeed restless, as quake-jittery residents oBrawley, El Centro, and Mexicali can attest. The most recent swayi ng began between Febru ary 8 and 21, 2008 as a swarm o500 temblors shook the ground beneath Guadalupe Victoria and the Cerro Prieto Volcano. The largest, on February 8, measured M5.4. It was powerul enough to knock out electricity and cellphone ser- vice to over hala million people in Mexicali. This initial sha ke was ollowed by two modest atershocks three days later on February11, registering M5.1 and M5.0. More rumbles occurred on Febru- ary 27 when six earthquakes measuring between M3.0 and M4.1 hit along the Imperial Fault. These harbingers raised the blood pressure othe scientifc community, which sponsored a “Great Southern Caliornia Shakeout” drill on November 13, 2008. The drill simulated a mock M7.8 temblor with an epicenter beneath Bombay Beach along the eastern shore othe Salton Sea. Four months later, between March 21 and 24, 2009, a handul Photo 1: Quake damage to Calexico’s red-tagged Hotel de Anza. All photos by Paul Remeika
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Educational Bulletin #10-2A publication of the Desert Protective Council www.dpcinc.org
“Earthquakes Shake Valley.” “Quakes Jar Wide Area.” “Quake
Biggest Known in Desert.” “Twin Quakes Jolt Area.” “AtershocksKeep Desert on Edge.” “5.6 earthquake rattles region.” “DesertQuakes Shake up the World o Geology.” “Earthquake centeredon active, dangerous ault.” “7.2 quake rocks region.”
On a scale worthy o Genesis, a zig-zag pattern o en echelon strike-slip aults such as the Cerro Prieto, Imperial, and San
Andreas tears the land horizontally in a northwest right-lateralsense, osetting in opposite directions the northernmost sea-
oor spreading centers that have migrated up the Gul o Cali-ornia. Like the seams on a baseball they defne pull-apart basinsalong a plate boundary as Alta and Baja Caliornia rit and rat
obliquely away rom mainland Mexico, opening up a new seaway flled by the Sea o Cortez. In doing so, new crust is generated
beneath these basins at Cerro Prieto, Brawley Seismic Zone, andat the Salton Buttes.
Other dynamic aults, such as the brutally-powerul San Ja-cinto, Elsinore, Laguna Sa lada, and Sierra Juarez, stretch and thinthe crust through dip-slip (vertical) motion with subordinate
strike-slipping. They control elongated northwest-trending desertmountain ranges like the ault-bounded Sierra Cucapá, with its
asymmetric sedimentary basin (Laguna Salada), both canted intothe depths o the Salton Basin incompletely buried in a vast arena
o sand and sea. These aults have uplited the Peninsular Rangesand widened the valley, making it one o the lowest and driestplaces on the North American continent.
As a daily consequence o aulting, or indigestion, earthquakes
are dangerous natural hazards that annual ly assault the troubled
belly o the Salton Basin. This restlessness has triggered at least 34earthquakes greater than M6.0 in the basin since 1850. That’s anaverage o one such strong earthquake every 4.6 years. Since 1954the area has experienced 50 shakers measuring M5.0 or greater
and thousands o lesser events. It is one o the most geologicallyintense places on Earth. Up until April 4, 2010, it had been 23
years since the last large temblor occurred within the basin— theM6.6 Superstition Hills Earthquake on November 24, 1987.
Dancing With DemonsThe Salton Basin is indeed restless, as quake-jittery residents
o Brawley, El Centro, and Mexicali can attest. The most recentswaying began between February 8 and 21, 2008 as a swarm o 500
temblors shook the ground beneath Guadalupe Victoria and theCerro Prieto Volcano. The largest, on February 8, measured M5.4
It was powerul enough to knock out electricity and cellphone service to over hal a million people in Mexicali. This initial shake waollowed by two modest atershocks three days later on February
11, registering M5.1 and M5.0. More rumbles occurred on February 27 when six earthquakes measuring between M3.0 and M4.1
hit along the Imperial Fault. These harbingers raised the bloodpressure o the scientifc community, which sponsored a “Great
Southern Caliornia Shakeout” drill on November 13, 2008. Thedrill simulated a mock M7.8 temblor with an epicenter beneathBombay Beach along the eastern shore o the Salton Sea.
Four months later, between March 21 and 24, 2009, a handu
Photo 1: Quake damage to Calexico’s red-tagged Hotel de Anza. All photos by Paul Remeika
o medium-sized M3.1-M3.3 quakes and several hundred smaller
atershocks rattled the Salton Buttes. One stretch recorded a shockevery hour or 48 hours. In the middle o all this mayhem an M4.8
thumper in the early morning hours o March 24, ollowed later by
an M3.5 bump in the aternoon o Apri
8 punched in where the San AndreasFault meets up with the spreading center
beneath Bombay Beach. The swarm con-tinued or two more weeks triggering an
unprecedented 400 microquakes.South o the border, more shaking
abruptly jolted Guadalupe Victoria and
Cerro Prieto on March 29 (M4.2), Apri
11 (M4.2), April 12 (M4.3), and June2 (M3.0). Between November 1 and 32009, the area o Heber and Calexico
also got exciting when low-level seismicswarms along the Imperial Fault registering between M3.0–M4.1 gave residents
a renewed appreciation or the power onature. They were ollowed by a wrench-
ing M5.8 quake December 30 near CerroPrieto and the Imperial Fault. This quake
was elt as ar away as Kern County, Cali-ornia. Most o the shaking occurred inand around Mexicali where 90,000 peo-
ple lost power or hal-an-hour. On Janu-ary 9, 2010, March 31, and April 4, the peace and quiet o the same
general area (Guadalupe Victoria) was shattered again at the M4.1M4.2, and M4.3 levels, respectively: ominous warning signs that in-
dicated a volcanically restless magma chamber beneath Cerro Pri-
Above: Photo 2: Larry McCaffery straddles a crack on Mexico Route 2 south of Cerroel Centinela. Phillip Carskaddan on right. Below: locations of faults, earthquakes, andphotos discussed in the text. Graphic by Phillip Carskaddan and Paul Remeika
to extensive leakage rom broken canallevees, aqueduct spillage, liqueaction, or
ironically, a lack o running water. Whenrocks suddenly break along a ault and a
massive temblor is unleashed, it can be aharrowing experience. Due to the inten-sity o ground shaking many rightened
residents moved into the streets to shelter
in tents and vehicles. By the next morning,100 rumbling atershocks in the M3.0–M4.5 range had rayed everyone’s nerves.
