Diego Melgar | University of California, Berkeley (original) (raw)
Papers by Diego Melgar
Amount of seconds until first warning was issued for 2011 Mw 9.0 Tohoku-oki earthquake. The conto... more Amount of seconds until first warning was issued for 2011 Mw 9.0 Tohoku-oki earthquake. The contours show seconds until the S-wave arrival. The region within the 0 s contour is the blind zone where no warning is available. This illustration shows that the warning was issued before the S-wave reached the coastline.
Scenario ruptures and ground motion simulation are important tools for studies of expected earthq... more Scenario ruptures and ground motion simulation are important tools for studies of expected earthquake and tsunami hazards during future events. This is particularly important for large (M w 8+) and very large (M w 8.5+) events for which observations are still limited. In particular, synthetic waveforms are important to test the response of earthquake and tsunami warning systems to large events. These systems are not often exercised in this manner. We will show an application of the Karhunen-Loève (K-L) expansion to generate stochastic slip distributions of large events with an example application to the Cascadia subduction zone. We will discuss how to extend the static slip distributions obtained from the K-L expansion to produce kinematic rupture models and generate synthetic long-period displacement data at the sampling rates of traditional Global Navigation Satellite Systems (GNSS) stations. We will validate the waveforms produced by this method by comparison to a displacement-based ground motion prediction equation obtained from GNSS measurements of large earthquakes worldwide.
Geodesy, the oldest science, has become an important discipline in the geosciences, in large part... more Geodesy, the oldest science, has become an important discipline in the geosciences, in large part by enhancing Global Positioning System (GPS) capabilities over the last 35 years well beyond the satellite constellation's original design. The ability of GPS geodesy to estimate 3D positions with millimeter-level precision with respect to a global terrestrial reference frame has contributed to significant advances in geophysics, seismology, atmospheric science, hydrology, and natural hazard science. Monitoring the changes in the positions or trajectories of GPS instruments on the Earth's land and water surfaces, in the atmosphere, or in space, is important for both theory and applications, from an improved understanding of tectonic and magmatic processes to developing systems for mitigating the impact of natural hazards on society and the environment. Besides accurate positioning, all disturbances in the propagation of the transmitted GPS radio signals from satellite to receiver are mined for information, from troposphere and ionosphere delays for weather, climate, and natural hazard applications, to disturbances in the signals due to multipath reflections from the solid ground, water, and ice for environmental applications. We review the relevant concepts of geodetic theory, data analysis, and physical modeling for a myriad of processes at multiple spatial and temporal scales, and discuss the extensive global infrastructure that has been built to support GPS geodesy consisting of thousands of continuously operating stations. We also discuss the integration of heterogeneous and complementary data sets from geodesy, seismology, and geology, focusing on crustal deformation applications and early warning systems for natural hazards.
W phase moment tensor inversion has proven to be a reliable method for rapid characterization of ... more W phase moment tensor inversion has proven to be a reliable method for rapid characterization of large earthquakes. For global purposes it is used at the United States Geological Survey, Pacific Tsunami Warning Center, and Institut de Physique du Globe de Strasbourg. These implementations provide moment tensors within 30–60 min after the origin time of moderate and large worldwide earthquakes. Currently, the method relies on broadband seismometers, which clip in the near field. To ameliorate this, we extend the algorithm to regional records from high-rate GPS data and retrospectively apply it to six large earthquakes that occurred in the past 5 years in areas with relatively dense station coverage. These events show that the solutions could potentially be available 4–5 min from origin time. Continuously improving GPS station availability and real-time positioning solutions will provide significant enhancements to the algorithm.
The 2015 M w 8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment ... more The 2015 M w 8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment of that subduction zone. It generated strong ground motions and a large (up to 11 m runup) tsunami which prompted the evacuation of more than 1 million people in the first hours following the event. Observations during recent earthquakes suggest that these phenomena can be associated with rupture on different parts of the megathrust. The deep portion generates strong shaking while slow, large slip on the shallow fault is responsible for the tsunami. It is unclear whether all megathrusts can have shallow slip during coseismic rupture and what physical properties regulate this. Here we show that the Illapel event ruptured both deep and shallow segments with substantial slip. We resolve a kinematic slip model using regional geophysical observations and analyze it jointly with teleseismic backprojection. We find that the shallow and deep portions of the megathrust are segmented and have fundamentally different behavior. We forward calculate local tsunami propagation from the resolved slip and find good agreement with field measurements, independently validating the slip model. These results show that the central portion of the Chilean subduction zone has accumulated a significant shallow slip deficit and indicates that, given enough time, shallow slip might be possible everywhere along the subduction zone.
