Spectral-Element Moment Tensor Inversions for Earthquakes in Southern California

We have developed and implemented an automated moment tensor inversion procedure to determine source parameters for southern California earthquakes. The method is based upon spectral-element simulations of regional seismic wave propagation in an integrated 3D southern California velocity model. Sensitivity to source parameters is determined by numerically calculating the Fréchet derivatives required for the moment tensor inversion. We minimize a waveform misfit function, and allow limited time shifts between data and corresponding synthetics to accommodate additional 3D heterogeneity not included in our model. The technique is applied to three recent southern California earthquakes: the 9 September 2001, M_L 4.2 Hollywood event, the 22 February 2003, M_L 5.4 Big Bear event, and the 14 December 2001, M_L 4.0 Diamond Bar event. Using about half of the available three-component data at periods of 6 sec and longer, we obtain focal mechanisms, depths, and moment magnitudes that are generally in good agreement with estimates based upon traditional body-wave and surface-wave inversions

Rapid Imaging of Earthquake Ruptures with Combined Geodetic and Seismic Analysis

Rapid determination of the location and extent of earthquake ruptures is helpful for disaster response, as it allows prediction of the likely area of major damage from the earthquake and can help with rescue and recovery planning. With the increasing availability of near real-time data from the Global Positioning System (GPS) and other global navigation satellite system receivers in active tectonic regions, and with the shorter repeat times of many recent and newly launched satellites, geodetic data can be obtained quickly after earthquakes or other disasters. We have been building a data system that can ingest, catalog, and process geodetic data and combine it with seismic analysis to estimate the fault rupture locations and slip distributions for large earthquakes

Southern California Seismic Network: Caltech/USGS Element of TriNet 1997-2001

The California Institute of Technology (Caltech), the United States Geological Survey (USGS), and the California Department of Conservation, Division of Mines and Geology (CDMG) are completing the implementation of TriNet, a modern seismic information system for southern California. TriNet consists of two elements, the Caltech-USGS element and the CDMG element (Mori et al., 1998). The Caltech-USGS element (Caltech-USGS TriNet) concentrates on rapid notification and archiving of data for seismological applications, while the CDMG element is focused on the needs of engineering users (Hauksson et al., 2002). All three. TriNet agencies are working toward facilitating emergency response and long-term mitigation of earthquake hazards in cooperation with other agencies. The technical development of Caltech-USGS TriNet is sufficiently different from the CDMG element of TriNet to warrant a separate description. This paper provides a technical overview of the design principles of Caltech-USGS TriNet. These principles were based on a document that stated the scientific requirements of TriNet (Jones et al., 1997). We also describe the implementation of these principles using modern technology. The implementation consisted of station deployments, establishing communications links, and developing and implementing new hardware and software for data processing and information distribution. Thus, the Caltech-USGS TriNet is an integrated project extending across many disciplines, using basic ground-motion data and seismological algorithms to generate in near real-time a sophisticated earthquake knowledge base following earthquakes in southern California. Caltech-USGS TriNet applies advanced technology to record both small and large earthquakes on scale. The latest generation of broadband and strong-motion sensors with 24-bit digitizers is used to acquire high-fidelity ground-motion data. Real-time communication is a requirement to facilitate rapid processing and notification about seismicity for emergency management. The data acquisition systems are designed to ensure redundancy and automated processing of data. To accomplish automation, high-speed computers and advanced software form the inner workings of the Caltech-USGS TriNet system. Adopting the commercial database Oracle is an important foundation of our data management system. The automated flow of data into an accessible data center and the automatic population of the database is part of our new seismic network design and is an essential feature of Caltech-USGS TriNet. The TriNet real-time systems and database have been operating online for more than two years, processing real-time data currently from more than 375 stations, or more than 1,200 high sample-rate data channels. Many of these capabilities were tested in the 1999 M_w 7.1 Hector Mine earthquake. New postprocessing and catalog-generation approaches have also been implemented in 2001. Caltech-USGS TriNet is one of the first U.S. regional seismic networks that uses digital technology on a scale of 200 or more stations, with both broadband and strongmotion sensors. In comparison, the IRIS Global Seismic Network consists of 108 stations, with plans for a total of 150 stations (Hutt and Bolton, 1999). Previous digital networks, such as TERRAscope (Kanamori et al., 1997) and the Berkeley Digital Seismic Network (BDSN) (Gee et aL, 1996), have been smaller than TriNet, with about 20 stations each. TriNet also benefits from the experience of other seismic networks around the world. The K-Net in Japan is another example of large-scale deployment of a digital network, although it is focused on strong motions (Kinoshita, 1998). Extensive developments of strong-motion networks in Taiwan and associated near-real-time processing of data employ somewhat different technology but have similar goals for information products following large earthquakes (Teng et al., 1997)

