Category Archives: Geology

Landslide Threats in Southern California

Southern California has many areas that are susceptible to landslides in many forms. The past couple of weeks have demonstrated that these slides can have dramatic affects on the regions infrastructure. While most slides aren’t that preventable, the damage they can create can be mitigated.

Understanding the signs of a pending landslide are fairly easy and should not be ignored. If you live in an area where landslides are possible, look for these signs:

  • Ground cracks, particularly ones that appear to pull apart
  • Sinking areas or changes in ground level
  • Unexplained leaks in pipes
  • Tilting poles
  • New cracks appearing in a structure
  • New springs or areas where water seems to drain without appearing on the surface

These signs are important to look out for. The first one, ground cracks, are the most obvious. They tend to arcuate and numerous. The largest ones may mark the head of the slide, though that is not a precise indicator of how large a slide may become. Ground cover itself doesn’t prevent deep seated landslides but it can help with smaller surficial slope failures. If you see any of these signs, please contact an engineering geologist or other local official to help assess the likelihood of a failure. Doing so can help prevent injury, loss of life, and property damage.

Mudslides, such as those that recently blocked Interstate 5 in Grapevine Canyon and State Route 58 in Cache Creek Canyon, are more common on steeper slopes with loose material. These tend to happen more commonly in areas which have burned recently, leaving little plant material and a soil that is less porous than it was previously. Those conditions, combined with a heavy rainfall, can turn that material into a thick mud which can easily move larger objects, such as boulders and trucks. As such, these slides can be very dangerous and fast moving.

The California Geological Survey has put together a series of maps to help determine the likelihood of a slope failure. These maps cover most of the metropolitan regions of California and are a great resource. They should, however, only be used as a guideline for potential slope failures. A more exact analysis should be determined through a geological report on your specific location.

Earthquakes and Movies

Lately, there has been quite a bit of press about the recent “San Andreas” movie. To me, this movie sets back the general public’s knowledge and understanding about how earthquakes create damage in Southern California.

Some basic stereotypes exist in the movie, many of which are completely false. Starting with the magnitude of the earthquake in the movie – No fault line in Southern California is capable of anything larger than about an 8.2. The only one truly capable of such an event, the San Andreas Fault, is also many miles from Los Angeles and is mostly separated from the Los Angeles Basin by the San Gabriel Mountains. Anything larger than a 9.0 is in the domain of “megathrusts” or subduction zones. In California, the only subduction zone is the southern end of the Cascadia Subduction Zone, which ends at the “Mendocino Triple Junction” just offshore of Cape Mendocino. It last produced something close to a 9.2 or so in January 1700.

Tsunamis, especially ones of great height, are also not in the forecast for a large earthquake here in Southern California. Tsunamis are created by the large scale displacement of water, similar to sloshing in a bathtub. Move your hand below the water quickly, you create a wave on the surface. Usually, tsunamis that are related to earthquakes are caused by the movement of the fault itself, typically megathrust faults underneath the ocean. We just don’t have those in Southern California. Even the largest tsunami generated by such a fault may only be tens of feet high, certainly not hundreds of feet.

Big cracks just don’t open up in the land from earthquakes, certainly nothing like those represented in the movie. Fissures are created by earthquakes, however. These fissures are usually the result of settlement or fault movement. They aren’t that large either way.

Structural damage is also not going to be as great as represented. Mind you, a large magnitude earthquake centered in the Los Angeles Basin will do a great deal of damage. Water mains, sewer mains, gas lines, power lines, and other utilities will be compromised in many locations creating shortages and, in some cases, fires. Buildings may collapse or be damaged beyond repair. The underlying geology will determine some of the damage extent. The rest will be determined by building type and its susceptibility to seismic waves. Either way, the skyscrapers in Downtown Los Angeles won’t be toppling like trees anytime soon. I’d still stay away from the area after a major event though, as there would be an immense amount of glass and debris creating hazards for travel.

Keeping all this in mind, and also keeping with the theme that Southern California officials have been doing, use this opportunity to prepare yourself for a major earthquake. They can strike at any time and will create problems for all of us that live, work, and visit this region. The best way to survive a major catastrophe is to be prepared. Part of that preparation is to know your region, know the routes, and know where the problem areas may be following a major earthquake.

For further information, I highly recommend contacting your local Emergency Services agency in your city and county. They have a great deal of resources to help you prepare for an event like a major earthquake.

Image of the Week – 4/26/15

Salt crystals in Badwater Basin - Death Valley, CA
Salt crystals in Badwater Basin – Death Valley, CA

Image of the Week – 1/19/15

Normal fault in Cretaceous turbidite deposits in the La Jolla Shores area.
Normal fault in Cretaceous turbidite deposits in the La Jolla Shores area.

