Shengji Wei

The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries

Detailed geophysical measurements reveal features of the 2011 Tohoku-Oki megathrust earthquake. Geophysical observations from the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki, Japan earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the Mw 8.8 2010 Maule, Chile earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation—a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event.

The 2014 Mw 6.1 South Napa earthquake : a unilateral rupture with shallow asperity and rapid afterslip

The Mw 6.1 South Napa earthquake occurred near Napa, California, on 24 August 2014 at 10:20:44.03 (UTC) and was the largest inland earthquake in northern California since the 1989 Mw 6.9 Loma Prieta earthquake. The first report of the earthquake from the Northern California Earthquake Data Center (NCEDC) indicates a hypocentral depth of 11.0 km with longitude and latitude of (122.3105° W, 38.217° N). Surface rupture was documented by field observations and Light Detection and Ranging (LiDAR) imaging (Brooks et al., 2014; Hudnut et al., 2014; Brocher et al., 2015), with about 12 km of continuous rupture starting near the epicenter and extending to the northwest. The southern part of the rupture is relatively straight, but the strike changes by about 15° at the northern end over a 6 km segment. The peak dextral offset was observed near the Buhman residence with right‐lateral motion of 46 cm, near the location where the strike of fault begins to rotate clockwise (Hudnut et al., 2014). The earthquake was well recorded by the strong‐motion network operated by the NCEDC, the California Geological Survey and the U.S. Geological Survey (USGS). There are about 12 sites within an epicentral distance of 15 km that had relatively good azimuthal coverage (Fig. 1). The largest peak ground velocity (PGV) of nearly 100  cm/s was observed on station 1765, which is the closest station to the rupture and lies about 3 km east of the northern segment (Fig. 1). The ground deformation associated with the earthquake was also well recorded by the high resolution COSMO–SkyMed (CSK) satellite and Sentinel-1A satellite, providing independent static observations.

Report on the August 2012 Brawley Earthquake Swarm in Imperial Valley, Southern California

The 2012 Brawley earthquake swarm occurred in the Brawley Seismic Zone (BSZ) within the Imperial Valley of southern California (Fig. 1). The BSZ is the northernmost extensional segment of the Pacific–North America plate boundary system. Johnson and Hill (1982) used the distribution of seismicity since the 1930s to outline the geographical extent of the BSZ, defining boundaries of the BSZ as shown in Figure 1. Its north–south extent ranges from the northern section of the Imperial fault, starting approximately 10 km north of the United States–Mexico international border and connecting to the southern end of the San Andreas fault, where it terminates in the Salton Sea. Larsen and Reilinger (1991), who defined a similar geographical extent of the BSZ, argued that the BSZ was migrating to the northwest, which they associated with the propagation of the Gulf of California rift system into the North American continent. During the seismically active period of the 1970s, the BSZ produced close to half of the earthquakes recorded in California (Johnson and Hill, 1982; Hutton et al., 2010). However, for two decades following the 1979 Imperial Valley mainshock M_w 6.4 and its aftershock sequence, the BSZ was much less active. In general, the BSZ seismicity is indicative of right-lateral strike-slip plate motion accompanied by crustal thinning as well as possible associated fluid movements in the crust (Chen and Shearer, 2011). The 2012 Brawley swarm produced more than 600 events recorded by the United States Geological Survey (USGS)–California Institute of Technology (Caltech) Southern California Seismic Network (SCSN). Other monitoring instruments in the region, such as the Global Positioning System (GPS) network, creepmeters, and the Wildlife Liquefaction Array (WLA) also recorded signals from the largest events. In addition, Interferometric Synthetic Aperture Radar (InSAR) satellites collected images from space.

