Changes in Repeating Earthquake Slip
Berkeley Seismo Lab researchers Douglas Dreger, Taka'aki Taira, and Robert Nadeau, and colleagues from Yokohama City University in Japan have a new method for modeling the way slip is distributed along the San Andreas Fault near Parkfield, CA. The new method differs from the standard approach in that it uses very small repeating earthquakes to identify common features and remove uncertainties. The resulting model supports earlier findings, suggesting portions of the fault are locked.
As models improve and become more precise, they can reveal different fault features, which aids seismologists in developing their understanding of the mechanics of earthquake ruptures and how the fault behaves over time in response to stress increases and decreases. This is important, as it is these stress changes that inform seismologists about the seismic hazard associated with different parts of the San Andreas Fault.
Modeling earthquakes is a difficult task because the data seismologists record contains much more information than simply the slip on the fault. In some cases, seismologists can take a seismic record (e.g., from a seismogram) and try to break down what caused the wiggles to appear the way they do. This is tricky, as the record not only records the seismic source, but it also records the distortions caused by the surrounding geology as the energy moves from the hypocenter to the seismic station where the signal is recorded, as well as the instrument response for the device that does the recording.
A classic approach simplifies this task by reducing the problem to a one-dimensional model, where a seismic record is assumed to consist of only two variables: an earthquake source and general variable that accounts for everything else. If a seismologist defines the general variable well, then all changes can be said to result from the earthquake source.
However, scientists have realized that extracting certain features of an earthquake using such a simplified one-dimensional model can skew and give misleading results. In this new approach, BSL and Yokohama City seismologists collected information from small repeating earthquakes to model the earthquake source.
Repeating earthquakes are quakes with a similar shape that occur again and again on the same patch of fault. Since all the earthquakes can be thought to be simple repeats of each other, changes in the recorded seismograms become much easier to define.
The results of their investigation were promising and supported earlier findings; that near Parkfield there exists portions of the fault that are locked. This modeling method also shed some light about the drop in stress and slip that occurred after the 2004 magnitude 6.0 event in Parkfield. The overall performance of this method allowed seismologists to make interpretations on the nuances between different seismic records; something that could not be done before.
Read the entire article at Journal of Geophysical Research: Solid Earth, doi:10.1002/2015JB012562.
Featured Researcher: Chris Milliner
Chris Milliner is a postdoctoral scholar working with Roland Burgmann on problems related to active tectonics and fault zone deformation. His research focuses on using geodetic techniques to better understand how faults release strain throughout the earthquake cycle from the surface to upper mantle depths.
Prior to his arrival at UC Berkeley in September, Milliner completed his doctoral thesis in Geology at USC with Prof. James Dolan. His doctoral research involved using satellite images taken before and after earthquakes to quantify the magnitude and width of fault rupture at the surface. This research delivered a better method to resolve complex motion close to faults, previously difficult to accomplish with standard geodetic techniques, and provided vital data needed for better mitigating for the effects of zones of distributed ground rupture that pose a hazard to the built environment. At UC Berkeley Milliner hopes to apply this optical imaging method in combination with others (e.g., GPS and InSAR), in order to improve scientist’s understanding of fault slip behavior at large, crustal depths.
Moving forward Milliner is interested in learning how new and emerging geodetic datasets (e.g. lidar differencing) can inform us about tectonic processes at depth. Come August Milliner will be moving to JPL to take a second post-doctoral position, where he hopes to continue the collaborations he has forged with members of the Berkeley Seismological Laboratory.
Complicated Creep where Three Plates Meet
Earthquakes with magnitudes bigger than 6 rock the tiny community of Petrolia in Humboldt County, CA with an astonishing frequency. The faults that produce these quakes are mostly offshore, part of the Mendocino Triple Junction where three tectonic plates meet. Their locations make them particularly difficult to study.
Aseismic creep, movement of the ground along a fault in the absence of earthquakes, is an important characteristic of faults. To get at the aseismic creep rates in the Mendocino Triple Junction, Kathryn Materna and colleagues analyzed so-called repeating earthquakes - earthquakes that occur when the same patch of ground fails over and over again. Materna’s code processed eight years of Cape Mendocino earthquakes, comparing the seismic signatures (waveforms) of each event and sorting them into “families.” From each family, Materna inferred a rate of creep on a specific section of a fault.
In her presentation “Measuring aseismic slip through characteristically repeating earthquakes at the Mendocino Triple Junction, Northern California,” which won an American Geophysical Union Outstanding Student Paper Award, Materna finds a high creep rate at the transform boundary between the Pacific and Gorda Plates. Closer to shore, beneath the North American Plate, the data suggest a more complicated and depth-dependent pattern of deformation. By analyzing the growing dataset of repeating earthquakes, Materna and coauthors are gaining new insights into the present-day tectonics of the Mendocino Triple Junction. Interestingly, their dataset also shows that creeping faults in the Mendocino Triple Junction can increase or decrease their creep rates in response to nearby large earthquakes.
The Lassen Volcanic Center sits atop a hydrothermal system that might one day be the scene of hydrothermal explosions. Researchers keep watch over this area, and a new form of monitoring may soon be added to their arsenal.
The Berkeley Seismology Lab’s Taka’aki Taira, along with Florent Brenguier (Université Grenoble Alpes) have developed a system that analyzes seismic “noise” -- vibrations without a distinct source. This recorded ambient noise, from six stations in the Northern California Seismic Network around Lassen Peak, is automatically processed to pinpoint changes in how fast seismic waves go through this area, which can indicate changes in tectonic stress in volcanic areas.
