Matthew Herman

Upper Plate Deformation in the Kaikoura Earthquake

Collaborators: Kevin Furlong (Penn State University), Harley Benz (USGS), Kirsty McKenzie (North Carolina)

A notable feature of the 2016 Kaikoura earthquake was the faulting visible at the surface, which in some places exceeded virtually all previously observed surface faulting magnitudes for faults of similar length! An important question is whether the Kaikoura earthquake deformation behavior is anomalous or not. Complicating the interpretation is the fact that this setting (specifically, the region above a subduction zone earthquake) is usually underwater. In contrast, the Kaikoura earthquake occurred in a setting where the usually offshore upper plate action was directly visible onshore. We investigated which characteristics of this event allowed these surface faults to slip with such extreme magnitudes, and it is likely that these processes are acting at many subduction zones globally, but are invisible because they occur underwater.

Pseudo-coupling and Subduction Earthquakes

Collaborators: Kevin Furlong (Penn State University), Rob Govers (Utrecht University)

An important component of evaluating earthquake hazards is measuring the rate of slip deficit accumulation on plate boundary faults. Where plates are frictionally locked on parts of the interface, the interface accumulates slip deficit at the rate of relative motion between the plates. These regions ("asperities") are where earthquakes typically nucleate and rupture. Outside asperities, although the interface might be able to slide at lower friction, proximity to locked areas prevents these regions from sliding at the full plate rate. These regions accumulate slip deficit, but at a lower rate than the relative plate motion rate, thus appearing partially locked despite no actual mechanical coupling across the interface.

This framework for modeling inter-seismic slip deficit accumulation and areas of partial slip deficit may help to explain several observations of recent earthquakes. Ruptures that propagate outside of fully locked areas through these "pseudo-coupled" regions can be larger than expected from the asperity dimensions alone. Earthquakes also show an intriguing pattern of slip increasing along with rupture length. Our models explain this observation as the magnitude of slip being limited by the pseudo-coupling produced by neighboring, unruptured asperities. Finally, our modeled deformation around pseudo-coupled regions up-dip of locked asperities in subduction zones are consistent with observed patterns of tsunamigenic slip, such as those that were gathered after the 2011 Tohoku earthquake (Herman et al., 2018).

Quantifying Geodynamic Forces

Collaborators: Rob Govers (Utrecht University), Nicolai Nijholt (UU, now at TU Delft), Lukas van der Wiel (UU)

There is an ongoing discussion about the relative significance of various geodynamic drivers in many rapidly deforming regions around the world. Various approaches to this problem include elastic block models, where deformation is partitioned onto discrete faults; viscous sheet models, where gravitational potential energy variations play the most important role; and hybrid models, where many geodynamic forces — some well established, others newly proposed — are incorporated. The primary constraints on these models are (a) velocity observations measured at GNSS stations and increasingly by satellite radar interferometry, and (b) stress orientations constrained dominantly by earthquake focal mechanisms. A major question is whether these observations are capable of distinguishing between the various geodynamic models.

I am developing a technique to systematically and quantitatively answer this question. With Nicolai Nijholt and Rob Govers, we have been developing 2-D finite element models of the Gibraltar and Aegean regions, which include major faults, subduction-related forces, other gravitational forces, and plate motion drivers. But to answer our research question, it is not sufficient to run just this one model anymore! To determine which of these forces can be constrained, we run 10s or 100s of thousands of these models, varying the parameters to see which produces the best fit. (For aficianados out there, we are using a Bayesian framework.)

Extension in Subduction Zones After Megathrust Earthquakes

Collaborators: Rob Govers (Utrecht University), Kevin Furlong (Penn State University)

The maps at the left show the substantial increase in normal faulting earthquakes after the 2011 Mw 9.0 Tohoku, Japan, earthquake. Although there were some normal faulting earthquakes prior to 2011, they were relatively infrequent and accounted for a small percentage of the total moment release. An important question for constraining seismic hazards and understanding the nature of earthquake cycle deformation processes is how long these extensional events will continue. We examine these events in context of relatively straightforward geodynamic earthquake cycle models representing the Japan subduction zone, and compare these model results to instances of normal faulting earthquakes triggered by large megathrust earthquakes globally.

This analysis shows that great megathrust earthquakes and triggered afterslip on the plate boundary promote normal faulting earthquakes, but the location and intensity of these aftershocks depends strongly on the spatial distribution and magnitude of co-seismic slip. Viscous relaxation of the deeper, warmer asthenosphere does little to the stress state of the elastic regions. Therefore, the system remains in a state favorable for normal faulting events until relocking of the plate interface and continued plate motion gradually re-establish the pre-earthquake compression. It turns out an important aspect of the stress evolution in the system is the degree of locking in the seismogenic sections of the plate interface. If the interface is widely locked, it is more difficult to trigger normal faulting seismicity. If instead the interface is characterized by locked patches, then extensional deformation is easier to produce. These two scenarios are hard to distinguish from direct observations, though, because they produce similar fault slip and surface deformation due to the pseudo-coupling effect (Herman and Govers, 2020).

