Magnitude 4.8 Earthquake (and 4.3 Aftershock) in Eastern Sierra Nevada near Big Pine

I wandered past our department seismometer (at Modesto Junior College in central California), and noticed some disturbances that didn't look like the record of students jumping up and down. They indeed turned out to be a pair of moderate earthquakes of magnitude 4.8 and 4.3 with an epicenter about 7 miles (10 kms) WNW of Big Pine, a small village in the Owens Valley east of the Sierra Nevada. A 4.8 magnitude can be a pretty sharp jolt, but this is a sparsely populated area, so I don't expect to hear of much in the way of damage.

The epicenter is just north of the Palisades Group of peaks on the eastern boundary of Kings Canyon National Park. They are the snowy glaciated peaks in the picture above. Big Pine is just to the left of the picture on the valley floor.

4.8 magnitude main shock

The fault that produced the quake shows right lateral strike slip motion. This means that the two sides of the fault moved sideways relative to each other, with the southwest side of the fault moving northwest. Since the Owens Valley is a fault graben, one might assume that the fault motion should have been vertical, i.e. lifting the mountains or dropping the valley, but the Sierra Nevada as a structural block is moving northwest relative to the rest of the continent, so this quake is consistent with that motion.

Sierra Nevada as a "microplate" moving northwest. Big Pine is labeled BP. Source: http://tahoequarterly.com/Features/Story/ice-fire-and-granite

The regional is seismically active. One of California's great historical earthquakes had an epicenter a short ways to the south at Lone Pine. The 1872 event had a magnitude estimated to be  as high as 7.9. The quake killed 27 people, nearly a tenth of the population of the village at the time. Ground ruptures extended from Owens Lake to north of Big Pine.

The 4.3 magnitude aftershock

Our seismometer is a simple classroom instructional model (from Wards Scientific), but we get good results with local quakes of moderate magnitude, and larger quakes worldwide.

The U.S. Geological Survey event page for the magnitude 4.8 quake can be seen here: http://earthquake.usgs.gov/earthquakes/eventpage/nc72592670#general_executive

Vagabonding on Dangerous Ground: Following the Cascadia Subduction Zone on Highway 101

Lake Crescent in Olympic National Park at the north end of Highway 101.

I'm sure that NO ONE in the various forms of mass media WOULD EVER stoop to using HYPERBOLE to increase their readership/viewership. You know, saying things like "THE BIG ONE IS COMING", that the GIGANTIC TSUNAMI is going to DESTROY EVERYTHING WEST OF I-5, that EVERYONE WILL DIE in the BIG EARTHQUAKE (or at some point decades afterward of old age). I would, for instance NEVER use capital letters in my opening paragraphs to try and catch the reader's eye. That being said, I really did have half of my trip completed before the media storm began concerning the chances for a magnitude 9 earthquake in the Pacific Northwest. I would have been describing my trip anyway, but the brouhaha over the possible earthquake and tsunami provides an opportunity to inject some level-headed geology into the discussion.

Route of Highway 101 through California, Oregon and Washington (red line). Interstate 5 runs parallel to Highway 101 just inland.

The fact is that the Pacific Northwest IS dangerous ground. But so is California. So is Oklahoma. And Minnesota. And Florida. And every bit of land across the world. There is nowhere that humans can live on this planet that is free of natural hazards. It doesn't work to go moving somewhere else, because no place is truly safe. The best we can do is to is to understand the nature of the threat, and to be prepared for the disasters when they happen.

The latest tempest involves a New Yorker article on the coming magnitude 9 earthquake along the Cascadia Subduction Zone. The basics of this potential earthquake have been known for nearly half a century, ever since the theory of plate tectonics came to be accepted by geologists. The details of a massive earthquake that took place in 1700 came to be known more than a decade ago. But it seems there is sometimes a tipping point when the public becomes aware of such hazards, and the New Yorker article seems to have done the trick. People in the northwest are talking earthquakes and tsunamis. The Cascades volcanoes must be feeling very neglected right now; they were once the main topic of any discussion of geologic hazards in the region.

