iPlex Lunch - spring-2017

April 5, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Petro Val - UCSD

Erosion during mountain building: Insights from the southern Central Andes

The temporal evolution of erosion over million-year timescales is key to understand the development of mountain ranges and adjacent fold-and-thrust belts. Models of orogenic wedge dynamics predict an instantaneous response of erosion to pulses of rock uplift while stream-power based models predict that catchment-wide erosion maxima lag behind a pulse of rock uplift. Other models, empirical data, and global compilations of exhumation data demonstrate the dependency of denudation and rock exhumation rate on climate. However, site-specific studies suggest that these relationships can be diminished by tectonics. This talk will contain two case-studies from the southern Central Andes, one involving a temporal record of 10Be-derived paleoerosion rates from 8-3 Ma and another constraining the along-strike erosion rate pattern using 10Be and gauge data from both flanks of the Andes (Argentina and Chile). These case-studies support that the attainment of peak erosion rates is lagged with respect to the rock uplift pulse and that modern erosion rates are better explained by tectonics.

April 12, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Lingsen Meng - UCLA

Improving back-projection imaging with a slowness-based aftershock calibration approach

Over the last decade, the development of large-scale dense seismic networks has promoted rapid progress in a broad spectrum of seismological sciences. High-frequency seismic waveforms from these large arrays have enabled back-projections (BP), an emerging tool to probe earthquake dynamics. BP is widely used to study large earthquakes, but its derived images are array-dependent. For the same earthquake, different arrays often produce different images, and it is difficult to judge which result should be trusted more. In this talk, I will present a new approach to effectively mitigate the BP uncertainties of large earthquakes based on their aftershocks. I introduced a slowness (ray parameter) error term calibrated by aftershocks to achieve consistency between BPs of different seismic networks. This correction accounts for the P-wave travel time errors in each receiver array, due to approximating 3D earth structure with 1D reference velocity model.

Such technological improvement in earthquake source observations allows me to address the questions of earthquake source dynamics in the case studies of recent large earthquakes. In the 2015 Mw 7.8 Gorkha earthquake, our refined source imaging reveals a narrow unilateral eastward rupture unzipping the lower bottom of the locked portion of the Main Himalaya Thrust. The limited rupture extent indicates that the Gorkha earthquake in Nepal is possibly a medium-size event resulted from heterogeneous loading stress during the inter-seismic period of larger earthquakes. We have also performed a joint seismic and geodetic investigation of the 2016 Mw 7.8 Kaikoura earthquake in New Zealand. We find that the earthquake spans over 100 km through stepping and branching of at least six distinct fault planes. The high-frequency radiations occur mainly on three shallow thrust faults located in the dilatational quadrants of rupture on the Hope-Kekerengu fault, consistent with the unclamping effect predicted by the dynamic Coulomb stress. This study demonstrates the capability of the BP method, enhanced by aftershock calibrations, to describe earthquake rupture kinematics in regions of complex fault systems.

April 19, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Sally McGill - California State University, San Bernardino

Distribution of slip rate & implications for earthquake ruptures within the so. San Andreas system

Differences between slip rates estimated for the San Andreas fault by geodetic versus geologic methods have sparked many new geologic slip rate estimates for this fault section over the past decade or so. These studies reveal that the southern San Andreas fault zone exhibits large variations in slip rate along strike, which can explain much of the previously noted differences between geologic and geodetic rate estimates. This talk will present new geologic slip rate estimates from Pitman Canyon, Badger Canyon and Plunge Creek along the San Bernardino section of the San Andreas fault, as well as work in progress at a slip rate site and paleoseismic site on the Banning strand of the San Andreas fault near North Palm Springs. The results show that the Holocene to latest Pleistocene slip rate on the San Bernardino and San Gorongio Pass sections of the San Andreas fault is quite low (<16 mm/yr), as compared to rates almost twice as fast on the Mojave section of the San Andreas fault and south of Indio. The slow slip rate on the San Bernardino and San Gorgonio Pass sections of the fault is likely due to plate boundary slip being accommodated by the San Jacinto fault and the Eastern California shear zone. The implications of these slip rate patterns for future earthquake ruptures on the San Andreas fault system remains poorly understood.