Filling Gaps - The 2014 Iquique and 2015 Illapel Earthquakes

Collaborators: Kevin Furlong (Penn State University), Gavin Hayes (USGS National Earthquake Information Center), Harley Benz (NEIC), William Yeck (NEIC), Jennifer Nealy (NEIC), William Barnhart (University of Iowa)

The oceanic Nazca plate descends eastward under South America at 70 mm/yr along the subduction trench that lies parallel to the coast of Chile. This subduction zone has a long history of hosting large earthquakes that produce severe shaking and extensive tsunamis. Some parts of the boundary last experienced a great (magnitude 8 or larger) earthquake over a century ago. Enough time has passed that these "seismic gaps" represent a significant great earthquake and tsunami hazard. I studied two events that ruptured seismic gaps in the Chilean subduction zone in detail, examining the processes that led up to the mainshocks, the characteristics of mainshock slip and aftershock distributions, and the possibility for future earthquakes.

On April 1, 2014, a magnitude 8.2 megathrust earthquake occurred offshore of northern Chile, generating severe shaking and producing a 2 meter tsunami in the Port of Iquique. This earthquake was preceded by four magnitude 6.0 and larger foreshocks in the two weeks before the big one. Should we have seen it coming? I used a technique called Coulomb stress change analysis to determine that these events may have sequentially triggered each other and led to the mainshock. I also carefully examined records of slow ground motion in Chile before the earthquake, as measured by high-precision GPS instruments. The GPS data suggest that there was other slip on the plate boundary during the foreshock sequence that did not produce large earthquakes, but may have aided in triggering the mainshock (Herman et al., 2016).

The 2015 Mw 8.3 Illapel, Chile, earthquake occurred in another seismic gap in the Chilean subduction zone. The ability to record and describe earthquake locations and sizes has improved by a huge amount over the past decade. For this event, I led a collaborative effort with scientists at the USGS and University of Iowa to combine and integrate results from different types of observations and improve our understanding more than the individual analytical techniques. The details of co-seismic slip during the earthquake, careful and precise aftershock locations, and physical models of the deformation in the subduction system suggest that the most vigorous aftershock activity occurs near the edges of the rupture zone and decays relatively quickly. The location and distribution of these aftershocks are highly sensitive to the location and magnitude of slip. In contrast, triggered events continue to occur farther from the rupture zone over a much longer time period. These events include both aftershocks on the interface, as well as other events, such as outer rise aftershocks occurring primarily next to areas where slip came closest to the trench (Herman et al., 2017).

Are Earthquakes Different Far From Plate Boundaries? The 2010-2016 Canterbury Earthquake Sequence

Collaborators: Kevin Furlong (Penn State University), Robert Herrmann (Saint Louis University), Harley Benz (USGS National Earthquake Information Center)

In September 2010, a magnitude 7.0 earthquake struck in the region west of Christchurch on the South Island of New Zealand. Although the South Island of New Zealand straddles the boundary between the Pacific and Australia plates, the earthquake source region is far from the plate boundary and has been notably devoid of seismicity over the last century. The September event was followed over the next two years by a sequence of over 10,000 aftershocks, including a deadly magnitude 6.1 earthquake beneath Christchurch in February 2011, another magnitude 5.8 event in June 2011, and a pair of magnitude 5.8 aftershocks in December 2011. An important question in the seismic hazard assessment of such low-seismicity areas is whether the behavior of this sequence can be understood through the same lens as more common plate boundary seismicity (like that of California, Turkey, or the Alpine Fault of New Zealand). To address this issue, I performed different types of seismological analyses on 150 of the largest earthquakes in the sequence to figure out how the faults slipped from the records of shaking. Perhaps our most important finding is that these earthquakes relieved stresses locally, but the broader region of eastern South Island still remains in the same state of stress that is responsible for these earthquakes (Herman et al., 2014).

Seismicity in South Island, New Zealand, also continued to occur for a much longer time period than is typically expected. One common way to think about the duration of an aftershock sequence is by looking how aftershock frequency decreases over time, which is typically proportional to 1/time (this is called "Omori's Law"). But this "Law" is really an empirical relation; there are no physics dictating that the aftershock rate must decrease in this fashion. In reality, stress changes from earlier earthquakes might trigger subsequent large earthquakes long after the aftershocks from the initial event should have decayed according to Omori's Law. Then these subsequent earthquakes will have decaying aftershock sequences of their own. I explored the seismological details of another magnitude 5.7 earthquake in Christchurch, New Zealand, which occurred in 2016, years after the last major event from the earlier Canterbury earthquake sequence. I used this earthquake to bring attention to other nearby areas that may still represent a seismic hazard in the Christchurch region (Herman and Furlong, 2016).