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2016
Saunders, JK, Goldberg DE, Haase JS, Bock Y, Offield DG, Melgar D, Restrepo J, Fleischman RB, Nema A, Geng JH, Walls C, Mann D, Mattioli GS.  2016.  Seismogeodesy using GPS and low-cost MEMS accelerometers: Perspectives for earthquake early warning and rapid response. Bulletin of the Seismological Society of America. 106:2469-2489.   10.1785/0120160062   AbstractWebsite

The seismogeodetic method computes accurate displacement and velocity waveforms by optimally extracting high-frequency information from strong-motion accelerometers and low-frequency information from collocated Global Positioning System (GPS) instruments. These broadband observations retain the permanent (static) displacement, are immune to clipping and magnitude saturation for large earthquakes, and are sensitive enough to record P-wave arrivals. These characteristics make seismogeodesy suitable for real-time applications such as earthquake early warning. The Scripps Institution of Oceanography (SIO) has developed an inexpensive microelectromechanical systems (MEMS) accelerometer package to upgrade established GPS stations. We compare the performance of our MEMS accelerometer with an observatory-grade accelerometer using an experiment at the University of California San Diego Large High-Performance Outdoor Shake Table. We show that the two types of accelerometers agree in frequency ranges of seismological and engineering interest and produce equivalent seismogeodetic estimates of displacement and velocity. To date, 27 SIO MEMS packages have been installed at GPS monitoring stations in southern California and the San Francisco Bay area and have recorded four earthquakes (M4.2, M4.1, and two of M4.0). The P-wave arrivals are distinguishable in the seismogeodetic observations at distances of up to similar to 25 km away but not in the GPS-only displacements. There is no significant permanent deformation for these small events. This study demonstrates the lower limit of detectability and that seismogeodetic waveforms can also be a reliable early confirmation that an event is not large or hazardous. It also raises the possibility of rapid magnitude estimation through scaling relationships.

Melgar, D, Allen RM, Riquelme S, Geng JH, Bravo F, Baez JC, Parra H, Barrientos S, Fang P, Bock Y, Bevis M, Caccamise DJ, Vigny C, Moreno M, Smalley R.  2016.  Local tsunami warnings: Perspectives from recent large events. Geophysical Research Letters. 43:1109-1117.   10.1002/2015gl067100   AbstractWebsite

We demonstrate a flexible strategy for local tsunami warning that relies on regional geodetic and seismic stations. Through retrospective analysis of four recent tsunamigenic events in Japan and Chile, we show that rapid earthquake source information, provided by methodologies developed for earthquake early warning, can be used to generate timely estimates of maximum expected tsunami amplitude with enough accuracy for tsunami warning. We validate the technique by comparing to detailed models of earthquake source and tsunami propagation as well as field surveys of tsunami inundation. Our approach does not require deployment of new geodetic and seismic instrumentation in many subduction zones and could be implemented rapidly by national monitoring and warning agencies. We illustrate the potential impact of our method with a detailed comparison to the actual timeline of events during the recent 2015 M(w)8.3 Illapel, Chile, earthquake and tsunami that prompted the evacuation of 1 million people.

2015
Galetzka, J, Melgar D, Genrich JF, Geng J, Owen S, Lindsey EO, Xu X, Bock Y, Avouac JP, Adhikari LB, Upreti BN, Pratt-Sitaula B, Bhattarai TN, Sitaula BP, Moore A, Hudnut KW, Szeliga W, Normandeau J, Fend M, Flouzat M, Bollinger L, Shrestha P, Koirala B, Gautam U, Bhatterai M, Gupta R, Kandel T, Timsina C, Sapkota SN, Rajaure S, Maharjan N.  2015.  Slip pulse and resonance of the Kathmandu basin during the 2015 Gorkha earthquake, Nepal. Science. 349:1091-1095.   10.1126/science.aac6383   AbstractWebsite

Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse similar to 20 kilometers in width, similar to 6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at similar to 3.3 kilometers per second over similar to 140 kilometers. The smooth slip onset, indicating a large (similar to 5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (>1 hertz; peak ground acceleration, similar to 16% of Earth's gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.

