Broadband Waveform Modeling Research Articles

Seismicity Analysis of the 2016 Ms5.0 Yunlong Earthquake, Yunnan, China and Its Tectonic Implications

We relocate earthquakes occurring 1 week before and 1 month after the 2016 Yunlong Ms5.0 earthquake by simultaneously using P (and S) travel times and waveform cross-correlation data in the double-difference relocation algorithm. We then use the well-relocated earthquakes as template events and scan through the continuous waveforms to search for and locate weak events by using the match and locate method. A total of 4602 events are detected by using 660 templates, which is ~ 3 times the number of events listed in the Yunnan Seismic Networks catalogue. Our refined catalogue reveals three stages of seismic activity and energy release processes during the 2016 Yunlong Ms5.0 earthquake sequence. We also refine the focal depths and mechanisms of 12M ≥ 3.0 earthquakes using the broadband waveform modeling method. Both the relocated hypocenters and focal mechanisms of the 12M ≥ 3.0 earthquakes delineate fault strike and dip angles of ~ 200° and ~ 75°, respectively, indicating that the seismogenic fault could be a NNE-striking blind or subfault located between the Weixi-Qiaohou Fault and Lancangjiang Fault. Our results could contribute to better understanding of local seismic hazard evaluations in the vicinity of Yunlong area of Yunnan, China.

Active backstop faults in the Mentawai region of Sumatra, Indonesia, revealed by teleseismic broadband waveform modeling

Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 “backthrust” sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as ‘unsticking’ of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults.

Double-ramp on the Main Himalayan Thrust revealed by broadband waveform modeling of the 2015 Gorkha earthquake sequence

The 2015Mw7.8 Gorkha earthquake sequence that unzipped the lower edge of the Main Himalayan Thrust (MHT) in central Nepal provides an exceptional opportunity to understand the fault geometry in this region. However, the limited number of focal mechanisms and the poor horizontal locations and depths of earthquakes in the global catalog impede us from clearly imaging the ruptured MHT. In this study, we generalized the Amplitude Amplification Factor (AAF) method to teleseismic distance that allows us to model the teleseismic P-waves up to 1.5 Hz. We used well-constrained medium-sized earthquakes to establish AAF corrections for teleseismic stations that were later used to invert the high-frequency waveforms of other nearby events. This new approach enables us to invert the focal mechanisms of some early aftershocks, which is challenging by using other long-period methods. With this method, we obtained 12 focal mechanisms more than that in the GCMT catalog. We also modeled the high-frequency teleseismic P-waves and the surface reflection phases (pP and sP) to precisely constrain the depths of the earthquakes. Our results indicate that the uncertainty of the depth estimation is as small as 1–2 km. Finally, we refined the horizontal locations of these aftershocks using carefully hand-picked arrivals. The refined aftershock mechanisms and locations delineate a clear double-ramp geometry of the MHT, with an almost flat décollement sandwiched in between. The flat (dip ∼7 degrees) portion of the MHT is consistent with the coseismic rupture of the mainshock, which has a well-constrained slip distribution. The fault morphology suggests that the ramps, both along the up-dip and down-dip directions, play a significant role in stopping the rupture of the 2015 Gorkha earthquake. Our method can be applied to general subduction zone earthquakes and fault geometry studies.

Interrogation of the Megathrust Zone in the Tohoku-Oki Seismic Region by Waveform Complexity: Intraslab Earthquake Rupture and Reactivation of Subducted Normal Faults

Results from the 2011 Mw 9.1 Tohoku-Oki megathrust earthquake display a complex rupture pattern, with most of the high-frequency energy radiated from the downdip edge of the seismogenic zone and very little from the large shallow rupture. Current seismic results of smaller earthquakes in this region are confusing due to disagreements among event catalogs on both the event locations (>30 km horizontally) and mechanisms. Here we present an in-depth study of a series of intraslab earthquakes that occurred in a localized region near the downdip edge of the main shock. We explore the validity of 1D velocity model and refine earthquake source parameters for selected key events by performing broadband waveform modeling combining regional networks. These refined source parameters are then used to calibrate paths and further simulate secondary source properties, such as rupture directivity and fault dimension. Calculation of stress changes caused by the main event indicate that the region where these intraslab events occurred are prone to thrust events. This group of intraslab earthquakes suggest the reactivation of a subducted normal fault, and are potentially useful in enhancing our understanding on the downdip shear zone and large outer-rise events.

