News

In the weeks following the magnitude 7.2 Easter Sunday earthquake that had much of southern California rocking and rolling more than it has in almost 20 years, many residents became acutely aware of every aftershock. For seismologists and geophysicists, it meant a wealth of new data - and new clues about whether this temblor could trigger a larger, more deadly earthquake along the densely populated San Andreas Fault.

UC San Diego's High Performance Wireless Research and Education Network (HPWREN) provided a steady stream of data in the two weeks following the quake, which occurred at 3:40 p.m. on April 4, its epicenter 39 miles (83 km) SSE of Calexico, in Baja California, Mexico. The earthquake - the Golden State's largest since 1992 - created a rolling, shaking sensation for almost 90 seconds throughout southern California and northern Mexico. At least three deaths and several hundred injuries were reported in Mexico.

"As part of the HPWREN partnership with many researchers, we have been generating a wide array of new and unique data from our network of sensors throughout HPWREN's roughly 20,000 square-mile area of connectivity," said Hans-Werner Braun, a research scientist with the San Diego Supercomputer Center at the University of California, San Diego, and co-founder of HPWREN, now in its tenth year. "For many residents this was a quite alarming event, and we are thankful that the devastation was not widespread. For us, however, this was very exciting because we are receiving the new, real-time data and hope to gain a much better understanding of what's going on beneath our feet."

HPWREN's network provided the generation and capture of data that measure the Baja earthquake in myriad ways. Added Braun: "We collectively have several new data sets that are being shared with various agencies in the region, with the hopes of developing even more effective hazard response efforts for various kinds of emergencies."

Solid to Liquid State
One such data set focused on the study of liquefaction, or the loss of strength as ground soil turns from a solid to a liquid state due to a massive stresses and forces that occur during a major earthquake. Measuring liquefaction at UC Santa Barbara's Wildlife Refuge site about 68 miles northwest of the epicenter, the peak ground acceleration recorded at the ground surface was just over 0.1g, as surface waves continued to shake the Imperial Valley for several minutes after the initial arrival of the earthquake.

"The M7.2 event produced an exciting and unique data set, showing the process of excess pore pressure generation that leads to the liquefaction of soils, which can cause significant damage to the built environment," said Jamie Steidl, a research seismologist at UC Santa Barbara. "It is through this type of data that scientists will be able to better predict liquefaction during earthquakes, and engineers will be able to better mitigate the damaging effects."

Early Warning, Rapid Modeling
In another measurement, HPWREN's connectivity with the California Real Time Network (CRTN), a grid totaling about 130 GPS receivers, allowed researchers to measure ground displacement just as the earthquake began. Scientists were able to compute positions of the GPS stations, once per second and with a latency of one second. The time series of these positions provide total displacement waveforms, which record both seismic (dynamic) and static, or permanent (coseismic) displacements.

In a message sent within the first hour of the initial event, Yehuda Bock, a geodesist with the UC San Diego/Scripps Institution of Oceanography's Institute of Geophysics and Planetary Physics (IGPP), noted that the CRTN's early warning system immediately picked up the dynamic shaking, recording peak-to-peak displacements of almost one meter, and up to 10 centimeters of permanent deformation in the horizontal plane.

"We've been building CRTN over the last 10 years, relying heavily on HPWREN to bring the 1 Hz data back in real time with a latency of about 0.4 seconds," noted Bock. "We've successfully tested throughput of up to 20 Hz data with HPWREN. Near-source regional GPS networks really came of age with this earthquake as an essential tool for early warning and rapid modeling of medium to large events. The real-time GPS data did not "clip" - or the signals were not distorted by the intensity of the earthquake - while most broadband seismic stations did clip for this earthquake, even at Piñon Flat Observatory (PFO), more than 100 miles (180km) from the epicenter."

Strainmeters Straining
According to Frank Wyatt at IGPP, the shaking was so strong that even the strainmeter at PFO couldn't track the greatest motions. "This earthquake demonstrated the power of real-time high-rate GPS data," said Bock. "They measure dynamic displacements directly, they do not clip, and they are also able to detect the permanent, or coseismic, surface deformation."

The sheer power of the Baja earthquake, which could be felt more than 200 miles away in Los Angeles, was also recorded by 17 broadband seismographic stations that span from San Diego as far north west as off the coast of Long Beach, and as far northeast as Riverside. Operated via HPWREN connectivity by the Anza Group at UC San Diego, all 17 stations in the network, including Monument Peak, the closest station to the event, had their broadband seismometer waveforms clipped due to the large magnitude of the quake.