April 26, 2017
noon - 1 a.m.
Geology 1810

Presented By:

• Alan Yong - USGS

Seismic Site Characterization: Mark, Measure & Cut for VS30

I present a review of the state-of-practice for estimating VS30, the time-averaged shear-wave velocity of the upper 30 m, as well as select developments for advancing measured or proxy-based VS30 methods. VS30 values have traditionally been derived directly from on-site array-based records of seismic travel-times. As a result of cost and/or environmental factors that restrict the mobilization of on-site recording arrays, remotely-derived proxy-based methods have been used to estimate VS30 values. Thus, proxy-based methods—commonly using map information: geology, slope, terrain, or their hybrids—serve as a stopgap solution until on-site measurements are available. Because of the indirect nature of these map-based methods, proxy-based VS30 estimates are known to have substantial uncertainties, whereas inter- and intra-method variability of measured VS30 values is typically 5-10%. To reduce uncertainties, Iwahashi et al. (2016) used a large data set of recently available measured VS30 values and an improved terrain framework to recalibrate their proxy-based VS30 model. For sites where cost isn’t a factor, there are advancements in the analyses of earthquake and microseismic data from seismic monitoring stations that allow estimations of VS30. Herrick et al. (2017) found P-wave-based VS30 estimates from earthquake sources correlate with array-based VS30 measurements in the VS30 range of 500-1500 m/s, while earthquake-based VS30 estimates outside of this range differ by more than 10 percent. Hassani et al. (2017) expanded the Hassani and Atkinson (2016) study, which compared VS30 to earthquake-based estimates of the dominant site frequency (fd), to fd based on microseisms and found the earthquake and microseismic fd estimates scale linearly. Lastly, Yong et al. (2017) found measured VS30 values correlate well with array-based VR40 (Rayleigh-wave phase velocity at the 40-meter wavelength) estimates, thus indicating VR40 is a potential proxy for estimating VS30 values.

May 3, 2017
noon - 1 a.m.
Geology 1810

Presented By:

• Zhongwen Zhan - Caltech

Here is the title and description, the same as in the GRL paper. Recurring large deep earthquakes in Hindu Kush driven by a sinking slab Hindu Kush subduction zone produces large intermediate-depth earthquakes within a small volume every 10–15 years. Here we study the last three M? 7 events within the cluster and find complex and diverse rupture processes. However, their main subevents appear to recur on the same fault patch, dipping 70° to the south. This recurrence requires an average of 9.6 cm/yr slip rate on the patch, much higher than the ~1 cm/yr surface convergence rate measured geodetically. The high slip rate is likely caused by significant slab internal deformation, such as localized slab stretching/necking. We infer that the Hindu Kush subducted slab below 210 km is sinking through the mantle at a vertical rate of 10 cm/yr.

May 10, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Haotian Xu - UCLA
• Zheng Xing - UCLA

1. Transdimensional approach applied to the measurement of higher mode surface wave (Haotian Xu)

The analysis of higher mode surface wave carries unique, independent constraints on the structure at depth and thus enhance resolution in the deep upper mantle, transition zone, and uppermost lower mantle. However, direct measurement of higher modes is challenging because their group velocities overlap significantly in a long frequency range. We developed a new non-linear waveform inversion technique using a transdimensional Bayesian approach to measure path-specific multimode Rayleigh wave dispersion. Fundamental mode, overtones as well as their uncertainties can be measured by this method. We applied our method to both synthetic and real data and reasonable results were obtained, demonstrating the feasibility of this method.

2. Assessing the Depth of the Pacific Lithosphere-Asthenosphere Boundary From Various Proxies With a Bayesian Approach (Zheng Xing)

Determining how the lithosphere-asthenosphere boundary (LAB) depth changes with ocean age is one of the keys to understanding the origin of oceanic plates. Previous studies have shown that inversion of surface wave dispersion curves for S-wave velocities yields LAB depth based on VS increase that depends on oceanic crustal age, consistent with thermal effects. However, depth changes in radial anisotropy used as LAB depth proxy do not display any age dependence. Although this could mean that the two proxies highlight different processes in the formation and evolution of oceanic plates (e.g. Hansen et al., 2016), the non-uniqueness of the inverse problem questions whether these two proxies are really seeing different rock properties. Here, we applied a model space search approach to two different datasets of surface wave phase velocity data to get an more quantitative analysis on uncertainties of the two LAB depth proxies.