Rupture Directivity Effect and Stress Heterogeneity of the 2013 Nantou Blind-Thrust Earthquakes, Taiwan

Abstract We investigated broadband evidence of the rupture directivity effect and estimated stress drop for the 2013 Nantou blind‐thrust earthquakes (the 0327 and 0602 crustal events) from three individual analyses using empirical Green’s functions: deconvolution source time function analysis, source spectral ratio analysis, and broadband waveform modeling of a strong‐motion generation area. Although these two adjacent events have similar focal mechanisms, the 0327 event shows only up‐dip directivity to the west, with an estimated stress drop of 9.0 MPa, and the 0602 event displays significant rupture directivity to the southwest, with an estimated stress drop of 14.2 MPa. The obvious variance in the rupture behavior and stress drop of these two nearby events, which might be related to immature blind faulting, suggests that this region requires additional study of the stress heterogeneity to better assess the seismic hazard.

Broadband waveform modeling with the spectral element method on Earth Simulator for the study of the structure at the top of the Earth's outer core

Broadband waveform modeling with the spectral element method on Earth Simulator for the study of the structure at the top of the Earth's outer core

Forward waveform modelling procedure for 1-D crustal velocity structure and its application to the southern Korean Peninsula

We propose a full-grid search procedure for broad-band waveform modelling to determine a 1-D crustal velocity model. The velocity model can be more constrained because of the use of broad-band waveforms instead of traveltimes for the crustal phases, although only a small number of event–station pairs were employed. Despite the time-consuming nature of the full-grid search method to search the whole model parameter space, the use of an empirical relationship between the P- and S-wave velocities can significantly reduce computation time. The proposed method was applied to a case in the southern Korean Peninsula. Broad-band waveforms obtained from two inland earthquakes that occurred on 2007 January 20 (Mw 4.6) and 2004 April 26 (Mw 3.6) were used to test the method. The three-layers over half-space crustal velocity model of the P- and S-wave velocities was estimated. Comparisons of waveform fitness between the final model and previously published models demonstrate advancements in the average value of waveform fitness for the inland earthquakes. In addition, 1-D velocity models were determined for three distinct tectonic regions, namely, the Gyonggi Massif, the Okcheon Belt and the Gyeongsang Basin, which are all located inside the study area. A comparison between the three models demonstrates that the crustal thickness of the southern Korean Peninsula increases from NW to SE and that the lower crustal composition of the Okcheon belt differs from that of the other tectonic regions.

Variable-Period Surface-Wave Magnitudes: A Rapid and Robust Estimator of Seismic Moments

Abstract We demonstrate that surface-wave magnitudes ( M s ), measured at local, regional, and teleseismic distances, can be used as a rapid and robust estimator of seismic moment magnitude ( M w ). We used the Russell (2006) variable-period surface-wave magnitude formula, henceforth called M s (VMAX), to estimate the M s for 165 North American events with 3.2 M w M w estimated from broadband waveform modeling (Herrmann et al. , 2008) were regressed against M s (VMAX). M w can be estimated from M s (VMAX) using the relationship: M w =1.91+0.66 * M s (VMAX) for 2 M s M w [ M s (VMAX)] with respect to M w [Waveform Modeling] was approximately ±0.2 magnitude units (m.u). The residuals between M w [ M s (VMAX)] and M w [Waveform Modeling] show a significant focal mechanism effect, especially when strike-slip events are compared with other mechanisms. Validation testing of this method suggests that M s (VMAX)-predicted M w ’s can be estimated within minutes after the origin of an event and are typically within ±0.2 m.u. of the final M w [Waveform Modeling]. While M w estimated from M s (VMAX) has a slightly higher variance than waveform modeling results, it can be measured on the first short-period surface-wave observed at a local or near-regional distance seismic station after a preliminary epicentral location has been formed. Therefore, it may be used to make rapid measurements of M w , which are needed by government agencies for early warning systems.