Melgar, D, Geng JH, Crowell BW, Haase JS, Bock Y, Hammond WC, Allen RM.  2015.  Seismogeodesy of the 2014 M(w)6.1 Napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike-slip fault. Journal of Geophysical Research-Solid Earth. 120:5013-5033.   10.1002/2015jb011921   AbstractWebsite

Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 M(w)6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

1999
Baer, G, Sandwell D, Williams S, Bock Y, Shamir G.  1999.  Coseismic deformation associated with the November 1995, M-w=7.1 Nuweiba earthquake, Gulf of Elat (Aqaba), detected by synthetic aperture radar interferometry. Journal of Geophysical Research-Solid Earth. 104:25221-25232.   10.1029/1999jb900216   AbstractWebsite

The November 22, 1995, M-w=7.1 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform in the Gulf of flat (Aqaba). Although it was the largest earthquake along this fault in the last few centuries, little is yet known about the geometry of the rupture, the slip distribution along it, and the nature of postseismic deformation following the main shock. In this study we examine the surface deformation pattern during the coseismic phase of the earthquake in an attempt to better elucidate the earthquake rupture process. As the entire rupture zone was beneath the waters of the Gulf, and there is very little Global Positioning System (GPS) data available in the region for the period spanning the earthquake, interferometric synthetic aperture radar (INSAR) provides the only source of information of surface deformation associated with this earthquake. We chose four synthetic aperture radar (SAR) scenes of about 90x90 km each spanning the rupture area, imaged by the ERS-1 and ERS-2 satellites. The coseismic interferograms show contours of equal satellite-to-ground range changes that correspond to surface displacements due to the earthquake rupture. Interferograms that span the earthquake by 1 week show similar fringe patterns' as those that span the earthquake by 6 months, suggesting that postseismic deformation is minor or confined to the first week after the earthquake. A high displacement gradient is seen on the western side of the Gulf, 20-40 km south of flat and Aqaba, where the total satellite-to-ground range changes are at least 15 cm. The displacement gradient is relatively uniform on the eastern side of the Gulf and the range changes are less than 10 cm. To interpret these results, we compare them to synthetic interferograms generated by elastic dislocation models with a variety of fault parameters. Although selecting the best fit fault parameters is nonunique, we are able to generate a group of simplified model interferograms that provide a reasonable fit to the coseismic interferogram and serve to constrain the location of the fault. The present analysis shows that if the rupture reached the Gulf-bottom surface, the mean sinistral slip along the fault is constrained to about 1.4 m. If surface rupture did not occur, the average sinistral slip is constrained to the range of 1.4-3 m for a fault patch buried 0-4 km below the Gulf-bottom Surface, respectively, with a minor normal component.

1994
Shen, ZK, Jackson DD, Feng YJ, Cline M, Kim M, Fang P, Bock Y.  1994.  Postseismic Deformation Following the Landers Earthquake, California, 28 June 1992. Bulletin of the Seismological Society of America. 84:780-791. AbstractWebsite

Accelerated strain followed the Landers and Big Bear earthquakes, returning to the normal rate only after a period of several months. We observed this strain throughout most of southern California using the Global Positioning System (GPS). Three GPS receivers operating continuously in fixed positions at Pinyon Flat, Jet Propulsion Laboratory (Pasadena), and Goldstone all recorded postseismic deformation in a relative sense. In addition, we established 16 sites where we deployed portable receivers occasionally over a period of about 6 months near the rupture zones of the earthquakes. Anomalous postseismic displacements ranged from 55 mm near the epicenter to a few millimeters far from the fault. We modeled the displacements, using dislocation theory, as due to variable slip on the faults that were displaced at the times of the earthquakes. The model suggests that the postseismic strain released the equivalent of about 15% of the seismic moment of the mainshock. While the strain released from the upper 10 km is about the same as what can be explained by direct effects of aftershocks, the major contribution of strain release comes from the lower layer, below 10-km depth. Significant afterslip or viscous relaxation must have occurred below 10-km depth to explain the observed deformation more than 100 km from the fault. One interpretation is that high stress on the margin of the co-seismic rupture zone drives the rupture to extend itself into unbroken rock below and along the initial rupture zone.