May 17, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Hui Huang - UCLA

Some large interplate earthquakes are found to be preceded by seismicity acceleration around the hypocenter. Whether it could be explained by cascade triggering or background slow slip is still controversial. Although some large earthquakes, e.g. the 2014 Mw 8.2 Iquique earthquake, are preceded by slow slip confirmed by geodetic observations, the possible precursory signals before most large earthquakes are unclear. In this talk, I will present our recent efforts to apply template matching methods to detect precursory microseismicity and repeating earthquakes, which are possibly driven by slow slip. The results show that abundant newly detected earthquakes down to M 1 appear to accelerate over a large area ~3-4 days before the 2015 Mw 7.8 Gorkha earthquake. This is more consistent with the scenario where small events are driven by background slow aseismic slip. Another example shows that both the repeater-inferred aseismic slip and the seismicity appear to accelerate within the last ~150 days before and in a large scale surrounding the 2015 Mw 8.4 Illapel mainshock. Our results highlight the possible role of slow-slip signals in unloading the impending rupture area.

May 24, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Peter Haproff - UCLA

Investigations of continental collisions often focus on thrust belts oriented perpendicular to the plate-convergence direction, excluding belts that surround the corners and flanks of a continental indenter despite being crucial to understanding the collisional process. Research of the Himalayan orogenic system, for example, has mostly focused on the east-trending thrust belt between the eastern and western syntaxes, leaving the north-trending Indo-Burma Ranges bounding the eastern margin of India inadequately examined. We present the first comprehensive geologic investigation of the Northern Indo-Burma Range (NIBR), the northernmost segment of the Indo-Burma Ranges and easternmost extension of the Himalayan orogenic system. By integrating field mapping with U-Pb geochronology, thermobarometry, and whole-rock geochemistry, we show that major Himalayan lithologic units and thrust faults extend across the eastern Himalayan syntaxis to the NIBR. The structural framework of the NIBR consists of a southwest-directed thrust belt cored by a hinterland-dipping duplex, similar to the rest of the Himalaya. However, the Northern Indo-Burma thrust belt is distinct based on (1) the absence of the South Tibetan detachment fault, (2) crustal shortening greater than 80 %, (3) a narrow orogen width of 7-33 km, (4) the exposure of an ophiolitic mélange complex as klippen, (5) and right-slip shear along the active range-bounding thrust fault. Furthermore, lithospheric deformation adjacent to the northeast corner of India is characterized by right-slip transpression partitioned between the thrust belt and orogen-bounding right-slip faults. Such a strain regime is interpreted to accommodate both contraction and clockwise rotation of Tibetan lithosphere around India, consistent with continuum deformation and rotation models rather than extrusion of internally-rigid blocks along strike-slip faults. We integrate our knowledge of the paleogeography of southern Asia and kinematics of indenter-induced continental deformation in a holistic model for the evolution of the entire Himalayan orogenic system.

May 31, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Joshua Gordon - UCLA Statistics

The spatial distribution of aftershocks in the Epidemic Type Aftershock Sequences (ETAS) model has been estimated under various parametric forms. Typical formulations include isotropic, anisotropic, and magnitude dependent. Nonparametric modeling procedures have been developed, such as in Marsan and Lengliné 2008, 2010, and more recently in Fox et al. 2016. In this study, we examine nonparametric extensions for the previously proposed parametric forms of the spatial response function. Simulation studies are conducted to assess the methods and they are demonstrated on an earthquake catalog for California from 1985 to 2005. Using the proposed extensions, multiple time-independent forecasts are developed, which provide five-year forecasts for magnitudes m ? 4.95 in the California CSEP testing region. Model performance is evaluated by the comparison of our models to the long-term forecast of Helmstetter 2007 using both Deviance and Voronoi Residuals. We show improved performance compared to Helmstetter 2007 in various regions while using a full nonparametric estimation and forecasting approach.

June 7, 2017
noon - 1 p.m.
Geology 1810

Presented By:

• Zagid Abatchev - UCLA

Seismic propagation forward models form the basis of seismic tomography and are one of the main constraints on the quality of seismic inversions. Many approaches exist to describe the first arrival wavefront propagation. We examine four classes of first arrival models, evaluating and comparing their failure modes and computational cost, as well as suitability for massively parallel GPU implementation.