Distinct velocity variations around the base of the upper mantle beneath northeast Asia

Both the global and regional P wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions. In particular, low-velocity anomalies have been observed beneath the descending high-velocity slabs in a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, make it difficult to substantiate this feature and evaluate the vertical extent of the low-velocity structure. From broadband waveform modeling of triplicated phases near the 660-km discontinuity for three deep events, we constrained both the P and SH wave velocity structures around the base of the upper mantle in northeast Asia. For the two events beneath the Southern Kurile, the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending Pacific slab. Our preferred models consistently suggest normal-to-lower P and significantly low SH velocities above and below the 660-km discontinuity extending to about 760-km depth compared with the global IASP91 model, corroborating previous observations for a slow structure underneath the slab. In contrast, both high P and SH velocity anomalies are shown in our preferred models for the Japan subduction zone region, likely reflecting the structural feature of a slab stagnant above the 660-km discontinuity. The velocity jumps across the 660-km discontinuity were found to be on average 4.5% and 7% for P and S waves under the south Kurile, and 3% and 6% under the Japan subduction zone. The respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under-slab areas. Based on mineral physics data, the depth-averaged ∼1.5% P and ∼2.5% SH velocity differences in the depth range of 560–760 km between the two regions could be primarily explained by a 350–450 K temperature variation, although the presence of about 0.5–1 wt.% water might also contribute to the subtle velocity variations near the base of the transition zone in the Southern Kurile. From our modeling results, we speculate that the slow structure in the Southern Kurile may be correlated to the low-velocity zone observed previously around the 410-km discontinuity under Northern Honshu. If this is the case, both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast Asia.

Broadband Waveform Modeling of Moderate Earthquakes in the San Francisco Bay Area and Preliminary Assessment of the USGS 3D Seismic Velocity Model

Abstract Recently, the United States Geologic Survey Earthquake Hazards Program developed a 3D seismic velocity model of Northern California based on geology, including a detailed model of the urbanized San Francisco Bay Area. In this study, we report comparisons of observed three-component broadband (0.03–0.25-Hz) waveforms with synthetic seismograms computed with the new 3D model using an elastic finite difference method. We selected a set of 12 moderate earthquakes ( M w 4.0–5.1) that occurred within the greater San Francisco Bay Area, having well-constrained source parameters and broadband recordings with high signal-to-noise ratios. The objective of this study was to investigate how well the 3D velocity model predicts observed waveforms and to identify features of the model that may require revision to improve the waveform fits and predictions of ground motions for future events. We show for the 3 September 2000 Yountville earthquake that reported source parameters accurately predict waveforms at two close strong-motion stations (approximately 10 km from the epicenter). By comparing synthetics for the average 1D model GIL7 (Stidham et al. , 1999) and the 3D structures we show that the effects of seismic wave propagation in the 3D model become important for frequencies at and above 0.1 Hz (periods less than 10 sec). Comparison of observed and synthetic seismograms for the 3D model consistently shows that the model predicts energy arriving earlier than is observed, particularly for the surface waves, indicating that the shear velocities in the upper crust must be reduced. We cross correlated the observed and synthetic waveforms and recorded the delay time and linear correlation for best alignment of the data and delayed synthetic. The results indicate that generally the 3D model predicts the observed waveforms well (mean linear correlations 0.41) and includes features that arise from the interaction of the wave field with 3D structure, especially the major sedimentary basins in San Pablo Bay, San Francisco Bay, Santa Clara Valley, and Livermore Valley. Simple conversion of the observed delay times for optimal alignment suggests shear velocities should be reduced by 4%–5% on average. Based on these findings, we conclude that the model is an excellent first step, suggesting that the overall structure of the model is accurate (i.e., the basin and discontinuity geometry). However, the velocities must be reduced to improve the observed timing of surface-wave arrivals.