Rupture Process of the 2004 Sumatra-Andaman Earthquake

The 26 December 2004 Sumatra-Andaman earthquake initiated slowly, with small slip and a slow rupture speed for the first 40 to 60 seconds. Then the rupture expanded at a speed of about 2.5 kilometers per second toward the north northwest, extending 1200 to 1300 kilometers along the Andaman trough. Peak displacements reached ~15 meters along a 600-kilometer segment of the plate boundary offshore of northwestern Sumatra and the southern Nicobar islands. Slip was less in the northern 400 to 500 kilometers of the aftershock zone, and at least some slip in that region may have occurred on a time scale beyond the seismic band

The Making of the NEAM Tsunami Hazard Model 2018 (NEAMTHM18)

ABSTRACT: The NEAM Tsunami Hazard Model 2018 (NEAMTHM18) is a probabilistic hazard model for tsunamis generated by earthquakes. It covers the coastlines of the North-eastern Atlantic, the Mediterranean, and connected seas (NEAM). NEAMTHM18 was designed as a three-phase project. The first two phases were dedicated to the model development and hazard calculations, following a formalized decision-making process based on a multiple-expert protocol. The third phase was dedicated to documentation and dissemination. The hazard assessment workflow was structured in Steps and Levels. There are four Steps: Step-1) probabilistic earthquake model; Step-2) tsunami generation and modeling in deep water; Step-3) shoaling and inundation; Step-4) hazard aggregation and uncertainty quantification. Each Step includes a different number of Levels. Level-0 always describes the input data; the other Levels describe the intermediate results needed to proceed from one Step to another. Alternative datasets and models were considered in the implementation. The epistemic hazard uncertainty was quantified through an ensemble modeling technique accounting for alternative models' weights and yielding a distribution of hazard curves represented by the mean and various percentiles. Hazard curves were calculated at 2,343 Points of Interest (POI) distributed at an average spacing of ∼20 km. Precalculated probability maps for five maximum inundation heights (MIH) and hazard intensity maps for five average return periods (ARP) were produced from hazard curves. In the entire NEAM Region, MIHs of several meters are rare but not impossible. Considering a 2% probability of exceedance in 50 years (ARP≈2,475 years), the POIs with MIH >5 m are fewer than 1% and are all in the Mediterranean on Libya, Egypt, Cyprus, and Greece coasts. In the North-East Atlantic, POIs with MIH >3 m are on the coasts of Mauritania and Gulf of Cadiz. Overall, 30% of the POIs have MIH >1 m. NEAMTHM18 results and documentation are available through the TSUMAPS-NEAM project website (http://www.tsumaps-neam.eu/), featuring an interactive web mapper. Although the NEAMTHM18 cannot substitute in-depth analyses at local scales, it represents the first action to start local and more detailed hazard and risk assessments and contributes to designing evacuation maps for tsunami early warning

Seismological observations of upper mantle anisotropy [pt. 1] ; Source spectra of shallow subduction zone earthquakes and their tsunamigenic potential [pt. 2]