The Juan de Fuca and Gorda crustal plates, among the world's smallest. The Cascadia Subduction Zone is the black line with the converging arrows. Many of the most important Cascade volcanoes are shown as red triangles.Source: NASA

So our journey quite literally was an exploration of nearly the entire subduction zone, as seen from U.S. Highway 101 and Highways 1 and 99 in British Columbia. As can be seen in the diagram above, the zone begins near Cape Mendocino in Northern California and extends to Vancouver Island in British Columbia, Canada. The highway parallels the coastline, and follows the coastal terraces and beaches in many places, especially in Oregon. We spent ten days making our way north from San Francisco to Canada and northern Washington, and three days heading home again along Interstate 5. We camped when spots were available, and stayed in hotels (if we had to).

Interstate 5 is a freeway from Mexico to Canada. It's a fast and efficient way to get from one place to another (except for the traffic jams in Seattle and Portland). But it's also not very interesting over much of its length. Highway 101 is more a patchwork of freeways, highways, and two-lane roads, and because of the challenges of the geology it is a route to be followed in a more leisurely manner. It is far more scenic, and for that manner, more dangerous in the geologic sense. It's never a surprise to see signs referring to falling rocks and tsunami evacuation routes.

Side view of the Cascadia Subduction Zone (source: U.S. Geological Survey)

The source of all the geological mayhem is one of the smallest crustal plates in the world. The Juan de Fuca plate forms a few hundred miles out in the Pacific Ocean where new basaltic lavas emerge from an oceanic divergent boundary. The plate moves east at a rate of few inches per year. It then dives beneath the North American Continent, where the buildup of frictional stress results in earthquakes, both big and small. Eventually the plate begins to melt, releasing plutons of molten rock that sometimes leak out at the surface, forming the Cascades volcanoes like Rainier, St. Helens, Crater Lake and Shasta.

Source: http://crew.org/sites/default/files/cascadia_subduction_scenario_2013.pdf

I imagine you could be thinking "what's the difference between this blog series (Dangerous Ground), and my just completed series (Driving Across the Most Dangerous Plate Boundary)? The facetious answer is, of course, that in this series, we will be driving along the plate boundary instead of across it. But the more serious answer is that the ancient Cascadia Subduction Zone in Central California is extinct. It stopped activity tens of millions of years ago. The subduction zone of the Pacific Northwest is presently active, and capable of causing catastrophic damage. And the landscape is different in many ways. In the series I plan to talk about the geology, of course, but I also find it to be a land of majestic beauty. That's the thing about geological hazards; they may be dangerous and cause human catastrophes, but they also result in some of the most spectacular landscapes on the planet.

4.4 Magnitude Earthquake near Pinnacles National Park

 A 4.4 magnitude earthquake at a depth of about 10 kilometers struck Central California near Pinnacles National Park early this morning. We got an excellent seismic record of the event at Modesto Junior College, as can be seen in the picture above.

Earthquakes of this magnitude are generally felt, but are not likely to cause serious damage. The shake map generated by the U.S. Geological Survey shows that extent of the region where the quake was felt.

The earthquake looks to have occurred on the San Andreas fault, but in detail it was more than two miles away from the fault. The movement was compressional (a "reverse" fault), rather than lateral, as would be expected on the San Andreas. The odd looking ball below is how seismologists are able to tell the type of causative fault. It is generated by the network of seismometers across Northern California, measuring whether the first wave or motion of the quake is compressional or extensional.

There is a small percent chance (5% or less) that this moderate/small event is a foreshock, or precursor earthquake to a larger event. Such moderate events are good reminders that the people who live in earthquake country must be prepared for large earthquakes. Check your emergency supplies, read up on earthquakes, and be prepared with an emergency plan for your family.

4.2 Magnitude Earthquake near San Juan Bautista

What else are you going to call a restaurant situated right on the San Andreas?