Source model of the 2005 Miyagi-Oki, Japan, earthquake estimated from broadband strong motions

AbstractWe estimate the source model of the 2005 Miyagi-Oki earthquake, which consists of two source patches called “strong motion generation areas (SMGAs)”, to explain the broadband strong motions. The location of the first SMGA is determined from the arrival time differences between the first motion and the first pulse in theP-wave portion. Based on the broadband waveform modeling obtained using the empirical Green’s function method, we estimate the parameters of the two SMGAs. The estimated location of the two SMGAs coincides with the two major large slip areas inferred from the inversion of strong-motion and teleseismic waveforms, which implies that the broadband strong motion of the 2005 Miyagi-Oki earthquake mainly radiates from the asperities. The deeper SMGA has a larger stress drop (34.1 MPa) than the shallower one (17.6 MPa). Comparison of the locations of SMGAs between the 2005 and 1978 Miyagi-Oki earthquakes suggests that they do not overlap with each other. The interplate earthquakes, including this event, were found to have smaller SMGAs than those predicted from the relationship of crustal earthquakes for a given seismic moment. This implies that the stress drop of the SMGAs for interplate earthquakes is larger than that for crustal earthquakes.

Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States

SUMMARY Traveltime tomographic models of the LA RISTRA transect produce excellent waveform fits if we amplify the damped images. We observe systematic waveform distortions across the western edge of the Great Plains from South American events, starting about 300 km east of the centre of the Rio Grande Rift. The amplitude decreases by more than 50 per cent within array stations spanning less than 200 km while the pulse width increases by more than a factor of 2. This feature is not observed for the data arriving from the northwest. While the S-wave tomographic image shows a fast slab-like feature dipping to the southeast beneath the western edge of the Great Plains, synthetics generated from this model do not reproduce the waveform characteristics. However, once we modify the tomographic image by amplifying the velocity contrast between the slab and adjoining mantle by a factor of 2‐3, the synthetics produce observed amplitude decay and pulse broadening. In addition to the traveltime delay, amplitude variation due to wave phenomena such as slab diffraction, focusing and defocusing provide much tighter constraints on the geometry of the fast anomaly and its amplitude and sharpness as demonstrated by a forward sensitivity test and snapshots of the seismic wavefield. Our preferred model locates the slab 200 km east of the Rio Grande Rift dipping 70 ◦ ‐75 ◦ to the southeast, extending to a depth near 600 km with a thickness of 120 km and a velocity of about 4 per cent fast. In short, adding waveform and amplitude components to regional tomographic studies can help validate and establish structural geometry, sharpness and velocity contrast.

Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band seismic waveform modelling

SUMMARY The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31 ◦ S. Earthquake locations define the subhorizontal subducted oceanic Nazca plate at depths of 90‐120 km. Another seismic region is located within the continental upper plate with events at depths 1.80, th = 45‐55 km) than those of the eastern Sierras Pampeanas (Vp = 6.0‐6.2 km s −1 , Vp/Vs < 1.70, th = 27‐35 km). In addition, we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes. Finally, we estimated average P and T axes over the CHARGE period. The entire region showed P- and T-axis orientations of 275 ◦ and 90 ◦ , plunging 6 ◦ and 84 ◦ , respectively.

Evaluation of slab images in the northwestern Pacific

Recent P wave travel-time tomographic studies using data from the International Seismological Centre (ISC) catalog determine a large-scale subhorizontal high velocity anomaly in the northwestern Pacific subduction zones and it has been interpreted as imaging stagnant slab in the upper mantle transition zone (∼400 to 700 km). The limited resolution of the travel time tomographic studies in this depth range, however, makes it difficult to evaluate accurately the vertical and lateral extent of a stagnant slab. A broadband waveform modeling of triplicated regional seismic waves which are very sensitive to the transition zone structure is useful to evaluate the velocity structure along the propagation paths and therefore to constrain the spatial distribution of anomalies. This study thus compares tomographic images from the model of Obayashi et al. (1997) with results of the regional waveform modeling by Tajima and Grand (1998). The ISC tomographic model shows the largest lateral extent of high velocity anomaly in the layer of 478 to 551 km depths although part of this spread is likely due to the deteriorated resolution in that depth range. The waveform modeling suggests that the strong high velocity anomaly associated with a stagnant slab exists below 525 km with its maximum intensity in the top 50 km and decreases with increasing depth to vanish at 660 km. These results along with a recent global SH velocity model SAW12D of Li and Romanowicz (1996) which has the strongest high velocity anomaly in a depth range 500–550 km may be integrated into an image of a stagnant slab. The anomalous velocity structure associated with a stagnant slab has its maximum intensity not immediately above the 660 km discontinuity but in a depth range ∼100 km above it. This feature appears to be consistent with a thermo-chemical model of down-going slab in which a larger velocity contrast with the surrounding mantle is expected at a shallower depth of the transition zone. The ISC tomographic model and waveform modeling consistently show that the deflected slabs are not laterally continuous but are separated into a few subregions. Beneath the northeastern China where the resolution is good, the slab related anomaly above the 660 km discontinuity is accompanied by its downward extension into the lower mantle.