We demonstrate a flexible strategy for local tsunami warning that relies on regional geodetic and... more 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 Mw8.3 Illapel, Chile earthquake and tsunami that prompted the evacuation of 1 million people.
Interferometric synthetic aperture radar (InSAR) is a key tool for the analysis of displacement a... more Interferometric synthetic aperture radar (InSAR) is a key tool for the analysis of displacement and
stress changes caused by large crustal earthquakes, particularly in remote areas. A challenge for traditional
InSAR has been its limited spatial and temporal coverage especially for very large events, whose dimensions
exceed the typical swath width of 70–100 km. This problem is addressed by the ALOS-2 satellite, whose
PALSAR-2 instrument operates in ScanSAR mode, enabling a repeat time of 2 weeks and a swath width of
350 km. Here we present InSAR line-of-sight displacement data from ALOS-2/PALSAR-2 observations
covering the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock that were acquired within 1 week of
each event. The data are made freely available and we encourage their use in models of the fault slip and
associated stress changes. The Mw 7.3 aftershock not only extended the rupture area of the main shock
toward the east but also left a 20 km gap where the fault has little or no coseismic slip. We estimate this
unslipped fault patch has the potential to generate a Mw 6.9 event.
Detailed geodetic imaging of earthquake rupture enhances our understanding of earthquake physics ... more Detailed geodetic imaging of earthquake rupture enhances our understanding of earthquake physics and induced ground shaking. The April 25, 2015 Mw 7.8 Gorkha, Nepal earthquake is the first example of a large continental megathrust rupture beneath a high-rate (5 Hz) GPS network. We use GPS and InSAR data to model the earthquake rupture as a slip pulse of ~20 km width, ~6 s duration, and with peak sliding velocity of 1.1 m/s that propagated toward Kathmandu basin at ~3.3 km/s over ~140 km. The smooth slip onset, indicating a large ~5 m slip-weakening distance, caused moderate ground shaking at high >1Hz frequencies (~16% g) and limited damage to regular dwellings. Whole basin resonance at 4-5 s period caused collapse of tall structures, including cultural artifacts.
Geophysical Research Letters
Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response sys... more Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response
systems during medium to large events. The 2014 Mw6.1 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.
Journal of Geophysical Research
GPS instruments are noninertial and directly measure displacements with respect to a global refe... more GPS instruments are noninertial and directly measure displacements with respect to a
global reference frame, while inertial sensors are affected by systematic offsets—primarily tilting—that
adversely impact integration to displacement. We study the magnitude scaling properties of peak ground
displacement (PGD) from high-rate GPS networks at near-source to regional distances (~10–1000 km),
from earthquakes between Mw6 and 9. We conclude that real-time GPS seismic waveforms can be used to
rapidly determine magnitude, typically within the first minute of rupture initiation and in many cases
before the rupture is complete. While slower than earthquake early warning methods that rely on the
first few seconds of P wave arrival, our approach does not suffer from the saturation effects experienced
with seismic sensors at large magnitudes. Rapid magnitude estimation is useful for generating rapid
earthquake source models, tsunami prediction, and ground motion studies that require accurate information
on long-period displacements.
Journal of Geophysical Research
Rapid near-source earthquake source modeling relying only on strong motion data is limited by ins... more Rapid near-source earthquake source modeling relying only on strong motion data is limited by instrumental offsets and magnitude saturation, adversely affecting subsequent tsunami prediction. Seismogeodetic displacement and velocity waveforms estimated from an optimal combination of high-rate
GPS and strong motion data overcome these limitations. Supplementing land-based data with offshore
wave measurements by seafloor pressure sensors and GPS-equipped buoys can further improve the image of the earthquake source and prediction of tsunami extent, inundation, and runup. We present a kinematic source model obtained from a retrospective real-time analysis of a heterogeneous data set for the 2011Mw9.0 Tohoku-Oki, Japan, earthquake. Our model is consistent with conceptual models of subduction zones, exhibiting depth dependent behavior that is quantified through frequency domain analysis of slip
rate functions. The stress drop distribution is found to be significantly more correlated with aftershock locations and mechanism types when off-shore data are included. The kinematic model parameters are then used as initial conditions in a fully nonlinear tsunami propagation analysis. Notably, we include the horizontal advection of steeply sloping bathymetric features. Comparison with post-event on-land survey
measurements demonstrates that the tsunami’s inundation and runup are predicted with considerable accuracy, only limited in scale by the resolution of available topography and bathymetry. We conclude that it is possible to produce credible and rapid, kinematic source models and tsunami predictions within
minutes of earthquake onset time for near-source coastal regions most susceptible to loss of life and damage to critical infrastructure, regardless of earthquake magnitude,
Geophysical Research Letters
he combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data ... more he combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data set that is sensitive to the entire spectrum of ground displacement and the full extent of coseismic slip. In this study we derive earthquake magnitude scaling relationships using seismogeodetic observations of either P wave amplitude or peak ground displacements from five earthquakes in Japan and California ranging in magnitude from 5.3 to 9.0. The addition of the low-frequency component allows rapid distinction of earthquake size for large magnitude events with high precision, unlike accelerometer data that saturate for earthquakes greater than M 7 to 8, and is available well before the coseismic displacements are emplaced. These results, though based on a limited seismogeodetic data set, support earlier studies that propose it may be possible to estimate the final magnitude of an earthquake well before the rupture is complete
Journal of Geophysical Research
Computation of tsunami models in the region adjacent to large earthquakes immediately after ruptu... more Computation of tsunami models in the region adjacent to large earthquakes immediately after rupture initiation remains a challenging problem. Limitations of traditional seismological instrumentation in the near field and concern by tsunami modelers regarding the nonuniqueness of source inversions and the use of indirect observations have in the past been hurdles for such efforts. Employing near-field data from the Mw 9.0 2011 Tohoku-oki data, we test source models obtained from newly developed algorithms and multisensor data. We demonstrate the ability of such source models determined from land-based coseismic data from the combination of GPS and strong-motion sensors to forecast near-source tsunamis. We also demonstrate that rapid ingestion of offshore shallow water (100–1000 m) wave gauge data substantially improves the earthquake source and tsunami forecast. To assess the success of such tsunami models, we rely on detailed comparisons to 2000+ tsunami survey measurements collected after the event. We argue that deployments of shallow water wave gauges coupled with land-based geophysical sensors can, in the future, provide enough information to issue timely and accurate forecasts of tsunami intensity immediately or shortly after rupture initiation of large earthquakes.
Geophysical Research Letters, 2013
Rotational along with translational and strain measurements are essential for a complete descript... more Rotational along with translational and strain measurements are essential for a complete description of the motion of a deformable body in a seismic event. We propose a new seismogeodetic approach where collocated high-rate GPS and accelerometer measurements are combined to estimate permanent and dynamic coseismic ground tilts at a point, whereas at present, only dynamic tilts are measured with either a dense seismic array or an expensive ring laser gyroscope. We estimate point tilts for a five-story structure on a shake table subjected to 13 earthquake strong motion records of increasing intensity. For the most intense record from the 2002 M7.9 Denali earthquake, we observe a peak-to-peak dynamic tilt of 0.12° and a permanent tilt of 0.16° for the structure's roof. Point tilts derived from networks of collocated GPS and accelerometers can be used to estimate the rotational component of the seismic wavefield for improved earthquake source characterization.
Geophysical Research Letters
Rapid characterization of finite fault geometry and slip for large earthquakes is important for m... more Rapid characterization of finite fault geometry and slip for large earthquakes is important for mitigation of seismic and tsunamigenic hazards. Saturation of near-source weak motion and problematic integration of strong-motion data into displacements make this difficult in real time. Combining GPS and accelerometer data to estimate seismogeodetic displacement waveforms overcomes these limitations by providing mm-level three-dimensional accuracy and improved estimation of coseismic deformation compared to GPS-only methods. We leverage collocated GPS and accelerometer data from the 2011 Mw 9.0 Tohoku-oki, Japan earthquake by replaying them in simulated real-time mode. Using a novel approach to account for fault finiteness, we generate an accurate centroid moment tensor solution independently of any constraint on the slab geometry followed by a finite fault slip model. The replay of GPS and seismic data demonstrates that robust models could have been made available within 3 min of earthquake initiation.
Geochemistry Geophysics Geosystems
The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the S... more The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the Salton Trough, California provides a unique data set to investigate a new seismogeodetic approach that combines Global Positioning System (GPS) and accelerometer observations to estimate displacement and velocity waveforms. First in simulated real-time mode, we analyzed 1–5 Hz GPS data collected by 17 stations fully encircling the swarm zone at near-source distances up to about 40 km using precise point positioning with ambiguity resolution (PPP-AR). We used a reference network of North American GPS stations well outside the region of deformation to estimate fractional-cycle biases and satellite clock parameters, which were then combined with ultrarapid orbits from the International GNSS Service to estimate positions during the Brawley seismic swarm. Next, we estimated seismogeodetic displacements and velocities from GPS phase and pseudorange observations and 100–200 Hz accelerations collected at three pairs of GPS and seismic stations in close proximity using a new tightly coupled Kalman filter approach as an extension of the PPP-AR process. We can clearly discern body waves in the velocity waveforms, including P-wave arrivals not detectable with the GPS-only approach for earthquake magnitudes as low as Mw 4.6 and significant static offsets for magnitudes as low as Mw 5.4. Our study shows that GPS networks upgraded with strong motion accelerometers can provide new information for improved understanding of the earthquake rupture process and be of critical value in creating a robust early warning system for any earthquake of societal significance.
