Seismic Records from the Nevada Earthquake Swarm, as Recorded at Modesto Junior College

My Twitter feed and text messages went crazy the other night, early in the morning of December 28. For the first time in a number of years, an earthquake was felt widely in the Modesto area as a series of magnitude 5+ quakes struck an isolated region between Hawthorne, Nevada, and the ghost town of Bodie in eastern California. The first two quakes, both measuring 5.7 on the magnitude scale struck within four minutes of each other. A magnitude 5.5 event followed about an hour later. More than a hundred aftershocks, ranging as high as magnitude 4, have taken place in the last few days.

The quakes caused only minor damage, mainly to older buildings in Bodie, and I have heard no reports of any injuries. But dozens of my students and colleagues felt the quake in areas ranging from Fresno , the Bay Area, and Sacramento. I was far away, in central Oregon, and it took me several days before I could get home and check our seismometer at Modesto Junior College. To my relief, there had been no power outages, and the unit was functioning just fine. It clearly caught the three magnitude 5 events, and one can see the magnitude 4.0 nearly hidden by the waves of the first event.

There is nothing fancy about the seismometer; it's really a teaching tool, and not part of any networks. It does a good job of catching major events worldwide, and most 4+ events in the California/Nevada region. There is a display monitor outside my lab (the unit is inside the storage area on the third floor of the Science Community Center), and visitors can get a reaction on the monitor by jumping up and down outside the window. The computer application allows me to isolate any recorded events and analyze them. The records can be stretched out to help distinguish different wave arrivals.

Above is the arrival of the first two events, still somewhat compressed to fit them on one page. It is clear that they were very close in magnitude. The third event, shown below, has been stretched out a bit to emphasize the individual waves (notice the minutes scale across the bottom to compare the two pictures).

The swarm took place in a complicated region east of the Sierra Nevada where numerous fault lines accommodate the motion of the Sierra Nevada block to the northwest. Strike-slip faults (both left and right lateral) are part of the structural framework. The Sierra Nevada is a solid mass of rock considered by some to be a microplate between the Pacific and North American plates. The area of the swarm is part of the broken up crust that formed in the wake of the northwest movement of the Sierra block. Swarms have occurred in this area in the past, including one in 2011 (the largest event in that one was a magnitude 4.4).

The University of Nevada, Reno has lots of information on the swarm at http://www.seismo.unr.edu/

Late to the Party, but here's the 7.1 Magnitude Alaska Earthquake as Recorded from Central California

An earthquake, large by California standards, but almost moderate by Alaska standards, hit on Sunday, January 24 southwest of Anchorage. It measured 7.1 on the magnitude scale (moment magnitude). The quake was on a strike-slip fault (lateral motion), and was thankfully relatively deep (128 km/80 miles). Deep is good because the waves had to travel 80 miles just to get to the surface; it's like being 80 miles away from the epicenter of a shallow quake. Because of this, the damage was minor.

Our recording device is a simple classroom demonstration seismometer, but it has proven capable of catching a good record of large distant quakes, and smaller local ones. It's available for less than $1,000 at science supply outlets like Wards Science.

Seismogram of Napa 6.0 Magnitude Earthquake from Modesto

One of the nice things about the new Science Community Center at Modesto Junior College is that we have been given some wonderful tools for the teaching of science. We had a modest seismometer at the old facility, but the software ran off Windows 97, and had not been updated since. We purchased a new seismograph unit from Wards Science, and although it is a simple apparatus, it has been giving us some excellent data on quakes worldwide. We got a good reading on today's event in the Napa Valley, a magnitude 6.0 shaker, the largest in the Bay Area in 25 years. That was the Loma Prieta Earthquake of 1988, a magnitude 6.9 quake that killed dozens, and collapsed bridges and overpasses in Oakland and San Francisco.

The bottom image is the raw data. Each horizontal line on the screen represents one hour of recording time, and so about two days are represented. The waves reverberated through our area for about eight minutes or so. The computer program allows us to isolated the event and manipulate the appearance of the waves, so the upper image is the same data, only spread out. Modesto is located about 100 miles from Napa as the crow flies, and the largest waves were off-scale. I'm pretty sure there were people in Modesto who felt the event.

The View of Japan's 9.0 Earthquake from Modesto, California, Part 2

The other day I provided a few screen shots of the seismic record that we captured in Modesto of the Sendai Earthquake in Japan. As I noted in that post, our seismometer is an old teaching model with deeply outmoded computer software, and all I was able to show were some screen shots. Well, I loaded the program on an older computer in the department, transferred the earthquake data to a floppy disk (remember those??), transferred the data onto a third computer where I could transfer the data to a CD-ROM, and finally was able to send the output to a printer. So today I offer the complete record. It's kind of astounding. The top picture is the initial minutes of the quake. You can see the amplitude of the waves rapidly increasing until they were off-scale. The notations on the left are the elapsed minutes, four minutes to the inch.

So what did the whole thing look like? At four minutes per inch, the print job took eight pages. I pasted them together and hung them on the wall, as seen below. This was a big earthquake, as if anyone needed to be told...