Zhongwen Zhan

Supershear rupture in a Mw 6.7 aftershock of the 2013 Sea of Okhotsk earthquake

Supershear rupture down below Deep earthquakes occur in complex fault zones more than 70 kilometers below Earths surface. Although they do not pose much of a hazard, they can still provide valuable insight into how earthquakes rupture. Zhan et al. found that a magnitude 6.7 aftershock of the largest deep earthquake recorded to date—the 2013 magnitude 8.3 Sea of Okhotsk earthquake—ruptured faster than the velocity of its seismic waves. This so-called “supershear” rupture shows that deep earthquakes have diverse rupture mechanisms and have several routes to dissipate energy. Science, this issue p. 204 Earthquakes that occur hundreds of kilometers below the surface may have more than one rupture mechanism. Earthquake rupture speeds exceeding the shear-wave velocity have been reported for several shallow strike-slip events. Whether supershear rupture also can occur in deep earthquakes is unclear, because of their enigmatic faulting mechanism. Using empirical Greens functions in both regional and teleseismic waveforms, we observed supershear rupture during the 2013 moment magnitude (Mw) 6.7 deep earthquake beneath the Sea of Okhotsk, an aftershock of the large deep earthquake (Mw 8.3). The Mw 6.7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second, suggesting efficient seismic energy generation. Comparing it to the highly dissipative 1994 Mw 8.3 Bolivia earthquake, the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes.

The Pawnee earthquake as a result of the interplay among injection, faults and foreshocks

The Pawnee M5.8 earthquake is the largest event in Oklahoma instrument recorded history. It occurred near the edge of active seismic zones, similar to other M5+ earthquakes since 2011. It ruptured a previously unmapped fault and triggered aftershocks along a complex conjugate fault system. With a high-resolution earthquake catalog, we observe propagating foreshocks leading to the mainshock within 0.5 km distance, suggesting existence of precursory aseismic slip. At approximately 100 days before the mainshock, two M ≥ 3.5 earthquakes occurred along a mapped fault that is conjugate to the mainshock fault. At about 40 days before, two earthquakes clusters started, with one M3 earthquake occurred two days before the mainshock. The three M ≥ 3 foreshocks all produced positive Coulomb stress at the mainshock hypocenter. These foreshock activities within the conjugate fault system are near-instantaneously responding to variations in injection rates at 95% confidence. The short time delay between injection and seismicity differs from both the hypothetical expected time scale of diffusion process and the long time delay observed in this region prior to 2016, suggesting a possible role of elastic stress transfer and critical stress state of the fault. Our results suggest that the Pawnee earthquake is a result of interplay among injection, tectonic faults, and foreshocks.

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.

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.

Diverse rupture processes in the 2015 Peru deep earthquake doublet

The nearby Mw 7.5 and Mw 7.6 events in a deep earthquake doublet under Peru have diverse stress drops and radiation efficiency. Earthquakes in deeply subducted oceanic lithosphere can involve either brittle or dissipative ruptures. On 24 November 2015, two deep (606 and 622 km) magnitude 7.5 and 7.6 earthquakes occurred 316 s and 55 km apart. The first event (E1) was a brittle rupture with a sequence of comparable-size subevents extending unilaterally ~50 km southward with a rupture speed of ~4.5 km/s. This earthquake triggered several aftershocks to the north along with the other major event (E2), which had 40% larger seismic moment and the same duration (~20 s), but much smaller rupture area and lower rupture speed than E1, indicating a more dissipative rupture. A minor energy release ~12 s after E1 near the E2 hypocenter, possibly initiated by the S wave from E1, and a clear aftershock ~165 s after E1 also near the E2 hypocenter, suggest that E2 was likely dynamically triggered. Differences in deep earthquake rupture behavior are commonly attributed to variations in thermal state between subduction zones. However, the marked difference in rupture behavior of the nearby Peru doublet events suggests that local variations of stress state and material properties significantly contribute to diverse behavior of deep earthquakes.

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.

Trench motion-controlled slab morphology and stress variations: Implications for the isolated 2015 Bonin Islands deep earthquake

The subducted old and cold Pacific Plate beneath the young Philippine Sea Plate at the Izu-Bonin trench over the Cenozoic hosts regional deep earthquakes. We investigate slab morphology and stress regimes under different trench motion histories with mantle convection models. Viscosity, temperature, and deviatoric stress are inherently heterogeneous within the slab, which we link to the occurrence of isolated earthquakes. Models expand on previous suggestions that observed slab morphology variations along the Izu-Bonin subduction zone, exhibited as shallow slab dip angles in the north and steeper dip angles in the south, are mainly due to variations in the rate of trench retreat from the north (where it is fast) to the south (where it is slow). Geodynamic models consistent with the regional plate tectonics, including oceanic plate age, plate convergence rate, and trench motion history, reproduce the seismologically observed principal stress direction and slab morphology. We suggest that the isolated ~680 km deep, 30 May 2015 M_w 7.9 Bonin Islands earthquake, which lies east of the well-defined Benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined Benioff zone, can be explained by Pacific slab buckling in response to the slow trench retreat.

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.

Absence of remote earthquake triggering within the Coso and Salton Sea geothermal production fields

Geothermal areas are long recognized to be susceptible to remote earthquake triggering, probably due to the high seismicity rates and presence of geothermal fluids. However, anthropogenic injection and extraction activity may alter the stress state and fluid flow within the geothermal fields. Here we examine the remote triggering phenomena in the Coso geothermal field and its surrounding areas to assess possible anthropogenic effects. We find that triggered earthquakes are absent within the geothermal field but occur in the surrounding areas. Similar observation is also found in the Salton Sea geothermal field. We hypothesize that continuous geothermal operation has eliminated any significant differential pore pressure between fractures inside the geothermal field through flushing geothermal precipitations and sediments out of clogged fractures. To test this hypothesis, we analyze the pore-pressure-driven earthquake swarms, and they are found to occur outside or on the periphery of the geothermal production field. Therefore, our results suggest that the geothermal operation has changed the subsurface fracture network, and differential pore pressure is the primary controlling factor of remote triggering in geothermal fields.

Lower Mantle Substructure Embedded in the Farallon Plate: The Hess Conjugate

The morphologies of subducted remnants in the lower mantle are essential to our understanding of the history of plate tectonism. Here we image a high-velocity slab-like (HVSL) anomaly beneath the southeastern U.S. using waveforms from five deep earthquakes beneath South America recorded by the USArray. In addition to travel time anomalies, the multipathing of S and ScS phases at different distances are used to constrain the HVSL model. We jointly invert S and ScS traveltimes, amplitudes, and waveform complexities to produce a best fitting block model characterized by a rectangular shape with a 2.5% S wave velocity increase and tapered edges. While the Farallon slab is expected to dip primarily eastward, the HVSL structure apparently dips 40° to 50° to the SE and appears to be related to the eclogitized Hess conjugate.