Journal of Geophysical Research, Apr 1, 2013
Computation of displacements from strong motion inertial sensors is to date an open problem. Two ... more Computation of displacements from strong motion inertial sensors is to date an open problem. Two distinct methodologies have been proposed to solve it. One involves baseline corrections determined from the inertial data themselves and the other a combination with other geophysical sensors such as GPS. Here we analyze a proposed automated baseline correction algorithm using only accelerometer data and compare it to the results from the real-time combination of strong motion and GPS data. The analysis is performed on 48 collocated GPS and accelerometers in Japan that recorded the 2011 Mw 9.0 Tohoku-oki earthquake. We study the time and frequency domain behavior of both methodologies. We find that the error incurred from automated baseline corrections that rely on seismic data alone is complex and can be large in both the time and frequency domains of interest in seismological and engineering applications. The GPS/accelerometer combination has no such problems and can adequately recover broadband strong motion displacements for this event. The problems and ambiguities with baseline corrections and the success of the GPS/accelerometer combination lead us to advocate for instrument collocations as opposed to automated baseline correction algorithms for accelerometers.
Seismological Research Letters
The recent great earthquakes of 26 December 2004 (Sumatra, Indonesia, Mw 9.2), 26 February 2010 (... more The recent great earthquakes of 26 December 2004 (Sumatra, Indonesia, Mw 9.2), 26 February 2010 (Maule, Chile, Mw 8.8), and 11 March 2011 (Tohoku-oki, Japan, Mw 9.0) have once again brought to the forefront the urgent need for early tsunami warning. These warnings mostly rely on magnitude and location of an earthquake. A large/great magnitude subduction-zone earthquake with rupture area extending up to the trench is potentially a tsunamigenic event. The appropriate magnitudes for tsunami warning are those that are based on long-period seismic waves (Abe 1979), e.g., the moment magnitude, Mw (Kanamori 1977).
Recently, W-phase (the long-period wave that arrives between P and S waves) has been used to compute Mw (Kanamori and Rivera 2008; Hayes et al. 2009). This magnitude can be determined in a relatively short time. For example, the first moment tensor solutions of the Tohoku-oki earthquake, based on inversion of W-phase at teleseismic distances, became available in 20 min (Duputel et al. 2011). For this reason, Mw computed from W-phase is especially useful for tsunami alerts at distant sites. It is at local distances that early tsunami warning becomes difficult. Even then, Mw based on inversion of W-phase recorded at regional distances may be useful. Tests show that Mw of Mexican subduction thrust earthquakes, based on W-phase recorded on broadband, regional seismograms, can be estimated in ∼7 min after the occurrence of the event (Pérez-Campos et al. 2010)...
Responses to recent great earthquakes and ensuing tsunamis in Sumatra, Chile, and Japan, with the... more Responses to recent great earthquakes and ensuing tsunamis in Sumatra, Chile, and Japan, with the resulting loss of life and damage to infrastructure demonstrate that our ability to ascertain the full extent of slip of catastrophic earthquakes and their tsunamigenic potential in the first minutes after the initiation of rupture is problematic. Regional GPS networks such as those in western North America and Japan are complementary to seismic networks by being able to directly measure displacements close to the source during large earthquakes in real time. We report on rapid modeling of two large earthquakes, the 2003 Mw 8.3 Tokachi-oki earthquake 100 km offshore Hokkaido Island using 356 GEONET stations and the 2010 Mw 7.2 El Mayor-Cucapah earthquake in northern Baja California using 95 CRTN stations in southern California about 75 km northwest of the epicenter. Working in a simulated real-time mode, we invert for finite fault slip in a homogeneous elastic half-space using Green's functions obtained from Okada's formulation. We compare two approaches: the first starts with a catalog of pre-defined faults, while the second uses a rapid centroid moment tensor solution to provide an initial estimate of the ruptured fault plane. In either case, we are able to characterize both earthquakes in less than two minutes, reducing the time necessary to obtain finite fault slip and moment magnitude for medium and greater earthquakes compared to traditional methods by an order of magnitude.