Image of the Week – 11/11/14

Bautista Canyon between Hemet and Anza in Riverside County. This follows the San Jacinto Fault.
Bautista Canyon between Hemet and Anza in Riverside County. This follows the San Jacinto Fault.

Image of the Week – 11/1/14

On the trail to Mt San Antonio along Devils Backbone.
On the trail to Mt San Antonio along Devils Backbone.

Image of the Week – 9/15/14

Old alignment of Hwy 2, now offset by about 2 feet horizontally and 5 feet vertically.
Old alignment of Mexico Hwy 2, now offset by about 2 feet horizontally and 5 feet vertically.

M4.6 near Big Bear Lake, CA

At 9:59am on July 5, 2014, a M4.6 earthquake struck near Big Bear Lake. The focus was 5.4 miles deep.  If you felt this earthquake, the USGS wants to know! Watch for debris on the roadways in that area (State Highways 18, 38, 138, 173, 189, and 330).

Shakemap showing intensity.
Shakemap showing intensity.
Map showing area where people felt the quake and at what intensity.
Map showing area where people felt the quake and at what intensity.

April 2010 Baja Quake – Part 2

In July 2010, I was able to go to Baja California with a friend. Part of our route traversed Federal Highway 2 (Mexico), which suffered some damage from the April 2010 quake. Hwy 2 had been repaired but the adjacent old alignment had not been. I was rather amazed at the amount of offset from this earthquake. I observed about 2′ of horizontal and about 5′ of vertical offset at the highway crossing.

East of the fault, bridge embankments on the Hwy 2 Mexicali Bypass show signs of settling.
East of the fault, bridge embankments on the Hwy 2 Mexicali Bypass show signs of settling.
Offset right of way fence next to Hwy 2.
Offset right of way fence next to Hwy 2.
Old alignment of Hwy 2, now offset by about 2 feet horizontally and 5 feet vertically.
Old alignment of Hwy 2, now offset by about 2 feet horizontally and 5 feet vertically.
Side view of the old alignment showing the vertical offset.
Side view of the old alignment showing the vertical offset.
Breaks in the soil to the north. Two distinct scarps can be noted here.
Breaks in the soil to the north. Two distinct scarps can be noted here.
At the break, the old alignment now has a steep ramp and crack.
At the break, the old alignment now has a steep ramp and crack.

M5.1 Earthquake hits La Habra

From the USGS – Updated:

http://earthquake.usgs.gov/earthquakes/eventpage/ci15481673#summary

intensity

lahabraci15481673_ciim

Event Time

  1. 2014-03-29 04:09:42 UTC
  2. 2014-03-28 21:09:42 UTC-07:00 at epicenter
  3. 2014-03-28 21:09:42 UTC-07:00 system time

Location

33.919°N 117.944°W depth=7.5km (4.6mi)

Nearby Cities

  1. 1km (1mi) S of La Habra, California
  2. 4km (2mi) W of Brea, California
  3. 5km (3mi) NNW of Fullerton, California
  4. 6km (4mi) E of La Mirada, California
  5. 546km (339mi) W of Phoenix, Arizona

Tectonic Summary

A M5.1 earthquake occurred at 9:09PM on March 28, 2014, located 1 mile easy of La Habra, CA, or 4 miles north of Fullerton, CA. The event was felt widely throughout Orange, Los Angeles, Ventura, Riverside, and San Bernardino counties.  It was preceded by two foreshocks, the larger of M3.6 at 8:03pm.  The demonstration earthquake early warning system provided 4 second warning in Pasadena.

There have been 23 aftershocks as of 10:00PM on March 28, the largest of which was a M3.6 at 9:30PM, and was felt locally near the epicenter. The aftershock sequence may continue for several days to weeks, but will likely decay in frequency and magnitude as time goes by.

The maximum observed instrumental intensity was VII, recorded in the LA Habra and Brea areas, although the ShakeMap shows a wide area of maximum intensity of VI. The maximum reported intensity for the Did You Feel It? map was reported at VI in the epicentral area.

This sequence could be associated with the Puente Hills thrust (PHT).  The PHT is a blind thrust fault that extends from this region to the north and west towards the City of Los Angeles.  It caused the M5.9 1987 Oct. 1 Whittier Narrows earthquake.

Previously, the M5.4 2008 Chino Hills earthquake occurred in this region.  It caused somewhat stronger shaking in Orange County and across the Los Angeles Basin.

The moment tensor shows oblique faulting, with a north dipping plane that approximately aligns with the Puente Hills thrust.

The demonstration earthquake early warning system provided 4 second warning in Pasadena.

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