Imaging the distribution of transient viscosity after the 2016 Mw 7.1 Kumamoto earthquake

Crustal rock strength from outer space The response of crustal rock to stresses is challenging to estimate yet vital for determining risks from events such as earthquakes. Moore et al. take advantage of the recent Mw 7.1 Kumamoto earthquake in Japan to determine the rheology of crustal rocks in the region. The observed inversion of the crustal strain rates demonstrates that certain areas have stiff rock and others (e.g., under the Aso volcanic complex) have much weaker rock. The results match up with expectations, which means that the method can successfully measure rock properties over a wide range of strength and large spatial and temporal scales. Science, this issue p. 163 The combination of GPS and InSAR data after the Kumamoto earthquake in Japan allows for an estimate of regional rock rheology. The deformation of mantle and crustal rocks in response to stress plays a crucial role in the distribution of seismic and volcanic hazards, controlling tectonic processes ranging from continental drift to earthquake triggering. However, the spatial variation of these dynamic properties is poorly understood as they are difficult to measure. We exploited the large stress perturbation incurred by the 2016 earthquake sequence in Kumamoto, Japan, to directly image localized and distributed deformation. The earthquakes illuminated distinct regions of low effective viscosity in the lower crust, notably beneath the Mount Aso and Mount Kuju volcanoes, surrounded by larger-scale variations of viscosity across the back-arc. This study demonstrates a new potential for geodesy to directly probe rock rheology in situ across many spatial and temporal scales.

Earthquake Centroid Locations Using Calibration from Ambient Seismic Noise

Earthquakes occur in complex geology, making it difficult to determine their source parameters and locations because of uncertainty in path effects. We can avoid some of these problems by applying the cut-and-paste (CAP) method, which allows for timing shifts between phases, assuming a 1D model, and determines source parameters. If the travel times or lags of the phases due to path effects are known relative to a reference model, we can locate the events’ centroid with surface waves without knowledge of the 3D velocity structure. Here, we use ambient seismic noise for such a calibration. We cross correlate the seismic stations near the earthquake with stations 100–300 km away to obtain the 10–100-s surface wave Green’s functions. The new method is tested in southern California to locate the 2008 Chino Hills earthquake, which proves consistent with the epicenter location from P waves. It appears possible to use the location offset between the high-frequency P-wave onset relative to the centroid to provide a fast estimate of directivity.

Surface Wave Path Corrections and Source Inversions in Southern California

The cut-and-paste (CAP) method for retrieving earthquake source parameters has proven useful in many regions. Since CAP breaks three-component data into Pnl and surface wave segments and models them separately, imperfect 1D Green’s functions can be effectively used to model the records and determine source parameters. The resultant time shifts for different segments, for example, Pnl or surface waves, can provide valuable data for improving the velocity model or Green’s functions. Moreover, if these shifts can be known from empirical path calibrations or estimated from a tomographic map, we can recover both source mechanism and location with an extended procedure CAPloc (Tan et al., 2006). Here we present a complete workflow from path calibration to source inversion in southern California. In particular, we use the CAP results from about 160 events with known locations to derive the Rayleigh and Love wave group velocity maps. We take a tomographic approach to solve for lateral velocity variations in 10 km size cells in a uniformly 12 km thick seismogenic layer. Velocity variations of up to 15% are observed and the patterns correlate well with some geological features. Synthetic surface waves for 2D cuts throughout the model with a finite difference approach show significant improvement over 1D synthetics in fitting surface wave travel times, but little difference in waveforms, which suggests 1D synthetics are sufficient in modeling with simple time shifts. This simplification allows source inversion for both mechanism and location to be easily obtained with CAPloc. Finally, we test the effectiveness of CAPloc in determining source parameters, including both mechanisms and locations with such calibration maps in poorly monitored situations. In particular, we focus on two station inversions with two pioneer stations PAS and GSC, which have appropriate recordings since 1960. Considerable success is achieved when the events are bracketed.

Source Mechanism and Rupture Directivity of the 18 May 2009 MW 4.6 Inglewood, California, Earthquake