Amount of seconds until first warning was issued for 2011 Mw 9.0 Tohoku-oki earthquake. The conto... more Amount of seconds until first warning was issued for 2011 Mw 9.0 Tohoku-oki earthquake. The contours show seconds until the S-wave arrival. The region within the 0 s contour is the blind zone where no warning is available. This illustration shows that the warning was issued before the S-wave reached the coastline.
Scenario ruptures and ground motion simulation are important tools for studies of expected earthq... more Scenario ruptures and ground motion simulation are important tools for studies of expected earthquake and tsunami hazards during future events. This is particularly important for large (M w 8+) and very large (M w 8.5+) events for which observations are still limited. In particular, synthetic waveforms are important to test the response of earthquake and tsunami warning systems to large events. These systems are not often exercised in this manner. We will show an application of the Karhunen-Loève (K-L) expansion to generate stochastic slip distributions of large events with an example application to the Cascadia subduction zone. We will discuss how to extend the static slip distributions obtained from the K-L expansion to produce kinematic rupture models and generate synthetic long-period displacement data at the sampling rates of traditional Global Navigation Satellite Systems (GNSS) stations. We will validate the waveforms produced by this method by comparison to a displacement-based ground motion prediction equation obtained from GNSS measurements of large earthquakes worldwide.
Geodesy, the oldest science, has become an important discipline in the geosciences, in large part... more Geodesy, the oldest science, has become an important discipline in the geosciences, in large part by enhancing Global Positioning System (GPS) capabilities over the last 35 years well beyond the satellite constellation's original design. The ability of GPS geodesy to estimate 3D positions with millimeter-level precision with respect to a global terrestrial reference frame has contributed to significant advances in geophysics, seismology, atmospheric science, hydrology, and natural hazard science. Monitoring the changes in the positions or trajectories of GPS instruments on the Earth's land and water surfaces, in the atmosphere, or in space, is important for both theory and applications, from an improved understanding of tectonic and magmatic processes to developing systems for mitigating the impact of natural hazards on society and the environment. Besides accurate positioning, all disturbances in the propagation of the transmitted GPS radio signals from satellite to receiver are mined for information, from troposphere and ionosphere delays for weather, climate, and natural hazard applications, to disturbances in the signals due to multipath reflections from the solid ground, water, and ice for environmental applications. We review the relevant concepts of geodetic theory, data analysis, and physical modeling for a myriad of processes at multiple spatial and temporal scales, and discuss the extensive global infrastructure that has been built to support GPS geodesy consisting of thousands of continuously operating stations. We also discuss the integration of heterogeneous and complementary data sets from geodesy, seismology, and geology, focusing on crustal deformation applications and early warning systems for natural hazards.
W phase moment tensor inversion has proven to be a reliable method for rapid characterization of ... more W phase moment tensor inversion has proven to be a reliable method for rapid characterization of large earthquakes. For global purposes it is used at the United States Geological Survey, Pacific Tsunami Warning Center, and Institut de Physique du Globe de Strasbourg. These implementations provide moment tensors within 30–60 min after the origin time of moderate and large worldwide earthquakes. Currently, the method relies on broadband seismometers, which clip in the near field. To ameliorate this, we extend the algorithm to regional records from high-rate GPS data and retrospectively apply it to six large earthquakes that occurred in the past 5 years in areas with relatively dense station coverage. These events show that the solutions could potentially be available 4–5 min from origin time. Continuously improving GPS station availability and real-time positioning solutions will provide significant enhancements to the algorithm.
The 2015 M w 8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment ... more The 2015 M w 8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment of that subduction zone. It generated strong ground motions and a large (up to 11 m runup) tsunami which prompted the evacuation of more than 1 million people in the first hours following the event. Observations during recent earthquakes suggest that these phenomena can be associated with rupture on different parts of the megathrust. The deep portion generates strong shaking while slow, large slip on the shallow fault is responsible for the tsunami. It is unclear whether all megathrusts can have shallow slip during coseismic rupture and what physical properties regulate this. Here we show that the Illapel event ruptured both deep and shallow segments with substantial slip. We resolve a kinematic slip model using regional geophysical observations and analyze it jointly with teleseismic backprojection. We find that the shallow and deep portions of the megathrust are segmented and have fundamentally different behavior. We forward calculate local tsunami propagation from the resolved slip and find good agreement with field measurements, independently validating the slip model. These results show that the central portion of the Chilean subduction zone has accumulated a significant shallow slip deficit and indicates that, given enough time, shallow slip might be possible everywhere along the subduction zone.