"Fortunately, all of the ANZA seismic network is also equipped with strong motion accelerometers, which provided excellent on-scale recordings," said Frank Vernon, an IGPP research geophysicist and co-founder of HPWREN. "The primary use of these data is for rapid earthquake information determination. In addition, these data were prominently displayed for multiple television broadcasters in the two weeks following the event."

The Baja earthquake was also measured using longbase strainmeters, which produce high-quality data by measuring very small deformations of the Earth's surface over hundred meters baselines. Operated by UC San Diego's IGPP and connected via HPWREN, the devices provided seismologists with new data to further explore the possibility that the Baja event or one like it could trigger an even larger event on the faults extending into the highly populated regions of Mexico and California, most notably on the San Andreas Fault.

"Within minutes of the strong shaking, we were able to log into the HPWREN data-streams from the seven strainmeters alongside the Salton Sea and near Anza," said Frank Wyatt, of Scripps Institution of Oceanography. "Our findings were then reported to representatives of the agencies charged with assessing the hazards associated with the earthquake; HPWREN connectivity was crucial to the ongoing monitoring of this important information."

New Tool for Creating Ground Shaking Intensity Maps?
The U.S. Geological Survey (USGS) analyzes seismic data in real time in an effort to generate maps in the epicentral region that determine which areas shook the most. First-responders use this information to help rescue people trapped in collapsed structures. The generation of these "shake maps" is not instantaneous. Different aspects of the seismic data need to be analyzed separately, and the resulting map is often just a preliminary one that is later improved after more data have been analyzed. Even after several iterations, the resulting map can still be inaccurate.

The Earth's surface is like a speaker in your car. When it vibrates, it radiates sound. Earthquakes not only generate seismic waves that are felt for hundreds of miles away, but the surface shaking near the epicenter also generates low-frequency sound waves, called infrasound, that travel through the atmosphere for hundreds of miles. HPWREN's network allowed geophysicists to record infrasound from this earthquake at two arrays of microphones in southern California. Operated by the Laboratory for Atmospheric Acoustics (L2A) at UC San Diego, these arrays are located near the Marine Corps Air Station at Miramar and at PFO, near Palm Desert. The data from these arrays were transmitted in real time via HPWREN. The data analysis that occurred about an hour after the event suggested that the areas that shook the most in the epicentral region extended for about 43 miles (70 km) to the U.S. border.

"Timely retrieval and analysis of this data is important because it facilitates rapid dissemination of information that pertains to events of scientific interest and societal significance, such as sonic rumbles or booms," said Kris Walker, of UC San Diego's Institute of Geophysics and Planetary Physics group. "Recent advances in both infrasound data recording and analysis have improved our ability to use infrasound as a tool for learning more about ground-shaking processes. For example, real-time analysis of data from infrasound arrays located in seismically active areas may provide an independent and relatively direct measure of the intensity of surface shaking for Magnitude 7 or larger earthquakes and assist hazard response efforts."

The L2A group is also developing a website that will contain advisories from triggers that run on the infrasound array data in near real-time. "Such advisories will likely be useful to the local community," noted Walker. "Every year there are typically one or two sonic disturbances, often called "mystery booms", that generate hundreds of phone calls to government authorities and widespread interest for days."

Visit the web for full details on HPWREN-related activities on the Baja earthquake.

About HPWREN
The High Performance Wireless Research and Education Network (HPWREN) is a National Science Foundation-funded network research project, which also functions as a collaborative cyberinfrastructure on research, education, and first responder activities. It includes creating, demonstrating, and evaluating a non-commercial, prototype, high-performance, wide-area, wireless network in the San Diego, Riverside, and Imperial counties. The network includes backbone nodes at the UC San Diego and San Diego State University campuses, and a number of hard-to-reach areas in remote environments.

About SDSC
As an organized research unit of UC San Diego, SDSC is a national leader in creating and providing cyberinfrastructure for data-intensive research. Cyberinfrastructure refers to an accessible and integrated network of computer-based resources and expertise, focused on accelerating scientific inquiry and discovery. SDSC is a founding member of TeraGrid, the nation's largest open-access scientific discovery infrastructure.

Media Contacts:
Jan Zverina, SDSC Communications, 858 534-5111 or jzverina@sdsc.edu
Warren R. Froelich, SDSC Communications, 858 822-3622 or froelich@sdsc.edu
Comment: Hans-Werner Braun, HPWREN and SDSC, hwb@ucsd.edu
Comment: Frank Vernon, HPWREN and SIO, flvernon@ucsd.edu