A 4.2 magnitude earthquake struck at 10:26 PM PST tonight about 3 miles south of the town of San Juan Bautista in the Coast Ranges of Central California (the official USGS report is here). It was preceded by a magnitude 3.6 foreshock at 10:21 PM. It was felt over a fairly large region, but quakes of this size rarely cause damage. The San Andreas is the major active fault present in the region, although the zone includes a few other fault strands, including the Vergeles fault, which is listed on the state map as having been active within the last 700,000 years.

Mission San Juan Bautista and the scarp of the San Andreas fault

Mission San Juan Bautista is quite famously built on a scarp of the San Andreas, and was at the south end of the fault rupture that occurred during the 1906 San Francisco earthquake (magnitude 7.8). To the south, the fault is creeping on a continual basis, as described in some of my recent posts.

There is a very slight chance (5% or less) that this quake is a foreshock to a larger magnitude event, as the San Andreas fault has over a century of accumulated stress. There is no way to say until a larger quake takes place. It is good to have moderate quakes like this once in a while to remind all of us of the necessity of being prepared. The larger quakes, when they come, will cause extreme damage to our infrastructure, meaning power outages lasting for days, disruptions of transportation corridors and communications (no internet!). Always have an emergency kit with extra water, food, radio, flashlight, batteries, and first aid supplies in your house and in your car.

Source: Southern California Earthquake Data Center

I've lived in California for most of my life, but it's been quite a few years since I've felt any (and yes, be careful what one wishes for). I was in Hollister on Saturday waiting to feel a quake, but was out of luck...

When a Magnitude 6.9 Earthquake is an Aftershock....

There wasn't a whole lot of information when I wrote about the 8.8 earthquake in Chile in the earliest hours of the morning. Information is starting to flood in now from many sources, and my first impression is media coverage is a bit better than it was for the Haiti quake and other earlier events. When I say "better", I don't mean more cameras, I mean evidence of an effort by the reporters to educate themselves about the basic science of earthquakes.

My first thought is for the people who have been affected, both in Chile and across the Pacific basin. A quake this big is going to have huge impacts. The aftershocks are going to be an issue: my USGS notifications are set for any quakes bigger than 5.5 and there have been 11 of them so far, with the largest at 6.9. For perspective, 6.9 is the size of the Loma Prieta earthquake here in California from 1989. It killed 4 dozen people and did around $10 billion in damage. The quake in Haiti was a somewhat larger 7.0. Aftershocks are going to continue for a long time, over a region about 600 miles long (the size of the fault zone that shifted in this quake).

The tsunami is a huge threat. The size of a tsunami in any particular place is dependent on many factors, including the size of the earthquake (which was colossal), how the quake transferred energy to the ocean water (unknown to me at this point), and the shape and depth of the coastline where the tsunami strikes. All I can say to my Pacific Basin friends is that when the Civil Defense folks set off the sirens, take it seriously! Hawaii in particular has a tragic history with tsunamis, and there has been a vast amount of coastal development since the last major tsunami (in 1960, also caused by an earthquake in Chile). There will be a big problem of people wandering down to the beach to watch the tsunami, which can only be described as an act of idiocy. Get to higher ground or higher floors. If the warnings are inaccurate, you've only lost a bit of time, but if they are accurate, your life will be saved. Don't go to the coast until the "all clear" is given, because there will be more than one surge of water. You can't swim your way out of a tsunami; they are one of the most dangerous of geologic events, as the Indonesian tsunami of 2004 demonstrated.

Magnitude 8.0 quake in Samoa

A magnitude 8.0 quake has struck in the Samoa Islands of the Pacific. Andrew Alden at About.com is on top of this with some details here. The official USGS report is here. I only want to add that my cheap little classroom seismometer is still going off-scale more than an hour after the event. It doesn't mean that the ground is still heaving in Samoa, but instead is showing that the entire earth is reverberating as the waves pass through the entire planet, reflecting and refracting off the interior boundary layers and discontinuities. One might compare it to the gonging of a bell (the "earthquake") and the ringing tones that follow for a few moments.

You can check out the current shaking as it is recorded in California right here. Click on any of the stations to see the wave activity.