In a new paper, highlighted in last week’s SpringerOpen blog, Taira and Brenguier track the seismic velocity for the Lassen Volcanic Center over more than four years. The differences in seismic velocity over time show the effects of the M 5.7 Greenville earthquake as well as the stresses associated with snow (changes in groundwater and surface loading.)
But the monitoring system could be improved. The current system outputs a daily velocity change. With more computing power, researchers could perform the massive cross-correlation computations required to give an hourly update, enabling researchers to detect changes in velocity that accompany and perhaps precede volcanic activity.
Read more about this topic at the SpringerOpen blog.
Complex Behavior of a Nevada Earthquake Swarm
For five months in 2008, a swarm of shallow earthquakes alarmed residents as they shook the Mogul neighborhood of West Reno, NV. Earthquake depths as shallow as 1 km below the Earth’s surface were widely felt, even at low magnitudes. Scientists rapidly moved to install temporary seismic instrumentation in west Reno for the purpose of recording the quakes, from the larger earthquakes down to micro-seismicity. The biggest quake, with a moment magnitude of 4.9, was unusual. This earthquake's sense of slip - the relative motion of the rock on each side of the fault with respect to the other side - was different from the so-called "dip-slip" faults that geologists had mapped in the area.
In a paper to be published in JGR: Solid Earth, Berkeley Seismology Lab post-doc Christine Ruhl and colleagues at University of Nevada, Reno and Boston University have mined the high-quality earthquake data set collected during the Mogul sequence to elucidate the structure of the faults active in the swarm. Ruhl and others used a technique called double-difference relocation to precisely determine the location of as many earthquakes as possible, including the tiniest recorded quakes in the swarm. They also computed focal mechanisms for more than a thousand earthquakes, ranging in magnitude from 4.9 down to magnitude zero. Focal mechanisms, the basis of the "beach ball diagrams" shown in the figure at right, tell seismologists which way the faults slipped and how the faults are oriented.
The team discovered more about the behavior of the swarm, revealing distinct clusters within the sequence and that it was driven by fluid beneath the surface. Fluids often play an important role in earthquake triggering; increased pore pressures facilitate rupture and enable swarms of seismicity to occur. Their work also revealed more about the faulting beneath west Reno: the quakes delineated the structure of the previously unknown Mogul fault, which may be evolving into a strike-slip fault zone, where motion is primarily horizontal.
Says Ruhl, “Temporary deployments during unusual earthquake sequences were essential for our very detailed study of the Mogul earthquakes and will continue to be an important tool in investigating the evolution of complex seismic sequences in the future.”
For researchers: View the paper at JGR: Solid Earth.
Earthquake Early Warning for California
Earthquake Early Warning (EEW) is a method of rapidly identifying an earthquake in progress and transmitting alerts to nearby population centers before damaging ground shaking arrives.
The first few seconds of the initial P-wave arrivals at one or more stations are used to detect the event. A warning of imminent shaking can be used to activate automatic safety measures, such as slowing down trains, isolating sensitive factory equipment, or opening elevator doors. We envision the alerts will be sent directly to the public via cell phone, computer, television, or radio. The Berkeley Seismological Laboratory, together with its project partners, is collaborating to build a single, integrated, end-to-end system for testing Earthquake Early Warning in California.
- Earthquake Early Warning at the Berkeley Seismo Lab
- ShakeAlert: Earthquake Early Warning
- California Integrated Seismic Network Earthquake Early Warning Project
Research Groups at the Berkeley Seismo Lab
The Global Seismology Research Group at UC Berkeley is a part of the Department of Earth and Planetary Science and our research focuses on structure and dynamics of the deep earth, from the crust to the inner core. We tackle theoretical wave propagation problems in complex 3D media, including forward modeling and tomographic inversion for elastic as well as anelastic structure. In order to better understand the chemical and thermal state of the mantle and the processes operating therein, we seek to apply the latest findings of the mineral physics community within the context of our seismic probing. We also study earthquake source mechanisms and scaling laws, as well as global seismic moment release and its relation to plate tectonics. One of our recent research directions concerns the Earth's "hum" and the insights it brings to ocean/atmosphere/solid earth interactions.
The Active Tectonics research group is part of the Department of Earth and Planetary Science and our research focuses on problems relating to fault zone processes and crustal deformation. We rely on geodetic measurements using GPS and InSAR, investigations of seismicity, examination of tectonic landscapes, and field mapping of geologic structures. Such observations can be used to constrain models of the first-order mechanics of an actively deforming region, such as the San Andreas fault system, the Sumatra-Andaman subduction zone, volcano deformation on the Big Island of Hawaii, or the India-Eurasia collision zone. Many of our efforts are focused on elucidating the various components of the earthquake cycle and related rheological properties of lithospheric materials. We also consider repeating micro-earthquakes and deeply seated non-volcanic tremors to improve our understanding of the behavior of shallow and deep fault zones, respectively. Our approach is interdisciplinary, integrating geodetic, geomorphic, geologic, and seismological observations along with theoretical modeling.
The The Earth Imaging group uses a wide variety of seismological techniques to image 3D Earth structure in an effort to understand the dynamic processes responsible for deformation, volcanism and earthquakes at the Earth's surface.
The Realtime Seismology group is interested in all aspects of rapid geophysical data characterization. The desire for realtime information is motivated by hazard mitigation objectives, and the development of such techniques drives fundamental research into earthquake source processes.
The Seismic Source Group focuses on the use of seismic waveform data to investigate seismic sources (tectonic and non-tectonic), wave propagation, and various geophysical inverse problems. Additionally, research is conducted to develop robust automated procedures to analyze earthquakes as they occur and to report strong shaking levels on a local and regional scale.