We demonstrate a flexible strategy for local tsunami warning that relies on regional geodetic and... more 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 Mw8.3 Illapel, Chile earthquake and tsunami that prompted the evacuation of 1 million people.
Interferometric synthetic aperture radar (InSAR) is a key tool for the analysis of displacement a... more Interferometric synthetic aperture radar (InSAR) is a key tool for the analysis of displacement and
stress changes caused by large crustal earthquakes, particularly in remote areas. A challenge for traditional
InSAR has been its limited spatial and temporal coverage especially for very large events, whose dimensions
exceed the typical swath width of 70–100 km. This problem is addressed by the ALOS-2 satellite, whose
PALSAR-2 instrument operates in ScanSAR mode, enabling a repeat time of 2 weeks and a swath width of
350 km. Here we present InSAR line-of-sight displacement data from ALOS-2/PALSAR-2 observations
covering the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock that were acquired within 1 week of
each event. The data are made freely available and we encourage their use in models of the fault slip and
associated stress changes. The Mw 7.3 aftershock not only extended the rupture area of the main shock
toward the east but also left a 20 km gap where the fault has little or no coseismic slip. We estimate this
unslipped fault patch has the potential to generate a Mw 6.9 event.
Detailed geodetic imaging of earthquake rupture enhances our understanding of earthquake physics ... more Detailed geodetic imaging of earthquake rupture enhances our understanding of earthquake physics and induced ground shaking. The April 25, 2015 Mw 7.8 Gorkha, Nepal earthquake is the first example of a large continental megathrust rupture beneath a high-rate (5 Hz) GPS network. We use GPS and InSAR data to model the earthquake rupture as a slip pulse of ~20 km width, ~6 s duration, and with peak sliding velocity of 1.1 m/s that propagated toward Kathmandu basin at ~3.3 km/s over ~140 km. The smooth slip onset, indicating a large ~5 m slip-weakening distance, caused moderate ground shaking at high >1Hz frequencies (~16% g) and limited damage to regular dwellings. Whole basin resonance at 4-5 s period caused collapse of tall structures, including cultural artifacts.
Geophysical Research Letters
Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response sys... more Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response
systems during medium to large events. The 2014 Mw6.1 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.
Journal of Geophysical Research
GPS instruments are noninertial and directly measure displacements with respect to a global refe... more GPS instruments are noninertial and directly measure displacements with respect to a
global reference frame, while inertial sensors are affected by systematic offsets—primarily tilting—that
adversely impact integration to displacement. We study the magnitude scaling properties of peak ground
displacement (PGD) from high-rate GPS networks at near-source to regional distances (~10–1000 km),
from earthquakes between Mw6 and 9. We conclude that real-time GPS seismic waveforms can be used to
rapidly determine magnitude, typically within the first minute of rupture initiation and in many cases
before the rupture is complete. While slower than earthquake early warning methods that rely on the
first few seconds of P wave arrival, our approach does not suffer from the saturation effects experienced
with seismic sensors at large magnitudes. Rapid magnitude estimation is useful for generating rapid
earthquake source models, tsunami prediction, and ground motion studies that require accurate information
on long-period displacements.
Journal of Geophysical Research
Rapid near-source earthquake source modeling relying only on strong motion data is limited by ins... more Rapid near-source earthquake source modeling relying only on strong motion data is limited by instrumental offsets and magnitude saturation, adversely affecting subsequent tsunami prediction. Seismogeodetic displacement and velocity waveforms estimated from an optimal combination of high-rate
GPS and strong motion data overcome these limitations. Supplementing land-based data with offshore
wave measurements by seafloor pressure sensors and GPS-equipped buoys can further improve the image of the earthquake source and prediction of tsunami extent, inundation, and runup. We present a kinematic source model obtained from a retrospective real-time analysis of a heterogeneous data set for the 2011Mw9.0 Tohoku-Oki, Japan, earthquake. Our model is consistent with conceptual models of subduction zones, exhibiting depth dependent behavior that is quantified through frequency domain analysis of slip
rate functions. The stress drop distribution is found to be significantly more correlated with aftershock locations and mechanism types when off-shore data are included. The kinematic model parameters are then used as initial conditions in a fully nonlinear tsunami propagation analysis. Notably, we include the horizontal advection of steeply sloping bathymetric features. Comparison with post-event on-land survey
measurements demonstrates that the tsunami’s inundation and runup are predicted with considerable accuracy, only limited in scale by the resolution of available topography and bathymetry. We conclude that it is possible to produce credible and rapid, kinematic source models and tsunami predictions within
minutes of earthquake onset time for near-source coastal regions most susceptible to loss of life and damage to critical infrastructure, regardless of earthquake magnitude,
Geophysical Research Letters
he combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data ... more he combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data set that is sensitive to the entire spectrum of ground displacement and the full extent of coseismic slip. In this study we derive earthquake magnitude scaling relationships using seismogeodetic observations of either P wave amplitude or peak ground displacements from five earthquakes in Japan and California ranging in magnitude from 5.3 to 9.0. The addition of the low-frequency component allows rapid distinction of earthquake size for large magnitude events with high precision, unlike accelerometer data that saturate for earthquakes greater than M 7 to 8, and is available well before the coseismic displacements are emplaced. These results, though based on a limited seismogeodetic data set, support earlier studies that propose it may be possible to estimate the final magnitude of an earthquake well before the rupture is complete
Journal of Geophysical Research
Computation of tsunami models in the region adjacent to large earthquakes immediately after ruptu... more Computation of tsunami models in the region adjacent to large earthquakes immediately after rupture initiation remains a challenging problem. Limitations of traditional seismological instrumentation in the near field and concern by tsunami modelers regarding the nonuniqueness of source inversions and the use of indirect observations have in the past been hurdles for such efforts. Employing near-field data from the Mw 9.0 2011 Tohoku-oki data, we test source models obtained from newly developed algorithms and multisensor data. We demonstrate the ability of such source models determined from land-based coseismic data from the combination of GPS and strong-motion sensors to forecast near-source tsunamis. We also demonstrate that rapid ingestion of offshore shallow water (100–1000 m) wave gauge data substantially improves the earthquake source and tsunami forecast. To assess the success of such tsunami models, we rely on detailed comparisons to 2000+ tsunami survey measurements collected after the event. We argue that deployments of shallow water wave gauges coupled with land-based geophysical sensors can, in the future, provide enough information to issue timely and accurate forecasts of tsunami intensity immediately or shortly after rupture initiation of large earthquakes.
Geophysical Research Letters, 2013
Rotational along with translational and strain measurements are essential for a complete descript... more Rotational along with translational and strain measurements are essential for a complete description of the motion of a deformable body in a seismic event. We propose a new seismogeodetic approach where collocated high-rate GPS and accelerometer measurements are combined to estimate permanent and dynamic coseismic ground tilts at a point, whereas at present, only dynamic tilts are measured with either a dense seismic array or an expensive ring laser gyroscope. We estimate point tilts for a five-story structure on a shake table subjected to 13 earthquake strong motion records of increasing intensity. For the most intense record from the 2002 M7.9 Denali earthquake, we observe a peak-to-peak dynamic tilt of 0.12° and a permanent tilt of 0.16° for the structure's roof. Point tilts derived from networks of collocated GPS and accelerometers can be used to estimate the rotational component of the seismic wavefield for improved earthquake source characterization.
Geophysical Research Letters
Rapid characterization of finite fault geometry and slip for large earthquakes is important for m... more Rapid characterization of finite fault geometry and slip for large earthquakes is important for mitigation of seismic and tsunamigenic hazards. Saturation of near-source weak motion and problematic integration of strong-motion data into displacements make this difficult in real time. Combining GPS and accelerometer data to estimate seismogeodetic displacement waveforms overcomes these limitations by providing mm-level three-dimensional accuracy and improved estimation of coseismic deformation compared to GPS-only methods. We leverage collocated GPS and accelerometer data from the 2011 Mw 9.0 Tohoku-oki, Japan earthquake by replaying them in simulated real-time mode. Using a novel approach to account for fault finiteness, we generate an accurate centroid moment tensor solution independently of any constraint on the slab geometry followed by a finite fault slip model. The replay of GPS and seismic data demonstrates that robust models could have been made available within 3 min of earthquake initiation.
Geochemistry Geophysics Geosystems
The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the S... more The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the Salton Trough, California provides a unique data set to investigate a new seismogeodetic approach that combines Global Positioning System (GPS) and accelerometer observations to estimate displacement and velocity waveforms. First in simulated real-time mode, we analyzed 1–5 Hz GPS data collected by 17 stations fully encircling the swarm zone at near-source distances up to about 40 km using precise point positioning with ambiguity resolution (PPP-AR). We used a reference network of North American GPS stations well outside the region of deformation to estimate fractional-cycle biases and satellite clock parameters, which were then combined with ultrarapid orbits from the International GNSS Service to estimate positions during the Brawley seismic swarm. Next, we estimated seismogeodetic displacements and velocities from GPS phase and pseudorange observations and 100–200 Hz accelerations collected at three pairs of GPS and seismic stations in close proximity using a new tightly coupled Kalman filter approach as an extension of the PPP-AR process. We can clearly discern body waves in the velocity waveforms, including P-wave arrivals not detectable with the GPS-only approach for earthquake magnitudes as low as Mw 4.6 and significant static offsets for magnitudes as low as Mw 5.4. Our study shows that GPS networks upgraded with strong motion accelerometers can provide new information for improved understanding of the earthquake rupture process and be of critical value in creating a robust early warning system for any earthquake of societal significance.