One of the most important developments in observational seismology in the last 10 years is the worldwide increase in the number of broadband instruments and seismic networks, as well as the improved access to the data-set that these seismometers provide. A data-set of this magnitude offers nearly unlimited possibilities for research into earthquake source processes and Earth structure. The work presented in this thesis involves the application of different methods to seismological recordings, as well as an interpretation and discussion of the results. In Chapter I, I take advantage of the very broadband nature and small spacing of the stations of TERRAscope, one of the first digital broadband seismic networks, to determine dispersion curves for long period surface waves. This enables us to invert for an upper mantle S-wave velocity model for southern California. The Rayleigh wave, SV, model is about 4% slower than the model developed for tectonic north America. If the correction for higher modes I performed on our Love wave data measurements is accurate, the resulting SH velocity model shows about 5% anisotropy (transverse isotropy) in the upper mantle beneath southern California. In Chapter 2, I perform measurements of shear-wave splitting on a unique data-set obtained from temporary arrays located above the Nazca subduction zone in South America. Data from SKS, and local S-wave data from deep and intermediate depth earthquakes, were used to develop a model of the anisotropy in this region. The above slab component of anisotropy in the western region, where the slab is at a depth of about 300 km and up is oriented NS and its delay time is limited to about 0.3 sec. This direction agrees with the shortening direction of the Andes and is orthogonal to that predicted by a comer flow model. To the east, the stations have EW aligned fast directions and possibly sample the Brazilian craton. The below slab component samples a zone of EW aligned anisotropy, as well as trench parallel aligned anisotropy. The trench parallel directions can be explained by the retrograde motion of the slab in south America, and I speculate that the EW direction could suggest a tear in the slab, or a local EW orientation because of local buckling of the slab under NS compression. In Chapter 3 I use this same method for the data of the TriNet array. Here I find an overall pattern of consistent directions of the polarization direction of the fast SKS waves, the fastest P-wave velocities and the World Stress Map maximum horizontal compressive stress directions. This suggest that the pattern of anisotropy is generally uniform in the crust and lithospheric mantle, in a layer with an overall thickness of 100 to 150 km. The alignment of most fast directions can be explained by plate-tectonic, extensional and compressional events. We also examine the detailed lateral and vertical variations of anisotropy in this region. Chapter 4 is focussed on the differences in source spectra and tectonic setting between tsunami earthquakes, which excite anomalously great tsunamis and 'regular' shallow subduction earthquakes. We find that these unusual events have several characteristics in common: low energy release at short periods, centroid location close to the trench, updip rupture, relatively small accretionary prism, sediment subduction and a well-developed horst and graben structure of the oceanic plate close to the trench. We speculate that these events can nucleate in an unusually shallow part of the subduction zone, where sediments normally exhibit stable sliding behavior, because of the contacts between the horsts and the overriding plate. Because the earthquakes are so shallow, and there is some sediment being subducted, part of the rupture goes through sediments, making the source process slow. The true displacement (and thus the tsunami height) of these events may be underestimated because the elastic constants of the fault zone are not taken into account when converting seismic moment into displacement

SITE RESPONSE ACROSS THE LOS ANGELES BASIN FROM AMBIENT NOISE RECORDED BY A HIGH DENSITY ARRAY

Sedimentary basins, such as the Los Angeles basin, can substantially amplify ground motion and increase its duration. To account for site response and develop better seismic hazard assessment and mitigation, it is essential to determine site characteristics across the basin at a high spatial resolution. We investigate site response within the Los Angeles Basin through the application of the microtremor Horizontal-to-Vertical Spectral Ratio (HVSR) approach on 3-component broadband waveforms from the Los Angeles Syncline Seismic Interferometry Experiment (LASSIE). LASSIE is a dense array of 73 broadband seismometers that were active for one month, transecting the Los Angeles basin at 1 km spacing from La Puente to a dense cluster in Long Beach. The spectral peak amplitudes and peak frequencies of the HVSR curves both show variation across the LASSIE network, even between stations that are spaced only 1 km apart, emphasizing the importance of micro-zonation. Our results show an average resonance period at the basin center of 6 to 10 sec. Secondary intermittent shorter period peaks are also observed near the basin edge and may be explained using basin edge resonance, the presence of small scale basins, and/or topographic effects based on their location within the array. Amplified shaking from resonance is characterized by long period HVSR peak amplitudes ranging from 2 to 5.5. The HVSR peak frequencies are higher than those predicted by a simple quarter wavelength function using a weighted velocity from 1-D profiles through the Southern California Earthquake Center Community Velocity Model – Harvard, especially near the center of the basin. Possible factors that may contribute to this discrepancy are oversimplifications used in the calculation, errors in the velocity model, resonance due to a more complex interaction between waves and physical complications introduced by the large dimensions of the LA basin

Tsunami Earthquakes

The original definition of “tsunami earthquake” was given by Kanamori [37] as “an earthquake that produces a large size tsunami relative to the value of its surface wave magnitude (M_S)”. Therefore, the true damage potential that a tsunami earthquake represents may not be recognized by conventional near real-time seismic analysis methods and may only become apparent upon the arrival of the tsunami waves on the local coastline. Although tsunami earthquakes occur relatively infrequently, the effect on the local population can be devastating, as was most recently illustrated by the July 2006 Java tsunami earthquake. This event (moment magnitude M_w=7.8) was quickly followed by tsunami waves two to seven meters high, traveling as far as two kilometers inland and killing at least 668 people