Modelling D″ structure beneath Central America with broadband seismic data

Deep South American events recorded at distances between 70° and 96° are used to constrain the lower mantle structure beneath Central America. Employing data from the TERRAscope, BDSN, IRIS and CNSN (Canadian National Seismic Network) broadband arrays, we obtain a 1-D shear model with a 200 km thick D″ layer with a 3% velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of SH-polarized S, ScS, and Scd. The shear structure above D″ is similar to the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981. Preliminary Reference Earth Model. Physics of Earth & Planetary Interiors 25, 297–356) in terms of dT dΔ and S-SKS differential times. SV synthetics for this model, assuming a PREM core, produce a satisfactory fit to the data, except at the larger ranges to the north, where the waveforms display rapid variation. This feature is indicative of lateral variation of the type suggested by Grand (1994), where the discontinuity becomes less well defined. Synthetics computed for a simplified 2-D cross-section through this structure produce the Scd phase and simple S at larger distances. While Scd is easily observed in the shear wave data, there is little evidence of its counterpart in the P data. By testing several models with different P-velocity jumps at the same depth as the D″ jump in shear structure, we conclude that the velocity jump must be less than 1%. In fact, PREM provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower-mantle structure beneath Central America shows clear evidence for a discontinuity in S velocity but not in P.

Analysis on rupture feature of a great complicated earthquake in Kamchatka

The M s=7.3 earthquake of June, 8 1993, off the eastern coast of Kamchatka was very complicated in the rupture history. The rupture feature of this event was discussed by the broadband waveform modelling method as well as the combining analysis on the subevent stack and the quasi-time difference. The results suggest that the rupture propagation of the event was in a strong unidirection and its main rupture processes can be expressed as: rupture nucleation→NEE→near east by north→near east by south→stop, from deep to shallow.

Seismic structure near the inner core‐outer core boundary

A model of P‐wave velocity of the Earth's core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad‐band waveform modeling of core phases at distances from 130° to 160°. Differential travel times between PKPBC (Cdiff) and PKPDF (TBC–TDF) are about 0.5 sec shorter than those computed from PREM at distances from 146° to 152° and increase with distance to nearly the same as those from PREM at 155°. Differential travel times between PKPCD and PKPDF (TCD–TDF) at distances from 130° to 143° are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (ABC/ADF) at distances 150° to 156° are about 40% larger than those from PREM. These observations require a smaller P‐wave velocity gradient (0.0005 sec−1) in the lowermost 300 km of the outer core than that from PREM (∼0.00065 sec−1), a slower P‐wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec−1) in the uppermost 300 km of the inner core.

Broadband waveform modelling of anomalous strong ground motion in the 1989 Loma Prieta Earthquake using three‐dimensional geologic structures

Strong motion recordings of the 1989 Loma Prieta earthquake show a large, coherent SH wave displacement pulse across much of the San Francisco Bay area. This distinctive ground motion has been modelled using approximate, broadband synthetics for a double‐couple centroid source in a 3‐D crustal model for the region. The synthetics indicate that (1) horizontal bending and focusing of SH waves by the lateral crustal velocity contrast across the San Andreas fault and (2) the source radiation pattern, contribute significantly to the observed azimuthal variation of the SH pulse and hence to the regional variations of strong ground shaking in the period range of 1 to 8 sec. In addition, the scattering of surface‐reflected shear waves by deep basins under the Santa Clara Valley may contribute to increased coda amplitude and duration observed for sites around Oakland.The wave modelling also supports the earlier conclusion that an increase in intensity in portions of San Francisco and Oakland is in part the result of energy reflected from the base of the crust [Somerville and Yoshimura, 1990].