On 18 May 2009, an M_w 4.6 earthquake occurred beneath Inglewood, California, and was widely felt. Though source mechanism and its location suggest that the Newport–Inglewood fault (NIF) may be involved in generating the earthquake, rupture directivity must be modeled to establish the connection between the fault and the earthquake. We first invert for the event’s source mechanism and depth with the cut-and-paste method in the long-period band (>5 s). Because of the low velocity shallow sediments in the Los Angeles (LA) basin, we use two velocity models in the inversion for stations inside and outside the LA basin. However, little difference is observed in the resolved source mechanism (M_w 4.6, strike 246°/145°, dip 50°/77°, rake 17°/138°) and depth (7 to ~9 km), compared to an inversion using the standard southern Calfornia model. With the resolved source parameters, we calibrate the amplitude anomaly of the short-period (0.5–2 Hz) P waves with amplitude adjustment factors (AAF). These AAFs are used as corrections when retrieving source mechanisms of the smaller aftershocks using short-period P waves alone. Most of the aftershocks show similar source mechanisms as that of the mainshock, providing ideal empirical Green’s functions (EGFs) for studying its rupture process. We use a forward modeling approach to retrieve rupture directivity of the mainshock, consistent with movement on the NIF with rupture toward the southeast. Although we focus on P waves for analyzing rupture directivity, the resolved unilateral pattern is also confirmed with the azimuthal variation of the duration of SH waves observed in the basin. The high rupture velocity near the shear velocity and relatively low stress drop are consistent with the hypothesis of rupture on a mature fault.

CAPjoint, A Computer Software Package for Joint Inversion of Moderate Earthquake Source Parameters with Local and Teleseismic Waveforms

Accurate earthquake source parameters such as fault mechanism, depth, and moment magnitude are not only important in seismic‐hazard assessment, but also are crucial to studies of earthquake rupture processes and seismotectonics. Although large earthquakes (M_w 7+) may cause substantial damage, they occur less frequently. In contrast, moderate earthquakes (M_w 5.0–6.5) occur with much higher frequency and may occur on faults not geologically identified. Some of the moderate earthquakes cause damage in densely populated communities, especially in developing countries (Baumbach et al., 1994; Hamzehloo, 2005). For example, the 2011 M_w 5 earthquake in Lorca, Spain (Pro et al., 2014), the 2012 M_w 5.9 Ferrara earthquake sequence in northern Italy (Malagnini et al., 2012), the 2010 M_w 5 Suining earthquake in Sichuang Province of China (Luo et al., 2011), and the 1998 M_w 5.7 Zhangbei earthquake in Hebei Province of China (Li et al., 2008) all caused substantial economic loss and casualty. Compared with events larger than M_w 7, rupture processes of these moderate events can be approximated as point sources, which are usually described with a centroid moment tensor (CMT) because the rupture duration is usually shorter than the period used in the waveform inversion.

Locating earthquakes with surface waves and centroid moment tensor estimation

Traditionally, P wave arrival times have been used to locate regional earthquakes. In contrast, the travel times of surface waves dependent on source excitation and the source parameters and depth must be determined independently. Thus surface wave path delays need to be known before such data can be used for location. These delays can be estimated from previous earthquakes using the cut-and-paste technique, Ambient Seismic Noise tomography, and from 3D models. Taking the Chino Hills event as an example, we show consistency of path corrections for (>10 s) Love and Rayleigh waves to within about 1 s obtained from these methods. We then use these empirically derived delay maps to determine centroid locations of 138 Southern California moderate-sized (3.5 > M_w > 5.7) earthquakes using surface waves alone. It appears that these methods are capable of locating the main zone of rupture within a few (~3) km accuracy relative to Southern California Seismic Network locations with 5 stations that are well distributed in azimuth. We also address the timing accuracy required to resolve non-double-couple source parameters which trades-off with location with less than a km error required for a 10% Compensated Linear Vector Dipole resolution.

The discovery of a conjugate system of faults in the Wharton Basin intraplate deformation zone

A conjugate system of faults (shear and reactivated fracture zones) accommodates intraplate deformation in the Wharton Basin. The deformation at well-defined, narrow plate boundaries depends on the relative plate motion, but how the deformation takes place within a distributed plate boundary zone remains a conundrum. This was confirmed by the seismological analyses of the 2012 great Wharton Basin earthquakes [moment magnitude (Mw) 8.6], which suggested the rupture of several faults at high angles to one another. Using high-resolution bathymetry and seismic reflection data, we report the discovery of new N294°E-striking shear zones, oblique to the plate fabric. These shear zones are expressed by sets of normal faults striking at N335°E, defining the direction of the principal compressional stress in the region. Also, we have imaged left-lateral strike-slip faults along reactivated N7°E-oriented oceanic fracture zones. The shear zones and the reactivated fracture zones form a conjugate system of faults, which accommodate present-day intraplate deformation in the Wharton Basin.