Journal of Geophysical Research, Apr 1, 2013
Computation of displacements from strong motion inertial sensors is to date an open problem. Two ... more Computation of displacements from strong motion inertial sensors is to date an open problem. Two distinct methodologies have been proposed to solve it. One involves baseline corrections determined from the inertial data themselves and the other a combination with other geophysical sensors such as GPS. Here we analyze a proposed automated baseline correction algorithm using only accelerometer data and compare it to the results from the real-time combination of strong motion and GPS data. The analysis is performed on 48 collocated GPS and accelerometers in Japan that recorded the 2011 Mw 9.0 Tohoku-oki earthquake. We study the time and frequency domain behavior of both methodologies. We find that the error incurred from automated baseline corrections that rely on seismic data alone is complex and can be large in both the time and frequency domains of interest in seismological and engineering applications. The GPS/accelerometer combination has no such problems and can adequately recover broadband strong motion displacements for this event. The problems and ambiguities with baseline corrections and the success of the GPS/accelerometer combination lead us to advocate for instrument collocations as opposed to automated baseline correction algorithms for accelerometers.
Seismological Research Letters
The recent great earthquakes of 26 December 2004 (Sumatra, Indonesia, Mw 9.2), 26 February 2010 (... more The recent great earthquakes of 26 December 2004 (Sumatra, Indonesia, Mw 9.2), 26 February 2010 (Maule, Chile, Mw 8.8), and 11 March 2011 (Tohoku-oki, Japan, Mw 9.0) have once again brought to the forefront the urgent need for early tsunami warning. These warnings mostly rely on magnitude and location of an earthquake. A large/great magnitude subduction-zone earthquake with rupture area extending up to the trench is potentially a tsunamigenic event. The appropriate magnitudes for tsunami warning are those that are based on long-period seismic waves (Abe 1979), e.g., the moment magnitude, Mw (Kanamori 1977).
Recently, W-phase (the long-period wave that arrives between P and S waves) has been used to compute Mw (Kanamori and Rivera 2008; Hayes et al. 2009). This magnitude can be determined in a relatively short time. For example, the first moment tensor solutions of the Tohoku-oki earthquake, based on inversion of W-phase at teleseismic distances, became available in 20 min (Duputel et al. 2011). For this reason, Mw computed from W-phase is especially useful for tsunami alerts at distant sites. It is at local distances that early tsunami warning becomes difficult. Even then, Mw based on inversion of W-phase recorded at regional distances may be useful. Tests show that Mw of Mexican subduction thrust earthquakes, based on W-phase recorded on broadband, regional seismograms, can be estimated in ∼7 min after the occurrence of the event (Pérez-Campos et al. 2010)...
Responses to recent great earthquakes and ensuing tsunamis in Sumatra, Chile, and Japan, with the... more Responses to recent great earthquakes and ensuing tsunamis in Sumatra, Chile, and Japan, with the resulting loss of life and damage to infrastructure demonstrate that our ability to ascertain the full extent of slip of catastrophic earthquakes and their tsunamigenic potential in the first minutes after the initiation of rupture is problematic. Regional GPS networks such as those in western North America and Japan are complementary to seismic networks by being able to directly measure displacements close to the source during large earthquakes in real time. We report on rapid modeling of two large earthquakes, the 2003 Mw 8.3 Tokachi-oki earthquake 100 km offshore Hokkaido Island using 356 GEONET stations and the 2010 Mw 7.2 El Mayor-Cucapah earthquake in northern Baja California using 95 CRTN stations in southern California about 75 km northwest of the epicenter. Working in a simulated real-time mode, we invert for finite fault slip in a homogeneous elastic half-space using Green's functions obtained from Okada's formulation. We compare two approaches: the first starts with a catalog of pre-defined faults, while the second uses a rapid centroid moment tensor solution to provide an initial estimate of the ruptured fault plane. In either case, we are able to characterize both earthquakes in less than two minutes, reducing the time necessary to obtain finite fault slip and moment magnitude for medium and greater earthquakes compared to traditional methods by an order of magnitude.