Within 48 hours the atershock total hadclimbed to 700, reaching 3,000 in the twoweeks ater the main quake. Structural
damage to personal property, businesses,and agriculture throughout Mexicali and
surrounding communities was estimatedat over $300 million. Incredibly, the earth-
quake only injured about 200 people, con-tributing indirectly to two deaths. By April27 over 6,000 atershocks had been recorded, making seismolo-
gists nervous as this seemed to be a higher rate than normal.North o the border, the entire downtown district o Calexico
was declared o-limits due to tons o alling wreckage rom col-lapsed ceilings, stucco walls, cladding, broken glass panes, retain-
ing walls, and roofng tiles. Two-story buildings sustained morestructural distress than single story structures. Many were red-agged, including the aging Hotel de Anza which suered irrepa-
rable damage (Photo 1, pg. 1). The town’s main water clarier wasthrashed along with 120 miles o broken water and sewer lines.
Many mobile homes were knocked o their oundations, therewere widespread power and telephone service outages, the Impe-
rial Airport terminal was put out o commission, and westboundlanes o Interstate 8 were temporarily closed due to cracked pave-
ment. Property damage exceeded $91 million. Schools were stillclosed at the beginning o May.
The El Mayor-Cucapah Earthquake was a breakaway margin
event, whereby tectonic orces literally rip the terrain apart, verti-cally and horizontally. Mountain ranges, such as the Sierra Cu-
capá and Sierra El Mayor tend to lean to the east or to the west asthey seek equilibrium like icebergs aoat on the ocean surace.
This is a common occurrence along the edge o an expanding ri tboundary such as the west side o the Imperial-Mexicali Valley.
On June 14, a M5.7 atershock whipsawed the desert fve miles
south o the tiny community o Ocotillo. It was centered in alarge cluster o atershock sequencing at the northern terminus
o the ault rupture west o Cerro el Centinela (Signal Mountain),bunching up along the eastern side o the Jacumba Wilderness
Area north o the border. This post-quake dirge o activity is com-monly associated with crustal stretching. In Ocotillo, the temblorshattered windows and cracked walls, broke pipes, and toppled
book cases. In San Diego, it briey interrupted the Padres baseballgame at Petco Park.
Geologic afcionados can visit the ault scarp without much e-ort. The easiest place, shown in photo 2, is located along old Mex-
ico Highway 2 in the saddle south o Cerro el Centinela, about 15miles west o Mexicali. Here resh cracks in the asphalt can beexamined and traced northward across the Caliornia-Mexican
border into the Yuha Badlands. It is here that clearly disturbed
ground along the Borrego strand o the Laguna Salada Fault re
veals cumulative vertical displacements o about two eet, withlateral osets even more. Two months ater the main event, High-
way 2 is still under repair as the ground continues to adjust. An-other location is about 10 miles south along the eastern side o the
Laguna Salada playa (Photo 3). I the lakebed is dry, it is worththe eort. Ater a one-mile hike up an unnamed sot sandy washthe 8-oot-high ault scarp is located on the Borrego Fault con-
trolling a small strike valley where the Sierra Cucapá splits intowest and east mountain ranges. The ground is extensively cracked
with rupture lines cutting crysta lline basement as well as unconsolidated gravel veneers and arroyo bottoms. At the ault scarp
the land east o the ault has dropped dramatically, while the wesside has risen, blocking all drainage channels in the arroyos. Thi
is a spectacular example o a shutter ridge, since older arroyos areshingled one on top o the other, altered rom similar past activityalong the ault. Stay tuned or more shaking!
Editor’s note: As i to underscore Paul Remeika’s last sentencein this piece, the Salton Basin landscape jolted again as we went
to press. A M5.4 earthquake occurred in southern Caliornia a4:53 pm (PDT) on July 7, its epicenter 13 miles north-northwest o
Borrego Springs. The ear thquake, triggered by the M7.2 El MayorCucupah Earthquake, occurred on the Coyote Creek segment othe San Jacinto Fault. The earthquake exhibited sideways hori-
zontal motion to the northwest, consistent with slip on the SanJacinto ault. It was ollowed by more than 60 atershocks o M1.3
and greater during the frst hour. Seismologists expect continuedatershock activity.
Paul Remeika is a retired Caliornia State Park Ranger, and is an
expert on the paleontology and geology o the Anza-Borrego DesertHe authored the best-selling book Edge o Creation: the Geologyo Anza-Borrego Desert State Park. He spends quality time as aField Program Instructor and Guide or the Anza-Borrego Founda-
tion and Institute. His feld trips are very popular and include hisspecial Desert Hostel visits onto the Colorado Plateau which are usually booked months in advance. He is currently completing reelance
research on ossil ootprints rom the Colorado Desert.
Photo 3: Large fault scarp clearly visible east of the Laguna Salada playa. PhillipCarskaddan (left) and Larry McCaffery shown for scale.